The MUNCASTERsteam-engine models

EDGAR T. WESTBURY glances back with a modern eyetoosome classic models of the past

I N THE COURSE of the long historyOf MODEL ENGINEER-now, in-cidental ly, approaching 60

years-many notable designs anddescriptive articles have been pub-lished which have established tradi-t ions or marked milestones ofprogress in model engineering.

Not only are these remembered byold readers but they are often thesubject of considerable discussion.and requests for further informationabout them are constantly encountered.A few of the authors of these featuresare still with us, and in one or twocases continue to contribute articles;but most of them have passed on andare no longer available to provideeither new designs or guidance ontheir earlier ones.

Many readers have suggested thatthe M.E. should reprint some of theseearlier features, but while this mightbe a good idea, from certain aspectsthere are several reasons why thepolicy has not been adopted by thisjournal. In the first place, althoughthe designs for models do not neces-sarily become outdated the mode oftreatment, including methods of con-struction, is subject to certain changesas workshop equipment and techniqueimprove.

In modern settingTo many modern readers, reprinting

of old articles or designs may seemto be a policy of retrogression, or atleast stagnation; it may even givethe impression that there is a dearthof new ideas in model engineering-which is far from being the case.Therefore the idea of verbatim re-prints of articles is not considereddesirable, but there is much to be saidfor the revival of old designs in amodem setting.

There can be few model engineerswho have not heard of such pioneersas Henry Greenly, C. S. Lake, FredWestmoreland, H. H. Groves, GeorgeGentry, or the traction engine special-ist “ Frost-spike,” who have beenacknowledged masters in a very wideand versatile field of design. Lastbut not least, the name of H. Mun-

MODEL ENGINEER

caster is well remembered as a special-ist in the design of all types of steamengines, whose excellent drawings ofmany types of stationary and marineengines in early volumes of the M.E.,and also in the handbook M o d e lStationary Engines, published nearlyhalf a century ago, provided scopefor the talents of innumerable con-structors.

H. Muncaster was a practicaldraughtsman who not only had awide experience of steam-engine designbut also obviously had a love anddevotion to his craft, and to allthings mechanical. In the introduc-tion to his book he pays due homage

1-A simpleoscillating

engine

to the many early engineers andinventors who contributed to thedevelopment of the steam-engine,and also gives a complete answer tothose who would condemn modelengineers for “living in the past.”

“For the purposes of the modelengineer,” he states, “it does notfollow that the most recent and perfectengines are most suitable; on theother hand, some of the older enginesform subjects better adapted and morefitted as prototypes for models, beingmore picturesque and providing betterobject lessons.”

With which precepts I whole-heartedly agree, and also with hisfurther comments that many of themost popular so-called models “ haveno prototype in reality, but neverthe-less may be useful in illustrating someof the points of the steam-engine, aswell as providing a simple motor,where only a small amount of power isrequired.” No model engineer, there-

270

fore, need despise the crude and primitive types of models producedby beginners, so long as they lead on tothe more realistic types which wereMuncaster’s speciality.

The simplest form of engine de-scribed by Muncaster is one having asingle-acting oscillating cylinder (Fig.1) and this will commend itself tomany readers, not only on accountof its simple construction, but alsobecause it can be built without castings.It is of the type which would now beclassed as “ inverted ” vertical, havingthe cylinder below the crankshaft,though in the early days the practiceof locating the cylinder at a low level-firmly bolted to the floor if possible-was considered normal and orthodox.

THE PILLARThe main structural component of

this engine is the pillar, shown inFig. 2, the lower portion of which isof rectangular section, with extendedfeet at the sides for mounting on theflat square baseplate. At the top end,the section is also rectangular and iscross bored to form a housing for thesingle main crankshaft bearing. .Inthe centre, it is turned to circulartapered form, with simple ornamenta-tion in the form of a beading near thelower end.

To save material in making thispart, the foot at the base may bemade separate and silver-soldered on;or screwing and sweating wouldprobably be satisfactory. This shouldbe done before machining and Isuggest that the front and rear sidesshould then be faced quite flat andtrue by filing, or any other convenientmethod, after which the two endsmay be marked out, exactly centralboth ways, and centre-drilled so thatthe part can be mounted betweencentres for turning.

The cross holes for the main bearingand the cylinder trunnion may nowbe drilled, and it is essential thatthese should be exactly square withthe pillar face, so it will be advisable,after marking out their positions, toset the pillar up on the faceplate ofthe lathe for these operations.

An alternative to the pillar as the

21 FEBRUARY 1957

support for the crankshaft bearing isgiven in Fig. 3. This consists of anA-frame cut from sheet metal, withthe bearing housing at the apex,either bushed or otherwise reinforcedto provide extra bearing surface. Itdoes not, however, incorporate theport block or other cylinder mountingand it is not explained how this shouldbe fitted. For this particular type ofengine, I do not consider it so elegantin appearance as the pillar, neitherdoes it simplify construction.

CYLINDERThe designer suggests that the

cylinder (Fig. 4) may be made from apiece of brass tube, with the flange,end cap and portface soldered on,

These conditions can be assured bymachining, as the bore and flangecan be dealt with at one setting;a D-bit is recommended for taking thefinal cut in a blind bore, after whichit should be finished by lapping withaluminium oxide or brick dust ona short copper or aluminium lap-not one which extends the full lengthof the bore, as this will tend to makeit tapered or bell-mouthed.

If the cylinder is machined from thesolid the smallest diameter of barwhich can be used is 1 in. A gooddeal of material will. be left aroundthe base, after turning as much aspossible of the outside, and this willhave to be filed or machined away,leaving the portface to be faced flat.

working to the same index reading onthe topslide for each cut, the shapemay be made practically circularand flush with the upper turned part,needing only a clean-up with a smoothfile to take off the sub-angular corners,and a final polish with emery cloth.

The trunnion is fitted to a tappedhole in the face of the portblock,and it is most essential that thisshould be dead square with the cylin-der axis. It could well be drilled andtapped while the cylinder is set upon the faceplate’, to ensure this; thehole should not go right through intothe bore, though some constructorsmay find difficulty in tapping a shortblind hole.

If it does go through, however,

but in practice it will usually be foundjust as easy to machine it from thesolid, as the manipulation of smallpieces, which have to be accuratelylocated and soldered simultaneously,is not as simple as it looks. In eithercase, however, it is essential that thebore of the cylinder should be exactlycircular and parallel, the flange facedsquare with it, and the portface deadflat and parallel to the axis.

21 FEBRUAltY 1957

The components of the engine

A very efficient way of doing this isto make a short mandrel, of a size tofit neatly in the cylinder, and fix thisin an angle-plate or a short piece ofangle iron, on the lathe faceplate.

Clamp the cylinder endwise on this,checking it first to see that it is parallelwith the faceplate; it can then beturned into any position to‘ machinethe portface of “nibble” away therest of the surplus material. By

271

make certain that the trunnion studdoes not project into the bore whentightly screwed in and that there areno burrs left on the inside to interferewith the free movement of the piston.It is an advantage to machine a shallowrecess around the tapped hole torelieve the centre of the face; alter-natively, this may be done on thecorresponding face of the pillar.

Little need be said about the cylinder

MODEL ENGINEER

Muncaster

steam-engine

models . . .MACHINED AWAY

cover (also shown in Fig. 4), as thisis a simple job which can be turnedat one setting. The spigot should fitneatly in the cylinder bore, and thehole drilled centrally to a working fitfor the piston rod. It is attached tothe cylinder flange by three 3/32in.or 8 B.A. screws.The piston assembly(Fig. 5) is built up in three pieces,the rod, of 3/32 in. dia. bright mild-steel, being screwed on each end totake the solid piston at one end andthe crankhead bearing on the other,k;&;Fse pieces bemg of brass or

In the drawing, the piston is shownas a plain parallel disc, machined tofit closely in the cylinder, but I stronglyrecommend, at least to those withlittle experience in these matters, thata groove should be machined in itfor packing with graphited asbestosor cotton yarn. The advice I havegiven in articles on other engines,that the final machining of the pistonshould be done after it has beenscrewed tightly on the rod, still holdsgood. Final adjustment of the lengthof the rod, so that the piston juststops clear of the end of the bore atthe extremity of its stroke, can bestbe done on assembly.

CRANKSHAFTThe crankshaft is built up with a

web made from rectangular brass bar,into which the main journal and crank-pin are screwed. As an alternativeform of construction a disc can be. used, and this would not only improvethe appearance but could also bebalanced if desired. In either case,however, it is essential that both thetapped holes should be square with

the web and parallel with each other.No details are given of the flywheel,which is shown as a solid disc, but Irecommend that a spoked flywheelwith a heavy rim should be fitted.

The main bearing is in the form ofa plain bush, made to press tightlyin the cross hole at the top of thepillar, and the centre hole in the endof the latter is drilled through into thebush to serve as an oil hole. It isnow in order to assemble the partstemporarily, to ascertain that every-thii works freely and smoothly,without binding or tight spots, andthat the piston clears at both endsof the cylinder.

MODEL ENGINEER

Set up for rna;F&;g the cylinder

PORT LOCATION AND TIMINGThe entire success of an oscillating

cylinder engine depends on the accur-ate location of the steam-ports, andthis is where many constructors failto get the best results, as it is by nomeans easy to mark out and drillholes exactly in the right place. Boththe size and position of the two holesin the stationary portblock are depend-ent on their radius from the trunnioncentre, in conjunction with the maxi-mum distance of swing at extremecylinder angularity-which, incident-ally, is not the same thing as half thepiston stroke.

In this engine, the maximum dis-tance of swing under these conditionsat 3/8 in. radius is 3/16 in., so the portsshould be drilled at 3/16 in. centredistance apart, and as the blank spacebetween them should be exactly thesame as the port diameter, this dimen-sion should be 3/32 in.; on no accountdrill larger holes as this would onlyresult in steam wastage between theports.

Even with the utmost care in locat-ing the holes in the portblock, how-ever, there is still a possibility of errorin the position of the single hole in thecylinder face, which may completelynullify all efforts to produce a correctlytimed engine. I suggest, therefore,adopting an unconventional methodof drilling-these holes, which not onlykills two birds with one stone,. as itwere, but also ensures positively

272

that they are correctly located inrelation to each other,

First of all, the hole m the cylinderis marked out as correctly as possibleand drilled undersize, say 5/64 in.or No 48 drill, the hole being con-tinued right through the ‘oppositewall of the cylinder. The engine isthen assembled and the crankshaftturned to swing the cylinder tomaximum angle in one direction,where it is clamped in place by thenut on the trunnion stud with asuitable distance piece.

LAPPINGBy running the drill through the

hole in the cylinder the position ofthe hole in the block may be spottedor drilled to full depth, after whichthe cylinder is shifted to the otherextreme position by turning the crank,and the operation repeated. Theports are then opened out to 3/32 in.or No 42, and the hole in the outercylinder wall closed by a plug screwedor soldered in.

Finally the two side holes in theportblock, forming steam and ad-mission connections, are drilled tomeet the ports and tapped to takescrewed pipes, the faces of both cylin-der and portblock then being lappedon a piece of plate glass to produce atruly flat and smooth finish.

When finally assembled, a lightspring is fitted to the trunnion andthe locknuts are adjusted to hold thecylinder against the block, but withno more tension than is necessary_tokeep it in steamtight contact againstthe working steam pressure. Theengine will run in either direction,according to which of the two con-necting pipes is connected to thesteam line, so that it could readilybe made reversible by fitting a change-over cock.

If made according to directions andcarefully finished, this should notonly be a satisfactory working model,but also a handsome and dignifiedone.

l To be continued.

----

ADDITIONS TO THE LATHE

Instructions for making centringdevices ; chucking accessories; toolholders and cutter bars; dividingappliances.; simple milling attach-ments; aids to screwcutting; andsteadymg appliances are to be foundin Edgar T. Westbury’s Lathe Acces-sories.-

Priced 3s. 6d., postage 3d. (U.S.A.and Canada $1.00), it can be obtainedfrom Percival Marshall and Co. Ltd,19-20, Noel Street, London, W.1.

The MUNCASTER steam-engine models

EDGAR T. WESTBURY is reviewing some classic models ofthe past in the light of modern techniques

EADERS will no doubt haveR noticed that the drawings ofthe simple engine described

in the last article were not fullydimensioned, and just in case thereshould be any complaints aboutthis I wil l anticipate them bysaying that Muncaster, in com-mon with many pioneer modeldesigners, did not consider. itnecessary to give more than a fewleading dimensions on drawings.

I have, however, added a scalewhich should be helpful in supplyingthe deficiency, in conjunction eitherwith a simply-made scale rule or apair of proportional dividers.

The present-day engineer is accus-tomed to drawings which have everydimension, including in many caseslimits and clearances, fully marked,as this is absolutely necessary inindustrial production, where differentparts have to be made, or even suc-cessive operations carried out onsingle parts, by individual workersout of direct touch with each other.

oscillatingengines

DOUBLE-ACTING OSCILLATINGUse initiative CYLINDER ENGINE

In cases where all the machiningand fitting on a one-off job are in thehands of a single constructor, how-ever, meticulous marking of everyessential and non-essential dimensionis by no means so important. WhileI should be the last to condoneinaccurate or slipshod work in anykind of model engineering, I believethat one can become a slave of theblueprint, and it is good engineeringpractice to work occasionally to whatthe professional engineer would con-sider inadequate drawings or speci-fications, if only because it helps oneto cultivate a sense of proportion.

Unlike the previous engine, theconstruction of this example, illus-trated in Fig. 7, favours the use ofcastings for the main structural parts,though they can be produced bycutting from the solid and fabrication.For instance, the hollow base, D ,could be made by soldering or brazinga rectangular frame of strip materialto a flat plate, and the bearing sup-ports or A-frames, F, could be madeby bending strip material to theprofile, as seen in the end view, andsoldering or brazing in the curvedsupporting rib or crossbar.

In the field of natural history,reasonably accurate re-constructionsof prehistoric animals were madefrom fragments of fossil bones longbefore they could be verified by furtherevidence; and many model engineershave produced good work withnothing more in the way of informa-tion than a few rough sketches andpossibly a photograph or two.

It is most important that theseframes should stand exactly per-pendicular to the base when fittedand that the top surfaces which formthe bearing seatings should be exactlythe same height and in alignment.While it is possible to correct errorsin these respects by packing under thefeet of the frames or the bearings, thisis certainly not a practice to beencouraged.

7 MARCH 1957 337

Continued from 21 February 1957, pages 270-272

I must confess that I have not agreat deal of patience with the typeof reader who makes a major issueout of a missing dimension or aslight discrepancy in a drawing; themore complicated drawings become,the more difficult it is to avoid minorerrors which escape the most carefulchecking, and the more likely they areto deter the timid beginner fromtackling construction.

2-Double-acting

If the cylinder, A, is made fromstock material, the circular portfacemay be made from a piece of 7/8in.dia. bar, filed or machined to fitclosely to the side of the barrel andpreferably secured by silver soldering.At the same time, short half-roundpieces can be fitted above and belowthis face. It will be seen that a bossor spigot is provided on the oppositeside of the barrel to receive the pointof a pivot screw, and this shouldsimilarly be fixed, exactly in diametricalignment with the portface.

The drawing shows a raised beadingin the centre of the cylinder; this isnot a necessity, but is desirable bothfrom the point of appearance and alsoto stiffen the cylinder wall. Formachining the bore and facing oneend flange, I recommend mountingon an angle plate with a strap bearingon the spigot, but care should betaken not to apply so much pressureas to risk distortion of the barrel.The other end flange is faced bymounting on a mandrel.

Mark off centresIt is absolutely essential that the

centre indentation in the spigotshould be dead in line with the holein the centre of the portfaee, and inorder to ensure this, it would be asound policy to mark off these centreswith a surface gauge, with the cylinderresting on the machined flange faceto locate the horizontal line, andthen mounting it on a mandrel restingin V-blocks for the vertical line. Theintersections on both sides are thencentre-drilled, and the Cylindermounted between centres for machin-ing the portface.

Flanged covers are secured, eachby four screws, to the ends of thecylinder, and as the engine is double-acting the upper cover is fitted with agland, or “ stuffing-box” as it wastermed by the early engineers. It isimportant that this should be exactlycentral and truly tapped; these con-ditions can be ensured by using themethods which I have described forengines of my own design.

No exact details are given of the

MODEL ENGINEER

Muncastermodels . . .

piston, but from other engine dimen-sions it is clear that this should be5/16 in. wide, and a groove 3/16 in. widex 1/8 in. deep may be turned in it totake graphited packing. The rod is5/32 in. dia., and the crankheadbearing, which is screwed to theupper end, must be split as shown inFig. 8, to enable it to be assembledon the crankshaft.

The cylinder is mounted betweenthe fixed portblock, B, and the pivotblock, C, both of which are screwedto the base between the feet of theA-frames, and it is most essential thatthe centres of these should be deadin line and that the face of the block,B, is squarely located so that theportface of the cylinder beds trulyagainst it while being quite free toswing when the pivot screw is properlyadjusted.

This is perhaps a somewhat difficultcondition to ensure, at least for thebeginner, and I suggest that onemethod of doing so is to fix the blockstogether temporarily by sweating, withan aligning dowel in the pivot holes(leave the tapping of the pivot blockpro tem), and facing off the basesurface by machining or filing exactlysquare with the face of the portblock.Even this, however, will be of noavail if the mounting surface of thebase is not dead flat and true; it is alsonecessary to locate the two blockscorrectly in relation to each otherwhen they are screwed in position.

Aligning mandrelTo do this, an aligning mandrel is

made from a dead straight piece of5/32 in. silver-steel rod, long enoughto extend across the base and passthrough both blocks. After squaringup their positions, the portblockshould be fixed first by its two screwsand then the pivot block; in each caseit will be possible to spot the positionsof the tapping holes in the base fromthose in the blocks to avoid the riskof error.

Small clamps are useful for holdingthe parts in place during these opera-tions and if not already available theyshould be made or obtained rightaway, as they will certainly be neededfor innumerable subsequent jobs.

When fitted to complete satis-faction, the hole in the pivot blockmay be tapped and a locking screwfitted to the top (though I shouldpersonally consider that a locknut on.the screw would be more satisfactory);the portblock is also equipped with acentre stud to fit the hole in thecentre of the cylinder portface.

MODEL ENGINEER

The crankshaft., G, may be eitherbuilt up or machined from the solid;it is not necessary to give details ofeither method as they have beendescribed on innumerable occasionsin connection with other engines. Imay observe, however, that this offersquite a good opportunity for thebeginner to get some practice inmarking out and machining anorthodox type of crankshaft from apiece of flat bar, which should be ofabout 1 in. x 3/8 in. section.

I would, however, suggest a slightmodification to this component, asdrawn; it would appear that the onlyend location of the shaft is thatprovided by the flywheel on one sideand the driving pulley on the other.I do not consider this to be very good

practice, as although it serves itspurpose, in the event of either fittingbeing shifted, accidentally or other-wise, it would be possible for thecrankhead to be forced out of align-ment.

A much better method would be toprovide positive end location on theinside of the main bearings, either byenlarging the idle portion of thejournals to form locating collars, orbetter still, extending the bearingsurface inwards up to the webs of thecrank. Incidentally, I do not under-stand why it should be necessary toplace the bearing frames so far apartin this engine as it is a cardinalprecept in any engine design to supporta crankshaft as close up to the websas possible.

0 h I 2 3 4 FIG. 7

Inches8 -8

338 7 MARCH 1957

Fig, 7 : General arrangement of double-actingoscillating engine, and details of cylinder

Extreme right, Fig. 8:Details of the split bear-ing for piston rod

Right, Fig. 9: Portblock,showing formation of theinlet and outlet passageson both sides (to suitthe twin-cylinder engine)t.++. .+++tt_7‘I Left, Fig. 10: Arrange-ment of complete plantwith vertical boiler, show-ing cylinder portblock,reversing valve, etc.

I I I I

FIG. 10

The main bearings, E, are of theplummer block type, and correctpractice dictates that they should besplit, though this is not a practicalnecessity in a small engine as solidbearings can be threaded on from thetwo ends of the crankshaft. Theymust, of course, be correctly alignedbut this is easily ensured if they aremade and fitted properly, and anysmall discrepancy which may causethe shaft to run tight when finallyfitted can be corrected by running areamer or lap through both bearingswhen they are screwed down in situ.(This, of course, pre-supposes thatthe shaft journals are properly mach-ined or assembled in exact alignment).

CYLINDER PORT LOCATIONIn drilling the steam ports in the

cylinder and portblock, it is notpracticable to use the simple methodof ensuring their correct locationrecommended in the previous article,as the former ports do not lead directlyinto the cylinder but communicatewith oblique ports drilled from thecylinder ends. It could, perhaps, bedone by drilling straight-through holesand plugging them up afterwards,but there might be some risk of leak-age with relatively inaccessible plugs.

The method of port formation re-commended by Muncaster, however,

7 MARCH 1957

is ingenious, and with careful measure-ment and marking out should givevery satisfactory results. In this case,the cylinder ports are located bymeasurement, and an annular grooveis formed in the fixed portblock, B,concentric.with the pivot pin, and atthe same radius as the two cylinderports (Fig. 9).

At the top and bottom positionsthe groove is blocked or “stoppedoff ” by fitting plugs, which are facedoff flush with the face of the block;and at right angles to these positions,holes are drilled in the groove tocommunicate with the inlet and exitpipes, one of each thus serving forthe two ends of the cylinder.

It will be seen that when the engineis assembled with the cylinder incontact with the block, and the pistonon dead centre, both cylinder portswill be blanked off by the stop pegs;but when the crank is turned to swingthe cylinder to its extreme angularposition (indicated by the line, ab)the top cylinder port will be placed,via the annular groove, in communica-tion with the right-hand horizontalpassage, and the lower port with theleft-hand passage.

At the other extreme end of theswing, the communications will bereversed; thus the conditions areestablished for admitting steam to

339

FIG.6.”.

each end of the cylinder in turn andreleasing it to exhaust, on alternatestrokes of the piston. As in the caseof the single-acting engine, optimumtiming is obtained when the “ stop ”of the groove lines up exactly with thecylinder port at dead centre and isexactly the same width as the port.

The ports in the cylinder and theoblique drilled passages should be1/8 in. dia., and the entry at the mouthof the cylinder in each case shouldbe unobstructed, which calls forchamfering the edge of the cylinderand the spigot of the cover at thispoint. The portfaces should be care-fully lapped and may with advantagebe relieved in the centre as recom-mended for the previous engine. Thinpaper gaskets should be fitted to thecovers to ensure steam-tight joints.

A TWIN-CYLINDER ENGINEThe design of this engine may be

adapted to form a twin or “ double ”engine by simple modifications andadditions to the components. Thebase will, of course, have to beenlarged, and a third bearing supportfitted to take a two-throw crank,with the crankpins located at 90 deg.as shown in the general arrangementdrawing, Fig. 10, of a complete double-cylinder engine with vertical boiler,in elevation.

The drawing shows a modifiedform of bearing support, but mostcomponents are identical with thoseof the single engine. A double-sided portblock is fitted between thetwo cylinders with both its facesgrooved and ported as shown inFig. 9.

This drawing is admittedly some-what sketchy and lacking in detailbut it should be readily understood ifconsidered in conjunction with thedrawing of the single-cylinder ar-rangement (see Fig. 7).

Some further details of the twin-engined plant will be given in thenext instalment.

l To be continued.

MODEL ENGINEER

’ ! .-,

The MUNCASTERsteam-engine

modelsEDGAR T. WESTBURY is reviewing some classic models of the

past in the light of modern techniques

Continued from 7 March 1957. pages 337 to 339

IN THE ISSUE of February 21, itwas stated that an oscillatingengine can be made to reverse

its direction of rotation simply bychanging over its steam and ex-haust connections; this applies toall engines timed to work withoutlap or lead, whether single ordouble-acting, and with any num-ber of cylinders, with the exceptionof those having flat slide valves.

It is therefore possible to equipany of these engines with a simplereversing control, consisting of a four-way change-over cock or valve, andsuch a control is indicated in the planview of the double engine, Fig. 11.Details of the reversing valve aregiven in Fig. 12, where it will be seento consist of two essential parts, astationary portblock, with four portsand their connecting pipes, and arotating valve with interrupted annu-lar groove just like the portblockshown in Fig. 9. These parts arelapped to fit truly together and heldin friction-tight contact by a springacting on the centre pivot.

MODEL ENGINEER

When the valve is turned into sucha position that the blanked portionsclose two diametrically opposite portsin the block, communication to thecylinders is shut off; this is the “ stop ”position. By moving it in a clockwisedirection,. however, steam is admittedto the pipe ‘marked R, and exhaustconnected to L; while moving it theother way reverses these connections.I would suggest, in order to improvethe seal of the valve face and makeit less critical in angle of movement,that the width of the groove stopshould be increased by fitting a pegof larger diameter, or other means.

A VERTICAL BOILERThe boiler recommended for either

of these engines is shown in sectionin the elevation drawing, Fig. 13, andin plan view, Fig. 14. It is preferablymade in copper, about 3/64 in. thick,or 18-gauge, for the shell, with end-plates 1/16 in. thick or 16-gauge, whichcan be beaten or spun to the shapeshown. For relatively low pressures,which should be perfectly satisfactoryfor these engines, riveting and softsoldering will be safe enough, thoughas a precaution in case the boilershould ever run dry the joints of theinner flue and cross tubes, whichare in contact with the flame, maywith advantage be silver soldered.

The flue is l-1/4in. dia. by 16-gauge,with cross tubes 3/8 in. dia. by 20-gauge,and the steam-pipe, which may either

3-Simpleslide-valve

engines

be brazed directly into the top of theboiler or fitted with a union joint, is3/16 in. dia.Whatever other fittings are attachedto the boiler, a safety-valve muston no account be omitted; this maybe of a simple type, combined with thefiller plug, as shown in Fig. 15. Theboiler should be hydraulic tested tostand 50 p.s.i. without leakage ordistortion, and the safety-valve spring(which should be of rustless steel orphosphor-bronze) adjusted so that itlifts at 35 lb. Firing may be byspirit, paraffin or gas, as described in _previous articles.

SIMPLE SLIDE-VALVE ENGINESThe majority of steam-engines,

large and small, are equipped withslide-valves for the distribution ofsteam, and of these, the simpler typesemploy an eccentric (which is essen-tially a form of crank) for directoperation of the valve in fixed phaserelation to the piston stroke.

To anyone who intends to buildengineering models of any kind, Iconsider that a practical under-standing of the simple slide-valveengine is essential, and one of thefirst essays in construction should bedevoted to producing a workingmodel of such an engine. Judging bythe many queries received on thissubject, it would seem that the steam-engine does not receive the attentionit deserves in elementary technical

Right, Fig. 11: Plan r -

the bottom) cylin-der and the rever-

crankshaft and (at

Left, Fig.12: The h

FIG. II

-.

education, as many intending con-structors do not appear to havemastered its basic principles. I feelsure, therefore, that more experiencedhands will not grudge a little spacedevoted to this important subject.

In my own articles on steam-engines, I have always recommendedbeginners to start right away onbuilding a slide-valve engine, withoutworrying about the conventional oscil-lating engine, which is generallyregarded as the first stepping stone toprogress in construction.

My reason for this advice is notsimply because it is possible to attainhigher efficiency with the slide-valve-though this is an undisputed fact-but because the latter gives facilitiesfor the observation of valve events,also for checking and experimentingwith timing; in this way it teachesthe constructor more about enginefunctions than is ever possible withan engine in which the means ofsteam distribution are both invisibleand immutable.

When once the principles of thedirect-acting slide-valve have beenmastered, one can, if one so wishes,go on to study the more complicatedvalve gears which enable the ex-pansion of steam to be controlled orrotation to be reversed.

Muncaster quite rightly devotescareful attention to the elementaryprinciples of slide-valve operationand gives an isometric section of asteam-engine cylinder and steam-chest which I reproduce here (Fig. 16)to illustrate the essential features.The piston, P, is shown at about halfstroke, moving in the direction of thearrows, and the slide valve, V, is inthe appropriate position in relationto it at this stage.

Steam is admitted under pressureto the steam chest, C, tilling the spacearound the back of the valve, whichhas a flat face in contact with astationary flat surface in which three

ports open into passages, SS, leadingto the respective ends of the cylinder,and the central port, E, is in com-munication with atmosphere, or theexhaust pipe system.Both the piston and the slide-valveare connected mechanically to theexternal working parts by rods whichpass through packing glands, FG, toavoid leakage of steam at the openmgsin the cylinder and steamchest.

At the position illustrated, steam isbeing admitted to the rear or closedend of the cylinder and forcing thepiston outwards; the valve mean-while is moving in the oppositedirection, so that it will cut off thesteam supply as the piston nears thefront end of the cylinder. Duringthis period, the other side of thepiston is in communication, throughthe passage, S, and the cavity in thecentre of the valve, with the exhaustport, E, so that the steam in this space,which has already done its work, isdisplaced by the piston and is freeto escape.

By the time the piston has com-pleted its outward stroke, the slide-valve has moved to commence open-ing the front end of the cylinder tolive steam and the rear end to exhaust,so that the motive force on the pistonis reversed and it starts on its returnstroke.

An engine of this type is said to bedouble-acting, as power is applied tothe piston on both forward andreturn strokes; it is the orthodoxarrangement for most types of steam-engines, though single-acting engines,in which power is produced on oneside of the piston only, are employedfor certain duties, particularly whereit is desirable to keep the weight ofworking parts as low as possible forthe attainment of high speed.

Fig. 16: Section through cylinderand steam-chest of slide-valve engine

Fig. 14: Sectional plan of boiler,showing cross-tubes

Below, Fig. 13:vertical boiler

Section of simple a twin engine

LAP AND LEADIn early steam engines it was usual

to make the closing faces or lips ofthe slide-valve on either side of the

FIG. 13.

23tports are formed.

Right, Fig. 17: Dia-grams illustrating“ lap ” and ” lead,”showing correspond-ing positions of thecrank and eccentric(clockwise rotation)

Below, Fig. 15: Asimple safety-valve

The two outer

FIG. 17

VAWEJ I-LAP

j--___-

)-421 MODEL ENGINEER

central cavity exactly the same width junction with the previous drawing,as the steam ports they controlled. should make them quite clear.The valve was timed to move at 90 It will be seen that the lips of thedeg. in advance of the crank, so that valve are extended in width (or,the ports commenced to open exactly strictly speaking, length, in the direc-at dead centres and a full piston tion of travel) so that they overlapstroke was occupied both for steam the ports, SS, when in mid-travel, asadmission and exhaust. seen on the right; the extent of the

It was soon found, however, that overlapping is termed “lap.” Suchbetter working efficiency could be a valve, if timed to move at 90 deg.obtained by advancing both the in front of the crank, would giveopening and closing points, but not delayed steam opening and therebynecessarily in the same ratio. Early reduce efficiency; to compensate this,

Fig. 18: Horizontal mill engine to 1 in. scale

Incidentally, I may mention thatin my directions, for timing steam-engines in the past I have occasionallybeen criticised for recommending“ rule of thumb ” methods-in otherwords, timing in situ by turning thecrank to dead centres and checkingup on the actual valve position. Itmight be considered more “ scientific ”to deal in exact angles of advance,and this would be absolutely necessaryin an engine having the eccentricsmachined integral with the crank-

F IG. 18.

steam opening could be used tocushion the piston movement, thusbringing it smoothly to rest at the endof its stroke and also to ensure thatthe maximum effort was available tostart it on its return. By cutting offthe steam supply before the pistonreached the end of its power stroke,the expansive poroperties of the steamcould be used to complete the stroke,so that by the time it was released toexhaust most of its energy had beenusefully expended.

the timing of the valve is advancedby shifting the eccentric so that itbegins to open the steam-port slightlybefore the crank reaches dead centre;the amount of opening at the latterpoint, as shown on the left, is knownas “ lead.”

The means employed to attain this result are by designing and timingthe valve to produce what is knownas “ lap and lead.” I have been askedso many times to explain these termsthat I consider it worth while toreproduce another of Muncaster’sdiagrams (Fig. 17) which, in con-

The diagrams above the valvesections in each case show the relativepositions of crank and eccentric toproduce the required timing; on theleft, the crank is on its dead centreand the eccentric approximately 120deg. in advance of it-in the usualterminology, the “ angle of advance ”is taken as the amount extra to theoriginal 90 deg. in front of the crank,so in this case it would be reckoned as30 deg. The right-hand diagram showsthe eccentric at mid-stroke, still leadingthecrankbythesameamount,ofcourse.

MODEL ENGINEER 422

shaft, or otherwise rigidly pre-located.But in actual practice the measure-

ment of angles on very small enginecomponents is extremely difficult-one might easily make an error oftwo or three degrees in the locationof a keyway on a small shaft, forinstanceand, moreover, it is notalways advisable to regard valvesetting as being “immutable as thelaws of the Medes and Persians.”

l To be continued

WIN A MYFORDSUPER SEVEN LATHE

Readers are reminded that theclosing date for the MODEL ENGI-NEER competition, the prize forwhich is a Myford Super Sevenlathe, closes on April 5..

21 MARCH 1957

b_ /

The MUNCASTER By

steam-engine modelsEdgar T.

Westbury

4-Horizontal Stationary Engines

IN DESCRIBING the function ofthe slide-valve and the effectsof lap and lead [Fig. 17, March

12], no particular mention wasmade of exhaust timing. It wouldbe a mistake to regard this asinsignificant, but it is generallysatisfactory to allow it to keep inphase with the steam admission,as it must inevitably do so in asimple slide-valve, and it is usualto make the inside edges, in otherwords the width of the valvecavity, such that they just, and onlyjust, span the inside edges of thecylinder ports at mid travel (right-hand diagram).

Continued from 21 March 1957. pages 420 to 422

Thus the slightest movement eitherway opens one or other of the cylinderports to exhaust. Occasionally,however , engines are t imed to givee i t h e r p o s i t i v e o r n e g a t i v eexhaust lap, by narrowing or wideningthe valve cavity; the latter is the morecommon and its object is usually to

Right, Fig. 20: Across section ofsteam-chest and aportface, showingthe port dimensionsBelow, Fig., 19: Ahorizontal mill en-gine, 1 in. bore x1 in. stroke, withbar crosshead slides

avoid excessive “ cushioning ” orcompression, or to eliminate risk ofback pressure in the exhaust system.

The conventional stationary or so-called “ mill ” engine forms an ex-cellent exercise in steam-engine con-struction, and is deservedly popular.Several examples of these engines were

MODEL ENGINEER

488 4 APRIL 1 9 5 7

Ad

Above, Fig. 23: Detailsof the crank and eccentric

Below,bore xdesigned by Muncaster, all generally

similar in major features but differingin size and certain details. The first,shown in Fig. 18 [March 21], isscaled down from a fairly large engine,12 in. bore x 18 in. stroke, in theproportion of 1 in. to l ft.

No dimensions are given on theactual drawing, but Muncaster givesa table of dimensions worked out toexact scale. I have taken the libertyof modifying some of these to giveround fractional figures, as mostconstructors would undoubtedly wishto use standard drills, reamers andstock materials wherever possible;but general proportions have beenclosely adhered to.

The cylinder dimensions for the1 in. scale model are 1 in. bore x1-1/2 in. stroke. Piston rod diameter3/16 in.,crosshead fin. wide insidefork, with crosshead pin 3/16 in. dia.Connecting rod, circular section, fish-bellied, 3/16in. dia. at the two ends,swelling to 1/4in. centre; length be-tween bearing centres 2-5/8 in. Crank-shaft 7/16 in. dia., with journals reducedto 3/8in. x 1/2in. bearing length;crankpin 1/4 in. dia.‘ x 5/16 in. long.Flywheel 5 in. dia. with rim section1/2 in. square. Eccentric throw 7/64 in.,rod circular section, tapered from1/8 in. to 7/64 in. Valve spindle 7/64 in.dia . ,valve t ravel 7/32 in . Port

7/16in., exhaust 5/32in. wide. Valve

5/16 in., lead l/100 in. Steam inlet1/4 in. dia., exhaust outlet 5/16 in.

As, the constructional methods forthis and the second engine aregenerally similar, they may be con-sidered together. In the latter case,illustrated in Fig. 19, leading dimen-sions are given on the drawing. Bothengines are intended to be builtfrom castings, though fabrication ofmost of the components is practicable.

The main components are mounted

4 APRIL 1957 4 8 9

on a long box-section bedplate, withmachined facings where required, andit is desirable to machine these inorder to ensure accurate location andsecure mounting of the parts, but inthe absence of proper facilities theymay be filed and scraped, accuracybeing checked by means of a surface

It will be seen that both engineshave an outboard crankshaft bearing,or “pillow block,” which must beaccurately lined up with the otherbearing mounted on the bedplate;for this reason the actual foundationon which the latter is mounted mustalso be flat and true. It may be madefrom a thick slab of well-seasonedhardwood, with a cavity cut out toclear the flywheel.

24: A horizontal engine, 1-1/2 in.in. stroke, with a slipper crosshead

.

MODEL ENGINEER

dimensions: steam-ports 5/64in. x

travel 7/32in. ,lap 3/64in., cavity

, .

Left, Fig. 21: Details of the flywheel. Above, Fig. 22: Details of the connect-ing rod. Below, Fig. 25: An engine of similar dimensions with trunk crosshead

Many constructors, no doubt, wouldprefer to make the engine self-

solid in articles on the Unicorn millengine.

when nearly all fitters had some

contained by extending the cast bed-plate to carry the outboard bearing;

Muncaster suggests forging partsknowledge of smith’s work; but thisseems to be practically a lost art

I have designed most of my enginesof this nature, which is very soundadvice-or at least it was in his time,

nowadays. There are, however, many

to avoid the need for extraneous partsl Continued on page 515

which have to be lined up. A singlebedplate with facings for all essentialparts, machined off to the same levelthroughout, greatly facilitates accurateconstruction.

The cylinders of both engines arebolted down to the bedplate, beingprovided with cast feet or flangedbrackets, and the bearings, which areof the split plummer block type, aresimilarly fixed. Details of the steam-chest, flywheel, connecting rod, crankand eccentric sheave, with its strap,are given in Figs 20 to 23.

The main difference in the two en-gines is in the type of crossheademployed; the first example has dieblocks fitted to the extended ends ofthe crosshead pins and workingbetween girder-shaped guide barsmounted on pillars fixed to the bed-plate, while the other has a slippertype crosshead working on singleguide bars each side. Both types arewell established in full-size practice;they require very careful lining up inrespect of height and parallelism withthe cylinder axis to ensure smoothmovement of the piston rod.

In this respect they may possiblybe more difficult to fit than the trunktype of crosshead, in which alignmentis automatic if machining is properlycarried out; on the other hand, theyare sometimes preferred because theyoffer facility of adjustment to com-pensate wear or initial errors.

The crosshead of the first engine isforked to admit the little-end eye ofthe connecting rod, but in the secondarrangement matters are reversed byforking the end of the rod to span theslipper crosshead block. This type ofrod may be a little more difficult toshape than the previous type, but itlooks very nice when properly carriedout and I have given instructions onhow to produce forked rods from the

MODEL ENGINEER 490 ‘4 APRIL 1957

Reversing by radio Continued from page 499

Oil-bath lubrication is of courseemployed, and I recommend the useof a light oil, such as Shell Vitrea, soas to keep down losses due to fluidfriction.. It is of course necessary tofit an au vent or breather to the topof the box, but I have not shownthis as ‘its position will have to bearranged so as not to interfere withany control gear fitted to the cover.The box should be filled to a depthof 1/2 in.

Various modifications of the detailsof the gearbox may suggest themselvesto readers but the general design, in

which all really essential features areincorporated, can be guarapteedreliable and efficient.

One point which should be notedabout reverse gears of this type isthat the gears are only under loadwhen actually in reverse, which willnormally be no more than a verysmall proportion of the total workingtime; when running ahead, the gearsrun idly and the only losses are thosedue to running friction and oil drag,which can be kept very low; thusthere. is no appreciable reduction inthe performance of the boat.

_. :

FOR SHIP MODELLERS -John N. C. Lewis, in Ship Modeller’s

Logbook, has produced the ideal bookfor the enthusiast. He shows thereader how to make simple, decora-tive, miniature and scenic models,including a clipper ship in a bottleand an Arabian Baggala.

There are details of clench andcarve1 built models, and the textis enlivened by stories of eighteenthcentury smuggling and the work ofthe Revenue Cutters.

Obtainable from Percival Marshalland Co. Ltd, price 12s. 6d., postage1s. (U.S.A. and Canada $3.00).

Muncaster.models . . ,Continued from page 490

components in model engines whichcould be produced more efficientlyand economically by hand forgingthan by any other method, and arevival of this skill would be wellworthwhile.

The crankhead bearings or “ bigends ” of both engines are of the solidbox type, with split brasses of rect-angular section fitted. These wereusually secured by a tapered gib andcotter, which enabled the bearings tobe adjusted for wear; but Muncastershows them simply located in placeby a screw, which passes through anotch in the rear half-bearing toprevent its moving sideways. The fronthalf would presumably have side lipsto engage the sides of the box framefor the same purpose; removal of thescrew would enable the rear half toslide back enough to withdraw it overthe collar of the crankpin.

Governor gear is fitted to the firstengine, and could be adapted to thesecond- if desired; some further in-formation on this subject will begiven later.

Two further examules of horizontalstationary engines are illustrated inFigs 24 and 25. Each has its owndistinctive features,, but many of theworking parts are sunilar or identical;the cylinder dimensions are l-1/2in.bore x l-1/2in. stroke, which may beconsidered on the large side by manyconstructors, but it would be quitepracticable to reduce the scale to halfthese dimensions or even less ifdesired.

The‘ first of these engines has a4 APRIL 1957

crosshead of the slipper type, workingon a slideway machined on the topsurface of the bedplate, with keepplates secured by three studs or boltson each side. A rather unusualmethod of fixing the cylinder is em-ployed, the underside of the castingbeing provided with four projectionsor feed, into which setscrews or studsare screwed from the inside of thebedplate.

In addition, two vertical lugs areextended upwards from the bedplateand drilled horizontally to take ex-tensions of two of the cylinder coverstuds. This undoubtedly gives addit-ional strength to resist workingstresses, but may be regarded as“ gilding the lily,” to use a. popularmisquotation.

The height of the cylinder, cross-head and main bearing centres mustall be exactly the same, and carefuladjustment in machining or fitting willbe necessary to ensure this. A vewslight error in the height of the shaftcentre would not be harmful; but anymisalignment of cylinder and cross-head would cause binding of theworking parts, and possibly glandtrouble.

Errors of this kind are oftenbotched up by makng the parts asloppy fit, which certainly enables theengine to run, with sundry clanks andgroans, but such slovenliness shouldnever be tolerated in model engineer-ing.

In the second example, Fig. 25, abored “trunk ” guide is employed,similar to that which I have describedfor the Theseus and Perseus engines,and this enables all uncertainty in thealignment of cylinder and crosshead tobe eliminated,. so long as machining isproperly carried out. The trunk inthis case is cast integral with thebaseplate, therefore, unless specialmachining facilities are available, itwill be necessary to mount the casting

515

on the lathe saddle, packed up to theexact centre height, for boring theguide seatings and facing the rearflange.

It can then be swung round forboring the main bearing housings,the caps of which should previouslyhave been fitted, and facing boththeir inner and outer sides. All theseoperations can b e carried out with

boring bars between ‘centres, andfitted with suitable cutter bits. Thecentre lines of the trunk and mainbearings must be exactly at rightangles to each other.

It will be noted that the bearingsof both engines are of the same type,also the crankshaft, which may beeither machined from the solid orbuilt up as indicated in the detailsincluded in Fig. 24. The journals andcrankpin in this case are made apress fit in the webs, and accuracyshould be checked after assembly,positive location then being ensuredby fitting pegs or dowels endwise ass h o w n .

The front end cover of the cylinder(Fig. 25) is sandwiched between theflanges of the cylinder and the trunk;it must be accurately machined onboth sides, including the boring ofthe- gland, to avoid introducing align-ment error. I have described how toensure this in connection with previousengines.

In other respects, the componentsof the two engines are identical,including’ the connecting rod andcrankhead bearing, eccentric sheave,strap and rod (Fig. 24), and the slide-valve with its rod and knuckle (shownin Fig. 25); it will be seen that themethod of attaching the valve to therod, to allow free “ floating ” location,is the same as in the Theseus andPerseus engines; namely, by reducingeither the width or diameter of the rodto fit a slot cut in the back of the valve.

l To be continued.

MODEL ENGINEER

The MUNCASTERsteam-engine models

EDGAR T. WESTBURY is bringing a modern eye to bear on

A LTHOUGH the horizontal typeof engine has always beenfavoured for s tat ionary

work, the alternative direct-actingform of engine having the cylinderlocated vertically above- the crank-shaft has some advantages wherefloor space is l imited, and isgenerally considered more suitablefor running at high speed than theformer type.

It is, of course, more common inmarine practice than stationary work,but both on land and sea it has beenextensively used for auxiliary purposessuch as driving electrical generators,ventilating and forced draught fans,and centrifugal pumps for circulatingwater in condensing plant, or dockdrainage.

One of the earliest engines in thisgeneral class was introduced a fewyears after the Nasmyth hammer

, made its appearance,. and because ofits structural similarrty to the lattermachine it was customary to referto it as the “ steam-hammer” type.

The salient features of such engines,an example of which is illustrated inFig. 26, include a relatively smallbedplate on which is mounted a

1 symmetrical pair of cast columns,usually of channel or hollow-boxsection, and these in turn support thecylinder assembly. In outline, thestructure bears a resemblance to thatof a lighthouse, tapering more orless gracefully from the cylinder headto the base, to give maximum rigidityagainst both dead load and workingstresses.

The inside faces of both the columnsare flat near the top end, and havemachined surfaces which serve a scrosshead guides. The working partsare generally similar to those ofhorizontal engines, except in certainpoints of detail which may be in-fluenced by their disposition and orderof motion.

In one respect, this particularengine may be regarded as an anacho-nism, in that while its main structurefollows the “ steam-hammer ” tradi-, tion, it is fitted with a piston valve,a feature which did not become

18 APRIL 1957

some classic models of the pastContinued from 4 April 1957. pages 488 to 490

popular until later developments,and particularly higher steam pres-sures. made it desirable. However.Mucaster knew steam-engine practicebetter than I ever shall and I wouldnever dispute his authority over suchdetails.

A piston valve is nothing more thana slide-valve having a circular insteadof a flat face, but this alteration inshape involves characteristics whichmay have advantages or limitationsaccording to circumstances. First ofall, it is capable of controlling portsall round its circumference instead of

5-Vertical

stationary

engines

over a limited width of face, and thusit can give much more rapid andefficient valve events than a normalflat valve, though this feature is notalways used to full advantage.

Secondly, it is not pressed hardagainst the portface by the steampressure, and therefore works withmuch less friction, especially wherehigh working pressure is employed;this is perhaps its most importantpractical advantage.

But because of being pressure-balanced it is not self-seating, andunless it is very carefully fitted to thebore of the steam-chest or liner, it isliable to leakage, much more so thanthe flat valve. Many small piston-valve engines have been found lessethcient than those with flat valvesfor this reason, especially when wearhas taken place; large engines havepiston rings fitted to the valve toavoid leakage, but this is hardlypracticable in a model.

Thirdly, piston valves may be

555

adapted to control steam admissioneither on their outer end faces (asin the case of the flat valve), or theinner faces, which would normallycontrol exhaust events. The latterarrangement, known as “inside ad-mission,” is generally preferred as itenables the steam-chest and passagedesign to be simplified, though itmakes no difference to efficiency solong as design is adapted to suit.

It will, of course, be clear that inthis case steam lap must be providedby reducing the width of the clearanceportion o f the valve, correspondingwith the cavity of the flat valve, andthe total length of the valve must besuch that it exactly covers the portsin the steam-chest, unless exhaust lapis specified-in other words, normal“ line for line ” exhaust timing.

Piston valves generally allow thecylinder steam passages to be madeshorter and more direct, thus im-proving thermal efficiency by reducingthe dead volume at the ends of thestroke and also the conducting surfacearea of the passages. They do not,however, provide the same facility forvisual valve timing as the flat valve,and it is necessary to adjust theirposition by exact measurement inmost cases.

The piston valve of the engine shownin Fig. 26 is of the inside admissiontype, the main steam inlet being inthe centre of the steam-chest’and theexhaust being taken out from twoports at the extreme ends to a passageshown in the plan section BB. It isdriven by a rod which passes upthrough a clearance hole in the centrefor most of the length of the valve,thus giving a small amount of sidefreedom for self-centring in the gland,but the uvner end is screwed into ashort tapped hole and a locknut. isprovided s o that lateral positionadjustment can be obtained.

It should be noted that for aninside admission valve the eccentrictiming must be adjusted so that ittrails behind the crank instead ofleading it. The angle of advance, however, is still in the same direction,so that for a valve with fairly orthodoxlap and lead, calling for 30 deg. angle

MODEL ENGINEER

i

Muncastermodels.

of advance, the setting will be 90 - 30= 60 deg. behind. the crank in thedirection of engine rotation.

Details of the piston valve and thetwo short half-liners, which arepressed into opposite ends of thesteam-chest, are shown in Fig. 27.Three ports are shown in each of thehalf-liners, giving a large total area,and the sides of the ports are cutobliquely to minimise ridge formationon the valve as a result of wear.Alternatively, a greater number ofround holes may be used,, and per-sonally I should favour this method.

A groove is turned in the outside ofthe liner to form an annular passagewhen it is inserted in the steam-chest.The liners must be accurately locatedto give the designed port timing inconjunction with the valve dimensions.A stainless steel valve with bronzeliners is recommended.

MAIN COLUMNSIn order to ensure accuracy in

cylinder location and guide alignment,I recommend that the columns shouldfirst be machined on the guide facesand then clamped together for facingthe top and bottom surfaces. Whenerecting the columns, they shouldfirst be bolted to the bedplate with agauge block between the guides tolocate them the correct distance apart.To locate the cylinder, the machinedcrosshead, or a dummy made to thesame dimensions, may be fitted to thepiston-rod to ensure correct alignment.

If s t raightforward machiningmethods are u s e d accuracy should bepositive, but it is not advisable totake anything for granted and routinechecks should be made at all stagesof assembly!

SINGLE-COLUMN VERTICALENGINE

The “ steam-hammer ” type ofengine is suited equally well forrunning in either direction, as thecrosshead guides are symmetrical andof equal bearing area each side; butthis is but rarely called for in stationarywork. Even marine engines do notoften run for very long, periods inthe reverse direction. In such cases,a lighter but quite adequate form ofstructure can be adopted in which only

one cast column is employed, and thecrosshead is of the slipper type, having

its major bearing surface on thesoleplate, which slides on the face ofthe column.

MODEL ENGINEER

An engine of this type is illustratedin Fig. 28. It is intended to run in aclockwise direction, looking at theend of the shaft as seen in the right-hand elevation. Note that when thepiston is on the up-stroke, the thrust,which is tending to straighten out thepiston rod and connecting rod link-age, presses the crosshead against thecolumn; but on the down-stroke, thetendency is to increase the angularityof the linkage and thus the cross-head is still pressed against thecolumn.

If the engine rotation is reversed,the side thrust on both strokes is inthe opposite direction so that thecrosshead will pull away from thecolumn and bear against the keepplates, which are of much smallersurface area than the column face,besides having to rely on their retainingstuds for security.

Above, Fig. 27 : Details of thepiston valve and ported half-liners

Below, Fig. 26 : A vertical engine of the symme-trical doable-column or “steam hammer ” type

v5 5 6 18 APRIL 1957

,

As the offset support of the cylinderby a single cast column leaves thestructure somewhat weak to resistalternate upward and downwardstresses, the opposite side is stayed bymeans of a single machined steelcolumn (sometimes more than one isused) which, though light in section,has greater inherent strength thancast iron. As this is usually somewhatout of the vertical plane, both itslength and the angle of its seating atboth ends must be carefully adjustedto hold the cylinder assembly exactlyperpendicular.

GUIDE FOR MAKINGSMALL WIRE SPRINGS

HE simple arrangement shownT below enables a short run ofwire springs to be made withouthaving recourse to normal spring-making machinery.

This drawing does not show theinterior details of the cylinder, butthese may be similar to the previousdesign, using either a flat slide or apiston valve. It is fitted with agovernor, mounted directly on theshaft and acting on the enginethrottle valve. The rather unusualposition of the eccentric, immediatelyadjacent to the flywheel, avoids

A wooden block, A, has a handleand several holes of different diameter,C, drilled through into the V-shapedopening. The wire, Br is then threadedthrough the appropriate hole, passedunder a core rod, E, and secured ina chuck carrying the rod. The dia-meter of the rod is chosen to suitthe internal. diameter of spring re-quired.

After making the wire taut andcompleting a few turns by manipu-lating the wooden tool, the corerod is rotated mechanically until the

18 APRIL 1927 557

excessive projection of the shaft atthe governor end. Engines similar tothis have been used extensively fordriving dynamos, though they weresuperseded by enclosed engines inlater years.

Brickbat departmentTo those readers who, despite my

explanations in the March 7 issue,have chastised me for not givingcomplete details with full dimensionsof all these engine designs, I wouldlike to point out that the drawingsare copied as exactly as possible fromMuncaster’s original published de-signs, and this is what has beenasked for by many readers over aperiod of several years.

Fig. 28 : A vertical engine of the singlecolumn type, with shaft governor

The descriptive matter is my own,but if I attempt to amplify the draw-ings in any way their individualitywill be lost; in any case, the amountof work involved and the spaceoccupied would be out of proportionto the popular appeal, which isbound to be specialised to someextent.

The value of Muncaster’s designslies in his genius for adapting typicalexamples of all kinds of full-sizeengines to reproduction in miniaturewhile retaining true prototype charac-ter; exact details are of lesser im-portance, but my previous articles onsteam-engine construction shouldmake up any deficiencies in thisrespect.

To be continued

correct length of spring is reached.The spring is of the closed variety,but by driving a pin or nail throughone of the holes, D? open springs canbe made with spacing as desired.

The holes, C, will probably wearout quickly so that the wooden toolmay need freauent replacement.

Digested f r o m “ A Device forForming Small Lots of Wire Springs,”in “ Wire Industry,” 1956 (October)

page 921. The illustration is reproducedby courtesy of the publishers. q

MODEL ENGINEER

.

The MUNCASTERsteam-engine

models -By Edgar T. Westbury

-HE term “ simple ” as generally

1 applied to steam-engines doesnot denote simplicity in the

mechanical sense, but may be morefully defined as “ simple expan-sion,” or, in other words, the useof available steam pressure range

in o n e s t a g e . This does notnecessarily mean a single cylinder;such engines may have any numberof cylinders, but each of them isfed with live steam at full pressure,and they each exhaust directly toatmosphere or into a condensingsystem. All the engines so fardescribed in this series comewithin this definition.

6-In this articlethe simple andcompound twinengines are dealt

with(Continued from 18 April, pager 555 to 557)

At a very early stage in the develop-ment of the steam-engine-in factalmost as soon as any very substantialsteam pressures above that of theatmosphere began to be used-it wasfound that in rder to extract themaximum mec anical power fromhhigh pressure steam and avoid therejection of exhaust steam while itstill contained useful energy, a greaterrange of expansion than could con-veniently be obtained in a singlecylinder was necessary.

Even with valve gears designed to’give very short cut-off, the pressureat the end of the piston stroke wasundesirably high, and the logicaldevelopment, therefore, was to trans-fer this steam to a second cylinderwhere a further stage of expansionwas carried out before the steamwas finally rejected. The best-knownpioneers in this were Woolf andHornblower, who succeeded in im-proving the economy of steam-engines considerably by this means.

In the further development of the“ c o m p o u n d ” engine, as it wascalled, further stages of expansionwere added, producing “ triple-ex-pansion ” and “ quadruple-expansion ”engines which became popular invery high powers, though the practicaladvantages of the fourth expansionstage have always been in dispute.

It will be clear that as the pressureof the steam is lowered at each. stage,i t s volume is correspondingly in - creased (remember Boyle’s law-PV

MODEL ENGINEER

equals a constant ?) and, therefore,the sizes of the cylinders have to beprogressively enlarged by a ratiodetermined by the pressure drop ineach stage; it is, of course, desirablein a multi-cylinder engine that eachcylinder should contribute a fairlyequal amount of power at the drivingshaft. For this reason, compound orstage-expansion engines are designedfor a definite pressure range andproduce their best results when inputpressure is kept fairly constant.

I have made this lengthy-but, Ihope, lucid--explanation because Ihave received a good deal of corres-pondence on engines of more thanone cylinder lately, and there has beensome confusion as to what constitutesa compound engine and its practicaladvantages in particular cases. Severalreaders, including some from remoteparts of the world, have asked myadvice on the respective merits ofsimple and compound engines fordriving prototype model power boats.

-

t

Fig. 3 I: Twin-cylinder compound engine with h.p. piston valve and 1.p. slide-valve

634 2 MAY 1957

This is not an easy question to answerin definite terms, as so much dependson circumstances, not the least im-portant of which are the conservationof heat and the reduction of pressureat the final exhaust outlet.

In small engines generally it is verydifficult to avoid heat loss, as the“ scale ” conductivity of the metalis high and the effectiveness of heatinsulation, which can be obtained bylagging, is low. Some heat loss isalmost inevitably incurred in thetransfer of steam from one cylinderto the next by way of an “ eductionpipe ” as it is termed. Incompleteelimination of back pressure is com-mon, even if a condenser is fitted, asit is extremely difficult to produce ahigh vacuum such as can be obtainedin large steam plants. Incidentally,it may be mentioned that the latterfactor has an important bearing onthe difficulty of exploiting the potentialadvantages of the steam turbine insmall sizes, though large multi-stageturbines have a great advantage overreciprocating engines, because theyenable progressive expansion from

Above, Fig. 30:Plan view o f thetwin simple engine

Right, Fig. 32: (Top)cylinder elevation(compound engine) ;(centre) sectionalplan of cylinderblock; (lower) under-side plan of cylinder

block

Below, Fig. 29: Atwin-cylinder simpleengine with central

[slide-valves

h.p. end to exhaust outlet (withouttransfer pipe losses) to be obtained.

The fact that locomotive engineers,in this country’ at least, have neveradopted compound engines to anygreat extent suggests that their ad-vantages, where exhaust is releasedto atmosphere, are somewhat dubious;much the same applies to traction.engines, though some of the com-pound types have earned a goodreputation for efficiency. In marineand stationary practice, however,where surface and jet condenserscapable of producing high vacuumare invariably used, there is noquestion about their superiority.

Whether small-scale engines of anytype, working on pressures up toabout 100 lb. and little or no exhaustvacuum, can employ compoundingwithout the 1.p. cylinder becomingmerely a passenger, remains to beproved. In the case of a twin double-acting engine, the compound type isnot positively self-starting unless fittedwith a “ simpling valve ” for tem-porary admission of live steam to the

MODEL ENGINEER

1.p. cylinder, and this may be animportant practical consideration inmany cases.

The engine illustrated in Fig. 29 isof a type well suited to model marinework, and its size would be suitablefor fairly powerful prototype boatssuch as tugs, pinnaces or fast packetliners up to 6ft or more in length.Its structure is of a type not at alluncommon in light marine practice;I have worked on engines similarlyconstructed (but of the compoundtype) in naval picket boats, thoughtotal enclosure of these engines wasbecoming more common at the time.No cast columns are employed, thecylinders being supported by fivevertical turned steel pillars-two atthe front and three at the back (seeplan view, Fig. 30). The latter wereused for the attachment of a horizontalmember, which supports the lower endof the slide bars, and their top endswere secured to flat faces on the backof the cylinder gland bosses.

Demands accurate workThis form of assembly calls for

careful location of the pillars, bothat the bedplate end and in the cylinderbase flange. It also calls for accurate’lining up of the slide bars, so that theyare exactly parallel with the cylinderaxis, both crosswise and sideways.

A rather unusual form of cylindercasting is employed, having a channel-shaped steam chest, open at the twoends, the pair of castings being boltedtogether at the centre by four’screwsor bolts in a vertical flange. Thisgives better accessibility to the portfaces than the use of a single cylinderblock, while on the other hand it issimpler and more rigid than the com-mon arrangement which uses separatesteam chests-generally on the outsideinstead of between the cylinder.

The cylinders have separate spigotedcovers top and bottom, and the endsof the steam chests are closed bysingle rectangular covers, the lowerof which embodies the stuffing-boxesfor both valve rods ; the upper is fittedwith guides for their “ tail” ends.

While this is by no means a difficultform of construction, it demandsspecial treatment, with due care inmachining and assembly to ensuresuccess. The two cylinder castingsshould be bored and faced separately,the length over outside of flangesbeing made as near exactly the sameas possible. Next the vertical jointfaces should be machined, either byfacing on an angle-plate in the latheor by milling or shaping. In the lattercase, the port face may be machinedat the same setting, thus ensuring thatit is parallel with the joint face. Butwhatever method is employed, thesesurfaces should all be parallel to thecylinder axis.

MODEL ENGINEER

Lapping of the port face is not aseasy as for more conventional typesof cylinders, as only a narrow lap,with rectilinear motion, can be oper-ated i n the space. But the majorerror liable to occur is slight con-vexity of the surface,, which is afault in the right direction. The portsand passages may be dealt with in theusual way, and are just as accessibleas in other types of cylinders.

After bolting the two cylinderstogether, the end faces should bechecked to ensure that they coincideto form an exactly flat continuoussurface-any slight discrepancy beingcorrected by lapping. More drastictreatment by machining may becalled for if there is greater differenceof level.

This engine is shown fitted withsingle eccentrics for the individualslide valves; reversing may be obtainedby using slip eccentrics, but forpositive reverse control, link motionwith double eccentrics may be fitted;this may call for a slight increase in

I

the space between the centre mainbearings. As the bearing surface isample, no harm would be done byreducing their width slightly, and alsoreducing the width of the eccentricstraps and sheaves.

A twin compound vertical engineThis is of the symmetrical double

cast column or “ steam hammer ”type, as shown in Fig. 31, but apartfrom this feature several of the con-structional details are similar tothose of the previous design. Thecylinder block is again divided ver-tically in the centre, with a boltedflange joint, but, of course, is unsym-metrical as the left-hand (h.p.) cylinderis smaller than the right-hand (1.p.)cylinder, in the ratio of 2 to 3, in borediameter. This engine is practicallytwice the size of the simple engine asdimensioned in the drawings, butcould quite easily be reduced to thesame size.

l Continued on page 656

Fig. 33 : Rear view of compound engine, and details of eccentric straps

636 2 MAY 1957

MUNCASTEER MODELS . . . continued from page 636

The steam chest arrangement ofthis engine is very ingenious in design,as it allows of a single chest for bothh.p. and 1.p. cylinders-rather unusualin compound engines-thus eliminat-ing the need for an eduction pipewhich, as already pointed out, may be

Routemaster buses tohave Leyland units

THE mechanical equipment intwo new prototype vehicles

for London Transport has beenproduced by Leyland Motors fortrial by L.T.E. engineers.

One, a double-decker for theGreen Line coach services, will betested in a few weeks’ time onroute 721 from Brentwood toAldgate. It is said to be the lastword in double-deck passengercomfort. The other will operateas a bus on a central area service,route 85, Putney Bridge to King-ston.

The vehicles are Leyland versionsof the new Routemaster, the bus ofthe future evolved by LondonTransport’s road vehicle engineers,led by Mr A. A. M. Durrant,C.B.E., chief mechanical engineerfor the Road Services.

Designated RM3 and RM4, thetwo new motlels incorporate theLeyland 125 h.p. diesel engine, atype already fitted to over 2,000Leyland buses in service in London.

Designed as a double-deckercoach for the Green Line routes,RM4 is equipped with fully-automatic gear change equipment,the gears sorting themselves outautomatically and changing up ordown according to the speed andload on the bus. If he desires, thedriver can over-ride the automaticmechanism and select gears merelyby moving a miniature lever.

Folding doors are providedaround the rear platform-and seatspacing is the same as for otherGreen Line coaches. Consequentlyit seats only 57 people-as com-pared with 64 for RM3-but stillane more than the present RTbuses.

Although all running units, suchas engines, axles and springing,have been manufactured by Ley-land to their own detailed design,they conform to the overall designlaid down by London Transport.One feature which they will havein common with other RM modelsis special heating for upper andlower saloons.

a source of heat wastage, especiallyas it often extends the full length ofthe cylinder block. This improvementis effected by using an inside-admissionpiston valve for the h.p. cylinder anda flat, slide valve for the 1.p. cylinder.

Entry to the piston-valve housing isby way of a port in the side of thecylinder casting between the “ lands ”of the valve and exhaust escapes at theends of the valve, directly into themain steam chest space, whence it isadmitted to the 1.p. cylinder under thecontrol of the flat slide valve. It isfinally exhausted by way of the cavityin this valve, in the normal manner.

Details of the piston valve and theported half-liners are similar to thoseof the single-cylinder vertical engine,(Fig. 26 in the April 18 issue). Someconstructors may prefer to fit asingle continuous liner for the fulllength of the housing, which willserve exactly the same purpose-andit is not necessary to chamber out thecentre larger than the working dia-meter, though this is usually done inlarge engines to avoid the formationof ridges in the bore of the liner.

Fig. 32 shows three views of thecylinder block, namely side elevation,sectional plan and underside plan.The former shows the position of theexhaust and steam admission ports atthe front and back of the enginerespectively; each of these has an ovalflange joint face. In the sectional, plan,it will be observed that the steamadmission port goes directly into thepiston valve housing while the exhaust

port communicates with the centreport of the 1.p. cylinder port face.

The underside view clearly shows theshape of the lower steam chest coverand the method of attachment bv sixstuds and nuts, as well as the positionof the valve rod guide bracket, whichis presumably cast on the cover,though this is not as clear as it mightbe; the sectional elevation (Fig. 31)shows it rather differently, and thetop view in Fig. 32 omits it altogether.

In view of the difficulty of aligningthe guides with the glands, which inthe case of the h.p. valve also have tobe exactly concentric with the housing,I should favour making two separateguide brackets and bolting them inplace on the cover after lining up.Valve rod guides are not consideredessential on many model steam-engines, particularly where the rodhas a tail end guide, as very littleside thrust is encountered but theyare almost universally fitted on largeengines.

The rear view of the engine (Fig. 33)shows a belt-driven governor whichregulates the speed by means of athrottle valve of the axial piston type.Further details of these items will begiven later. The cast columns, to-gether with details of the guidesurfaces and crosshead, are as for theengine shown in Fig. 26. There isplenty of room in the ‘centre of thisengine for fitting double eccentricsif desired, and either one or two inter-mediate main bearings may be fitted.

l To be continued

A CHILDREN’SLOCOMOTlVE . . .continued from page 646

BOILERThe outer casing of the boiler can

be made from 22-gauge sheet, anymetal except aluminium will do. A9:! in. length of 24 in. stovepiping, ora tin canister of same dimensions,will be quite O.K. Just slit one enddown for 34 in. length, make a cross-cut at the end of the slit and openit out to the shape shown in the cross-section. Fit a throatplate of the samekind of material, like that shown forthe larger Virginia boiler, and tix itwith three or four rivets at eachside. No brazing is needed on thecasing. .

The backhead is flanged up from16-gauge sheet copper and a 7% in.length of Qin. x 22-gauge coppertube is silver soldered to it. Don’t use

656

anything thicker; if this thin tubeisn’t available. roll up a bit of 22-gauge sheet copper to the givendiameter, allow &in. overlap, put afew & in. rivets in to hold the edgesin close contact and silver solder thejoint. The front end is closed by aflanged disc of 16-gauge copper silversoldered in. Two 5/32 in. water-tubescan also be silver .soldered in asshown, but they are not absolutelyessential on an engine of this kind.

The front end of the casing isfurnished with a smokebox frontcarrying a dummy door and adorn-ments. This may be a casting, or itcan be built up by cutting a 16-gaugeplate to the shape of the smokeboxfront (drawing next week if all’s well)and silver soldering a flange to it. Thisshould be a tight push fit in the endof the casing, but is not fixed as it isremoved while getting up steam. Thechimnev liner is a 1 in. lencth o fi$ in. ihin tube silver soldered intoa hole drilled 3 in. from the end ofthe casing.

l To be continued

MODEL ENGINEER 2 MAY 1957

,

.

The MUNCASTER . . ;& ”

steam-engine models ’ jWestbury

7-Entablature or table engines Continued from 2 May 1957. pages 634 to 636

IN THE EARLY STAGES o f i t sevolution, the steam-engine as-sumed various forms, some of

which, though now obsolete, areof great interest to the modelconstructor. Design was influencedby several factors, including ex-pediency or convenience in thematerials and methods of con-struction available at the time;and also by traditional structuralstyles.

I have already explained that thepreferred position for the cylinder-which was then usually the heaviestsingle component-was on, the flooror bedplate; the difficulty of producingaccurate straight slides, capable ofresisting side thrust without unduefriction, favoured the use of parallelmotion for guiding the piston rod,or indirect action with long con-necting rods, such as in beam, return-crank and steeple engines.

The class of engine known as the“ entablature ” or “ table ” enginemade its appearance very early in the1800s, but became most popular inthe middle years of the century. Somevery fine examples of these engines byMaudslay and other prominent makerswere shown at the 1851 Crystal PalaceExhibition; they varied a good deal indesign, some being of the indirect-acting type, with the crankshaft belowthe cylinder and the piston rod ex-tended upwards to a rather spiderycrosshead from which motion wastransmitted by side connecting rodsto the crank.

Architectural termOthers, such as the types illustrated

here, had the cylinders mounted onthe bedplate and the crankshaf tmounted on an elevated platform-probably the first attempt at whatcame to be known as “ direct-acting ”engines.

In common with many other termsin engineering, the word “entablature”is borrowed from architecture, beingin fact a legacy from the classicGraeco-Roman era. Its definition (toquote a standard architectural text-book), is “ the horizontal member ormembers supported’ by the columns,and including the cornice, frieze andarchitrave.”

All engines in this class, therefore,

MODEL ENGINEER

have the common feature of a flatelevated table supported by four(sometimes more) columns-compar-able in fact to the humble kitchen table.Many of the classic examples of theseengines had fluted Corinthian columnswith decorated capitals and mouldededges to the entablature.

SIMPLE “ BASIC ” DESIGNThe engine illustrated in Fig. 34

represents one of the simplest possibledesigns in this class, being reducedalmost to the point of austerity, yetfully in character. It is of the direct-

acting type, having a sliding crossheadwith bar slides. Both the baseplateand the entablature are made fromflat plate, the former being 1/4 in. thickwith chamfered top edges, and thelatter 1/8 in. The columns may bebuilt up, as indicated in the partsection, the ends being shouldereddown to form studs, with separatecapitals and pedestals fitted to them.

This is economical with material,but I have a predilection for making .things in one piece where this ispossible by straight-forward machiningand I think I should prefer to turn

Fig. 34: A simple type of single-cylinder entablature engine

rtl

16 M A Y 1957 *

Fig. 35: Side view and motion-workplan of the doubleentablature engine with parallel motion crosshead.

them from 1/2in. square bar, leavingflanges about 1/16 in. wide unmachinedat each end, with simple mouldingsadjacent to them and the rest of theshank tapered from 5/16 in. to 1/4 in.dia. Needless to say, the lengthbetween flanges must be the same inall cases, and not only must the flangesbe square with the sides of the en-tablature but the latter must also havesquare corners, not rounded; asMuncaster says, “it is a sound rulein architecture that no cylindricalpart appears about the square cap ofa column.”

Brass is often used for the structuralparts of these steam-engine modelson the grounds of appearance, oravoidance of rust, but in the proto-types nearly all parts were made ofcast or wrought iron. The cylinderis 3/4 in. bore x 3/4 in. stroke, the design,including slide-valve, etc., followingconventional practice. Piston andvalve rod glands have oval flangesand the sides of the stuffing-box onthe cylinder cover have flat surfacesfor the attachment of the slide bars.

The latter are splayed outwards atthe top, with horizontal lugs to fasten

16 MAY 1957

I ’

to the underside of the entablature,but the sliding surfaces must beexactly vertical and parallel to thecylinder axis both ways. It is probablethat these were intended to be forgedto shape, and machined or filed onlyon the working surfaces; but mostconstructors will probably find itbest to cut them from the solid.

The crankshaft, of the overhungtype, has a 1/4in. dia. main journaland runs in plummer block bearingsmounted on the flat surface of theentablature; the web or crank dischas a pin 5/32in. dia. fitted at 3/8in.radius, either by screwing or pressingin. A spoked flywheel, 3 in. dia., isfitted and it may be observed that ifcharacter is to be as faithful as possibleboth the rim and spokes should bethinner than is usual in modernpractice.

The eccentric is attached to the shaftas close as possible to the mainbearing, thus serving as an end locat-ing collar, and this should enable therod to be lined up with the valve rodwithout bending, which is always un-sightly and frequently quite un-necessary. Forked ends are employed

The two further examples of enginesin this class, illustrated in Figs 35, 36and 37, are both the “ double ” type;having overhung cranks at either endof the shaft and a central flywheel;but the design could quite easily beadapted as a single-cylinder engine ifdesired. They are also much moreelaborate in design than the foregoingexample and best suited to con-struction from castings, though many,if not all, of the components could beproduced by machining from solid orfabrication by brazing and soldering.The latter methods are often preferredby constructors who wish to obtainthe utmost accuracy in details such asfluting and other forms of decoration,which was such an attractive featureof these old engines.

701 MODEL ENGINEER

Fig. 36: End view and thecylinder plan of double engine

on both the connecting rod an theeccentric rod; the crosshead is of H-section with grooved faces to embracethe slide bars. Despite its simple construction, this design can be madeinto quite a handsome and dignifiedmodel and an efficient worker.

TWIN-CYLINDER OR “DOUBLE”ENGINES

.!./

Very little descriptive matter wasfurnished by Muncaster on theseengines; the drawings were consideredto be self-explanatory, at least to thosereaders who were sufficiently exper-ienced to be likely to take an interestin their construction. Taken in con-junction with previous examples ofdesign and functional details, Ithink that most of the essential in-formation will be found in the draw-ings.

The major difference between theengine shown in Figs 35 and 36, andthat in Fig. 37, is that the former wasfitted with parallel motion to thepiston crossheads and the latter withslide bars; this would probably, butnot necessarily, be a guide to period,as the earlier engines were less likelyto be fitted with sliding crossheads.

In the particular type of parallelmotion illustrated, the geometry issimple and obvious; it was used onmany types of engines, both hori-zontal and vertical, though in thelatter case the ends of the radius rodswere more often anchored frombrackets fixed to the walls of theengine house than from columns; in.a few cases, however, short rodsanchored to lugs extended from the

Fig. 37: Another double

sides of the cylinders were used.The use of a deep bedplate or

plinth with the cylinder partly orcompletely sunk into it, in conjunctionwith an equally deep entablature, notonly enhances the dignified appear-ance of the engine, but by enablingshorter columns to be used, increasesthe rigidity of the structure.

In the two engines shown in Figs 35to 37 it appears that two separate plinthsare employed, and also separateentablatures in the form of box girders;but I should prefer to employ asingle plinth and a rectangular frameentablature; even in an engine havingonly one cylinder this form of con-struction would give maximumstrength and simplify lining up.

Note that the columns are notturned to a straight taper but areslightly convex or “ fish-bellied “;this is correct to architectural tradi-tions and improves the appearance solong as it is not overdone. In thedetails of column pedestals andcapitals on the right of Fig. 37, theexact shape of mouldings, etc., isshown, most of the dimensions beinggiven m 32nds of an inch. Theconnecting rods have gib and cotterfixings for both the crankhead and

MODEL ENGINEER

. ,.

crosshead bearings, the latter beingforked. A belt-driven governor isfitted, operating a butterfly throttleon the steam supply line whichconnects to the two cylinders by ahorizontal branch pipe.

When the drawings of these engineswere published one or two of thedetails were mildly criticised bymeticulous students of period steam-engine design; for instance, it wassuggested that the flywheels had toogreat a radial depth of rim, and thatfour spokes should be used insteadof six, also that the crankshaft shouldbe of square section, turned only onthe journals, with the flywheel andeccentric sheaves staked on.

I have no doubt that the criticswere well informed, but apparentlyMuncaster made some concessions tosimplicity in castings and machiningprocedure-just as I do myself-andif every example of a model purportingto be a true period piece were ascorrect as his drawings I for one shouldbe very well satisfied. Incidentally, thedrawings for Figs 35 and 36 weremade by my col league, J . N.Maskelyne, and are a fine example ofengineering draughtsmanship.

l To be continued

engine with sliding crossheads, showing details of columns

n.

702 16 MAY 1957

The MUNCASTERsteam-engine models - 8

Continued from 16 May 1957, pages 700 to 702

In this instalment EDGAR T. WESTBURY discusses governors and control gear

SEVERALL of the engine designsin this series have includedgovernor gear, which is a

necessary fitting for engines whichhave to maintain a fairly constantspeed under varying load conditionsand, in fact, practically any enginewhich is not under the constantsupervision of the driver.

Nearly all steam-engine governorsare of the centrifugal type, based onthe original pendulum governor ofJames Watt, and, in the earlier enginesat least, were usually made as a self-contained unit, located at a con-venient position on the engine to bedriven by belt or gears, and forconnecting up to a throttle controlvalve. I have described governormechanism in connection with theUnicorn and Tangye engines as well asin a separate artical [MODEL ENGINEER,27 October 1955] dealing with agovernor unit suitable for the Vulcanbeam engine or other early types.

Although the basic principle of thegovernor is very easy to grasp-indeed, it is obvious to anyone whohas observed the forces exerted inrotating masses-a full explanation ofthe theoretical considerations involvedin its design would take up far morespace than could be spared here.Advanced text books on mechanicsusually devote one or two pages ofmathematical formulae to the pheno-mena of centrifugal force (for example,see Ganot’s Physics, para. 55), but

MODEL ENGINEER

I propose to deal only with simplepractical applications of the governorin the examples illustrated by Mun-caster.

As first designed by James Watt, therotating ball weights of the governornot only produced the positive operat-ing effort, under the effect of centri-fugal force, but also the negative orrestoring effort, under the effect ofgravity; no extra weights or springswere used, and the vertical shaftarrangement was essential for itsoperation. Muncaster states that inthis form it cannot be recommendedfor small engines, and my own ex-perience supports this view; the reasonof course is that in a small size; thegravitational effect is reduced to amuch greater extent than the frictionin the working parts.

The action of a governor relyingon the weight of the balls alone torestore the control on reduction ofspeed would be very erratic or“ sticky.” Some governors, such asthe Porter type, have an additionalmoving weight, which slides on thecentre shaft and supplements therestoring force, but even this is limitedin its effects and has been foundinadequate in small sizes. It is alsonecessary in most cases to increasethe positive effort by running smallgovernors at higher speed than thefull-size types.

The obvious method of increasingthe restoring effort is by fitting some

ind of spring. In theory, this com-

plicates matters because there is adefinite difference between the actionof a spring and that of gravity, butin practice governors fitted withsprings work quite well. The governorillustrated in Fig. 38 has a com-pression spring fitted on the centre’shaft, which exerts pressure betweenthe fixed upper yoke from which theweight arms are suspended, and thelower sliding yoke carrying the groovedcollar which operates the control lever.

This design has several features incommon with the governor of theTangye engine previously referred to,but the method of articulating thelinks is somewhat different. Insteadof forking the top ends to embracethe ball, the latter is made hollowand the link is made ball-ended tofit inside it; in either case the pivotpin passes diametrically through thecentre of the ball. This is a very neatarrangement, but it decreases theweight of the ball substantially so thatit must be made larger to produce thesame centrifugal effect.

To transmit the movement of thesliding yoke to the governor leverwith the minimum friction, a horse-shoe-shaped thrust collar i s fitted tothe groove, with extended pivotsengaging the eyes of short arms whichstraddle the yoke and are pinnedto the lever shaft. These details areillustrated in Fig. 39. The governorshaft runs in a long vertical bearing,in a bracket mounted on the engineframe, and is bevel-geared to the

Left, Fig. 39: De-tails of the link pivots and thrustcollar for the ver-tical-shaft governor __

Right, Fig. 40: Ahorizontal spring-loadedgovernor suit-able for high-speed

engines

778 30 MAY 1957

\

drive shaft which runs in a longho r i zon t a l bea r ing .

For high-speed engines, such asthose used for driving electricalgenerators, it is often found con-venient to fit the governor directlyon the end of the main shaft, thusavoiding the necessity for either beltor gear drive, and making a compactarrangement. In this case springreturn is an obvious necessity, and itis also desirable to reduce the numberof working parts in the mechanism,thus cutting down wear, friction andany tendency to rattle. The governor(Fig. 40) fulfils these conditions, as itentails the use of only two pivotedjoints and a single sliding member,with a totally-enclosed central com-pression spring. This type is suitablefor the engine shown in Fig. 28.

It will be seen that as the ballsmove outwards under the effect ofcentrifugal force, the bell crank leversto which they are attached press

against a fixed thrust washer, causingthe entire governor assembly to movebodily to the left. The grooved collaron the sleeve operates the governorlever through a thrust collar, as inthe previous example, though this isnot shown. In the drawing, the shaftextension which carries the governoris shown as screwed into the end ofthe main engine shaft, but there arepractical objections to this arrange-ment, and I would suggest modi-fying it.

Apart from the risk of the extensionunscrewing if he shaft ran in a clock-wise direction or in the event of suddenacceleration or stopping, there mightbe difficulty in ensuring true runningof the extension shaft. It would bebetter to make this integral with theshaft or devise some more positivescheme of fixing and alignment.

It is hardly necessary to add thatthe governor assembly should besymmetrical and in balance at all

,

speeds; this is of special importancenot only when it is fitted on a shaftextension as in this case, but alsowhere it projects a long way above asingle vertical bearing as in theprevious example.

THROTTLE VALVESThe design of the control gear which

regulates engine speed under theinfluence of the governor is of equalimportance to that of the governoritself. The simplest method ofgoverning is by means of a throttleor restricting valve, which may be ofany type so long as it is capable ofbeing operated with the minimumeffort; it does not need to be capableof shutting completely, as it is gener-ally a supplement to the main enginestop-valve, which should be quitesteamtight when closed.

The type of throttle valve shown inFig. 41 is commonly used in full-sizepractice and is recommended by

Left, Fig. 38: Aspring-loaded ver-tical-shaft gover-nor, bevel-gearedfor belt or direct

drive

Right, Fig. 41:Details of thesliding piston-type throttlevalve for gover-

nor control

MODEL ENGINEER

MUNCASTERMODELS . . .

Muncaster for small engines. It issimilar to that specified for theTangye engine, but I have found itmore difficult to employ successfullythan types which have a rotary move-ment, such as barrel or butterfly valves.The reason for this is that it takes lessb effort to rotate a shaft in a packedgland than to slide it bodily throughthe gland, as in this case.

However, the example illustratedis certainly capable o f working suc-cessfully on the larger models forwhich it is obviously intended, havinga bore diameter of 3/8in. Steam mustbe admitted from the left-hand(horizontal) branch, the vertical pas-sage being connected to the engine.It is fitted with a liner having threeports which open into an annularpassage, so that pressure is even allround the valve and there is notendency to press it against one sideof the liner. The sliding piston shouldbe a smooth, easy fit in the liner, thetwo parts preferably being of dis-similar metals, such as steel and castiron, or brass and hard bronze, togive good wearing properties. A holemust be drilled through the pistonto balance the pressure on the uppersurface, otherwise it will be difficultto move owing to inequalities in thisrespect or through the trapping ofsteam or warer. Care must be taken tofit the cover, with its central gland,in exact concentric register with theliner, and the piston and rod alsoconcentric with each other.

The “ power ” of a governor maybe defined as the positive effort whichit is capable of exerting on the controlvalve. It must obviously be capableof overcoming any frictional or otherresistance encountered in the controlgear, and it is desirable to have amargin of power in hand to ensurereliable action. The power can beincreased in two ways: by increasingthe weight of the balls, and by in-creasing the rotational speed. Largeslow-sped engines call for heavygovernor weights unless the governoris geared up. But in high-speedengines a governor which appearsmuch too small may be just as effective.

It should be noted that the forceavailable to operate the control gearis influenced by the means by whichmotion is transmitted from the

governor. Sometimes, in order toobtain sufficient amplitude of move-ment at the valve rod or spindle froma relatively small governor movement,multiplying levers are used in theconnecting mechanism, and the effec-tive force is, therefore, reduced ininverse ratio to the increased move-ment. Thus the governor power mustbe adequate to cope with this.

If, however, either the effectiveweight or speed of a governor isincreased, it will come into operationat a lower speed and thus, if the enginespeed is to remain the same, equili-brium must be restored by increasingthe strength of the return spring orcounterweight. Erratic action or“ hunting ” (alternate rise and fall ofengine speed) may be caused byfriction in the control gear or byfaulty governor design-this includestoo great a multiplication of levermovement-so that the control valvemoves over too great a range with aslight increase of speed. Steadiness ofcontrol is sometimes improved byfitting a damping device, such as anair or liquid dashpot. Speed rangecan be adjusted by varying the springtension, or fitting an external springwhich can be adjusted while running.

ACCURACY OF CONTROLAlthough a governor is often

assumed to be capable of keeping theengine speed exactly constant, thisis not so in practice because thegovernor cannot effect any change inthe throttle position until some changeof speed has taken place. There mustobviously be some margin of error inany type of governor, depending on itsdesign and control linkage, but morestill on workmanship and eliminationof friction. For most stationaryengines of small size, a margin offive per cent. deviation from the setspeed is fairly reasonable, but forspecial purposes, such as generatingelectricity, closer accuracy is necessary,and the permissible variation may beless than one per cent. in some cases.

Geared or positively-coupled gover-nors are preferred for accurate control,as belt-driven governors may beliable to variable slip-and theirreliability may be open to question.Cases have occurred where brokengovernor belts have caused seriousaccidents, sometimes with loss of life.Nevertheless belt driven governorsgave good results on steam-enginesfor many years.

EXPANSION GOVERNORSWhile throttle governing is effective,

and is probably the most satisfactorymethod on small engines where rigidsteam economy is not the first con-sideration, it tends to waste steamby the wiredrawing effect of the

throttle valve, which results in lower-ing the working pressure of the steambefore it enters the engine cylinder.To obtain the best efficiency the steamshould be admitted at full pressure,but cut off earlier in the pistonstroke so that it can be used ex-pansively and its energy utilised to thebest effect. This is done in mostlarge engines by using governors whichvary the timing of the valve gear.

The governors themselves may be ofnormal type though they are some-times of special design; they mayoperate on the normal slide or pistonvalve through linkage comparable tothat of reversing gear, or on “ trick ”valves with elaborate porting arrange-ments. Corliss and drop-valve engineshave trip devices which close the steamadmission valve suddenly at differentcut-off points under governor control.

A very simple and effective ex-pansion governor is that fitted directlyon the engine shaft-often in theflywheel-and controlling a movableeccentric, not only by reducing thethrow but also advancing the timing.If only the throw and, consequently,the valve travel were reduced, itwould certainly result in earlier cut-off, of steam, and, therefore, beeffective in speed control; but itwould be uneconomical because theadmission point would be retardedor, in other words, lead reduced inthe same proportion. To compensatefor this, therefore, the eccentric isadvanced as its stroke is reduced.

As expansion governors are aspecialised type in which only alimited number of readers are likelyto be interested, I do not propose todescribe them further. But for thebenefit of those seeking further in-formation: it may be noted that theywere fully dealt with by Muncasterin a series of articles in Vol. XXVI(January to June 1912) of M O D E L

ENGINEER.l To be concluded

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MODEL ENGINEER 780/

30 MAY 1957

i

\

The MUNCASTERsteam-engine

models - 9By Edgar T. Westbury (Continued from 30 May 1957. pages 778 to 480)

IN the construction of modelswhich have any pretensions tofidelity in respect of type or

period, correctness is essential-not only in the general style andcharacter of the main structurebut also in the minor. details andfittings. I have referred to this inearlier articles, but a further reviewof this matter may be desirable,especially in view of the errors

which are of ten seen in modelengines entered in exhibitions.

These models may be broadlyclassified as follow:

First, the type of engine which Ihave described as a “ utility ” engine.Many constructors wish simply tobuild an engine which will do a job,such as driving the screw propeller orpaddles of a power boat or a stationaryplant of any kind; they are not particu-

, larly worried as to whether it conformsto any particular period or style, andin many cases the engine worksbehind the scenes, where it is notvisible or obtrusive. In such cases theonly things that matter are functionalefficiency and mechanical soundness.

Secondly, there is the freelance typeof model which is intended to begenerally representative of some periodor phase of engine development, butnot an exact copy of a prototype.

Thirdly, the true scale model ofan actual full-size engine, whetherperiod or contemporary type.

It is, in my opinion, the secondclass of model which presents theworst pitfalls to the unwary con-structor because certain features in itmust always be optional, and may bedetermined to some extent by sim-

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plicity or convenience. The discreetconstructor of a true scale modelwill take pains to obtain authenticinformation about the actual prototypebefore he undertakes construction,and his task consists of making anaccurate copy-at least in visibledetails. But in freelance models, notonly may general design and propor-tions be at fault, but a model which isbeyond criticism in these respectsmay be let down by small details.

Perhaps the worst kind of fault isan anachronism; that is, somethingobviously out of period, such as a“ marine ” type bolted crankheadbearing on an early nineteenth centurybeam engine. Another very seriousfault in freelance models is to representthem as models of actual prototypes-ethically, this borders on thefraudulent-but it is often done !How often, for instance, have we seenalleged models of Stephenson’s Rocketwhich bore only a very sketchyresemblance to this famous engine.

In Muncaster’s models, the need tosimplify construction never led himto commit crimes of this nature;both the general character and detailwork in his designs were true to typeand period though the use of certainready-made parts, such as slotted-head screws, was allowed to a greaterextent than I should consider per-missible in a model intended forexhibition. These parts, however,could be changed without affectingactual component design.

GLANDSPiston and valve rod glands are

very simple components, but theirappearance may make or mar a model.

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In utility engines, screwed glands maybe easiest to fit, and also to adiust inthe limited space available. But mostfull-size engines have flanged andstudded glands, the most commonbeing the oval type as illustrated inFig. 42 (the proportionate dimensions,to suit various sizes of rods, are givenin the accompanying table).

Very large engines sometimes havecircular-flanged glands, with three,four or more studs; the measurements,however, may be much the same, withthe exception of the minor axis g.The internal bevel at the mouth of thespigot (shown in dotted lines on thisdrawing) acts as an internally inclinedplane to wedge the packing towardsthe piston rod; an included angle ofabout 120 deg. (or normal drill-pointangle) will be suitable-also for thecounterbored gland recess or neckbush. This applies to either screwedor flanged glands.

BEARINGSSeveral of the engines in this series

have been shown with pedestal bear-ings of the split “ plummer block ”type, for bolting down to a flat baseor entablature. The proportions ofthese are shown in Fig. 43, and thecorresponding table.

In full-size practice, the bearinghousings were often fitted with arectangular bottom “ brass,” the fittingof which may be somewhat difficultin a model. Unless meticulousadherence to detail is considerednecessary, a half-round brass maygenerally be regarded as suitable, ifprevented from rotating by a peg or-other means. In many cases, bushingsmay be omitted, and the shaft rundirect in the pedestal casting, of castiron or bronze.

An alternative type of main bearing,applicable where it is cast integralwith the engine bedplate, is illustratedin Fig. 44. Here the horns of thehousing extend well above the shaftcentre, and are fitted with a keep plate,both the halt-brasses being rectangu-lar. The external shape of the castkeep plate shown in this illustrationgives a much better appearance thanthe plain strip of steel bar so commonlyfitted, and is also more rigid.

In many horizontal engines, pedestalbearings having both the split bushingsand their housing set at an angle areemployed, with the object of avoidingmaximum thrust being taken on thedividing line; a typical example isseen in the Unicorn engine.

CONNECTING RODSVarious cross-sections of rods have

been used in steam-engines; theearliest types had forged rods ofround or rectangular section, but manyclassic examples of beam engines laterhad cast-iron rods of cruciform-or

MODEL ENGINEER

ribbed section. Still later, whenmachining facilities were improved,round and rectangular rods againbecame popular, but were now bright-finished all over. The bearings ofthese were usually of the “ gib andcotter” type, at least at the crank-head, and often at the crosshead.

Two typical examples o f thesebearings are seen in Fig. 45; the firsthas a rectangular strap embracing thetop and bottom sides of the rod end,and bolted in position. The splitbearing is of rectangular shape to fitthe frame thus formed, with sideflanges for location, and the innerhalf has a tapered groove in the backso that it can be secured, or weartaken up, by the wedge or “ cotter”which passes through slots in thestrap.

In the second example, the strap iseliminated, the rod having an open-ended slot with a gap piece securedby a bolt. The wedge method isagain employed for adjustment, butin this case is drawn in by means of abolt against a tapered face in the roditself. Many other variations of thesebearings were used, but the sameprinciple of adjustment was applied.

STOP VALVESIn many steam-engine models, real-

ism is marred by the use of crudely-shaped auxiliary fittings, such asdrain cocks or stop valves, which mayserve their purpose but are quite outof keeping with the rest of the design.An example of a stop valve which iscorrect both internally and externallyis shown in Fig. 46. It is best adaptedfor construction from castings, or atleast one for the body, but could befabricated or machined from the solid.

One or two important points about

Fig. 43: Standard ” Plummer block ”type pedestal bearings

the functonal design may be noted;first, that the actual valve head i sseparate from the stem and free torotate upon it, being retained in placeby a tangential pin which intersectsthe groove in the stem. It may bemade with a spherical curve outside,and also on the inside thrust face,. asshown; this gives it a self-aligmngproperty, though the “mitred” 90-deg. head and flat internal thrustface is more common.

Secondly, the valve seating in thebody is raised, so that metal is availablefor re-facing if it becomes necessary,and grit or scale tends to fall awayfrom the seating instead of remainingto become imbedded when the valvecloses. Last, but not least, the threaded

MODEL ENGINEER

Left, Fig. 44: Analternative type ofpedestal bearing,cast integral withthe engine bedplate

Right,‘ Fig. 45: Al-ternative types of” gib and cotter ”connecting-rod bear-

ings

842

portion of the valve stem is madelarger in diameter than the shankwhich passes through the gland, anda washer or neck bush preventspacking from jamming in the thread.

The main object of this form ofdesign is to prevent any likelihood ofthe spindle being unscrewed out of thevalve and getting blown into the faceof the operator-not by any meansan impossibility with some types ofstop valves used in the past. These, Ibelieve, have long been banned onfull-size engines and boilers by Board -of Trade regulations.

Both right-angled and straight-through types of stop valves are usedon engines, according to convenience,the latter being more difficult to makein small sizes, though methods ofdoing so have been described severaltimes in MODEL ENGINEER. In eithertype, the entry side should be underthe valve head, so that the gland is notunder pressure when the valve isclosed.

REVERSING GEARIt has already been mentioned that

the majority of stationary engines aremade to run in one direction only, asthe need to reverse them simply does

13 JUNE 1957

STANDAPD GLAND

Left, Fig. 46: A stan-dard type of right-angled s top valvefor the manual con-trol of steam supply

Right, Fig. 47: Dia-gram showing thepositions of eccen-tric in relation tocrank, for forwardand reverse rotation

not arise. Certain types of engines,however, such as those for hauling,hoisting or winding, need to bereversible-and this also applies tomarine engines of all kinds.

In dealing with oscillating engines,it was seen that rotation can bereversed simply by changing over steamand exhaust connections, but this isnot practicable in the normal type ofslide-valve engine, especially if it isprovided with lap and lead forefficient operation. A piston-valveengine with no lap or lead, however,can be reversed in this way, and it isoften done on engines for smallcranes, ash hoists and ships steeringgear, for the sake of simplicity and

. convenience. Generally, however,reversing entails shifting the phase of

the valve gearing through an angle of180 deg., minus twice the angle ofadvance.

This is illustrated in Fig. 47 whichshows the positions of the eccentric,relative to the crankpin, for bothdirections of rotation. The exactangle, of course, will depend on theamount of lap and lead employed,and for an inside-admission pistonvalve the angle of advance will be onthe minus side of 90 deg. from thecrankpin in each case.

The simplest method of obtainingthe required phase shift is by using asingle eccentric, not rigidly fitted tothe shaft but capable of a limitedangle of movement, so as to take upthe relative angles shown whenmoved to the limit of travel either way.

Fig. 48: A horizontal engine fitted with Stephenson link reversing gear

\”With this arrangement, an engineonce started in a given direction willautomatically take up its correcttiming so as to continue to run in thatdirection.

The “ slip ” eccentric, as it is called,is very popular for simple modellocomotives and marine engines, andhas also been applied to quite largeengines in conjunction with somemeans of disconnecting the slidevalve and operating it by hand .formanoeuvring. Such engines are notpositively self-starting but this appliesto all single-cylinder engines in anycase, and is not necessarily a practical,disadvantage. A typical example isthe Trojan marine engine.

There are several types of positivereversing gears used on steam engines,most (if not all) of them being de-signed not only to effect the requiredphase shift, but also to vary theadmission cut-off, so as to employexpansion to best advantage-in otherwords, to “notch up,” the notchesbeing those in the reversing gearcontrol quadrant. Of these, the earliestbest known, and still most universallypopular, is the so-called Stephensonlink gear, though its title has oftenbeen held in dispute, and is sometimesascribed to Howe, who was employedby George Stephenson.

Muncaster gives an example of ahorizontal engine fitted with this formof link gear (Fig. 48) which mayserve as a typical illustration of thegeneral design and proportions of itsworking parts.

To be concluded

ADDITIONS TO THE LATHEInstructions for making centring

holderschucking accessories; tooland cutter bars; dividing

simple milling attach-ments; aids to screwcutting; andsteadying appliances are to be foundin Edgar T. Westbury’s Lathe Acces-

Priced 3s. 6d., postage 3d. (U.S.A.and Canada $l.00), it can be obtainedfrom Percival Marshall and Co. Ltd,19-20, Noel Street, London, W.1.

13 JUNE 1957 843 MODEL ENGINEER--