Current Collection SystemsPart 3 Section 8

August 2008

8.1 IntroductionThere are probably as many ways of currentcollection for model locomotives as there are wheelarrangements. All the methods available have theiradvocates and modellers should select a systemthat they find easiest to apply to a particularlocomotive.

Pick-up systems fall roughly into three groups.• Commercial plunger types where a spring-

loaded contact bears on the back of the flange• Wiper systems where a wire or strip bears on

the wheel• Contact is made through the axle to the bearing

bush. This requires either metal wheels orplastic centered wheels to be fitted with ashorting strip.

8.2 Plunger pick-upsThere are several suppliers of plunger pick-ups andalso some kits come with the manufacturer's owndesign. As the name suggests, a plunger is pressedagainst the back of the wheel. A spring is used toapply the pressure (Photo 8.1).

The installation requires a hole in the frame to fitthe body of the pick-up or to clear the plunger. Theposition of the hole can be important. If there is anyform of suspension then the hole should be on thehorizontal centreline of the axles, or nearly so. Seealso the data sheets at the end of this section.

When using plunger pick-ups, it is important thatthe wire that connects the pick-up to the motor isflexible, so that it does not restrict the movement ofthe plunger. The current drawn by modern motorsis small, and so a fine gauge can be used. The wireshould be stranded rather than solid.

Photo 8.1 Typical plunger pick-up arrangements in a chassis under construction. (Photo K. Sheale)

Photo 8.2 Brass wire pick-up soldered to a copper-clad strip bonded to the chassis. Several bendshave had to be introduced to reach the back of theflanges.(Photo R. Alderman)

8 Current Collection Systems

8.3 Wiper pick-upsWiper pick-ups are probably the simplest and mostadaptable type. They can be fitted in many placesand can be almost invisible.

One of the best method of mounting them is touse a piece of copperclad attached to the chassis.This can be sleeper strip or cut from a sheet. If youdo not wish to glue the strip to the chassis, double-sided copperclad can be used and soldered on. Takecare not to bridge the gap and create a conductingpath between the two sides of the copperconductors.

The wiping contact is a matter of choice. 0.45 mmdiameter brass wire is a good choice and easilysoldered to the copperclad strip. Nickel silver orphosphor bronze can equally be employed (a source

3-8-1Author: Bob Alderman

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of the former being 0.012 in. guitar E strings).The geometry of the wire will depend on the

installation. In Photo 8.2 it is apparent thatseveral bends are needed to clear the bottom of thechassis then reach over to the back of the wheel.The end in contact with the wheel can be either theend of the wire pressing into the flange or shapedas in the illustration to sweep the back of thewheel. This is a matter of personal choice, neithermethod having a great advantage over the other.

The contact pressure on the wheel should not betoo heavy. Treat the wire as a cantilever spring. Asimple rule of thumb is if the length of the wirefrom its fixed point is taken as 'X' units, ignoringany bends, then the deflection required to bear onthe wheel is about 0.2X units (Figure 8-1).

Sometimes it may prove difficult to thread thepick-up wire through brake gear without ittouching. A simple solution to this is to insulate it,using insulation stripped from some fine electricalconnecting wire, or the thin insulated tubing usedin electronic work. Such insulation is threadedover the pick-up wire.

An alternative to rubbing on the back of thewheel is to have the wiper rub on the tread, thetheory being that it is self-cleaning. This is notalways easy from inside the chassis, and can leadto a convoluted pick-up wire. If possible try tomount it on the outside of the chassis. Theillustration (Photo 8.3) shows a pick-up fixed to thehead of an 8BA bolt threaded through an insulatedbush in the chassis. The bush was made from the

Photo 8.3 In this variation the pick-up is largelyhidden from view by the locomotive body. (PhotoR. Alderman)

Photo 8.5 Slater's driving wheel shorted using twophosphor bronze strips. (Photo F. Lewis)

Photo 8.4 Another variation of rim scraper pick-ups is to solder a wide strip of copperclad betweenthe frames and score it into four electrical sections.The outer strips provide a means of soldering tothe underframe and the inner strips provide amounting point. The outer strips and the frameedges have an insulating layer to prevent the pick-ups shorting to the chassis. (Photo K. Sheale)

body of a plunger pick-up but styrene tube canserve equally well. In the example illustrated, thebolt is slightly free to move in the bush allowingone set of wheels to move on the compensation. Themotor wire is soldered to the inside end of the bolt.

8.4 Axle/bush contactThis system is used in conjunction with either splitaxle systems or the 'American' pick-up system. Forthese systems the pick-up via the axle/bush contactrequires an uninsulated wheel.

All-metal wheels are available in both insulatedand uninsulated versions. For the split axle systemthe uninsulated version is used throughout. Forthe 'American' system half the wheels areuninsulated and half insulated. For plastic centredwheels like Slater's it is necessary to fit a shortingstrip between the rim and the boss. Photo 8.5shows a Slater's driving wheel shorted from tyre tocentre bush. The metal parts are drilled to accept a

Figure 8-1 Pick-up wiper deflection.

X

0,2

Current Collection SystemsPart 3 Section 8

August 2008

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a short circuit. The outer sleeve shown is optional.The axle carrying the final drive from the motor

needs to be treated differently. There arecommercial axles having double insulation leavingthe centre electrically dead. These allow the finalgearwheel to be mounted directly on the axle. Thedesign as shown in Figure 8-2 is only suitable if thefinal drive gear is bored out to fit over theinsulating sleeve or if the gearwheel is made froma load bearing plastic which is electrically non-conducting.

A conventional drive can be installed if the finalgearwheel is offset and the insulation cuts arelimited in length. The arrangement is shown inFigure 8-3. The amount that the motor and gearingcan be offset depends on the space available in thelocomotive body and the method of mounting. Mostfinal gears are about 10 mm wide including theboss, so it should be just possible to insert aninsulation break in the space available. If the finaldrive gear forms part of a motor-gearbox design itis still possible to use the system providing thegearbox final drive bearings, which will most likelybridge the split, are of insulated material.

A further method that is only suitable for tenderor bogie axles, which do not require quartering, isshown in Figure 8-4. The axle is sawn in half, andthe two halves are glued together with epoxy resin,

Figure 8-3 Final drive gear offset to allow room foran offset axle spilt while retaining the squared andshouldered axle.

Figure 8-2 Split axle system suitable for squaredand shouldered driving axles.

Figure 8-4 Split axle system suitable for bogieand tender wheels

copper wire or, as in the illustration, a phosphorbronze strip. This can be readily soldered if it istinned before fitting and a large hot soldering ironis used. The soldering should only take moments,and must be done quickly to avoid softening anddistorting the plastic centre of the wheel. If desired,the spokes can be relieved to accept the strip, asshown in the illustration.

In this type of system, current passes from theaxle through the bearing. For plain bearings this isnot usually a problem because the bearings are notflooded with lubricant so that it forms a continuousinsulating film. There is sufficient, continuous,metal-to-metal contact to carry the necessarycurrent. However, with ball races, the area ofcontact between the ball and the race is very small,and even a small current can cause sparking,oxidation, and erosion that can destroy the bearingin a short time. The solution in such cases is toinsulate the bearings from the axle (the inner racerotates with the axle so no rubbing is involved),and current is collected from a wiper acting in agroove on the axle.

8.4.1 Split axle systemA number of methods of producing split axles usingepoxy resin adhesive as the securing compoundhave appeared in the model press. Most require theuse of a lathe and an assembly jig to hold thecomponents in position while the resin cures.

There are two methods that do not require theuse of a lathe and cover most requirements. Amethod suitable for axles not required to carrygearwheels is shown in Figure 8-2.

Drill two holes in line with each other throughthe axle and join with a saw cut. Pack the slot soformed with epoxy resin and allow to set. When itis hard, saw across the axle from opposite sides intoeach hole. Pack the cuts with more epoxy resin andallow it to set. Care needs to be taken to ensurethat no metal swarf is trapped in the cuts to cause

Drill through Optional insulated sleeve

Back to back

First saw cut

Second saw cut

Gear Axle

Offset

Half axle

Vent

Plastic sleeve

Gap

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ensuring that a continuous film of resin occupiesthe space left by the saw cut. A plastic sleeve holdsthe two halves together in a jig at the correct back-to-back dimension. The small hole, approximately1 mm diameter, is to allow excess epoxy resin toescape when the two half-axles are pushed into thesleeve.

8.4.2 Frames for split axle locomotivesSplit axle arrangements using metal bearings orhornblocks require that the frames be electricallyisolated from each other. However, if insulatedhornblocks are used (Slater's or similar - see Photo8.6), this is not necessary.

Modern locomotive kits using underframesetched in relatively thin brass can easily bemodified for split axle pick-up. Figure 8-5 showsthe methods used. The frames should first beassembled using the kit spacers. These are only tohold the frames in position, and will later beremoved, so solder them only lightly or, if possible,screw them in place. Fix mounting nuts to thefootplate. Use a steel screw well oiled to hold thenut in place when soldering it, to ensure that thescrew comes out again. Cut printed circuit board(PCB) spacers and groove them for insulation gapsand secure them to the footplate, using themounting nuts. Put the frame assembly on to thePCB spacers and align it by eye.

Put the wheels in the frames and spin them toensure clearance at the splasher cutouts. When

Figure 8-5 Insulating the frames

satisfied, solder the frames to the PCB spacers.Either remove the original metal frame spacers, orsplit them and support them with an insulatedbacking material. Check that the frames areelectrically isolated from each other.

A similar method using double-sided PCBspacers set between the frames is shown in thescrap view at the bottom left of Fig. 8-5. Because ofthe close proximity of the footplate, it is useful toput a thin layer of isolation along the top edge ofthe frame or on the underside of the footplate toprevent electrical contact. Other methods includeusing frame spacers made from Tufnol, Perspex orsimilar materials.

When using insulated frames, it is also importantto ensure that any other components that bridge

Photo 8.6 Slater's insulated hornblocks(Photo K. Sheale)

Insuation Gap

Insuation Gap

Insuation

B

Motor support plate

Footplate

Copper face

Frame

A

C

GapSingle sided copper

laminate frame spacer

Angle solderedto frame and spacer

C

A

View A-A Elevation C-C

Plan B-B

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Figure 8-6 An insulated drawbar

the frames, such as inside cylinders and motionbrackets, or brake beams, include insulating gapsto avoid short circuits occurring that way.

These methods can also be used for locomotiveswith ordinary axles and pick-ups to ensure that thebody is electrically neutral. Hence, when doubleheading, operators are not greeted with the sight oftwo stationary locomotives with couplings glowingred hot.

8.4.3 'American' wiring systemThis system is designed to eliminate the use of pick-ups rubbing against the wheels of a locomotive butis only applicable to tender engines and some tankengines. The locomotive wheels are insulated on oneside only and power is picked up from the rail on theuninsulated side. The tender wheels are insulatedon the opposite side and the uninsulated side actsas the return to the other rail. One terminal on themotor is connected to the locomotive chassis and theother is taken to the tender chassis via a connectionbetween the engine and tender. Examples areshown in Photos 8.7 and 8.8.

Although the system is simple it has certaindrawbacks. For example, unless the locomotive andtender chassis are insulated from the bodies,shorting will occur if the bodies make contact on asharp curve. Locomotives with long overhangingcab roofs may come in contact with high-sidedtenders under similar circumstances. If the fallplate between the locomotive and tender is

modelled, it must be insulated. A thin layer ofstyrene sheet or insulating tape on the underside isadequate for the purpose. Unless the locomotivecouplings are insulated, double heading can alsocause a short circuit if both locomotives are wiredthe same way round.

In this system, current is transferred betweenthe locomotive and tender using a wire as shown inPhoto 8.7 (which might be disguised as a waterhose) or by means of an insulated drawbarconnector. The simple arrangement shown inFigure 8-6 that is used by Far Eastern brasslocomotive builders for the American marketmakes use of an insulated drawbar as a built-inconnector. The tender carries a pivot pin fixed

Photo 8.7 The connection system fitted to an old model converted from three rail. The tops of the chassismembers have a thin insulating layer attached to ensure that the locomotive and tender bodies are electricallyisolated. The mounting screw holes in the frame stretchers have been enlarged to allow an insulating sleeve tobe installed. Insulating washers are fitted under the nuts. The locomotive and tender are semi-permanentlyconnected by the wire, which just uses a simple 'chocolate-block' connector to link the two. The wheels are castiron and therefore conducting (Photo K. Sheale)

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firmly to, and making electrical contact with, thebody. The locomotive has a similar pin located in aninsulated block carrying the drawbar. Two holesmay be provided as shown, one for the normalconnection and a longer one to allow for use ontight radius curves, although if building one's ownlocomotive for track curves of known radius, thismay be unnecessary. The light wire spring fits a

groove in the pin and prevents the drawbar slidingoff the pin, although on good track the tendershould not bounce around and this should not benecessary. The act of coupling automatically makesthe electrical connection. If combined withinsulated chassis, the system provides a simplepick-up system that does not require checking andadjustment.

Photo 8.8 The system can be adapted for tank engines having a bogie. The chassis is from a Southern M7 0-4-4T. An insulated block has been fitted into the frame at the rear and it carries the bogie pivot post. This makescontact with the bogie frame. The bogie wheels are insulated on the opposite hand to the drivers and providethe return path. The bogie sideplay is slightly restricted to prevent the leading bogie wheels from coming intocontact with the steps below the cab and creating a short circuit when traversing curves down to 4ft radius.The wheels are cast iron (Photo K. Sheale)