DESIGN AND FABRICATION OF ALLWELDED BRIDGE CRANES

By W. G. WEHR, General SuperintendentThe Cleveland Crane and Engineering Company, Wickliffe, Ohio

AS manufacturing processes have become more com-plex and straight line production the rule ratherthan the exception, the electric traveling bridge

crane has become a production tool. Equipment of thisnature is subjected to continuous service, making it essen-tial to reduce maintenance to a minimum, increaseefficiency, eliminate costly production delays, and inmany cases to use speeds which only a few years agowere considered impractical. While safety is alwaysimportant, it is doubly so in the case of a crane sinceit is always in operation over workmen's heads. Thedevelopment of roller bearings and application of weld-ing to crane construction has enabled these demands tobe met.

The conventional crane design which has always beenfollowed, for many years consisted of riveted box girders,heavy cast end trucks and cast trolley side frames andheavy structural members which were either bolted orriveted together. The modern welded design providesa welded box girder with end trucks and trolley framescompletely welded of structural shapes into two separatemonosteel units.

In order to illustrate the evolution from the old rivetedmethod of construction to the modern method of weld-

Figure 1

1c»

ing, we will take up separately the principal units whichgo to make up a complete crane,—namely, (A) thebridge girders; (B) the end trucks and (C) the trolleyframes.

The bridge girders are usually the largest and heaviestpart of a crane, and always the longest unsupported mem-bers. In the design of a girder, resistance to deflection isof course essential. But in modern high speed cranes,the factor of rigidity has become so important that thewelded girder will eventually completely replace theolder type. A box girder consists of a top and bottomcover plate and two web plates with diaphragms locatedat frequent intervals along the girder. In the riveteddesign, the cover plates are attached to the web platesby chord angles and the diaphragms attached to the webplates in the same manner. In the welded design, theweb plates are attached directly to the cover plates byfillet welds, the chord angles being eliminated. It is alsopossible to attach the diaphragm to the top cover plate aswell as the web plates, a feature of design impossiblewith riveted construction.

Figure 1 illustrates clearly the difference betweenthe two types of construction. Every time a rivet is usedit means the strength of the parts to be joined is reducedin an amount equivalent to the size of the rivet hole.Fabricating by welding employs the full strength of thecontiguous parts, so from a design standpoint, the grosssection is the net section. It follows therefore that ade-quate strength can be obtained with less material, result-ing in a reduction in gross weight which is vitallyimportant, and seems to be one answer to the question sooften heard, "Why drag around a lot of dead weightfor 25 years?" As bridge speeds have increased, thegirders are required to absorb the greater impacts, thehigher acceleration forces and stresses, resulting frombringing the crane to a quick stop. All these forces tendto loosen rivets, resulting eventually in mis-alignmentand high maintenance costs.

It can be seen from figure 1 that in the welded girderthe web plates can be placed further apart with a givenwidth cover plate than is possible with the riveted girder,and since the chord angles have been eliminated, thedistance from the neutral axis to the center of gravity ofthe cover plate is increased by an amount equivalent to"D" . It is obvious that this produces a stronger andmore rigid section by being able to utilize the materialto give the greatest stiffness and strength with the mini-mum deadweight and greatest mechanical strength per

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Figure 2

pound of metal. The trolley wheel load is imposed onthe girder through the rail which is mounted on the topcover plate. Since the diaphragms are welded to theunderside of the top cover plate, this load is carrieddirectly to the girder section by means of the diaphragms.Since it is impossible to rivet the diaphragms to the coverplate, this load may not be properly distributed, whichresults in subjecting the cover plate to severe additionalstress caused by deflection. P'or the riveted girder as awhole, this becomes a weaving action that grows morepronounced with the service life of the crane, and asthe weaving increases the girder weakens. This saggingand weaving cannot occur in a welded girder, becauseall points are rigidly connected, with no possibility of thecover plate slipping at the diaphragms. The welded con-struction produces a cell-type girder with the top platerigidly connected to all four sides of each cell, (i. e. toboth web plates and to two reinforcing diaphragms).This construction is of extreme importance, because allconnecting edges of each cell are rigidly joined, excepttwo—i. e. between the diaphragms and the bottom plate.Each cell, therefore, will resist stresses and shocks inevery direction,—horizontally, vertically, or diagonally.For this reason, a welded girder, with its many compo-nent cells, will withstand strains and skewing and twist-ing action to a far greater extent than is possible with ariveted girder, and what is more important, the actualdeflection of a welded girder under load can be accuratelycomputed, this has been proven by tests. The amountof slippage in a riveted connection depends on so manyfactors that the deflection of a riveted girder cannotbe computed with any degree of accuracy.

Figure 2 shows a 120' span welded girder in theprocess of fabrication. All welded girders are airtightand watertight, this gives a permanent resistance tointerior corrosion, which is another feature impossible

Figure

3

Figure 4

to obtain with riveted construction.The two bridge girders are supported and connected

together at the ends by heavy end trucks, in which aremounted the crane wheels. Next in importance to a rigidgirder for maintaining alignment is the connection ofthe girders to the end truck. Consider for a momentthe forces involved when a long span, high speed crane,with its trolley located at one extreme end and fullyloaded, is either accelerated to full speed or brought toa quick stop. The unloaded end naturally tends to comeup to speed faster or come to a stop quicker. This skew-ing action, unless properly resisted, would quickly resultin mis-alignment, causing excessive maintenance of allmachinery. Welded design offers the best solutionto this problem. The ends of the girder are notchedduring fabrication,—see Figure 3. During assemblythe end truck and girder are lined-up before the endtie connection is made. The final result is clearly shownin Figure 4. In the design of any large welded struc-ture, it is important to make adequate provision for theadjustment which is essential in final assembly to takecare of any distortion and shortening due to welding.A good illustration of this is provided in the end tieconnection. A wide gusset plate is first bolted to theend truck. This gusset plate extends through the notchof the girder. Regardless of the amount of gap betweenthe web plates of the girder and the gusset plate, a posi-tive and permanent connection is made by welding thewebs and gusset plate together by means of a long hori-zontal shear bar which can be clearly seen in Figure 4.Figure 3 also shows how this type of welded connectionis applied in attaching the end of the girder to the sideof the end truck. After welding, the gusset plate be-

Figure 5

R E - E N P 0 R C I N G R I B S / , - H E A V Y CONTINUOUS/ GUSSET PLATE

WELDED ONE PIECE. END TRUCKLIBERALLY EQUIPPED WITH DIAPHRAMS

SEARING HOUSINGSEAT CARFULLV

MACHINED

MAY, 1939 Page 7

LARGE HEAVYROLLER BEARINGS

Figure 6

comes a permanent part of the girder. The assembly inthe field is simple, and permanent alignment of thegirders is assured.

The end truck, which is shown in Figure 5, is con-structed in much the same way as the girders. The boxsection is made up entirely of rolled steel plates andshapes, including the proper number of interior stiffen-ing diaphragms. The wide surfaces of the trucks aredesigned to accommodate the gusset plates of the notchedgirders. The older method of end truck constructionwas to use a one piece casting. The problems of castinga long end truck, due principally to the abrupt changesin section, required the use of a far heavier section thannecessary. The principal source of weakness in a cast endtruck occurs at the bearing seats in the notched end. Inthe welded truck, the bearing seats are made from rolledsteel of known strength which can easily be strengthenedby reinforcing ribs located so as to absorb the shocks andloads from all directions.

Welding enables the designer to ignore the limitationsof the foundry, an advantage so great that it is hardlynecessary to elaborate on it.

The hoisting machinery of any crane should bemounted on as rigid a base as can be constructed withinthe limits of clearance and mobility.

Welded trolley frames are rolled steel sections weldedinto one complete unit. (See Figure 6.) During fabri-cation, parts of the trolley frame are welded separately.Final assembly is then obtained with a minimum amountof welding, keeping the locked up stresses to a minimum,because welding distortion and shrinkage has alreadytaken place in the component parts. Here again the de-

Figure 7

signer can easily provide for this factor as was possiblein the connection of the girders to the end trucks.

Where machinery is to be mounted, (See Figure 7,)structural members are thickened by suitable pads, so thatthrough bolts will have ample bearing area.

After the welding is complete, the trolley frames arecarefully machined as one piece units, so that accuratecenters of gears and shafts will be permanently maintain-ed. No flexible couplings are used. Where mis-align-ment does not, and cannot exist, it is unnecessary toprovide for it.

The desirability of applying roller bearings to heavymachinery introduced a problem that demanded a newtype of machine assembly. The precision bearing madeup of tolerances in the magnitude of ten thousandths ofan inch, required an overall assembly of machinerywithin the same tolerances, otherwise damaged bearingsand races would cause the immediate failure of thewhole machine. Prior to that time, slight mis-alignmentof parts had frequently gone unnoticed because it had no

Figure 8

serious consequence. The bronze bearing could wearitself in such a manner as to accommodate minor dis-tortions. If gears chattered and journals and bushingsshowed excessive wear, and had to be replaced, it wasconsidered a necessary evil. The welded structure isthe obvious solution to the problem.

Note the many stiffeners and braces welded and be-coming part of the one piece steel structure, and since itwould be necessary for the foundry to produce a onepiece casting to be comparable, it is not hard to imaginethe many difficulties. Attention is also called to thelocation of the double acting spring bumper welded oneach trolley side frame. The shock of striking the endstops is transmitted through the side frame rather thanthrough the wheels and bearings.

In welding, generally we are able to use worked steelwith known analysis and consistent strength. The factthat shapes and plates are rolled, produces a quality ofmaterial which can never be obtained by a casting. Dif-ferent types of rolled steel can be used where specialqualities of material are required. A notable example of

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Figure 9

this is a welded gear. See Figure 8. The rim of the gearis high carbon steel to provide the resistance to wear andlong life of the teeth so desirable. The web and thehub can be of lower carbon content, providing the neces-sary shock absorbing qualities also desired. It must bekept in mind that it is necessary to anneal high carbonmaterial that has been welded. Many times a cast steelgear blank will be almost completely finished before in-herent flaws in the casting show up. While with rolledsteel material the resulting welded gear is homogeneousthroughout.

The rapid improvement in welding electrodes, provid-ing a weld metal of 20% - 25% elongation, consistently,enables the designer to take full advantage of his ma-terials. Proper welding equipment for the job is essential,and in many cases the successful fabrication of a weldedstructure requires special equipment. A good example isshown in Figure 9, which shows two automatic weldingheads mounted on a motor driven gantry crane,—one-located on each side of the girder, making the two filletwelds simultaneously, which connect the bottom coverplate to the two web plates of a crane girder. The speedof crane can be varied to suit the work.

A simple illustration of the advantage of welding indesign is shown in Figure 10, showing a welded bridgeline shaft bracket. The bracket consists of two plates anda channel section. The two plates are welded to the webplate of the girder. The bearing housing is bolted tothe channel for easy removal. During assembling theshaft and bearing are properly lined up, after which thechannel is securely welded to the two plates. The partsalways remain in place, eliminating the possibility of mis-alignment of the line shaft. Maintenance of line-shaftbearings therefore becomes little more than lubrication.

In the case of the bridge crane, which has been describ-ed in this article, attention is called to the extensive andalmost exclusive vise of fillet welds. Structural shapesand plates come from the mill true to shape within es-tablished mill tolerances. The use of fillet welds enablesthe welding designer to ignore these irregularities to alarge extent.

The freedom from limitations on the welding de-signer is so great that, in effect, the modern weldery is inthe position of being able to fabricate complicated me-chanical structures, by means of acetylene torch, shearand welders almost as easily as if the raw material werewood, rather than rolled steel.

Figure10

MAY, 1939