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TYPES OF TRACKLAYERS.

3rd April 1919, Page 8
3rd April 1919
Page 8
Page 9
Page 8, 3rd April 1919 — TYPES OF TRACKLAYERS.
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An Interesting Comparison of the Various Methods of Pivoting the Truck Frames and Mounting the Driving Sprockets.

By M.I.Auto-E.

IN VIEW OF developments in the Tank-type or chain-track tractor, a comparison of the frames which carry the load-supporting rollers is both interesting and instructive. These tractors are fitted

with side frames each with a front idler for the chain track and a series of flanged rollers travelling upon the lower run of the track. Such side frames, with but few exceptions, are provided with pivots, which allow vertical movement of the said frames relatively to the main frame of the Vehicle. In the diagrams, the pivots for the side frames are es reprented in solid black, so that the side frames are free to move, relatively to the main frame of the tractor, about such black disc. In each of the six diagrams the letter c indicates the chassis or main frame, d the chain-track driving sprocket, f the truck frame or side traction frame, and p the pivot. In the Holt machine (Figs. 1 and 2) the load-Supporting rollers are mounted on truck frames, which are pivoted to the main frame approximately centrally of such truck frames. The Holt chain-track driving sprocket is journalled on the main frame and is entirely independent of the trucks. In Fig. 3 the C. L. Best Co.'s tractor is shown. The side "traction. frame" is fulcrumed upon the rear axle for the driving sprocket. Fig. 4 shows the Cleveland tractor, also with the side traction frames

pivoted aboilt the rear axle, but, in this example, the traction frames are continued to the front of the main frame. In Fig. 5, both the front idler and the rear driver are journalled on the side traction frame ; this represents the Bullock or "Creeping Grip" construction. The driving sprocket is operatively engaged by a lantern wheel. Fig. 6 shows another form of drive to a toothed driving sprocket journalled in. the side traction 'frame. Upon the pivot is a sleeve, having two toothed wheels, one of which is chain driven from the main frame and the other of which drives a chain wheel coaxial with the chain track driving sprocket. These various• methods of connecting the side frames to the main frame have all the same principal object ; namely, to provide for vertical movements of the supporting rollers to enable the track to pass more smoothly over obstacles ; they therefore corre

spend to the springing of the axles of a motorcar, but with this difference that, in the latter case, each wheel is lifted individually as it strikes the obstacle ; whereas, in the chain-track vehicle, all the rollers on the side frame must pivot together with that frame. With central pivoting (see diagrams 1, 5 and 6), when the forward end of the track rises upon an obstacle, the pivot and, consequently, the main frame at that point will be raised approximately one-half of the height of the obstacle, and as the tractor advances further raised to the entire height of the obstacle as the pivot itself passes over the latter, to be thereafter gradually lowered to the half height as the last roller passes over the obstacle and further

lowered to its normal position as the sloping run of the chain passes up to the driving sprocket. Thus the rise and fall take place under similar conditions. In the case of the multiple-articulated trucks, diagram 2, since the truck can yield intermediately, the height of rise and fall is reduced, the chain-track being subject to a contrary flexure which allows it to maintain. contact with the ground on each side of a moderate obstacle. (See Fig. 51 of L. A. Legros' paper on "Traction on Bad Roads"; I.Mech.E., January, 1918). With end pivoting of the frame about the sprocket axle (see diagrams 3 and 4), the initial lift imparted to the frame is reduced by the increased deflection of the spring due to the movement of the centre of suppert, that is to say, the distance by which the leading roller overhangs the centre roller. The advance of the vehicle reduces this overhang until the centre roller passes over the obstacle, at which moment the spring carries its normal load and the main frame has been raised by the full height of the obstacle. Further advance of the vehicle transfers a portion of the load on to the frame-pivot, thereby relieving the spring, which, ansequently, expands sufficiently to bring the leading roller again into action at about the moment when the last roller is passing over the obstacle. The main frame of the vehicle has then to be lowered through the entire height of the obstacle during the upward travel of the sloping run of the chain. Apparently this sudden fall of the vehicle will produce a much greater jolt than in the former system because the height of the fall is double. On the

other nand, the latter system affords, a more gradual rise to offset the drawback of the sudden fall.

So many designers have been studying the tracklayer during the war that great developments in this type of tractor are expected to be forthcoming.

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