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Art Tractors too Powerful ? Drawbar Attachment. The Editor, THE COMMERCIAL MOTOR.

9th October 1913, Page 15
9th October 1913
Page 15
Page 15, 9th October 1913 — Art Tractors too Powerful ? Drawbar Attachment. The Editor, THE COMMERCIAL MOTOR.
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Which of the following most accurately describes the problem?

11245] Sir,—Further to the interesting article by " Verax " in your issue of 2nd inst. there are many points to be considered—beyond those to which he draws attention— in the design of the most-efficient tractor and trailer to. be used under the Heavy Motor Car Order of 1904. By this Order, the total limit of weight of tractor and trailer is 6-1 tons, and the tractor must not when unladen weigh more than five tons, the unladen definition being without fuel, water or accumulators, and sometimes the body ....whatever this may: mean in the case of tractors not carrying load. Most of the trailers on the road weigh nearly two tons with body, unladen, and ean carry a load of six tons, making a, gross weight of eight tons, i.e., four tons on each axle, according to the Order. The problem, therefore, is. to construct a tractor, weighMg unladen under five tons, to haul at five miles or more per hour, a total load of eight tons, up hills of as much as 1 in 6 gradient.

Large heating surface, contrary to " Verax's statement, is in a steam tractor one of the first considerations. Haulage capacity can be obtained by means which I willedescribe later.

The boiler of a. tractor under the Heavy Motor Car Order is at all times too small; fire-boxes give out prematurely, owing to this. An increased heating surface means longer life to the boiler, irrespective of the horse-power developed under the eircuire stances. Increased cylinder dim.ensions in the same way mean longer life to the engine, since lower revolutions will give the same result as -higher revolutions with smaller cylinder dimensions. Moreover, both these factors give the necessary power for fetching the load out of difficult places.

Turning to questions of road adhesion and haulage capacity, if the road adhesion is less than the tractive effort, the driving wheels of a tractor must necessarily slip, and it is remarkable how little, on one side or the other, in the balance of these two factors, will affect the result. I can remember a threaton wagon with steel tires slipping on a concrete floor ; by placing one sheet of newspaper under one of the wheels, the necessary adhesion was obtained, and only at the first part of contact of the steel-tired driving-wheel did the sheet of paper sliew evidence of any deterioration ; the remainder of the track of the wheel over the single sheet of the newspaper left no evidence of its passage. In the War Office tractor trials of 1909, it was noticed thatwith the Broom and Wade tractor, although of the same gear ratio and driven by a singlecylinder engine giving an uneven torque as compared with a competitor with a four-eylindere:d engine, and although the weight -of the Broom. and Wade tractor was at least oneton lighter than any of the other competitors, less slip took place at its driving wheels on greasy ground when hauling an eight-ton trailer. The secret of this non-slip was due to the angularity of the attachment of the drawn load to the tractor hauling the load. During tests that were made before the trials, a 10-ton trailer was hauled easily up a 1-in-9 gradient with a tractor not weighing more than three tons, without any slip of the driving-wheels.

If, therefore, the tender of a steam tractor were divided, SO as to allow the drawbar to be attached direct to the axle of the tractor at an angle throwing part of the load hauled on to the axle of the tractor, increased adhesion would be obtained, and the higher horse-power utilized. The more-powerful tractor, under the-se ciaeumstane,es, would show the beet results, and there would be no need for " Verax's " query—" Are some tractors too powerful ? "—Yours -faithfully, T. C. AVELING, A ssoc.M.InSt-C.E. 91, Canal Street, Birmingham.


Organisations: War Office
Locations: Birmingham

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