THE "PT" TRACTOR WHEEL.
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A Combination of Wheel and Chain Tread.
By HENRY STURMEY.
THE SHORTCOMINGS of the wheel for work on soft surfaces where heavy traction is required are well known, and the chain-tread system has been largely adopted as an alternative. But the chain-tread system gives the idea of complication to the uninitiated, and, moreover, entails the use of many more parts, so that it is not surprising that inventive effort has been 'devoted the world over to arrive at a system which, whilst retaining the principle of the wheel, shall yet meet the objections to it. The latest device of this kind has been introduced in America, where it is known as the "P.T." wheel, which aims to combine the features of the chain tread with those of the rolling wheel. The "P.T." wheel may broadly be described as a wheel encircled by a chain tread of peculiar construction, the "shoes" of which are automatically brought into a horizontal position beneath it by Contact with the ground. At all times the weight of the wheel and of the load supported by it is spread over a ground area equal to the combined surface of two shoes, which, supposing a shoe to measure 15 ins. by 10 ins., would give a supporting area of 300 square ins, per wheel, equal to supporting a 3,000-lb. tractor on the two wheels, with a ground pressure of only 5 lb. per square inch, and at the same time, by reason of the construction of the shoes, 'bringing four grip bars projecting on each side into ground contact.
This "P.T." (Positive Traction) wheel is constructed in two halves of pressed sheet steel, having the peripheries separated by a series of stout rollers running on short pins. The treads are not hinged together as in a chain tread, but are separate and independent of eacheether. The inner, or upper surfaces, or (shall we say ?) the backs of the tread?blocks, which are single castings, are formed like the teeth of a chain wheel and are furnished at their apices with cross studs, or pins, which serve to attach them to a series of strong spiral springs, the other ends of which at tach to fitments arranged in series around the centre of the hub of the hollow wheel. Thus, normally, each block is-held by its particular spring, with each face of its tooth-like back pressed in contact with one of the aforementioned rollers.
No movement takes place as the wheel revolves until the shoes in tern come into ground contact, when the weight upon that end which touches the ground first, brings the whole shoe into horizontal contact with the ground, 'and, when the wheel has revolved far enough for the shoe to be relieved of weight, the action of the spring by which it is cen.
trally held draws it into its normal inoperative position on the wheel again. The effect of this -action is that there are always two shoes in horizontal contact with the ground beneath the wheel and supporting the weight, and, as one is relieved and brought out of action, another is brought into action to take its place. The power put into the wheel by the engine is delivered through the rollers, which are, for the time being, in engagement with the teeth" of the two shoes in ground contact and the drive is effected by pressure against them, the shoes of which they form a part being held down by the weight upon them and anchored to the ground by the crossbars on their contact faces. The action is equivalent to that which takes place with the usual continuous chain tread. The rollers, acting as cogs, engage with the tooth-like portions of the pads or shoes, whilst, as with the chain tread, the smooth periphery of the wheel rolls smoothly over the flat tracks provided for the purpose on the upper faces of the pads on each side of the teeth. In the whole wheel there are only rolling movements, except for the slight sliding that occurs in changing from forward to reverse to the extent of about in. clearance allowed between the rollers and the re-action surfaces of the pads.
Another feature of the "P.T." wheel is that its tractive capacity is not a function of the diameter, but, rather, a function of the pad size; • so that it is not necessary to employ such „large diameters of wheels as when plain wheels are used, because, providing the surface of the pads is the same, the traction will be the same, whether they be fitted, say, to a 36-in. or a 50-in. wheel.
"PT." wheels are designed and supplied for use with either hub or chain-driven wheels, of 36-in., 40-in., 48-in., 54-in. and 60-in, diameters, and are made by Wm. Wharton, Jr., and Co., of Easton, Pa. So far as we are aware they are not yet on the British market.