Tractor Trendsetters: Allis-Chalmers WD
Written by Larry Gay
Allis-Chalmers introduced the WD tractor in mid-1948 as the replacement for its popular WC model. Along with more power, the WD added three new features, the first being a hand-operated transmission clutch which stopped the tractor’s travel, but permitted the PTO to continue operating. The second new feature was the Traction Booster hydraulic system which provided weight transfer to the rear wheels when the tractor was used with the Single Hitch-Point mounted plow and other implements. However, these were features that were similar to those offered by competitive tractors. The third new feature, Power-Shift rear wheels, was unique and made the Allis-Chalmers WD tractor a trendsetter.
Rear wheel tread had been a challenge since 1924 when International Harvester introduced the first row-crop tractor, the Farmall. The rear wheel tread of the Farmall was 74 inches which enabled it to straddle two rows when cultivating. However, this wide tread resulted in side draft when pulling a 2-bottom plow. By the mid-1930s, International Harvester still had the side draft problem as the F-20 and F-30 had tread widths of 74.5 inches and 77.25 inches, respectively. The Allis-Chalmers WC tractor had a 65-inch wheel tread for plowing and by reversing the rear wheels, a 76-inch wheel tread for cultivating.
The Oliver Hart-Parr 18-27 and the Farmall F-12 tractors used large diameter rear wheels mounted on a straight axle which provided clearance when cultivating and an adjustable wheel tread of 60 to 74 inches for the Oliver and 44 to 78 inches for the Farmall F-12. However, reversing the rear wheels or sliding the wheels to a new position on the straight axle required a jack to raise the rear wheel off the ground, often some blocks, and lots of physical effort to move a large rear wheel usually filled with liquid ballast.
The Power-Shift rear wheels were built with four diagonal rails attached to the rims. Moveable stops were attached to the rails and the center disk of the wheel was clamped to each of the rails. After loosening the clamps and positioning the stops in the desired position, engaging the tractor’s drivetrain provided the power to cause the rim to move relative to the center disk. This provided five rear wheel treads of 56, 61, 66, 71, and 76 inches, providing a narrow setting for plowing and a wider setting for cultivating. Now tractor power could be used instead of manual labor to adjust the tractor’s rear wheel tread.
The Power-Shift rear wheels were standard equipment on the Allis-Chalmers WD tractor and many of the succeeding models of A-C tractors. The later, larger A-C models offered them as optional equipment. Other tractor manufacturers offered Power-Shift rear wheels as optional on selected models.
Larry Gay is the author of four tractor books published by the American Society of Agricultural and Biological Engineers, including Guide to Oliver Tractors and Guide to Ford Tractors. These books may be obtained from ASABE at 800-695-2723 or asabe.org, click publications and then click book catalog.