METHOD of selecting sbaetiasfraicntogrsy for the transmissions and final drives of pneumatic-tired tractors that depends on a knowledge of average operating conditions is reported by John Borland.
The well-known method of rating bearings on a fatigue basis is, according to tests carried out by Mr. Borland, a reasonably accurate way of predicting bearing life when loading conditions are definitely established. However, the tables that have been compiled by the bearing manufacturers are satisfactory for determining bearing life only of bearings subjected to a constant load at a constant speed.
Since tractor transmissions are subjected to as many different loading conditions as there are speed changes provided in the tractor, tractor transmission bearings cannot be selected directly from these tables. They must be used in conjunction with the formula for determining a factor called weighted life. This formula takes into consideration the life factors of the bearing at the different loads and the per cent of time for which each factor applies.
To calculate bearing loads by the author's method, it is necessary to know the weight of the proposed tractor in operating condition, including the operator and water in the rear tires; the speed in each gear; and the tire size. The tangential force on the rear wheels in each speed can be obtained in terms of per cent of total tractor weight from a curve derived by Mr. Borland from data he collected. This tangential force on the rear wheels multiplied by the rolling radius will give the torque on the rear axle. From this torque, the bearing loads can be calculated.
Mr. Borland also discusses other factors that have an effect on bearing life in addition to the imposed loads.