PTO Shaft GeometryBy
PTO vibration is caused by the universal joints and is especially pronounced when cornering. The output velocity of a single U-joint operating at an angle fluctuates, even though the input velocity is constant. This output velocity fluctuation becomes greater as the angle of the U-joint is increased. To eliminate the velocity fluctuation and hence the PTO vibration, two universal joints are used. The velocity fluctuation caused by the first U-joint is cancelled by the second U-joint, only if both are operated at the same angle and are in phase with each other.
Since most tractor/implement PTO‘s use the two U-joint system, why may there still be PTO vibration? It is because the U-joints are not operating at the same angle. Most tractors with a 540 rpm PTO have a distance of 14 inches from the end of the tractor PTO shaft to the hitch point. Most implements, however, have a distance greater than 14 inches from the hitch point to the end of the implement input shaft. This design is necessary to allow sufficient telescoping action of the PTO shaft, so the shaft does not bottom out on sharp turns and does not allow the two parts of the PTO shaft to separate when the PTO is in line with the drawbar. This geometry however, does not allow the U-joints to operate at equal angles, when cornering.
Many implement manufacturers are supplying a tractor hitch extension with their implements. This extension increases the distance from the end of the tractor PTO shaft to the hitch point. The implement and PTO shaft are designed around this new geometry so the U-joints will operate at equal angles. This geometry is commonly called the EQUAL ANGLE HITCH. Existing equipment can be modified to obtain an equal angle hitch. For example, a 540 rpm PTO driven machine is hitched to a tractor. The distance from the end of the tractor PTO shaft to the hitch point is the standard 14 inches. The distance from the hitch point to the input shaft of the machine is measured and found to be 20 inches, making the overall distance 34 inches (14 and 20). For the PTO to operate at equal angles the hitch point should be 17 inches (34 divided by 2) from both the end of the tractor PTO and the input shaft of the machine.
A 3 inch extension is bolted to the tractor drawbar and the machine hitch is shortened 3 inches. It should be noted the overall distance of 34 inches has not changed and the original PTO shaft can still be used. However, the machine cannot be operated unless the drawbar extension is bolted to the tractor drawbar.
As mentioned earlier it is important to phase the U-joints correctly. Proper phasing of the U-joints allows the velocity fluctuation caused by the first U-joint to be cancelled by the second. Improper phasing compounds the velocity fluctuations and will cause severe vibration. The two parts of many PTO shafts are designed to fit together the correct way only.