Imbalance in the propeller shaft causes vibration or noise, like in tires. The propeller shaft turns faster than tires by the gear ratio of the differential gear. Therefore, the frequency of the vibration or noise increases, and is often accompanied by a booming noise.
Main causes of Imbalance
To adjust the imbalance in the propeller shaft, a vibration meter or other instrument is necessary. So, performing the adjustment is difficult in ordinary workshops. Therefore, at first, inspect and repair the portions which may cause imbalance.
(1)Run-out of the propeller shaft
Run-out is the same thing as the offset of the rotational center, which causes the unbalanced rotation.
(2)Run-out of the differential companion flange and propeller shaft center flange
If there is run-out in each flange, the rotational center of the propeller shaft is altered.
(3)Offset centering of propeller shaft and differential companion flange
If the propeller shaft and differential companion flange do not fit together properly, centering is altered and the balance is lost.
Run-out causes first order vibration in all cases.
The propeller shaft joint has some angle due to the structure of a vehicle, so when a propeller shaft rotates, a vibrating force is generated. Vibration frequency is the second order component of the propeller shaft. This vibrating force is generated due to the torque fluctuation in the output shaft against the input shaft, and secondary couple (a type of force) applied to each joint.
In the case of a Hooke’s joints, the joint angle generates torque fluctuation in the input shaft. This torque fluctuation is generated twice per one propeller shaft revolution.
Torque fluctuation generated in the No.1 joint can be canceled at the No.2 joint by equalizing the two joint angles (a,b) and combining the phase of the hook as shown in the illustration. In this case, the amplitude of the torque fluctuation generated in the No.1 and No.2 joints should be the same. Therefore, the joint angle of the No.1 and No.2 joints should be the same and the output/input shaft should be parallel.
Regarding the three-joint propeller shaft, torque fluctuation cannot be completely canceled. So the joint angle has to be as small as possible.