Diagnosing Second-Order Driveline Vibration

Second-Order Driveline Vibration Theory



A faulty universal joint (U-joint) may cause a vibration that occurs twice for each rotation of the propeller shaft. This type of vibration is called a second-order vibration.
Second-order driveline vibrations are independent of runout or balance of a driveline component.
The following description of basic U-joint theory will help you to understand where second-order drivel me vibrations originate and why they occur.
- As the propeller shaft rotates, the U-Joint speed up and slowdown twice for each rotation of the propeller shaft.
- The acceleration and deceleration of the U-Joint is not visible. If there is vibration in the U-Joint, the acceleration and deceleration will be audible and tactile.
- Compare the U-joint in a vehicle to a universal-type socket. When a universal-type socket is used to tighten a bolt, the socket will bind and release as the socket turns toward 90 degrees. The bind and release occurs twice for each revolution of the socket.
- The U-joint in a vehicle works in the same way as the universal-type socket. The bind and release effect is directly proportional to the angle that the U-joint operates: the greater the angle, the greater the effect.
- Because the transmission output speed is constant, the binding and releasing of the U-Joint is better described as an acceleration and deceleration which occurs twice for each revolution of the propeller shaft.
- If the propeller shaft is running slowly, the accelerating and decelerating effect is visible. The acceleration and deceleration may create a vibration due to the fluctuations in force that are generated at high speeds.