U-Joint problems, as a rule, are of the progressive nature. They generally accelerate rapidly and result in ruined components.
Some recognizable signs of u-joint deterioration are:
- U-joint looseness
- U-joint discoloration due to excessive heat buildup
- Inability to purge all four trunnion seals
- An audible noise or squeal from the driveline
The most common reasons for u-joint wear are lack of lubrication, inadequate lube compound quality, inadequate initial lubrication or failure to lubricate properly and often enough.
On Quick Disconnect applications, excessive bearing rotation could cause premature wear of components involved. The causes of rotation are:
- Use of low quality components
- Improper torque on retaining strap bolts
- Failure to firmly seat both bearing assemblies in the end yoke saddles before the strap bolts are tightened
- Dirty bearing saddles.
To avoid lubrication-related problems:
Lube all fittings including those that are often over looked, out-of-sight, dirt covered or difficult to reach
Know how some fittings appear different from regular chassis lube fittings and require a special needle nose attachment for the grease gun.
Don't overlook slip yoke lubrication
Use correct lube technique. New lube must flow from all four bearing seals
Use correct lubricant. It should be a recommended type, such as NLGI Grade 1 or 2 with EP additives and high temperature resistance
New u-joints must be lubricated when assembled into the driveshaft yokes
Observe recommended lubrication cycle (see chart below)
Generally, a lubrication problem is one of two types -- brinelling or end galling. The grooves made by the needle roller bearings on the trunnion of the cross are known as brinelling. Brinelling can also be caused by too much torque for the capacity of the u-joint used. End galling is a displacement of metal at the end of the trunnion and can also be related to angularity problems. Both of these problems can be caused by lack of lubrication.
Problems which are not a result of lubrication are associated with the installation, angles, and speed of the driveshaft. Fractured parts caused by torque, fatigue and bending are associated with overload, excessively high u-joint angles and driveshaft lengths exceeding critical speed limitations.
Vibration is a driveshaft problem that can either transverse or torsional.
Transverse vibration is the result of unbalance acting on the supporting shafts as the driveshaft rotates. When a part having an out-of-balance, or heavy side, is rotated an unbalanced force is created that increases with the square of the speed. The faster the shaft turns, the greater the unbalance force acting on the shaft.
The force produced by this out of balance condition tends to bend the supporting members. As the supporting members have a natural frequency of vibration similar to a swinging pendulum, a violent vibration may exist at certain periods when the speed of rotation and the natural frequency of supports coincide.
Each end of the shaft must be balanced individually as each support is responsive to an out of balance condition in portion of the shaft it supports. Out-of-balance affects operating conditions only when rotating.
Transverse vibration caused by a driveshaft out-of balance will usually emit sound waves that you can hear and mechanical shaking that you can feel. The force from out-of-balance increases with speed, not torque load. The driveshaft speed is determined by vehicle speed and the vibration is demonstrated best by road testing the vehicle to operating speed, disengaging engine, and checking vibration while coasting with engine noise eliminated.
Torsional vibration, although similar in effect to transverse vibration, is an entirely different motion. The transverse vibration is a bending movement where as torsional vibration is a twisting motion.
The energy to produce to torsional vibration can occur from the power impulses of the engine or from improper u-joint angles. This type of vibration is difficult to identify in road testing but certain characteristics do exist. It causes a noticeable sound disturbance and can occasionally transmit mechanical shaking.
Torsional vibrations can exist at one or more periods any place in the operating range and tend to be more severe at lower speeds. Changes in torque load (part-to-full throttle) usually affect the vibration. The non uniform velocity obtained when a u-joint operates at an angle produces torsional vibration. In a driveline having two or more joints in series, it is desirable to have the individual joint angles arranged such that the net result minimizes non uniform velocity characteristics over the system.
It is practically impossible to maintain the desired joint angles throughout the operating range. Therefore, it is necessary to determine some maximum limit of torsional excitation which can be considered as generally acceptable.
The amount of torsional excitation which can be accepted without causing excessive disturbance depends upon operating speed and characteristics of supporting structures and other units in the driveline and drivetrain system.
Other vibrational problems in a driveshaft could be caused by worn or damaged u-joints. These joints must be constantly maintained according to manufacture's lubrication specifications.
Re lubrication cycles vary depending on the service requirements and operating conditions of the vehicle. A recommended re lube cycle for various types of service are shown in the following table:
Type of Service
Other Driveline related issues: