Usually found on the rear axles of SUVs and RWD cars as well as light trucks, propshaft universal joints are responsible for prevention of damage to the shaft. Without them, the up and down movement of the rear axle in relation to the gearbox would result to snapping or bending of the shaft. While the common location of the universal joints is at each end of the propshaft , there are sometimes universal couplings at the center.
Understand the Universal Joint
Most modern universal joints consist of a steel cross with lubricated needle bearings in four arms held in place by caps and circlips. The lugs on each yoked end of the two shafts engage with the four arms, pivoting on the roller bearings. However, the bearings will finally cause wearing into the surface of the cross and the rotation of the propeller shaft will move from the center.
How to Detect Problems
The joints in most modern cars are permanently sealed and, therefore, cannot be opened. Older cars allow the dismantling of the joints from the propshaft for checking. They can then be replaced if worn, or just lubricated if in good condition. Your car may, however, have grease nipples on the propshaft joints, in which case you should check for the greasing interval from the manufacturer's service.
One of the most obvious signs of a worn joint is vibration on the propshaft which worsens with the increase of the wear. You may also pick out a knock which occurs regularly when you accelerate or decelerate. An additional sign of something have given inside the joint bearings is gathering of rusty dust around the cross. If there is no dust, however, check for wear by firmly holding one side of the joint and trying to turn the other side. Any movement in the joint is evidence of a problem.
You can make another check by putting a screwdriver between the cross and the yoke and trying to lever it. If there is any play, there is a problem. One disadvantage with most of the modern cars is that it is impossible to separate universal joints from the propshaft. When there is wearing of the joints, the propshaft must be replaced.
Some of the universal joint problems may originate from reassembly. For instance, there may incorrect phasing of some truck propeller shafts. Since it results to harmonic vibration occurring in different conditions of load and speed, the problem is not easy to detect. In order to prevent it, consider indexing the propeller shaft to its mating flanges and other individual parts before the replacement of the joints.
Damage to the propeller shaft can be easily avoided by using a universal joint press, which is usually held by hand. Before removal, you should also soak the propshaft universal joints in penetrating oil. In addition, the needle bearings should be packed with chassis grease while the propeller shaft loops should always be lubricated before assembling. If there is a stiffness of the universal joints after installation, tap the propeller shaft loops with a hammer to align the four arms containing the needle bearings.