U-joints are often used where a flexible, rotating means of transmitting power is needed. They are inexpensive and durable as long as a few guidelines are followed.
Virtually all U-joint failure is the result of three primary causes
Lack of lubrication
Operating beyond the maximum design angle
Contamination of the bearings
Ironically, lack of lubrication is more likely in a U-joint with a grease fitting. Sealed joints are adequately packed with grease and sealed. This makes them preferable in most situations.
To make a U-joint that can be greased, seals must allow grease to flow out when new grease is added. If this type joint is not properly lubricated on a regular basis, it can run out of lubricant. Another problem, it is easier for contaminants to enter the joint through the zerk fitting and seals. This is why almost all OEM U-joints are now sealed.
As the angle of U-joint operation increases so does the load on the joint. For this reason design engineers limit the angle at which U-joints must work. Modifying vehicle suspensions by raising or lowering can increase driveshaft angle and cause repeated U-joint failures.
On stock vehicles, when a larger angles must be used, engineers implement two piece driveshafts or switch to constant velocity (CV) joints. With a two piece driveshaft the angle is split among three U-joints, decreasing the individual angle on any one.
U-joint angle is also effected by a bent chassis or even broken, bent or worn motor mounts. Repeat failure of U-joints is very often a symptom of such issues. A frame and alignment specialty shop can measure driveshaft angles and determine if the are excessive.
Contamination of U-joints can occur when the vehicle is driven through high water or deep mud. Disassembling and cleaning the joint is not practical. Once water or mud gets pass the U-joint seal the joint will fail.
When a U-joint fails, it may show any of several symptoms. A clunk, when shifting into reverse or drive is one such symptom. Breakdown of lubrication results in wear to the rollers and the U-joint cross. This wear produces slack in the joint and a clunk when the driveshaft changes direction.
At this stage the U-joint must be replaced promptly. Failure to replace the U-joint can cause damage to the driveshaft, transmission, differential and transfer case, if present. A broken U-joint will also disable the vehicle and almost always ruins the driveshaft.
U-joints can also fail by the rollers seizing. Seized rollers may not show up as slack in the driveshaft. A symptom of seized rollers is vibration, especially on acceleration. Another symptom may be a squeak noise, when the vehicle is driven slowly. Again prompt replacement can prevent damage to other components.
Replacing U-joints must be very carefully accomplished. Many driveshafts are constructed of aluminum. A very tiny dent in a driveshaft or any distortion of the mounting ears can throw the shaft out of balance and ruin it. Because of the weight and extremely high speed at which they operate driveshaft balance is critical.
Beating universal joints in and/or out with a hammer will almost always damage the driveshaft. Quality shops use special pullers to remove u-joints and a support fixture and press to reinstall them. The shaft should also be marked before disassembly so it can be properly indexed on re-assembly.
Driveshafts often operate at three to four times the speed of the vehicle. For this reason they are very precisely balanced. Improperly installing a U-joint can drastically effect this balance. Aftermarket U-joints that do not fit properly can also severely effect balance.
When a driveshaft is out of balance it will cause vibration. This vibration can damage the rear differential pinion bearings and the transmission. Driveshafts can sometimes be re-balanced by a driveline specialty shop. Other times they must be replaced, at considerable expense.
With careful handling and prompt replacement of worn U-joints the driveshaft will normally last the life of the vehicle. As with most things, recognizing the symptoms and taking quick action will prevent considerable expense.
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