What Does A U-Joint Do?

A simple single piece driveshaft has two u-joints. The u-joints serve several different purposes:

• One serves as the link between the driveshaft and the transmission
• The other is the link between the driveshaft and the differential
• They help deliver the power from the engine to the differential
• For trucks and older cars, they allow the driveshaft to flex as needed so the rear axle can move up and down.

In some cases newer trucks often have two piece driveshafts. These usually have three u-joints, with one at each end, and one in the middle.


Signs Of A Bad U-Joint
If you notice one or more of the following signs, your u-joint probably needs replacing;

• Squeaking or clunking noise when shifting from forward to reverse or vice versa
• Vibrations felt through the whole vehicle while driving at high speeds
• Fluid leak from the rear of the transmission or transfer case
• Fluid leak from the front of the differential

What happens when u-joints fail?
1. Leaking Transmission
The vibrations caused by a bad u-joint cause the driveshaft’s stub shaft to rotate out of round. This causes the transmission’s rear seal to fail.  Fluid will leak out of the transmission. If the transmission loses some fluid, it can’t shift right. It would only do hard shifts. If the fluid gets low enough, the transmission will eventually fail. The same thing can happen to a transfer case.
2. Leaking Differential
A bad u-joint at the rear of the driveshaft can ruin the pinion seal, in the same way the transmission seal can be ruined.
3. Damaged Bearings
If they go on long enough, the vibrations from a bad u-joint can damage any of these bearings:

• The transmission output bearing (if there is no transfer case)
• The transfer case output bearing
• The pinion bearing

4. Broken Driveshaft
A bad u-joint will eventually fall apart if it’s not replaced. When they fall apart, which happens when you’re moving, the driveshaft will become disconnected from the transmission or differential. 
5. Serious Collision

All of the above highlight that ignoring a failing u-joint can lead to more expensive problems down the road! 

What are the most common causes of u-joint failure?

1. Lack Of Grease
This applies to both greasable and maintenance free universal joints.
Lack Of Grease Due To Vibration
U-joints don’t like vibration. A driveshaft that is out of balance can cause enough vibration that the life of the u-joints will be shortened. The vibration forces out the grease and causes the metal components to grind against each other. Eventually this leads to failure.
Lack Of Grease Due To Excessive Heat
U-joints don’t perform well under extremely high temperatures. The grease inside them supplies lubrication to the metal components. If too much heat builds up, it can cook the grease so that it fails to protect the metal. This can happen if a u-joint is located close to a source of heat, such as a muffler.

2. Wear And Tear
Many universal joints will wear out over time. This happens especially if you put a lot of strain on your u-joints on a regular basis. 
When a u-joint is subjected to high levels of strain for long periods of time, the metal components can become thin. They can also develop cracks. This reduces their ability to transfer torque properly, which is what causes u-joint failure.
Wear And Tear Due To High Loads
U-joints are designed to handle certain amounts of load. If you put more weight on your vehicle than it’s capable of carrying, your u-joints will have to work harder. This can lead to accelerated wear, which could result in failure after time.
Wear And Tear Due To Improper Installation
Proper installation is important for u-joint longevity. The caps and cups must be properly aligned to prevent damage. Also, make sure that your bolts are torqued down to their proper specifications.

3. Steeper Than Ideal Operating Angles
U-joints are designed to handle a certain amount of operating angle. If you exceed the operating angles that these u-joints were designed for, they’ll struggle to handle their load. That will cause them to fail prematurely.
This is a common issue with lifted vehicles. The driveshaft operating angles become steeper if:

• The vehicle is lifted
• The pinion angles are incorrectly set

How to look after U joints

Preventative maintenance is the key to maximizing the performance and durability of any universal joint. 

1. Lubrication
Greaseable U-Joints
Greasing the grease-able style of u-joints regularly is the best way to prevent failure. It ensures that they stay lubricated and functional. Yet, it’s possible for a u-joint to fail even if it’s well greased. U-joints are exposed to the elements, which means moisture can make its way inside them. When that happens, the u-joint can seize and fail.
Maintenance Free U-Joints
Some maintenance free u-joints have a teflon coating inside in addition to grease. This helps to prevent corrosion. This should be enough to keep your joints well lubricated for their entire service life. But sometimes they still fail. If they do, it may be because of improper installation. Inspect u-joints during installation to make sure that the u-joint caps installed correctly. They also may fail due to excessive vibration.

2. Inspect the yoke that holds your Universal Joint for cracks and stress marks.
3. Check your seals for signs of leakage and replace if seals are leaking.
4. Periodically check for drive shaft play, an indication of Universal Joint or seal failure.
5. Inspect drive shaft for cracks, stress marks, dents and bends and replace if necessary.