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U-Joint Replacement

Helping put a fresh, positive spin on power

If you're lucky enough to have the rear set or all of the wheels providing the motion to get your vehicle going then you may already know about u-joints. The u in u-joint stands for universal. According to the sixth definition of the word universal from Webster himself, this means universal as in being adaptable to many sizes or mechanical uses, or in this case—universal rotation.

Supporting Role

U-joints are usually found at either end of a driveshaft. Their purpose is to transfer engine power and torque from the transmission to the differential and drive wheels as the suspension travels. As the driveshaft spins it has to do so as the suspension moves up and down. The universal joint allows the driveshaft to put a positive spin on engine power and handle the twist of torque regardless of whether the road is smooth, or the going gets rough.

Spin Out

All that spinning under power eventually causes even the toughest u-joint to wear out. At the heart of a u-joint is an x-shaped cross. The cross spins around in the driveshaft yoke thanks to four caps that sit on top of the tips of the cross. Inside each of these caps are tiny needle bearings and high temperature grease. Through the miracle of gravity and centrifugal force, the grease travels from inside the cross as the u-joint spins, and keeps these needle bearings happy and alive as they spin along.

Over time the grease can spin out permanently. Wear can set in, causing the needle bearings to eat into the hardened steel of the cross. Symptoms of worn u-joints include clunking when you put the vehicle in gear, or vibration that matches transmission output speeds. At best, a worn u-joint causes vibration and clunky operation. At worst, a suddenly broken u-joint can cause an accident. If a front u-joint suffers catastrophic failure, the vehicle can launch on the driveshaft much the same way a pole-vaulter uses a pole to launch into the air.

X-Ray Vision

While clunking, rattling, and a grumbling vibration at speed are telltale signs of worn u-joints, there is no way to tell if a u-joint is worn by looking at it from the outside. Removing the driveline and moving the u-joints by hand can reveal crunchy movement or impending failure, but the only way to be sure is to replace the u-joints when the time comes. Since most u-joints come from the factory full of grease, once they're done—they're done. Some replacement u-joints have grease fittings for service while others, like the OEM units, are lubed for life. Replacing u-joints is a relatively easy affair once the driveshaft is out of the vehicle. Whether the classic hammer, socket and vice method or a relatively inexpensive u-joint tool is used—the method is the same. Follow along the step-by-step guide and put the universal back into your u-joints.