Mechanic Glossary

Universal Joint

A cross-shaped mechanical coupling with needle bearings on each end, allowing a driveshaft to transmit power at an angle.

A Universal Joint (U-joint) is a mechanical coupling that connects the driveshaft to the transmission and differential yoke. It consists of a steel cross-shaped body (journal) with cup bearings packed with needle rollers on each of the four ends. As the rear wheels go over bumps and the differential moves up and down, the U-joint lets the driveshaft spin constantly while bending at different angles. This prevents the shaft from binding.

U-joints fail when the grease inside the needle bearings dries out or escapes. Moisture enters the cups, rusting the needles. The bearings seize, causing a high-pitched squeaking noise that changes with vehicle speed, or a loud metallic clunk when you shift the car from Park to Drive.

Frequently Asked Questions

A squeaking or chirping sound that sounds like a cricket, which increases speed as you drive. If it is severely worn, you will hear a loud clunk when shifting into gear or feel a heavy vibration under your seat at highway speeds.
Raise the vehicle, grab the driveshaft near the joint, and try to shake it up and down. There should be absolutely zero play. If you see the cross shift inside the bearing cups, the U-joint is worn out and must be replaced.
Some aftermarket U-joints feature grease zerk fittings and must be greased during every oil change. Factory U-joints are sealed and cannot be greased; they are designed to be run until they wear out and are replaced.
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