What is a Universal Joint on a Car?

Jack R. Nerad | Jan 29, 2021

If you have driven for a while, you have probably heard a mechanic or do-it-yourselfer use the term "universal joint." Often they'll include the name of this mechanical part in a car in a statement like this: "I think it's a universal joint" or "The u-joints are going bad."

what is a universal joint

Of course, car parts aren't like aging vegetables. They don't "go bad." Nevertheless, when you hear something like this, you naturally want someone to explain the answer to the question: what is a universal joint on a car?

The answer is important because some types of vehicles won't move at all without functioning universal joints. Further, understanding the proper operation and maintenance of u-joints can save you from a giant repair bill, and that's something we'd all like to avoid.

U-joint failure is typically gradual and, left unchecked, can be expensive to repair or even dangerous while you're driving. The good news is there are telltale signs that your u-joints need some professional attention.

What Is a Universal Joint?

Universal joints are X-shaped parts typically made of steel with a bearing cap at each end that allows the vehicle's rigid driveshaft to connect to the transmission and rotate freely.

They are part of the system that enables the rotating crankshaft in a vehicle's engine to transfer that rotating motion to the rear wheels in a rear-wheel-drive vehicle. Because they provide a flexible connection at the driveshaft's ends, they allow the driveshaft to move up and down and at an angle to the engine as the vehicle encounters uneven road surfaces.

For example, pickup trucks have universal joints at both ends of their driveshafts. Typically, the u-joints connect to yokes that allow for a driveshaft's fore-and-aft movement and the up-and-down movement for which the universal joints compensate. Without universal joints or some similar system, it would be impossible for a vehicle to have a suspension that offers any substantive wheel travel. The driveline would bind with every bump and pothole.

Front-wheel-drive vehicles have constant velocity joints, or CV joints. They perform a similar function to universal joints.

What Does a Universal Joint Do?

Because vehicles don't travel on surfaces as flat as a billiard table, driveshafts must be able to move up and down with the suspension while continuing to rotate to ensure uninterrupted power flow to the drive wheels. The universal joints enable this movement while allowing the driveshaft to continue to turn.

The use of U-joints makes up for the height misalignment between the transmission and the rear axle that drives the rear wheels. Since a vehicle's rear axle and rear wheels move independently of the rest of the vehicle, a direct driveshaft from the engine to the axle would bind or flex as the vehicle encounters bumps. The u-joints compensate for that motion, keeping the driveshaft spinning freely. As the driveshaft rotates, the flexible "universal" action of the u-joints automatically compensates for misalignment.

What Maintenance Is Necessary?

It's obvious that the vehicle's universal joints get a serious workout any time the car is moving. The faster the vehicle goes and the more its rear axle travels up and down, the greater that workout becomes. This suggests that u-joints require maintenance.

The bearings of each u-joint are greased at the factory during assembly. Some are "lubed for life," and they cannot be serviced with additional lubrication. Other universal joints have grease fittings that allow the addition of grease at service intervals.

In either type, the lubrication can diminish over time, and the "dry" bearing results in wear-causing metal-to-metal contact. You can lubricate universal joints with a grease fitting, but u-joints without that ability will simply wear out and need replacement. Happily, their service life is quite lengthy.

What Are Symptoms of U-Joint Failure?

It is advisable to pay attention to any signs your vehicle's universal joints are failing because, in the worst-case scenario, the driveshaft could separate from the vehicle, damage it severely and perhaps cause an accident. Even before total failure, worn u-joints can lead to costly transmission issues and could leave you stranded by the side of the road.

Here are some early warning signs of u-joint failure:

  • Squeaking noise when moving from a standstill
  • Clunk sound when shifting
  • Vehicle vibration when underway
  • Fluid leaks from the back of the transmission
  • Banging sound accompanied by complete loss of power

To gauge if one or more of your vehicle's u-joints is nearing failure, you can perform a simple test. With your foot on the brake, shift the car into reverse and then, after a moment, shift into drive. If you hear and feel a clunk, a worn u-joint is the probable cause. The clunk occurs because a failing u-joint allows too much play at its connection, causing a momentary stall and then rapid engagement that registers as a clunk.

What Should You Do?

If you think your vehicle's u-joints are nearing the end of their useful life, have them replaced as soon as possible. The repair is not very costly and is far less expensive than the repairs required if you ignore them and suffer transmission problems or a driveshaft separation.

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