A universal joint (U-joint, also called a Cardan joint or Hardy Spicer joint) is a mechanical coupling that connects two rotating shafts whose axes meet at an angle, letting torque keep transferring even when the shafts aren’t in a straight line. On a vehicle, the most common place to find one is at each end of the driveshaft, where it lets the shaft keep spinning as the suspension moves the axle up and down relative to the transmission.
What a U-Joint Is Made Of
A standard cross-type U-joint has three parts, and every source that describes one agrees on the same three:
- Two yokes — U-shaped brackets, one bolted to the driving shaft and one to the driven shaft.
- A cross (also called a spider or journal) — a rigid, X-shaped piece with four arms (trunnions) set 90° apart.
- Needle bearings in bearing caps — caps packed with small roller bearings fit over each of the cross’s four arms, letting both yokes pivot on the cross with minimal friction.
Each yoke connects to opposing arms of the cross. Because the cross sits between them and both yokes can pivot independently on it, the joint can bend through an angle while still carrying rotational force straight through.
How It Works
The power path through a U-joint is straightforward:
- The driving shaft turns its yoke.
- That yoke pushes on the cross.
- The cross pushes the second yoke.
- The second yoke turns the driven shaft — even though it isn’t perfectly aligned with the first one.
This is what lets a driveshaft keep transmitting power from a transmission that’s fixed to the frame to a differential that’s constantly moving up and down with the axle. Without the joint, a rigid shaft would bind or break the moment the two ends stopped being perfectly in line.
The Trade-off: A Single U-Joint Doesn’t Spin at a Constant Speed
This is the part most people don’t expect: when a U-joint is operating at an angle, the output shaft doesn’t rotate at a perfectly steady speed — even if the input shaft does. The output speeds up and slows down twice per revolution, and the larger the operating angle, the more pronounced that speed variation becomes. This isn’t a defect; it’s inherent to how a single cross-and-yoke joint geometrically handles an angle.
Two consequences follow directly from this:
- Most driveshafts use two U-joints, not one — one at each end — specifically because a second joint, installed correctly, can cancel out the speed variation the first one introduces. For that cancellation to actually work, the two joints have to be installed in the correct rotational relationship to each other, known as phasing. A driveshaft with two U-joints that aren’t properly phased will still vibrate, even though both joints are mechanically fine.
- Some applications need a joint that doesn’t have this limitation at all. A double Cardan joint pairs two U-joints back-to-back in a compact assembly to cancel the variation internally — common on 4WD front driveshafts, which often run at steeper angles than a rear driveshaft. A constant velocity (CV) joint goes further, using a different internal geometry (ball bearings riding in curved grooves rather than a cross-and-yoke) to maintain truly constant output speed even at sharper angles — which is why front-wheel-drive axles and independent suspensions use CV joints rather than simple U-joints.
Where You’ll Find Them
The driveshaft is the primary location — connecting the transmission (or transfer case, on 4WD/AWD vehicles) to the differential, and accommodating the suspension’s up-and-down travel as the axle moves. U-joints also show up in a couple of secondary spots worth knowing about: steering columns, where they let the steering shaft bend around engine-bay components on its way to the steering rack, and PTO (power take-off) shafts on tractors and other farm/industrial equipment, transmitting rotary power at a fluctuating angle between the machine and an implement.
Signs a U-Joint Is Wearing Out
A U-joint’s needle bearings run dry or wear over time, and when they do, it shows up as one or more of: a clunk when shifting into Drive or Reverse, vibration that builds with speed, or a squeaking/grinding noise from underneath the vehicle that changes with engine speed rather than road speed. This deserves a full symptom-and-diagnosis treatment on its own — this article’s job is to explain what the joint is and how it works, not to walk through failure diagnosis.
Quick Reference
| Part | Job |
|---|---|
| Yoke (×2) | Connects to the driving/driven shaft |
| Cross / spider | Central pivot point, 90° trunnion arms |
| Needle bearing caps | Let the yokes pivot on the cross with low friction |
| Second U-joint (phased) | Cancels the first joint’s speed variation |
| Double Cardan joint | Two U-joints combined to cancel variation internally — common on 4WD front shafts |
| CV joint | Different geometry, maintains constant speed at sharper angles — used on FWD/IRS axles |
Related reading: What Is a Carrier Bearing? · Universal Joint vs. Carrier Bearing: Types and Function Compared · Driveline vs. Driveshaft vs. Propeller Shaft: What’s the Difference?
