A universal joint, often called a U-joint, lets a rotating shaft send power to another shaft even when the two shafts are not in a straight line. In cars and trucks, U-joints are commonly found in the driveshaft that connects the transmission to the rear axle. They matter because the axle moves up and down with the suspension while the engine and transmission stay mostly fixed.
Without a flexible joint, the driveline would bind, vibrate, or fail when the shaft angle changes.
Key Facts
- A universal joint transmits torque between two shafts whose centerlines meet at an angle.
- Power in a rotating shaft is P = Tω, where P is power, T is torque, and ω is angular speed.
- Torque is T = Fr, where F is tangential force and r is the radius from the shaft center.
- A single U-joint does not keep the output shaft speed perfectly constant when the joint angle is not zero.
- Larger joint angles increase speed variation, vibration, wear, and heat.
- Two U-joints used with equal and opposite angles can cancel most speed variation in a driveshaft.
Vocabulary
- Universal joint
- A mechanical coupling that allows two rotating shafts to transmit torque while their axes are at an angle.
- Driveshaft
- A rotating shaft that carries power from the transmission or transfer case to the axle or differential.
- Yoke
- A fork-shaped part of a U-joint that connects to a shaft and holds the cross bearings.
- Cross
- The central X-shaped part of a U-joint that connects the two yokes and allows them to pivot.
- Torque
- A twisting effect that can cause rotation, measured in newton meters or pound feet.
Common Mistakes to Avoid
- Assuming a U-joint is a constant-velocity joint is wrong because a single U-joint changes output speed slightly during each rotation when the shafts are angled.
- Ignoring the joint angle is wrong because a steep angle increases vibration, bearing load, and wear even if the joint still turns.
- Installing the two yokes out of phase is wrong because misaligned yokes prevent the speed variations from canceling and can cause driveline vibration.
- Thinking lubrication is optional is wrong because the needle bearings need grease or a sealed bearing design to reduce friction and prevent failure.
Practice Questions
- 1 A driveshaft transmits 180 N m of torque at an angular speed of 120 rad/s. What power is being transmitted in watts using P = Tω?
- 2 A tangential force of 400 N acts at a radius of 0.035 m on a U-joint yoke. What torque is applied using T = Fr?
- 3 A rear-wheel-drive vehicle has one U-joint at the transmission and one at the differential. Explain why the two joint angles should usually be equal and opposite rather than random.