Drive shafts and U-joints provide an efficient and durable means of transferring engine torque to the drive axles. U-joints allow drive axles to move up and down with the suspension while the shaft is rotating so power can be transmitted even when a drive shaft is not at the same angle as its connection point.
Some drive shafts also have a slip joint, a yoke that slides onto the transmission output shaft splines or drive shaft splines, allowing the drive shaft to make minor length changes as the vehicle suspension height changes. U-joints and slip joints function to save wear and tear on your vehicle’s transmission and gear boxes.
U-joints typically look like a + sign, or a cross, and are made of high strength hardened steel. They have four steel bearing cups with tiny needle bearings placed around the wall of the cup. These cups press onto four circular steel bearing surfaces, which the needle bearings contact, and contain special lubricant.
There is a seal on each cup to prevent lubricant from escaping. The cups rotate on each bearing surface and allow variable movement based on the angle. The cups are retained into each arm of a yoke, and there are two arms on each yoke. A yoke is present at the transmission, at the gear box, and on the drive shaft.
Yokes are also present on axles with a U-joint on the main axle shaft and the shorter stub shaft axle at the wheel.U-joints used on most modern vehicles are lubed for life from the factory and do not require periodic lubrication. Even if a vehicle’s U-joints cannot be lubricated, they should be inspected at every oil change.
SUVs and pick-up trucks sometimes have lubrication fittings on drive shaft slip joints. These should be lubricated when the vehicle is serviced. Replacement U-joints can also come with lubrication fittings, so it is important to inspect all of a vehicle’s U-joints for grease fittings even if grease fittings were not originally equipped on the vehicle.
Symptoms of a bad U-joint include a squeaking sound while moving at slow speeds or when accelerating from a stop, a clunking noise when shifting into any gear, a shuddering or vibrating sensation when accelerating or driving, clicking noise near the front of the vehicle when turning, or evidence of rust coming out of the grease seals.
If your vehicle shows any of these symptoms, have it inspected as soon as possible by an ASE certified technician. Neglecting the warning signs of a bad U-joint may cause the drive shaft to separate from the vehicle which may leave you stranded, result in more expensive repairs, and possibly damage the vehicle.
The most common cause of U-joint failure is the lack of or loss of lubrication. If you are considering buying a used vehicle that is equipped with a drive shaft (front and/or rear) with U-joints, inspect the drive shaft for damage, and inspect the U-joints, including front end U-joints present on four-wheel drive vehicles.
For looseness, noises, broken grease seals, and damage. The vehicle should also be test driven, and if equipped with four-wheel drive, it should be engaged to listen for any unusual noises.