Universal (U) Joints - Axle & Driveshaft
Drive shafts and universal joints are present on most rear-wheel drive and four-wheel drive vehicles. A universal joint (U-joint) is a mechanical joint used for connecting shafts that rotate and are aligned at different angles to each other. The U-joint compensates for the misalignment between the shafts in any direction. U-joints can be used when the alignment between the shafts is about 30 degrees or less and ideally as close to alignment as possible. Universal joints are used on the front axle shafts of some vehicles, as well as front and rear drive shafts. They are a connection point allowing the smooth delivery of torque.
Constant velocity (CV) joints are a different kind of connecting joint, similar to U-joints. CV-joints also compensate for steering changes, and are found on front-wheel drive and all-wheel drive vehicles, and on many four-wheel drive vehicles.
The drive shaft and U-joints connect the transmission to the rear drive axle on most rear-wheel drive vehicles. Many four-wheel drive vehicles also use drive shafts with U-joints, with one drive shaft between the transfer case and rear drive axle, and a second drive shaft between the transfer case and the front drive axle. A drive shaft is also known as a propeller shaft. Some four-wheel drive vehicles also use U-joints to connect front axle shafts to the front wheels, and these U-joints allow torque to be provided to the front wheels when they are turning.
Drive shafts and U-joints provide an efficient and durable means of transferring engine torque to the drive axles. U-joints allow drive shafts 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, have an ASE certified technician 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.