A propeller shaft, or propshaft, connects the rear axle to the gearbox on front-engine rear-wheel-drive cars. At each end of the propshaft there is a universal joint which allows the rear axle to move up and down, in relation to the gearbox without bending or snapping the shaft. Some propshafts also have a universal coupling fitted at the centre.
A universal joint consists of a cross-shaped 'spider' with needle roller bearings held on its four arms by caps and circles. The yoked ends of two shafts have lugs which engage with the spider arms and can pivot on the roller bearings. In most modern cars the joints are sealed for life and cannot be lubricated. In older cars, they can be dismantled from the propshaft, checked and lubricated or replaced. If the propshaft joints on your car have grease nipples, check with the manufacturer's service schedule for greasing intervals.
The main drawback with many current car models is that the universal joints cannot normally be separated from the propshaft. When the joints wear, the propshaft must be renewed. A worn joint causes the propshaft to vibrate. A sure sign of something breaking up inside the bearings of a joint is rust-colored dust around the spider. Fit a new joint or shaft immediately. To check for wear when no dust is evident, grip one side of the joint firmly and try to turn the other side against holding pressure. There should be no play in the joint at all.
Rear-wheel drive universal joint:
Failure of the needle rollers is the commonest cause of wear in this type of Car's universal joint, usually due to lack of lubrication. Before checking the joint, make sure that any movement is not due to slackness in the bolts holding it to its mating flange.
Some symptoms of Failing Universal Joint
Squeaking noise when starting to move:
The bearing components of each universal joint are greased at the factory, but may not have grease fitting to allow further lubrication after the vehicle is put into use, limiting their service life. Since the bearing portion of each universal joint twists a small amount with each rotation of the driveshaft the grease can evaporate, or be thrown out of the bearing cup. The bearing becomes dry, metal-to-metal contact is made, and the Universal Joint bearings will squeak as the drive shaft rotates. The squeak usually cannot be heard once the vehicle moves faster than 5-10 mph because of other vehicle noise.
Transmission fluid leaks:
Transmission fluid leaking from the rear of the transmission is often the result of a badly worn universal joint. The vibration noted above has now worn the transmission tail shaft bushing and damaged the transmission output shaft seal, which then leaks transmission fluid. If fluid is suspected of leaking from the transmission, the transmission should be inspected to determine the source of the leak, and the appropriate repair made.