A propeller shaft, or propshaft, connects the rear axle to the gearbox on front-engined rear-wheel-drive cars.
At each end of the propshaft there is a universal joint (UJ), 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.
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A universal joint consists of a cross-shaped 'spider' with needle roller bearings held on its four arms by caps and circlips. 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. As wear increases, vibration worsens.
Another sign of wear is a clonk when you accelerate or decelerate, or a regular knock when the car is moving.
A sure sign of something breaking up inside the bearings of a joint is rust-coloured 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.
Make another check by inserting a large screwdriver between the yoke and the spider and levering it to see if there is any play. If you feel any play, fit a new joint or propshaft.
The symptoms of a worn joint — the clonks or apparent play — are very similar to those for excessive backlash in the crown wheel and pinion.
Be sure that any play you do feel is in the joint, and not the rear axle.
Some specialist engineering companies will machine out the yokes of modern propshafts to accept joints that can be fitted in the same way as on earlier designs.
If a sealed joint fails, it is worth seeking out such a company and comparing the cost of their services against that of a new propshaft.
Generally, if one joint on the prop-shaft is worn, the others will also need renewal, and it is best to do them all at the same time.
Rear-wheel drive U-joint
Failure of the needle rollers is the commonest cause of wear in this type of 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. Then check the joint for both radial and lateral play.
Grip the flange tightly with one hand and try to turn the propshaft with the other; look for movement at the spider ends, under the circlips. With a strong screwdriver or steel bar, lever the joint laterally. again looking for movement at the spider ends.
Doughnut joints are used on the inner ends of the drive shafts on some front-wheel-drive cars, and also on some rear-engined cars.
The joint consists of a flexible rubber ring held at six regular intervals by bolts to which the drive shaft and driven shaft are attached.
The rubber can perish or split after a high mileage, causing vibration in the transmission and eventual failure if not attended to quickly. Examine particularly round the bolts for signs of splitting. If you find any, fit a new doughnut joint.
Rubber spider joint
Early BLMC Minis were fitted with rubber-covered spider joints at the inner ends of the drive shafts. The spider is secured within the lugs of the two forked shaft ends by four U-bolts.
Inspect the joints after every 5,000 miles for signs of oil saturation and age deterioration. Check for loose nuts and bolts, and oval wear marks round the bolt holes.
The first sign of wear in the joint is usually a swelling of the rubber. Watch also for a small hole or circle at the end of any of the rubber arms. It is a sign that the rubber is becoming detached from the spider.
Fit a complete new joint immediately you spot any wear.
If deterioration of the joint has been caused by oil saturation, have the drive-shaft oil seal replaced by a garage.
Checking drive-shaft oil seals
There is little you can do to check the oil seals on the drive shafts of frontwheel-drive cars, except a precautionary inspection to see if there is oil leaking where the shaft enters the transmission.
In extreme cases, the oil forms a puddle below the leaking seal. If this happens, the shaft must be removed and a new seal fitted - a task best undertaken by a garage.
On some cars the inner end of the shaft is hidden behind a rubber gaiter. Check the gaiter for splits or other signs of damage. Generally, if the gaiter is in good condition, the seal is also sound.
Some front-wheel-drive cars have their oil seals in axle-shaft carriers on either side of the differential housing. You can easily check for leakage here.
Clean any oil from the area thoroughly and leave the car for a couple of hours to see if oil reappears - and exactly from where it emerges.
If there is a leak, have new seals fitted as soon as possible. Until they are fitted, check the transmission oil level at least once a week.
Checking constant-velocity joints
Constant-velocity, or CV, joints are fitted generally to drive shafts on front-wheel-drive cars, where movement has to be transmitted through both the transmission and the steering.
The CV joint normally consists of a central spider, six steel balls, a cage for the balls and an outer cup. The balls engage grooves in both halves of the joint, keeping them together, but allowing free alteration of the operating angle.
During normal service, CV joints do not require attention, but make regular checks of the rubber gaiters in which they are encased for signs of wear or damage. Even the slightest damage or tiniest hole will let in dirt and grit which will quickly ruin the joint.
Wear in the joints causes a clonking noise when the car is turned under power on full steering lock. The knock gets progressively worse until eventually it can be heard on lesser steering locks.
The only solution is to have the CV joint renewed.