How to check U-Joints

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.

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 between two parts such as the teeth of gears. data-term-id="1310">backlash