Cam or timing belts are made from rubber, reinforced with cord or fibre glass, which are notched along the inner face to form equally spaced teeth. The belt is driven by the crankshaft pulley and drives the camshaft pulley or in the case of double overhead cam engines, pulleys. The pulleys have teeth around their circumference that mesh exactly with the teeth on the cam belt to ensure that when the valve timing is set it is maintained. This arrangement is very common on modern overhead valve engines as it is comparatively cheap, quiet and efficient way of driving the camshaft when it is mounted far away from the crankshaft.
This diagram shows a cam belt fitted to a single overhead camshaft engine. The pulley at the bottom is attached to the crankshaft and the larger pulley at the top is attached to the camshaft. The cam belt is tensioned by a tensioner wheel situated to the left of the belt aided by the idler wheel located above it to the right. When a cam belt is replaced it is best practice to replace the tensioner and idler at the same time. On some vehicles the water pump is driven by the cam belt and it again good practice to replace this when the belt is changed
Cam belts have a life span and must be checked and replaced in accordance with manufacturer's guidelines. Cam belt failure will result in breakdown and may cause significant internal engine damage, this is because when a cam belt breaks, the camshafts will stop turning but the crankshaft will continue to rotate. Most modern engines are know as 'interference' engines, which means that if the valve timing is not maintained then the pistons will strike any open valves.
This diagram shows that as the piston reaches the highest point of the cylinder it has impacted with a valve which is stuck in the open position as the camshaft is not turning. The valve will now be bent and the cylinder head will need to be removed and the
valve(s) will need replacing.
It is worth considering that if a cam belt is replaced as part of routine maintenance then the cost would be between 5 and 10% of the repair costs following cam belt failure.
Cam belts can fail for a variety of reasons: foreign bodies can enter the drive, oil leaks can contaminate the belt, pulleys can seize or become misaligned, belt tension can be incorrect and the belt can be affected by temperature extremes.