by James Dreamer

Created on: July 21, 2010

A timing belt is a band consisting of fiber and rubber used in an internal combustion engine to connect the crankshaft to the camshaft (or camshafts, depending on engine design there can be up to four) in such a way as to maintain an exact rotational position in relation to each other. Unlike the V-belts used on most power equipment, a timing belt is equipped with teeth along its inner circumference that mesh with the grooves on each pulley to eliminate slippage.

Most modern day internal combustion engines have a crankshaft (the shaft which converts the oscillating motion of the pistons into rotational energy), and at least one camshaft (the shaft which when rotated, opens and closes the valves at the proper time to allow the fuel/air mixture to enter the cylinders and the burned exhaust gases to exit). "At the proper time" are the key words here. Because the piston creates the vacuum to pull the mixture into the cylinder and pushes the exhaust gases out after detonation, the valves must open at precisely the right moment. This is referred to as valve timing. It should not be confused with ignition timing, which is when the spark occurs to detonate the fuel.

For the valve to open at the correct time, the camshaft and crankshaft must remain synchronized or "timed" in relation to each other. In a four cycle automotive engine, the camshaft is required to rotate exactly once for every two rotations of the crankshaft. To make this occur, the crankshaft pulley is one half the size of the camshaft pulley. These pulleys can be connected together using several techniques. Gears that mesh with each other, a chain between sprockets on each shaft, or a belt between pulleys on each shaft. One requirement in all systems is that there is no slippage.

For the purposes of this article, we are studying the belt between pulleys system or timing belt. In this system, the crankshaft and camshaft each have attached to their end a pulley whose outside is made up of grooves. These grooves run perpendicular to its circumference like the grooves on the edge of a coin. The crankshaft pulley has half as many grooves as the camshaft pulley. For example, if the crank pulley has eighteen grooves then the cam pulley will have thirty-six. If there are multiple camshafts each of their pulleys will be identical.

A timing belt is used to connect these pulleys together. Because it contains fiber imbedded in the rubber, it resists stretching. Its teeth mesh with the grooves in each pulley making it impossible for one to rotate without the other. This insures that the valve timing remains constant. Hence the name - timing belt.

Timing belts must be replaced at certain intervals as recommended in the owners manual. If this is not done, the belt will fail and the engine will abruptly stop. It has now become a huge paper weight and will remain so until the belt is replaced and any damage to pistons and valves is repaired. A belt should also be replaced if it gets contaminated with oil or coolant.

A timing belt will fail one of two ways. The teeth will become worn to the point where they strip off and can no longer keep the pulleys timed. Or, the belt will become so week that it simply breaks apart. Too many miles or chemical contamination can cause either scenario. Its best to keep it renewed for trouble free driving.

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