A timing belt, or cam belt (informal usage), is a part of an internal combustion engine that controls the timing of the engine's valves. Some engines, like the flat-4 Volkswagen air cooled engine, and the straight-6 Toyota F engine use timing gears.
Timing belts replace the older style timing chains that were in common usage until the 1970's and 1980's (although in the last decade there has been some reemergence of chain use).
The term "timing belt" is sometimes used for the more general case of any flat belt with integral teeth, although such usage is a misnomer since there is no timing or synchronization involved.
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In the internal combustion engine application, the timing belt / chain connects the crankshaft to the camshaft(s), which in turn controls the opening and closing of the engine's valves. A four-stroke engine requires that the valves open and close once every other revolution of the crankshaft.
The timing belt / chain does this. It has teeth to turn the camshaft(s) synchronised with the crankshaft, and is specifically designed for a particular engine. In some engine designs, the timing belt may also be used to drive other engine components such as the water pump and oil pump.
Gear or chain systems are also used to connect the crankshaft to the camshaft at the correct timing. However gears and shafts constrain the relative location of the crankshaft and camshafts. Even where the crankshaft and camshaft(s) are very close together, as in pushrod engines, most engine designers use a short chain drive rather than a direct gear drive.
This is because gear drives suffer from frequent torque reversal as the cam profiles "kick back" against the drive from the crank, leading to excessive noise and wear. Fibre gears, with more resilience, are preferred to steel gears where direct drive has to be used. A belt or chain allows much more flexibility in the relative locations of the crankshaft and camshafts.
While chains and gears may be more durable, rubber composite belts are quieter in their operation (in most modern engines the noise difference is negligible), are less expensive and are mechanically more efficient, by dint of being considerably lighter, when compared with a gear or chain system. Also, timing belts do not require lubrication, which is essential with a timing chain or gears.
A timing belt is a specific application of a synchronous belt used to transmit rotational power synchronously.
Timing belts are typically covered by metal or polymer timing belt covers which require removal to carry out inspection or replacement. Engine manufacturers recommend replacement at specific intervals.[1] The manufacturer may also recommend the replacement of other parts, such as the water pump, when the timing belt is replaced because the additional cost to replace the water pump is negligible compared to the cost of accessing the timing belt.
In an interference engine, or one whose valves extend into the path of the piston, failure of the timing belt (or timing chain) invariably results in costly and, in some cases, irreparable engine damage, as some valves will be held open when they should not be and thus will be struck by the pistons.
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