Timing Belt Information
Timing belt A timing belt, timing chain or cam belt is a part of an internal combustion engine that controls the timing of the engine's valves. Some engines use timing gears. The term "timing belt" is also used for the more general case of any flat belt with integral teeth. Such belts are used for power transmission or to interchange rotary motion and linear motion, where either high loads or maintaining a specific drive ratio are important. A common non-automotive application is in linear positioning systems. Such belts have also been used in efforts to make a cleaner, lower-maintenance bicycle transmission but have never become popular in this application.
Gear or chain systems can also be 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. Fiber 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. Camshaft drives, whether gears, belts or chains are also able to even out wear, since the chain or belt can be made such that the number of teeth on the belt is coprime to the number of teeth on the crankshaft and camshaft sprockets, thus ensuring that each tooth on sprocket doesn't end up on the same tooth on the belt repeatedly.
While chains and gears may be more durable, rubber 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. A timing belt is a specific application of a synchronous belt used to transmit rotational power synchronously.
Timing belts are typically inaccessible and difficult to inspect. Replacement at specific intervals is recommended by the manufacturer. 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. Failure of the timing belt will seize the engine and often leads to damage that is uneconomic to repair.
In the internal combustion engine application, the timing belt 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 turn of the crankshaft. The timing belt does this. It has custom teeth to turn the camshaft(s) synchronized 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.
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