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 internal 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.
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.
A gearing system can 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. A belt or chain allows much more flexibility in the relative locations of the crankshaft and camshafts. Furthermore, belts are cheaper than a gearing system. While chains may be more robust, rubber belts are quieter. A timing belt is a specific application of a Belt_(mechanical) 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 leave the engine non-functioning. Depending on the design of the engine, the piston and valve paths may "interfere" with one another and incorrect timing in their movements may result in the piston and valves colliding. (Such designs are also called "interference head" or "interference engines". Conversely, non-interfering engines are called "free-wheeling" or "non-interference" engines.)
In interference designs, regular service is especially important as incorrect timing may result in the pistons and valves colliding and causing extensive engine damage and therefore costly repairs. The piston will likely bend the valves or if a piece of valve or piston is broken off within the cylinder, the broken piece will cause severe damage within the cylinder, often also affecting the crankshaft. In some newer engines, timing belts are designed to last the effective life of the engine. When a timing belt is replaced, care must be taken to ensure that the valve and piston movements are correctly synchronized.
A timing belt is typically rubber with high-tensile fibers (e.g. fiberglass or Kevlar) running the length of the belt.
[. - "Contrary to what you might think, rubber timing belts do not stretch with accumulated mileage and wear. They are reinforced with strands of fiberglass which makes them virtually unstretchable. After making the crankshaft to cam drive circuit millions of times, the strands can become brittle and may begin to break. Eventually the reinforcing cords give way, the belt snaps and the engine quits."]
Rubber degrades with higher temperatures and with contact with motor oil and antifreeze. Thus the life expectancy of a timing belt is lowered in hot or leaky engines. Newer or more expensive belts are made of temperature resistant materials such as "highly-saturated nitrile" (HSN). Older belts have trapezoid shaped teeth. Newer manufacturing techniques allow for curved teeth that are quieter and last longer.
Aftermarket timing belts may be used to alter engine performance.
The first known timing belt was used in 1945.
[. - "In earlier engines, camshafts were often gear-driven off the crankshaft. Later on, powerplant designers developed chain drives in OHV (overhead valve) configurations that allowed some flexibility in the placement of the camshaft so that shorter pushrods could be used, all for more performance and efficiency. Those engines with long chains sometimes tended to whip about and cause problems. The only alternative was a noisy and complicated multi-gear train until the cogged rubber synchronous timing belt was invented in 1945. Though it was once considered the hallmark of a cheap engine, now it is used in distinguished automobiles such as Acuras, Volvos and Porsches."] The German Goggomobil microcar was the first mass produced vehicle to use a timing belt in 1950. The first American vehicle to use a timing belt was the 1966 Pontiac Tempest.