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A toothed belt, timing belt, cogged belt, cog belt, or synchronous belt is a flexible belt with teeth moulded onto its inner surface. Toothed belts are usually designed to run over matching toothed pulleys or . Toothed belts are used in a wide array of mechanical devices where high power transmission is desired.
Toothed belts are used widely in mechanical devices, including , and many others. A major use of toothed belts is as the timing belt used to drive the within an automobile or motorcycle engine.
As toothed belts can deliver more power than a friction-drive belt, they are used for high-power transmissions. These include the primary drive of some , notably later ; and the supercharger used for drag racing.
Microlight aircraft driven by high-speed two-stroke engines such as the Rotax 532 use toothed belt reduction drives to allow the use of a quieter and more efficient slower-speed propeller. Some amateur-built airplanes powered by automotive engines use cog belt reduction drive units.For example : Bede BD-5, Dieselis aircraft , and Pennec Gaz'Aile 2
One failure mode is gradual wear to the tooth shape, which may eventually lead to slippage over rounded teeth. The belt often continues to work, but the relative timing between shafts changes.
The catastrophic failure mode is caused by delamination between the belt and its fabric reinforcement. Although this may be caused by age and wear, it is often accelerated by mistreatment of the belt, often during initial installation. Overloading the belt by bending it to a narrow radius is a common cause of damage, either by bending out of the belt's designed axis, twisting, levering it into place with tools, bending in the correct axis but to a too small radius, or even a belt in storage. Another cause, particularly with natural rubber belts, is contamination by oil, especially to the edges where the reinforcing fabric is exposed and can cause a wick effect.
It is extremely rare for a timing belt to break. More common is for the belt to delaminate, disconnecting the fabric strength member from the teeth that ride on the pulleys. The belt is then often thrown from the pulleys and may be further damaged, cut, or broken. Although worn teeth may be detectable by careful inspection, internal deterioration is not considered to be reliably detectable and so the observance of service lifetimes is important.
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