 | ||
A timing belt, toothed belt, cogged belt or cog belt, or synchronous belt is a non-slipping mechanical drive belt. It is made as a flexible belt with teeth moulded onto its inner surface. It runs over matching toothed pulleys or sprockets.
Contents
When correctly tensioned, they have no slippage and are often used to transfer motion for indexing or timing purposes (hence their name). They are often used in lieu of chains or gears, so there is less noise and a lubrication bath is not necessary.
Applications
Toothed belts are used widely in mechanical devices, including sewing machines, photocopiers and many others. A major use of toothed belts is as the timing belt used to drive the camshafts within an automobile engine.
As toothed belts can also deliver more power than a friction-drive belt, they are also used for high-power transmissions. These include the primary drive of some motorcycles, notably later Harley-Davidsons. Also the supercharger used for dragsters.
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.
Construction
These belts are made of a flexible polymer over a fabric reinforcement. Originally this was rubber over a natural textile, but developments in material science have had a substantial effect in increasing the lifetime of these belts. This included changes from natural to synthetic rubber and polyurethane and also the adoption of nylon, Kevlar or other aramid fibres and carbon fibres in their reinforcement.
Failure
Toothed belts have two failure modes, one gradual and one catastrophic. Both increase their risk over time, so it is common for highly-stressed belts to be given a service lifetime and to be replaced before this.
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 too small a radius, or even knotting 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, or cut. 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.