Toothed belt

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Supercharger drive belt in a dragster Dragster Kompressor 2005.jpg
Supercharger drive belt in a dragster

A toothed belt; timing belt; cogged belt; cog belt; or synchronous belt is a flexible belt with teeth moulded onto its inner surface. It is sometimes designed to run over matching toothed pulleys or sprockets. Toothed belts are used in a wide array of mechanical devices, where high-power transmission is desired.


Design and application

Timing belts, [1] toothed belts, [2] cogged or cog belts, [3] and synchronous belts [4] are non-slipping mechanical drive belts. They are made as flexible belts with teeth moulded onto their inner surface. The belts run over matching toothed pulleys or sprockets. [5] [6] [7] When correctly tensioned, these type of belts 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.

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 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 motorcycles, notably later Harley-Davidsons; and 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. [8]

Other names

A gilmer belt was a brand or trade name for a mechanical belt used for transferring power between axles in a machine. The gilmer belt was originally sold by the L. H. Gilmer company after 1949, [9] and represents one of the earliest toothed belt designs. Gilmer belts use trapezoidal teeth to engage matching grooves on toothed pulleys in order to maintain synchronicity between moving parts. [10] Belts are no longer sold under the Gilmer name, although enthusiasts are still likely to refer to toothed belts by the gilmer name. [11]


Toothed 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 steel, nylon, Kevlar (or other aramid fibres), and/or carbon fibres in their reinforcement. [2]

Belt failure

A new belt, already damaged by knotting 2001 honda accord timing belt-terabass.jpg
A new belt, already damaged by knotting

Toothed belts have two failure modes, one gradual and one catastrophic. There is an increased risk of either over the lifetime of the belt, so it is common for highly-stressed belts to be given a service lifetime and to be replaced before this failure can occur.

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, cut, or break. 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.

See also

Related Research Articles

Clutch Mechanical device

A clutch is a mechanical device that engages and disengages power transmission specially from drive shaft to driven shaft.

Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device preserves the input power and simply trades off input forces against movement to obtain a desired amplification in the output force. The model for this is the law of the lever. Machine components designed to manage forces and movement in this way are called mechanisms. An ideal mechanism transmits power without adding to or subtracting from it. This means the ideal mechanism does not include a power source, is frictionless, and is constructed from rigid bodies that do not deflect or wear. The performance of a real system relative to this ideal is expressed in terms of efficiency factors that take into account departures from the ideal.

Pulley grooved wheel to support movement and change of direction of a taut cable

A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell that does not transfer power to a shaft, but is used to guide the cable or exert a force, the supporting shell is called a block, and the pulley may be called a sheave.

Gear Rotating circular machine part with teeth that mesh with another toothed part

A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth, which mesh with another toothed part to transmit torque. A gear may also be known informally as a cog. Geared devices can change the speed, torque, and direction of a power source. Gears of different sizes produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The rotational speeds, and the torques, of two meshing gears differ in proportion to their diameters. The teeth on the two meshing gears all have the same shape.

Roller chain

Roller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire- and tube-drawing machines, printing presses, cars, motorcycles, and bicycles. It consists of a series of short cylindrical rollers held together by side links. It is driven by a toothed wheel called a sprocket. It is a simple, reliable, and efficient means of power transmission.

Transmission (mechanics)

A transmission is a machine in a power transmission system, which provides controlled application of power. Often the term 5-speed transmission refers simply to the gearbox, that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device.

Continuously variable transmission Automatic transmission that can change seamlessly through a continuous range of effective gear ratios

A continuously variable transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. The flexibility of a CVT with suitable control may allow the engine to operate at a constant RPM while the vehicle moves at varying speeds.

Manual transmission Type of transmission used in motor vehicle applications

A manual transmission is a multi-speed motor vehicle transmission system, where gear changes require the driver to manually select the gears by operating a gear stick and clutch.


In mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive is sometimes mistakenly called a freewheel, but is otherwise unrelated.

Timing belt (camshaft)

A timing belt, timing chain, or cambelt is a part of an internal combustion engine that synchronizes the rotation of the crankshaft and the camshaft(s) so that the engine's valves open and close at the proper times during each cylinder's intake and exhaust strokes. In an interference engine the timing belt or chain is also critical to preventing the piston from striking the valves. A timing belt is usually a toothed belt—a drive belt with teeth on the inside surface. A timing chain is a roller chain.

Overhead camshaft engine

An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion chamber in the engine block.

Sprocket Toothed wheel or cog

A sprocket, sprocket-wheel or chainwheel is a profiled wheel with teeth, or cogs, that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which radial projections engage a chain passing over it. It is distinguished from a gear in that sprockets are never meshed together directly, and differs from a pulley in that sprockets have teeth and pulleys are smooth except for timing pulleys used with toothed belts.

Belt (mechanical) Method of connecting two rotating shafts or pulleys

A belt is a loop of flexible material used to link two or more rotating shafts mechanically, most often parallel. Belts may be used as a source of motion, to transmit power efficiently or to track relative movement. Belts are looped over pulleys and may have a twist between the pulleys, and the shafts need not be parallel.

Chain drive

Chain drive is a way of transmitting mechanical power from one place to another. It is often used to convey power to the wheels of a vehicle, particularly bicycles and motorcycles. It is also used in a wide variety of machines besides vehicles.

Belt-driven bicycle

A belt-driven bicycle is a chainless bicycle that uses a toothed synchronous belt to transmit power from the pedals to the wheel.

Rover KV6 engine

The KV6 automotive petrol engine has a 24-valve quad-cam V6 configuration, and a pressurising variable-length intake manifold to add hot spots throughout the rev range. Variants exist in 2.0 to 2.5 litres capacities. These were built initially by Rover Group, then by Powertrain Ltd. KIA manufactured KV6 in Korea under licence. Production moved from the UK to China in 2005, re-designated NV6.

Reduction drive

A reduction drive is a mechanical device to shift rotational speed. A planetary reduction drive is a small scale version using ball bearings in an epicyclic arrangement instead of toothed gears.

An idler-wheel is a wheel which serves only to transmit rotation from one shaft to another, in applications where it is undesirable to connect them directly. For example, connecting a motor to the platter of a phonograph, or the crankshaft-to-camshaft gear train of an automobile.

Motorcycle components and systems for a motorcycle are engineered, manufactured, and assembled in order to produce motorcycle models with the desired performance, aesthetics, and cost. The key components of modern motorcycles are presented below.

Drum motor

A drum motor is a geared motor drive enclosed within a steel shell providing a single component driving pulley for conveyor belts.


  1. "Huco Timing Belts and Pulleys". Huco Dynatork.
  2. 1 2 USpatent 5807194,"Toothed belt",published 15 September 1998, assigned to Gates Corporation
  3. in Contact !, Experimental Aircraft and Powerplant Newsforum for Designers and Builders, n°55, Dieselis Aircraft, A Prototype Aircraft with a Diesel Engine
  4. "PowerGrip HTD and timing synchronous belts". Gates Corporation.
  5. "ENG-10, Camshaft (Timing) Belt and Balance Shaft Belt Tension - Checking and Adjusting". Clarks Garage. Retrieved 2014-02-27. This should be checked at the midpoint between the cam sprocket and the crankshaft sprocket.
  6. "Replacing Chrysler 2.2 and 2.5 liter engine timing belts". AllPar. Retrieved 2014-02-27. Sprockets on the cam and intermediate shaft are twice the diameter of the sprocket on the crank, so for every two turns of the crankshaft the cam and intermediate shaft is turned one time.
  7. "Timing Belt Pulley Pitch Diameter & Outside Diameter Charts". Pfeifer Industries. Retrieved 2014-02-27. sprocket pitch circle
  8. For example : Bede BD-5, Dieselis aircraft "Archived copy". Archived from the original on 2013-07-01. Retrieved 2013-05-12.CS1 maint: archived copy as title (link), and Pennec Gaz'Aile 2
  9. Synchronous Belts - Part 1 (PDF), Gates Rubber Company, archived from the original (PDF) on 2016-08-09
  10. Definition of a Gilmer belt, Isky Racing Cams, archived from the original on 2010-11-24, retrieved 2011-01-16
  11. "Uniroyal to Sell Unit to Gates", The New York Times, 1986-05-22