Part of an internal combustion engine

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A timing belt being installed

A timing chain (installed around the two circular cam sprockets)

In a piston engine, either a timing belt (also called a cambelt) or timing chain or set of timing gears is a perishable component used to synchronize the rotation of the crankshaft and the camshaft. This synchronisation ensures that the engine's valves open and close at the correct times in relation to the position of the pistons.

Design

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In most piston engines, the camshaft(s) are mechanically connected to the crankshaft. The crankshaft drives the camshaft (via a timing belt, timing chain or gears), which in turn actuates the intake and exhaust valves.[1] These valves allow the engine to inhale air (or an air/fuel mixture) and exhale the exhaust gasses.[2]

The most common devices to transfer the drive are toothed rubber belts, metal timing chains or a set of gears. The teeth of the belt/chain/gears mesh with both the crankshaft and camshaft(s), thereby synchronising their motion.

In many older overhead valve engines, the camshaft is located in the block near the crankshaft, therefore a simple gear system is often used to drive the camshaft. Overhead camshaft engines mostly use timing belts or timing chains, since these are better suited to transferring drive over larger distances. Timing belts were common on mass-production cars until the s and s,[citation needed] however since the s timing chains have become more common due to the replacement intervals required when using timing belts.

Toothed rubber belts

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A toothed rubber timing belt, also known as a synchronous belt, is a crucial component in an internal combustion engine. It is made of durable rubber and features teeth on its inner surface that mesh with corresponding grooves on the crankshaft and camshaft pulleys. These teeth ensure precise synchronization between the rotation of the crankshaft and camshaft, which is essential for the correct timing of the engine&#;s valves in relation to the movement of the pistons.

The phrase "timing belt" usually refers to a rubber toothed belt. The advantages of timing belts are typically a lower cost, reduced friction losses,[citation needed] less noise and that belts traditionally do not require lubrication.[4] The main disadvantage is that belts wear over time, therefore belt replacement is recommended at specific intervals.[5][6] Replacement of the engine's water pump at the same time is often recommended, since the water pump is also subject to wear and easily accessed during the replacement of the timing belt.

Timing belts are typically located in front of the engine and are often behind a cover for protection against dust and debris. However a few engines since have used "wet timing belts", whereby the belt is lubricated by engine oil to reduce friction.[7] In some engine designs the timing belt may also be used to drive other components, such as the water pump and oil pump.

Construction

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A timing belt is typically made from rubber, although some belts are instead made from polyurethane or neoprene.[8][9] The structure of the belt is reinforced with corded fibres (acting as tension members)[10] and the toothed surface is reinforced with a fabric covering.[11]

Rubber degrades with higher temperatures, and with contact with motor oil. Thus the life expectancy of a timing belt is lowered in hot or leaky engines. Also, the life of the reinforcing cords is also affected by water and antifreeze, so it is important that belt that can be exposed to water is able to drain the water away quickly.

Older belts have trapezoid shaped teeth leading to high rates of tooth wear.[citation needed] Newer manufacturing techniques allow for curved teeth that are quieter and last longer.

Manufacturer-specification timing belts may stretch at high rpm,[citation needed] retarding the cam and therefore the ignition.[12] Stronger aftermarket belts will not stretch and the timing is preserved.[13] When designing the timing belt, a wider belt increases its strength however a narrower belt reduces weight and friction.[14]

The usual failure modes of timing belts are either stripped teeth (which leaves a smooth section of belt where the drive cog will slip) or delamination and unraveling of the fiber cores. Breakage of the belt, because of the nature of the high tensile fibers, is uncommon.[15] Often overlooked, debris and dirt that mix with oil and grease can slowly wear at the belt and materials advancing the wear process, causing premature belt failure.[16]

History

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Toothed belts were invented in the early s, for use in textile mills.[17] The first known automobile engine to use a timing belt was the American Devin-Panhard racing car, used an engine converted from pushrods to overhead camshafts through the use of a toothed belt made by the Gilmer Company.[18][19] This car won the Sports Car Club of America (SCCA) National Championship in .[20]

The Glas was the first mass-produced vehicle to use a timing belt. The Pontiac OHC Six engine was the first US mass-produced vehicle to use a timing belt,[21][22] while the Fiat Twin Cam engine was the first mass-produced engine to use a timing belt with twin camshafts.

Timing chains

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Are you interested in learning more about Automotive Timing Belts? Contact us today to secure an expert consultation!

Metal timing chains have become more widespread in car engines produced since the s, due to the lack of the regular maintenance task of replacing a rubber timing belt. While the chains themselves are subject to minimal wear, lubrication of the chain or failure of the tensioner and chain guides can cause maintenance issues. By contrast to rubber-based timing belts which give no indication of snapping while one is driving, early symptoms of a timing chain system becoming worn exist, and these include a rattling noise from the front of the engine.[23]

Most pushrod engines, where the crankshaft and camshaft are very close together, 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.[citation needed]

Timing gears

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Timing gears are used in various overhead valve engines, due to the close proximity of the camshaft to the crankshaft.

Fiber or nylon covered gears, with more resilience, are often used instead of steel gears where direct drive is used. However, commercial engines and aircraft engines use steel gears only, as these other materials can fail suddenly and without warning.[24]

Effects of failure

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Two valves bent due to a broken timing belt

Failure of the timing system will prevent an engine from running. Many modern cars use interference engines, which can suffer catastrophic damage in the event of timing system failure,[25] since the loss of synchronization between the crankshaft and the camshaft will cause the valves to collide with the pistons.

Warning signs that a timing chain needs to be replaced include:[26]

• Later model VVT applications will generate engine codes and check engine lights prior to engine rattle
• Older, pre-VVT applications will generate engine rattle

Aside from the belt/chain itself, also common is a failure of the tensioner, and/or the various gear and idler bearings, causing the belt/chain to derail. Furthermore, in engines where the timing belt drives the water pump, failure of the water pump can cause the pump to seize, which can break the timing belt or chain. For this reason, timing belts and chains are often sold as part of a kit with the water pump, tensioner, and idler pulleys so that these parts can be replaced to prevent timing belt or chain failure due to failure of these parts.

See also

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References

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Timing Belt and Timing Chain &#; A brief history

A car&#;s engine design is beautiful. Many components inside the car have to move and rotate and move in perfect time and symmetry to get the engine to run without any flaws. Chains and belts are like bridges that connect integral parts of the engine to others. This is important because, for the engine to function, crankshafts and camshafts need to rotate at precise intervals, and chains and belts help achieve this movement.

Brief History

For most internal combustion engines, the crankshaft, that holds the pistons, spins around. The crankshaft further connects to the camshaft that opens and closes the valves. There are various ways you can connect the camshaft to the crankshaft, one of them being the timing chains and timing belts.

When timing chains and belts did not exist, gears did the job of synchronizing shaft rotations in combustion engines. However, as time progressed, designers realized that they were not as effective for car engines. Even though the gear system was solid and reliable, but it has two disadvantages.

One of them was that the manufacturing of the gears was very costly, and the other was that the gears were loud. Hence, instead of having multiple gears fit into each other, car engine makers resorted to using timing chains.

With the introduction to timing chains, there was a shift in car engine design. In this car design, the crankshaft sprocket had teeth on it, which the timing chain could attach to. Following that, all American engines came with timing chains.

However, like everything, evolution has to take its course. Timing chains, although solid and durable, were also expensive to mold since they consisted of metal. This led to the introduction of rubber belts. These timing belts were rubber base, and connected to the crankshaft and camshaft, and did the same job as timing chains.

Why Some Cars have Timing Belts and Others have Timing Chains?

Many car manufacturers install timing belts instead of timing chains. One of the reasons why this is so is because they are cheaper to make. Toyota relies heavily on timing belts instead of chains because not only are they reliable in the Toyota engine, but also because every engine they make is non-interference.

This means that if the timing belt breaks, it will not cause the pistons to do any damage. However, if you take faster engine cars, the pistons are higher and the values were lower. This meant that if the rubber belts broke when driving fast, this would cause damage to the engine.

Both, the rubber timing belts, and metallic timing chains serve the same purpose but have different advantages and disadvantages. Many people can argue whether belts are better than chains, but if you want to get a car that requires the least expenditure and maintenance, then look for one with a timing chain. Chains, unlike rubber, do not deteriorate over time and last longer.

Contact us to discuss your requirements of Audi Timing Belt Replacement. Our experienced sales team can help you identify the options that best suit your needs.