TIMING BELT and TIMING CHAIN

Timing Belt and Timing Chain

Timing Belt and Timing Chain

Belt and Chain Drive System

A synchronous belt drive system is currently the most commonly used technology in Europe for modern engine overhead camshaft drives. Great efforts on material development and drive systems design optimisation have been made over the past years to enable life time timing belt drive systems. Many timing belt drives capable for engine life time have been successfully introduced to the market for several years.

Timing belt drives have significant advantages when compared to chain drives.

Some examples are excellent elongation behaviour and good damping characteristics under load, which allow a timing belt to achieve an accurate control of the camshaft timing over the entire engine lifetime. This guarantees the highest efficiency with respect to fuel consumption and emissions.

Another important aspect is the ability for a belt drive system to incorporate additional components such as water pumps and injection pumps within a single layer drive. This leads to significant packaging advantages.

This article deals with two additional fundamental advantages of a belt drive system against a chain drive, namely reduced friction losses and better acoustic behaviour.

The main purpose of both timing belt and chain drives is to drive the camshaft of combustion engines synchronously with to the crankshaft. Each error in synchronization between camshaft and crankshaft causes deterioration of fuel consumption and emissions.

General design requirements are similar for both technologies. All drives have to be durable, compact, silent, and efficient and must be optimized for system and maintenance costs.

Automotive timing belts are a composite of heat resistant Nylon fabric, heat resistant rubber compound and high tensile glass cord. Due to the nature of this product and the involved material combination it is able to compensate high dynamic shock loads without permanent elongation and wear.

Conversely a chain, constructed of numerous metal links has a very high stiffness and minimal damping characteristic. This leads to an inability to resist high dynamic shock loads without wear and irreversible elongation.

Chain elongation caused by wear over time (up to 10 times more than a belt) is very critical to emission control. An example for this behaviour is given by the use of timing belts in "unit injector" diesel engines. On such high dynamic, highly loaded systems, chain technology cannot compete with timing belts.

Tensioners in the slack side span maintain the system tension and compensate for wear and elongation of either belt or chain over the system life time. On belt drive systems the tension is applied by hydraulic or spring loaded devices through plastic or steel pulleys utilizing ball bearings. All belt tensioning devices are characterized by the ability to operate fully independent from the engine oil circulation. On chain systems the tension is applied to the chain by hydraulic tensioners, in most cases through plastic friction guides. Chain tensioners need a direct connection to the engine oil pressure system.

Chain drives require all spans to be guided by additional guides to minimize chain span vibrations. Belt drive systems require guidance by additional pulleys only ...