Toothed belt
Timing ( in engine technology and synchronous belt or timing belt known ) are drive belts with teeth that run positively in toothed pulleys. They combine the properties of a chain and of a flat belt.
Design and operation
On the inside of the belt teeth of rubber ( chloroprene rubber, HNBR) or plastic ( polyurethane) are formed which engage in a particular gear. This design has over Flat or V- belt that work only with force concluded the following advantages:
- By the positive engagement of the teeth are higher forces transferable
- Because the slip teeth makes impossible toothed belt may also be used to control
The actual force is transmitted by the embedded in the toothed belt tension cord, which is usually made of glass or aramid fibers ( rarely from steel cables ). The inside of the toothed belt is made of abrasion -resistant fabric in order to protect the existing elastomeric teeth from wear.
For special shapes are both internally and externally applied to the timing belt teeth. These can have different distances. By suitable deflections and tooth shapes very different behavior of the transmission is possible. Combinations with snails and racks are possible.
Timing belts are eg standardized in:
- DIN ISO 5296: tooth pitches MXL, XXL, XL, L, H, XH, XXII ( trapezoidal tooth in inch graduations ).
- DIN 7721: tooth pitches T 2.5, T 5, T 10, T 20 ( trapezoidal teeth in metric divisions).
- ISO 13050: tooth pitches H8M, H14M, S8M, S14M, R8M, R14M ( curved teeth in metric divisions).
Use
Timing belts are generally used for two purposes:
- For the transfer of large belt forces, where Flat or V- belt would slip through (for example, as a substitute for chain drives ), or
- For adjusting drives, in which the teeth prevents twisting drive wheel and driven wheel to each other.
Timing belts for adjusting drives
Valve control with four-stroke engines
A typical application of the polymer fabric of the toothed belt is used as an alternative to the timing chain for driving the camshaft (s ) of four-stroke engines. For accurate timing of any change in the angular position of the camshaft and crankshaft must occur.
Advantages over timing chains
- Noise
- At low load high life
- Lower mass, so higher speeds possible
- By attaching outside of the engine easy replacement and inspection for damage
- No lubrication required ( self-lubricated )
- Closer wrap possible
- No expensive (oil pressure loaded ) tensioner required
- Little elongation during the lifetime, thus precise control
- Lower costs
Disadvantages compared to control chains
- Increased wear, since only self-lubricating
- Lower load limit / elongation at break ( can suddenly tear )
- Belt tension should be checked regularly
- May be due to lack of resistance oil outside only be mounted on the engine, so longer overall length of the motor ( shaft seals additionally required). Exception: the Ford EcoBoost three-cylinder engine with 1.0 liter displacement - from 2012 used in the Ford Focus - has a running in oil bath toothed belt.
- Higher maintenance costs ( regular change all 40000-240000 km, with many engines and time- intensive work is needed to; draining the coolant expansion of radiator, coolant pump, alternator, etc.)
- Not age- determined
Tear the timing belt, the valves are opened at the wrong time or remain open and can use the piston head collide (not with " free runners "). The can cause severe damage or even total loss. These reasons, motors with particularly highly loaded valve trains not equipped with timing belts. use timing chains, bevel or spur gear whose reliability is significantly higher. was the first passenger car series motor with toothed belt drive 1000cc applies the glass of 1962. In aircraft engines find valve actuators with toothed belts and / or chains no use.
Tool and machine
Timing belts are used in positioning drives, since they operate reproducibly due to the positive ( synchronous ) coupling. They can be performed with virtually no clearance and deliver - if they have a steel insert - no load-dependent position deviation. Typical application fields for timing belt drives are positioning with servo motors, handling modules and other linear actuators that are used in printers, packaging machines or in industrial robots.
Timing belt as the driving force
If a timing belt is used less because of its precise floating properties, but the transmission of torque in the foreground, one speaks of being used as a drive belt. With motorcycles, for example, timing belts are used more and more often to drive the rear wheel, so at Harley- Davidson, OCC, Buell, Kawasaki and the BMW F series. The advantage over the conventional steel chains consists in elimination of lubrication, which means less maintenance and longer life. Furthermore, the drive can be constructed without clearance due to the elasticity of the belt. The broader construction and over a steel chain slightly higher power loss fall on strong motors negligible.
Even when driving bikes find their application timing belt, but lead despite its advantages ( largely maintenance-free, change interval: 22000-30000 miles) in Europe a niche. To maintain the required belt tension, find belt tensioner ( chain tensioners modeled ) or integrated into the bottom bracket chain tensioner use. Usually bicycle belts are combined with hub gears; but there are also belt and Tretlagerschaltungen or combinations thereof conceivable. There is also a timing belt circuit.
Others
Rubber crawlers, such as those used in small construction machinery and model vehicles are, though similar in structure, but are not designated as a toothed belt.