Timing belt
Toothed belts (also called synchronous belts or timing belts in engine technology) are drive belts with teeth that run positively in toothed pulleys . Timing belts combine the properties of a chain and a flat belt .
Structure and functionality
On the inside of the belt, teeth are molded from an elastomer that mesh with a special gear . The materials used for the teeth are rubber, chloroprene rubber , hydrogenated acrylonitrile butadiene rubber ( HNBR ) or plastic ( polyurethane ).
This design has the following advantages over V- belts or flat belts , which only work with a force fit :
- Due to the positive fit of the teeth, high forces can be transmitted with lower preload.
- Because the toothing prevents slippage , toothed belts can also be used for control.
The force in the toothed belt itself is transmitted through the embedded tensile cord, which is stiff compared to the elastomer and usually consists of glass or aramid fibers , less often of steel cables. The inside of the toothed belt is coated with an abrasion-resistant fabric to protect the teeth from wear.
In the case of special shapes, teeth are applied to the toothed belt both inside and outside. These can each have different distances. With suitable deflections and tooth shapes, very different behavior of the transmission is possible. Toothed belts can be combined with worms and racks .
Timing belts are standardized in:
- DIN ISO 5296: tooth pitches MXL, XXL, XL, L, H, XH, XXH (trapezoidal toothing in inch pitches)
- DIN 7721: tooth pitches T 2.5, T 5, T 10, T 20 (trapezoidal toothing in metric pitches)
- ISO 13050: tooth pitches H8M, H14M, S8M, S14M, R8M, R14M (curved teeth in metric pitches)
In sewing machines around 1960, there were also toothed belts made from a ribbon-shaped roll of cord with C-brackets made of round steel wire as teeth that were pressed on comprehensively at intervals.
The teeth of a toothed belt run transversely to its circumferential direction, similar to the rungs of a ladder. A grooved belt ( V-ribbed belt ), however, has to engage with a pulley on its inner side along a plurality of parallel grooves. The cross-cut grooved belt has a zig-zag profile and is a type of V-belt.
use
Toothed belts are generally used for two purposes:
- Transmission of large belt forces where V-belts or flat belts would slip (e.g. as a replacement for chain drives)
- Adjusting drives in which the teeth prevent the drive wheel and the driven wheel from rotating with respect to each other.
Toothed belts as actuators
Valve control in four-stroke engines
A typical application for polymer fabric timing belts is as an alternative to timing chains for driving the camshaft (s) of four-stroke engines . The angular position of the camshaft and crankshaft must not be changed to ensure that the timing is exactly adhered to . For the first time, a series engine with toothed belt drive was used from 1962 in the Glas 1004 ; before that, the principle had already been used in the Devin Panhard racing car .
Advantages over timing chains
- Low noise
- Long service life with low loads
- Lower mass, therefore higher speeds possible
- Because it is attached outside the motor, it can be easily replaced and checked for damage
- No lubrication necessary (self-lubricating)
- Closer wrap angles possible
- No expensive (oil pressure loaded) chain tensioners required
- Hardly any elongation during the service life, therefore precise control
- Lower costs
Disadvantages compared to timing chains
- Higher wear as it is only self-lubricating
- Lower load limit / elongation at break (can suddenly tear)
- Belt tension must be checked regularly
- Due to the lack of resistance to oil, it can only be attached to the outside of the engine, therefore longer overall length of the engine (additional shaft seals required), unless it is a toothed belt (made of special elastomer) in oil, such as in the Ford EcoBoost three-cylinder engine with 1, 0 liter displacement - used in the Ford Focus from 2012 onwards .
- Higher maintenance costs (regular replacement every 40,000 to 240,000 km, many engines require time-consuming and costly work: draining the coolant , removing the radiator, coolant pump , alternator, etc.)
- Not resistant to aging
If the toothed belt breaks, the valves are opened at the wrong time or remain open and can collide with the piston crown (not with " freewheels "). This can lead to severe damage up to a total loss. For this reason, engines with particularly heavily loaded valve trains are not equipped with toothed belts. They use timing chains , vertical shafts or spur gears , the reliability of which is considerably higher. Valve drives with toothed belts and / or chains are not used in aircraft engines .
Tool and mechanical engineering
Toothed belts are used in positioning drives because they work reproducibly due to the form-fitting (synchronous) coupling. They can be designed largely free of play and - if they have a steel insert - hardly any load-dependent positional deviation. Typical fields of application for toothed belt drives are positioning systems with servomotors , handling modules and other linear drives that are used, for example, in printers , packaging machines or in industrial robots .
Toothed belts as drive belts
If a toothed belt is used less because of its precise positioning properties, but rather the transmission of torque is in the foreground, it is called a drive belt. In motorcycles, for example, toothed belts are increasingly used to drive the rear wheel, for example in Harley-Davidson , OCC , Buell , Kawasaki and the BMW F series . The advantage over conventional steel chains is that there is no need for lubrication, which means less maintenance and a longer service life. Furthermore, due to the elasticity of the belt, the drive can be designed without play. The wider design and the somewhat higher loss of power compared to a steel chain are hardly significant in the case of powerful engines.
Timing belts are also used to drive bicycles , but despite their advantages (largely maintenance-free, 2–3 times longer service life than a chain drive under the same conditions, efficiency equal to a new chain), they have a niche existence in Europe. To maintain the necessary belt tension, belt tensioners (modeled after chain tensioners) or chain tensioners integrated in the bottom bracket are used. Bicycle timing belts are usually combined with hub or bottom bracket circuits; but belt circuits are also conceivable. When cycling, comparatively little continuous power is transmitted, but very high starting forces occur which strain a toothed belt, which proportionally reduces the efficiency and causes both plastic toothed pulley and belt to wear out.
Others
Rubber crawlers , as used in small construction machines and model vehicles, are constructed in a similar way, but are not referred to as toothed belts.
literature
- Thomas Nagel: toothed belt drive. Properties, standardization, calculation, design . Hanser, Munich 2008, ISBN 978-3-446-41380-1
- Raimund Perneder: Manual of toothed belt technology. Basics, calculation, applications . Springer, Berlin 2009, ISBN 978-3-540-89321-9
- Autodata (Ed.): Zahnriemen 2010. Petrol and diesel vehicles 2000–2010. - Examination, recruitment, exchange
Web links
- 14th conference "Toothed belt drive", page 9 , TU Dresden, Institute for Precision Engineering and Electronic Design, 29./30. September 2010; accessed February 17, 2018
See also
Individual evidence
- ↑ 10th conference "Toothed belt drives", p. 2 (TU Dresden, 13/14 September 2005)
- ^ Toothed belts in oil, Christiane Brünglinghaus, November 21, 2013 at Springer Professionell , accessed on February 17, 2018
- ↑ When the timing belt ages, available from Hans W. Mayer, January 19, 2010 at faz.net, February 17, 2018