Timing chain
The timing chain is a component of the camshaft drive in four-stroke engines . It connects the crankshaft with the camshaft .
use
The camshaft of four-stroke engines must be driven at half the crankshaft speed - the situation is different when driving the cam drum of radial engines . A timing chain is required wherever the camshaft is not driven by other machine elements such as push rods ( NSU Max ), toothed belts , vertical shafts or spur gears.
history
Due to the elaborate manufacturing process, vertical shafts are mostly reserved for motorsport. The roller chain as a timing chain was a standard component in engines from around 1910. The toothed belt has been the most frequently used drive element since the early 1970s. However, individual manufacturers are sticking to the timing chain for traditional reasons. Mercedes-Benz z. B. fundamentally rejects the timing belt and has only used timing chains since the 1950s, before that also spur gears and vertical shafts , as did Porsche for the boxer engines.
Roller chains are predominantly used as timing chains, earlier also sleeve chains and tooth chains are used in aircraft engines and US OHV engines.
Function and wear
Timing chains must run tight. For this purpose, chain tensioners (tensioning rails) are used, which are spring-loaded or operated by oil pressure to ensure the correct tension and on the free running lengths - called "slack strand" (the strand that transfers the forces is the "load strand") - the swing of the chain in the bending direction ( Prevent "whipping").
The slide rails are wearing parts that should be replaced after about 120,000 to 300,000 km. If the tensioning mechanism fails, the chain can skip, and in many engines the pistons collide with the valves and the result is engine damage .
The service life of a timing chain depends heavily on the operating conditions and the maintenance of its tensioning and guide elements. The highly stressed single roller chains are partly worn after 80,000 to 100,000 km. Duplex chains are more durable and can achieve mileages of over 500,000 km. The wear of a timing chain can be determined by the automatic change of the engine timing within a certain mileage. The timing chain becomes longer as the rollers and chain links wear out. However, the post-tensioning causes the camshaft sprocket to twist in relation to the crankshaft sprocket. The angular offset is dependent on the change in length of the timing chain. The engine therefore runs more restlessly with a heavily worn timing chain because the gas exchange is staggered. If the chain is very badly worn, the pitch no longer matches the gears. The rollers then run against the teeth during operation and cause not only a louder noise but also heavy wear on the gears.
An example of an engine with frequent problems with elongated timing chains is the EA 111 from VW. The resulting engine failures often occur after a mileage of 60,000 km.
literature
- Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden 2005, ISBN 3-528-23933-6
- Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag GmbH, Braunschweig 2000, ISBN 3-14-221500-X
