Timing belt
Encyclopedia
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
that controls the timing of the engine's valves
Poppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
. Some engines, such as the flat-4
Flat-4
A flat-4 or horizontally opposed-4 is a flat engine with four cylinders arranged horizontally in two banks of two cylinders on each side of a central crankcase...
Volkswagen air-cooled engine, and the straight-6 Toyota F engine
Toyota F engine
The Toyota F series engine was a series of OHV inline-6 cylinder engines produced by Toyota between 1955-1992. They are known for their high amount of torque at low RPM, massive cast iron blocks and heads and also their high reliability. The F Engine had one of the longest production runs of any...
use timing gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
s. Timing belts replaced the older style timing chains that were common until the 1970s and 1980s (although in the last decade there has been reemergence of chain use for many carmakers). Some manufacturers, such as BMW
BMW
Bayerische Motoren Werke AG is a German automobile, motorcycle and engine manufacturing company founded in 1916. It also owns and produces the Mini marque, and is the parent company of Rolls-Royce Motor Cars. BMW produces motorcycles under BMW Motorrad and Husqvarna brands...
, are known for utilizing timing chains, because of their durability. The term "timing belt" is sometimes used for the more general case of any flat belt
Belt (mechanical)
A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the...
with integral teeth, although such usage is a misnomer since there is no timing or synchronization involved.
Engine applications
In the internal combustion engine application, the timing belt/chain connects the crankshaftCrankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
to the camshaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
(s), which in turn controls the opening and closing of the engine's valves. A four-stroke engine
Four-stroke cycle
A four-stroke engine, also known as four-cycle, is an internal combustion engine in which the piston completes four separate strokes—intake, compression, power, and exhaust—during two separate revolutions of the engine's crankshaft, and one single thermodynamic cycle.There are two...
requires that the valves open and close once every other revolution of the crankshaft. The timing belt/chain does this. It has teeth to turn the camshaft(s) synchronised with the crankshaft, and is specifically designed for a particular engine. In some engine design
Design
Design as a noun informally refers to a plan or convention for the construction of an object or a system while “to design” refers to making this plan...
s, the timing belt may also be used to drive other engine components such as the water pump and oil pump (internal combustion engine)
Oil pump (internal combustion engine)
The oil pump in an internal combustion engine circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine...
.
Gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
or chain
Roller chain
Roller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire and tube drawing machines, printing presses, cars, motorcycles, and simple machines like...
systems are also used to connect the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
to the camshaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
at the correct timing. However, gears and shafts constrain the relative location of the crankshaft and camshafts. Even where the crankshaft and camshaft(s) are very close together, as in pushrod engines, most engine designers 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. Fibre gears, with more resilience, are preferred to steel gears where direct drive has to be used. A belt or chain
Roller chain
Roller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire and tube drawing machines, printing presses, cars, motorcycles, and simple machines like...
allows much more flexibility in the relative locations of the crankshaft and camshafts.
While chains and gears may be more durable, rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
composite
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
belts are quieter in their operation (in most modern engines the noise difference is negligible), are less expensive and more efficient, by dint of being lighter, when compared with a gear or chain system. Also, timing belts do not require lubrication, which is essential with a timing chain or gears. A timing belt is a specific application of a synchronous belt
Belt (mechanical)
A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the...
used to transmit rotational power synchronously.
Timing belts are typically covered by metal or polymer timing belt covers which require removal for inspection or replacement. Engine manufacturers recommend replacement at specific intervals. The manufacturer may also recommend the replacement of other parts, such as the water pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
, when the timing belt is replaced because the additional cost to replace the water pump is negligible compared to the cost of accessing the timing belt. In an interference engine, or one whose valves extend into the path of the piston, failure of the timing belt (or timing chain) invariably results in costly and, in some cases, irreparable engine damage, as some valves will be held open when they should not be and thus will be struck by the pistons.
Indicators that the timing chain may need to be replaced include a rattling noise from the front of the engine.
Timing
When an automotive timing belt is replaced, care must be taken to ensure that the valve and piston movements are correctly synchronized. Failure to synchronize correctly can lead to problems with valve timing, and this in turn, in extremes, can cause collision between valves and pistons in interference enginesInterference Engines
An interference engine is a type of 4-stroke internal combustion piston engine. Depending on the design of an engine, piston and valve paths may "interfere" with one another as a result of incorrect timing in their movements. An interference engine is a type of 4-stroke internal combustion piston...
. This is not a problem unique to timing belts since the same issue exists with all other cam/crank timing methods such as gears or chains. To help alleviate a timing belt from being incorrectly installed, the timing belt had painted marks on the surface of the belt that line up with markings on the camshaft gears and crankshaft gear so that the belt is perfectly aligned and properly positioned.
Failure
Timing belts must be replaced at the manufacturer's recommended distance and/or time periods. Failure to replace the belt can result in complete breakdown or catastrophic engine failure. The owner's manual maintenance schedule is the source of timing belt replacement intervals, typically every 60,000 to 90,000 miles (approx 96,000 to 144,000 kilometres). It is common to replace the timing belt tensioner at the same time as the belt is replaced.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. Correct belt tension is critical - too loose and the belt will whip, too tight and it will whine and put excess strain on the bearings of the cogs. In either case belt life will be drastically shortened. Aside from the belt itself, also common is a failure of the tensioner, and/or the various gear and idler bearings, causing the belt to derail.
Construction and design
A timing belt is typically rubber with high-tensileTension (mechanics)
In physics, tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression. As tension is the magnitude of a force, it is measured in newtons and is always measured parallel to the string on which it applies...
fibres (e.g. fiberglass
Fiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...
or Twaron
Twaron
Twaron is the brandname of Teijin Aramid for a para-aramid. It is a heat-resistant and strong synthetic fibre developed in the early 1970s by the Dutch company AKZO, division Enka, later Akzo Industrial Fibers. The research name of the para-aramid fibre was originally Fiber X, but it was soon...
/Kevlar
Kevlar
Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed at DuPont in 1965, this high strength material was first commercially used in the early 1970s as a replacement for steel in racing tires...
) running the length of the belt as tension members.
Rubber degrades with higher temperatures, and with contact with motor oil
Motor oil
Motor oil or engine oil is an oil used for lubrication of various internal combustion engines. The main function is to lubricate moving parts; it also cleans, inhibits corrosion, improves sealing, and cools the engine by carrying heat away from moving parts.Motor oils are derived from...
. Thus the life expectancy of a timing belt is lowered in hot or leaky engines. Newer or more expensive belts are made of temperature resistant materials such as "highly saturated nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
" (HSN). The life of the reinforcing cords is also greatly affected by water and antifreeze. This means that special precautions must be taken for off road applications to allow water to drain away or be sealed from contact with the belt.
Older belts have trapezoid
Trapezoid
In Euclidean geometry, a convex quadrilateral with one pair of parallel sides is referred to as a trapezoid in American English and as a trapezium in English outside North America. A trapezoid with vertices ABCD is denoted...
shaped teeth leading to high rates of tooth wear. Newer manufacturing techniques allow for curved teeth that are quieter and last longer.
Aftermarket
Aftermarket (automotive)
The automotive aftermarket is the secondary market of the automotive industry, concerned with the manufacturing, remanufacturing, distribution, retailing, and installation of all vehicle parts, chemicals, tools, equipment and accessories for light and heavy vehicles, after the sale of the...
timing belts may be used to alter engine performance
Engine tuning
Engine tuning is the adjustment, modification or design of internal combustion engines to yield optimal performance, to increase an engine's power output, economy, or durability....
. OEM timing belts "will stretch at high rpm, retarding the cam and therefore the ignition. Stronger, aftermarket belts, will not stretch and the timing is preserved. In terms of engine design, "shortening the width of the timing belt reduce[s] weight and friction".
Usage history
The first known timing belt was used in 1945.The German Glas 1004 was the first mass produced vehicle to use a timing belt in 1962. The first American vehicle to use a timing belt was the 1966 Pontiac Tempest
Pontiac Tempest
The Pontiac Tempest was an entry-level compact produced by the Pontiac Motor Division of General Motors, introduced in September 1960 for the 1961 model year....
. In 1966, OPEL started production of the Slant Four
Slant Four
The Slant Four is a type of car engine manufactured by Vauxhall Motors and in modified form by Lotus Cars. Unveiled in 1966, it was one of the first production overhead camshaft designs to use a rubber toothed belt to drive the camshaft from the crankshaft , a method developed in 1956 by Bill...
overhead cam four-cylinder design which used a timing belt, a configuration that is now used in the vast majority of cars built today.