A timing belt, timing chain or cambelt is a part of an internal combustion engine that synchronizes the rotation of the crankshaft and the camshaft(s) so that the engine's valves open and close at the proper times during each cylinder's intake and exhaust strokes. In an interference engine the timing belt or chain is also critical to preventing the piston from striking the valves. A timing belt is usually a toothed belt -- a drive belt with teeth on the inside surface. A timing chain is a roller chain.
Many modern production automobile engines use a timing belt[i] to synchronize crankshaft and camshaft rotation; some engines, particularly cam in block designs, used gears to drive the camshaft, but this was rare for OHC designs. The use of a timing belt or chain instead of gear drive enables engine designers to place the camshaft(s) further from the crankshaft, and in engines with multiple camshafts a timing belt or chain also enables the camshafts to be placed further from each other. Timing chains were common on production automobiles through the 1970s and 1980s, when timing belts became the norm, but timing chains have seen a resurgence in recent years. Timing chains are generally more durable than timing belts - though neither is as durable as gear drive - however, timing belts are lighter, less expensive, and operate more quietly.
In the internal combustion engine application the timing belt or chain connects the crankshaft to the camshaft(s), which in turn control the opening and closing of the engine's valves. A four-stroke engine requires that the valves open and close once every other revolution of the crankshaft. The timing belt 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 designs the timing belt may also be used to drive other engine components such as the water pump and oil pump.
Gear or chain systems are also used to connect the crankshaft to the camshaft 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 or nylon covered gears, with more resilience, are often used instead of steel gears where direct drive is used. Commercial engines and aircraft engines use steel gears only, as a fibre or nylon coated gear can fail suddenly and without warning.
A belt or chain allows much more flexibility in the relative locations of the crankshaft and camshafts.
While chains and gears may be more durable, rubber composite 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 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, 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.
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 engines. 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.
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, especially in interference engines. The owner's manual maintenance schedule is the source of timing belt replacement intervals, typically every 30,000 to 50,000 miles (50,000 to 80,000 km). 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. Often overlooked, debris and dirt that mix with oil and grease can slowly wear at the belt and materials advancing the wear process, causing premature belt failure. 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.
A timing belt is typically rubber with high-tensile fibres (e.g. fiberglass or Twaron/Kevlar) running the length of the belt as tension members. The belt itself is constructed in sturdy materials such as molded polyurethane, neoprene or welded urethane with various standard, non-standard or metric pitches. The distance between the centers of two adjacent teeth on the timing belt is referred to as the pitch.
Rubber degrades with higher temperatures, and with contact with motor oil. 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" (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 shaped teeth leading to high rates of tooth wear. Newer manufacturing techniques allow for curved teeth that are quieter and last longer.
Aftermarket timing belts may be used to alter engine performance. OEM timing belts may 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".
The first known timing belt was used in 1945. In the 1950s, Bill Devin built a Panhard racing special with a highly modified engine using Norton Manx cylinders and heads and a toothed rubber belt driving the overhead camshaft on each Norton cylinder head. Devin's Panhard special won the Sports Car Club of America (SCCA) National Championship in 1956.
The 1962 Glas 1004 was the first mass-produced vehicle to use an overhead camshaft engine with a timing belt.Fiat's 124 twin cam engine developed in the early 1960s was the first mass production engine to have belt-driven twin camshafts. In 1966, Vauxhall Motors started production of the Slant Four single overhead cam four-cylinder design which used a timing belt, a configuration which became one of the most common.
A timing chain, also called a sync chain, is used on tandem bicycles, to synchronize crank movement between the captain (front rider) and stoker (rear rider). As with auto engines, proper tension of the timing chain is necessary to prevent premature wear of the drivetrain components. Tension is adjusted by means of an eccentric bottom bracket.
Example of an Audi belt that broke before the recommended replacement interval.
In earlier engines, camshafts were often gear-driven off the crankshaft. Later, powerplant designers developed chain drives in OHV (overhead valve) configurations that allowed some flexibility in the placement of the camshaft so that shorter pushrods could be used, for more performance and efficiency. Those engines with long chains sometimes tended to whip about and cause problems. The only alternative was a noisy and complicated multi-gear train until the cogged rubber synchronous timing belt was invented in 1945. Though it was once considered the hallmark of a cheap engine, now it is used in distinguished automobiles such as Acuras, Volvos and Porsches.