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A timing belt, timing chain or cam belt 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 a belt that usually features teeth on the inside surface, while a timing chain is a roller chain. Most modern production automobile engines use a timing belt or chain to synchronize crankshaft and camshaft rotation; some engines instead use gears to directly drive the camshafts. The use of a timing belt or chain instead of direct 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 direct gear drive – however, timing belts are lighter, less expensive, and operate more quietly. ==Engine applications== 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Timing belt (camshaft)」の詳細全文を読む スポンサード リンク
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