page_title: Gear train
section_title: Belt drives
hierarchical_section_title: Gear train / Belt drives
caption_attribution_description: Front view of timing gears in a Ford Taunus V4 engine in a SAAB 96. The (fibre) balance shaft gear (lower left) has lost its teeth due to bearing failure. The other small (steel) gear is the crankshaft gear. The largest (fibre) gear is the camshaft gear. It has twice the diameter of the crankshaft gear so it rotates at half the speed.
context_page_description: A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
  Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission of rotation from one gear to the next.
  The transmission of rotation between contacting toothed wheels can be traced back to the Antikythera mechanism of Greece and the south-pointing chariot of China. Illustrations by the Renaissance scientist Georgius Agricola show gear trains with cylindrical teeth. The implementation of the involute tooth yielded a standard gear design that provides a constant speed ratio.
  Features of gears and gear trains include:
  The ratio of the pitch circles of mating gears defines the speed ratio and the mechanical advantage of the gear set.
  A planetary gear train provides high gear reduction in a compact package.
  It is possible to design gear teeth for gears that are non-circular, yet still transmit torque smoothly.
  The speed ratios of chain and belt drives are computed in the same way as gear ratios. See bicycle gearing.
context_section_description: Belts can have teeth in them also and be coupled to gear-like pulleys. Special gears called sprockets can be coupled together with chains, as on bicycles and some motorcycles. Again, exact accounting of teeth and revolutions can be applied with these machines.
  For example, a belt with teeth, called the timing belt, is used in some internal combustion engines to synchronize the movement of the camshaft with that of the crankshaft, so that the valves open and close at the top of each cylinder at exactly the right time relative to the movement of each piston. A chain, called a timing chain, is used on some automobiles for this purpose, while in others, the camshaft and crankshaft are coupled directly together through meshed gears. Regardless of which form of drive is employed, the crankshaft-to-camshaft gear ratio is always 2:1 on four-stroke engines, which means that for every two revolutions of the crankshaft the camshaft will rotate once.
caption: Valve timing gears on a Ford Taunus V4 engine — the small gear is on the crankshaft, the larger gear is on the camshaft. The crankshaft gear has 34 teeth, the camshaft gear has 68 teeth and runs at half the crankshaft RPM.
  (The small gear in the lower left is on the balance shaft.)
exif: {}