The toothed, or synchronous timing belt is used for driving camshafts, balance shafts, water pumps and diesel injection pumps.
It has an inner woven core made from fiberglass, Kevlar, or steel braid, coated with synthetic rubber or neoprene. The teeth, which may be square or curved, are molded to close tolerances to match the drive teeth on the crankshaft and timing gears. A molded plastic cover protects the belt from oil or water contamination.
Timing belts have a high working efficiency due to the low friction properties of their construction. This means they require no lubrication and are silent in operation. Timing belts are inexpensive to manufacture, they're heat and wear resistant and have a service life of 50,000 to 100,000 kilometers or 30,000 to 60,000 miles.
Although it stretches little in use, the tension of the timing belt is important. This is normally set with an adjustable idler pulley that applies tension via a spring. This pulley is fixed to the engine by a fastener. Adjustment is performed manually after the timing belt is installed.
Some manufacturers use a spring and oil damper as an automatic belt tensioner. This type of tensioner is effective at reducing timing belt chatter noise as the belt is always under pressure, even as it stretches. A heavy spring acts against a piston attached to a tensioner pushrod. This is mounted so that the tension pulley can apply pressure perpendicular to the back face of the belt. The cylinder is filled with silicone oil, and ball valves allow the piston to be forced out by the spring but prevent the piston from moving rapidly inwards. In operation the spring provides the force that keeps the timing belt tensioned, and the piston valves prevent loss of tension.