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A toothed pulley is the type of pulley used with a cog belt. Looking much like any pulley with teeth or cogs machined into the outside perimeter in place of the more common deep V associated with a common V type fan belt, the toothed pulley relies on the teeth to provide adequate drive power instead of the tightness and friction used with a V belt. Often used in automotive and machinery designs, the toothed pulley is available in two common tooth styles, a Gilmore style and a high torque drive (HTD) design. A Gilmore tooth is a flat and square tooth design with a typically very shallow cut in the pulley, while the HTD tooth is a deeper and semi-rounded tooth profile that allows more power to be applied without slipping the belt.
In many applications, belts are the preferred method of powering or driving a machine or component. This is due to lighter reciprocating weight as compared to a chain or gear drive. In order to drive a toothed belt, the device requires a toothed pulley. Toothed belts are preferred over other similarly-sized belts for their thin construction style. The thin belt does not hold heat in the same manner that a thick rubber belt does while providing equal or superior strength.
In high-performance applications, the Gilmore-type belt is often the preferred belt due to the whining noise it makes when in motion over the surface of a toothed pulley. Often called a blower whine due to the famous sound a Gilmore belt-equipped supercharger makes on a performance engine, the sound is actually created by the air trapped underneath the belt and not from the belt or toothed pulley. Some pulleys are drilled to place a small hole in each groove or tooth of the pulley to allow the trapped air to escape, thus greatly eliminating the sound.
Measuring the fit of a toothed belt is much different than measuring a V-type belt. A V-style belt is sized by measuring the pulleys at the depth of the V. The toothed pulley is measured around the flat side of the pulleys to arrive at a belt size. The toothed pulley is also machined at an angle with two pulleys using opposed angles, known as a pitch. Instead of tightening the belt by forcing the pulleys apart, a cog belt is held in place by running on the two differently-angled pulleys; this pitch allows the belt to attempt to slide off of one pulley while the other toothed pulley slides the belt right back into position.