Timing pulleys are used with timing belts and have teeth for driving power. They come in Gilmore and HTD styles. Timing belts are preferred due to their slim construction and lack of heat retention. Gilmore belts are popular for high performance applications due to the whining noise they make. Measuring a timing belt is different from a V-belt, and tensioning is achieved through the pitch of the sprocket pulleys.
A timing pulley is the type of pulley used with a timing belt. Looking very much like any pulley with teeth or gears machined into the outer perimeter instead of the more common deep V associated with a common V-type fan belt, the timing pulley relies on the teeth to provide adequate driving power instead of sealing and of friction used with V-belts. Often used in automobile and machinery designs, the timing pulley comes in two common tooth styles, a Gilmore style and a High Torque Drive (HTD) design. A Gilmore tooth is a flat, square tooth design with a typically very shallow cut in the pulley, while the HTD tooth is a deeper, 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 the lighter reciprocating weight compared to a chain or gear drive. To drive a toothed belt, the device requires a toothed pulley. Timing belts are preferred over other belts of a similar size due to their slim construction style. The thin belt does not retain heat the same way as a thick rubber belt, while still providing equal or superior strength.
In high performance applications, the Gilmore-style belt is often the belt of choice due to the whining noise it makes when moving across the surface of a sprocket pulley. Often called a fan hiss because of the famous sound a Gilmore belted supercharger makes on a high performance engine, the sound is actually created by the air trapped under the belt and not by the belt or sprocket. Some pulleys are drilled to place a small hole in each groove or tooth on the pulley to allow trapped air to escape, thus greatly eliminating sound.
Measuring the fit of a timing belt is very different from measuring a V-belt. A V-belt is sized by measuring the pulleys to the depth of the V. The timing pulley is measured around the flat side of the pulleys to get a size of the belt. The sprocket is also machined at an angle with two pulleys using opposite angles, known as a pitch. Instead of tensioning the belt by pulling the pulleys apart, a toothed belt is held in place by sliding over two differently angled pulleys; this pitch allows the belt to attempt to slip off one pulley while the other sprocket pulley pulls the belt back into place.
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