Fundamentals of timing pulleys and beltsTemitayo Oketola | October 11, 2021
Ever since Bill Devin invented the first timing belt drive in 1954, the application and demand for timing pulley and belt drives have continued to increase. Engineers typically opt for timing pulley and belt drives over conventional power transmission drives primarily because of their unique design, which offers several advantages.
A timing pulley is a type of pulley configuration that features teeth and pockets on the outside diameter of the pulley’s body. These teeth and pockets mate with a timing belt having the same pitch in a synchronous drive, preventing pulley-belt misalignment and ensuring smooth power transmission.
This article covers the basics of timing pulley and belt drives. It will also present some key design considerations and calculations for specifying timing pulley and belt drives.
Timing pulley and belt advantages
Conventional power transmission pulleys and belts have a smooth surface, making it more likely for the belt to slip off the track of the pulley. They also usually require a significantly great pretension to transmit torque without slip.
Timing pulley and belt drives solve this slippage challenge since they have mating teeth and pockets that keep the belt and pulley firmly attached. Their unique design also gives them a higher torque carrying capacity and allows them to operate with lower noise and vibration than conventional pulley and belt drives. Timing belt and pulley drives ensure a more synchronous rotation when two shafts are linked together. Timing belts are much quieter than roller chains, which also offer highly synchronized power transfer, and also don't require lubrication.
However, these advantages will only be felt if engineers specify the right timing pulley and belt drive system. Engineers might prefer other power transfer technologies if the cost or complexity of a time pulley and belt are too great, or if a clutch is required.
Terminologies and parameters for timing pulleys and belts
Figure 2 shows a typical timing pulley with some of its important parameters. It will be used to explain the following terminologies.
Pitch is the distance from one tooth’s center to the adjacent tooth’s center, and it is measured on the pulley pitch circle (or sprocket pitch circle). When specifying a timing belt and pulley drive system, it is recommended that engineers choose timing pulleys and belts such that they have the same pitch since timing pulleys and belts having different pitch dimensions cannot mate successfully.
2. Pitch diameter
Pitch diameter is the dimension of the straight line through the center of the pulley to the pitch circle, as shown in Figure 2.
3. Outside diameter
The outermost surface of a timing pulley is the part of the pulley in direct contact with the bottom surface of the timing belt when they mesh. The diameter formed by this outermost surface is called the outside diameter, and it can be calculated using:
U = distance from the sprocket pitch circle to the outermost surface of the timing pulley
4. Speed ratio
As its name suggests, the speed ratio is the ratio of the rotational speed of the driven pulley (measured in revolutions per minute) to the rotational speed of the driving pulley. It can also be obtained by dividing the number of teeth of the larger pulley by the number of teeth of the smaller pulley, as shown below:
Common timing belt materials
Timing belts are usually a rubberized polymer, and may be filled with synthetic or organic fibers for additional strength, or may include other fillers to enhance performance based on individual application.
Common timing pulley materials
With the broad range of material options available today, it can be overwhelming for engineers to specify a material for their timing pulley design. The truth is that there is no perfect, one-size-fits-all material for timing pulleys; the ideal choice will depend on the application requirements.
The following subsections present three commonly used engineering materials for timing pulley design.
Aluminum is among the most commonly used material for timing pulley design primarily because of its durability and lightweight. This material also has good tensile strength, making it suitable for applications that demand timing pulleys with high load-carrying capacity.
Aluminum is also corrosion-resistant; when exposed to moisture (or oxygen), it forms a thin oxide film that prevents further oxidation, and in turn, corrosion. This makes aluminum the ideal metal for applications where the timing pulley will be subjected to moist environments.
Steel generally offers superior strength and durability and is generally more affordable than several metals, including aluminum. However, keep in mind that steel is heavier (specifically 2.5 times denser) than aluminum. Steel is also harder than aluminum and cannot generally be pushed to the same extreme dimensional limits without cracking or deforming.
Plastic combines the lightweight and corrosion resistance advantage of aluminum and the affordability advantage of steel. As a result, they are ideal for simple applications in which the timing pulley might be exposed to corrosive fluids, combustible gases, and solvents.
While this article presents helpful information and calculations about timing pulley design, there are several other things to consider when specifying a timing pulley and belt drive for a particular application.
Engineers are advised to reach out to timing pulley manufacturers to discuss their application requirements.
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