Pulleys are the most common pieces of equipment found in many devices and machinery today. They come in many different designs and capabilities to serve different application requirements.

Timing pulleys are among the most popular pulleys on the market. They are typically used in power transmission systems where maintenance of speed ratio and prevention of misalignment are important design considerations. Yet, many product designers and engineers are often plagued with challenges when designing efficient timing pulley systems.

What follows are some basic definitions, sizing considerations and suitable pulley materials for efficient timing pulley systems. Keep in mind that this article presents calculations using the metric system of measurement.

## Key parameters for timing belt pulleys

### 1. Determine the torque requirement

As with other engineering design problems, product designers must first know about their timing belt-pulley application requirements, such as the output torque required to do the work.

Once this output torque requirement has been determined, proceed to choose the drive system. As a rule, choose a drive motor with an input torque greater than output torque requirements.

### 2. Identify the speed ratio and select the pulley

When a timing pulley drive has two gears, the smaller gear revolves faster than the large one. “Speed ratio” is used to describe the difference in speed between these two gears. It is calculated by dividing the angular velocity of the output gear by the angular velocity of the input gear, as shown in the equation below.

TBP_eq1

Where:

ω1 = angular speed of smaller gear

ω2 = angular speed of larger gear

Say a design engineer needs the smaller pulley to spin three times faster than the larger one, then the speed ratio would be 3:1. The good thing is that the speed ratio also shares a mathematical relationship with the number of teeth of the timing pulleys, as shown in the equation below.

TBP_eq2

So once the engineer knows the speed ratio, the number of teeth required on the smaller (or larger) gear can be easily determined. Still considering the previous scenario, if the design engineer had a 12-tooth pulley on shaft #1, then a 36-tooth pulley would be needed on shaft #2 to satisfy the speed ratio requirement.

### 3. Calculate the required belt length

The required belt length can be calculated using:

TBP_eq3

Where:

C = distance between the centers of the two shafts

D1 = Pitch diameter of pulley #1

D2 = Pitch diameter of pulley #2

Engineers are also advised to check online calculators provided by pulley manufacturers. However, keep in mind that this belt length formula does not consider pretension on the belt, so there might be a need to adjust pulley diameters (or shaft spacing) for the pulley system to work efficiently.

## Material types

When it comes down to material selection, a product designer’s options are near-endless. There is no perfect, one-size-fits-all engineering material for timing pulleys; the right choice typically depends on system requirements and environmental conditions.

Aluminum, one of the most utilized materials in pulley fabrication, offers high tensile strength, thermal stability and durability. It is ideal for making timing pulleys that will be exposed to high loads and extreme temperatures. Aluminum is also naturally corrosion-resistant, so timing pulleys made of aluminum can be used in moist environments.

Like aluminum, steel also offers high strength, thermal stability and durability. However, a major drawback with steel when compared to aluminum is its density. Steel is much heavier (about 2.5 times denser) than aluminum, so it might not be the best material choice for lightweight designs.

Plastics timing pulleys are also very commonly used, especially for lightweight applications in environments where little to no maintenance is needed.

## Summary

Although this article presents useful information about timing belts and pulley design, timing pulley designers are strongly advised to reach out to manufacturers or suppliers. They typically walk engineers through gathering other important specifications needed for timing belt and pulley design.