Timing belts — also known as synchronous belts or toothed belts — are most often used in transporting, indexing, and positioning applications where high torque or force transmission and high acceleration rates are required. Unlike V-belts, which rely on friction between the belt and the pulley for power transmission, timing belts have positive engagement between the belt teeth and the pulley teeth, so the possibility of slip is eliminated. This makes timing belts very efficienct at transmitting power and gives them good positioning accuracy.

Belt manufacturers typically offer their own exclusive or patented tooth designs, but even proprietary designs are based on one of three basic profiles—trapezoidal, curvilinear, or modified curvilinear. Each one offers benefits in power transmission, speed capability, or backlash.

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Although timing belt tooth profiles are based on just three basic designs, belts with the same type of tooth profile and the same tooth pitch may not be interchangeable. This is due to variations in tooth dimensions between manufacturers’ designs, and in some cases, even within a given manufacturer’s product line.

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Trapezoidal belt tooth profiles

Belts with a trapezoidal tooth profile are probably the most widely used in timing belt applications, especially for linear positioning and conveying applications. They have good force transmitting capabilities and low backlash. But the trapezoidal tooth shape results in high stress concentrations at the belt-pulley interface, which can lead to high wear rates when the transmitted torque or speed is high.

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Curvilinear belt tooth profiles

The curvilinear tooth profile was developed to alleviate the stress concentrations found in trapezoidal profiles and improve on torque and speed capabilities. Curvilinear profiles also have a larger tooth depth than trapezoidal designs, so belt ratcheting is less likely. And the smoother transition that curvilinear teeth provide during mesh means these profiles are quieter than their trapezoidal counterparts. The tradeoff, however, is that curvilinear designs have higher backlash than trapezoidal profiles.

The curvilinear tooth profile was developed by Gates, and belts with this profile were referred to as high torque drives (HTD). Although various manufacturers use different names, the HTD designation is still quite common for belts with curvilinear tooth profiles.

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Modified curvilinear tooth profiles

Further improvements to the curvilinear tooth profile resulted in the modified curvilinear design. This profile has a smaller tooth depth and a greater flank angle than the original curvilinear version, giving the modified curvilinear design the highest torque and force transmitting capabilities among the three tooth profiles. It also allows the areas of the belt between the teeth to share the load with the teeth that are engaged with the pulley. This means that belts with modified curvilinear teeth are the least likely to experience ratcheting, even under extreme loads, making them common in processing applications that require very high torque transmission at high speeds.

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Imperial or metric?

It’s important to note that belts with a trapezoidal tooth profile are available in both imperial and metric dimension. However, belts with curvilinear and modified curvilinear tooth profiles are available only in metric dimensions.

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Image credit: B&B Manufacturing • Feature image credit: Optibelt