Like a cross between traditional belting and chain, timing belts include a main body and engaging surfaces — a.k.a. teeth. They're used to synchronize motion on everything from copiers to conveyors. They are also useful (because of their flexibility) in tight places that don't necessarily need timing and (because of their power density) in portable designs that must be kept light.

The shape, material, and design of synchronous belt teeth greatly affect output motion and performance. Upper limits are determined in part by tooth shear strength, in turn defined by tooth cross-section, pulley engagement dynamics, and material. Cost-effective rubber often carries the highest load, so many industrial belts are made of neoprene. On the other hand, newer polyurethane often works where rubber fails. It is more resistant to harsh chemical environments, allows more specialized profiles and machining, and perhaps most importantly, can boost tooth shear strength.

In short, a belt's tooth shear strength must be high enough to withstand the application's highest torque and shock. So, this strength must exceed the highest possible load condition — in general, maximum force plus any pretension.

Q&A

What's the most common tooth profile?

Trapezoidal-profile teeth are the original; their design is derived from the spur gear. They're common because they offer precise positioning, especially with newer, modified trapezoidal-tooth profiles that improve engagement.

Are there other tooth shapes?

For the most part, fully rounded profiles (often generically labeled by trademarked ‘high-torque drives’ or HTDs) have superceded trapezoidal profiles. They distribute tooth loads more evenly on the belt tensile member and boost tooth shear strength — to carry greater load than trapezoidal designs. In some cases, they double or triple horsepower ratings.

One option, belts with modified curvilinear tooth designs, optimize pressure angles and tooth depth to improve load capacity and prevent ratcheting. Many of these belts are designed for automotive applications; note here that their special sizes and tensile cords (which change pitch line differential) make them unsuitable for industrial applications.

Can belt teeth actually cause problems?

Yes — backlash. Clearance between the belt teeth and pulley grooves is needed to allow belt teeth to enter and exit grooves with minimum interference. So, there is a bit of free play in most synchronous belts.

In static systems, though errors during transport are acceptable, end of move positioning must be consistent. Dynamic systems are more demanding: They must move precisely at every advancement in motion, even if load varies during operation. For these reasons, sometimes backlash is minimized with belt tooth designs that require less clearance for pulley engagement.

This month's handy tips provided by Bob Hamilton, B&B Mfg. Inc. For more information, visit motionsystemdesign.com and click on Belts and Chains in the Component Zone, or visit bbman.com.