What Is a round Belt?

Lori Kilchermann

A round belt is a soft rubber device used in low-torque applications. Resembling a rubber O-ring, the round belt is designed to operate in a V-groove of 60 degrees for optimal friction and bite. The often-elastic qualities of the round belt contribute the lack of need for a tensioner pulley in most applications. The size of the round belt can be altered by cutting the belt and splicing the two ends together using a variety of methods.

The best lathe belts are commonly made of a reinforced rubber material that will resist fraying and stretching.
The best lathe belts are commonly made of a reinforced rubber material that will resist fraying and stretching.

Many styles of round belts are often used in tools, such as electric drill motors, milling machines and lathes. A round belt is also commonly found in vacuum cleaners, kitchen appliances and limited automotive uses. Designed to provide optimal grip in a low-torque application, the round belt also is widely used in applications where the belt is required to twist, such as the case of two crossing jack-shafts.

Unlike a typical V-belt commonly referenced as a fan belt, the rubber used to manufacture round belts is very soft and pliable. This soft consistency allowed the belts to be stretched around two pulleys or shafts and operated without the aid of a belt tensioning pulley. The tendency of the rubber to become even stickier, therefore providing more grip as it becomes heated due to friction caused from minor slippage, makes this belt a good choice to use on a lathe or drill motor.

While the round shape of the belt fits snugly into a U-groove pulley, the optimum shape of a pulley when using a round belt is a V-pulley. Moreover, a 60-degree V is commonly recommended for use with the round-shaped belt. The U-grooved pulley is typically resigned to guide pulleys or non-critical pulley drive systems. Occasionally, a U-groove pulley will be used with the softest rubber belts as the U-shaped groove takes advantage of maximum contact surface for the sticky belt and slippage is typically not an issue.

An advantage that a round belt has over a typical V-belt is in its ability to be re-sized by cutting and trimming material away. The rubber is cut and the required amount needed to make the belt the correct length is removed. The two ends of the trimmed belt are reconnected using various methods, including gluing, melting and stapling. When gluing a belt, it is imperative that a quick-setting glue, such as contact cement, be used to adhere the ends of the belt back together.

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