Overhung Load for Belt Drive Systems with Speed Reducers

Overhung load (OHL) is a force exerted perpendicular to a shaft beyond the outermost bearing.

When that force exceeds the equipment's rated capacity, shafts and bearings become overloaded and fatigue more quickly.  For this reason, speed reducer manufacturers publish OHL ratings.

When designing or retrofitting a belt drive attached to a speed reducer, designers should review the load against published OHL limits to ensure drive success. Manufacturers use different formulas and ratings, so it is important to get information directly from the speed reducer manufacturer.

If the OHL is too high, the belt drive should be redesigned to reduce the load. Some general design guidelines include:

• Selecting larger diameter sprockets
• Minimizing the belt width
• Mounting the sprockets as close as possible to the outermost bearing

Once the drive is redesigned, the OHL calculations should be rerun to ensure that the load falls within the speed reducer manufacturer’s published limits.

What is Belt Pull?

Belt pull estimates the force a belt exerts on a shaft. It is the force applied to a shaft by the two belt spans entering and exiting a pulley.  Belt pull does not account for pulley weight or pulley location on a shaft.  While related, bell pull should not be considered to be interchangeable with bearing load or over hung load values. 

Belt pull values can be calculated using Gates Design Flex® and Design IQ® software programs, or by using formulas found in the Gates drive design manuals.

Overhung Load

To prevent premature failures, manufacturers typically publish overhung load values for the speed reducers they offer. When designing a belt drive for use on a reducer, it is important to review the maximum published overhung load value to prevent overload of the bearings and shaft.

Along with their overhung load ratings, each speed reducer manufacturer publishes unique overhung load equations and constants. Therefore, it is crucial to obtain the correct overhung load calculation procedure from the reducer manufacture and to use their corresponding ratings.

If the overhung load is too high, the designer may consider increasing the pulley diameters, reducing the belt width, and/or mounting the pulley closer to the bearing. Following design adjustments, the overhung load calculations should be re-run to ensure that the drive is acceptable.