Belt Drive Design Tension Ratios

As they operate, belt drives use span tension differentials to create a net effective pull. The two belt spans are defined as the tight side span (higher tension value) and the slack side span (lower tension value). The drive loads, as well as installation tension, impact the span tensions.

The tension ratio is defined as:

Tension Ratio = Tight Side Span Tension (lb) / Slack Side Span Tension (lb)

Based on different operating characteristics, belt drives types have different design tension ratios. Assuming 180° wrap on both pulleys, here are some typical design tension ratios:

• 4:1 for Micro-V® Belt Drives
• 5:1 for V-belt Drives
• 8:1 for Synchronous Belt Drives

Assuming other drive parameters to be equal, larger tension ratios result in lower belt pull values.  

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.