How to Calculate the Secondary Moment in a Universal Joint

A well designed use for a universal joint keeps the operating angle constant and less than 25 degrees.

A universal joint is a mechanical connection between two rotating shafts. They transmit movement, power or both and you can see them, for example, on hydraulic heavy construction equipment. The product of a force and the distance at which it acts is called a moment, and when two forces act together they're called a couple and their collective moment is a torque. The calculation of the secondary moment translates the spinning driving shaft's torque (moment) to the couple's collective torque (moment).

Things You'll Need

• Driving torque, provided by the driving shaft
• Shaft operating angle, i.e. the angular difference between the shafts

Instructions

1. Calculate the tangent of the operating angle. For an example, with an operating angle of 20 degrees, the tangent is approximately 0.36.

2. Calculate the sine of the operating angle. Continuing with the operating angle of 20 degrees, the sine is approximately 0.34.

3. Multiply the result from step 1 by the driving torque. With an example driving torque of 100 Newton-meters (N-m), the result is 36 N-m. This is the secondary couple or secondary moment of the driving shaft.

4. Multiply the result from step 2 by the driving torque. Continuing with a 100 N-m driving torque, the result is 34 N-m. This is the secondary couple or secondary moment of the driven shaft.

References

• Koyo: Driveshafts for Industry
• SDP/SI: Universal Joints
• Splung: Moments and Couples