What is the Relationship Between Input Shaft Angle and Torque in U-Joints?

[BretMan]

Hello,

I need to select an appropriate electric motor for a drive mechanism which includes a U-Joint. It's understood that rotational variances occur in U Joints as the angle of the input shaft increases relative to the output shaft. Can someone refresh my memory as this relates to torque? In other words, is there a percentage increase in torque to drive the input shaft as its angle increases to say 15, 30, 45, 90 degrees relative to the output shaft that stays at 0 degrees? If so, what's the percentage increase at each increment - or at least a general rule of thumb?

Many thanks.

Bret

 

[SteamKing]

It's not so much the change in torque that is the problem with shafts coupled by U-joints. The life of the joint is greatly reduced once the angle increases beyond a very small amount, unless the torque applied to the joint is greatly reduced.

For instance, if a given shaft and joint is rated to transmit 300 ft-lb of torque at an angle of 3 degrees, the same shaft will only be capable of transmitting 100 ft-lb of
torque if the angle increases to 10 degrees.

For larger angles, U-jointed shafts are not practical.

 

[BretMan]

Thanks SteamKing. Good thing that I asked. I seem to remember now about this limitation but didn't remember it to be as dramatic. I may need to reconsider the electric motor. However, don't you think that at least the wear issue could be reduced by going with lower RPMs , say under 100 RPM or even a manual crank?

 

[SteamKing]

It depends on what kind of motor you want to use. If you have a motor turning at high speed (say 1800 rpm) and you wish to reduce the output speed to 100 rpm, then the speed reduction will also result in a torque magnification. For instance:

1 ft-lb at 1800 rpm = 18 ft-lb at 100 rpm

It's hard to suggest anything without knowing more about how your drive mechanism is situated between the motor input and the output of the drive mechanism.

 

[BretMan]

What I'll do then is jig a physical test with a torque wrench, create some resistance constant, and see what the readings are with each change of the angles. I'll post my findings.