Power transmission coupling technology has been in existence for hundreds of years, various coupling products are available but normally are in two different very distinct areas. Either high speed low torque or lower speed higher torque. Add into that equation the actual flexibility of the couplings themselves.
This distinction means couplings like beam couplings or Lovejoy U joints, which are used in industrial applications like conveyor belts and industrial equipment, run at high speed with no internal components because they are only able to be flexible for 4°. Add to that they are only capable of a small amount of torque means they cannot be used in anything outside of that, if the application doesn’t fit then the engineer has to design something around the coupling.
The other kind of coupling is the universal joint or u joint for short. This joint is predominately used in vehicles and off road applications or anywhere where there is a need for either higher torque and/or a higher angle of misalignment than a beam coupling can offer. This flexibility comes at a price, as the universal joint cannot run at high speeds like the beam coupling and to make it more flexible has internal components called bearings.
Uni joint bearings have to be greased often to maintain their inner workings, if not they fail and have to be replaced. While bearings themselves are not expensive, the continual maintenance and replacing when they either fail or wear out is.
So the increased flexibility also has another price, vibration as the farther out of alignment the uni joint is the vibration because it isn’t torsionally rigid the oscillation at speed sends vibration through the attached components, this vibration also hastens the failure/replacement of the bearings. So while a universal joint is more flexible than a beam coupling, it only is by a few extra degrees so anything higher than 8° means the bearings in the coupling start to wear faster.