What is the difference between a u-joint and a constant velocity joint?
The difference is in the theory of operation. A single universal joint operating at an angle, will transmit inconsistent velocity at the output. Two universal joints that are in phase and the input and output shafts are on the same plane, create constant velocity. This means that the output velocity will be identical to the input velocity. The inconsistent velocity will occur after the first joint and be counteracted by the second. For the constant velocity joint to maintain equal angles at all times, a centering device of some kind is typically used.
