Kinematics
The universal joint allows transmitting a rotary motion between two concurrent shafts. However, a universal joint is not a constant velocity device; if the drive shaft rotates at a constant speed, the driven shaft speed will be subject to a recurrent variation, the more angular shafts, and the more sensitive the variation.
In this case, transmitting a motion between two non-concurrent shafts requires the use of a shaft composed of, at least, two universal joints and, generally, one of them a sliding joint.
The constant velocity conditions of a propeller shaft composed of two universal joints are the following:
1) The central shaft forks (1) ought to be aligned, except for some special cases.
2) The flexion angles ß1 and ß2 of the two joints ought to be identical as regards both the disposition W (fig. 1) and the disposition Z (fig. 2).
3) The drive shaft, the central shaft and the driven shaft ought to be on the same plane.
