All the joints in this catalogue are of the Cardan type (i.e two yokes connected by a cross). It is a geometric characteristic of this joint that when operating at an angle, the rotary motion of the driving yoke is not reproduced at the driven yoke. If the rotary motion of the driving yoke is assumed constant then the driven yoke moves faster and slower within one revolution, this motion variation in most drive systems is unacceptable, and is cancelled by having a second or more joints. The majority of drive systems have two joints which are combined in a shaft as shown in Fig. 1 with the joints aligned as illustrated.
For this construction of shaft cancellation of the joint motion variation in the system overall requires that:
(a) The joint angles are equal.
(b) The joint angles lie in the same plane.
Drive layouts satisfying these rules are shown in Figs 2 & 3.