4.4 Ball Joints

The transmitting elements in ball joints are balls which run independently along guide tracks. It is assumed that the compressive force P is evenly distributed among all m-active balls. This number of balls m depends on whether the joint is divided according to the Rzeppa principle (concentrically) or the Weiss principle (radially), see Fig. 4.26.

1. Rzeppa type m = z,

2. Weiss type m= z/2

Fig. 4.26: Number of active balls m in joints with z balls.

The dividing planes between the input and output members are then either con-centric with (Fig. 4.26a) or radial to the longitudinal axis of the joint (Fig. 4.26b). In the Rzeppa arrangement, all the balls act for each direction of rotation whereas with the Weiss arrangement only half the balls act. In the universal torque equation for joints (4.16), for the Rzeppa arrangement m = z, and for the Weiss arrangement m = z/2.

In addition to the way in which the planes run relative to the longitudinal axis of the joint, the shape of the tracks in this direction is also critical; this is the "effective geometry" in Sects. 2.3.1 and 2.3.2.

Where there are curved tracks the joint can only articulate, there can be no plunging. Straight and helical tracks allow articulation and plunge of the joint. Ball joints can therefore be further categorised as fixed joints and plunging joints.