Birfield Joint

The Birfield joint, manufactured by Hardy Spicer Limited, is based on the Rzeppa principle (Fig. 26.17). In this construction the joint's performance has been improved further by incor­porating converging ball tracks, which do not rely on a controlled ball cage to maintain the intermediate ball members on the median plane (Fig. 26.17B). This Joint uses an inner (ball) input member driving an outer (cup) member. Torque is transmitted from the input to the output member again by six intermediate ball members, which fit into curved rack grooves formed in both the cup and spherical members. Articulation of the joint takes place due to rolling of the balls in between the inner and outer pairs of curved grooves.

Constant velocity conditions are achieved by an intermediate member, formed by a ring of six balls placed in the median plane (Fig. 26.17B) of the shape of the curved ball tracks generated in both the input and output joint members. 

Fig. 26.17. Birfield. Rzeppa type constant velocity joint. A. Side and end view. B. With 30 degrees shaft angularity.

In order to obtain a controlled movement of the intermediate balls, the tracks (grooves) are formed on semicircles in both halves, inner and outer members. The centres are on either side of the joint's geometric centre by an equal distance (Figs. 26.17A and 26.18). Once the inner member aligns inside the outer one, the six matching pairs of tracks from grooved tunnels in which the balls are sandwiched. The inner and outer track arc offset centre from the geometric joint centre is so chosen to give an angle of convergence (Fig. 26.18) marginally larger than 11

Fig. 26.18. Birfield Rzeppa type joint showing ball track convergence.

degrees.

This is the minimum angle required to positively guide and hold the balls on the median plane over the entire angular inclination movement of the joint.

The ball tracks in the inner and outer members are slightly elliptical in section, having effectively two centres of curvature (Fig. 26.19) instead of a single semicircle arc. The radius of curvature of the tracks on each side of the ball at the four pressure angle contact points is larger than the ball radius. The pressure angle is equal in the inner and outer tracks due to which the balls are all under pure compression at all times so that the limiting stress is raised and consequently the loading capacity of the balls is also raised.

The ratio of track curvature radius to the ball radius is known as the conformity ratio, which is selected to achieve a 45 degrees pressure angle point contact. This provides effective and durable service in transmitting the torque from the driving to the driven half members of the joint (Fig. 26.19). When the balls move under load to and fro in their respective tracks, a certain amount of roll and slide takes place. With a pressure angle of 45 degrees, the roll to sliding ratio is roughly 4:1 which is sufficient to minimize the contact friction during any angular movement of the joint.

Fig. 26.19. Birfield joint track groove profile. 

Both the inner drive and outer driven members of the joint have spherical external and internal surfaces respectively. Also, the six ball intermediate members of this joint are posi­tioned in their respective tracks by a cage which has the same centre of arc curvature as the input and output members (Fig. 26.17C). The space between the spherical surfaces of both male inner and female outer members is taken up by the cage, which also provides the central pivot alignment for the two halves of the joint when the input and output shafts are inclined to each other (Fig. 26.17B). Additionally the overall alignment of all the balls on the median plane is provided by the cage.

The efficiency of these joints is high, ranging from 95% to 100% with the joint working angle 45 degrees and zero respectively. Losses are mainly due to the internal friction between the balls and their respective tracks, which depends on ball load, speed and working angle, and by the viscous drag of the lubricant.

The diagnosis of front wheel drive constant velocity joint wear or damage can be narrowed down by turning the steering to full lock and driving the vehicle round in a circle. If the steering or transmission now indicates signs of excessive vibration or clunking and ticking noises are heard coming from the drive wheels, then further investigation of the front wheel joints is necessary. Split rubber gaiters used to protect the constant velocity joints can considerably shorten the life of a joint because of exposure to the weather and abrasive grit entering into the joint mechanism.

26.2.6.