625 Birfield joint based on the Rzeppa Principle

Alfred Hans Rzeppa (pronounced sheppa), a Ford engineer in 1926, invented one of the first practical

Alfred Hans Rzeppa
Fig. 6.33 Early Rzeppa constant velocity joint

constant velocity joints which was able to transmit torque over a wide range of angles without there being any variation in the rotary motion of the output shafts. An improved version was patented by Rzeppa in 1935. This joint used six balls as intermediate members which where kept at all times in a plane which bisects the angle between the input and output shafts (Fig. 6.33). This early design of a constant velocity joint incorporated a controlled guide ball cage which maintained the balls in the bisecting plane (referred to as the median plane) by means of a pivoting control strut which swivelled the cage at an angle of exactly half that made between the driving and driven shafts. This control strut was located in the centre of the enclosed end of the outer cup member, both ball ends of the strut being located in a recess and socket formed in the adjacent ends of the driving and driven members of the joint respectively. A large spherical waist approximately midway along the strut aligned with a hole made in the centre of the cage. Any angular inclination of the two shafts at any instant deflected the strut which in turn proportionally swivelled the control ball cage at half the relative angular movement of both shafts. This method of cage control tended to jam and suffered from mechanical wear.

Joint construction (Fig. 6.34) The Birfield joint, based on the Rzeppa principle and manufactured by Hardy Spicer Limited, has further developed and improved the joint's performance by generating converging ball tracks which do not rely on a controlled ball cage to maintain the intermediate ball members on the median plane (Fig. 6.34(b)). This

Birfield Ltd
Fig. 6.34(a-c) Birfield Rzeppa type constant velocity joint

joint has 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 track grooves formed in both the cup and spherical members. Articulation of the joint is made possible by the balls rolling inbetween the inner and outer pairs of curved grooves.

Ball track convergence (Figs 6.34 and 6.35) Constant velocity conditions are achieved if the points of contact of both halves of the joint lie in a plane which bisects the driving and driven shaft angle, this being known as the median plane (Fig. 6.34(b)). These conditions are fulfilled by having an intermediate member formed by a ring of six balls which are kept in the median plane by the shape of the curved ball tracks generated in both the input and output joint members.

To obtain a suitable track curvature in both half, inner and outer members so that a controlled movement of the intermediate balls is achieved, the tracks (grooves) are generated on semicircles. The centres are on either side of the joint's geometric centre by an equal amount (Figs 6.34(a) and 6.35). The outer half cup member of the joint has the centre of the semicircle tracks offset from the centre of the joint along the centre axis towards the open mouth of the cup member, whilst the inner half spherical member has the centre of the semicircle track offset an equal amount in the opposite direction towards the closed end of the joint (Fig. 6.35).

When the inner member is aligned inside the outer one, the six matching pairs of tracks form grooved tunnels in which the balls are sandwiched.

The inner and outer track arc offset centre from the geometric joint centre are so chosen to give an angle of convergence (Fig. 6.35) marginally larger than 11°, which is the minimum amount necessary to positively guide and keep the balls on the median plane over the entire angular inclination movement of the joint.

Track groove profile (Fig. 6.36) The ball tracks in the inner and outer members are not a single semicircle arc having one centre of curvature but instead are slightly elliptical in section, having effectively two centres of curvature (Fig. 6.36). 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 and is so chosen so that track contact occurs well within the arc grooves, so that groove edge overloading is eliminated. At the same time the ball contact load is taken about one third below and above the top and bottom ball tips so that compressive loading of the balls is considerably reduced. The pressure angle will be equal in the inner and outer tracks and therefore the balls are all under pure compression at all times which raises the limiting stress and therefore loading capacity of the balls.

The ratio of track curvature radius to the ball radius, known as the conformity ratio, is selected so that a 45° pressure angle point contact is achieved, which has proven to be effective and durable in transmitting the torque from the driving to the driven half members of the joint (Fig. 6.36).

As with any ball drive, there is a certain amount of roll and slide as the balls move under load to and fro along their respective tracks. By having a pressure angle of 45°, the roll to sliding ratio is roughly 4:1.

Birfield Rzeppa
Fig. 6.35 Birfield Rzeppa type joint showing ball track convergence
Joint Rzeppa
Fig. 6.36 Birfield joint rack groove profile

This is sufficient to minimize the contact friction during any angular movement of the joint.

Ball cage (Fig. 6.34(b and c)) Both the inner drive and outer driven members of the joint have spherical external and internal surfaces respectively. Likewise, 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. 6.34(c)). The cage takes up the space between the spherical surfaces of both male inner and female outer members. It provides the central pivot alignment for the two halves of the joint when the input and output shafts are inclined to each other (Fig. 6.34(b)).

Although the individual balls are theoretically guided by the grooved tracks formed on the surfaces of the inner and outer members, the overall alignment of all the balls on the median plane is provided by the cage. Thus if one ball or more tends not to position itself or themselves on the bisecting plane between the two sets of grooves, the cage will automatically nudge the balls into alignment.

Mechanical efficiency The efficiency of these joints is high, ranging from 100% when the joint working angle is zero to about 95% with a 45° joint working angle. Losses are caused mainly by internal friction between the balls and their respective tracks, which is affected by ball load, speed and working angle and by the viscous drag of the lubricant, the latter being dependent to some extent by the properties of the lubricant chosen.

Fault diagnoses Symptoms of front wheel drive constant velocity joint wear or damage can be narrowed down by turning the steering to full lock and driving round in a circle. If the steering or transmission now shows signs of excessive vibration or clunking and ticking noises can be heard coming from the drive wheels, further investigation of the front wheel joints should be made. Split rubber gaiters protecting the constant velocity joints can considerably shorten the life of a joint due to exposure to the weather and abrasive grit finding its way into the joint mechanism.

6.2.6 Pot type constant velocity joint (Fig. 6.37) This joint manufactured by both the Birfield and Bendix companies has been designed to provide a solution to the problem of transmitting torque with varying angularity of the shafts at the same time as accommodating axial movement.

There are four basic parts to this joint which make it possible to have both constant velocity characteristics and to provide axial plunge so that the effective drive shaft length is able to vary as the angularity alters (Fig. 6.37):

Outer cylindrical Outer Control

Dot member <.tra:Qhl cage

Outer cylindrical Outer Control

Dot member <.tra:Qhl cage

Pot Joint Ball Cage
R. — Inner cage centre and radii Ro -■■ Outer cage centre and radii

Fig. 6.37 Birfield Rzeppa pot type joint

1 A pot input member which is of cylindrical shape forms an integral part of the final drive stub shaft and inside this pot are ground six parallel ball grooves.

2 A spherical (ball) output member is attached by splines to the drive shaft and ground on the external surface of this sphere are six matching straight tracked ball grooves.

3 Transmitting the drive from the input to the output members are six intermediate balls which are lodged between the internal and external grooves of both pot and sphere.

4 A semispherical steel cage positions the balls on a common plane and acts as the mechanism for automatically bisecting the angle between the drive and driven shafts (Fig. 6.38).

It is claimed that with straight cut internal and external ball grooves and a spherical ball cage which is positioned over the spherical (ball) output member that a truly homokinetic (bisecting) plane is achieved at all times. The joint is designed to have a maximum operating angularity of 22°, 44° including the angle, which makes it suitable for independent suspension inner drive shaft joints.

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    What is pressure angle in track of rezeppa joint?
    3 years ago

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