backstreet: Constant-velocity joint

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3D rendering of the internals of a simple CV joint

Simple 6-ball CV joint
Constant-velocity joints (aka homokinetic or CV joints) allow a rotating shaft to transmit power through a variable angle, at constant rotational speed, without an appreciable increase in friction or play. They are mainly used in front wheel drive and all wheel drive cars. However, rear wheel drive cars with independent rear suspensions typically use CV joints at the ends of the rear axle halfshafts. Audi Quattros use them for all four half-axles and on the front-to-rear driveshaft (propeller shaft) as well, for a total of ten CV joints.
Contents
1 Before the CV joint
2 The first CV joints
3 Modern CV joints
4 Fault-finding and diagnosis
5 References
6 See also
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Before the CV joint
Early front wheel drive systems such as those used on the Citro?n Traction Avant and the front axles of Land Rover and similar four wheel drive vehicles used Hardy-Spicer (universal) joints, where a cross-shaped metal pivot sits between two forked carriers. These are not CV joints as, except for specific configurations, they result in a variation of the transmitted speed. They are simple to make and can be tremendously strong, and are still used to provide a flexible coupling in the propeller shafts, where there is not very much movement. However, they become "notchy" and difficult to turn when operated at extreme angles, and need regular maintenance. They also need more complicated support bearings when used in drive axles, and could only be used in rigid axle designs.
The first CV joints
As front wheel drive systems became more popular, with cars such as the Mini using compact transverse engine layouts, the shortcomings of Hardy-Spicer joints in front axles became more and more apparent. Based on a design by Alfred H. Rzeppa which was filed for patent in 1927[1] (a CV joint, the Tracta joint [2], designed by Pierre Fenaille was filed for patent in 1926 [3]), constant velocity joints solved a lot of these problems. They allowed a smooth transfer of power despite the wide range of angles through which they were bent. Driveshafts using CV joints are self-supporting along their length, and do not need additional supports (although very long shafts such as the right-hand driveshaft on the Citro?n CX or Peugeot 205 have an intermediate bearing that supports the inboard joint).
Modern CV joints
Two different types of CV joint are used on the driveshafts of modern cars. At the "inboard" end, where the shaft only moves up and down with the movement of the suspension, a "Triax" (also known as "Tripod") joint is used. This joint has a three-pointed yoke attached to the shaft, which has barrel-shaped roller bearings on the ends. These fit into a cup with three matching grooves, attached to the differential. Since there is only significant movement in one axis, this simple arrangement works well. These also allow an axial 'plunge' movement of the shaft, so that engine rocking and other effects do not preload the bearings. A typical Tripod joint has up to 50mm of plunge travel, and 26 degrees of angular articulation[4].
At the "outboard" end of the driveshaft a slightly different unit is used. The end of the driveshaft is splined and fits into the outer "joint". It is typically held in place by a circlip. The shaft fits in the center of a large, steel, star-shaped "gear" that nests inside a circular cage. The cage is spherical but with ends open, and it typically has six openings around the perimeter. This cage and gear fit into a grooved cup that has a splined and threaded shaft attached to it. Six large steel balls sit inside the cup grooves and fit into the cage openings, nestled in the grooves of the star gear. The outer shaft on the cup then runs through the wheel bearing and is secured by the axle nut. This joint is extremely flexible and can accommodate the large changes of angle when the front wheels are turned by the steering system; typical Rzeppa joints allow 45-48 degrees of articulation, while some can give 52 degrees.
An alternative is to use a pair of universal joints fitted back to back, which form a Double Cardan Joint. This is a robust type of CV joint and is used where high torque loads and/or high articulation angles are experienced.
These joints are very strong, and are usually highly overspecified for a given application. Maintenance is usually limited to checking that the rubber gaiter (dust/weather boot) that covers them is secure and not split. If the gaiter is damaged, the molybdenum disulfide grease with which the joint is packed will be thrown out. The joint will then pick up dirt, water, and road deicing salt and cause the joint to overheat and wear. The grease can also contaminate the brakes. In worst case, the CV joint may disjoin causing the vehicle to stop moving or lock up, rendering...(and so on)