Constant-velocity joint

3D rendering of the internals of a simple CV joint

Simple 6-balls 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.

Early front wheel drive systems such as those used on the Citroën 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 strictly CV joints as they result in a variation of the transmitted speed except for certain specific configurations). These 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.

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, constant velocity joints solved a lot of these problems. They allowed a smooth transfer of power despite the wide range of angles they were bent in. 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 Citroën CX or Peugeot 205 have an intermediate bearing that supports the inboard joint).

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 rollers 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.

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.

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 MoS₂ (molybdenum disulfide) grease that the joint is packed with 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 the car incapable of steering. Damaged CV joint gaiters will usually cause a car to fail a vehicle inspection.

Faultfinding and diagnosis

Constant velocity joints are usually reliable and largely trouble-free. The two main failures are wear and partial seizure.

Wear in the outer joint usually shows up as vibration at certain speeds, a bit like the vibration caused by an unbalanced wheel. To determine if the joint is worn, a driver should find a big empty parking lot and drive the car slowly in tight circles, left and right. Worn joints will make a rhythmic clicking or cracking noise. Wear in the inner joints shows up as a "clunk" or "pop" when applying power or, if severe, when lifting off the throttle.

Partial seizure causes a strange "pattering" sensation through the suspension. It is caused by the joint overheating, which in turn is usually caused by the outer joint gaiter/boot having split, allowing the joint to throw out its grease. If caught in time, one can clean the joint carefully, repack with grease and replace the gaiter/boot. Kits which include the grease, gaiter/boot, and retaining clips are available from most automotive manufacturers. Some universal gaiters/boots are split lengthwise enabling them to be fitted without having to disassemble the wheel hub and CV joint.

See also

• Universal joint
• Thompson coupling