Concept

# Constant-velocity joint

Summary
A constant-velocity joint (also called a CV joint) is a mechanical coupling which allows the shafts to rotate freely (without an appreciable increase in friction or backlash) as the angle between the two shafts moves within a certain range. A common use of CV joints is in front-wheel drive vehicles, where they are used to transfer the engine's power to the wheels, even as the angle of the driveshaft varies due to the operation of the suspension. The predecessor to the constant-velocity joint was the universal joint (also called a Cardan joint), which was invented by Gerolamo Cardano in the 16th century. Universal joints are simple to produce and can withstand large forces, however as the angle of operation increases, universal joints often become "notchy" and difficult to rotate. The first type of constant-velocity joint was the Double Cardan joint, which was invented by Robert Hooke in the 17th century. This design uses two universal joints offset by 90 degrees, which cancels out the velocity variations in each joint. Many other types of constant-velocity joints have been invented since then. Double Cardan joints are similar to double Cardan shafts, except that the length of the intermediate shaft is shortened leaving only the yokes; this effectively allows the two Hooke's joints to be mounted back to back. DCJs are typically used in steering columns, as they eliminate the need to correctly phase the universal joints at the ends of the intermediate shaft (IS), which eases packaging of the IS around the other components in the engine bay of the car. They are also used to replace Rzeppa style constant-velocity joints in applications where high articulation angles, or impulsive torque loads are common, such as the driveshafts and halfshafts of rugged four-wheel drive vehicles. Double Cardan joints require a centering element that will maintain equal angles between the driven and driving shafts for true constant velocity rotation. This centering device requires additional torque to accelerate the internals of the joint and does generate some additional vibration at higher speeds.
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