How a CV Joint Works

"CV joint" is short for constant-velocity joint, an example of a type of mechanisms that connects two intersecting rotating shafts making an angle with one another, especially when this angle varies in service. CV joints are now very widely used in front-wheel drive cars at the connection of a half-axle with a wheel. They transmit torque evenly when the wheel moves in steering or suspension. For a brief description of a typical CV joint, see the link to the Wikipedia article given in the References.

Neither this article, nor those I searched for on Google, actually explained how the CV joint worked. This article presents my best understanding at the moment, which is certainly not complete.

Nearly parallel shafts may be joined with flexible couplings, which provide a small amount of angular, torsional and longitudinal flexibility to ease alignment difficulties. There are many types of flexible couplings, such as Thomas or Oldham couplings. The Thomas coupling uses a flexible laminated steel ring, the Oldham a cross-keyed steel slider. Such couplings do not provide sufficient angular flexibility for automotive applications, but are widely used in machinery.

Universal joint, or U-joint, is the general name for a mechanism that transmits torque from one shaft to another when the shafts meet at a point but may be at a considerable angle with one another. If the angle is constant, bevel gears satisfy the requirement very well, but cannot be used when the angle varies in service. A simple U-joint for small shafts can be a length of rubber tubing slipped over the ends of the shafts. Clearly, this cannot be used for serious machinery. The most familiar U-joint is the Hooke, Cardan or Hardy-Spicer joint. The ends of the shafts are forked in a U-shape, and joined by a cross-shaped piece with four bearings, two in each fork. This works very well when the angle between the shafts is small, and its functioning is easy to understand. U joints of this kind are used in the Hotchkiss rear-wheel drive for cars, with one at each end of the propeller shaft. The drive flexibility accommodates the springing of the rear wheels.

The difficulty with the Cardan joint is that the output angular velocity of the driven shaft is not constant when the angle between the shafts is not small, but pulsates twice in every revolution. Generally, Cardan joints are restricted to angles of 15° or less. As a result, the Cardan joint is not satisfactory for transmitting torque to a front wheel, when the wheel may turn at up to 30° or even more on its kingpin, not to mention the angular motion with such a short half-axle when the wheel moves up and down. The solution to this problem is a new kind of joint, invented around 1928 by A. H. Rzeppa at Ford Motors. This is the ball-and-groove CV joint for which input and output angular velocities are equal at all angles of rotation. A similar joint is the Bendix-Weiss joint.

The Rzeppa joint consists of a spherical ball and socket connected by six steel balls that run in longitudinal grooves in the ball and socket, and which are held in a cage between the ball and socket. One shaft is connected with the socket, the other with the ball, usually by a splined shaft allowing some longitudinal motion. The cage always assumes a position making equal angles with the input and output shafts. This is probably the result of the shape of the grooves in which the balls move (as in the Bendix-Weiss joint, which uses four balls and a central ball to handle the thrust), but I do not know the reason for this exactly in the Rzeppa joint. As the joint rotates, the individual balls move backwards and forwards along the grooves, with greater amplitudes for greater angles.

The position of the balls is indicated in the figure on the right. The input shaft rotates the balls, which drive the output shaft. Since the relation of the balls to the shafts is the same for input and output, they must revolve at the same rate. When the angle between the shafts is zero, the balls are in the equatorial plane and only rotate about the axis, not moving backwards and forwards in the grooves. Centrifugal force would keep the balls in the desired location, but I am not sure if it can be relied upon to do this.

CV joints are usually lubricated with MoS₂ grease, and give remarkably little trouble.

Wikipedia article on the CV joint.

C. Carmichael, ed., Kent's Mechanical Engineers' Handbook, (New York: John Wiley & Sons, 1950), 12th ed., Design and Production Volume, p. 15-28.

Composed by J. B. Calvert
Created 25 January 2007
Last revised