By Dr. Jody Muelaner
A universal joint is a connection between two objects, typically shafts, that allows relative rotation in two axes. It is made up of two revolute joints with perpendicular and intersecting axes.
When shafts are connected using a universal joint, each shaft terminates in a revolute joint with its axis perpendicular to the shaft’s rotational axis. This allows rotary motion to be transferred between the shafts while allowing misalignment in both remaining rotational degrees of freedom. A single rotational degree of freedom is constrained (the shaft rotation) as well as all relative translations, giving a universal joint two degrees of freedom (2-DOF).
The universal joint is not a constant-velocity joint. If the input shaft is rotating at a constant velocity, the output shaft’s velocity will oscillate. They will have the same average velocity but the output shaft’s velocity will be somewhat higher or lower than this average at any given time. The amount of oscillation in the output shaft depends on the amount of misalignment between the shafts, if the shafts are coaxial then the output shaft will in fact have a constant velocity.
It is possible to create a constant velocity joint by combining a number of universal joints. A double Cardan joint is an arrangement of two universal joints, with a short connecting shaft between them, 90° out of phase with each other. If any bend angle is shared equally between the two universal joints, then the two joints will cancel out velocity oscillations so that the final output shaft has a constant velocity. However, the oscillation of the intermediate shaft will cause vibrations and supports are required to maintain the equal angles.
Universal joints have been extensively used in vehicle drivetrains — but are being replaced by constant velocity joints. Universal joints are now rarely used to transmit power to the front wheels of vehicles, except for some heavy duty off-road vehicles. They remain widely used for drive shafts, although constant velocity joints are even starting to be used for these applications. Universal joints also have many other uses in mechanical control systems and industrial machinery. Universal joints allow large angles between shafts. For slight misalignment between shafts, a flexible coupling can be an alternative to a universal joint.
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