More about Universal Joints

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Universal joints allow two attached coplanar shafts to turn in multiple directions. They are widely used in drive shaft applications and situations where shaft force needs to transmitted through various angles. Multiple universal joints can be employed to provide relatively smooth motion, and correct the jerking caused by a single joint.

A wide range of metals and synthetics are used in the manufacture of universal joints, although synthetics are generally suitable only low stress and force applications. Common materials used include stainless steel, steel, naval brass and other similar alloys. Universal joints achieve optimal life and performance when booted; a boot is a synthetic sleeve that helps the joint retain lubrication, prevents lubricants from drying out and protects the moving part from wear and from foreign contaminants.

Universal joints are generally rated for operating angles and maximum speeds (measured in rotations per minute) and torque. As the angle of the connected shafts gets larger, the working torque that can be achieved by the joint becomes lower. Additionally, some shafts may be rated for maximum speeds, but not for maximum torque, and vice versa. Other important characteristics of universal joints are length, diameter of the joint and any fastening specifications. Certain joints may have holes bored in either end in order to provide a fitting for the shaft.

Generally, universal joints are one of three varieties: pin and block, ball and socket, or needle bearing. A pin and block joint consists of a central block with four pin attachments—two for each shaft. The pins are perpendicular to one another, and the block is held between the ears of both shafts. A ball and socket universal joint includes a central ball that is fit between the two shafts. The shafts have special ear fittings that enable them to move freely around the ball, permitting an extensive range of movement. Needle bearing universal joints are similar to pin and block joints, except that a needle bearing is held between the shaft ears.