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A universal joint, or u-joint, is one of many names given to a type of gimbal-based joint design that connects two rods and allows them to bend in virtually any direction while spinning. Other names for the design include Cardan joint, which is the common term in Europe; the now-archaic Hooke's joint; and Hardy-Spicer joint, named for a pair of inventors who developed a variant on the traditional design. A universal joint is typically used in situations where long shafts are rotating in place and transmitting power, such as an automobile's drivetrain.
The idea of a joint that leverages the way gimbals work has been around since the Classical eras of ancient Greece and Rome. Its application in the transfer of motion, as it is used in modern vehicles, was not conceived until around the middle of the 16th century by Italian scientist Gerolamo Cardano, for whom the joint is still named in Europe. Others subsequently refined the design, and by the 19th century several variations were patented and remain familiar in the 21st century.
The articulation of a universal joint can be modeled using the thumb and forefinger of each hand. First, make a letter U with each thumb and forefinger and bring them together — but not touching — in a perpendicular fashion, with one set facing vertically and the other horizontally. Then, as if an imaginary ball is being held at the four distinct points, it is possible to emulate the wide range of motion a universal joint is capable of, by moving the hands side-to-side and up-and-down.
In actual u-joints, the U shape on the end of each rod is created by forking the end of each piece of metal, like a trident missing the middle tine. These pieces connect to a disk that has round studs coming out at four equal points along its circumference. The studs connect to the ends of each U and become the points of articulation for the joint.
Though highly versatile and effective in a wide variety of applications, universal joints do suffer from a propensity to wear out quickly. This is due to the fact that they are in constant use whenever a vehicle is operating. A properly functioning u-joint should have only a minimal amount of excessive movement, or play. In a passenger or commercial vehicle, too much play is often signaled by a loud clunking sound when shifting the transmission into gear, and noticeable vibration at low speeds.
U-joint failure will render a vehicle inoperable, so any unusual sounds emanating from underneath the cabin should be checked by a mechanic as soon as possible. A universal joint replacement is typically not a hugely expensive repair, as vehicle drivetrains are easily accessible and replacement parts reasonably cheap. In fact, it is considered a do-it-yourself repair for the moderate at-home amateur mechanic, with instructions available on the Internet and in guidebooks.