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A universal joint or U joint is a joint in a rigid
rod that allows the rod to 'bend' in any direction. It consists of a pair of ordinary hinges located close together, but oriented at 90° relative to each other.
Universal joints are common wherever a drive shaft needs to turn a corner; a drive shaft with a universal joint can freely rotate through the
universal joint, and no gears are required to couple the two ends. The most obvious example of this application of a universal
joint is in the drive shaft of automobiles.
However, when the two shafts are at an angle other than 180°, the driven shaft does not rotate with the same uniform speed as
the drive shaft; the more the angle goes toward 90° the jerkier the movement gets (clearly, when the angle is 90° the shafts
would even lock).
To prevent the jerky rotation of the axle or drive shaft there usually are two U-joints in a three part shaft assembly. The
second U-joint will convert the jerky movement back to an even, uniform speed of the third shaft.
homokinetic joint
Another way to prevent jerky movement is to use a Constant Velocity (CV) or 'homokinetic' joint ('homo' meaning 'same', 'kinetic' meaning 'movement' or motion).
A homokinetic joint has the same function as a U joint but is constructed with a cage and steel balls moving in grooves, inside a
'dome' (see picture).
- drive shaft from the transmission,
- steel balls (in this case 6) in a 'cage'. The balls run in grooves in the dome.
- cage, splined to the driveshaft
- spherical 'dome' and outer driveshaft, part of the hub of the wheel.
History
The concept of the universal joint is based on the design of gimbals, which have been in use since antiquity. The first person known to have to suggested its use for
transmitting motive power was Girolamo Cardano, an Italian
mathematician, in 1545, although it is unclear whether he produced a working model. In
Europe, the device is often called the Cardan joint or Cardan shaft. Robert Hooke produced a working universal joint in 1676, giving rise to
an alternative name, the Hooke's joint. It was the American car manufacturer Henry Ford who gave it the name universal joint.
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