Universal joint

A universal joint, U joint, Cardan joint or Hardy-Spicer 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.

Related Topics:
Cardan - Joint - Hinge

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Universal joints are common wherever a driveshaft needs to turn a corner; a driveshaft 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 driveshafts of automobiles, a technology known as the Hotchkiss drive.

Related Topics:
Driveshaft - Automobile - Hotchkiss drive

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However, when the two shafts are at an angle other than 180°, the driven shaft does not rotate with the same speed as the drive shaft; the more the angle goes toward 90° the jerkier the movement gets (clearly, when the angle β = 90° the shafts would even lock).

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The angular speed ω2 of the driven shaft, as a function of the angular speed of the driving shaft ω1 and the angle of the driving shaft φ1, is found using:

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:omega_2 = rac{omega_1coseta}{1-sin^2etasin^2phi_1}

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and the angular acceleration,

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:lpha_2 = rac{omega_1^2sin^2etacosetasin 2phi_1}{1-sin^2etasin^2phi_1}

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To prevent the jerky rotation of the axle or driven 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 - if both the driving and the driven shaft are parallel.

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