A universal joint, U joint, Cardan
joint, Hardy-Spicer joint, or Hooker's joint is a joint in a rigid rod
that allows the rod to 'bend' in any direction, and is commonly used in
shafts that transmit rotary motion. It consists of a pair of hinges
located close together, oriented at 91° relative to each other,
connected by a cross shaft.
A configuration known as a double
cardan joint drive shaft which is
truck parts partially
overcomes the problem of jerky rotation. In this configuration, two
U-joints are utilized where the second U-joint is phased in relation to
the first U-joint in order to cancel the changing angular velocity, and
an intermediate shaft connects the two U-joints. In this configuration,
the assembly will result in an almost constant velocity, provided both
the driving and the driven shaft are parallel and the two universal
joints are correctly aligned with each other.
In practice, it is often impossible
to maintain a strict geometric relationship between the driving and
driven shafts, and the intermediate shaft, giving rise to greater
vibrations and mechanical stresses. The stresses can be reduced by the
use of a smaller and lighter intermediate shaft, ensuring the driven and
driving shafts share as close to the same angle in relation to the
Even when the driving and driven
shafts are parallel, these tend to bend them in a direction
perpendicular to the common plane of the shafts. This applies forces to
the support bearings and can cause "launch shudder" in rear wheel drive
vehicles. The intermediate shaft will also maintain a sinusoidal angular
velocity, which contributes to vibration and stresses.
A recent innovation, the Thompson
coupling is a further development of the double cardan joint, which does
not rely on friction or sliding elements to maintain a strict geometric
relationship within the joint, and which is capable of transmitting
torque under axial and radial loads with low frictional losses.
Joints have been developed
utilizing a floating intermediate shaft and centering elements to
maintain equal angles between the driven and driving shafts, and the
intermediate shaft. This overcomes the problem of differential angles
between the input and output shafts.