A universal coupler is a specialized rotary joint used to allow a rotating split shaft to deflect along its axis in any direction. In other words, the universal coupler would allow a boat drive shaft exiting a gearbox standing dead level to drive a propeller orientated at 22° from horizontal. This flexibility is achieved by constructing the joint with two U-shaped yokes joined by a cross-shaped hub. One yoke is attached to the end of each portion of the split shaft and joined with the cross hub, with the U-sections orientated at 90° to each other. This arrangement allows the horizontal primary shaft to drive the inclined shaft with no undue friction or loss of speed or drive output potential.
Many mechanical applications require an inclined drive output from a horizontally-orientated drive source. This poses a problem when rigid drive shafts are used, a problem most commonly solved by using a universal coupler. Also know as a universal joint, Cardan joint, or U-joint, this coupler allows a solid or hollow, two-piece drive shaft to "bend" in any direction and at a wide range of deflection values without losing any of the original drive potential.
The universal coupler is actually relatively simple. Each coupler consists of two U-shaped yokes, one each for the ends of the two halves of the shaft. To join the two shaft halves, the two yokes are brought together, orientated at 90° to each other and joined by a cross hub. The cross hub typically takes the form of a disc or ball with two round shafts passing through it at right angles forming a cross shape. The flat "ears" of each yoke U have holes drilled through them through which the shafts of the cross hub are placed. This allows the primary and secondary shafts to hinge on the cross hub and rotate smoothly even if orientated at different angles.
These couplers feature excellent performance in addition to the drive orientation versatility that they allow. The universal coupler does not have any adverse effect on drive speed, does not generate any significant friction, and is robust and capable of transferring large torque loads. The universal coupler is widely used in industrial, marine, and automotive applications, and may even be found in socket tool sets. Used with the socket ratchet handle, they allow the tools to be used in tight spaces where deflected torque inputs are required.