Driving shafts and driven shafts used to transfer torque (transmission to differential/rear-end, differential/rear-end to wheel, etc.) in a vehicle’s drivetrain are typically aligned at an angle to each other rather than being linear. Moreover, their geometry is dynamic as the suspension moves up and down. To accommodate the angled drivelines, it is necessary to connect the shafts with some type of flexible coupling to “bend” the torque being transferred.
Universal-joints and constant velocity (CV) joints both enable torque “bending”, but U-joints have severe limitations with large angles. When a U-joint is bent, two of the arms on the center cross (spider) travel in one elliptical path while the other two arms follow a different elliptical path. This causes a change in speed between the driving and driven shafts that results in vibration. The vibrations increase in amplitude as the angle increases – excessive angles lead to binding of the shafts as well.
CV joints handle joint angularity differently. One type of CV joint, used as an outboard joint, is designed so that when it bends, its steel ***** held in grooves or slots are positioned so that they always travel in a circular path exactly half way between the joint angle. This circular path is what ensures constant velocity by eliminating the cyclic variations in speed that a U-joint experiences when it operates at more than a few degrees off-center. Another type of CV joint, used an inboard joint, is designed to move or plunge in and out to compensate for changes in driveshaft length that occur as the suspension moves up and down – something a U-joint can not do.
Thus, a CV joint performs essentially the same tasks as a U-joint, (and then some) yet better, and is designed to be totally maintenance free – provided it is properly sealed and remains that way.