When a driveshaft must rotate through an angle, a universal joint is used to transfer torque across that angle. Universal joints can become faulty over time and in most cases when they start to exhibit symptoms of wear the joints will need replacing.
Symptoms of a bad universal joint
A worn universal joint (also known as a U-joint) is the most common driveshaft problem. This problem usually makes itself known with abnormal noises. Symptoms may include squeaking, grinding, clicking or clunking sounds when the car is in gear. Before the problem becomes severe, these symptoms may appear at first only when the car is in reverse.
Universal joint problems
Many times, the grease inside a universal joint will dry out. This leads to small indentations being worn by the roller bearings into the part of the joint known as the cross. As the bearings are forced to roll across the indentations, a loud grinding or chirping may be heard. When these problems become evident, it is time to replace the universal joint.
Most driveshafts employ two U-joints. The joints permit the vehicle to have changes in the alignment between the transmission and the differential. Essentially, the universal joint works by connecting two yokes at 90-degree angles to a cross. The yokes are free to pivot, and so the joint can spin while the driveshaft changes its angle.
Universal joints come in three basic types. A cross-and-roller is the most common, and is also known as a cardan universal joint. Sometimes, when a cross-and-roller joint has to be driven across a sharp angle, the output speed can fluctuate with each revolution. In these cases, to eliminate the problems associated with torsional vibrations, another type of U-joint is used -- the constant velocity joint, or CV joint. A third type of universal joint, the ball-and-trunnion joint, serves the same purpose as a CV joint.
If a vehicle's driveshaft had to go from the transmission to the differential in one piece and had no means of changing its angle, then every time the differential moved (with respect to the transmission) huge amounts of stress would be placed on the driveshaft, possibly snapping it. Moreover, without universal joints, it would be pretty near impossible to get the drive train in a straight line from the engine to the transmission to the differential.