The U-joint allows two rotating shafts to operate at a slight angle to each other. The original joint was developed in the sixteenth century by a French mathematician named Cardan. In 1902, Clarence Spicer modified Cardan's invention for the purpose of transmitting engine torque to an automobile's rear wheels. By joining two shafts with U-shaped forks (called yokes) to a pivoting cruciform member, the problem of torque transfer through a connection that also needed to compensate for slight angular variations was eliminated.

There are three operating U-joint considerations that automotive engineers must keep in mind when designing rear-wheel drivelines: speed variations, phasing of U-joints, and canceling angles.

Speed Variations

Although simple in appearance, the universal joint is more intricate than it seems. This is because its natural action is to speed up and slow down twice in each revolution when operating at an angle. The rate of speed varies, depending on the steepness of the U-joint angle.

The universal joint operating angle is derived by taking the difference between the transmission installation angle and the drive shaft installation angle. To align better with the rear axle, the transmission is installed at an angle 5 degrees off the true horizontal line. The drive shaft installation angle is 8 degrees off the true horizontal. The difference between the transmission installation angle and the drive shaft installation angle is 3 degrees. Therefore, the universal joint operating angle is 3 degrees. When the universal joint is operating at an angle, the driven yoke speeds up and slows down twice during each drive shaft revolution. This acceleration and deceleration of the universal joint is known as speed variation or fluctuation.

The four speed changes are not normally visible during rotation. They might be felt as torsional vibrations due to improper installation, steep or unequal operating angles, and high speeds.

This is more easily understood after examining the universal joint action. A universal joint is a coupling between two shafts not in direct alignment, usually with changing relative positions. It would be logical to assume the entire unit simply rotates. This is true only for the universal joint's input yoke.

An ellipse is merely a compressed form of the circle. The output yoke's circular path looks like an ellipse because it can be viewed at an angle instead of straight on. This effect can be obtained when a coin is rotated by the fingers. The height of the coin stays the same even though the sides seem to get closer together.

This might seem to be a merely visual effect, but it is more than that. The U-joint rigidly locks the circular action of the input yoke to the elliptical action of the output yoke. The result is similar to what would happen when changing a clock face from a circle to an ellipse. The 12, 3, 6, and 9 o'clock are the same, but in between the time reading changes. At 2 o'clock on the circle, it is 2:20 on the ellipse.

Speed variation is more easily visualized when looking at the travel of the yokes by 90-degree quadrants. The input yoke rotates at a steady or constant speed through the complete 360-degree turn. The output yoke quadrants alternate between shorter and longer distance travel than the input yoke quadrants. When one point of the output yoke covers the shorter distance in the same amount of time, it must travel at a slower rate. Conversely, when traveling the longer distance (but only 90 degrees) in the same amount of time, it must move faster.

Because the average speed of the output yoke through the four 90-degree quadrants (360 degrees) equals the constant speed of the input yoke during the same revolution, it is possible for the two mating yokes to travel at different speeds. The output yoke is on an irregular track. The resulting acceleration and deceleration produces a fluctuating torque and torsional vibrations. This acceleration and deceleration is characteristic of all Cardan universal joints. The steeper the U-joint angle, the greater the fluctuations in speed. Conversely, the smaller the angle, the less the speed change.

Phasing of Universal Joints

Speed variations must be canceled at exactly the same point in drive shaft rotation. The two driving yokes at opposite ends of the drive shaft must be at the same point of rotation.

Single-piece drive shafts have the universal joint yokes welded into position when they are manufactured. On a two-piece drive shaft, a technician might encounter problems if care is not exercised, because the center universal joint must be disassembled to replace the center support bearing. The center driving yoke is splined to the front drive shaft. If the yoke's position on the drive shaft is not indicated in some manner, the yoke could be installed in a position that is out of phase. Manufacturers use different methods of indexing the yoke to the shaft. Some use aligning arrows. Others machine a master spline that is wider than the others. The yoke and shaft cannot be reassembled until the master spline is aligned properly. When there are no indexing marks, the technician should always index the yoke to the drive shaft before disassembling the universal joint. This saves time in the reassembly procedure. Indexing requires only a light hammer and center punch to mark the yoke and drive sahft.

Canceling Angles

Vibrations can be reduced by using canceling angles. Carefully examine the illustration, and note that the operating angle at the front of the drive shaft is offset by the one at the rear of the drive shaft. When the front universal joint accelerates, causing a vibration, the rear universal joint decelerates, causing a vibration. The vibrations created by the two joints oppose and dampen the vibrations from one to the other. The use of canceling angles provides a smoother drive shaft operation.