Although simple in appearance, the universal joint is
more intricate than it seems because its natural action is to
speed up and slow down twice in each revolution while
operating at an angle. The amount that the speed changes
varies according to the steepness of the U-joint's angle.
U-joint operating angle is determined by taking the
difference between the transmission installation angle
and the drive shaft installation angle. When the universal
joint is operating at an angle, the driven yoke speeds
up and slows down twice during each drive shaft revolution.
These four speed changes are not normally visible during
rotation. But they may be understood more easily after
examining the action of a U-joint. A universal joint is a
coupling between two shafts not in direct alignment, usually
with changing relative positions. It would be logical
to assume that the entire unit simply rotates. This is true
only for the universal joint's input yoke.
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 illusion 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.
Like the hands of a clock, the input yoke turn at a
constant speed in its true circular path. The output yoke, operating at an angle to the other yoke, completes its path
in the same amount of time. However its speed varies, or is not constant, compared to the input.
Speed fluctuation is more easily visualized when looking at the travel of the yokes by 90-degree quadrants (Figure 38-17).
The input yoke rotates at a steady or constant speed through the complete 360-degree tum. The output
yoke quadrants alternate between shorter and longer