two yokes (fig. 5-3). The two yokes are set at right
angles to each other and their open ends are connected
by the journal. This construction permits each yoke to
pivot on the axis of the journal and also permits the
transmission of rotary motion from one yoke to the
other. As a result, the universal joint can transmit
power from the engine through the shaft to the rear
axle, even though the engine is mounted in the frame at
a higher level than the rear axle, which is constantly
moving up and down in relation to the engine.
Figure 5-3.Simple universal joint.
A peculiarity of the conventional universal joint is
that it causes a driven shaft to rotate at a variable speed
in respect to the driving shaft. There is a cyclic
variation in the form of an acceleration and
deceleration of speed (fig. 5-4). Two universal joints
are placed in a drive shaft to eliminate the speed
fluctuations of the shaft while the shaft is at an angle to
the power source. The universal joints are placed at a
90-degree angle to each other and one counteracts the
action of the other while in motion.
Three common types of automotive drive shaft
universal joints are used on rear-wheel drive vehicles:
cross and roller, ball and trunnion, and double-cardan
(constant velocity) universal joints.
Cross and Roller Universal Joint
The cross and roller design is the most common
type of drive shaft U-joint. It consists of four bearing
caps, four needle roller bearings, a cross or journal,
grease seals, and snap rings (fig. 5-5).
The bearing caps are held stationary in the drive
shaft yokes. Roller bearings fit between the caps and
the cross to reduce friction. The cross is free to rotate
inside the caps and yokes. Snap rings usually fit into
grooves cut in the caps or the yoke bores to secure the
bearing caps and bearings. There are several other
methods of securing the bearing caps in the yokes.
These are bearing covers, U-bolts, and bearing caps.
Figure 5-4.Speed fluctuations caused by conventional universal joints.