Chevrolet Light Duty Truck 1973 Service Manual page 232

4 -8 REAR S U S PEN SIO N AND DRIVELINE
THEORY OF OPERATION
Universal Joints
The simple universal joint is basically two Y-shaped
yokes connected by a crossmember called a spider. The
spider is shaped like an X and arms that extend from it
are called trunnions. See figure 18.
The spider allows the two yoke shafts to operate at an
angle to each other. When torque is transmitted at an
angle, through this type of joint, the driving yoke rotates
at a constant speed while the driven yoke speeds up and
slows down twice per revolution. This changing of
velocity (acceleration) of the driven yoke increases as the
angle between the two yoke shafts increases. This is the
prime reason why single universal joints are not used for
angles greater than three to four degrees. At four
degrees, for example the change of velocity is
degrees it is 3%. If the universal joint were set at 30
degrees and the driving yoke were turning at 1000 RPM
the velocity of the driven yoke would change from 856
RPM to 1155 RPM in one quarter of a revolution. In the
remaining quarter revolution the velocity would change
from 1155 RPM to RPM.
866
On a one-piece drive shaft this problem can be
eliminated by arranging two simple universal joints so
that the two driving yokes are rotated 90 degrees to each
other. However the angle between the drive and driven
yokes must be very nearly the same on both joints for
this to work. Refer to figure 19. This allows the alternate
acceleration and deceleration of one joint to be offset by
Fig. 18—Simple Universal Joint
PROPELLER SHAFT
Theory of Operation...................
Component Parts Replacement
At ten
.5%.
INDEX
............................................. 4-8
............................................. 4-9
the alternate deceleration and acceleration of the second
joint. When the two joints do not run at approximately
the same angle, operation can be rough and an
objectionable vibration can be produced.
Universal joints are designed to consider the elfects of
various loadings and rear axle windup, during accelera­
tion. Within the design angle variations the universal
joints will operate safely and efficiently. However, when
the design angles are exceeded the operational life of the
joints may decrease.
The bearings used in universal joints are the needle
roller type. The needle rollers are held in place on the
trunnion by round bearing cups. The bearing cups are
held in the yoke by either (depending on the
manufacturer) snap rings or plastic injection. These
joints usually are lubricated for life and cannot be
lubricated while on the vehicle.
Constant Velocity Joint—Double Cardan
Joint
As mentioned previously, the simple universal joint will
operate efficiently through small angles only. Also, two
simple universal joints phased properly and operating
through the same angle will transmit constant velocity.
When a large angle is encountered in a driveline, a
simple universal joint will introduce two vibrations in
each revolution. It is in this situation that a constant
velocity joint is used.
Essentially, the constant velocity joint is two simple
universal joints closely coupled by a coupling yoke,
phased properly for constant velocity.
A centering ball socket between the joints maintains the
relative position of the two units. This centering device
causes each of the two units to operate through one-half
of the complete angle between the drive shaft and
differential carrier. See figure 20.
Fig. 19—Universal Joints Arranged for Constant
Velocity
LIGHT DUTY TRUCK SERVICE MANUAL