the propeller shaft hold it in a centered position.
displacement is allowed by outward movement of the
Variations in length are permitted by the longitudinal
balls on the trunnion pins. This type of universal joint is
movement of the balls in the body grooves, and angular
recognized easily by the flexible dust boot that covers it.
Section III. CONSTANT VELOCITY JOINTS
driving engagement will, at all times, bisect the angle
Characteristics of Operation.
between the driving and the driven shaft.
a. The speed fluctuations caused by the
conventional universal joints described in paragraph 28-5
do not cause much difficulty in automotive propeller
a. Rzeppa (Fig. 28-5). The Rzeppa joint is a ball-
shafts where they have to drive through small angles
In front-wheel drives where the wheels are
bearing type in which the balls furnish the only points of
cramped up to 30 degrees in steering, velocity
driving contact between the two halves of the coupling.
fluctuations present a serious problem. Conventional
The details of the component parts, adapted for use in a
universal joints would cause hard steering, slippage, and
front driving axle, are shown In figure 28-5. The inner
tire wear each time the vehicle turned the corner.
race (driving member) is splined to the inner axle shaft;
Constant-velocity universal joints, which eliminate the
the outer race (driven member) is a spherical housing
pulsations, are used exclusively to connect the front axle
that is an integral part of the outer shaft; the ball cage is
shaft to the driving wheels.
fitted between the two races. The close spherical fit
between the three main members supports the inner
b. The conventional universal joint produces
shaft whenever it is required to slide in the inner race,
velocity fluctuations because the journal connecting the
relieving the balls of any duty other than the transmission
two yokes does not allow free movement other than a
of power. The movement of the six balls is controlled by
pivoting action. Velocity fluctuations occur because the
the cage. The cage positions the balls in a plane at right
journal tilts back and forth (wobbles) as the joint rotates.
angles to the two shafts when the shafts are in the same
This tilting movement is translated into rotary movement
line. A pilot pin, located in the outer shaft, moves the
and, when the journal tilts toward the output shaft, it adds
pilot and the cage by a simple leverage in such a manner
to the speed of the output; and when the journal tilts
that the angular movement of the cage and the balls is
away from the output shaft, it subtracts from the speed
one-half the angular movement of the driven shaft.
and the output shaft rotates slower than the input shaft.
When the driven shaft is moved 20 degrees, the cage
The only time that the speeds of the two shafts are equal
and the balls move 10 degrees. As a result, the balls of
is when the journal lies in the plane that bisects the angle
the constant-velocity universal joint are positioned from
between the two shafts. As stated in paragraph 28-5,
the top view, to bisect the angle formed.
this occurs only four times during each revolution.
b. Bendix-Weiss (Fig. 28-6). The Bendix-Weiss
c. It can be seen that a universal joint transmitting
joint also uses balls that furnish points of driving contact,
constant velocity must be designed to permit the point of
but its construction differs from that of the Rzeppa in that
driving contact between the two halves of the coupling to
the balls are a tight fit between the two halves of the
remain in a plane that bisects the angle between the two
coupling and no cage is used. The center ball rotates on
shafts. If this is accomplished, some arrangements must
a pin inserted in the outer race, and serves as a locking
be made for the points of the driving contact to move
medium for the four other balls. The driving contact
laterally as the joint rotates. Keeping this in mind, it will
remains on the plane that bisects the angle between the
be easier to understand the principles of constant-
two shafts, but it is the rolling friction between the four
velocity joints that are in universal use today. Three
balls and the universal
types used in Army vehicles are: Rzeppa, Bendix-Weiss,
and Tracta. These types are discussed separately to
show that, in all, a plane passed through the points of the