Universal
Joint
A
universal joint is a joint in a rigid rod that allows it to
bend in any direction. Universal joints are used to transmit
rotary motion between two shafts that are not in straight
line. In the simplest kind of universal joint, also known
as a Hooke’s joint, Cardan joint, or Hardy-Spicer joint, the
ends of the shafts to be joined end in U-shaped yokes. They
dovetail into each other and pivot flexibly about an X-shaped
spider. This construction allows side-to-side and up-and-down
movement, while still transmitting rotary motion.
In rear-wheel drive vehicles a Hooke’s joint provides a satisfactory
link between the rear axle transmission and the wheels, accommodating
the up-and-down motion of the wheels over bumps in the road.
However, in the case of front-wheel drive there is a complication
because of the fact that the front wheels not only have to
move up and down but must also be able to turn through large
angles for steering. A Hooke’s joint employed on a front drive
vehicle results in the drive being transmitted in a series
of uneven jerks when the wheels are turned through an angle
for steering. The dual role of the front wheels makes it necessary
to use a constant-velocity universal joint which transmit’s
the drive at an even speed, irrespective of the direction
in which the wheels are turned.
In one type of constant-velocity drive there is an inner spherical
member attached to the drive shaft with six semicircular section
grooves machined lengthwise along its outer surface. The driven
shaft member is a spherical cup with similar grooves machined
on its inner surface. The drive is transmitted by six steel
balls housed in the grooves of the inner and outer members,
part of the ball in each groove. A spherical ball cage with
six holes locates and controls the lengthwise position of
the balls in the grooves. The ball cage also locates the driving
and driven shaft members. The ball cage moves the balls along
the grooves so that the centers of the balls always lie in
a plane bisecting the angle between the driving and driven
shafts. This is the essential condition for true constant
velocity (I.e., transmission of drive without any speed variations
in the driven shaft).
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