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| UJ
joint care |
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Rear-wheel
driven vehicles, which include the majority of trucks
and buses, usually have a universal joint (UJ) at
each end of the propshaft. Cars with independent rear
suspension have either a UJ or a CV-joint at each
end of the driveshaft linking the differential unit
to the rear wheels. UJs have an easier life than CV
joints because the rear wheels don't have to steer.
Because of this, they also have a far simpler construction.
This results in a cyclic movement being transmitted
to the wheels, which doesn't seem to matter on non-steering
wheels.
UJ
maintenance consists, mainly, of a regular inspection
to make sure the unit is not dry. On some cars there
is still a grease nipple, whereas on others there's
no greasing provision, because the unit has been filled
with grease, and sealed. This means that UJs often
suffer from inadequate lubrication, and running dry
is also the main reason why these units fail. Interestingly
enough, a UJ can also fail from too much lubrication.
If you pump too much grease in through the nipple,
the sheer volume will destroy the seals at the entrance
to the cups, resulting in grease leaking out and dirt
getting in.
 There
is only one way that you can inspect a UJ properly,
and that is to remove the shaft from the vehicle, an
operation that usually means you have to remove four
bolts and pull the driveshaft rearwards away from the
gear-box. This should, preferably, be done with the
rear of the vehicle jacked up, otherwise some of the
gearbox oil will run out. Move each joint by hand to
feel for signs of stiffness or excessive play. The cross-pin
rotates in rollers and it should be free to rotate,
but with no play in any other direction. The joint should
be replaced if the rollers are dry and worn out, or
if the joint feels stiff and notchy. Do not try to revive
a notchy joint by greasing, because it is a signal that
the rollers are worn, or their contact surfaces are
ridged. On the road, a joint needing replacement will
either clonk or snatch when the drivetrain is jerked
with the throttle in first or second gear.
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