Unfortunately this is
where the problem lies. The transfer of drive happens very quickly
and so happens for only a short length of time because, as soon as the
wheels have stopped slipping, they are turning equally once more
and so are the plates inside the coupling. This then stops the
slicing of the silicone fluid and the heat subsides.
If
the tyre treads are of different depths then this will result in
the plates never being able to spin at the same speeds and so the
heat is built up by the constant cutting of this silicone fluid.
Now
this does not affect the viscous coupling itself as this is
designed for heat to be present due to the way the viscous
coupling works but, as the transfer box and its components are made of metal, the heat is conducted to the rest of the
transfer box over a period of time.
So
what does this heat affect then ?? -
1.
As
the heat travels through the transfer box it reaches the clutch
rings. As mentioned in the TRANSFER
BOX section there are 2 types of clutch ring - rings with
teeth on the inside (front rings) and ring with teeth on the
outside (rear rings). The front rings contain clutch like material
on each side whereas the rear rings do not.
Front Clutch Rings with
section of Clutch material
When
the heat reaches these clutch rings the clutch material is
affected. The result is the clutch material starts to break up and
fall apart leaving the surface of the front rings uneven. The
Pressure Plate then has to apply more force to the clutch rings in
order to keep them pressed together and maintain the drive to the
rear axle.
As
this pressure increases over time, the sealing gasket between the
viscous coupling and the front of the box starts to weaken and
then fails completely.
This
gasket is what keeps the Power Assisted Steering fluid of the
transfer box and the oil section of the transfer box separate.
With
this sealing gasket now gone, the 2 fluids are able to mix. The
power of the PAS pump fills up the transfer box with PAS fluid and
forces it up and out of the breather hose that is situated by the
coolant tank.
This is the most common failure of
the transfer box and is easily prevented -
As
mentioned, it is the CONSTANT heat from the viscous
coupling that travels, through conduction, along the transfer box
to the clutch rings and this is caused by the viscous coupling
plates constantly spinning at different speeds, causing the fluid
in the coupling to heat up all the time.
To
solve this problem, the plates need to be rotating at the same
speed which will stop the heating up process. To do this, all 4
wheels must be rotating at the same speed. This can only occur if
the wheels and tyres are of the same diameter and so it is very
important to make sure that all 4 tyres have their tread depth
within 2mm of each other as this will minimise the differences in
rotational speeds and thus the heat within the transfer box.
To
achieve this practically, it is recommended to rotate your wheels
(front to back) every 1500-3000 miles
2.
A similar problem that can occur with the transfer box itself
centers around the sealing gasket mentioned above.
Again
it is heat related. As the heat travels along the transfer box it
not only affects the clutch rings but can affect the sealing
gasket directly. The sealing gasket will deteriorate over time if
exposed to extended periods of heat causing it to fail, again
allowing the PAS fluid to enter the transfer box and force the oil
up the breather pipe.
It
has been discussed about using a different material for the
sealing gasket but unfortunately this can lead to problems under
cool conditions which is generally normal transfer box temperatures.
Again,
to solve this problem, it is necessary to rotate the wheels so
that the tread depth of all 4 wheels is within 2mm of each other
as this will stop the heat build up. To
achieve this practically, it is recommended to rotate your wheels
(front to back) every 1500-3000 miles
