Borg Warner Transfer Box: Description and Operation - Part 16
TRANSFER BOX
DESCRIPTION AND OPERATION
9
Viscous coupling (VCU)
The VCU comprises a short cylinder which contains
an inner shaft with slotted discs attached to its outer
surface, and a similar set of discs attached to the
inner surface of the cylinder. Both sets of discs are
arranged so that they interleave alternately and in
close proximity to each other. The VCU is sealed,
and filled with a type of silicone jelly which has the
property of increasing its viscosity with rises in
temperature and shear forces.
Variations in speed between the front and rear
output shafts are transmitted through the VCU, with
the speed differential occurring between the inner
shaft and the cylinder. In normal road conditions
where the speed variation between front and rear
shafts is low, the difference in rotational speed
between the VCU discs is also low. As a result, the
shear forces acting on the silicon jelly are marginal
and offer little resistance to the different rotational
speeds of the output shafts.
In cases where large rotational speed differences
occur between the front and rear output shafts, such
as in rough terrain conditions, the speed variation
between the discs is high with a subsequent
increase in the shear forces acting on the viscous
jelly. The resulting increase in viscosity generates
sufficient shear resistance to force both sets of discs
to rotate at similar speeds, reducing axle slippage
and loss of traction.
