The time this process takes depends on how easily the
synchro ring moves and the rate of frictional energy
transfer between the two synchronizer surfaces. Higher
viscosity lubricants slow the sliding of the synchro ring on
the input shaft and require a longer time for the oil to be
squeezed out from between the mating synchronizer
surfaces. After the lubricant is squeezed out, the
coefficient of friction of the lubricant determines the rate
of frictional energy transfer between the two surfaces.
Slippery lubricants such as hypoid gear oils can take too
long to synchronize the gears, which promotes
synchronizer wear. Red Line MTL and MT-90 has a
coefficient of friction which is greater than conventional
oils, allowing a quicker transfer of frictional energy. The
graph below shows the desirable friction curve
demonstrated by Red Line MTL compared to
conventional lubricants. Note how the coefficient of
friction is greater for the MTL than all others except the
motor oil. Some motor oils have an adequate dynamic
coefficient of friction, but most have problems with the
static and low velocity coefficient of friction which can
result in clashing.
Clashing of the gears (actually clashing of the
synchromesh gears, since the drive gears are always in
contact) can occur if excessive shift effort is used in
order to shift the locking ring into place before the
surfaces have achieved equal speed. It may also occur
at the end of a reasonably smooth shift if an instability
exists in the coefficient of friction, causing stick-slip to
occur. In this form of clashing, the ring slides on the
mating synchro gear, but a sudden slippage causes a
grinding of the gears. In order to prevent stick-slip from
occuring and the gear clashing which results, the
coefficient of friction should inflect downward as the
relative speed drops to zero.
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