Twin Disc MG-5170DC, Service Manual

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Twin Disc, Incorporated
Operation
Marine Transmission Service Manual #1020667
Manual Control Valve — Primary
When a shift to the primary position is desired, the control valve lever is
moved.
The shift causes the control valve stem to rotate and assume the
position indicated in Figures 9 and 10. The pressurized oil in chamber B is
directed through ports G and K to passages D and J. Passage D is aligned
with a passage directing main pressure oil to the primary clutch. Pressurized
oil from port K travels through passage J and enters chamber L through an
orifice in the orifice plate. The orifice in this plate meters the oil for a steady,
smooth pressure rise in chamber L. As chamber L fills with oil, the pressure
rate-control piston moves against the springs until the piston is stopped by a
shoulder in the valve body. This causes the pressure in chamber B to rise to
clutch engaging pressure. When in primary, passage E remains at atmospheric
pressure because slot F remains open to sump.
When a shift is made from primary to neutral, the valve stem is rotated to the
position illustrated by Figures 7 and 8.
Under these conditions, passage D is
connected to sump by slot F. Passage J also is connected to sump by port H
in the valve stem. Because passage D is connected to slot F, oil drains rapidly
from the primary clutch to sump. Because passage J is now at atmospheric
pressure, the oil pressure in chamber L unseats the steel ball against the
compression spring permitting a rapid oil drain from chamber L to sump and
allowing the pressure-rate control piston to move back against the orifice
plate. The primary clutch is now disengaged and main system pressure reduced
to neutral pressure.