Specifications
Power Train Systems Operation 33
Modulating valve movement during clutch fill
When a new direction is selected by the operator, the
selector spools open up a circuit to the new clutch
piston. System pressure drops as the new clutch
piston is stroking. This drop in supply pressure causes
a force imbalance on the modulating valve / reaction
slug pressure becomes smaller. Since Force 1 is still
the same, Force 1 forces the modulating valve to the
right until the end of the modulating valve opens the
load piston cavity to drain.
The load piston oil dumps to drain and the load piston
immediately moves to the right (shown in fig.2).
This action is called "load piston reset". It happens
very quickly in comparison to the time needed for the
clutch piston to fully stroke. Therefore the modulating
valve and load piston are ready to start controlling the
clutch pressure in a smooth upward manner once the
clutch piston finishes stroking.
Modulation of clutch to top pressure
As long as reaction slug pressure is greater than load
piston pressure, oil will flow across the orifice from
Area 2 to Area 1. As oil flows to the load piston the
springs will continue to compress, allowing the load
piston to move to the left. As the load piston moves to
the left the spring force increases and load piston
pressure increases.
As press (LP) increases the modulating valve will
cause a corresponding increase in clutch pressure in
order to keep the forces balanced. In simple terms as
the load piston strokes to the left the clutch pressure
rises to maximum system pressure.
This controlled rise in clutch pressure takes about 0.7
sec. and is shown in fig.4. It occurs immediately after
the clutch piston completely strokes (end of clutch fill).
Figure 2
Figure 3
MODULATION
Figure 4