Gravely 950001, Maintenance Manual

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Trencher Valve Circuit Description
When the PTO is engaged to power the trencher oil
flow from the “a” port of the PTO valve is directed to the
VLVA port of the trencher valve.
Pilot pressure builds against the pressure
compensated valve Item 7 to shift its spool diverting
pressure and flow to the trencher motor via coupling
“A”. See circuit sketch 1 below for flow path in this
hypothetical initial condition.
In the initial condition as shown above all flow would
return from the trencher motor to power the drive
wheels, this flow rate would most likely be excessive.
The drive wheels would attempt to travel faster than the
trenching chain cutting into the ground will permit,
causing system pressure to rise. When the system
pressure reaches 150 bar (2200 psi.) sequence valve
Item 5 opens, causing the pressure compensated valve
Item 7 to modulate, thereby directing surplus flow into
the trencher return line by-passing the trencher motor
via the over center valve Item 4. This “bleeds-off” the
surplus flow directly back to tank.
NOTE: The over center valve Item 4 is in the fully open
position at this point. With its setting of 3000 psi and a
4.25:1 ratio, it opened at 705 psi.
The pressure compensated valve Item 7 continues to
modulate as the ground conditions vary between hard
and soft. See circuit sketch 2 for regulated balanced
flow.
Notice in the circuit sketch above that a pilot pressure
signal is now applied to both ends of the spool. This
causes the pressure compensated valve to modulate
automatically according to the load, thereby varying the
amount of oil flow that is diverted for bleed-off to tank.
Note also, that a percentage of oil is bled-off to tank
and the remainder continues onto the drive wheels in
the normal way. This is because the over center valve
Item 4 is set to 210 bar (3000 psi) i.e. above the
pressure required to by the trencher and the drive
wheels. Full system pressure up-stream of the
restrictions, needle valve Item 2 and the pressure
compensated valve Item 7 is applied to the over center
valve to allow bleed off.
CIRCUIT SKETCH 1
(Initial Condition)
1
5
CIRCUIT SKETCH 2
(regulated flow)
Bleed-Off
Pressure Compensated
Valve
5
CIRCUIT SKETCH 3
(Trencher reverse flow)
5
1