Komatsu 830E, Shop Manual

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L7-4 Hoist Circuit L07027
HOIST PILOT VALVE
The hoist pilot valve (1, Figure 7-4) is located in the
hydraulic component cabinet directly behind the
operators cab. The hoist pilot valve spool is spring
centered to the hold position. The valve is controlled
directly by the operator through a lever and cable
arrangement. The control lever is located between
the operator and center console. When the operator
moves the lever, the pilot valve spool moves and
directs pilot flow to the appropriate pilot port on the
hoist valve causing the main spools to direct working
pump flow to the hoist cylinders.
The hoist pilot valve is equipped with a one way load
check valve which allows free flow from the center
passage to bridge core and prevents reverse flow.
The hoist pilot valve is also equipped with a power
down relief valve (8). The power down relief valve is
located between the power down control port and
return galley. The power down relief valve limits
power down pressure at 1500 psi (10.5 MPa).
HOIST-UP LIMIT SOLENOID
The Hoist-up Limit Solenoid (2, Figure 7-4) is used in
the hydraulic circuit to prevent maximum hoist cylin-
der extension.
This solenoid valve is “normally open” between the
hoist pilot valve raise port and the hoist valve raise
pilot port. The return-to-tank port is “normally closed”.
The solenoid is controlled by the proximity switch
(hoist limit switch) located inside the rear frame rail
near the body pivot and above the right rear suspen-
sion. When the solenoid is signaled by the proximity
switch, the raise pilot port on the hoist valve is
closed. The hoist pilot valve raise port is opened to
return to tank.
PILOT OPERATED CHECK VALVE
The Pilot Operated Check Valve (7, Figure 7-4) is
opened by power down pilot pressure to allow oil in
the raise port to bypass the hoist up limit solenoid for
initial power down operation while the solenoid is
activated by the hoist limit switch.
COUNTERBALANCE VALVE MANIFOLD
The counterbalance valve manifold (12, Figure 7-2)
is mounted at the rear of the pump/hoist valve mod-
ule. The counterbalance valve (13) controls the pres-
sure (like a relief valve) of the oil in the annulus area
of the hoist cylinders when the body approaches the
maximum dump angle. The valve restricts the maxi-
mum pressure build-up by relieving pressure in
excess of 3000 psi (20.7 MPa), preventing possible
seal damage. Also see Figure 7-5 for a schematic
view of the counterbalance valve.
There are 2 pilot pressures that can open it (cylinder
head raise pressure & rod return pressure. If there is
no raise pressure, it will take 3000 psi of rod end
return pressure to open it. If there is theoretically 666
psi or more of raise pressure, it is wide open for the
return oil. In between these 2 maximums the ratio of
raise:return pressure is 1:4.5 to open the return flow
Quick disconnect fittings (14) installed on the mani-
fold allow service personnel to dump a load in the
truck body if the engine, hoist pump, or other hoist
circuit component is inoperable, by connecting hoses
to the quick disconnects of an operable truck.
FIGURE 7-4. HOIST PILOT VALVE
1. Hoist Pilot Valve
2. Hoist-Up Limit
Solenoid
3. Control Cable
4. Supply From Hoist
Valve
5. Pilot Pressure to
Hoist
6. Return Line
7. Pilot Operated Check
Valve
8. Power Down Relief
Valve