CONTROL VALVES
SUB-SECTION 4D
When the telescope lever is in the neutral, centered.
1. Discard all old O-rings and back up rings. Wash all parts
position, the oil delivered to the valve by the telescope
in a clean mineral oil solvent and place them on a clean
pump is prevented from entering the telescope circuit by
surface for inspection.
the control and compensator spools. Since the oil is pre-
2. Carefully remove burrs by light stoning or lapping. Be
vented from passing through the valve, pressure will build
certain that there is no paint on mating surfaces of valve
up in the inlet and spool sections. This pressure is then fed
body. Check all parts for woof. Replace if necessary.
through the orifice in the compensator spool to the right
end of the spool. When the pressure the right end of the
3. Inspect the valve spool and bore for burrs and scoring. If
spool exceeds the force imposed on the spool by the
scoring is not deep enough to cause objectionable leakage,
compensator spring, the spool will shift to the left. When
the surfaces can be stoned or polished with crocus cloth. If
the spool is shifted to the left, the inlet port is connected to
scoring is excessive the valve body and spool must be
the tank port, and the pump delivery is diverted to tank at
replaced. Check the valve spool for freedom of movement
a very low pressure.
in the bore.
When the control spool is shifted, pump flow is metered
4. Check the relief valve for smooth operation in its bore.
across an orifice formed by the hole in the spool and the
The valve should move by its own weight.
body of the valve and into a passage within the spool. The
oil flowing into the spool will first lift the appropriate load
ASSEMBLY. To assemble the valve, proceed as follows:
check, and then flow onto the circuit through the work
port. The oil returning from the circuit will flow into the
other work port, lifting the load check at the opposite end
NOTE
of the spool, which opens a passage to the tank port.
Coat all parts with clean hydraulic oil to facilitate
reassembly and provide initial lubrication. Petroleum
The pressure in the circuit, imposed by the load, is fed
jolly can be used to hold O-rings in place during reas-
through a pilot pressure passage, which contains two check
sembly.
balls and an orifice, to the left end of the compensator
spool. The pressure at the left end of the spool adds to the
force applied to the spool by the compensator spring and
1. Install the O-ring and back up ring on
the relief valve
tends to shift the spool to the right. Pump pressure, on the
plug, with O-ring nearest the relief valve.
Install the relief
other hand, is fed through the orifice in the center land of
valve in its bore, then install the spring on
the relief valve.
the compensator spool to the right end of the spool.
lnstall and tighten the relief valve plug. Do
not over-tighten
plug.
As load changes occur, a corresponding pressure change will
2. Install new O-rings in each end of the spool bore.
be felt in the left end of the compensator spool, while
changes in pump pressure will be felt on the right end of
3. Install the spool in the bore from the end cap end.
the spool. These pressures, and the force applied by the
compensator spring, will interact with each other to shift
4. Install the end cap and attaching screws. Tighten the
the compensator spool either right or left. Increases in load
screws to 15 to 20 foot-pounds (2.1 - 2.8 m-kg).
pressure or decreases in pump pressure will cause the spool
5. Install the retainer plate, wiper, and wiper retainer over
to shift to the right. This tends to block the opening be-
the spool, and secure with screws.
tween the inlet and tank passage, forcing a greater volume
of oil to pass through the valve. Decreases in load pressure
or increases in pump pressure will cause the opposite to
occur. Thus, a constant flow of oil is, delivered by the valve,
CONTROL VALVE (36Z840)
regardless of changes in either load or pump pressure.
DESCRIPTION
The pressure acting on the left end of the compensator
Control valve 36Z840 is actually two individual pressures
spool also acts against the lower end of the main relief
compensated, directional control valves which are joined
poppet. When the load pressure exceeds the setting of the
together to form a single assembly. Each valve does, how-
poppet, the poppets unseats, venting the pressure on the
ever, function independently to control only one of the
left end of the compensator spool. When this occurs, the
two telescope cylinders in the boom (see Figure 4D-13).
compensator spool will immediately shift to the left,
opening the inlet port to the tank port. This will divert the
NOTE
main flow of oil back to tank, reducing the pressure in the
circuit. The relief poppet reseats when the pressure in the
The following description applies to one section of
circuit drops below the setting of the relief poppet, and
the valve only. The operation of the other valve
normal compensator operation resumes.
section is identical to the one described.
4D-17
