COUNTERBALANCE VALVES
SUB-SECTION 5F
6. Lubricate the valve body bore with clean hydraulic oil,
position could be caused by contamination or a worn
and install the poppet subassembly. Install the cap and the
piston and/or seat. Proceed as follows to determine the
cause:
four capscrews which secure the cap to the valve body.
7. Tighten the capscrews to 45 ft-lbs (6.2 m-kg) in the
A. Raise and lower the boom several times to flush any
sequence shown in Figure 5F-3.
contamination from the valve which could prevent
the piston and/or check valve from seating properly.
COUNTERBALANCE VALVE (36Z838)
B. Elevate the boom to approximataly 45 degrees and
DESCRIPTION The counterbalance valve is installed
return the control lever to the neutral position.
between the boom hoist control valve and the velocity fuses
on the boom hoist cylinders. The valve is mounted on the
C. Allow the boom to remain in this position for several
inner side of the right hand sidestand, just below and for-
minutes and note whether or not the boom lowers.
ward of the boom pivot point (see Figure 5F-5).
D. If the boom lowers, clean the entire charge of hy-
The purpose of this valve is to prevent cylinder movement
draulic oil thoroughly using a filter buggy, and re-
until pressure has been developed in the boom hoist circuit.
place the return line filters. After the oil has been
The valve operates as follows (see Figure 5F-4):
cleaned, repeat Steps A through C. If the boom still
lowers, the problem is most likely a worn piston or
When the boom hoist lever is in the raise position, oil from
check valve, and the valve should be repaired.
the control valve is supplied to the counterbalance valve,
and the pressure in the valve will increase until it is suf-
2. If the boom cannot be lowered, the problem is due to
ficient to unseat the check valve, allowing the oil to flow
the piston not shifting. This can be caused by blockage in
through the valve to the blind end of the cylinders.
the pilot line or orifice, or excessive leakage past the piston
O-ring. Proceed as follows to determine the cause:
When the control lever is in the neutral position, oil pres-
sure on the top of the check valve is insufficient to hold the
A. Elevate the boom to approximately 45 degrees, and
check valve open. The check valve will then be seated by
return the control valve to neutral.
the spring inside it, blocking the flow of oil from the blind
B. Remove the tube connected to the pilot port of the
end of the cylinders.
valve.
When the control lever is in the lower position, oil flows to
C. If the piston O-ring is worn, seepage will be observed
the rod end of the cylinders, while the oil from the Mind
at the pilot port. If no seepage is observed, remove
end of the cylinders is prevented from returning to tank by
the valve and pilot tube, and check for blockage.
the piston, causing the pressure in the circuit to increase. A
tuba connecting the rod end of the cylinders to the pilot
REMOVAL. To remove the counterbalance valve, proceed
port of the counterbalance valve allows this pressure to be
as follows (see Figure 5F-5):
sensed by the valve. Pressure at the pilot port will cause oil
to flow through an adjustable orifice, which determines the
opening rate of the piston, until sufficient pressure is
applied to the end of the piston to unseat it. When the
Extreme caution must be exercised when removing
piston is unseated, oil in the blind end of the cylinders is
the counterbalance valve from the machine, as this
allowed to return to tank.
valve is the only hydraulic means of preventing the
boom from lowering once the boom has been ele-
When the control lever is returned to the neutral position,
vated. The following procedure describes a method of
the pressure in the rod and of the cylinder and at the pilot
removing the valve with the boom in the boom rest,
port drops below the pressure required to hold the piston
which is the preferred method. However, if circum-
open, thereby allowing the springs above the piston to seat
stances do not allow removal of the counterbalance
it. As the piston closes, the oil at the end of the piston is
valve with the boom in the boom rest, a positive,
returned to the pilot port via the small check valve in the
mechanical means must be used to prevent boom
cap end of the valve. The mall check valve by-passes the
movement while repairing the counterbalance valve.
adjustable orifice, allowing rapid seating of the piston.
1. Lower the boom onto the boom rest, and secure it to
TROUBLESHOOTlNG. The problems that can be
the boom rest with the retaining pin.
experienced with this valve fall mainly into two areas;
namely, the valve will not hold the boom in a raisad
NOTE
position or the boom will not lower. To determine the
The counterbalance valve can be removed by reaching
cause of valve malfunction, proceed as follows:
down between the boom and right hand sidestand
from the shroud behind the operators cab.
1. The inability of the valve to hold the boom in a raisad
5F-4
