Service Manual User Manual

Hydraulic System
9-31
Model 3606 • Origin 10/99
d. Extend/Retract Cylinder Counterbalance Valve
The extend/retract cylinder (Fig. 9– 19) includes two
counterbalance valves (Fig. 9– 20). The valves serve two
main functions. One of the functions is to help prevent
component damage.
The other counterbalance valve function can be
described as follows:
• If the vehicle is travelling and the boom runs into a
solid object, pressure will build up on the base end
of the cylinder. Without a counterbalance valve, the
cylinder rod would bend, damaging the cylinder.
With the counterbalance valve installed, the
pressure will build up until it reaches 4000 psi (276
bar) and then the counterbalance valve will be
internally piloted open, allowing the pressure to
return to the reservoir. This condition will only exist
if the cylinder has been extended. If the cylinder is
already fully retracted, the counterbalance valve will
have no effect and damage can result.
• Should any of the hydraulic lines going to the
extend/retract cylinder (Fig. 9– 19) fail, there will be
a loss of hydraulic system pressure to the cylinder.
Example:
If there was a hydraulic line failure in the
pressurized retract line during retracting, the
counterbalance valve would lose pilot pressure,
closing off flow returning to the reservoir. The oil in
the base end of the cylinder would then be trapped,
which would immediately stop boom retraction,
preventing an elevated load from uncontrolled
retraction. The load can be lowered safely to the
ground by following the
Emergency Boom Lowering
procedures in
Section 3 Boom
of this manual.
IMPORTANT: DO NOT attempt to reset a counterbal-
ance valve cartridge. In the event that a counterbalance
valve cartridge was disabled for emergency boom lower-
ing or any unauthorized adjustments are ever made to
this cartridge, remove and replace the cartridge with a
new part. Failure to replace this cartridge with a new part
may alter the holding characteristics of the counterbal-
ance valve, creating an unsafe condition for vehicle oper-
ation.
Figure 9– 20. The extend/retract cylinder counterbalance valves serve two main functions.
MS2230
Front of Vehicle
Extend/Retract
Cylinder
Counterbalance
Valves
(set at 4000 psi,
or 276 bar)