SkyTrak 8042 Service Manual - Manual (Page 475)

Hydraulic System
8.60 Model 8042, 10042, 10054 Legacy Rev. 10/03
8. Controlling the force applied to the service brake
pedal controls the amount of braking power.
9. If the first application of force applied to the service
brake pedal was not enough to deliver the braking
required, the one way orifices at the work ports help
give the brake valve “pump up” ability. The one-way
orifices allow unrestricted flow to the brake lines but
also restrict the rate of decompression.
8.8.22 Boom Extend/Retract Circuit
Description
Hydraulic flow is applied in the boom extend/retract circuit
by the hydraulic piston pump through the main control
valve. System pressure is directed to either side of the
extend/retract cylinder by the shifting of the spool valve in
the main control valve. The spool valve is shifted by the
operator joystick and its associated control cable.
a. Boom Extend Position (refer to page 8.20)
When the joystick is in the boom EXTEND
position, the spool valve is shifted by the cable,
so flow is directed through extend/retract spool
valve ports, through the counterbalance valve to
the base end of the extend/retract cylinder. On the 10054
model, the extend lockout valve will prevent boom
extension beyond 42 feet unless the outriggers are down.
Refer to Section 9.7.43, “10054 Outrigger Circuit and
Troubleshooting.” Return oil from the rod end of the
cylinder exits through the check valve. The oil will pass
from the cylinder to the spool valve, to the oil cooler, to the
return filter and then to the reservoir.
If system pressure exceeds 3250 psi (224 bar), the
extend/retract port relief will open and allow hydraulic oil
to return to the reservoir. If the return filter is plugged, the
oil returning to the reservoir bypasses the filter when the
internal pressure of the filter reaches 25 psi (1,7 bar).
b. Boom Retract Position (refer to page 8.22)
When the joystick is in the boom RETRACT
position, the extend/retract spool valve is shifted
by the cable so that system pressure is directed
through the spool valve, to the rod end of the
extend/retract cylinder. The counterbalance valve is
piloted open by high pressure, allowing return oil from the
base end of the cylinder to flow through the spool valve,
to the oil cooler, to the return filter and then to the reservoir.
If system pressure reaches 3250 psi (224 bar), the main
relief valve opens, allowing hydraulic oil to return to the
reservoir. If the return filter becomes plugged, return
hydraulic oil will bypass the filter when pressure reaches
25 psi (1,7 bar) and return to the reservoir unfiltered.
The extend/retract cylinder includes a counterbalance
valve and a pilot-operated check valve. The valves serve
two main functions. One of the functions is to help prevent
component damage. The other valve function can be
described as follows:
• If the vehicle is traveling 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 3250 psi
(224 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 cyl-
inder is already fully retracted, the counterbal-
ance valve will have no effect and damage can
result.
• Should any of the hydraulic lines going to the
extend/retract cylinder fail, there will be a loss of
hydraulic system pressure to the cylinder. For
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 procedures
in the appropriate Legacy Owners/Operators
Manual.
IMPORTANT: DO NOT attempt to reset a
counterbalance valve cartridge. In the event that a
counterbalance valve cartridge was disabled for
emergency boom lowering 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 counterbalance valve, creating an
unsafe condition for vehicle operation.
8.8.23 Boom Lift/Lower Circuit Description
Hydraulic flow is applied in the lift/lower circuit by the
hydraulic piston pump through the main control valve.
System pressure is directed to either side of the left and
right lift/lower cylinders by the shifting of the spool valve
in the main control valve. The spool valve is shifted by the
operator joystick and its associated control cable.
MA7220
MA7230