SkyTrak 8042 Service Manual - Manual (Page 476)

8.61 Model 8042, 10042, 10054 Legacy Rev. 10/03
Hydraulic System
a. Boom Lift Circuit (refer to page 8.24)
With the joystick in the boom lift position,
pressure shifts the lift/lower spool valve in the
main control valve to direct system pressure
through the counterbalance valve to the base
end of both the left and right lift/lower cylinders.
Return oil from the rod end of the cylinders flows through
the pilot-operated check valve 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 main
relief will open, allowing 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.
b. Boom Lower Circuit (refer to page 8.26)
When the joystick is in the boom lowering
position, pressure shifts the lift/lower spool valve
in the main control valve. Oil is then directed to
the pilot-operated check valve to the rod end of
the left and right lift/lower cylinders. System pressure also
pilots open the counterbalance valve in the base end of
the lift/lower cylinders.
The open counterbalance valve allows oil from the base
of the cylinders to flow to the main control valve, to the oil
cooler, to the return filter and then to the reservoir.
If system pressure exceeds 3250 psi (224 bar), the main
relief will open, allowing 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 counterbalance and pilot-operated check valves
serve another main function for safety purposes. This
function can be described as follows:
• Should any of the hydraulic lines routed to the lift/
lower cylinders fail, there will be a loss of hydrau-
lic system pressure to the cylinder. For example,
if a hydraulic line failure occurred in the pressur-
ized lowering line during lowering, the counter-
balance 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 lowering
and prevent an elevated load from falling to the
ground uncontrolled. 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 the
counterbalance valve cartridge. In the event that the
counterbalance valve cartridge was disabled for
emergency boom lowering, or if any unauthorized
adjustments are ever made to the cartridge, remove and
replace the cartridge with a new part. Failure to replace
the cartridge with a new part may alter the holding
characteristics of the counterbalance valve, creating an
unsafe condition for vehicle operation.
8.8.24 Frame Sway and Stabilizer Circuit
Description
Hydraulic flow is applied in the frame sway circuit by the
hydraulic piston pump through the main control valve.
System pressure is directed to either side of the frame
sway 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. The stabilizer
circuit is connected to the frame sway circuit. For a
description of the stabilizer circuit, refer to Section 10.13,
“Boom Extend System Hydraulic Circuit Operation and
Troubleshooting.”
a. Frame Sway LEFT (refer to page 8.28)
With the joystick moved to the left, the spool
valve in the frame sway section of the main
control valve assembly is shifted.
System pressure is directed to the spool valve in
the main control valve, and from there to the base end of
the frame sway cylinder. Oil from the rod end of the frame
sway cylinder flows through a check valve, unseated by
system pressure, to the stabilizer cylinder, back to the
main control valve, to the oil cooler, to the return filter and
then to the reservoir.
b. Frame Sway RIGHT (refer to page 8.30)
With the joystick moved to the right, the spool
valve in the frame sway section of the main
control valve assembly is shifted.
System pressure is directed to the spool valve,
back to the frame sway cylinder, where it flows to the rod
end of the cylinder. Oil from the base end of the cylinder
flows through the check valve, which was unseated by
system pressure, then to the spool valve, to the stabilizer
cylinder, back to the main control valve, to the oil cooler,
to the return filter and then to the reservoir.
MA7200
MA7210
MA7160
MA7170