Frame Sway Circuit Description - JLG sky trak mmv Service Manual

Hydraulic System
a. Boom Lift Circuit (See page 9.28)
With the joystick in the boom lift position, pilot
pressure provided from the pilot select manifold
shifts the lift/lower spool valve in the main control
MT3820
valve to direct system pressure through the
counterbalance valve to the base end of the lift/lower cyl-
inder.
Return oil from the rod end of the cylinder flows through
the pilot-operated check valve to the spool valve, to the
return filter and then to the hydraulic fluid reservoir.
If system pressure exceeds 3500 psi (241 bar), the main
relief will open, allowing oil to return to the reservoir. If the
return filter becomes plugged, return hydraulic oil will by-
pass the filter when pressure reaches 25 psi (1,7 bar) and
return to the reservoir unfiltered.
b. Boom Lower Circuit (See page 9.32)
When the joystick is in the boom lowering posi-
tion, pilot pressure provided from the pilot select
manifold shifts the lift/lower spool valve in the
MT3830
main control valve. Oil is then directed to the pi-
lot-operated check valve to the rod end of the lift/lower
cylinder. System pressure also pilots open the counter-
balance valve in the base end of the lift/lower cylinder.
The open counterbalance valve allows oil from the base
of the cylinder to flow to the main control valve, to the re-
turn filter and then to the hydraulic fluid reservoir.
If system pressure exceeds 3500 psi (241 bar), the main
relief will open allowing oil to return to the reservoir. If the
return filter becomes plugged, return hydraulic oil will by-
pass 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
cylinder fail, there will be a loss of hydraulic system pres-
sure to the cylinder. For example, if a hydraulic line failure
occurred in the pressurized lowering line during lowering,
the counterbalance valve would lose pilot pressure, clos-
ing 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 proce-
dures in Section 4.3.10, Emergency Boom Lowering and
Retracting of this manual.
9.78
IMPORTANT: DO NOT attempt to reset the counterbal-
ance valve cartridge. In the event that the counterbal-
ance 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.
9.8.27

Frame Sway Circuit Description

Hydraulic flow is applied in the frame sway circuit
(see page 9.36 and page 9.40) by the front section of the
hydraulic gear pump through the unloader valve. The rear
section of the pump provides hydraulic flow through the
accumulator charge valve to the unloader valve. System
pressure is directed to either side of the frame sway cyl-
inder by the shifting of the spool valve in the main control
valve. The spool valve is shifted by pilot pressure from the
secondary function manifold, regulated by the position of
the joystick.
a. Frame Sway LEFT (See page 9.36)
With the two frame sway select buttons (front
and left) depressed and held, and the joystick
moved to the left, the spool valve in the frame
MT3760
sway section of the main control valve assembly
is shifted by pilot pressure provided from the pilot select
manifold. 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, unseat-
ed by system pressure, to the stabilizer cylinder, through
the solenoid valve, and returned to the return filter and the
reservoir via the tank port on the stabilizer cylinder.
b. Frame Sway RIGHT (See page 9.40)
With the two frame sway select buttons (front
and left) depressed and held, and the joystick
moved to the right, the spool valve in the frame
MT3770
sway section of the main control valve assembly
is shifted by pilot pressure provided from the pilot select
manifold.
System pressure is directed to the spool valve, then to the
stabilizer cylinder, through the solenoid 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, and returned to the re-
turn filter and the reservoir via the tank port on the stabi-
lizer cylinder.
Model MMV Rev. 12/04