Boom Extend/Retract Circuit Description; Boom Lift/Lower Circuit Description - JLG sky trak mmv Service Manual

9.8.25 Boom Extend/Retract Circuit
Description
Hydraulic flow is applied in the boom extend/retract circuit
(see page 9.20 and page 9.24) 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 extend/retract
cylinder by the shifting of the spool valve in the main con-
trol valve. The spool valve is shifted by pilot pressure pro-
vided from the pilot select manifold, regulated by the
position of the joystick.
a. Boom Extend Position (See page 9.20)
When the joystick is in the boom EXTEND posi-
tion, the spool valve is shifted by pilot pressure
in the pilot select manifold so flow is directed
MT3800
through extend/retract spool valve ports,
through the counterbalance valve to the base end of the
extend/retract cylinder. 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 return filter and
then to the reservoir.
If system pressure exceeds 3000 psi (207 bar), the ex-
tend port relief will open and allow hydraulic oil to return
to the reservoir. If the return filter is plugged, the oil re-
turning to the reservoir bypasses the filter when the inter-
nal pressure of the filter reaches 25 psi (1,7 bar).
b. Boom Retract Position (See page 9.24)
When the joystick is in the boom RETRACT po-
sition, the extend/retract spool valve is shifted by
pilot pressure in the pilot select manifold so that
MT3810
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, al-
lowing return oil from the base end of the cylinder to flow
through the spool valve to the return filter and then to the
reservoir.
If system pressure reaches 3500 psi (241 bar), the main
relief valve opens, allowing hydraulic oil to return to the
reservoir. If the return filter becomes plugged, return hy-
draulic 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.
Model MMV Rev. 12/04
The other valve functions 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 of the cylinder. Without a counterbal-
ance valve, the cylinder rod would bend, damag-
ing the cylinder. With the counterbalance valve
installed, the pressure will build up until it
reaches 4000 psi (276 bar) and then the counter-
balance 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 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 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 Section 4.3.10, Emergency Boom Lowering
and Retracting 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.
9.8.26

Boom Lift/Lower Circuit Description

Hydraulic flow is applied in the lift/lower circuit (see
page 9.28 and page 9.32) by the front section of the hy-
draulic 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 lift/lower cylinder
by the shifting of the spool valve in the main control valve.
The spool valve is shifted by pilot pressure provided from
the pilot select manifold, regulated by the position of the
joystick.
Hydraulic System
9.77