SkyTrak 6036 Service Manual - Manual (Page 167)

Section 9 Hydraulic System
Model 6036/6036T S/N 9B0500 thru 14833 9-17
The secondary control valve is the same type of
valve as the cable controlled main control valve
and operates the same, only controlling secondary
functions and the relief valves have different
settings. Refer to paragraph 9.2.1.
9.4.2 Grille Tilt and Slave Circuit
a. Description
Hydraulic pressure is applied in the grille tilt and
slave circuit by the 15 gpm section (front half) of
the tandem pump, which draws its fluid through a
suction screen in the reservoir, Fig. 9.12. Pres-
sure is supplied through the sequence valve and
the load sensing steering priority valve to the
secondary control valve. Pressure is directed to
either side of the grille tilt and slave cylinder
pistons by the shifting of a spool in a directional
control valve found in the secondary control valve
assembly. The spool is shifted by the grille and
frame tilt control lever and its associated control
cable. Control lever positions for tilting the fork
are as follows:
Center Position
When the grille and frame tilt control lever is
placed in the center or neutral position, the
directional control valve spool is positioned so that
pump supply pressure is directed through ports D
to A, Fig. 9.12, to the return filter and reservoir.
If the return filter becomes clogged, hydraulic oil
will bypass the filter when the pressure reaches 10
to 15 psi
(0,69 to 1,03 bar).
Up Position
When the grille and frame tilt control lever is
placed in the “up” position, the directional control
valve spool is positioned so that pump supply
pressure is directed through ports E to B, Fig.
9.12, to the base (extend) end of the grille tilt and
slave cylinders. The extension of the slave
cylinder is fixed by the position of the boom so that
only the grille tilt cylinder is extended to tilt the
grille down. If supply pressure reaches 2700 -100/
+150 psi (186 -6,9/+10,3 bar), the grille tilt “up”
relief valve will open allowing hydraulic oil to
return to the return filter and reservoir, Fig. 9.12.
Return oil from the rod side of the grille tilt cylinder
piston is directed back to the directional control
valve through ports C to F, Fig. 9.12, to the return
filter in the reservoir. If the return filter becomes
clogged, hydraulic oil will bypass the filter when
the pressure reaches 10 to 15 psi (0,69 to 1,03
bar).
Down Position
When the grille and frame tilt control lever is
placed in the “down” position, the directional
control valve spool is positioned so that pump
supply pressure is directed through ports E to C,
Fig. 9.12, to the rod (retract) end of the grille tilt
and slave cylinders. The extension of the slave
cylinder is fixed by the position of the boom so that
only the grille tilt cylinder is retracted to tilt the
grille down. This supply pressure also pilots open
the counterbalance valve, Fig. 9.12. The open
counterbalance valve allows return oil from the
base end of the cylinder to return through ports B
to F, Fig. 9.12, of the directional control valve to
the return filter and reservoir, Fig. 9-12. If the
supply pressure reaches 2700 -100/+150psi (186 -
6,9/+10,3 bar), the grille tilt “down” relief valve,
Fig. 9.12, will open allowing hydraulic oil to return
to the return filter and reservoir. If the return filter
becomes clogged, hydraulic oil will bypass the
filter when the pressure reaches 10 to 15 psi (0,69
to 1,03 bar).
The counterbalance valve in the grille tilt cylinder
serves two other main functions. One prevents
component damage and the other is for safety.
These functions can be described as follows:
• If the boom is being raised and the grille or
load comes in contact with an immovable
object, pressure will build up on the base
end of the cylinder. If the counterbalance
valve was not installed, the rod of the
cylinder would bend damaging the cylin-
der. With the counterbalance valve
installed, the pressure will build up until it
reaches 2000 psi (137,8 bar) and then the
counterbalance valve will be internally
piloted open, allowing the pressure to
return to reservoir. This condition will only
exist if the cylinder has been extended
which is the normal position of the cylinder
when lifting a load.
• Should any of the hydraulic lines going to
the grille tilt cylinder fail, there will be a
loss of hydraulic system pressure to the
cylinder.
Example: If there was a hydrau-
lic line failure in the pressurized grille tilt
down line during tilting with the load
elevated, the counterbalance valve would
loose 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
tilting and prevent an elevated load from
falling off the forks. If a broken line
condition would ever exist, immediately
retract the boom and lower the load safely
to the ground and correct the condition.