9-17
When spool (A) is shifted to the lifting position,orifice(6) is closed,and orifice(7) is opened and
chamber(1) is connected with chamber(3) through passages(37 and 38):passages to the tank are
closed.As orifice (11) is opened and orifice(10) closed,chamber(5) is connected to the tank.
Chambers(3 and 4) are connected with each other through orifice(9). Chamber(13) is connected with
chamber (3) through passage (12),chamber(7),and passages(37 and 38),so the fluid from the pump
flows into chamber(13) from chamber(3) where it comes through chamber(4) and orifice(4) and
orifice(9).Through passage(16) and chamber (19) it flows further into orifice(20) in the poppet(I) of
spool (H)(unloading valve 2).Then a pressure difference is built up at orifice (20) and poppet (I) is
shifted leftwards to disconnect chambers(39 and 21) from each other.
Pump delivery pressure is imposed upon the right-hand side of spool(B)( unloading 1) and combined
force of the pump delivery pressure and the force of spring© upon the left-hand side.As pushed areas
on both sides are the same,spool(B) is shifted rightwards and closes orifice(35).Consequently pump
delivery pressure rises along with the pressure in passage(25)
When the pressure in passage(25) exceeds the combined force of that of spring (E) and the cylinder
Holding pressure which holds check valve poppet(D) closed,the poppet is shifted leftwards
connecting passage(250 and chamber(24) with each other.Then the pump delivery fluid flows into the
cylinder through port C via.Chamber(4),orifice(9),chamber(3),passage(25),and chamber (24),and
activates the piston.The pump delivery fluid also flows into chamber(18) through passage(17) and
shifts spool (H)(unloading valve 2),while chamber(22) is disconnected from port T4 but is connected
with chamber(19).Thus pressure difference between chambers(21 and 39) is equalized out and poppet
(I) is shifted rightwards,making the pressure in chambers(39,21,and 22) the same as that in
Chamber (13).Suppose that the piston push-up pressure is 100 Kgf/㎠(1422 psi),and the pressure in
chambers(24 and 26) is also 100 Kgf/㎠(1422 psi),then the pressure in passage(25) is 101 Kgf/㎠
(1437 psi):100 Kgf/㎠(1422 psi)+ 1 Kgf/㎠(14 psi)-the force of spring E converted into hydraulic
pressure imposed upon the left-hand side of spool(B)(unloading valve 1) is maintained at the
combined pressure of the pressure in chamber(13):101 Kgf/㎠(1437 psi) and that by spring C:
7 Kgf/㎠(100psi),that is ,108 Kgf/㎠(1563 psi).
In this way,the pump delivery pressure is always maintained at a level higher than the piston push-up
pressure by 7 Kgf/㎠(100 psi),so the pressure difference between ,in front of,and behind orifice(9):
Pressure difference between chambers(4 and 3) is maintained at a constant 8 Kgf/㎠(114psi)
Independent from cylinder push-up pressure or fluid volume delivered by the pump.Therefore the
fluid volume which passes orifice(9) is constantly controlled at a level conforming to the open area of
the orifice.This means that subtle control or damping of shocks at the starting and ending of lifting
Operation is easily achieved by adjusting the opening of orifice(9).If the spool is shifted enough to
fully open orifice(9), the pressure difference between,in front of,and behind the orifice[between
chambers(4 and 3)] is flattened out.So pressure imposed upon both sides of spool(B)( unloading 1) is
equalized and the spool is shifted rightwards by the spring force.Then all fluid from the pump flows
into the cylinder to push up the piston at full speeds.
If the pump should stop delivering fluid resulting form a stalled engine,pump breakdown,etc.,the
check valve poppet(D) closes immediately to block the passage from the cylinder,thus lowering of the
implement is prevented.
3) Lowering position
When spool(A) is shifted to the lowering position,orifice(6) is opened,orifice(7) is closed,and
chamber(1) is connected to the tank,while orifice(11) is closed,orifice(10) is opened,chamber(5) is
Connected to the tank,while orifice (11) is closed,orifice(10) is opened,chamber(5) is connected to
chamber(4), and the passage to the tank is closed.Chamber(3) is connected to chamber (2).
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