Yanmar ViO12-2A, Service Manual

Page: 124 / 409

6 . HYDRAULIC SYSTEM
6- 3-1
6
-3 Circuit Operation
6
-3-1 Boom
1) Boom “Up”
(1) Pilot oil flow
When the boom control lever is pulled back to raise the
boom, the oil from the pilot pump P3 flows through the
port A of the cut-off valve and the port 3 of the pilot valve
(RH) to the port a3 of the boom section to move its
spool.
(2) Oil flow from hydraulic pump
The oil discharged from the piston pump P1 flows to the
boom section through the port P1 of the inlet section and
the parallel passage and flows through the port A3 to BU
of the boom cylinder to extend its cylinder rod, raising
the boom. The return oil from BD of the boom cylinder
flows back to the hydraulic oil tank through the port B3 of
the boom section, the port T of the control valve and the
return filter.
2) Boom "Down"
(1) Pilot oil flow
When the boom control lever is pushed forward to lower
the boom, the oil from the pilot pump P3 flows through
the port A of the cut-off valve and the port 1 of the pilot
valve (RH) to the port b3 of the boom section to move its
spool.
(2) Oil flow from hydraulic pump
The oil flows in the opposite direction to the oil flow in the
boom up operation with regard to the boom cylinder.