Lowering the lift cylinders can be interrupted by
returning the lift spool into the neutral position.
This closes the connection between lift cylinder and
tank and also interrupts pilot supply to the selector
valve, causing it to close. As a consequence lift
cylinder pressure builds up at the spring side (=
pilot side) of the pilot operated check valve,
resulting that the pilot operated check valve closes
as well. See Figure 5.
Gland Lubrication
To provide lubrication to the seals in the gland, the
inside of the lift cylinder rod is filled with hydraulic
oil. When the cylinder extends to the highest
position, some of the oil moisturizes the gland seals.
The area between shell and rod is closed off by a
1 MPa (145 psi) check valve, which is located at the
lower cross member of the outer mast. The function
of the 1 MPa (145 psi) check valve is to retain air
pressure at the rod side.
When the cylinder extends, the increasing air
pressure between shell and rod pushes the oil
through the holes into the inside of the rod. When
the cylinder retracts, the reducing air pressure at
the shell side allows oil to return form the rod inside
to the highest possible position on the rod outside,
providing lubrication to the gland seals.
1. LIFT CYLINDER
2. LOWERING CONTROL VALVE
3. PILOT OPERATED CHECK VALVE
4. CYLINDER SIDE
5. TO LIFT SPOOL
6. PILOT PRESSURE FROM LIFT SPOOL
7. SELECTOR VALVE
8. PILOT SIDE
9. EMERGENCY LOWERING VALVE
Figure 5. Pilot Operated Check Valves Schematic
Description And Operation
4000 SRM 1664
4
