Two different holding valves are utilized on the
is used for positioning the main poppet valve which
crane, one in the lift cylinder and one in the
is being held against its seat by heavy springs. The
telescope cylinder. Both valves are installed in the
pilot pressure is sensed from the lowering side of
port block of their respective cylinder.
the hydraulic circuit. As the directional control valve
is opened to the lower position, pressure is applied
The holding valve is ported to allow hydraulic flow
to the pilot piston. The pilot piston moves against
in two directions. To raise a load, the oil must
the pilot release poppet until it unseats the pilot
unseat the free flow poppet valve. In this case, oil
release poppet valve which ports oil to the return
pressure must overcome the main poppet spring.
side of the directional control valve.
To do this, the oil flows through the oil ports of the
pilot release poppet valve which applies pressure
The free flow check valve is provided for venting
to the inside face of the free flow poppet. When the
the pressure side of the pilot piston.
main poppet unseats, the oil flows past the main
poppet seat to the component to be worked.
A vent is provided for venting the area between the
pilot piston 0-rings should weepage occur. A
When a load is to be lowered, as in retracting the
second vent is provided at the adjustment end of
boom, better control is necessary because of the
the pilot release poppet valve should weepage
weight of the load. For this reason a pilot
occur past the 0-ring seal into the spring area.
Do not damage the O-rings during
To remove the holding valve, unscrew it from
installation of the holding valve. If the
holding valve turns freely then gets hard
to turn, then easy to turn; remove the
holding valve and check the O-rings.
They have probably been damaged by a
Check the inside of the port block for any
sharp edge of a port.
sharp edges or burrs and remove as necessary with
The holding valve should turn by hand
Install new O-rings onto the holding valve, refer
until compression of the O-rings begins.
Carefully install the holding valve into the port
Lubricate the holding valve and O-rings with
block until fully seated.
clean hydraulic oil.