When the brake valve is in the neutral position, oil from the top of the swing brake is allowed to pass through the center of
the spool. The oil escapes from the spool through two pilot holes to the drain port. When the foot brake pedal is
proportionately pressed, oil is routed around the groove on the spool through the pilot holes and is routed to the brake.
When pressure builds to apply the brake, pressure also builds against the spool of the brake valve. This pressure will act
against the greater surface area between the port end and the spool and tend to cut off the pressure port and compress
the spring proportionate to the pressure applied to the pedal.
When the foot pedal is fully pressed, the pressure is routed directly to the brake. Since the spool is completely against the
ported end cap there is no surface area to act upon, therefore full system pressure is available.
Cylinder Holding Valves
Three different holding valves are utilized on the crane, one in each lift cylinder and one in the telescope cylinder. The
valves are installed in the port block of their respective cylinder.
The overcenter valve is a normally closed valve used in the boom lift circuit to act as a fourth holding valve and to provide
smooth operation during circuit activation. This valve provides static overload relief and thermal expansion relief as the
control valves in the lift circuit have open centers.
An overcenter valve is basically a counterbalance valve with a pilot override, or assist, on the relief valve section. The load
is raised by a free flow of oil through the check section. With the control valve centered, the load is locked (relief must be
set at least 1.3 times higher than the maximum load induced pressure).
To lower the load, pilot pressure is required which effectively reduces the relief valve setting. This allows the load to be
safely and smoothly controlled with minimum energy loss. If the load tries to run ahead of the pump, the relief section will
throttle or close to prevent runaway.
Two Position Selector Valve
The two position selector valve is used to direct a regulated flow of pressure. When in the disengaged (knob pulled out)
position, the flow is routed in a port A through the valve and out the IN port. In the engaged (knob pushed in) position the
flow of oil is allowed to enter the IN port and exit through the B port.
The valve is held in the normally closed position by a spring. When the solenoid is energized, the plunger assembly forces
the spool to shift which opens the valve. De-energizing the solenoid causes the spring pressure to shift the spool to its
normally closed position.