JLG G6-42P Service Manual - Manual (Page 495)

DESCRIPTION AND OPERATION OF THE MICO OPEN
CENTER HYDRAULIC POWER BRAKE VALVE
The MICO Open Center Brake
Valve will provide hydraulic power
braking when installed in an open
center hydraulic circuit. It can be
used in conjunction with other
hydraulic devices such as power
steering, also installed in the same
circuit. Using a single pump to
provide flow and pressure, the brake
valve should be installed in the
system circuitry, in series, between
the pump relief valve and the other
hydraulic devices. The entire pump
flow is directed thru the brake valve
and is available to actuate the
downstream devices. The brake
valve requires a very small volume
of oil for its operation; therefore, it
does not interfere with the rest of the
circuit, nor does usual actuation of
the downstream hydraulic devices
affect operation of the brake valve . Full system pressure is always
available for operating the rest of the
system.
The MICO Brake Valve provides
a single-fluid system using the
systems hydraulic oil to operate the
brakes. Synthetic rubber wheel-
cylinder cups must be used to
prevent swelling.
This valve reduces the braking
effor t to any required degree
depending on pedal ratio. Brake
pedal force is directly proportional to
brake line pressure, thus giving a
sense of feel in the operation of the
brakes. Manual braking is always
available whenever the hydraulic
power system is not functioning.
This is accomplished by a
mechanical follow through within the
brake valve. A longer pedal stroke,
usually with increased pedal effort,
will be expected when braking in this
condition.
The manner in which the hydraulic
power brake valve funtions will be
discussed in the following
paragraphs.
Oil from the pump flows through
the brake valve and on to the
downstream hydraulic unit as
indicated (Figure 3). Sections of the
valve which are communicated to the
return port and the tank when the
valve is not operating are shown
(Figure 2). The compensating port
(Figure 2) provides for thermal
expansion of the oil in the brake lines
and the master cylinder.
The residual valve (Fig. 2), if used,
maintains pressure of 12-20 PSI in
the brake lines when the pedal
(3)
Schematic of Brake Valve in Neutral
Brake Valve Actuated -- Power Steering in Neutral
FIGURE 2
RETURN LINE
PRESSURE
BRAKE LINE
PRESSURE
FLOW THRU
PRESSURE
RETURN LINE
PRESSURE
BRAKE LINE
PRESSURE
FLOW THRU
PRESSURE
CONTROLLED
PRESSURE
Return Port to Tank
Manual Brake Actuating Sleeve
Compensating Port
Check Valve Sleeve Guide
Residual Valve
To Brake Pedal
Pressure Regulating Spring
Orifice Pressure Regulating Spool
Land “A”
Land “B”
Inlet Port From Pump
Land “C”
Master Cylinder Piston
Master Cylinder Reaction Chamber Flow Thru Port
Orifice
Return Line Pressure
To Brake Pedal
FIGURE 3