INTRODUCTION 2250 SERVICE/MAINTENANCE MANUAL
1-24
Published 07-19-16, Control # 249-01
Monitoring the Main Pressure
Each system’s digital display screen displays the monitored
system pressure. The system pressure displayed on the
system screen is charge pressure or greater. System
pressure can also be checked at each pressure sender
diagnostic coupler with a 689 bar (10,000 psi) high-pressure
gauge when the pump is stroked.
Basic Operation
See Figure 1-10 and Figure 1-11 for the following.
When a control handle is moved from the neutral position, an
input voltage in the handle command direction is sent to the
programmable controller (PC). The PC sends a variable plus
or minus 0 to 2.8 V output that is applied to the pump’s
external electrical displacement control (EDC). The output
current magnetizes an armature and starts to block one of
the orifice ports, depending on the command direction.
Blockage of flow at the exhaust side of the right orifice port
causes a pressure difference across the spool. This pressure
difference overcomes the resistance of the spool spring and
moves the spool proportionally to pressurize the top servo
pistons. The fluid from the bottom servo pistons is routed to
the tank. This tilts the swashplate, stroking the pump in the
selected command direction. As the swashplate tilts, the
chamber spring is pulled in the opposite direction of the
spool with linkage. This centers and maintains the spool in a
the neutral position until the 1 bar (16 psi) chamber spring
pressure is reached.
In travel pumps, the pressure-relief and pressure-limiting
sections of the multifunction valves respond when relief
pressure is reached. If the travel pump pressure exceeds the
preset pressure limit, the pump de-strokes to prevent
overheating of the system fluid. The hydraulic fluid pressure
overcomes spring resistance in the pressure-limiting relief
valve (1), shifting the spool to open a line for fluid pressure.
The servo check valve (2) is spring loaded with an opening
pressure of 52 bar (750 psi). Hydraulic fluid from the
pressure-limiting relief valve flows through the exhaust port
of the displacement control valve (3). The exhaust port has a
restricted orifice that develops pressure for the servo control
cylinder (4) to pressurize and limit the system pressure to de-
stroke the pump. When rapid loading produces pressure
spikes, the system-relief valve (5) shifts. This allows high-
pressure fluid to return to the tank through the charge pump’s
relief valve (6). Alternatively, fluid transfers to the low-
pressure side of the closed-loop system through the charge
flow make-up check valve (7).
In other system pumps, pressure limiting is controlled
through the relief valve section of the multifunction valves
only. The flow control orifice (8) is removed from the pump by
the EDC. The servo check valves are removed from the
pump, and the lines to the servo control cylinders are
plugged. These changes permit the pump to react quicker to
the control handle’s commands.
The pressure limiting-relief valve (1) serves as a pilot valve
to open the system-relief valve (5) when the desired relief
pressure setting is reached. For example, if a pressure
imbalance occurs on both sides of the flow restrictor (9), the
pressure-limiting valve opens and the system-relief valve
relieves system pressure.
Hydraulic fluid is directed to the tank through the relief valve
(7), or the flow is transferred to the low-pressure side of the
system through the make-up check valve (8).
Each variable-displacement motor, except the travel motor,
begins operation at maximum displacement (high-torque,
low-speed) and shifts to minimum displacement (low-torque,
high-speed) if the torque requirement is low. The motor
remains in maximum displacement until the servo PC valve
(10) receives a command from the pressure-control pilot
(PCP) valve (11) to direct system pressure and flow from the
shuttle valve (12) to the minimum displacement side of the
servo cylinder (13) that shifts the motor.
As the PCP valve opens in proportion to the output voltage
received from the PC, pilot line pressure is directed to shift
the servo PC valve. After overcoming the adjustable valve
spring (14) and the valve spring (15), the servo PC valve
shifts and directs fluid to stroke the motor to minimum
displacement output. If the load at the motor shaft increases,
force on the adjustable valve spring increases. This shifts the
servo PC valve to de-stroke the motor to maximum
displacement for safe load handling.
Optional drum 1 motor, boom hoist motor, and the single
motor drive for split rear drum also have a pressure-
compensating override (PCOR) valve (16) that is enabled
when system pressure of 340 bar (4,930 psi) is reached.
When system pressure exceeds the PCOR setting, the valve
shifts to direct flow from the shuttle valve into the maximum
displacement side of the servo cylinder.
The PCOR valve overrides the command from the servo PC
valve, increasing motor displacement, output torque, and
reducing output speed. When the PCOR valve closes,
control of the motor returns to the servo PC valve.
The optional dual motor drive for the split rear drum shaft
(Figure 1-28
) has one bi-directional, variable-displacement
motor with a 12 V proportional solenoid control along with
one fixed-displacement motor mounted in tandem.
Proportional motor displacement is set to override to
maximum (high torque) at 310 bar (4,500 psi), preventing
high-pressure motor failure.
The travel motor servo is opposite of the other system
motors. The travel variable-displacement motors begin
operation at minimum displacement (low-torque, high-
speed). The motor shifts to maximum displacement (high
torque, low speed) when starting torque is required and back
to minimum displacement when in motion if load is below a
preset pressure of 260 bar (3,770 psi). Depending on the
motor system, the servo uses internally or externally
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