INTRODUCTION 16000 SERVICE/MAINTENANCE MANUAL
1-16
Published 05-09-17, Control # 014-28
opposite direction of spool with linkage. This centers and
maintains spool in a neutral position until the 15 psi (1 bar)
chamber spring pressure is reached.
In travel pumps, the pressure relief and pressure-limiting
sections of multifunction valves respond when relief
pressure is reached. The pressure limiting function of travel
pumps is set not to exceed 6090 psi (420 bar). If travel pump
pressure exceeds preset pressure limit, pumps de-stroke to
prevent overheating of system fluid.
Hydraulic fluid pressure overcomes spring resistance in
pressure limiting relief valve (1, Figure 1-10
), shifting spool to
open a line for fluid pressure. Servo check valve (2) is spring
loaded with an opening pressure of 750 psi (52 bar).
Hydraulic fluid from pressure limiting relief valve flows
through exhaust port of displacement control valve (3).
The exhaust port has a restricted orifice that develops
pressure for servo control cylinder (4) to pressurize and de-
strokes pump to limit system pressure. When rapid loading
produces pressure spikes, system relief valve (5) shifts. This
allows high-pressure fluid to return to tank through charge
pump relief valve (6). Alternatively, fluid transfers to low-
pressure side of closed-loop system through charge flow
make-up check valve (7).
In other system pumps, pressure limiting is controlled
through relief valve section of multifunction valves only. Flow
control orifice (8) is removed from pump EDC. Servo check
valves are removed from pump and lines to servo control
cylinders are plugged. These changes permit the pump to
react quicker to control handle commands.
The pressure limiting relief valve (1) serves as pilot valve to
open system relief valve (5) when desired relief pressure
setting is reached. For example, if a pressure imbalance
occurs on both sides of flow restrictor (9), pressure limiting
valve opens and system relief valve relieves system
pressure. Hydraulic fluid is directed to tank through relief
valve (7) or the flow is transferred to low-pressure side of
system through the make-up check valve (8).
Pump displacement depends on engine driven pump speed
through pump drive and swashplate tilt angle. The engine
provides power for work, while the swashplate tilt angle
provides speed control. Engine speed is set and controlled
with hand or foot engine throttle.
Each variable displacement motor, except travel, begins
operation at maximum displacement (high torque, low
speed) and shifts to minimum displacement (low torque, high
speed) if torque requirement is low. The motor remains in
maximum displacement until servo PC valve (10) receives a
command from PCP valve (11) to direct system pressure and
flow from shuttle valve (12) to minimum displacement side of
servo cylinder (13) that shifts motor. As PCP valve opens in
proportion to output voltage received from the node
controller, pilot line pressure is directed to shift servo PC
valve. After overcoming adjustable valve spring (14) and
valve spring (15), servo PC valve shifts and directs fluid to
stroke motor at minimum displacement output. If the load at
the motor shaft increases, force on adjustable valve spring
increases. This shifts servo PC valve to de-stroke the motor
to maximum displacement for safe load handling.
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