INTRODUCTION 2250 SERVICE/MAINTENANCE MANUAL
1-26
Published 11-06-15, Control # 040-13
requirements for fan motors and pilot fluid pressure for
accessory valve operation.
All main pumps are variable displacement, axial piston
pumps that operate in a bi-directional closed-loop system.
Each pump contains:
• Charge pump
• EDC (Electrical Displacement Control)
• Cylinder block where pistons are positioned axially
around a drive shaft
• Charge pressure relief valve
• Two multifunction (relief) valves
Each system pump has a gerotor type gear charge pump
that is internally mounted on the end of each pump system
driveshaft. System charge pump draws fluid directly from
suction manifold and delivers it to closed-loop system at a
charge pressure of approximately 350 psi (24 bar). Charge
pressure depends on engine load/speed, pressure relief
valve settings, and hydraulic system efficiency.
When a system control handle is moved, the PC sends a
variable plus or minus 0 to 2.8 volt output to pump EDC as
required for handle command direction. Pump EDC tilts
swashplate to stroke pump in the command direction. Pump
pistons move within cylinder block as the block rotates. The
longer stroke of each piston draws in return fluid from system
motor. As the stroke shortens, hydraulic fluid is pushed out of
pump piston cylinders into hydraulic piping to the motor.
Hydraulic fluid displaced by motor returns through piping to
inlet side of system pump. Swashplate tilt angle determines
volume of fluid that can be pumped to the motor. Increasing
swashplate tilt angle increases piston stroke length, allowing
more fluid to be pumped to the motor.
Table 1-1
Multifunction Valve Pressure Limit Settings
Each pump has two multifunction valves that consist of
system relief valve and charge flow make-up check valve.
Pump system multifunction valves control maximum system
pressure and protect each pump system from damage by
limiting pressure spikes in each operating direction. When
preset loop system pressure is reached, multifunction valves
limit system pressure by de-stroking pump or transferring
fluid from high-pressure side to low-pressure side. Maximum
pressure setting of multifunction valves for each pump is
listed in Table 1-1
. Limits should not be reached unless there
is a failure in the system.
Charge Pressure
Charge pressure in each closed-loop system is preset at
approximately 350 psi (24 bar) with a relief valve in charge
pump. If the charge pressure is set too high, the hydraulic
system could be damaged. Charge pressure must be at
preset value as lower pressures can cause a slowing or
stopping of operation. When a system control handle is in
neutral the digital display screen indicates system charge
pressure.
If boom/luffing jib charge pressure system drops, the brake
begins to apply at approximately 295 psi (20 bar) for boom
hoist and 260 psi (18 bar) for luffing jib. Brakes are fully
applied at 219 psi (15 bar) for boom hoist and 140 psi (10
bar) for luffing jib.
Hydraulic Motors
See Sauer-Sundstrand Service Manual or Rexroth Service
Manual for a description of a hydraulic motor.
Variable displacement low torque/high speed, bent axis
piston hydraulic motors are used in the travel, boom hoist,
and load drum systems. The swing system motor is a fixed
displacement, low torque/high speed, bent axis piston
hydraulic motor. Each motor contains a cylinder block,
pistons, output shaft, and internal flushing valve. Motors in
load drum and boom hoist systems have a PCP (Pressure
Control Pilot) valve that controls output speed/torque of the
motor.
Motor cylinder block axis is tilted at an angle to output shaft
with pistons fitted axially around its axis. The internal end of
output shaft has a large flange face similar to pump
swashplate. The motor piston ends are connected to output
flange face and do not ride around the axis of rotating flange
face like the pump pistons.
Hydraulic fluid from pump enters inlet side of motor and
places a force against pistons. The retained piston ends
place a thrust against output flange with a rotational torque
that turns output shaft.
This also rotates the cylinder block on bent axis, while tilt
angle to flange face moves the pistons as they rotate.
Hydraulic fluid displaced by the motor pistons, exits outlet
side of motor and returns to inlet side of pump.
SYSTEM FUNCTION PRESSURE (BAR)
Load Drums
(1, 2, and 3)
Hoist 6,090 (420)
Lower 2,900 (200)
Boom Hoist
(Drum 4)
Up 6,090 (420)
Down 6,090 (420)
Luffing Jib
(Drum 5)
Up 6,090 (420)
Down 6,090 (420)
Swing
Left 6,090 (420)
Right 6,090 (420)
Travel
Forward 6,090 (420)
Reverse 6,090 (420)
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