TSE DP20-4H, Owner'S Manual

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Hydraulic Displacement Control (HDC)
HDC principle
An HDC is a Hydraulic Displacement Control. Pump swashplate position is proportional to the input
command and therefore vehicle speed or load speed (excluding influence of efficiency), is dependent
only on the prime mover speed or motor displacement.
The HDC control uses a hydraulic input signal to operate a porting spool, which ports hydraulic pressure
to either side of a double acting servo piston. The hydraulic signal applies a force input to the spool
which ports hydraulic pressure to either side of a double acting servo piston. Differential pressure across
the servo piston rotates the swashplate, changing the pump’s displacement from full displacement in
one direction to full displacement in the opposite direction. Under some circumstances, such as
contamination, the porting spool could stick and cause the pump to stay at some displacement.
A serviceable 125 μm screen is located in the supply line immediately before the control porting spool.
HDC control
P400520
HDC schematic
P400519
X1
F00B F00A
Feedback from
Swashplate
T P
X2 M14
HDC operation
HDC’s are hydraulically driven control which ports hydraulic pressure to either side of a porting spool,
which pressurizes one end of the servo piston, while draining the other end to case. Pressure differential
across the servo piston moves the swashplate.
A swashplate feedback link, opposing control linkage, and a linear spring provide swashplate position
force feedback to the hydraulic pressure. As hydraulic pressures in the operating loop change with load,
the control assembly and servo/swashplate system work constantly to maintain the commanded position
of the swashplate.
The HDC incorporates a positive neutral dead band as a result of the control spool porting, preloads from
the servo piston assembly, and the linear control spring. Once the neutral threshold point is reached, the
swashplate is positioned directly proportional to the control pressure.
When the control input is either lost or removed, or if there is a loss of charge pressure, the spring loaded
servo piston will automatically return the pump to the neutral position.
Service Manual
MP1
Operation
© Danfoss | July 2017 AX00000244en-US0103 | 17