User Manual

000 293 646 0 - SYS-H XERION 3300 - 04/05 23
1 Overall hydraulic system
PFC pump
391
Description of function Downstroking of the PFC pump is performed under the following conditions:
– ·a control unit is actuated to neutral position, no volume flow is required.
– ·an additional control unit is actuated into the fine control range, a very small
volume flow is required.
– ·in parallel operation, a control unit is actuated to the neutral position or to
the fine control range. A smaller volume flow is required.
The pump must perform a downstroke whenever a smaller volume flow is
needed. The downstroking process is started when the pressure on the right
face end of the volume flow controller (772) is higher than on the left face (in the
spring space). In this case, the volume flow controller (772) is moved to the left
against the compression spring (21), in this process opening the control edge
(25). This results in a rising pressure on the top face of the variable-displace
-
ment pump hydraulic cylinder (377) that sets the swash plate (17) to a flatter
position against the control spring (15) – the volume flow is reduced.When a
control unit is actuated in the fine control range (a very small volume flow is to
flow to the consumer), the load pressure at input (7) drops. This changes the
force ratio at the volume flow controller (772) so that it is actuated to the left
according to the pressure drop, against the compression spring. The control
edge (25) is opened, thus subjecting the variable-displacement pump hydraulic
cylinder (377) to pressure. The swash plate (17) is set to a flatter position and
the pump performs a downstroke until the volume flow requirement is met.
Parallel operation of two control
units
When operating two control units in parallel, the load pressure values are 140
bar and 100 bar. In this process, the pump pressure adjusts to 170 bar. (Load
pressure 140 bar + 20 bar margin pressure). Both the volume flow controller
(772) and the swash plate (17) are in a constant position. The pump provides the
volume flow required for both consumers.When the second control unit (load
pressure 100 bar) is now actuated in the neutral position, the volume flow must
decrease since only the first hydraulic circuit remains to be supplied. This is
achieved by increasing the pump pressure for a short time following the shut-
down of a hydraulic circuit, since the high volume flow flows to the first control
unit only. The volume flow controller (772) is now actuated to the left, the control
edge (25) opens and the pressure on the top face of the control ram rises. The
swash plate (17) is set to a flatter position, thus reducing the volume flow.