HYDRAULICS 1.59 Downstroking of the pump The volumetric load controller (3) and swash plate (13) are in a constant position. The pump is downstroked under the following conditions: A control valve is moved to the neutral position; The pump delivers the volumetric flow required for the two consumers. no volumetric flow is required. During operation, one control valve is moved to If the second selective control valve, 100 bar (1450 psi) load pressure, is now moved to the neutral position, the volumetric flow has to be reduced as now only the first selective service has to be supplied. the neutral position or to the precision control range. A smaller volumetric flow is required.
The pump must always be downstroked when a smaller volumetric flow is required. This is achieved by switching off one of the selected circuits. This causes the pump pressure to rise briefly, as the volumetric flow is now only delivered to the first selective control valve. The downstroking process is started when the pressure and the spring force of the pressure acting on the right face of the volumetric flow controller (3) is higher than that acting on the left hand face (in the spring chamber). The volumetric flow controller (3) is moved to the left, the control edge (18) opens and the pressure on the upper side of control plunger (10) is reduced.
In this case the volumetric flow controller (3) is moved to the left against the pressure spring (17) and control edge (18) opens. This leads to an increased pressure on the upper side of the control plunger (10), which reduces the angle of the swash plate (13) against the control spring (11) and the volumetric flow is reduced. The angle of the swash plate (13) is decreased so that the delivery volume is reduced. If a selective control valve is moved to the precision control range (a very small volumetric flow is to be delivered to the consumer), the load pressure at inlet port (2) drops.
The force conditions at the volumetric flow controller (3) change as a result so that it is pushed to the left, up against the pressure spring (17) in relation to the drop in pressure. The control edge (18) is opened so that control plunger (10) is pressurized. The angle of swash plate (13) is reduced; the delivery of the pump is reduced until the volumetric flow demand is satisfied. Example During parallel operation of two selective services, the load pressures are 140 bar (1508 psi) and 100 bar (1450 psi). The pump moves to a pressure of 160 bar (1537 psi) (140 bar load pressure + 20 bar stand-by pressure).