1.56 HYDRAULICS Pump goes into delivery Increased volumetric flow demand. Example: A selective control valve is actuated. The load pressure is 140 bar (1508 psi); the pump moves to a pressure of 160 bar (1537 psi) 140 bar load pressure + 20 bar stand-by pressure). When a control valve is actuated and thus increased delivery volume is demanded from the pump, the pump pressure drops slightly. If an additional second selective service is now actuated with a load pressure of 100 bar (1450 psi), the pump pressure drops slightly. The following conditions are created at the volumetric flow controller (3) : The pump pressure acts on the right hand face.
The load pressure and the spring force of the pressure spring (stand-by pressure) (17) acts in the spring chamber. The pressure on the upper side of control plunger (10) is also reduced as a result and the angle of swash plate (13) is increased so that the delivery volume is increased until the additional volumetric flow demand of the second service is satisfied. As the load pressure and the spring force of the pressure spring (stand-by pressure) (17) is larger than the pump pressure acting on the right face volumetric flow controller (3) is moved to the right up to the stop. As a result: The passage of the pump pressure to control plunger (10) is barred.
Control plunger (10) is linked to the tank (no pressure on the upper side of the plunger). Swash plate (13) is deflected by control spring (11). The pump now delivers a higher volumetric flow through the pump outlet (16). This process is referred to as ‘upstroking’. The volumetric flow of the pump is determined by the throttling effect of the open control spool cross-section or by the setting of the flow divider of a selective control valve. Lower volumetric flow demand If the control valve cross-section is reduced (the volumetric flow is to be reduced), the load pressure at inlet port (2) drops.
The force conditions at volumetric flow controller (3) change as a result, so that it is pushed to the left up against the pressure spring in relation to the drop in pressure. Control plunger (10) is pressurized and the angle of the swash plate (13) is reduced; the delivery of the pump is reduced until the volumetric flow demand is satisfied.