1.54 HYDRAULICS Stand-by position (engine started) All control valves are in the neutral position. The swash plate (13) is at maximum deflection. With the machine started, the pump delivers maximum volumetric flow via pump outlet (16) to the control spool of all the control valves. As the control spools are completely shutting off the oil flow, the pressure increases and acts on the right hand side of the volumetric flow controller (3) which is moved to the left against the pressure spring (stand-by pressure) (17). Control edge (18) is opened so that pressure acts on the upper side of control plunger (10).
Swash plate (13) is now moved back against control spring (11) into the minimum delivery position. This process is completed within 10 milliseconds. In this pump position Only the volumetric flow required to compensate leakage losses is generated. The pressure required for the initial actuation of a consumer is maintained. Since every control valve is actuated when in the neutral position, the inlet port (2) for the load pressure is pressure free. In order to move the volumetric flow controller (3); the pump pressure only has to overcome the force of the pressure spring (stand-by pressure) (17).
The pressure require to do this is 22 bar (320 psi) and is referred to as the ‘low pressure stand-by pressure’. The ‘low pressure stand-by pressure’ is slightly higher than the stand-by pressure of 20 bar (290 psi). Depending on the setting of the volumetric flow controller (3) and the pump leakage, the ‘low pressure stand-by pressure’ and the stand-by pressure may be equal. However, the stand-by pressure can never be higher than the ‘low pressure stand-by pressure’. The ‘low pressure stand-by pressure’ cannot be preset and may vary between machines. It changes with increasing leakage in the pump or system.
The pump remains in the ‘low pressure stand-by’ position until one of the valves is actuated. In this position, the pump requires very little drive energy.