Structure of a hydraulic system This simplified block diagram shows the division of hydraulic systems into a signal control section and a hydraulic power section. This signal control section is used to activate the valves in the power control section.
Hydraulic power section The diagram of the hydraulic power section is complemented in this case by a circuit diagram to allow correlation of the various function groups; the power supply section contains the hydraulic pump and drive motor and the components for the preparation of the hydraulic fluid. The energy control section consists of the various valves used to provide control and regulate the flow rate, pressure and direction of the hydraulic fluid. This drive section consists of cylinders or hydraulic motors, depending on the application in question.
Simple Hydraulic Circuit A simple open The equivalentcenter hydraulic circuit circuit schematic
Interaction of components The animations show the sequences in a basic hydraulic circuit in simplified form - the actuation and spring return of the final control element (4/2-way valve), the advance and return of the drive component (double acting cylinder) and the opening and closing of the pressure relief valve.
Circuit diagram: Return flow filter An oil filter situated in the return line to the tank has the advantage that the filter is thus easy to maintain. A disadvantage, however, is that contamination is removed from the hydraulic fluid only after it has passed through the hydraulic components. This configuration is often used.
Circuit diagram : Pump inlet filter With this configuration, the pump is protected from contamination. The filter is, on the other hand, less easily accessible. If these filters have a too fine mesh, suction problems and cavitation effects may occur. Additional coarse filters upstream of the pump are recommended.
Circuit diagram: Pressure line filter Pressure filters can be installed selectively upstream of valves which are sensitive to contamination; this also enables smaller mesh sizes to be used. A pressure-resistant housing is required, which makes this configuration more expensive.
Circuit diagram: Contamination indicator It is important that the effectiveness of a filter can be checked by a contamination indicator. The contamination of a filter is measured by the pressure drop; as the contamination increases, the pressure upstream of the filter increases. The pressure acts on a spring- loaded piston. As the pressure increases, the piston is pushed against a spring. There are a number of different display methods. Either the piston movement is directly visible or it is converted into an electrical or visual indication by electrical contacts.
Circuit diagram: Hydraulic power unit The illustration shows the detailed circuit symbol for a hydraulic power unit. Since this is an combination unit, a dot/dash line is placed around the symbols representing the individual units.
Pressure relief valve (1) In this design incorporating a poppet valve, a seal is pressed against the inlet port P by a pressure spring when the valve is in its normal position. In this situation, for example, an unloaded piston rod is executing an advance stroke and the entire pump delivery is flowing to the cylinder.
Circuit diagram: Pressure relief valve (2) As soon as the force exerted by the inlet pressure at A exceeds the opposing spring force, the valve begins to open. In this situation, for example, the piston rod is fully advanced; the entire pump delivery is flowing at the preset system pressure to the tank.
PRV used to limit system pressure This illustration shows a pressure relief valve within a basic hydraulic circuit (used to control a double acting cylinder). The resistances at the outlet (tank line, filter) must be added to the force of the spring in the pressure relief valve. See also the animation “Interaction of components” (at page 7).
PRV used to limit system pressure This illustration shows the same circuit as the previous illustration, but with the cut-away view of the PRV replaced by the appropriate circuit symbol.
(1A) Circuit without brake valve (Animation) One application of pressure relief valves is as brake valves; these prevent pressure peaks which may otherwise occur as the result of mass moments of inertia when a directional control valve is suddenly closed. The animation shows an (incorrect) circuit in schematic form in which the working line on the exhaust side has fractured due to the absence of a brake valve. The next animation shows the correct circuit.
(2) Circuit diagram: Brake valve This illustration shows the correct circuit for the problem in topic 1A. This circuit incorporates not only a brake valve on the piston-rod side but also a non-return valve on the inlet side via which oil can be taken in from a reservoir during the vacuum phase following the closure of the directional control valve. The following animation shows the events which occur in the two working lines.
Circuit with brake valve (Animation) The animation (2a) shows in schematic form the behavior of the PRV during the braking phase, while (2b) shows the behavior of the non-return valve (NRV) in the supply line and (2) shows the two events together in summary. The necessity of the brake valve can be demonstrated by the preceding animation.