The role of the main pressure limiter is to protect the pump from overpressure.
The pump generates a flow that must be directed at all times:
- To the hydraulic receivers.
- To the tank using a directional valve.
- To the tank using a pressure relief valve.
There are two types of pressure relief valve:
- Direct acting.
- Pilot operated.
The pressure relief valve with pilot operated is composed of a pilot stage in which there is a poppet valve (item 6) with a spring of high stiffness (item 7) adjustable by the screw (item 8) and a power stage in which is found a poppet valve (item 3) provided with a jet (item 5) with a low stiffness spring (item 4) which is not adjustable.
3- Operation (cylinder output phase)
The detailed symbol (figure B) is representative of the mechanism (figure A).
The simplified symbol (figure D) makes it easier to read the diagram, but does not show the jet (item 5) to be taken into account during troubleshooting.La figure C représente le développement des surfaces A & B du clapet (rep 3).
The pressure relief valve is set at 200 bar. This pressure is the addition of the hardness of the spring (item 7) 195 bar and the spring (item 3) 5 bar.
The oil from the pump moves the piston (item12) of the cylinder (item 11).
The pressure required to move the cylinder is 100 bar on the pressure gauge (item 13).
The pressure (100 bar) is established under the poppet valve (item 3), through the jet (item 5) a back pressure is returned to the poppet valve (item 3). The spring (item 4) closes the pressure relief valve.
A pressure of 100 bar also applies to the poppet valve (item 6) but the mechanical stress of the calibration spring (item 7) is greater than that of the pressure exerted on the poppet valve (item 6).
The pressure relief valve remains closed and all the flow of the pump (item 1) is directed towards the cylinder (item 12).
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4- Operation (cylinder phase in mechanical stop)
The cylinder (item 12) is in mechanical stop and the pump (item 1) is driven by the motor (item 2) to the working regime.
The oil instantly accumulates in the cylinder (item 12) and the pressure rises in the circuit.
The pressure exerted under the poppet valve (item 3) is brought back against the pressure on the same poppet valve via the jet (item 5). This back pressure is limited by the opening of the poppet valve (item 6) to 195 bar. A leak calibrated by the jet (item 5) returns to the tank by the poppet valve (item 6). At this moment, the flow of the pump lifts the poppet valve (item 3) under a pressure of 200 bar (195 + 5) and returns to the tank (item 10).
It should be noted that there is a large pressure difference (ΔP) across the pressure relief valve, which causes a rapid heating of the oil.
5- Pressure relief valve adjustment with fixed displacement pump
- Decrease the pressure relief valve.
- Start the pump, engine working speed (max).
- Operate a directional valve to put a cylinder in abutment.
- Tighten the pressure relief valve by checking the setting on the pressure gauge installed at the pump outlet. If the cylinder moves, stop the adjustment and wait for the stop.
6- Adjustment range
It is imperative to make the adjustment of the pressure relief valve with engine full speed for the pump provide the maximum flow of the installation.
Pressure valves have adjustment ranges. The opening pressure varies depending on the flow through them (Figure E). These pressure differences are due to the springs of the valves and vary according to the manufacturers.
Note: The pilot operated pressure relief valve reduces the setting range compared to the direct acting pressure relief valve.
7- Direct action or pilot operated ?
Direct acting pressure relief valve:
- For low flows.
- Important adjustment range.
- Not very sensitive to oil pollution.
- Speed of opening.
Pressure relief valve with pilot operated:
- Decrease the size of springs for large flows.
- Low adjustment range.
- Sensitive to pollution.