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Direct acting pressure relief valve
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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.
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There are two types of pressure relief valve:
- Direct acting.
- Pilot operated.
The direct acting pressure relief valve consists of a spring (item 2), adjustable by the screw (item 1). The poppet valve (item 3) rests on its seat, it is composed of a pusher serving both as a guide and damping when the valve closes. The channel (item 5) ensures the internal drainage of the spring.
3- Operation (cylinder output phase)
The symbol B is representative of the mechanical figure A.
The fixed displacement pump (item 4) driven by the engine (item 10) provides a flow rate of 30 l / min at idle speed and 60 l / min at the operating speed.
The hardness of the setting spring of the pressure relief valve (item 2) is set to 200 bar.
The oil from the pump will move the piston (item 12) of the cylinder (item 11).
The pressure required to move the cylinder is 100 bar on the pressure gauge (item 13).
The pressure is established on the poppet valve (item 3), but the mechanical force of the calibration spring (item 2) is greater than that of the pressure exerted on the surface of the pusher (item 3).
The pressure relief valve remains closed and all the flow of the pump (item 4) is directed towards the cylinder (item 12).
4- Operation (cylinder phase in mechanical stop)
The symbol D is shown in action to compare with Figure C.
The cylinder (item 12) is in mechanical stop and the pump (item 4) is driven by the motor (item 10) to the working regime.
The oil instantly accumulates in the cylinder (item 12) and the pressure rises in the circuit.
The pressure exerted on the push valve (item 3) reaches the value of the spring (item 2).
The flow moves the valve (item 3) and returns to the reservoir (item 7).
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.
Note: It is imperative to make the setting of the pressure relief valve with engine full speed for the pump to 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.
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