Operation of the pilot operated
The role of the pressure reducing valve is to protect a part of the hydraulic circuit by reducing the pressure.
- Protect the brake of a hydraulic motor.
- Provide reduced pressure to hydraulic joystick controls.
- Reduce the pressure of a control circuit.
There are two types of pressure reducting valve:
- Pilot action.
The pilot-operated pressure reducer is composed of two stages:
- A power stage (poppet valve 6).
- A pilot stage (poppet valve 8).
We find two springs:
- A spring of low stiffness not adjustable (item 7).
- A high stiffness spring (item 9) adjustable by the screw (item 10).
To note : The pressure reducers are externally drained to decompress the spring box of the pilot head. Indeed, there is a pressure on each side of the valve (M1 & M2) which prevents pilot head decompress leak (item 8) in internally.
3- Operation (cylinder output phase)
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The detailed symbol B is representative of the mechanical figure A.
The symbol D is a simplified symbol that does not show the pilot head and jet (item 11).
Figure C is the developed surfaces of the poppet valve (item 6).
The fixed displacement pump (item 1) driven by the engine (item 2) provides a flow rate of 30l / min.
The hardness of the setting spring of the pressure relief valve (item 3) is set to 200 bars.
The hardness of the setting spring (item 9) of the pilot head of the pressure reducer is adjusted by the screw (item 10) to 100 bars read on the pressure gauge M2. (95 bars (spring, item 9) + 5 bars (spring, item 7).
The pressure reducer is a valve normally open at rest, put in series on the circuit and piloted on the output.
The oil from the pump moves the cylinder piston (item 5).
The pressure required to move the cylinder is 50 bars found on the pressure gauge (M1 & M2), pressure that is created by the mechanical strength of the load.
The pressure of 50 bars is established under the valve (item 6 surface C), through the jet (item 11) a counter-pressure of the same value is established on the poppet valve (item 6 surface D) in addition to the spring of 5 bars (item 7). The poppet valve (item 6) is kept fully open and the entire flow of the pump (item 1) is directed towards the cylinder (item 5).
If we neglect the losses of pressure, we read 50 bars in M1 and M2
4- Operation (cylinder phase in mechanical stop)
The cylinder (item 5) 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 5) and the pressure rises in the circuit.
The pressure is established under the poppet valve (item 6 surface C), through the jet (item 11) a back pressure is established on the poppet valve (item 6 surface D) in addition to the spring of 5 bars (item 7). This back pressure is limited by the adjustment of the spring (9) of the pilot valve (item 8) is 95 bars.
At this moment, the pressure needed to lift the valve (item 6) is 100 bars (95bars + 5bars), the flow lifts the poppet valve (item 6) from the pressure reduction and closes the oil passage from A to B. The pressure read in M2 is reduced to 100 bar.
There will be a slight leakage at the poppet valve (item 8) discharged through the drainage port. It corresponds to the consumption of the jet (item 11) used to maintain the pressure reduction closed.
The flow rate supplied by the pump (item 1) will be brought back to the tank by the pressure relief valve (item 3) at 200 bars.
The value read on the pressure gauge M1 is 200 bars reduced to 100 bars read on the pressure gauge M2 when the cylinder is in mechanical stop.
To note : If a leak appears on the piston of the cylinder (item 5), the pressure in M2 will tend to drop, the poppet valve (item 6) will move slightly and let the flow necessary to compensate for this leakage without exceeding 100 bars.
5- Adjusting the pressure reducer
It is imperative to install a pressure gauge after the pressure reduction
- Loosen the pressure reducer.
- Start the pump, motor working speed (max).
- Tighten the pressure relief valve by checking the setting (200 bars) on the pressure gauge installed at the pump outlet (M1).
- Tare the pressure reducer. At 50 bars the cylinder moves, at this moment it is necessary to stop the adjustment and wait until the cylinder is in mechanical stop.
- Finish setting the pressure reducer at 100 bars read on the pressure gauge M2.
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