LS directional valve operation (Load Sensing) with individual pressure compensators

1. Role

The 4/3 LS directional valve is progressive, the control is proportional. The control can be manual, electric or hydraulic.

It is used in mobile applications (public works, agriculture, ports, etc.). It makes it possible to manage the speed of the receivers and to facilitate the driving of the machines. Simultaneous movements are possible depending on the options chosen (individual pressure balances), handling the machines is greatly facilitated.

The LS directonal valve can be connected to a fixed or variable displacement pump .

The 4/3 LS type distributor has 4 or 5 connections depending on the configuration chosen:

  • P: Pressure (Oil inlet).
  • T: Tank (Return to tank).
  • A & B: Receiver connections (cylinders or hydraulic motors)
  • LS: Pump control (Connection for variable displacement pump)

 Note: The LS pilot is not connected and remains blocked when the directional valve is configured to operate with a fixed displacement pump.

2. Operation : Stand-by

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In Fig. A is shown the 4/3 LS directional valve block: For didactic reasons, the directional valve is divided into 4 parts:

  • Rep 0: Entrance plate.
  • 2 slices of distributor .
  • Rep 16: Closing plate.

We find :

  • Rep P: Inlet (pump discharge).
  • Rep T: Return to tank.
  • Rep LS: Plug for connecting the signal to an LS pump.
  • Rep M: Manometer connection (pump pressure).
  • Rep 1: Fixed displacement hydraulic pump.
  • Rep 2: Heat engine.
  • Rep 3: Jet.
  • Rep 4: Tank.
  • Item 5: Pressure relief valve adjustment screw.
  • Rep 6: Pressure relief valve pilot head spring (190 bar).
  • Rep 7: Pressure balance spring (10 bar).
  • Rep 8: Plug. (To be modified for configuration with a variable displacement pump).
  • Rep 9: Manual control by lever.
  • Item 10: Adjustable spool stop (maximum flow rate adjustment).
  • Rep 11: Spool return spring (reset to neutral).
  • Rep 12: Machined drawer. The spool holes are represented by the dotted lines & the nozzles on the symbol.
  • Rep 13: Shuttle valve which allows to select the pressure of the most loaded cylinder.
  • Rep 14: Cylinder with a load that creates a displacement pressure of 100 bars.
  • Rep 15: Cylinder with a load that creates a displacement pressure of 50 bars.
  • Rep 17: Individual pressure balances.
  • Rep 18: Springs of individual pressure balances (5 bars).
  • LSA and LSB: Adjustable pressure relief valve installed on the LS pilot.
  • Shock valves: Non-adjustable shock valves.
  • M1: Pressure gauge installed at the pump outlet.
  • M2 and M3: Pressure gauges installed for explanations between the pressure balances and the spool. (It is impossible in reality to install pressure gauges at this location).

The fixed displacement pump (item 1) driven by the heat engine (item 2) delivers 100 l/min.

The center of the directional valve are plugged (closed), the LS pipe represented by the dotted lines on the symbol is decompressed to the tank by the jet of the spool. No pressure is added to the value of the pressure balance spring (item 7). All the oil from the pump returns to the tank via the pressure balance under 10 bars observed on the M1 manometer.

The pressure balances (item 17) work like pressure reductions valves . The pressure behind the scales is reduced to 5 bars observed on the M2 and M3 manometers.

Note: 4/3 LS valves have proportional control, ie depending on the instruction received by the valve, the spool moves more or less regardless of the type of control used. (Manual, hydraulic or electric)

3. Operation : Simultaneous movements

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Hydraulic and electrical training

On site in business and/or videoconference

We move with a simulation bench, the training sessions alternate theory and practice.

The simulation bench makes it possible to reproduce industrial and mobile assemblies.

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In Fig. B, the operator activates the 2 directional valve control levers (item 9). The spools (item 12) move progressively according to the instructions (10%) and compress the return springs (item 11). Moving the drawers uncovers the passages from P to A & B to T for each cylinder. At the same time, they direct the pressure information from line A (cylinder pressures) into the LS signal. The shuttle valve (item 13) shifts and closes the pressure balance (item 7). This phase is transitory and extremely rapid which is unnoticed by the operator (phase rise in pressure).

The oil is directed the cylinders by the directional valve spool without however letting the entire flow rate of the pump pass. The position of the control levers is identical (10%). Thanks to the shuttle valves (item 13), the highest pressure (that of the cylinder item 14: 100 bars) is brought back to the pressure balance (item 7) and is added to the 10 bar spring.

The pump saturates the directional valves with oil, the excess flow (90 l/min) returns to the tank via the pressure balance under 110 bars observed in M1. (100 bars created by the load (LS) + 10 spring bars).

At the same time, the individual pressure balances (item 17) receive the pressure information from the loads via the holes in the spools (item 12). Respectively 100 bars and 50 bars. These pressures are added to the springs of each pressure balance (5 bars). The pressure balances operate as pressure reductions, they allow the flow rate required by the uncovering of each spool to pass without exceeding 105 bars observed in M2 and 55 bars observed in M3.

We notice that the identical cylinders come out at the same time and at the same speed thanks to the individual pressure balances. (5 lpm)

For the same spool opening (item 12), the pressure difference is identical at the directional valve terminals. 

(∆p of 5 bars between M2 and M4 & ∆p of 5 bars between M3 and M5)

4. LSA & LSB operation

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In Fig. C, the cylinder (item 14) comes to a mechanical stop. Instantly the oil accumulates in the cylinder and the pressure rises. The pressure is limited by the LSA valve to 155 bars observed in M4. 155 bars are actually the addition of the LSA spring (150 bars) + the spring of the individual pressure balance (5 bars). When opening, the LSA valve allows a leak flow (+/- 1 l/min) to pass, which allows the pressure in the LS piloting to be limited to 150 bars. The pressure balance is at this moment 155 bars. The pressure compensator + the LSA valve behaves like a pilot-operated pressure reduction . It reduces the pressure from 160 bars in M1 to 155 bars in M2.

The ∆P  disappears at the directional valve terminals, the pilot pressure LS is brought back to the pressure balance of the input plate and is added to the 10 bar spring (item 7). The excess flow from the 94 l/min pump returns to the tank via the pressure balance under 160 bars observed in M1.

We see that the cylinder (item 15) continues its stroke, thanks to the individual pressure balance, the ∆P  at the directional valve terminals has not changed (5 bars).

5. Operation : Pressure relief valve

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In Fig. D, the cylinder (item 15) in turn comes to a mechanical stop. Instantly the oil accumulates in the cylinder and the pressure rises. The ∆P  disappears at the terminals of the directional valve, the shuttle valves move because the cylinder (item 15) forces more than the cylinder (item 14). The pressure at the pump outlet is brought back to the pressure balance by the LS control. At this moment the pressure balance closes, the oil accumulates in the circuit until it reaches 190 bars regulated by the pilot head (item 6). When the valve (item 6) opens, the pressure behind the nozzle (item 3) can no longer change because all the oil that manages to pass through it (for example 1 l/min) returns to the tank via the valve (item 6) under 190 bars. The pump delivers 100l/min, the oil accumulates in the circuit until it reaches 200 bars observed in M1 (190 bars of the valve(item 6) + 10 bars of the pressure balance spring). At this moment the pressure balance opens, the excess flow (98 l/min) returns to the tank.

The pressure balance (item 17) of the directional valve supplying the cylinder (item 15) is wide open since its value is at this moment equal to 200 pilot bars + 5 spring bars.

6.Shock valve operation

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In Fig. E, anti- shock valves are set above the main pressure relief valve and operate only when the valve is closed on a mechanical shock or sudden stoppage of a load with high inertia.

The shock produced on the cylinder (item 15) creates a pressure of 220 bars, which allows the anti-shock valve to open. The cylinder moves back, the opposite chamber is resupplied by the replenishment valve of the opposite anti-shock valve. (Zoom Fig. F)

6 valves antichocs

Note: Depending on the violence of the impact, the flow to be evacuated by the anti-shock valve changes. The opening pressure of the anti-shock valve varies according to the flow passing through it. (Adjustment range)


In conclusion

The 4/3 LS directional valve makes it possible to manage the speed of the receivers. This type of distribution makes it easier to drive machinery (excavators, cranes, etc.).

To improve ease of driving, it is useful to install LS directional valve with individual pressure balances which will allow the speed of the cylinders to be maintained when they operate simultaneously.

The LS dispenser can be completed with multiple options (shock valves, individual pressure balances, etc.). It can be configured to work with a fixed or variable displacement pump by modifying the inlet plate.

The pump must always saturate the LS distributors with flow. If the pump is undersized or if the pump has power regulation, it will be necessary to use Flow Sharing type distributors to maintain optimum driving comfort.

LS systems are sensitive to pollution, it is necessary to pay particular attention to filtration. Pressure filtration is strongly recommended.

Unlike the 6/3 distributor, the operator will not feel the effort of the machine except for the changes in noise caused by the rise in pressure in the circuit.



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