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N° 67

LS (Load Sensing) directional and receiver speed

Hydraulic training

 

 

1-Role.

The 4/3 LS directional valve is progressive or 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 operation of the machines. Simultaneous movements are possible. Depending on the options chosen (individual pressure balances for example), handling the machines is greatly facilitated.

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

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

 P: Pressure (Oil inlet).

 T: Tank.

      A & B: Receiver connections (hydraulic cylinders or motors)

      LS: Pump control (Connection for variable displacement pump)

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

 

 2-Operation: stand-by.

 

Directional valve LS
 

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

       - Rep 0: Input plate.

       - 2 slices of drectional valve.

       - Rep 16: Closing plate.

We find:

     - Item P: Inlet (pump delivery).

     - Item T: Return to the tank.

     - LS Rep: Plug used to check the LS signal or to connect the signal to an LS pump.

     - Item M: Pressure gauge connection (pump pressure).

     - Item 1: Fixed displacement hydraulic pump.

     - Item 2: Engine.

     - Item 3: Restriction.

     - Item 4: Tank.

     - Item 5: Pressure relief valve adjustment screw.

     - Item 6: Pilot head spring of the pressure relief valve (190 bars).

     - Item 7: Spring of the pressure balance (10 bars).

     - Item 8: Plug. (To be modified for configuration with a variable displacement pump).

     - Item 9: Manual control by lever.

     - Item 10: Adjustable spool stop (maximum flow adjustment).

     - Item 11: Spool return spring (reset to neutral).

     - Item 12: Machined spool. The spool openings are represented by the dotted lines & the restrictions on the diagram.

     - Item 13: Shuttle valve which selects the pressure of the most loaded cylinder.

     -Item 14: Cylinder with a load that creates a displacement pressure of 100 bars.

     -Item 15: Cylinder with a load that creates a displacement pressure of 500 bars.

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

The center of the directional valves is clogged (closed), the LS pipe represented by the dotted lines on the symbol is decompressed in the tank by the machining 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 reservoir via the pressure balance under 10 bars recorded on the M1 pressure gauge.

Note: The 4/3 LS directional valve is proportional, that is to say that depending on the setpoint received by the directional valve, the spool moves more or less regardless of the type of control used. (Manual, hydraulic or electric)

 

3- Operation: 1 single movement.

 

Directional valve Load Sensing opération
 
 

In Fig. B, the operator actuates the directional valve lever (item 9), the spool (item 12) moves progressively according to the instruction and compresses the return spring (item 11). The movement of the spool discovers the passage from P to A & B to T. At the same time, it directs the pressure information of pipe A (cylinder pressure) in the signal LS. The shuttle valve (item 13) shifts and closes the pressure balance (item 7). This phase is transient and extremely fast which is unnoticed by the operator (pressure build-up phase)

The oil is directed towards the cylinder through the directional valve spool without, however, allowing the entire flow of the pump to pass. 10 l / min are dosed by the operator, the cylinder comes out under a pressure of 100 bars created by the force to be overcome. This same pressure is brought back to the LS line by the machining of the spool and is added to the 10 bars spring of the pressure balance (item 7). The excess oil (90 l / min) returns to the tank via the pressure balance under 110 bars noted in M1. (100 bars created by the load (LS) + 10 bars of spring).

The cylinder extends at low speed with a flow rate of 10 l / min. There is a pressure difference (ΔP) 10 bars at the directional valve terminals (110 bars at the pump outlet - 100 bars in the cylinder).

If during the displacement of the cylinder the force on the rod increases, the pressure instantly increases in the cylinder to 150 bars (for example). By drilling the spool, the pressure in the signal also increases and hardens the spring of the pressure balance (item 7). The excess oil (90 l / min) always returns to the tank via the pressure balance but under a pressure of 160 bars (150 bars created by the load (LS) + 10 bars of spring).

It can be seen that the pressure difference at the terminals of the directional valve is always 10 bars, which makes it possible to maintain the flow rate of 10 l / min passing through the directional valve. The speed of the cylinder is maintained thanks to the conservation of the ΔP (Bernoulli’s theorem).

ΔP  : Pressure difference

 

4- Operation: simultaneous movements

 

 

Proportional directional LS valve
 

In Fig. C, the operator actuates the 2 control levers of the directional valve (item 9). The spools (item 12) move progressively according to the instructions and compress the return springs (item 11). The movement of the spools uncovers the passages from P to A & B to T for each cylinder. At the same time, they direct the pressure information for pipe A (cylinder pressures) into the LS signal. The shuttle valve (item 13) shifts and closes the pressure balance (item 7). This phase is transient and extremely fast which is unnoticed by the operator (pressure build-up phase).

The oil is directed towards the cylinders through the directional valve spool without however allowing the entire flow of the pump to 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 bars spring.

The pump saturates the directional valve with oil, the excess flow (78 l / min) returns to the tank via the pressure balance under 110 bars recorded in M1. (100 bars created by the load (LS) + 10 bars of spring).

We see that the cylinders come out at the same time but do not go at the same speed.

For the same opening of the spools (item 12), the ΔP is different. (10 bars for the cylinder (item 14) & 60 bars for the cylinder (item 15)).

The cylinder (item 14) comes out with a flow of 10 l / min while the cylinder (item 15) comes out with a flow of 12 l / min.

 

5- Operation: acceleration / stop.

 

 
directional LS valve and speed conservation
 

In Fig. D, the cylinder comes to a mechanical stop. Instantly the oil accumulates in the cylinder (item 15) and the pressure rises. The ΔP is erased at the terminals of the directional valve, the shuttle valves move because the cylinder (item 15) forces more than the cylinder (item 14). The pump outlet pressure is brought back to the pressure balance by the LS pilot. At this moment the pressure balance closes, the oil accumulates in the circuit until it reaches 190 bars set by the pilot head (rep 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 through the valve (item 6) under 190 bars. The pump delivers 100l / min, the oil accumulates in the circuit until it reaches 200 bars noted 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 (84 l / min) always returns to the tank.

The cylinder (item 14) continues its stroke and accelerates because the ΔP at the directional valve terminals increases to 100 bars.

In conclusion:

The 4/3 LS directional valve is used to manage the speed of the receivers. This type of distribution makes it easier to operate machinery (excavators, cranes, etc.). However, the speed of the receivers remains random when they are used simultaneously.

To improve ease of operation, it is useful to install LS distributors with individual pressure balances which will allow the speed of the cylinders to be maintained when they are operating simultaneously.

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

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

Operation Load Sensing directional valve (PVG type) : hydraulic training

 

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