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N°69

 Adjustment of an LS (Load Sensing) pump 

Hydraulic training

 
 

Load Sensing pumps are present in mobile applications. They make it possible to reduce energy consumption by adapting the flow rate to the pressure required for the operation of the machine.

1- Load Sensing regulation pump (sectional view).

 
how to adjust a pump with load sensing regulation
Adjustment procedure for a Load Sensing pump
 
 

In Fig. A is shown a didactic sectional view of a variable displacement pump fitted with constant pressure regulation and Load Sensing (LS) regulation.

In Fig. B is shown a simple hydraulic circuit. The pump symbol is representative of the mechanics shown in Fig. AT.

We find :

     - Rep 1: Pump block.

     - Item 2: Pump shaft.

     - Item 3: Shaft seal.

     - Item 4: Bearing.

     - Item 5: Pump plate.

     - Item 6: Piston pads.

     - Item 7: Pistons.

     - Item 8: Barrel.

     - Item 9: Spring.

     - Item 10: LS directional valve.

     - Item 11: Drain line.

     - Item 12: Counter-piston (increase in displacement).

     - Item 13: Return spring (maximum displacement at standby).

     - Item 14: Plate control piston (displacement reduction).

     - Item 15: Constant pressure valve spool.

     - Item 16: Constant pressure adjustment spring.

     - Item 17: Adjustment screw (constant pressure).

     - Item 18: Nozzle (decompression / stabilization of the control piston).

     - Item 19: LS valve spool. 

     - Item 20: LS valve adjustment spring.

     - Item21: Adjustment screw (LS).

     - Item 22: Plugs.

     - Item 23: Engine.

     - LP: Pressure relief valve.

     - V1: Double acting cylinder.                                            

     - M1 : Pressure gauge.

     - D1: LS directional valve.

     -Item 24 : Nozzle present on the LS directional spool.

     - Item 25 : Shuttle valve.

A pump fitted with a regulation (LS) is connected to 4 pipes:

     - A suction (S: Suction)

     - A discharge (P: Pressure)

     - Drainage (L or Y)

     - X piloting LS: Load Sensing

 

2- Load Sensing pump adjustment (LS).

The settings in the diagram are as follows:

Pressure relief valve (LP): 280 bars.

Constant pressure valve (Item 15): 250 bars.

LS valve (Rep 19): 25 bars.

The pressure relief valve (LP) installed on the manifold inlet plate protects the pump. It remove pressure peaks.

The constant pressure valve (Item 15) adjustable by the screw (Item 17) cancels the displacement of the pump at the maximum pressure of the circuit. (Cylinder in mechanical stop for example).

The LS valve (Item 19) adjustable by the screw (Item 21) ensures a stand-by pressure which allows "to start the pump in displacement".

     - 1: Read the hydraulic diagram and install a pressure gauge (0-400 bars) between the pump and the directional valve.

     - 2: Completely loosen the pressure relief valve (LP).

     - 3: Completely loosen the LS valve (Item 21) and constant pressure valve (Item 17) settings. (Be careful, there is often no stop on the adjustment screws).

     - 4: Start the pump.

     - 5: Gradually tighten the LS valve (Item 21) and the constant pressure valve (Item 17). The pump is transformed into a fixed displacement pump. The pump starts in displacement, all of its flow returns to the tank via the pressure relief valve (LP) at a low pressure observed in M1.

     - 6: Gradually tighten the pressure limiter (LP), the pressure must gradually rise on the M1 pressure gauge until it reaches 280 bars. The pressure relief valve is set.

       - 7: Loosen the LS valve until reaching 25 bars on the M1 pressure gauge, the pump cancels its displacement and don't give flow. Change the pressure gauge by taking a smaller caliber (0-60 bars to adjust the stand-by pressure to 25 bars). The LS valve is adjusted. Replace the 0-400 bar pressure gauge.

     - 8: Activate the directional valve (D1), wait for the cylinder (V1) to reach the mechanical stop.

     - 9: When the cylinder reaches the mechanical stop, gradually loosen the constant pressure valve (Item 17) until reaching 250 bars read on the M1 manometer. The constant pressure valve is set.

Warning! According to the manufacturers and for the sake of economy, on certain machines using high flow rates, the pressure relief valve does not always allow the entire flow of the pump to pass.

In this case, when we reach point 5 of the procedure, the pressure rises abnormally.

It is then necessary to overload the LS valve (Item 21), slowly and progressively alternate the tightening of the constant pressure valve (Item 17) and of the pressure relief valve (LP) until reaching 280 bars (point 6 of the procedure).

The rest of the procedure is the same.

Note: The pressure relief valve should always be set above constant pressure regulation (approximately 10%) to ensure that the pump is correctly canceling its displacement.

If the pressure relief valve is set too low, the pump does not completely cancel its displacement when the receiver reaches the mechanical stop. The remaining flow rate in the pressure relief valve and causes an abnormal rise in temperature of the hydraulic circuit.

 

How to adjust the hydraulic load sensing pump : training course

 

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