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 Proportional directional valve

 type 6/3 hydraulic

Hydraulic training

 
 

There are generally 2 types of proportional directional valve in mobile application.

     - The 4/3 LS (Load Sensing) type directional valve which allows the speed of the receivers to be managed (quick handling of the machine).

     - The 6/3 type directional valve which makes it possible to feel the force of the machine. On the other hand, machines using this type of distribution are more difficult to operate.

 
Distributeur 6 3 monobloc avec anotations 1
 

The 6/3 directional valves (6 internal ports and 3 positions) can be manually, hydraulic or electrically operated .

The 6/3 directional valve is connected to 4 pipes:

P: Pressure (oil inlet).

T: Tank (oil return).

A & B: 2 outputs connected to the receiver ( cylinder or hydraulic motor).

 

1- 6/3 type directional valve in stand-by.

 

 
 
directional valve 6/3 stand-by

In Fig. A is shown a 6/3 type directional valve in stand-by (1 section). The symbolization represents the mechanics.

We find :

     - Item 1: Fixed displacement pump .

     - Rep 2: Engine.

     - Item 3: Directional valve block.

     - Rep 4: Manual control lever.

     - Rep 5: Spool return spring.

     - Rep 6: Main pressure relief valve .

     - Rep 7: No return valve.

     - Rep 8: Spool.

     - Rep 8a: Progressivity slot.

     - Rep 8b: Center to follow *.

     - Rep 9: Double-acting cylinder.

In stand-by, the fixed displacement pump (item 1) is driven by the engine (item 2) at working speed. The pump flow (60 l / min) returns freely to the tank passing through the center to follow (item 8b) of the directional valve. There is no pressure on the M1 gauger except for the pressure drops * which are neglected in the explanation.

In stand-by, the oil returns to the reservoir from P to T, passing through the center to follow. Ports A & B are closed by the directional valve spool and immobilize the cylinder in position.

Note: The 6/3 directional valve is of "spool" design. That is, it is not waterproof internally. Ports A & B are closed in stand-by, but internal micro leaks are present. If the cylinder has a driving load, the cylinder will be immobilized but not blocked. To block the receiver, it is necessary to install pilot check valves between the directional valve and the cylinder.

 * Pressure drop : Pressure created by the friction of the oil in the piping and hydraulic components.

* Center to follow : If we have many directional valves , the arrow in the center  extends through all the centers of the directional valves.

Center to follow hydraulic directional valve

2-Full setpoint cylinder output.

 

hydraulic directional valve 6/3
 
 

In Fig. B, the control lever (item 4) is actuated at full setpoint. The spool (item 8) shifts to the left and compresses the return spring (item 5). The center to follow (ref 8b) closes completely, the spool discovers the passage of oil from P to A & B to T.

The oil which can no longer return to the reservoir through the center to follow (item 8b) accumulates in the circuit and rises to the pressure necessary to release the cylinder (item 9). The anti-drift valve, the spring value of which is negligible, rises, the entire pump flow (60 l / min) passes from P to A under a pressure of 100 bars observed in M1 (pressure created by the load of 1 ton) . The return oil from the cylinder (30 l / min) returns freely to the reservoir, passing through the distributor spool from B to T.

Hydraulic proportional directional valve 6/3
 

Fig. C: In this phase (valve fully open), all the pump flow is directed towards the cylinder which exits at maximum speed. When the jack encounters an additional force (2nd load), the pressure instantly increases to 200 bars in the circuit and the speed of the jack remains constant.

In this phase, the operator does not feel any effort while driving the machine.

Hydraulic proportional valve
 
 

In Fig. D, the cylinder (item 9) comes to a mechanical stop. The oil from the pump accumulates in the circuit and instantly rises in pressure until the pressure relief valve setting value is reached (item 6).

At this moment, the pressure relief valve opens and the oil returns to the reservoir from P to T. 250 bars are observed in M1 (value of the pressure relief valve setting spring).

The pressure relief valve protects the pump from overpressures. The passage of oil through the pressure relief valve causes an increase in the temperature of the oil. We are talking about lamination.

In this phase, the power requested from the heat engine (Item 2) is maximum.

Hydraulic power :

P = p*Q / 540

P : Power in Kw

p : Pressure in bar

Q : Flow rate in l/min

540 : Coefficient taking into account mechanical efficiency.

P = 250 * 60 / 540

P = 27.77 Kw

 

3-Cylinder retract : full setpoint.

 

Hydraulic directional valve (manual lever)
 

In Fig. E, the control lever (item 4) is actuated at full setpoint. The spool (item 8) shifts to the right and compresses the return spring (item 5). The center to follow (ref 8b) closes completely, the spool discovers the passage of oil from P to B & A to T.

The oil which can no longer return to the reservoir through the center to follow (item 8b) accumulates in the circuit and rises to the pressure necessary to bring in the cylinder (item 9). The anti-drift valve, the spring value of which is negligible, rises, the entire pump flow (60 l / min) passes from P to B under a pressure of 200 bars observed in M1 (pressure created by the load of 1 ton on an area twice as small). The return oil from the cylinder (120 l / min) returns freely to the reservoir, passing through the directional valve spool from A to T.

Note: We note that the return flow of the cylinder passing through the directional valve is in this case twice the flow of the pump. This is due to the section ratio * of the cylinder (R = 2). The volume of the cylinder chambers is different, which affects the return flow. This is why it is necessary to size the hydraulic directional valve according to the return flow of the cylinders and not only according to the flow of the pump.

Section ratio : Ratio between the piston surface and the annular surface (piston - rod).

4- Charging and progressivity.

 

 
 
Hydraulic training : directional valve

In Fig. F, the jack is requested by a driving load (freight elevator for example).

In stand-by, all the oil coming from the pump returns to the reservoir through the center to be followed by the directional valve.

When the lever (item 4) of the directional valve is actuated progressively (small setpoint), the spool moves slightly and discovers the passage of oil from P to A and B to T thanks to the progressivity slots (item 8a) while choking slightly the passage of the center to follow (rep 8b).

 The pressure of 100 bars created by the load is lodged behind the anti-drift valve (item 7). This valve, as its name suggests, prevents the jack from lowering when the instruction given by the operator is insufficient to take up the load. Without this valve, the oil from the cylinder returns to the tank through the center to follow and the cylinder goes down.

 At this moment, the oil returning to the tank through the center to follow force. The pressure rises at the pump outlet (for example 50 bars observed in M1). This pressure does not make it possible to counter the force of the jack which is loaded at 100 bars. (Necessary pressure created by the load to move the cylinder).

All the oil from the pump returns to the reservoir through the center of the directional valve and the cylinder does not move.

 
 
Hydraulic proportional directional valve 6/3: Traning course

In Fig. G, the operator increases the control set point of the directional valve, the center to be monitored constricts a little more and increases the pressure at the pump outlet to 100 bars. Part of the oil (2 l / min) lifts the anti-drift valve and is directed towards the cylinder which allows the load to be lifted. The rest of the pump flow returns to the tank through the center to follow (58 l / min).

When the cylinder is moving at low speed, the return flow from the annular chamber (small chamber) returns to the tank passing through the center of the directional valve (1l / min) which adds up to the 58 l / min passing through the center at to follow.

There is a pressure difference of 100 bars across the center to be monitored, which behaves like a nozzle installed in bypass.

Note: While the cylinder is moving, if the operator increases the directional valve setpoint, he changes the flow values ??passing through the center to follow and the cylinder accelerates.

 

 

 
 
Hydraulic training : directional valve 6/3

 In Fig. H, the operator keeps his setpoint, the cylinder encounters an additional force. The total load is now 200 bar. It is observed that the jack stops because the pressure necessary to overcome the force of the jack must rise to 200 bars, increasing the  ΔP  at the terminals of the center to follow. From 110 bars (arbitrary pressure) all the pump flow returns to the tank through the center to follow.

 

ΔP  : Pressure difference across a component.

According to Bernoulli's theorem , if the ΔP increases across the nozzle of the center to follow, the flow passing through it increases and therefore less oil is directed towards the cylinder.

The operator observes a slowing down or stopping of the jack, he must increase his setpoint to maintain his speed which gives him a feeling of the effort. (Feeling of the effort which is only visual + modification of the command).

In conclusion: The 6/3 distributor is a cheap proportional directional valve, it is reliable and not very sensitive to pollution. It makes it possible to manage the efforts of the receivers. The feeling of the effort by the operator can be an asset.

On the other hand, the speed of the receivers remains random if the loads change, the operator must constantly correct his setpoint to keep a constant speed.

 
 

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