Pilot application

[GHAZANCHAEI]

I am wondering if I can get a pilot line from one of main lines without controling the pressure or flow if the main line has a low pressure and low flow rate? (maximum pressure 3000 PSI and maximum flow rate 1GPM)
Secondly , if I want to connect two lines when the pressure in one or both of them drop to for example 50 PSI , what is the other option rather than using pressure switches and solenoid operated directional valve.

 

[budt]

I believe you are looking for a Shuttle Valve. See one here.

http://www.hydraulicspneumatics.com/200/eBooks/

Look in Chapter 18 page 3.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[IceStationZebra]

First question: You need to specify what you intend to do with the pilot pressure. The component you are connecting to would have to be rated for 3000psi regardless. You also need to know how much flow it will steal and if it is acceptable to lose that volume from your main line.

Second question: Do you need bang-bang on/off or can you live with some metering? (i.e. when you get to 60psi it is supplying some oil, but it is not fully open.)

http://www.parker.com/literature/Literature Files/IHD/LEsec1.pdf

This catalog has some logic valves that may work.

If you want to get more help post your schematic.

ISZ

 

[GHAZANCHAEI]

This is acucally applied in an autoequalizing system in a scissor car lift. I have 2 single acting cylinder which open the scissor with oil pressure, they are not mechanically linked. Right now there is a valve assembly which equalize 2 cylinders pressures and flows in order to synchronize them by 2%.
I will attach the schematic. It shows the element position when cylinders are opening . It means none of the solenoids are energized when We are pressing the up bottom.As soon as we pushed the down bottom Solenoids 3 are energized and valves 3 open up the oil way to the combiner and from there to the tank and cylinders return under the load.
For final position I set the pressure switches to 50 PSI . If both solenoid 3 are energized and one of these pressure switches or both of them reach to 50 PSI then Solenoid 4 is energized and there will be a connection between C1 and C2 to equalize the lines and cylinders return to zero position completely synchronized.
The problem with this design is that sometimes pressure in one line or both reach 50 PSI and again goes up and I think confusion happens in logic control system , because sometimes cylinders stop before they get to their final position.
Thank you for your advice

 

[hydtools]

If the cylinders stop, then either or both valves 3 de-energize and the check valve(s) block the flow from the cylinders. Sounds like an error in the electrical logic. Activating valve 4 is causing valve 3 to de-energize.

Ted

 

[GHAZANCHAEI]

Ted,
Solenoids 3 don't de-energized while we are pushing the down bottom even when the cylinders stop.
The reason for stopping the cylinders earlier than final position is for a hydraulic lock in the flow combiner because of an unbalanced pressure on two side of the pressure balance spool and not because of solenoid valves.
It exactly shows that the connection between C1 and C2 is not happening.

 

[hydromech]

I think the system works like this...

With the cylinders partly extended there is pressure in C1 and C2 and pressure seen at both pressure switches. To lower the load, solenoids 3 are fired and oil is allowed to drain through to the combiner and to return to P. The flow combiner needs flow in both legs to work, so if one of the cylinders bottoms out before the other the pressure will drop to zero. The logic cicuit will then fire solenoid 4 to allow oil to reach both legs of the combiner. The cylinder that lags behind will then lower under gravity until it too bottoms out. When there is no pressure on either pressure switch the lift must have reached the final position. The cylinders may not bottom out, there may a mechanical stop for the lift to rest on. Either way when there is no pressure on the pressure switches the lift is at its lowest point.

All flow dividers have an error of some degree and the only way to remove the error is to make both cylinders reach the end of their full/minimum stroke length.

The logic to control the circuit should be to fire solenoid 4 whenever there is an input from only one pressure switch. The pressue swtich setting must be lower than the pressure drop across the flow combiner. This will ensure that the pressure switches always see pressure when the lift is lowering. the pressure switch setting could set lower to allow for a lower pressure drop when the oil is warmer. Cold oil may give a pressure drop of 50-60 PSI, as the oil get warmer and thin the pressure drop could lower to a point where the switches dont always see enough pressure to operate.

Does the control logic need at least one switch to operate before it fires solenoid 4? What made you pick 50 PSI as the setpoint?

The system looks like it should work just fine...if the switches are set correctly.

Regards

Adrian

 

[GHAZANCHAEI]

The system works exactly like what Adrian said but pressure switches are configured for falling pressure and not rising pressure . It means when pressure is rising from 0 to 50 PSI they are not act ,just when pressure drop from for example 100 PSI to 50 PSI they will act.
I designed in a way that either one signal or both signal from pressure switches can energized solenoid #4.
Having said that I assume noun of solenoids and pressure switches are acting in dividing mode . They come to act just in returning the cylinders and in combining mode.
I set the 50 PSI just by test and error detection and correction method . Do you think I can calculate this number in an accurate way?
What I can't explain is that if we reach to 50 PSI in one line and let say 80 PSI in another line, first pressure switch will energized solenoid #4 and pressure will be equalized in both line let say gauge pressure shows 65 PSI and according to the pressure switch specification because pressure goes up signal won't change .
Ideally dese it make sence ? If pressure in the system falls down to the set point of the switch and again goes up . would we have the signal from then on?

 

[hydromech]

I think I see the problem...

When solenoid 4 opens to equalise, the pressure switch closes again and the logic assumes that the cylinders are synchronised again. This is the problem with measuring presure and not position.

If this is the problem, there is a cost effective way of fixing it. I just need time to think of the solution.

Regards

Adrian