Proportional directional valves 1

[kebecano]

Hi all, Could anyone familiar with these valves (Hytos in this case) help me figure out why the following happens: I have a hydraulic circuit where the principal components are: pump, Hytos proportional directional valve with closed center, a sequence valve which will actuate a cylinder (on extention) and when the cylinder reaches stroke end, the gear motor starts running (flow is now being diverted to it by the sequence valve). However, when doing the inverse procedure, when I move the valve past its center, therefore starting the retraction stroke of the cylinder( as the proportional control is turned from full flow paralel paths, to center and then moving towards cylinder retraction (crossed paths), the motor will start running albeit at a slower speed untill the cylinder is completely retracted and the gear motor then stops. I know its related to the differences between a proportional valve and an on/off one, but I just can't put my finger on it.

Thanks

 

[itsmoked]

Got a schematic?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[kebecano]

Sure do, is there anyway to attach the file??
thanks

 

[maytag]

I think the "flow amplification"of the fluid being returned to tank from the blind end of the cylinder is generating enough pressure to overcome the sequence valve setting-all prop valves throttle or meter flow(in this case some flow is directed to tank via the prop valve and some thru the sequence valve to the motor). You didn't mention the cylinder ratio-is it 2:1 or larger. Many manufacurers offer 2:1 spool configurations or custom spools-hope this helps. Putting a pressure gage in the cylinder extend line can help verify this. Good luck, Maytag

 

[itsmoked]

kebecano; Check this picture faq.
faq238-1161

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[kebecano]

hi all, this is the schematics I talked about, thanks a lot for any help.

 

[budt]

Have you checked rotation direction when the motors are running while the cylinder is retracting? Is it he same as when they are running as planned?


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[kebecano]

yup, motors run in the same direction 1st case, after the cylinder has completed extension, motors start running with speed varying according to flow delivered by the proport. valve (0-4.5gpm). 2) On cylinder retraction, motor will begin runing but at a reduced speed (approx. 0.5gpm) and motor stops runing once piston has completely retracted. But yes, in both occasions, the motor runs in the same direction.

BTW, the two directional valves that are shown on the schematics are set in a manifold. However, in this case only the proportional directional valve (closed center) is connected to the circuit.

Thanks for your tips

 

[hydromech]

When the cylinder is retracting there is more flow being pushed through the proportional valve. How much more depends on the area ratio in the cylinder.

Correctly designed proportional valves run with a high pressure drop to make them more responsive, in this case the high pressure drop creates a back pressure that is high enough to open the sequence valve and start the motors turning. When the cylinder stops there is no oil flowing so the motor stops turning.

Your options are to increase the size of the proportional valve, limit the flow rate of the pump, slow the cylinder down, decrease the size of the motors so they need more pressure to rotate or increase the opening pressure of the sequence valve.

Whatever you do to make the retraction operation work will effect the extension operation.

The schematic shows a pressure test point on the input side of the sequence valve. Put a pressure gauge and you will see the pressure rise quite high when the cylinder retracting.

Good luck

Regards

Adrian
Hydraulic System Engineering Consultant

 

[itsmoked]

Nicely stated hydromech!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[kebecano]

Thanks a lot Hydromech! That's exactly what I was looking for. Now you do realise that means there'll be more questions coming in the future as you seem to be well versed in hydraulics.

Thank you all

 

[maytag]

From your schematic it appears you have an unused DCV on the manifold. Tee into the A line(cylinder extend line)using the unused DCV station, install a check valve to block flow to the cylinder from this new line, fire the appropriate coil to retract and you can return your"overflow"back to tank thru this spare DCV. Installing a flow control you can tweak as to how much or how little flow you want.
Going this route, cylinder extend stays the same, cylinder retract speed can be increased and the bottleneck can be hopefully eliminated. As Hydro mentioned there are other options but they affect both extend and retract. Maytyag

 

[budt]

Another possible fix would be to pipe the Drain Line of the Sequence Valve into the flow line going to the Rod End of the cylinder. This would keep the Sequence Valve from opening while the cylnder is retracting but not affect its function on extend.
.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[hydromech]

Thanks Keith

Kebecano...you are welcome.

Posts like yours are few and far between. It's a pleasure to help.

Another possible solution would be to change the oil to one with a lower viscosity. That would reduce the pressure drop.

You say the motor turns slowly, that means the bleed off to through the sequence valve is quite low. It won't take much to stop it happening. The schematic shows a flow meter, can you use that to tell how much flow the motor is seeing when the cylinder is retracting?

Have you noticed that the speed of the motor slows are the system gets warmer?

Regards

Adrian