Hydraulic Cylinder Circuit not working as expected 1

[sammyb123]

Please see the cicuit here

http://uk.geocities.com/sean_bool/ocv.jpg

The DCV's operate simultaneously to operate the two cylinders. They are both loading on the annulus side.
Can you tell me why when I operate it, the pressure in line B is zero until the cyl 1 bottoms out then it climbs to the pressure required to lift cyl 2 (which requires nearly twice as much).
The screw on the Over Centre Valve (OVC) at line B is screwed right in.
It seems the valve is open to tank all the time there is flow but at no flow the OCV operates.

The OCV was put in because the Cyl 1 lifted first as it doesn't need as much pressure as cyl 2 and I tried to restrict Cyl 1 somehow. They are both supposed to come up more or less together but they are different sizes & loads hence different pressure requirements.
I thought by screwing in the OCV at line B should mean it "blocks" pressure until the pilot pressure from line A & the pressure in line B rises enough to overcomes the compressed spring pressure?

Even stranger is we have made 10 units which are acceptable but this one won't behave like the others - We have tried about 6 different OCV's with no difference. We haven't got the other units now to be able to swop those valves either.
The pump is on a 12v powerpack.
Any advice would be gratefully accepted
Thanks

 

[hydromech]

So the cylinders hang upside down, pulling the frame up as oil is pumped into the annulus.

With the pins out the weight of the frame and the load is supported on the annulus.

If this is the case, the OVC in line A is working as it should.

The total load is being supported by the OVC in line A.

With the pins out, if the minimum pressure setting is low enough the valve in line A will have opened already and the weight is being supported by the spool in the DCV.

If the valve has not opened already, it will be very close to opening and to get it to move the pressure required in line B will be almost zero.

Either way the pressure in line B will be near zero.

I would expect that the only time you see pressure in line B is when the load is lifted.

When you fire the solenoid valve to start lowering from a midway static position does it go with bang?

You'll get a smoother operation is you change the DCV to P blocked A + B to T and reset the OVC valve to 30% above the maximum load induced pressure.

That is not to say the system you have is wrong, it will just make it quieter.

Hydromech

 

[scarecrow55]

Thanks for all your help, we get the new overcentre valves tomorrow so I'll try & fit them & let you guys have the results.
I'll also think about you other comments re the DCV etc. when I look at the redesign.
Your comment re go with a bang, only the front little cylinder does that sometimes. I put it down to the post cocking over in it's "tubing". (yes that's another story)

 

[scarecrow55]

Feedback - fitted different manufactured OCV's and they all worked OK. Awaiting a report from the supplier of the originals.

 

[hydromech]

Success is often failure with another coat of paint...

 

[chicopee]

This should tell you a thing or two about oversea suppliers of valves. Best to buy American made valves-more expensive but more reliable. People can get killed from faulty hydraulic circuits and that I know first hand.

 

[hydromech]

I am NOT of the same opinion...

First off scarecrow55 and myself are in the UK and hydraulic equipment is very rareley sourced in the US.

Secondly, most motion control valves in the US have been designed and developed in Europe.

Thirdly, I have had my share of problems with US valves. I would say they are no better or worse the any other valves.

It is not a good idea to chuck in general statements like that, especially when you have not fully read all of the posts.


Hydromech