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

 

[itsmoked]

sammy; I'm not a hydraulics guy but could you have air in line B? Would seem to me that the pressure in B would stay low (~0) as the piston comes down because the OCV doesn't block air but once the cylinder bottoms the tiny leakage past the piston then provides something the OCV can deal with and regulate hence the pressure builds, (suddenly).

But how does the air return each cycle? Not sure...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[sammyb123]

Thanks itsmoked, but we have tried to "bleed" it now and again but it still didn't work. One thing though, when it was reading zero on the gauge I cracked a joint in line A and got a reading on the gauge. We still couldn't control it though.
I set the previous units by starting it then as the big cyl (1) started to rise, closed the OCV at the line B side and that slowed it down to a point when the little cyl (2) started to move. After that they both went up and down more or less together.

 

[sammyb123]

Just to clear up any misunderstanding big cyl (1) stared to rise should actually be retract. They are actually pulling the load.

 

[BigInch]

Looks to me like there is still no real loading on Cyl 1 happening before you get a loading on cyl2. The pump is at a full flow runout condition, until Cyl 1 hits the stop. Then pump flow decreases and pump discharge pressure increases and continues to increase, so loading cyl 2. When the actual movement of cyl2 starts, there is a further increase in discharge pressure which allows some load shifting to cyl1 and that process repeats itself. It looks like you have created (or not entirely rectified) a pressure imbalance between the two cylinders, caused by a poor relationship of the pressure distributed to each cylinder in relation to the different loads they have when each one is first, initially loading and, second, when overcoming static friction, and, third, have to change again to a slightly lesser load when they do get moving(moving or dynamic friction is less than static or starting friction). Have you noticed a "jerky" motion at all when the loads are moving?

 

[Cockroach]

Sounds to me like you have your hydraulic loop wrong. It is difficult to diagnose your problem directly in this forum, but clearly the OVC is not working as intended in that loop. You say that several different elements have been tried, the result being the same. I would say that the design of the loop is problematic, you must be having a bleed back through the lines to the sump, or as stated before, the system has air in it.

I would also put a check valve in the inlet line in order to eliminate the possibility of reverse flow from the cylinder. This is probably not your answer, but a suggestion for maintaining a pressurized cylinder. I assume that you have checked the lines for blockage, perhaps something over the filter port.

You must have an issue with your hydraulic loop. Sorry I couldn't offer a better suggestion, good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Airman23]

I think to have a solution for your problem...
I prepare it and post it in this forum asap.
See you in this forum...

Eng. Antonino Nicosia
Aeronautical Engineer specialized in Fluid Power field.

my website: web.tiscali.it/aeronavigandosulweb
(in the future there will be many programs about fluid power problems, thermal and structural problems)

 

[scarecrow55]

Thanks Airman, I have tried a bigger cylinder at Cyl 2 but cyl 1 still rises first. Tomorrow we are trying a different over centre valve - I'll keep you posted

 

[sammyb123]

FEEDBACK - I've managed to get hold of a OCV from a good system and put it on the one we are having trouble with and guess what - it works OK.
Yesterday we tried a different sort of OCV and that didn't work either.
The pressure gauge on line B for the good valve shows the pressure builds up (as I expected it should) more or less straight away after the cly 1 starts moving, thereby supplying enough pressure to lift cly 2. On the bad valves the pressure does not build up until the cyl is nearly up, suggesting the oil is byepassing/leaking or the spring is too weak. The hyd suppliers have tried 6 or so different OCV's now that were not working as expected.
At least this points to the valve as the problem.
Airman, I'd still like your solution please.
Thanks

 

[Airman23]

I am preparing my solution, but I can give only a summary of this solution in this forum, do you want give me your e-mail, so I send you a detailed solution for your problem.
Now I am working and then I hope to send you my solution before of this weekend...
However I have some questions for you, I post it in this forum asap.
See you in this forum..

Eng. Antonino Nicosia
Aeronautical Engineer specialized in Fluid Power field.

my website: web.tiscali.it/aeronavigandosulweb
(in the future there will be many programs about fluid power problems, thermal and structural problems)

 

[Airman23]

Is your e-mail that it is contained in the directory of this website (

http://uk.geocities.com/sean_bool/personalpageblue.html)

Eng. Antonino Nicosia
Aeronautical Engineer specialized in Fluid Power field.

my website: web.tiscali.it/aeronavigandosulweb
(in the future there will be many programs about fluid power problems, thermal and structural problems)

 

[sammyb123]

No, try my gearing site, Cheers Airman

http://uk.geocities.com/mikebull2001/green.html

 

[Airman23]

O.K. I contact you in the next week.
Pardon but in this moment I am very busy.
However I think to change your hydraulic cyrcuit...
see you on this site.


Eng. Antonino Nicosia
Aeronautical Engineer specialized in Fluid Power field.

my website: web.tiscali.it/aeronavigandosulweb
(in the future there will be many programs about fluid power problems, thermal and structural problems)

 

[hydromech]

My help may be little late but if it helps...

Do you know the pilot ratio of the OVC valves?

Even with valve fully shut if the pilot ratio is high enough the valve will open at low pressure.

A valve set at 3000 PSI/206 BAR with a pilot ratio of 8:1 will open at 345 PSI/25 BAR...on a pressure gauge with a high scale that might not even lift the needle off zero.

It is possible that you may have a valve with a different pilot ratio, that could explain why most of the systems work and this one won't

If the cylinder does not go over centre you may have been better off with a counterbalance valve or even a sequence valve.

Can the load on cyl 2 ever pull the cylinder out or cause the cylinder to overrun?

Hope this helps

Hydromech

 

[budt]

Sammy:

What is the manufacturer of the OCV? Is the non working the same make?


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[scarecrow55]

Thanks Guys, the pilot ratio is 4.25/1. Not being an hydraulics man what exactly does this pilot ratio control. What do you mean by the cylinder going overcentre (past mid stroke?)
I supposed the cylinder could pull back but it would mean it would have to "suck" the checkvalve in the OCValve (Line B)open. The DCV is closed ports in neutral.
The same supplier supplied all the valves. We are getting hold of 3 different supplier valves to try ASAP.

Forget the Handle I need two, one for work & one for home

 

[hydromech]

OK...The first thing...If an overcentre valve is not vented to atmosphere, otherwise known as pressure balanced, it may struggle to open if the DCV is closed in the normal position. Oil can leak into the spring chamber and if that oil cannot leak away through the DCV the valve will become hydraulically locked and could struggle to open.

OVC valves work best with DCV's that are open in the normal position...A and B to T.

The pilot ratio effectively states the pressure at which the pilot signal will open the valve. An OVC valve is basically a pressure relief valve with a pilot area to help the valve to open. If you remove the pilot function from an OVC valve it becomes a counterbalance valve. It will need the full pressure to open the valve.

Lets say an OVC will hold a static load of 1000 tons at 3000 PSI(206 BAR). The pressure is being held by the force of the spring in the OVC valve. When the DCV valve opens and oil begins to flow, oil pressure in the pilot line will make the valve open at the value of the ratio. with a ratio of 4.25:1, the 3000 PSI spring will open at 666.666 PSI. That is without any assistance from the load on the cylinder. As soon as the oil flow stops, the piot signal is lost and the valve closes very quickly an the actuator locks. Depending on the type of system valves with a ratios from 2:1 to 10:1 can be used.

The higher the ratio, the more unstable the system will be.

If a cylinder is on an assembly that moves through an arc, the cylinder may go over centre(Vertical). When a cylinder goes over centre, rather than being pushed by oil flowing into the bore, it will be pulled by load. So as it goes over centre the pressure and the load switch from the bore to the annulus. Hence the name for the valve.

On your system, if the cylinder is being pulled by the load the pilot pressure required to open the valve will be very low.

If an OVC is set to open at 3000 PSI and load induced pressure is 2100 PSI, that leaves a difference of 900 PSI. With a pilot ratio of 4.25:1, the system will only require an additional 212 PSI (900/4.25)to open the valve.

If I am reading the schematic correctly, the valve in line A should stop the cylinder running away.

Is it possible for you to block off the pilot signal to the valves?

Who made the valves?

Regards

Hydromech

 

[scarecrow55]

Thanks Hydromech, I'll study your post. The valves were supplied from Italy (don't know the manufacturer) by our Hydraulics supplier. This was a prototype circuit that had no OCV to start with. I didn't know what would happen so the Supplier suggested the OCV when we found the little cyl (2) rising first.
Although I suppose the other valve on line A would stop it going back, it is only there as that was the valve (a double single block thing) our supplier had at the time. We are leaving that side screwed right out.
Also we've just had feedback that the Italian company think that if the wrong springrate springs are in then that will cause it. They are posting some over, although I took some physical dims of the springs and they look the same to me.

 

[hydromech]

If the OVC valves are from Italy they are probably from Oil Control.

What orientation is cyl 1 in?

If the load on cyl 1 is pulling the cylinder out, and the valve on line A is set to minimum, it is no surprise that there is low pressure in line B.

Can you explain the application a bit more?

Thanks

Hydromech

 

[scarecrow55]

Both cylinders lift & lower a frame. If you imagine a triangular frame with a sliding post(leg)in the three corners. These posts are allowed to slide vertically in the frame. The hydraulic cylinders are connected thus - the rod end to the top of the post and the cly end fixed to the frame. There are infact 2 big cylinders (cyl 1) in two of the corners connected together with T pieces.
The load (2500 Kg)is on the frame (most towards the 2 big cyls), therefore gravity is trying to pull the rods out.
The finished positions are either up or down. no inbetween. When down it's on the ground so no load on the clys. When it's up pins are inserted so again no load on the cyls.
There are times though when we need to stop halfway so if the DCV went to neutral then it can't be allowed to run to tank - unless of course we utilise the OCV on line A.
I know this circuit is not the best now we have it, but it's evolving from the prototype.