Question about counterbalance valves 1

[freddyj]

My question related to the back pressure produced by the valve. From what I've learned so far on my own is that the Pilot ratio determines how much pressure is going to unseat the pilot and along with the bias spring determines the amount of back pressure that is maintained while the pilot is opened, correct? My application uses a counterbalance valve and the generated back pressure is taking away much needed net force. I have a 6:1 valve, would a 10:1 reduce the back pressure? If so by about how much compared to the 6:1?

 

[IceStationZebra]

The ratio is the pressure ratio between the two sides of the valve. If you are looking at cylinder force you need to take the differential area into account. Be aware that in some scenarios changing the ratio can lead to instability resulting in chatter or noise.

ISZ

 

[hydromech]

The first thing to say is that the valve you are talking about is an overcentre valve. Counterbalance valves have no pilot ratio and rely purely on line pressure to open them.

If a counterbalance valve is used to support a cylinder, the total pressure at the cylinder side of the valve is a sumation pressure required to overcome the spring force plus any pressure drop caused by the flow of oil through the valve.

Overcentre valves have external pilot ports to assist the opening of the valve. The pilot ratio is the differnece between the annular area of the main stage and the area of the pilot piston. Because the pilot signal is remote it has no effect on the pressure drop across the valve, when the valve is piloted open, it will open fully. When used on a double acting cylinder, the pilot signal pressure has a direct relationship with the spring pressure of the valve that it is opening and for that reason, high ratio valves can cause the cylinder operation to become very unstable. It is always best to use a low pressure ratio wherever possible, although application limitations may mean that a high pilot ratio is unavoidable. In these cases there are valves that have special pilot control features to suppplement the pilot signal and prevent the cylinder shaking that is often seen.

'hope this helps a little.

Adrian

 

[freddyj]

Hydromech, you are the first person to tell me that counterbalance valves have no pilot ratio. Everywhere I've looked so far (vendors, manufacturers, literature, etc.) seems to contradict that statement. However, everything else you stated helps alot. I was thinking of buying the 10:1 but you have caused me to have second thoughts. Thanks

Maybe 'Backpressure' was the wrong choice of words. What I meant by that is the resistance created on the opposing side of the stroke to keep the load under control. In my case I need as much Net Force in the piston rod area direction (down stroke, for stripping) as possible and the pressure created by the valve (in doing what it's supposed to do) is reducing that net force significantly.

Ice, differential area seems to make sense... Is this area independent of the pilot ratio? If that is the case then the counterbalance valve may not be the right choice for my application.

 

[hydromech]

OK perhaps most people do call an overcentre valve a counterbalance valve and vice versa. They are both motion control valve and do act in much the same way. It is like most other things in fluid power, the terminology can be as confusing as the the circuits themselves.

For me there is a clear difference between the two types of valves and their application. What Ice said is true to a certain point. If a cylinder is pulling a different load than it pushes the differential goes out of the window.

If your application is moving on the down stroke with a constant load then a 10:1 will be OK. The problems tend to occur when the resistive load drops away and the pilot is doing all of the work. The valve becomes over responsive and the cylinder will become unstable.

I am not sure that you have fully grasped the concept of motion control valves. It is the resistive load on the cylinder that is used to open the valve to let oil out. What will provide the greatest resistance to the cylinder down stroke, the stripping process or the motion control valve?

It is possible to get a similar result using a pilot operated check valve, although they do tend to be slightly agressive in closing and opening. They can induce shock to the system.

Adrian

 

[budt]

What you need is a Counter Balance valve that has INTERNAL and EXTERNAL Pilot functions so you get smooth action when moving to the work and then relieve all Back Pressure when acomplishing the work.

You can see an explanation of the three types of Counter Balance Valve pilot operation in my Basic Fluid Power book in Chapter 14 starting on Page 14-3.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[budt]

Oops, here is the link to the Ebooks.

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

look in the basic book.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[freddyj]

Yesterday I removed the counterbalance valve from the circuit to see how it would operate, so now both lines are going straight to the valve. I didn't mention that I am using a +/-10V proportional valve that uses positional feedback and a PID algorithm from a controller to precisely move the load in either direction. After doing this I've noticed that the problem is still there but now my positioning is significantly more accurate than with the counterbalance valve in place.

From what I can tell I don't need the counterbalance valve, but I still don't know what is causing such a high piston pressure during my downstroke. Things I've considered are the cylinder area ratio, which is nearly 2:1, or the proportional valve maximum flow. For example, during the UP stroke the pressures on both sides of the piston are about the same but during the DOWN stroke the bottom pressure is always half the top pressure no matter how much I increase it.

My conclusion/hypothesis, the line sizes and orifices are the same on both sides of cylinder therefore this behavior is perfectly normal for the design and if I want the bottom pressure to drop more I'd have to either beef up the lines or return straight to the tank, i.e. change the design. Hydromec, Bud, does that sound about right?

 

[PNachtwey]

There can be only one!
You should have mentioned you are using a counter balance valve with a motion controller in the first post. There can only be one valve that controls the flow. You have found out that counter balance valves interferes with the motion and cause errors that the motion controller tries to correct and often over correct.

I have ranted and raved about designs that include counter balance valves. I normally recommend simple blocking valves that are energized open for safety.

However, you still can have problems with cavitation on the cap end. I prefer having a fill valve in these cases. If you must use a counter balance valve use the lowest ratio possible and an don't size the counter balance valve too big. 3:1 will be better. If you must use a counter balance valve then make sure the design opens the counter balance valve relatively slowly. Don't connect the pilot to the cap side.

I got a call once from a control person trying to tune up one of hydraulic motion controllers. He had been trying to tune a system up for a couple of weeks and finally he called for help. He was tuning a system with a counter balance valve AND regen! The oil on the rod side didn't even go through the proportional valve! The counter balance valve controled the flow on the rod side when going down. There was nothing the control guy could do and there was nothing I could do to help him except to tell him the hydraulic design wouldn't work.

Do you see that Budt? I am not the only one that has seen counter balance valves interfere with motion control. I hope others that monitor this forum pay attention. Designing bang-bang circuits and servo circuits is quite different.

 

[hydromech]

Freddy...

You may well be correct about not needing a motion control valve, but only if you can allow the cylinder drop when it is not mechanically supported.

Proportional valves are designed to run with high pressure drops because it makes them more responsive. It may be that the spool in the directional control valve has over/under laps to compensate for the differetial in the cylinder. That may account for why the pressure is always high and why the pressure has not dropped after you removed the motion control valve.

Peter...again I see a very simplistic response. Blocking valves are not a reliable replacement for motion control valves! To control a cylinder the starts with pressure in bore which goes to zero as the cylinder approaches the centre position and then has the pressure increasing in the annulus side, the only safe solution is an overcentre valve.

Hydraulic valves are sensitive to viscosity and will obviously take a longer to accelerate and decelerate than an electronic command signal.

Electronic controls augment the performance of hydraulic systems but there will always be performance limitations in the hydraulic system that cannot be solved using electronics.

Adrian

 

[budt]

Peter asked:
"Do you see that Budt? I am not the only one that has seen counter balance valves interfere with motion control. I hope others that monitor this forum pay attention. Designing bang-bang circuits and servo circuits is quite different."

As you know from experience with a local company, I always recommend you or another moreloal person when I am asked to Design or work on a circuit with Proportional or Servo valves. I learned many years back that those types of Fluid Power circuits were out of my knowledge base and getting into trouble with one circuit was enough for me. I'm not a slow learner and have no problem saying to anyone that I am not proficient at a particular system design or trouble shooting job. No one has ever complained or given me a hard time when I say their request is out of my experience range. Of course, I always have the names of persons who can handle their request as you know from experience.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[PNachtwey]

Peter...again I see a very simplistic response. Blocking valves are not a reliable replacement for motion control valves!

First I don't consider a blocking valve or a counter balance valve to be a motion control valve. That is kind of like calling the on-off switch of your TV the volume control. So you don't like blocking valves, what is your solution then?

To control a cylinder the starts with pressure in bore which goes to zero as the cylinder approaches the centre position

I don't understand what you are trying to say here. What pressure goes to zero? Why doesn't it depend on the cylinder position? Are you referring to the cap pressure and the possibility the cap side pressure can cavitate?

and then has the pressure increasing in the annulus side, the only safe solution is an overcentre valve.

What you are telling freddyj is that he can't do servo control with hydraulics! An over center valve has dead band. That is not a solution for freddyj. freddyj wants to do servo control on a press. Counter balance and over center valves are not linear or servo quality. So what is your servo solution for freddyj?

It takes a little, but only a little, thought and care to make sure the pressure on both sides of the blocking valves are approximately even before opening the blocking valves otherwise the load can be dropped. This is done by using a small open loop command to raise the load slowly before opening the blocking valves. These interlocks can be hard wired or done within a PLC. Meanwhile the relays that keep the blocking valves open can be de-energized by any fault you care to wire up.

You should know that there are plenty of presses out there using servo control without counter balance valves.

 

[hydromech]

Peter...

Your background is electrical, you are not qualified to make the statement "First I don't consider a blocking valve or a counter balance valve to be a motion control valve."

See here...

http://www.oilcontrol.com/website/

To clarify...

Imagine a cylinder pushing an arm that pivots at the bottom. As the cylinder begins to push the arm the load in the full bore is at its maximum. As the arm moves towards the vertical/centre position the load decreases and eventually reaches zero when the arm is vertical and the load is acting vertically on the pivot at the bottom of the arm. As the arm continues to move it will tend to drag the cylinder and the pressure begins to rise in the cylinder annulus.

I am absolutely not telling freddy that he cannot do servo control with hydraulics. I am simply stating that to control a vertical load safely some sort of motion control valve is required.

All spool valves leak and to rely on them to support a load is simply foolish. If a blocking valve is used to lock the pressure the in the cylinder the line pressure between the blocking valve and the spool valve will decay. When the blocking is opened the line pressure increases rapidly and produces a damaging shock wave to the system.

If I were in freddy's shoes, I would leave the motion control valves off and let the directional control valve do the dynamic control and then lock the press mechanically when it is idle.

Adrian

 

[PNachtwey]

Peter...

Your background is electrical, you are not qualified to make the statement "First I don't consider a blocking valve or a counter balance valve to be a motion control valve."

So my back ground is electrical. That doesn't preclude knowledge in other areas. Hydraulic is a lot like electronics at a basic level. I suggest you do a search for my name + hydraulic motion control. When people use their real name I always do a search on Google to see who they are before I blow them off.

What exactly are you controlling with what you call a motion control valve? Counter balance valves don't control position, velocity or acceleration. If you aren't controlling position, velocity, acceleration then you aren't doing motion control, you are just slamming cylinders around.

http://www.oilcontrol.com/website/

I don't see how that is relevant. Are you a distributor?

I am absolutely not telling freddy that he cannot do servo control with hydraulics. I am simply stating that to control a vertical load safely some sort of motion control valve is required.

We have a difference in opinion about what a motion control valve is. At best you can say a counter balance valve is a flow control device.

I was hoping you would come up with a suggestion for making the system safer that doesn't include counter balance valves. I have provided mine.

All spool valves leak and to rely on them to support a load is simply foolish. If a blocking valve is used to lock the pressure the in the cylinder the line pressure between the blocking valve and the spool valve will decay.

I agree

When the blocking is opened the line pressure increases rapidly and produces a damaging shock wave to the system.

I mention above how I would solve the problem above. freddyj is an electrical person that can probably program a PLC so the blocking valves aren't opened until the pressure are equalized.

If I were in freddy's shoes, I would leave the motion control valves off and let the directional control valve do the dynamic control

Direction control valves are not servo valves. freddyj said he has a motion control card. He should be able to get his actuator to smoothly follow a target position within a few thousandths of an inch with a servo valve or a proportional valve with a servo quality spool. That can't be done with a directional control valve.

and then lock the press mechanically when it is idle.

At least we can agree here.

Peter Nachtwey

http://www.deltamotion.com

 

[hydromech]

Peter...

This will be my last comment on the subject as I feel that we have hijacked freddy's thread.

My comments about your background were tongue in cheek and I am sorry if caused offence.

I pointed you to the oilcontrol website to highlight the fact that the people who make the valves call them motion control valves for that is what the valves actually do.

All hydraulic valves are mechanical and their function is reliant on many factors that electronic controls are immune to. For that reasons precise electro-hydraulic control is not always possible when using motion control valves and servo/proportional control valve.

I would not fit a counterbalance valve to a servo cylinder on a flight simulator, the same as I would not fit a moog servo valve to backhoe loader. That does not mean that hydraulic valve are troublesome pains in the backside, which most of your comments seem to infer.

It's all about application!

Merry Christmas!!!

Adrian

 

[freddyj]

I appreciate the comments. All of you guys are great, the feedback is priceless no matter the viewpoint. Figured I'd fill you guys it on my current status. I've picked up on a few basic hydraulics concepts that have caused me to criticize the design of my cylinder.

The tolerances are tight as hell, the tooling load is about 1100lbs and it holds with no assistance from any valves whatsoever, I noticed that I wasn't getting a pressure reading with just the load so I decided to remove the lines to see what would happen and it just held itself up with friction. Left it there overnight and it barely moved at all. The bore is 11in. in diameter with a 9.5in rod, hollow in the middle so the areas are only 44.89in^2 and 24.3in^2 respectively. At 1500psi this produces a tremendous amount of force on either side but the downstroke has the tooling weight on top of the 1500psi driving the fluid out of the 1/2" line. Flow should be significantly higher during downstroke, which is why I get the unwanted pressure reading. I spoke to a hydraulics specialists that suggested changing the proportional valve for a directional valve. I did that and got the same pressure response. So I concluded that whatever valve I use is going to do the same thing because I'm driving the fluid out too fast either for the valves flow response or for the tube size I'm using. I was advised that the tube size is adequate so I came up with another potential solution.

I don't need all that net force all the time so I'm going insert a solenoid bypass circuit that dumps capside fluid directly in the the tank when I need to maximize downforce. I still expect some restriction but hopefully it will make some difference. I know there is some safety and stability issues that could arise by doing this, what should I look out for?

By the way the counterbalance valve is outta here, the proportional valve works so much better without it.

 

[hydtools]

Maybe I don't understand your piston construction. I calculate 95 sq in cap end and 34.1 sq in rod end.

Ted

 

[kcj]

Do you mean you have high pressures out of the rod side when going rod extend? That would be normal depending on the cut of the spool lands. That pressure is sort of by definition the 'control' part of motion control. Summation of forces, etc.
Run through Jack Johnson's VCCM equations to understand the concept. VCCM is for steady state motions and force, not dynamic accel/decel that peter talks about, but the concept is there.
Hollow rod doesn't matter, the area at end of the hollow spot still adds to the piston (closed) side area.

 

[freddyj]

Its difficult to explain the design without a drawing, sorry. It is actually hollow all the way through, rod and piston is bored out. Similar to a telescopic design only with one extension. It sits on the floor like a bottle jack and moves PM compaction tooling up and down. Time does not permit me to verbally describe the full design, even if I could, just take my word for it on the areas, they are accurate per our technical drawings.

Kcj, I mean that I have higher back pressure on the piston cap end during down stroke. I need net stripping force, down force. In my case case rod retracting force. Back pressure on the piston side during the retract takes away net force. For example, during a retract, if I have a reading of 1500psi on the rod side, I get 750psi on the piston cap. Using the area figures I gave earlier you can see what I end up with compared to what it could output. Ideally I'd like the full 18Ton output but naturally like you said backpressure is needed for control. If I cut this backpressure in half I go from 1.4Tons to 10Tons net force.

 

[PNachtwey]

The diameter of the whole through the rod would be nice to know. A link to a picture or drawing is worth 1000 words at this point. The only thing I of that would be this odd is a concentric powdered metal press. The hole through the middle of the rod must b 8 inches in diameter.

Kcj, I mean that I have higher back pressure on the piston cap end during down stroke. I need net stripping force, down force.

There will be back pressure on the cap side while retracting because there is flow and a pressure drop across the valve. The areas are about 2/1 so you can expect twice the flow through the cap side as the rod side. Since the pressure drop is proportional to the flow squared you can see the pressure drop across cap side land of the spool is going to be 4 times higher. but once contact is made there will be less back pressure as the material will

There are valves with 2:1 spools that may keep the pressure drop across the spool lands closer to 1 to 1.

What motion controller are you using? Doesn't it provide graphs of the positions, control outputs and pressures? I know ours would.