Proportional valves and dirt in oil. Good/No Good?

[Skogsgurra]

Hello. I am active in the electric/electronics field of the automation business and know precious little about hydraulics. So I may be using words that you do not fully understand. Please ask again if I do.

The problem: A press with hydraulic cylinders moving two tables up and down. Each table has its own hydraulic circuit with proportional valves that are controlled from a PLC with analogue output cards, +/- 10 V.

It is important that the lower table moves faster than the upper table or there will be a collision between the two when going down. The same thing applies when going up, but upper table has to be faster - of course.

Sometimes, the upper table doesn't move fast enough when going up and sometimes the lower table doesn't move fast enough when going down. Which results in collision and sometimes ruined products. But always reduced production since the machine has to be reset manually.

We have studied the machine for two days. We have recorded the control voltages for the valves and we have also recorded the feed-back signals from the valves (the spindle's actual position). Control voltage and feed-back are identical. A 7 V control voltage results in a 7 V feed-back and X V results in an X V feed-back. No ripple, no delay, no overshoot in the feed-back signals.

My question is: Is it time to get the hydraulics guys in and make them clean the system? They have taken oil samples and are showing codes like 13-4 (or something like that) and they say that there is no problem with the oil. What else could we do?

Good ideas needed.

Gunnar Englund

http://www.gke.org

 

[maytag]

You mentioned a "13-4" code on the oil test result- I'm at home now and don't have any books but I think they are referring to ISO 4406 and the 13 refers to the 5 micron particles and the 4 refers to the 15 micron particles-you work this 13 and 4 with a chart and it will give you the actual range of contamination-I think 13-4 would not be considered to be a BAD sample but sampling technique and other varibles may not show a true representation on the oil condition.
Was the feedback from the Parker valves on showing the spool position of the pilot stage? The cable resolvers still may be something for management to consider. Maytag
I went to your website but could not read it-is this in German?

 

[Ashereng]

maytag,

I think Gunnar's web stie is Norwegian?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[MikeHalloran]

ANY metal particles in a hydraulic system are cause for disassembly and manual cleaning of _everything_. Yes, that means disassembly and cleaning of every valve and cylinder, and disassembly and flushing and rodding out (lint- free cloth, please) every tube and hose.

Flushing just moves the particles around. Really strong flushing moves particles that wouldn't have done any damage in normal service into places where they are likely to cause misbehavhor.



Mike Halloran
Pembroke Pines, FL, USA

 

[Skogsgurra]

Thanks all.

I feel much more confident now. My task was/is to record electric signals in the machine in order to find out what the problem is. It was assumed that the problems were in the PLC and valve control circuits.

None of the recordings deviate from the ordinary when the fault occurs. They are absolutely identical - fault or not. So, since no one else (except management) seems to bother and since mechanics and hydraulics guys don't accept (don't understand) what the recordings say, I need to understand this better in order to get things moving.

Traces: 1 Enable 2 Actual spool position 3 Spool setpoint 4 Terminal 5 and 6 on "Parker box" (not quite sure what the Parker box does)

This looks the same regardless of fault or no fault.


Re my site; it is in Swedish. But Swedish and Norwegian are very similar. We can understand each other quite well. And about half of the Swedish words stem from medevial German, so it isn't entirely wrong to guess German either.




Gunnar Englund

http://www.gke.org

 

[budt]

How sad it is that Fluid Power is so far advanced in technology and so far behind in how few even have a vague understanding of how it works and what to do when it finally fails.

Sad that trouble shooting is left too electrical persons who can pinpoint the problem but have no training in what causes it.

Sad that a pump or other component must catastrophically fail and contaminate the total system when there is usually lots of indicators well in advance of the failure.

Sad when industry will pay someone with a High School education $100.00/Hr. to trouble shoot a hydraulic problem and think they are getting a bargain.

Someday? industry will decide the Fluid Power field needs trained dedicated persons to design and work on Fluid Power powered machines and will finally start enjoying all the benefits of Hydraulics and Pneumatics.

So far, I must admit, it seems no one is listening or at least no one is doing anything about the problem except changing everything possible to other means of doing the work that has been and should be best handled by Fluid Power.


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

http://www.fluidpower1.us

 

[Skogsgurra]

I have similar feelings. What I think is sad is that there's very little response from the Fluid Power people when I try to get information about the system or try to get them started in a constructive direction.

It is only when I ventilated (no pun) the problem here that I got any meaningful reaction. Does that mean that the competence out there is lacking? Or is this the normal way of handling problems? I have experienced this before. Although on a lesser scale and not over such an extended time - about one year without anything being done to solve the problem.

Gunnar Englund

http://www.gke.org

 

[budt]

Gunnar;

In my experience in the US it is the same as you see in Sweden. Actually I was under the opinion that Europe, I place Sweden in Europe and I trust that is true though itd been a long time since Geography classes, was far advanced over anything in the US.

Here it seems almost all circuit design is done by the Flluid Power salesman, I did that for 27 years, and even when a competitors product will do a better job they always use something from a manufacturer they represent. The end users depend on the salesman to design the circuit , assit in installing it, getting it started, trouble shooting it and doing any type of training the customer may desire or need. A simple case of shooting ones self in the foot in my opinion.

In 1980 I semi-retired and started doing the same thing I had done for 27 years as a Fluid Power salesman but getting paid by the hour instead of by commission. All my old customers still depend on me as before and I have enjoyed working with them without the hassle of an employer constantly pushing for more sales. I don't care who they buy their components from now.

At the same time it gave me the freedom to use any manufacturers component I determined was best for the application without getting negative feedback from a supervisor.

If anyone is interested I have an article that was in Hydraulics & Pneumatics magazine in the late 80's on my web site that outlines why I think industry is losing out by not having dedicated Fluid Power Engineers and Maintenance persons on staff.

I'm not holding my breath that there will be Fluid Power persons in plant in my lifetime but someone has to start making waves before anyone will even give it a thought.

Older but Hopefully Smarter:


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

http://www.fluidpower1.us

 

[Skogsgurra]

I know the feeling, Bud.

Used to be working for ABB and Siemens, but started on my own back in 1977. Tough, yes. But once you have built up a set of customers (takes five to ten years) you get on quite well. And I still do lots of work for ABB and Siemens. Mostly on "vintage drives" but also doing modern drives. Keeps you alert.

I really love doing what I do. And it takes me to places I would never visit otherwise and meet people I would never have met - it also makes me eat things I wouldn't ever have thought of eating...

Now, I'll have a go on your brain-teasers. Where's that Aspirin...?

Gunnar Englund

http://www.gke.org

 

[waross]

Hi Gunnar
I have an idea. I probably know less about you than hydraulics but like you have been forced to learn because someone else knew less. I wonder if it is a flow problem? There are two parts to this, cause and effect.
First, effect: If the valves are asking for more flow than is available, the sharing of the volume becomes dependent on the back pressure as well as the valve position. With equal loads, the piston extending will have a greater mechanical advantage and a lesser back pressure because the effective area of the piston retracting is reduced because of the shaft. The piston extending will therefore "hog" the fluid and move faster.
The why.
The first thing I would check would be the accumulators. There may be a ruptured bladder or a low nitrogen charge in one or more accumulators.
If the system pressure is below design pressure, the accumulators will not charge fully and will run out part stroke and the flow will be reduced.
The machine may work fine on a slow cycle, but if the there are too many up-down cycles in rapid succesion the accumulators will go flat.
The accumulators may be overcharged. The charge must be matched to the system pressure.
There may be a problem with the pressure control on the pump. Maybe the bypass valve leaks when it's hot. This would reduce the system pressure and lead to undercharging of the accumulators.
Maybe the pump is failing again.
I suggest monitoring the system pressure and correlate system pressure with table problems. If you can show a correlation then you can either say "The electronics are good" and leave, or stay and get involved with the hydraulics.
Maybe everything is allright but they are attemting to cycle the machine faster than it was designed to operate.
Please let me know how it turns out.
I used to service (electrically) some hydraulic equipment. At times the operater would be cycling the hydraulics much faster than normal and more often than the design provided for. The slowdown in operation was always abrupt and noticable when the accumulators ran out. Try to simulate the problem with a series of rapid up-down cycles when the machine is hot.
Respectfully
Bill

 

[Skogsgurra]

Bill,

It is an interesting observation that you know less about me than about hydraulics. And probably true, too. ;-)

Yes, your thinking and your experience with this is something to have in mind when/if this continues. I have told my customer what I think (and it turns out that they had the same thinking - just wanted a second opinion) and we have left the whole thing for the machine builders and hydraulics people. I do not know if I get involved again or if I ever will know what the solution is. But if I know, I will let you know.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Skogs,

re. Dirt in the system. We use a lot of Moog servo valves on our steam turbine throttles and governors, and on the gas turnines too. Dirt is the #1 enemy we have with these valves - we flush for a long time and use finer filters than supposedly necessary for these valves, but the additional planned maintenance which results does leave us with good uninterrupted runs between maintenance. If contamination enters the system we experience a lot of problems until cleanliness is restored.

Rather than sense spool position, can your control scheme use rod position? We use Balluff MicroPulse sensors - similar, but in our experience superior, to the Temposonics products mentioned earlier. The North American manufactured flameproof stainless steel BTL-5 design is virtually indestructable once mounted. One of ours bears the scars from being hit with a mell by a clanky trying to align the actuator to the valve, but it still works perfectly.

If you are relying on spool position only, then I would expect differing rates of travel depending on whether the fullbore or annulus is exhausting. However, if the problem has only just started and it worked previously, then the question is "what changed?" and the answer would seem to be the pump failure contaminated the system with metal shards. The initial step will be as Mike Halloran says - strip & clean everything. Once it is clean, consider sending the valves back to a specialist for overhaul - the Moog valves we use certainly aren't intended for field repair.

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[Skogsgurra]

Feedback.

Problem solved. But not by the hydraulics guys. A service company representing the supplier visited the site almost two weeks ago. They did "everything" - and found nothing wrong. They even wrote a report saying that there was nothing wrong. In the same report, they said that "because there was a dirt problem in another machine, they think it is a dirt problem in this one, too".

They simply had made up their minds once and for all: "The electronics was the culprit. There!"

The machine builder's service technician and an elder hydraulics guy took everything apart. Cleaned everything and flushed the system thoroughly. Filled new oil. Found lots of those brass particles. Machine runs like new after that operation.

But remember: "There's nothing wrong with the hydraulics!" Also remeber not to suspect the least likely part - the hydraulics!

Sorry if I sound sarcastic. But that is inevitable in situations like this.

Gunnar Englund

http://www.gke.org

 

[budt]

Gunnar:

We have a saying in the States, May be the same in the UK,

"You get what you pay for"

And I find that is about par for the course on one of these forums.

Happy you found the problem and got the machine running well.


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

http://www.fluidpower1.us

 

[maytag]

Gunnar,
I've worked as a mechanic in 3 large corp.-its always been an arguement between the crafts as to where the problem lies. Sounds like this knows no boundary. Maytag

 

[Skogsgurra]

Yes maytag, I agree. And we always sit down to discuss matters when we don't have clear-cut cases. But this was exceptional. Not an ordinary "those clankys! those sparkies!" thing.

We had all the facts. We communicated them effectively. Orally and in writing. We even explained what and how we were measuring and asked them to propose measurements that would enlighten them. But the hydraulics guys had made up their minds and decided that it wasn't their problem. The end customer hired them at least three times to do their job - and they didn't. This is over a span of around a year.

It is not an ordinary "between the crafts" argument. At least not as I see it.

Gunnar Englund

http://www.gke.org

 

[waross]

Hi Gunnar;
I've been waiting to hear what happened on this one. Did the fellows who eventually repaired the system offer any speculation as to the effect of the brass particles?
Was a control valve being affected directly, was a relief valve being affected and dropping system pressure?
Thanks for the feed back.
Like you and a lot of electrical guys I've learned a lot about mechanics and hydraulics on the spot to be able to show that a problem was not electrical.
Respectfully
Bill

 

[Skogsgurra]

I talked to these guys yesterday. They think, perhaps, that a check valve got clogged in some way. But not quite sure. The particles were removed from the system and the machine is running very well.

Gunnar Englund

http://www.gke.org

 

[MikeHalloran]

The particles you can see are generally not mobile enough to cause trouble. But think of them as sentinels.

The real troublemakers are just small enough to enter the (really small) clearances in a hydraulic valve, and too big to just flow on through. You can't see them, but if you've cleaned out all the visible particles and then cleaned some more, you've probably gotten most of them.



Mike Halloran
Pembroke Pines, FL, USA