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4-Run Method of Balancing

4-Run Method of Balancing

4-Run Method of Balancing

(OP)
Can anybody help me with the procedure of carrying out a 4-Run Balancing Method using Mils/sec as the vibration parameter. I am only familiar with using Mils instead. Also, please give me the equations for calculating the correction mass

RE: 4-Run Method of Balancing

(OP)
No, this is not for School..I want to balance a Flue-Gas Fan that keep coming out of Balance. I just want to know whether finding the correction mass has the same relation as when you use Mils for the vibration parameter.
Thanks.
I am a Graduate Engineer (Mechanical)

RE: 4-Run Method of Balancing

Save yourself hours of agony and get a proper analysis system that uses phase as well as magnitude.

The units you use don't really matter.


 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: 4-Run Method of Balancing

Is this a continuation of the posting that begin on Jan 26th?

At that time you had 'experts coming in to do the balancing' and now you are doing it yourself?

Anyway....to answer your questions

a)mils/sec is not a sensible unit for vibration measurements

mils are 0.001 inches displacement

mm/sec is a recognised unit in terms of vibration velocity

Do you understand the difference? If you can 4 run balance with mils you should be able to balance with other units.

b) Unbalance is unbalance no matter what units you measure vibration in  - simply put, the objective of balancing is to get the centre of mass to coincide (or get close to) with the centre of rotation - you do this by adding mass (ounces or grams, depending on your background) to the rotor assembly

Greg is right - get a setup that uses phase angle and you can balance much more quickly. if your vibration is not due to solely to unbalance (thermal effects/cracks/misalignment)and you go for a 4 run method, you are going to waste a lot of time and effort.  
 

RE: 4-Run Method of Balancing

(OP)
TPL

Thanks mate, and yes I am still on that January balancing issue. I have decided to do it myself and learn on my own through help from guys like you. Thank you so much, I am now confident to go to the Section Engineer and let him know that I will attend to the problem. As for other issues like misalignment, cracks and heat/thermal, I have checked and made sure, but though I have suggested the support structure to be stiffened, I am going to go for the balancing and hopefully get it right. The support structure is only going to be re-done in the next shut-down, 20th April.
Thank you all for your valuable input. I owe you all.

RE: 4-Run Method of Balancing

I am now confident to go to the Section Engineer and let him know that I will attend to the problem.

I do hope that you Section Engineer recognises that your confidence far outweighs your ability and decides to call in someone who really does know what they are doing.

 

RE: 4-Run Method of Balancing

(OP)
I understand TPL. My confidence is not coming out of reading the help I have got from you all only, though it was very insightful. I have a Balancing Jig at my disposal and throughout this week I have been working on it and learning balancing issues (mostly single plane). I have learnt a lot. I believe in challenge and strive to better myself always. I saw you mentioned "mils/sec is not a sensible unit for vibration measurements", it was a typo on my part, meant mm/s. I may sound like the wrong kind of guy for the job to you, but I assure you, I will not try/do anything that I cant do.
The 4-run method seems easier and faster than what one guy was doing on the FANS initially, i.e; putting weights randomly and hoping that by some trial-and-error stroke of light he gets the balance right.
There was not even a log where he kept recordings of what mass he placed where and what at what angles. I was not involved at all, just observed. But I took it upon myself to try and do a better job. We do not have a set-up that gives amplitude as well as phase, only amplitude.
Thanks.

 

RE: 4-Run Method of Balancing

"putting weights randomly and hoping that by some trial-and-error stroke of light he gets the balance right."

Yes, I've done a fair bit of that myself, particularly on 2 plane balancing.

OK, you need to identify 4 unknowns, phase and mag of the original out of balance (P0 and M0), and the phase and mag of the response of the system to a known trial mass in a known position (Pt and Mt). You might be lucky and be able to assume that Pt is 'obvious' but that is not true in general.

I suggest you make up an example where you know what each of those four unknowns are, and plot them on the phase plane. Then see if you can back-derive that plot from the 5 amplitudes - I am told there is a geometrical construction to do it but did it by observation and trial and error myself.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: 4-Run Method of Balancing

If you have your heart set on the 4-run method, then here is an online calculator for it:
http://www.vibronurse.com/NoPhaseBalance.html
In no way do I recommend that Web site nor do I suggest you use the 4-run method for a large fan. You would be better served by getting a vibration analyzer capable of measuring speed, filtered vibration and phase angle and then either use a single plane or 2-plane solution. I use a CSI 2120 analyzer, but there are plenty of others on the market. I always try to find the heavy spot, so that my trial weight will typically reduce vibration level and may be left in place and then a final trim weight is added. The 4-run method is OK i you are balancing a ceiling fan in your house, but it is not good for large fans.

Walt

RE: 4-Run Method of Balancing

Incidentally even if you only have a two channel scope you can figure out the phase, there is really no excuse for not measuring it.

Nice find Walt, but it is still clear as mud to me.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: 4-Run Method of Balancing

It seems to be a popular method mentioned recently on maintenanceforums.com   Victor Wowk is a big proponent.  Why some people preferit, I'm not sure.

The method is spelled out several places in cookbook fashion.  The reason why it works is not obvious.  With some help from WFoiles, I put together a vector analysis which shows why the method works  (attached).

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RE: 4-Run Method of Balancing

I always thought that the 4 run method was a solution along the lines of a Venn diagram setup.

The 4 run method will only work if most of the vibration is due to unbalance AND the unbalance remains constant throughout the 4 runs - if there is any change from run to run it won't work and then you are left with lots of head scratching.

I want to balance a Flue-Gas Fan that keep coming out of Balance

If you have a phase reference (tacho pulse or keyphasor or whatever you wish to call it) you can use previous balancing efforts to go for a one shot balance each time the vibration goes high - that is the way to impress people, otherwise you just have to go for a 4 run balance each time (although with a bit of experience, you might be able to modify it down to a 3 run exercise).

If this machine is continaully going out of balance, you don't really want to be welding weights on - I suggest the first step would be to drill a number of holes (say 12, to include the 4 needed for the 4 run balance and the others to get as close to the final solution as possible) around the circumference of the fan (or one on each blade) and use nuts bolts and washers as the weights


 

RE: 4-Run Method of Balancing

(OP)
Guys I am thrilled to report that I have done the 4-Run method on a big Fan (Flue Gas), and it worked!!
The Fan initially tripped and my Boss called me and asked if I was confident I would do it before knock-off time, I said YES. It took me about 2hrs and the Fan vibration levels dropped to 2mm/s (NDE) and 3.8mm/s (DE). The trip occured at 44.11mm/s (NDE) and 48mm/s (DE). So you can see that was indeed a dramatic improvement, i.e; comparing it with 16mm/s (NDE) and 12mm/s (DE) that the Guy from South Africa always managed. I do agree that we need to get a set-up that measures both maginitude and phase to make it easier but my company is a bit skeptical about spending cash since the economic slump we all going thru.
But thanks guys for the constructive critism (TPL) and help (TPL,GergLocock and Strong).
This is indeed an informative house.

RE: 4-Run Method of Balancing

Good result - well done

Keep it safe though

RE: 4-Run Method of Balancing

Quote (TPL):

I always thought that the 4 run method was a solution along the lines of a Venn diagram setup
TPL - I don't understand your comment. Was this a comment about my proof?  If so I would point out it is more of a proof than a cookbook recipe for the graphical solution. But if you study it you can see the recipe.

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RE: 4-Run Method of Balancing

I guess one more comment is required.  If you want to understand how to do it, the best way is to google and find a step by step procedure.  Once you know how the step by step procedure works, if you want to know why it works my proof explains that.  

Sorry I am not trying to be picky or pedantic or whatever. Just wanted to clarify what my attachment was.

Done beating my dead horse.

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RE: 4-Run Method of Balancing

I would recommend hiring a professional that allow you to follow what they do, a professional who doesn't use the 4 run method.  The 4 run method is very inefficient, and if you have to trim balance you have to start over with another 4 runs.  

If something changes, keeping track of phase and the vector changes helps greatly.  Vector plots of the runs helps explain what has happened and can be good in a report that can be used in the future for other balances of this or similar machines; try that with the 4-run method.

Regards,

Bill

RE: 4-Run Method of Balancing

Either what Bill says, or actually invest in one of the 2 or 3 day rotating machinery field balancing courses offered.  Not only do you learn the manual vector calculation method for single and multiplane balancing, but the instructors have usually been doing this for years and have a lot "tips" to share that will help make balancing easier.  

Finally, the instructor, after making sure you know how to do the manual balance calculations, will tell you that you really should be using one of the "canned" balance routines that are available with most analyzers as an option.  Being able to manually balance using the vector diagram methods will serve you well as a reality check of results when you make a mis-step with one of the "canned" programs.

The four run method can work, there is no question of that.  The main problem is that it is a method to use when you have no phase reference information.  Having no phase data indicates that you are going to do a balance without truly knowing if the machine really has a balance problem or if it has high vibration levels due to one of the many other possible sources of high vibration amplitude at 1X RPM.  If you have a soft foot, or resonance issue or an eccentric rotor or a bent shaft, the attempt at balancing will only frustrate you!  Worse than that, you will suffer a loss of credibility at your facility that will be difficult to overcome.  

Fully 75% of the times I have been hired to do field balancing of rotating machinery in my 30 years of experience I have found other problems to be the dominant source of the vibration.  Most times the balance procedure was not even necessary.

http://www.machinerywatch.com

RE: 4-Run Method of Balancing

What is the theory behind the 4 run no phase method?  It is not intuitively obvious to draw the second circle centered on the circumference of the first.  I've looked and looked and can only find the how to.  I've used the method successfully but didn't know the basis for the different steps.

Thanks Klyde

RE: 4-Run Method of Balancing

It is a bit of a slippery fish. As it happens the B&K blue book on vibrations does include an explanation and a diagram.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: 4-Run Method of Balancing

I have provided something of a graphical proof of the method attached to my message 19 Feb 09 21:27  

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RE: 4-Run Method of Balancing

mtigos,

I have read all of the discussions and I have not seen mentioned a very important step. This is a flu gas fan, a very large one. It was mentioned that it is always going out of balance.

If you have already thought of this please forgive me, but it strikes me that the first thing before balancing would be to clean the fan of all accummulated dirt on the impellar blades. After cleaning run the fan again to check the balance if it is still unbalanced excessively remove all weights and start from scratch. Usually the fan is very close to being balancd from the factory (it may have come with a trim wieght which in that case you will  probably be putting a balance wieght back in one of the spots). Balance your clean fan and then if it becomes unbalanced again clean it first and you should solve your problem without balancing. Balancing a dirty will always end up out of balance again soon.

Also, it was never mentioned whether the fan is an overhung impellar. If this is the case then you may be dealing with a two plane balance problem almost in all cases. There is a special procedure on the Emmerson/CSI web site under Dr. Know.

I trust this is helpful for you.
 

DANIEL C. SMAISTRLA
judasmai@optusnet.com.au
 
Equipment: Centrifugal Pumps and compressors; Steam and Gas Turbines; Reciprocation Engines and
Compressors

Specialties: Machinery diagnostics, design application of compressors, FEED,Detail Design,construction,commissioning and maintenance.

RE: 4-Run Method of Balancing

Having balanced many large fans it always surprises me that many engineers consider that it was all black magic when it really was just following the theory.  However, there are many pitfalls.  You do need an analyser but it is vital that you understand the vector method or things can go awry if the response is not exactly what the analyser expects.  You can rent analysers.  Although many out of balance problems can be corrected in the single plane the more difficult ones usually turn out to be two plane.  I recommend correcting the static or single plane first hence the need to understand what is happening - the analyser computer will try to do both in one step which can lead to big problems in tricky cases.  On large fans removing a couple (two plane) may require very large balance weights which must be exactly equal and diametrically opposite - several kg on each plane are not unusual.  Be careful -especially with weight attachment - make sure that you know what material the impeller is made from and use appropriate welding. In your case the my guess would be that you do have an issue with the support structure stiffness and are running close to resonance (which can make balancing very difficult with conventional methods).  Fixing the foundation could remove the problem long term.  There is another problem that occurs on a few large hot fans - expansion may not be symmetrical and the fan may have to be balanced hot for best results.  Check for other sources of vibration - impeller contact with the inlet cone or shaft seals when the fan is running at temperature.  Hope this is helpful.  In many cases you will need more than the theoretical minimum four runs.

RE: 4-Run Method of Balancing

As a PS to MachEng comments.  Be careful about removing all balance weight as this could result in dangerous out-of-balance when you try to run the fan.  Large balance weights are often applied by the manufacturer to correct fabrication asymmetry which is not unusual on big fans.  Therefore don't assume that the impeller with all weights removed will be close to being in balance.

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