What is the best practice for Field balancing over hung rotors? 1

[MachEng]

What is the best practice for the field balancing of overhung rotors,ie, pillow block mounted, direct mounted on the motor shaft and direct mounted vertical orientation?

I have an analyser balancing program that uses 4 mearsurement points. It is capable of balancing multiplane. It does give a procedure for a 2 plus plane balance but does not say anything about an overhung rotor application.

One description of a field balance job is a 500 kW motor mounted vertically with a fan impeller (shrouded) centre intake centrifugal type that is 500 mm wide and 3 metres in diameter. After diagnosing that the problem is unbalance at the 1X's RS frequency what would the specific technique be for balancing this machine best practice?


DANIEL C. SMAISTRLA
dajusmai@senet.com.au

Equipment: Centrifugal Pumps and compressors; Steam and Gas Turbines; Reciprocation Engines and

Compressors

Specialties: Vibration & Piping Pulsation analysis, Root Cause Analysis and Condition Monitoring

 

[electricpete]

1 - Here is an article on balancing overhung rotors you can buy for $25:

http://svd.sagepub.com/cgi/content/abstract/37/3/179

2 - Here is a fairly detailed article on balancing overhung rotors from an Enteract conference that I downloaded for free. These articles used to be available before Entek was bought out by Rockwell. Now they're not available on the website. There's no copyright listed so I don't think there's any problem with me uploading it.

http://home.houston.rr.com/electricpete/Enteract98_balancingOverhung.pdf

3 - There have been several discussions on this subject at maintenanceforums.com . It seems like the most recommended approach is something like this:
A - First attempt single plane static balance. This is enough to significantly reduce vibration in most cases.
B - Continue static balance as long as there is indication that both bearing vibration is in-phase. If the vibration starts to look like couple unbalance (180-out on opposite bearings), then perform couple balance.

One important reason for the above sequence is you can't do an effective couple balance until the static components is removed because the static unbalance creates a "false" couple.


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[electricpete]

One more link with some free info on this subject:

http://www.update-intl.com/VibrationBook5h.htm

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[electricpete]

I'm not sure if the terminology "single-plane static balance was exactly correct." What I meant was to monitor the motor top an bottom bearing and use the vector average of these two as input to a single-plane balance solution.

Since it is such a thin disk-like rotor, you likely would not require any couple correction.

You might try asking the question at maintenanceforums.com because there are a plenty of professional vibration analysts and balancers there.

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[svanels]

Nice documentation Pete, thanks

 

[Tmoose]

A small point - "Continue static balance as long as there is indication that both bearing vibration is in-phase. "

The generic assignment of in-phase and out-of phase 1X bearing casing vibration as static and couple respectively is not too well founded, in my opinion. As Figure 5 in Pete's link (

http://www.update-intl.com/VibrationBook5h.htm)

shows, a basic single plane unbalance might produce 1X vibration with phases 180 degree opposite. This is true of subcritical balancing or a hard bearing balance machine. It is equally true of supercritical balancing on machines with resonances in range or mounted on isolators (same thing).

Even the ENTERACT paper mentions this on page 11. "Do not attempt to determine the type of unbalance (static, couple, dynamic, etc.) based on comparative amplitude and phase data for bearings A and B. I'd personally extend the statement to more conventionally proportioned machines. Response to static and couple corrections, maybe, but basic vibration data, nope.

 

[electricpete]

I agree we can't determine static and couple of the overhung fan by looking at the bearings.

Stick with the articles to be safe. My comments may have been misleading. I was trying to describe what I heard on maintenanceforums.com but I may have garbled it a little bit.

I'll try to find the link when I get a chance but their search feature is down right now.



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[Tmoose]

Probably should have added that as long as long as the 1X vibration continues to decrease at each bearing and the phases remain consistent I would keep pumping on the single plane correction. That is, if each bearing was considered as a single plane problem, and the solution was the same. keep going.

 

[WCFoiles]

If your program allows, use vibration data from both bearings with either your static or couple shot. You can do a 2-plane balance if you have use both a static and a couple. Using the actual vibration data takes care of the possibility of having opposite phase angles on an overhung rotor.

Also, if you want to 'single plane balane' for the solution. Do single plane balance calculations on each bearing vibration (if you can't use both pieces of data at the same time). If the balance calculations are close, the single plane balance will work fine. If the single plane balances differ in solution by 180 degrees, it will take a two plane calculation or possibly a couple shot.

Regards,

Bill

 

[Zackman]

Bill gave some good advice but an overhung rotor, operating within the first lateral mode should respond very well to a single plane weight placement. Given your brief discription of the machine, I would assume that this machine operates below the first lateral resonance (critical). Typically you balance at the impeller. Not only is this where the wear or fouling is occurring but you can get the most effect (i.e. F=mrw^2).

It's not a good idea to start your balancing career using a computer program. Instead, consider using graphical methods. You'll be able to "see" exactly what you’re doing. You can always use the computer program to check your calculations.

Typically, the vertical stiffness is much greater than the horizontal stiffness at the bearing on these types of machines, so readings taken on the bearing housing with seismic transducers will show greater vibration levels in the horizontal direction. If not, you may want to reconsider balancing and look for other problems.

Anyway, if you still suspect imbalance, use the highest reading at each bearing (it really doesn't matter which direction but be consistent) since you are looking for a response with the first calibration weight placement.