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Balancing of Rotor during Electric Motors Overhauling
7

Balancing of Rotor during Electric Motors Overhauling

Balancing of Rotor during Electric Motors Overhauling

(OP)
Dear Experts;

I have a question regarding best practice of Motors overhauling. Do we require to balance the Motor rotor during every overhaul, when only bearings are replaced due to high vibration. Or is it only required when re-winding or some work is done on rotor?
The reason is that we are transforming our maintenance practices to best in class and i have heard that precision maintenance practices require that Motor rotors should be precise balanced everytime an overhaul is performed in workshop.
Also what is the criteria of installing sleeves in motor casing at bearing area when there is some clearance found? Do sleeves require even when a clearance of bearing outer race with motor casing is 0.01 or 0.02mm?
Need your expert opinion in this regard.

RE: Balancing of Rotor during Electric Motors Overhauling

It's a good engineering practice to re-balance the rotor to still lower tolerances during overhauls and rewinds.

Before mounting the bearings, fits on the shaft and housing must be checked and confirmed they meet the recommended tolerances.

Loose bearings on the housing/end brackets will result in creep, which will only worsen due to continuous slow machining of the housing/end bracket and will result in stator-rotor rub if left uncorrected and will damage both stator and rotor windings.

Muthu
www.edison.co.in

RE: Balancing of Rotor during Electric Motors Overhauling

(OP)
Thanks Edison123 for your reply.
From your post, I infer that every time a motor is brought to workshop for maintenance (Bearing replacement or rewinding) it should be balanced?
I have been reading other posts related to this matter and few of the experts suggested that, when only bearings replacement is required and we donot have any component of unbalance in vibrations, balancing is not normally required.

Also is there any tolerance values in bearing fits with housings, where we can skip machining and sleeving in housings?

Thanks and best regards,

RE: Balancing of Rotor during Electric Motors Overhauling

Are you a workshop separate from a plant that works on customer's motors? Or a workshop tied to a plant?

The standards may be slightly different. Repair shops may be held to a higher standard than customers hold themselves to.

For our workshop which is part of a single large industrial facility (no external customers), if we were doing bearing replacement, we would not necessarily balance the rotor. Especially if the endbells can be removed to replace the bearings without even pulling the motor, and all the fits check out fine, and we have no suggestion of severe unbalance (maybe vibration shows bearing defects but not high 1x) then we certainly wouldn't bother balancing the rotor before reassembly. If there is a mistake and we end up with high 1x during solo run after repair but before return to service, then we have an opportunity to go backwards and disassemble to do the balancing we should have (in this particular case) done the first time... but that's a pretty rare occurrence.

Some of our vertical motors can be a special case, if we expect they tend to vibrate during solo run without thrust load, we are more likely to balance those up front to avoid second guessing if/when the vibration is high during solo run.

We pay more attention to the fits than the balance. I'd never let an out of spec shaft fit go without sending it out for repair, and rarely let an out of spec housing go without repair. (I view the shaft fit as more critical than the housing). Also we have to send shaft out for repair, but we can fix the housings ourselves.

RE: Balancing of Rotor during Electric Motors Overhauling

(OP)
Thanks electricpete;
Our workshop is within plant and we do services of in-house motors repair for the plant.

Please confirm, if we have removed the rotor from the housing and done cleaning of windings and other important electrical checks and only replaced bearings, do we need to do rotor balancing in this case?

best regards,

RE: Balancing of Rotor during Electric Motors Overhauling

If motor or the machine it drives has special requirements related to the low level of vibrations, there is no reason to think about balance rotor or not. For example these are definitely motors from a grinder machines.
Perhaps power of the electric motor and its speed can be a good parameter.
We have a small workshop for rewinding motors up to 200kW, balance machine up to 1500 kg rotor weight and We usually check every rotor of motor over 50 kW and speed over 1400 revolutions or over 22kW motor with 3000 rpm on a balance machine .
The work of balancing does not require a lot of time and can help in the event that after assembly we have a some problem with vibrations.

RE: Balancing of Rotor during Electric Motors Overhauling

2
faisalakhan: You mentioned in your original post that the motor was removed from service for high vibration. Do you know the source of the vibration - by which I mean root cause, not necessarily the part where the damage showed up. If the vibration is ABSOLUTELY ATTRIBUTABLE (and ONLY attributable) to bearing failure, then MAYBE you can get away without a proper balance run on the rotor. If there is any reason to believe otherwise, than go ahead and perform the balance.

My background is both as an original equipment manufacturer (for machines ranging from 1 through 130 000 HP, operating at speeds anywhere from 0 - yes, continuous stall! - through 22 000 rpm) and as a repair facility not affiliated with a particular manufacturing site (i.e. our shop does work for everybody all over North America).

Bearing manufacturers and original equipment manufacturers have developed some fairly good go/no go tolerances with regard to journal (shaft) fits and housing fits for various bearing types and arrangements. Depending on where you are in the world, there may be some relevant "repair industry" standards you can leverage as well.

Converting energy to motion for more than half a century

RE: Balancing of Rotor during Electric Motors Overhauling

(OP)
Thanks all for your valuable comments and suggestions.
Very much Appreciated. Now I have understanding, which motors need balancing and which can be skipped and not affecting the reliability.

Best regards,

RE: Balancing of Rotor during Electric Motors Overhauling

Quote:

Thanks electricpete;
Our workshop is within plant and we do services of in-house motors repair for the plant.

Please confirm, if we have removed the rotor from the housing and done cleaning of windings and other important electrical checks and only replaced bearings, do we need to do rotor balancing in this case?

If the 1x vib was not high to begin with and it's an uncomplicated horizontal motor with bearings replaced due to symptoms of bearing defects, then my personal vote if the motor were in our shop would be don't balance it.

RE: Balancing of Rotor during Electric Motors Overhauling

From spending money, time and effort to strip down a motor/generator for an overhaul/rewind to re-balancing the rotor is not a big step. It's just plain common sense and good engineering practice.

Muthu
www.edison.co.in

RE: Balancing of Rotor during Electric Motors Overhauling

I was specifically asked my opinion (a second time) by the op so I gave my honest opinion. That is what I would recommend in our shop.

I'm not particularly interested in debating but since you have repeated your opinion without being asked, I gather you (edison) are attacking my opinion.

So I'll give some points to support my opinion:

  1. * I said the motor in question is horizontal motor without abnormal 1x. That means it didn't have a balance problem when it came into the shop. If it came in without a balance problem and without dirt on the rotor then it shouldn't have a balance problem when it leaves. If rotor is dirty and needs cleaning then maybe that's a different story which I should have mentioned.
  2. * An interesting excercize is to compute the load associated with unbalance. Let's say we have 3600rpm 100rpm motor with 6313 bearings whose rotor weight 400 pounds. ISO G1.0 balance spec would be 0.7 inch-ounce total. The centrifugal force from that unbalance at 3600rpm would be about 16 lbf. Let's say your unbalance is 20 times that ISO G1.0. That would be 320lbf. It's still less than the weight of the rotor. Let's add together weight of the rotor and the unbalance it would be 720 pounds totoal or 360 pounds per bearing. The 6313 rating C0 is 13,488 lbf. That loading created by the weight of the rotor plus unbalance at 20x higher than the ISO G1.0 limit is completely insignificant to the bearing life.
  3. * So why do we worry so much about vibration? Well, if you look at the ratio of force to vibration you'll find misalignment can be a lot higher than unbalance because the misalignment forces includes dc (static) forces which generate no vibration (those dc component of misalignment reaction forces are not detectable through vibration monitoring, but still contribute to beairng loading). The alternating components of misalignment forces which cause vibraton can be a lot lower than the dc components because they arise from secondary effects. If I have a machine with 0.2ips vibration from misalignment and a machine with 0.2ips vibration from unbalance the machine with vibration from misalignment is generating a lot more bearing forces.
  4. * We have learned to pay attention to vibration for reliability but viewing everything that causes vibration as equally bad is overly simplistic. In reality you'll get a lot more bang for your buck reducing vibration if it was originating from misalignment than if it was originating from unbalance.
  5. Unlike misalignment, unbalance has no dc component. It can't cause forces without vibration. Unbalance reveals itself in vibration, so it can't hide like misalignment. If there was not high vibration from unbalance when the machine came in then there was also not high forces from unbalance when the machine came in and there's no compelling reason to balance it.
I will say these things are not an exact science. There's room for judgement. Just because we've been told something, that doesn't mean it's true without exception. It certainly applies to me and my advice here, but I also believes it applies to the types of things often called good practices because they are handed down from old timers or because we've always done it that way.

There is a culture in motor shops to take pride in their work and that includes pledging the highest standards in things like unbalance. That makes the repair shop people look like they are thorough and conscientious, and no-one is ever going to fault them for that. And indeed if a repair shop tried to cut corners by skipping rotor balance I'm sure there are customers that would judge them quite harshly for that. So if I were in a repair shop supporting outside customers, I wouldn't even bother recommending to skip balance to a customer, we'd just do it every time (and pass on the cost of course!). But for an in plant shop those perception factors don't matter as much. We have more a focus on reliability and cost and if there is something we are doing that costs but does not add anything to reliability, we will look at reducing it. The incentives and goals are different for in plant shop than customer focused shop. Like I said in my very first post

Quote (electricpete, my first reply in this thread)

"The standards may be slightly different. Repair shops may be held to a higher standard than customers hold themselves to."

RE: Balancing of Rotor during Electric Motors Overhauling

Balancing of the motor rotor is recommended for every overhaul, regardless of the type of work carried out. At least check the vibration level according to ISO 10816 .

To speed up the measurement process, I advise you to make a simple balancing stand, which consists of a platform on spring vibration isolators and a simple measuring system.
Photos can be seen here : https://vibromera.eu/example/simple-but-effective-...

Here is one of my acquaintances for a couple of hours made a stand from improvised materials for balancing motor anchors ...

you can laugh at him, but it allows you to balance anchors with an accuracy of 0.1 mm per second or specific unbalance of 0.3 grams / millimeter.

RE: Balancing of Rotor during Electric Motors Overhauling

Quote:

you can laugh at him, but it allows you to balance anchors with an accuracy of 0.1 mm per second or specific unbalance of 0.3 grams / millimeter.

It's an interesting setup. Soft balance machine. If the mass supported on springs creates a resonant frequency far below balancing speed, then the spring force becomes negligible compared to mass acceleration forces. In that case we can write M*e = (m*r) where M is total mass (rotor mass PLUS the mass of other parts mounted on those springs like the wood, the drive motor etc), e is measured displacement of the moving mass, and (m*r) is the residual unbalance given as mass times radius (typically units of inch ounces).

0.1 mm/sec under some idealized assumptions means ISO G 0.1 (10 times lower residual unbalance than ISO G1.0).

I'm not familiar with units g/mm. But often residual unbalance limits are given in units of mass times radius, so maybe you meant 0.3 g*mm (?)
...converting that to more familiar units would be about 0.3g*mm * (ounce/30g) * (inch/25.4mm) ~ 0.0004 inch*ounce (!)
IF I am interpreting that correctly, that residual unbalance doesn't make any sense in terms of a goal worth pursuing nor a goal which is achievable on most industrial balancers, much less a homemade balancer....
  • ... as an example, let's assume the mass of the parts mounted on those springs is M=2kg. Then we calculate e = (m*r)/M = (0.3 g*mm) /(2,000g) = 0.00015mm = 0.15 micron. That is not a realistic dynamic distance to measure precisely as far as I know. (what kind of sensor is he using?)
  • note - for simplicity I included within M both the motor rotor mass and other masses mounted on top of the springs (sometimes called parasitic masses). Assuming the claimed e is to be associated with the motor rotor along, then splitting that 2kg into a separate motor mass and separate parasitic masses would make the distance to be measured even smaller
Maybe I have misunderstood something somewhere along the line (it wouldn't be the first time).

I don't know exactly what is meant by "anchor"... a winch motor rotor? How fast does it spin?

RE: Balancing of Rotor during Electric Motors Overhauling

Of course g*mm, thank you.
By motor anchor, I mean the rotor.

For the balancing process, a portable balancer Balanset-1A was used. You rightly noted that the correctly chosen rotation speed plays a significant role in such a setup. To ensure the sensors' sensitivity is adequate, a high rotation speed is required. Unfortunately, I don't have data on the rotation frequency in this specific example.


In 2010 our specialists modernized the stand for gyroscope rotor balancing. In the process of modernization the outdated measuring system of the stand was replaced by our device. As a result, it was possible to significantly reduce the residual unbalance of the balanced rotor and bring its level to 0.003 - 0.005 g*mm, which corresponded to the value of residual unbalanced mass equal to 0.2 - 0.3 mG. It should be noted that our equipment allows to achieve much higher characteristics of residual unbalance. However, in the process of implementation there were problems related to the fact that the Customer's existing technology for removal of residual unbalanced mass from the rotor did not allow to remove mass with a discreteness of less than 0.2 mG.

RE: Balancing of Rotor during Electric Motors Overhauling

Truly achieving balance quality better than G 6.3 is not just trim balancing the rotor until the balance machine's light go out, or the computerized print out reports a tiny residual unbalance.
All the arbors, shafting, and bearings must duplicate the centering of the "balanced" component when in service better than .001" Sometimes MUCH better.

Changing the centering by .0001" of a "perfectly" balance rotor would induce residual unbalance equal to G 1.0. for 3600 rpm service.
Commercial shafting mounted in pillow block bearings using set screws or eccentric locking collars collar are very likely to be off center over .002" .
If a pump rotor or fan is a slip fit on its shaft and is retained by set screws, tightening the screws will force the rotor off center by 1/2 the clearanace.
If I'm lucky the concentricity of the bearing seats and rotor location on the shaft might be as good as .001".

ISO 1940 has NOTES that recognizes that.
NOTE 4 There are limits for achievable residual specific unbalance eper depending on the set-up conditions in the
balancing machine, for instance: centering, bearings and drive.
NOTE 5 Small values of eper can only be achieved in practice if the accuracy of shaft journals (roundness, straightness,
etc.) is adequate. In some cases it may be necessary to balance the rotor in its own service bearings, using belt-, air- or
self-drive. In other cases balancing needs to be carried out with the rotor completely assembled in its own housing with
bearings and self-drive, under service condition and temperature.

API Standards that include balance requirements specify a multiple step sequence of measuring runout, qualifying fits, and more.
Because it is necessary.



https://rotorlab.tamu.edu/me459/Rotor%20Balancing/...

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