Motors meeting IEEE-841 standards have tighter vibration standards than normal. Is there an appropriate ISO G balancing standard level that one can balance the rotor at to ensure success (at least the 1x part)during final assembly and testing.

Regards,
Alan

IEEE841 limits vibration to 0.08 ips when mounted at least 25% away from resonance.

(I know you know that Alan but others might not).

Long answer: We have talked conversion of balance to vibration before and it has always been a dicey subject. ISO G1.0 corresponds to 1 mm/sec rms (0.06 ips pk/0) rotor movement in an idealized sense far above resonance. But very idealized and it's on the rotor and not the housing.

Short answer, I have no idea.

For a workshop balance, request "better than ISO 1940 G1.0", with the added proviso that "residual unbalance at each end of the rotor must be less than 50% of allowable G1.0" to guarantee that all remaining unbalance, although small, is not concentrated at one end. This is easily achievable by any competent workshop - if yours protests, go somewhere else. With this balance tolerance, you have plenty of capacity for absorbing coupling and keyway unbalance - not that this is an excuse not to get them right in the first place

The ISO 1940 GX.X tolerances are all remarkably slack when compared to machines categories and cuodl do with revision IMHO.

Forget trying to interpret the references to velocity contained in ISO1940 - they are a sort of explanation of how the values attached to the G number are derived. These cannot really have any direct relationship to vibration velocity measured on a machine casing when the rotor is built in to it - unless of course you want to get into a research program

quote:

ISO G1.0 corresponds to 1 mm/sec rms

rms? (Arne will not be happy when he will read this! )

We have four 22000 hp. refiner motors. The motor OEM demand that the vibration amplitude measured horizontally on each pedestal must not exceed 0.14 ips pk. at the operating speed. (1800 rpm) when motors are coupled with refiners and at full load. (0.09 ips pk. motor alone, not coupled)

We have no choice; in order to not exceed this limit, the motor rotors must be balanced according ISO 1940 G0.4

They are balanced as low as 0.05 mil pk. at 400 rpm! to give you an idea.

This message has been edited. Last edited by: Alec,

"balanced as low as 0.05 mil pk. at 400 rpm"
That sounds more like G0.05 on a soft bearing machine with small parasitic mass.

Simultaneously keeping one eye on the 0.05 MIL CG eccentricity and the other eye on 0.05 MIL tolerances on various machined parts, including the inner race concentricity would best be handled by an iguana or a chameleon. I can't do it.

Spending 30 minutes more on the balancing job to reach lower levels is nonsense?

These rotors are balanced on their journals on a IRD soft bearing machine with a IRD 290T analyser. I was in that machine shop in Montreal last year to check their entire work on one of our rotor.

Here are the numbers for the balancing.

Balancing grade : G0.4
Rotor weight : 18000 lbs.
Tolerance per plane : 360 gr-in.
Balancing speed : 403 rpm.

Original displacement :
Left plane : 3.53 mil (pk-pk) @ 264 deg.
Right plane : 1.97 mil (pk-pk) @ 66 deg.

Final unbalance :
Left : 0.155 mil (pk-pk) @ 133 deg.
Right : 0.091 mil (pk-pk) @ 335 deg.

Total weight added :
Left : 336 gr.
Right : 145 gr.

The balancing guy was a perfectionnist. He had said to me :"I like to be as close to perfect as I can be concerning the balance process , and I find when i float the rotor between the end stops i can achieve extremely low, repeatable readings, plus our customers are usually very happy with our results and consistently send their work to us....I suppose its importance would be a matter of opinion from balancer to balancer, but personally, I like to do the best possible work that I can, and this is as close to ideal balance condition as you can get."

Dear Alec, you must be aware that what I want is to handle apples as apples and so on. What I do not mind at all is that you experess the pre and post balance results as vectors in mils p-p and angels - that is a vibration measure taken in that balancing machine. Nice, but that is not what you buy from the balancing workshop, right? You have bought a residual balancing quality level, a balancing Grade. A totally different matter.

So what I miss, if I would have been present in that situation, is the complete pre and post unbalance information. You agreed on getting below 360 gr-in per plane. Fine. We can read traces of the info: What was added was 336/angle? and 145/angle and what was left on the rotor gr/angle both ends? And hence, what actual balancing Grade was achieved? That is what I would pay for.

Using a soft machine, and letting the end stops go away on both directions, means that the shaken rotor mass is the rotor plus the bearing roll cradles plus half the cardan if that is used. That is the mass to compare to the residual distance from zero to actual residual correction weights not inserted at last run. Knowing net rotor mass, we can deduce the final Grade.

To make a a good procedure fully valid, when using a cardan, I would also index the cardan 180 degrees and check the vectorial difference. That can be done at any time during the balancing process. Half of the vectorial difference is the error in the cardan. Indexing back can show how well the cardan mates to the rotor.

For a complete comment, if a "help shaft" is used to balance just a disk or similar, the indexing is crucially important. Else, you can spend money correcting the help shaft grinding in the disk... Sigh, it happens all the time. The balancer defending it by saying - Oh, don´t worry about that shaft, I test it once a year.
Regards Arne

what's a Cardan shaft?
Thx
Pete

Arne,

You're right! We buy a residual unbalance quality grade. The rotor was driven by a belt. I don't know about what angles the correction weights were added but the residual unbalance was +232 gr.in @ 17 deg. left plane and +114 gr.in @ 231 deg. right plane.

The permissible residual unbalance for the rotor weight only is supposed to be: 340 gr.in per plane. (360 gr.in = parasitic mass added)The rotor was then balanced at a grade of: 114+232=346 gr.in
(346 x 0.4)/680= G0.2
Is that correct?

pete, a cardan shaft is a drive shaft with what we call universal joints I believe

Excuse me. But is a big error to make confuse the G Grade of any standard with vibration level at machine work place. Balance procedure limits the rotor residual unbalance, the final dynamic force (the centrifugal force). The vibration level depends of machine assembly. Compare a good assembly with a ressonant one.

Ricardo Góz from Brazil

Roy - thanks.

Ricardo - Agreed. From my reading I believe the participants in this threads are aware of that point. But it is still a legitimate practical question to ask what balance level to use prior to assembly to give reasonable assurance of meeting specification vibration requirements after assembly on a non-resonant base (mounting requirements defined by NEMA).

Thanks for the input, sometimes it is comforting to see that their is no easy answer to something. Since this posting a little research has led me to believe that I should aim for G2.5 and if experience shows that this will not work, lower the level to G1.0. This means I may have trouble with smaller rotors, and very large ones. Trial and error!

Regards,
Alan

No - aim for G1.0 or better and if for whatever reason you cannot get there, then maybe relax your requirement.