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Author Topic:   balance levels ISO 1940
Borja
Junior Member
posted 03-12-2001 09:49 AM     Click Here to See the Profile for Borja   Click Here to Email Borja     Edit/Delete Message   Reply w/Quote
Hi everybody
My first question is related to balancing. I would like to know when we compare our vibration level in a electric motor measured in mm/seg with the balance quality grade G in the table of the ISO 1940 Standar, which format must we use to compare the vibration level?, I mean our measurement must be in peak to peak, peak o rms format? This is very important to us because, if the vibration due to unbalance is measured in peak to peak is 2,8 times greater than in rms. Could anyone help me?
thankyou

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electricpete
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posted 03-12-2001 02:23 PM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Borja:
Hi everybody
My first question is related to balancing. I would like to know when we compare our vibration level in a electric motor measured in mm/seg with the balance quality grade G in the table of the ISO 1940 Standar, which format must we use to compare the vibration level?, I mean our measurement must be in peak to peak, peak o rms format? This is very important to us because, if the vibration due to unbalance is measured in peak to peak is 2,8 times greater than in rms. Could anyone help me?
thankyou

I can't help you with ISO. I can tell you that NEMA MG-1 which governs most new motors in the USA specifies peak values for velocity measurements at the bearing housings and peak-to-peak values for displacements measured with prox probe.

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Greg Klein
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posted 03-12-2001 03:08 PM     Click Here to See the Profile for Greg Klein   Click Here to Email Greg Klein     Edit/Delete Message   Reply w/Quote
Electricpete,

Were can I get my hands on a copy of NEMA MG-1? If you don't mind, all I am looking for is the balance spec.s.

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Sasaki
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posted 03-12-2001 07:49 PM     Click Here to See the Profile for Sasaki   Click Here to Email Sasaki     Edit/Delete Message   Reply w/Quote
Borja san
The displacement by unbalance and vibration is not the same.
It is useless to discuss the difference of rms (severity) and P-P displacement, since it the criteria is a criteria.
Balance quality is not a prime factor of vibration, but only one of many.
Regards,

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Borja
Junior Member
posted 03-13-2001 03:40 AM     Click Here to See the Profile for Borja   Click Here to Email Borja     Edit/Delete Message   Reply w/Quote
Electricpete,
I am in the same case as Greg, where can I get a copy of the NEMA MG-1? This NEMA MG-1 is your standard when you balance motors, there in the USA?
An the last question, when you measure the unbalance of any machine, which values do you use to compare, I hope you use a standar...

Sasaki San
It,s clear enougth the diference between rms (severity) and P-P displacement, but if I measure for example the value of vibration at 49 Hz in a motor (it´s turning frecuency) this value must be in pp, p o rms? I hope you get what I mean..
Regards

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StanPolonski
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posted 03-13-2001 11:22 AM     Click Here to See the Profile for StanPolonski   Click Here to Email StanPolonski     Edit/Delete Message   Reply w/Quote
Balance and vibration are related only in how the rotating element and the installation it is in responds to the unbalance. For field balancing, when excessive vibration is attributed to unbalance, we would balance until we achieved less than 1 mil vibration, peak to peak, or .15 IPS, peak.

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jclough
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posted 03-13-2001 02:31 PM     Click Here to See the Profile for jclough   Click Here to Email jclough     Edit/Delete Message   Reply w/Quote
You can get the standards at www.nema.org. I'm sure they will refer you to Global out in Colorado.

The NEMA standard wg-1 part 7 (at least in my copy )states that velocity is expressed as in/sec pk.,displacement as inches p-p and acceleration as g's pk.

While the NEMA spec calls out vibration limits for assembled motors, keep in mind that unbalace is just one factor contributing to vibration. We still use ANSI S2.19-1989(based on the ISO) as the standard for balancing.

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Ricardo Goz
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posted 03-14-2001 05:49 AM     Click Here to See the Profile for Ricardo Goz   Click Here to Email Ricardo Goz     Edit/Delete Message   Reply w/Quote
Borja
Balance grade of ISO 1940 or other standard refer to correction of error in rotor mass distribuition. The attention is reduce the centrifugal forces generate by bad mass distribuition. With better balance, the rotor will generate smaller forces on bearings and lower vibration level.
But the vibration level will depend of complete machine mass (include the motor), of stiffness and damping of the mounting. Assemble ressonance, for example, can produce high vibration on well balanced motor.

Ricardo Goz, from Brazil

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electricpete
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posted 03-14-2001 02:35 PM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by jclough:

The NEMA standard wg-1 part 7 (at least in my copy )states that velocity is expressed as in/sec pk.,displacement as inches p-p and acceleration as g's pk.


I'm trying to understand the ). Isn't that the same as what I said? (except my copy says MG-1 on the front ) !)
electricpete

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Arne Lindholm
Junior Member
posted 03-18-2001 06:14 AM     Click Here to See the Profile for Arne Lindholm   Click Here to Email Arne Lindholm     Edit/Delete Message   Reply w/Quote
The secret of ISO Balancing Standard for Stiff Rotors ISO 1940/1 is not that difficult.

[Rotor mass in Kg] times [residual excentricity in microns] is equal to [residual unbalance in grams] times [the radius you leave on in mm].

It can be written G x e = B x r

The e is specified residual unbalance in the ISO 1940/1 to for instance maximum 8 microns at 3000 RPM or 16 microns at 1500 RPM for a Quality level of Q2.5 (applicable for most normal good machinery in a proactive approach of maintenance).

So whatever G and B and r you have, B x r divided by G must be below 8 microns.

Units are all in the SI system. Plain Kg, micron, gram and millimeters.

The simplest way to check is to insert a mass at a known radius and see what vector change in mm/s rms you made. Then you have "calibrated" your polar graph. Now look at the smallest vector you want to leave the rotor with. How large is that in comparison with the above change vector. There is what "e" you left on the rotor. This is for one plane.

Since first posting yesterday I have got several requests to explain the "calibrating procedure" so I will take numbers form a real case: A drive motor at a paper machine with coupling half mounted had a key sticking out (It was too long). So we tried to balance using a 100g mass at radius 60mm across. The horis. radial bearing reading was before 5.1 mm/s rms in angle 40 degrees at 1xRPM at 2800 RPM (motor at thick rubber feet on the workshop floor). With the 100g, vector was 0.2 mm/s rms in angle 65 degrees. From plotting these two readings in a polar plot I stated that it takes 100 g to move the vector approx. 5 mm/s rms on the plot (from tip of 5.1 to tip of 0.2). Hence, when I have still 0.2 mm/s rms, I can also say that I have left approx. 4 g on the rotor. Since the rotor mass was 120, this corresponds to a residual excentricity of e = 4 x 60 / 120 or approx. 2 microns.

Careful 1: This is only for one plane. Mail me if you want a text for two planes.
Careful 2: Use all radial hor and vert vectors on both bearings to check the quality.
Careful 3: This case was acceptable as a one plane case since the motor had almost
zero vibration when the coupling was not mounted and the keyway was filled with a half key according to ISO 8821. I would recommend to cut a key sticking out, but others decide sometimes. :-)
Careful 4: Also the US has accepted the ISO 1940/1. If you export, it may be an advantage to use it. Other US Standards are very near in their judgement and advice.
Careful 5: If you like oz, in and lb, the above works just as fine. The rotor does not know in which environment he has ended up in.
Best regards Arne (to mail me, remove /spam in mail address)

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Richard Burton
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posted 03-23-2001 04:22 PM     Click Here to See the Profile for Richard Burton   Click Here to Email Richard Burton     Edit/Delete Message   Reply w/Quote
Borja,

The answer to your original question is:

ISO Standard 2372
...vibration velocity has been selected as the significant parameter for characterizing the severity of machine vibration
...the RMS value of the oscillating velocity is used to measure vibration severity

Richard

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Ray Beebe
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posted 03-27-2001 11:35 PM     Click Here to See the Profile for Ray Beebe   Click Here to Email Ray Beebe     Edit/Delete Message   Reply w/Quote
Borja

The replies do not give the answer I think you are seeking.

The ISO1940 balance grades refer to quality of balance achieved in a balancing machine, on the assumption that satisfactory vibration due to unbalance will result when a rotor is replaced in its normal operating machine.

I do not believe that the numbers from the G grades relate directly to vibration in the normal machine at all. As others have said, other factors influence the vibration level. The stiffness of the machine's structure will not be the same as that of the balance machine (except in the large machines such as steam turbines where great care is taken to match up the stiffnesses).

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Ricardo Goz
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posted 04-03-2001 01:31 PM     Click Here to See the Profile for Ricardo Goz   Click Here to Email Ricardo Goz     Edit/Delete Message   Reply w/Quote
Balancing Standards and Balancing Procedures

Borja stand up a problem and is clear that the persons in this forum dont understanding or are unfamiliar to the Rotor Balance fundaments.

Regardless the standard, ISO, NEMA, JIS persons unknow that balancing is only a correction of mass distribuition of rotor for minimize the rotational forces on shafts, bearings and structures. Vibrations levels will depend too of suspension and assembly (base, coupling etc) and too of others forces in machine.

Other aspect is the non knowledge on measure of vibration. Persons dont know distingue or correlate peak, peak to peak and RMS.

Ricardo Goz from Brazil

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R Brook
Member
posted 04-05-2001 01:20 PM     Click Here to See the Profile for R Brook   Click Here to Email R Brook     Edit/Delete Message   Reply w/Quote
Borja,
First - Are you balancing rotors in a balance machine? If so, then you need to use the ISO1940 spec you referred to. The levels on that chart are use ONLY WITH A BALANCE MACHINE!
Second - Are you verifying that a rotor that was balanced and is now residing in a completely assembled machine meets a good vibration level?
Then Richard Burton's answer is correct.
It is obvious from many of the replies that the misconceptions over what a residual balance tolerance and resulting vibration level of a completed machine are still rampant. First, please clarify which case you are working with.
You are getting answers from both sides of the fence here that will do nothing more than confuse the issue.

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Arne Lindholm
Junior Member
posted 04-05-2001 01:51 PM     Click Here to See the Profile for Arne Lindholm   Click Here to Email Arne Lindholm     Edit/Delete Message   Reply w/Quote
Dear Ricardo, of course the balancing is to get the centre of gravity at the rotation axis. But when you measure the vectorial change as I describe to check the residual unbalance you can use any vibration unit in p, pp or rms to your liking, since the vector length for a known test mass is still the same unit as the residual unbalance, grams or oz or what have you. My test is surely not valid for nontypical testing situations where alignment or hydraulic unbalance at 1xRPM plays a major role. But the major reason for a remaining vibration vector at 1xRPM is unbalance. If, for a specific case, it is maybe NOT, then the test will also reveal that. I can describe that to anyone interested.

Dear R Brook, do I understand you correctly that you are suggesting that the fulfillment of an agreed ISO 1940/1 Quality Grade cannot be tested "in situ"? If so, it would be a great help if you could take the time to specify what is wrong in doing it. Any customer to a balancing job must be allowed to use available tools to check what has been delivered. In the balancing machine or in the finished machine. Checking it "in situ" the way I describe it above has been used since at least in the 50-ies in Europe and is very established in several branches. I am curious to learn what´s wrong with it and to learn the pitfalls. Regards Arne

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Arne Lindholm
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posted 04-12-2001 08:14 PM     Click Here to See the Profile for Arne Lindholm   Click Here to Email Arne Lindholm     Edit/Delete Message   Reply w/Quote
Dear Ken, with due respect, what you state above is wrong. You are comparing apples and banans and claim they are the same thing. As you have noticed yourselves, the reading of residual unbalance taken from the balancing machine and the real world vibration does not always compare. If it ever compares it is a masterpiece of arbitrary conditions.

Please note that the residual unbalance is a measure of offset of the centre of gravity. The Quality factor given for a balancing quality is only that offset in microns [e] multiplied with the speed in radians. Sofar you are correct that we can consider it to be a velocity, but a "virtual" one indeed.

However, since a given machine can be very rugged - take the extreme - a motorspindle in a milling machine - to the very flimsy and soft, maybe even often resonant case of a ventilation fan, you must realize that a given unbalance of the rotor will cause all from very small velocity measured in mm/s rms (maybe 0.04) up to a rather impressive number (maybe 9).

You can not, I repeat not, state that balancing quality Grade according to ISO 1940/1 is the same as the reading of 1xRPM in mm/s RMS. It is a misunderstanding.

I offer that you erase your posting, and I will erase mine, to avoid too much confusion.
Happy Easter and best regards Arne

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Ken Squires
Junior Member
posted 04-22-2001 12:51 PM     Click Here to See the Profile for Ken Squires   Click Here to Email Ken Squires     Edit/Delete Message   Reply w/Quote
In an effort to be brief and simple I may have caused some confusion to readers of my previous post, so I have deleted it and am trying to restate with a little more clarity on this subject.

The balance grade referred to in the ISO 1940 standard is equivalent to mm/sec RMS. In other words ISO balance quality grade G-1.0 is equal to 1 mm/sec RMS, grade G-2.5 is equal to 2.5 mm/sec RMS. These values are filtered to indicate 1x vibration only. Note also that this is the total value applied to the rotor. (For symmetrical rotors such as electric motors 1/2 of this value would be applied to each bearing plane)

This vibration value also only applies to that portion of 1x vibration relating to unbalance not the sum of the 1x vibration. There are other vibration problems that can cause an increase in 1x vibration which are not related to unbalance. The introduction of the ISO Standard states “…there is often no easily recognizable relation between the rotor unbalance and the machine vibrations under operating conditions.”

My experience with portable vibration equipment and hard bearing balance machines is that seldom do the two compare exactly because of the way the vibration signals are collected. Even between balancing machines the ISO Standard has recommended guidelines for an allocation of a percentage of error (totaling as much as 45% for G1.0) when performing balance evaluations.

As with any operation involving balancing (or balance evaluation), one has to be aware of the various sources of unbalance error. Many of these sources of error are identified in the standard. When evaluating a rotor, which was balanced in a balance machine while not in it’s service bearings, the bearing clearance should also be considered.

Now to get back to the original question as to how this relates to electric motors. If the purpose of evaluation is to determine balance quality of an electric motor using some form of velocity measurement then, in my opinion, the velocity measurements provided in the standard may be used as a guideline provided the motor is isolated from any other machinery. If the stated quality grade were G2.5 then I would expect to see less than 2.5 mm/sec. If the reading were less it would not necessarily mean the balance grade was below G2.5 but it would not be cause for alarm. A reading above 2.5 mm/sec would cause some serious question as to the balance quality grade being G 2.5. A more thorough investigation using some form of balancing method would then provide a more accurate representation of the balance.

The vibration readings collected would, in my opinion, be less than the quality grade and be in proportion to the quality grade. The collected readings would be dependent on equipment type, resolution used (collection specification), method and position of mounting, integration, number of interfaces between shaft and sensor, and the mass of the object (mass dampening of vibrations), bearing clearance, etc. Most of which would degrade the signal strength.

All balancing machines are calibrated with their appropriate sensors and without any type of calibration on a velocity measurement then errors must be expected but as stated above for an evaluation only there is a proportional relationship between the balance quality grade and the vibration so measured.

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Ricardo Goz
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posted 05-04-2001 08:42 AM     Click Here to See the Profile for Ricardo Goz   Click Here to Email Ricardo Goz     Edit/Delete Message   Reply w/Quote
Impressing, schoking

The Quality Grade G of the ISO 1940 dont implies in final vibration level.

How Ken Squires with his long experience agree with this? What is the correlation between field balance with the G of the ISO 1940?

The balance procedures applied in the rotor only limits the centrifugal forces that rotor will apply on bearings. The residual balance recommended by ISO 1940 only considere the rotor.

The final machine vibration level will depende of the other parameters, as total mass, mass moment of inertia, support stiffnes, damping, natural frequency ...

Several persons dont know interpret the units used in standard text and then are confuses.

Ricardo Góz from Brazil

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Arne.Lindholm
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posted 05-09-2001 07:25 PM     Click Here to See the Profile for Arne.Lindholm   Click Here to Email Arne.Lindholm     Edit/Delete Message   Reply w/Quote
Dear Ken, please do not take offense, but you are still writing something that is simply incorrect:

I quote your posting:
The balance grade referred to in the ISO 1940 standard is equivalent to mm/sec RMS.

IT IS CERTAINLY NOT. Even the Standard itself clearly says so. We should not allow this to spread to possibley untrained persons since it is not correct what you say.

Sorry for the unpolite approach, kindly remove or correct your statements. Best regards Arne

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Richard Burton
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posted 05-14-2001 05:03 PM     Click Here to See the Profile for Richard Burton   Click Here to Email Richard Burton     Edit/Delete Message   Reply w/Quote
Arne,

You sparked my curiosity, so I went back to my Level II manual written by Mr. Berry. He says: "When balancing a machine, you first refer to the tabulated information in (ISO 1940) Table 6.01A to determine what tolerance you should balance it.....Note that the quality grade number itself represents the maximum permissable circular velocity of the rotor center of gravity expressed in millimeters per second (mm/sec). For example, a quality grade G-6.3 corresponds to rotor velocity of 6.3 mm/sec RMS which corresponds to an equivalent .248 in/sec RMS (.351 in/sec peak)."

Maybe I'm reading this wrong(?), but it sure looks like he is saying that the tolerance table 6.01A is in velocity units. Please let me know if I'm reading it wrong.

Richard

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Arne.Lindholm
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posted 05-14-2001 08:22 PM     Click Here to See the Profile for Arne.Lindholm   Click Here to Email Arne.Lindholm     Edit/Delete Message   Reply w/Quote
Dear Richard, the velocity that you refer to is a theoretical construction based on the way the standard was designed. Given the rotor in a situation where it is not subject to ANY bearing forces, ANY torque, ANY other radial forces at all, then the velocity is identical to the reading you will get on a "virtual" bearing position. The moment you support the rotor in space, you get a much larger mass moved by the residual unbalance and the relevance of that number disappears.

Take a simple example, just one plane. Rotor mass 100 Lb, foundation on soft rubber is also 100 Lb. This rotor in a balancing machine with very very tiny bearings (so you can neglect them) is maybe balanced to Q6.3, which corresponds to 6.3 mm/s peak (not RMS, we cannot measure grams in RMS, it is always a peak, or "real" value) When you place that rotor in the real machine, the level is reduced to 3.15 mm/s peak, because the e remains the same but the mass is twice as large. (If you got RMS from the Berry document, it must be a typing error.)

Behind this is the expression:
Rotor mass in Kg times excentricity in microns is equal to residual unbalance in gram times its radius in millimeters.

We can take a further example: Check: 30 Hz - 1800 RPM, 6.3 mm/s peak is 33.44 microns (peak). If you prefer to measure the velocity in RMS, the reading is 47.28 microns (peak).
If the shaken mass is twice as large the readings are cut in half. 17 and 24 microns respectively.

Do you get it? Best regards Arne

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Richard Burton
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posted 05-15-2001 12:49 PM     Click Here to See the Profile for Richard Burton   Click Here to Email Richard Burton     Edit/Delete Message   Reply w/Quote
Arne,

I respect both Mr. Berry's and your opinion alot, so I pulled out our copy of ISO 1940 to read the source material for myself. You are correct about the RMS vs. Peak. I understand now about using Peak value for Table 1, since it is e(permissable) x radians per second. That makes sense.

However, I saw nothing in the standard to support only applying it for balance machines. Quite the contrary, it mentions both manufacturer and customer acceptance testing, and actually allows higher levels vibration for the customer(section 9). It even goes further, and recommends in section 6.2 to perform balancing of machines that require a G-0.4 level balance in their own bearings and housing, under service conditions and temperature, and also recommends using its own drive.

I can see where other 1 X RPM forces and system mass can affect the measured value, but using the same argument, the mass, stiffness and other possible problems of the balancing machine will affect the measured unbalance of the rotor (section 9.2).
I guess that, in my opinion, there is nothing in ISO 1940 that supports the claim that it can only be applied to balancing machines.

Once again, thanks for the units tip. I've been using the wrong conversion for years!

Richard

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Arne.Lindholm
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posted 05-15-2001 01:17 PM     Click Here to See the Profile for Arne.Lindholm   Click Here to Email Arne.Lindholm     Edit/Delete Message   Reply w/Quote
Hello Richard, respect is good, but knowing the facts is better. Never get impressed by somebody who pretends he or she "knows". What I am saying is that one should always apply a lot of source critical view to all statements. I might have said something real bad, and you should try to find it. In that process you really learn how it is.

You are absolutely correct that the standard does not specify were you "must" test the residual unbalance. For many years, there has been agreements on residual unbalance in the real machine and suppliers and owners have verified and adjusted it in the real machine. We try to cover a fantastic span of different types of machinery in a few sentences. That general grip is nice, but should be used with caution.

In summary, maybe, I would like to respond again to Borja and state he should use ISO 10816 and look at broad band vibration. If he needs to verify ISO 1940/1 balance quality, he should make a "fake" balancing procedure and see how much he decides to leave from any of the 1xRPM vector tip to the origin expressed in microns. I believe that for 95 percent of the cases, the system is linear enough to give a good estimation of the residual unbalance.
My posting to Bernard today can also give some aspects on this. Regards Arne

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Borja
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posted 05-24-2001 08:29 AM     Click Here to See the Profile for Borja   Click Here to Email Borja     Edit/Delete Message   Reply w/Quote
Hi everybody again!
I have seen a lot of postings because of my first question.
Yes, I know it has not an easy answer. The reason of my question was beacuse I work with machine-tools, and when there are vibrations I have to make a diagnosys.
For example: a grinding machine that doesn´t make a good finish surface. Normally this is caused by vibrations, so we have to chek all the turning elements: motor, spindle, grinding wheel..So we colect this vibrations and we analyze them using FFT with an OR25. In the spectrums of those vibrations, there are peaks (of course!) but the question always is: the amplitude of them is too large? The peaks are caused by unbalance or misaligment or others: bearings, resonance or chatter...But focusing in the peaks at 1xrpm due to unbalance, a field analyst has to say if that peak at 1xrpm is OK or NOT, the only standard talking about 1xrpm peaks is the ISO1940. So when I compare the peaks in a spectrum velocity-frecuency with the values of the standard I can say if the unbalance of the wheel is enough or not.Really a don´t care about the balancing of the motor in a balancing machine, what I really care is the vibrations generated by the motor (or other element)in the tool machine where they are mounted. What I am not very sure is in the fact that which units I have to use to check the vibration levels at 1xrpm. I should use peak or peak to peak values in the spectrum? to make the comparison with the ISO standar.
When you measure the 1xrpm vibrations in a machine (any kind)how can you say if they are too big or not? For me the only answer is using the ISO1940. The only question is if the spectrum has to be in velocity peak other peak to peak?
Thanks every body for your atention and answers.

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Herb
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posted 05-24-2001 10:10 AM     Click Here to See the Profile for Herb   Click Here to Email Herb     Edit/Delete Message   Reply w/Quote
I work with machine tools and they are often designed with structural resonant and shaft critical speed near to 1X rpm. This is not done intentionally but it often is the case. A specta with a 1x spike can have several sources. With machine tools the difference between good and bad can be very little.

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Ricardo Goz
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posted 05-25-2001 07:30 AM     Click Here to See the Profile for Ricardo Goz   Click Here to Email Ricardo Goz     Edit/Delete Message   Reply w/Quote
Borja
If you want to use a ISO standard for vibration level, the ISO 2372 is the correct. The ISO 1940/1 is only for the Balance procedure where G = 3.4 for "machine tools parts" and G = 1.6 for "machine tools drives" . But attention, ISO 1940/1 is for balancing the rotors.

For vibrations levels, the ISO 2372 classify machine tools as class I or class II, for vibrations measured in velocity mm/s RMS.

I recommend you to use your experience for establish your limits refered to the product quality. The no permissible vibration level is that compromise your quality.

In your spectra, peak = RMS x sqrt(2).

Ricardo Goz from Brazil

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