What level G?? 2

[bklauba]

Folks, if a 14" diameter body, rotating at 1000 - 1500 RPM, weighing about 40 pounds is balanced down into the 0.1 g - cm. range, what grade of G is that?? (Is there one?)

BK

 

[electricpete]

My calculations show that would be somewhere around G0.06 if there were such a grade (I'm not sure if there is any grade below G0.4).

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[electricpete]

Correction - around G0.006 if such a grade exists.

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[electricpete]

Here is my calculation:

ISO G1.0 corresponds to unbalance roughly 6*W/N. i.e.

U/(inch-ounce) = 6 * (W/lbm) / (N/rpm)
= 6 * 40 / 1250
= 0.192
0.192 inch ounce would correspond to ISO G1.0

You have (0.1*gram*cm)* ounce/(29*gram) * inch/(2.54*cm) = 0.00136 inch-ounce.

0.00136 inch-ounce / 0.192 inch ounce =0.007 which means you are 0.007 times the limit for G1.0 which places you around G0.007

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[electricpete]

Such a low level makes me wonder, how did you determine that residual unbalance of 0.1 g - cm ?

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[GregLocock]

That's about the size and weight of a wheel and tyre assembly. Standard tyre weights go down to 5g increments, at say 20 cm rim diameter, so that is 100 g cm. You want a THOUSAND times better than that. Ten, easy. One hundred, OK.

On the other hand, that is about the sort of precision we measure driveshafts to, at 5000 rpm. But we'd never claim to hit 1 g cm residual unbalance repeatedly. more like 10.





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Greg Locock

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[bklauba]

To electricpete:

The spec was something we were handed; we were not involved in deciding if it is req'd. From what I can see, they appear to need it.

BK

 

[Tmoose]

http://www.irdbalancing.com/downloads/TechPaper1BalQualityReqmts.pdf

>>REALLY<< Attaining G1.0 is much more than a few calibration runs in a balance machine, and I think claiming those low numbers should be done only after some serious soul searching, and tolerance analysis. I often look at Table 1A on page 4 and compare the mechanical centering repeatablity of the balance arbors, parts at assembly, etc, etc, etc. Over 1000 rpm ISO G 2.5 simply does not grow on trees. I would submit that other ISO documents recognize what a problem low G balancing is by the suggested adoption "inspection tolerance" of +15% for G-16 - G2.5, +20% of ISO G1 and +30% for G 0.4

 

[bklauba]

Well, these folks (and their customers) have a tendency to be a tad spec happy. I really appreciate the feedback!!

Now they tell us they never have made one that meets these specs. Well golly, imagine that.

BK

 

[bklauba]

New info: The data that is being looked at, so as to determine whether this device is balanced, is not just the 1X data, but rather an acoustic spectrum, ranging from 5 - 300 Hz, with the amplitude parameter being mN, 1200 being FS. The RPM used for balance test is ~ 500.

The data seems pretty clean in the 1X area, but there is quite a bit of signal, with minimal pattern (no clear ___ X's) in the 80 - 160 Hz range.

Does it make sense to look at this broad a spectrum, in order to merely achieve balancing? Is it not normal that balancing looks exclusively or chiefly at the 1X data?

BK

 

[GregLocock]

Normally you just look at 1st order for balancing. The other data is telling you something, if it changes in response to your balancing process then you have looseness or some other non-linearity in the system.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bklauba]

Thanks again!! I thought that was the case, and of course it makes sense, but I needed some support on this point.

Regards to all, and I tip my hat again to eng-tips.

BK