I have seen drawings where they want a roll balanced using a "G" amplitude.
(Example: Balance roll to 1900 rpm @ G2.5)
Anyone know if there is a conversion for this in Mils?

I believe (but may be wrong) but the G you are talking about actually stands for grade. In other words if it calls for a balance g2.5 this means balance grade 2.5 as set forth by ANSI and ISO (ANSI standard S2.19-1975; ISO standard 1940)

I agree it is probably a reference to an ISO balance grade.

In an abstract mathematical sense, ISO G2.5 stands for velocity v = 2.5 mm/sec pk/0. (not a real measurable velocity)

The associated eccentricity or displacement (again abstract mathematical and not measurable) would be e = d = v /[2*pi*f] = 2.5 mm/sec / [2*pi*1900/(60sec)] = 0.0126 mm = 12.6 microns pk/0. Also could be expressed as 12.6 micron * (1 mil / 25 micron) =0.504 mils where mil is a thousandth of an inch. This is not a displacement that likely would be measured but it represents a peak/zero number.

To convert to allowable residual unbalance (inch ounce), use the relationship m*r = e*M = 0.000504" *M where M is mass of rotor, e is eccentricity from above, m*r is unbalance. If rotor were 1000 pounds you would have 0.504 inch-pounds allowable residual unbalance. To convert to inch-ounce, multiply by 16 inch-ounce / inch-pound and the result is a max allowable residual unbalance of 8 inch-ounce.

A shortcut to all the above:
allowable inch-ounce = 6 * G * W / N
where:
G is ISO grade
W is rotor weight in pounds
N is speed in RPM.
for the example above 6 * 2.5 * 1000 / 1900 ~8 inch-ounce

All of this seems like fairly easy math with no need to look at the standard. But some cautions:
1 - Don't misinterpret the above velocity or displacment as something you can measure directly. Residual unbalance in inch pounts is something you can calculate from your final measured vibration level using your calculated influence coefficients based on results of adding trial weights.
2 - I believe the standard provides some additional info regarding how to allocate that residual unbalance for various situations of 2-plane, overhung rotors etc.

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

Pete and Roy are exactly right.

Goinb from balance grade to a predicted vibration level is not so simple. We can calcuate the force generated by imbalance, but the vibration level resulting from this force depends on the machine design, mass and stiffness of the structure, bearing parameters and other variables.

Consider for example a machine on soft isolation mounts. A little force from imbalance will result in a high vibration level on the bearing housing because the entire machine is easy to move. On the other hand, vibration levels on a large heavy machine may be low for the same amount of imbalance.

So which machine will fail first? The unit with the highest vibration levels may seem like the obvious answer, but because it's flexibly mounted, there's not much force on the bearings. The smoother machine is absorbing all the force in the bearigs, so it may well fail first.

Jon
Spintelligent Labs

Grono,
They are right about it being a balance qrade. We use it in our balance stand for most of the electric motor rotors and fans we balance in our shop. Here is a web link that will explain some of the grades and the applications they are used for.
http://www.plant-maintenance.com/articles/balancingqualitylimits.pdf

Ronnie

The basic math for this:
Unbalance: U=M.e or U=m.r
where M= rotor mass, e=eccentricity of your cg to center of rotation. Assume no Amplification Factor, 2e will be equal to your displacement peak to peak. Unfortunately there is no system without AF. m=mass at radius r. m normally will be your residual unbalance at r radius.

If you have G2.5, you can convert to displacement to get e using the Grade chart (need speed info).

If you know your AF at measurement location, you can estimate the displacement reading at location. You can use the impact test or adding trial mass to measure AF.

The G (not g) is Grade as iterated to by everyone. The 2.5 grade may be listed in gram/mm or ounce/inches. It isn't just as simple as putting a number on it: say 3 mils (0.003"). I have balance tolerance charts and they are readily available and even Electric Pete has put up some links to old IRD papers that have those charts and full explanations I do believe. Search some old post and you should find it or "search google - IRD balancing standards, technology, papers, etc... Hope this helps.