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AGMA Limits

The American Gear Manufacturers Association (AGMA) has issued Standard 9000-C90, which establishes, in graph form, the maximum potential unbalance as a function of operating speed and coupling weight. The graph is divided into seven zones; once a zone is selected, the class of balance is determined from the Chart 1 given below, which includes the machine sensitivity to coupling unbalance. AGMA's guidelines are commented by the author:

Shaft Flexibility. Long and slender shafts are very sensitive, while short and rigid shafts (such as electric motors) have a low sensitivity to unbalance.
Bearing Loads. The smaller the ratio between the coupling weight and total rotor weight, the less sensitive the machine is to residual unbalance. For example, single stage centrifugal pumps have a rotor that is seldom heavier than the coupling; hence pumps are very sensitive to the coupling's residual unbalance.
Machine and Foundation Rigidity. Heavy machines with a rigid frame attached to a solid foundation are not very sensitive to residual unbalance.
Resonance. Systems are designed to operate at speeds that are removed from lateral vibration resonances. The close a machine operates to any resonant frequency, the more sensitive it is to residual unbalances.
Coupling Length. Large shaft separations make machines and couplings sensitive to residual unbalance.
Shaft Configuration. The longer the span between bearings, as compared with the overhung portion on which couplings are installed, the more sensitive a machine is to unbalance. Once the class of coupling unbalance is determined, the maximum displacement of the mass center to the rotation center is determined from the chart below. This "displacement" method is slightly tedious to calculate.

The principle involved can be understood using the single disk,
and the following relations can be written:
e . W = M . R
where e = eccentricity from its geometrical axis
W= Weight of the mass (in pounds)
M= weight added at Radius R
Disk's unbalance is
Us = M . R
Total Disk Weight is (W + M)
It can be assumed that the unbalance is generated by the total disk weight (rather than the weight M), rotating at an eccentricity "e" from its geometrical axis.
Therefore:
Us = M . R = (W + M) . e
From these formulae, the theoretical displacement can be calculated:
e = Us / (W + M)
The significance of AGMA standard is that, rather than simply establishing limits for unbalances, it also incorporates all possible factors that can create a potential unbalance. Among the factors analyzed are:

Pilot surface eccentricity
Pilot surface clearance
Hardware displacement
Hardware weight differences

AGMA Standard 9000-C90
classes of Coupling Balance


AGMA Coupling Balance Class	Maximum Displacement of Principal Inertia Axis at Balancing Planes (rms micro inches)
4	over 32,000
5	0.2500
6	16,000
7	8,000
8	4,000
9	2,000
10	1,000
11	500
12	250

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