Tips and tricks

Balancing of assemblies


Is it necessary to balance the individual components of an assembled rotor? If so, what tolerances should be applied?

Naturally, the individual components of an assembly can all be balanced separately. However, the residual unbalances of the individual components will add up vectorially in the assembly. The angular positions of the residual unbalances of the individual components may be completely random and their respective magnitudes may add up completely in the worst case. In addition, there may be unbalances resulting from the fit (radial and lateral runout).

If the required balance quality of the assembly cannot be reached by balancing the components individually, the assembly, or at least its major subassemblies, should be balanced as a whole. After balancing, the assembly should not be dismantled anymore.

If dismantling is unavoidable, the position of the individual components relative to each other should be noted very carefully, and the components should be re-assembled in exactly the same position. In addition, you need to check for errors resulting from play. Even a fast-running electric motor mounted on anti-friction bearings constitutes an assembly in the sense discussed here.


Example 1

Let's consider the example of an electrical armature operating at a speed of n = 15,000 rpm. This armature is to be balanced to quality grade G 2.5. The permissible eccentricity error of the anti-friction bearing (inner race) is assumed to be 3 µm. Is it necessary to balance the armature with its service bearings?


Solution 1

The permissible eccentricity of the center of gravity resulting from n and G is eper = 1.6 µm. As the eccentricity of the anti-friction bearings is greater than the permissible eccentricity of the center of gravity, the answer is quite clear: You need to balance the armature with its anti-friction bearings.

Normally, the eccentricity of the center of gravity permitted for the complete assembly is also applied to the individual components. If there is a large difference in weight between the weights of the individual components, a different distribution may be preferable. If the armature used in the example above is fitted with a light-weight pulley, it may be advisable to balance the larger mass, i.e. the armature, more accurately, leaving a standard balance quality for the lighter part, i.e. the pulley (which may need to be changed more frequently) which can be achieved easily on a balancing adaptor.

Example 2

Let's assume that the mass of the armature is m1 = 39 in., that of the pulley m2 = 2.2 lbs, the eccentricity due to the fit is epa = 10 µm. A taper connection allows to play. The unbalance of the pulley is fully attributable to one balancing plane of the armature, as the pulley is overhung. Armature and balancing plane are virtually symmetrical. How should you distribute the permissible unbalance of the assembly, to allow the pulley to be balanced as individual part?

Solution 2

The permissible unbalance per balancing plane is:

2. The pulley can be balanced as an individual part (by index balancing) to approx. 5 µm. Add to this the eccentricity due to fit of 10 µm. In the worst case, both values will add up, so that a total eccentricity of the pulley eri = 15 µm has to be expected.

The unbalance of the pulley is therefore max. .043 inches.

3. To compensate the effect of the pulley, the a correspondingly higher balance quality has to be achieved for the armature. It may make sense to permit the full value of .161 inch in the second plane.

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