Runout tolerance and balancing 2

[CTengIS]

I heard/read in some places that controlling runout helps to prevent static unbalance, but not dynamic unbalance. Is it true? And if so, could anyone tell why it isn't useful for dynamic?

 

[greenimi]

And if so, could anyone tell why it isn't useful for dynamic?

Because does not take in consideration the homogeneity of the material.

 

[drawoh]

CTengIS,

If something is statically balanced, but dynamically unbalanced, it usually has an asymmetric nominal shape. I dont see how run-out helps.

If balancing matters, I would specify balancing, and testing to verify balancing. I would not worry about run-out.

--
JHG

 

[geesaman.d]

It is definitely useful for limiting dynamic (2-plane) imbalance. It is not a complete solution for limiting 2-plane imbalance.

While the magnitude of runout imbalance control might be 'better' for a single-plane imbalance case, I think it should work for both cases. The only thing that comes to mind is if the 2-plane imbalance cases being compared also require a higher degree of balance quality.

 

[Eric Gushiken]

I've heard that the forces of a rotating part increase with the square of the RPM. That could have something to do with it.

 

[NotecA]

Static imbalance of a rotor occurs without its rotation, in a stationary state. In this case, the rotor, under the action of gravity, rotates so that its "heavy point" is at the bottom. An example of a rotor with static imbalance can be seen in the figure:

Static imbalance of a rotor

Dynamic imbalance, unlike static, manifests only during the rotation of the rotor. An example of dynamic imbalance of a rotor is presented in the next figure:

Dynamic imbalance of a rotor


Let's simplify the explanation. In the case of static imbalance, if you imagine the heavy point on the top part of the rotor, a slight turn will cause it to move down under the action of gravity. To balance the rotor, it is enough to add weight on top until the rotor stops turning with the heavy point downwards.

Now, imagine that on one side of the rotor there is a heavy point at the top and on the other side, a heavy point at the bottom, both equal in mass. If you turn the rotor 90 degrees, these heavy points will balance each other, and the rotor will not tend to turn so that the heavy point is at the bottom because there are two of them and they compensate each other.

However, once you start spinning the rotor, the centrifugal force will act on both heavy points, pulling the rotor in opposite directions, resulting in counterphase vibration.

Source: Introduction to rotor balancing

Runout control can be effective for small and uniform rotors such as grinding disks or narrow pulleys. This is because their shape and structure make it easy to detect static imbalance and correct it without delving into the complexities associated with their rotation.
However, when it comes to long and especially inhomogeneous rotors, runout control is no longer as effective. The reason is that such objects require runout to be analyzed along their entire length and from different sides, which makes the task much more difficult. Long rotors can have different types of imbalances at different points in their structure, and one-sided runout control cannot fully detect and correct them.
In addition, if the rotor is made of material with non-uniform wall thickness, this adds an extra layer of complexity. Different wall thicknesses can lead to uneven mass distribution, which in turn causes dynamic imbalance. Runout control in this case will not provide a complete understanding of the problem, as it is unable to account for the internal mass distribution and its effect on rotor vibration during rotation.

 

[CTengIS]

Thank you all.
My thinking about this has been similar to what was said by geesaman.d:
"It is definitely useful for limiting dynamic (2-plane) imbalance. It is not a complete solution for limiting 2-plane imbalance."
Assuming the imbalance can be detected from inspection of geometric irregularities, I think a runout control could be the way to go, not only for static but also for the dynamic type of imbalance as described above by Vibromera. I acknowledge there might be other factors unrelated to runout like homogeneity of the material, as was mentioned. However, a runout check would still useful, equally for static and dynamic imbalance. Any other opinions?

 

[3DDave]

The former practices of symmetry and concentricity did this better, but those were deemed unnecessary as it was claimed position could do all the same things, which it cannot.

 

[CTengIS]

Hi 3DDave, I think you brought up a good point. Are you saying that ASME Y14.5 concentricity when it was available, did this better because it was not affected by roundness, so it was more dedicated to the exact type of variation that could affect balance and less prone to rejecting potentially functional parts? And how was symmetry useful for this purpose?

 

[Belanger]

it was claimed position could do all the same things

I don't think that's true. ASME knew perfectly well that concentricity and position had differences, but they felt that the confusion wreaked by concentricity and symmetry outweighed the benefits of those two symbols (in determining that they should be removed).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Burunduk]

John-Paul, the argument that the reason for removal was that the committee members decided that position could replace it, is not to be taken too seriously. He probably heard it from some recently popular youtuber that he likes.

 

[3DDave]

They claimed "confusion" as if no one is any longer confused about essential aspects of the standard.

If there was actual confusion then the committee should have spent a little effort in clarifying that instead of coming up with yet another term to group separate surfaces as a singular datum feature.