How do automatic ball balancers work, and are there other passive shaft balancing mechanisms?

[Jez]

I've recently seen 'ABB' mentioned in hand tools and washing machines, being automatic ball balancers apparently consisting of ball bearings in a race track with some damping fluid.

Do these only 'damp' the imbalance? I guess as the shaft experiences cyclic angular accelerations and decelerations then it'd damp those, is that it? So not really balancing the shaft masses by shifting CoG?

Are there any passive shaft balancers that can shift the CoG to balance rotary motion of unbalanced shaft masses?

[exchemist]

2 hours ago, Jez said:

I've recently seen 'ABB' mentioned in hand tools and washing machines, being automatic ball balancers apparently consisting of ball bearings in a race track with some damping fluid.

Do these only 'damp' the imbalance? I guess as the shaft experiences cyclic angular accelerations and decelerations then it'd damp those, is that it? So not really balancing the shaft masses by shifting CoG?

Are there any passive shaft balancers that can shift the CoG to balance rotary motion of unbalanced shaft masses?

I did not know what these were until I read your post but, having looked at a few references, e.g. this one: https://www.sciencedirect.com/science/article/abs/pii/S0094114X15001937. my understanding is the balls move so as to take up positions at which their mass tends to bring the axis of the total moment of inertia closer to the axis of rotation. They thus don't just damp the out of balance forces, but actually reduce them by shifting the CoG.  

Secondly, by doing this they reduce the lateral accelerations (i.e. vibration normal to the axis of rotation), not changes in angular velocity.

At least, that would be my reading of how they work. 

 

[Jez]

Yes I came across that paper too, but in the absence of seeing a full explanation I could not really tell what the idea did, or how it worked.

I mean, if you have a row of loose balls in an azimuthal slot, surely they'd all just work over to the heavier out-of-balance side of the rotation, under centripetal effects, and make the imbalance worse still?

That's the substance of my question. If you only have loose passive elements in a rotating system, surely they always make the imbalance worse?

[exchemist]

1 hour ago, Jez said:

Yes I came across that paper too, but in the absence of seeing a full explanation I could not really tell what the idea did, or how it worked.

I mean, if you have a row of loose balls in an azimuthal slot, surely they'd all just work over to the heavier out-of-balance side of the rotation, under centripetal effects, and make the imbalance worse still?

That's the substance of my question. If you only have loose passive elements in a rotating system, surely they always make the imbalance worse?

Evidently not, at least if their freedom of motion is suitably damped. But it seems a lot depends on the speed of rotation, i.e. the vibration frequency: at some speeds it indeed makes things worse but the idea seems to be that at design speed of operation it tends to cancel the out of balance forces. It is evidently highly mathematical and it looks to me as if one would need to be a rotating machinery engineering specialist to understand and explain it. For instance, look at this paper:

https://asat.journals.ekb.eg/article_22757_440eb67a81fd7bbd66129baee93f72cb.pdf  

I'm afraid I don't have the background to be able to follow this.  

The principle seems to have been discovered by a chap called Thearle in 1932. Perhaps if you look him up you may be able to find a simpler explanation of it. 

 

Edited May 31, 2023 by exchemist