Hello to all,

After having read most of the messages on this post site, I now realize that a large number of rotating equipment manufacturers do not give much thought about providing on-field balancing facilities on their rotors, be it a motor, a pump, a fan or whatever else.

In situ balancing of our electrical motors or generators has always been of very much concern to us. And, providing easy access to built-in balancing pre-drilled discs, is always present on all types of our equipment.

With the knowledge that trim balancing seems to be standard out in the field although our rotors were precisely shop-balanced prior to shipping, we have always incorporated such features in our design.

Yet, in spite of all our shop testing under load, it appears that somehow, once installed on other vendor's equipment, the need for trim balancing is ever present.

In other words, we provide in situ balancing facilities to compensate for the lack of designer's concern about the final installation at the end-user.

I have seen couplings so badly unbalanced that they litterally self destructed due to high vibrations.

And, not to mention A.C. motor driven vertical pump installation that ressemble something close to a tuning fork system.
Imagine a motor whose physical size is about 30 some inches in diameter fitted down to a 12 inch pipe flange some 10 feet off the ground.

I've also read on this forum some messages about adding weight onto the motor in order to change the resonant frequencies of similar installations ! And not to exclude some very exotic sets of mathematical calculations to attain the right weight size to circumvent the problems.

Gosh... I can't understand why equipement manufacturers have not addressed the problem where it lies... in the mounting design !!!

This kind of installation is similar to a gyroscopic phenomena and, if the motor is not ABSOLUTELY in balance to the n'th degree, it will behave like one of those toy wooden tops that wobble like mad just before stopping.

Well... just a reflexion from a retired rotating equipment design engineer that has had the 'opportunity ?' to witness such installations and got very involved in trying to correct those kinds of problems.

Yep... designing for balance at the drawing board... an art that seems to be ignored.

Still, when you think about it, this situation keeps us vib experts employed !

MarkoLeo

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

I have done a lot of large rotor balancing in various shops and this is one of my pet peeves also, Markoleo. I think that turbine/compressor manufacturers should design some sort of a balance correction ring which would allow weight removal either by drilling or grinding to make final corrections in the shop. With the insistence on tolerances such as 4W/N on these large rotors, we are always trying to make a 10 gram correction when the smallest weight we have to add to the part weighs 600 - 700 grams. This costs the end user in time while we piddle around trying to make these corrections with differential weights. Thanks for letting me on the soap box on your posting.

Dear Earl,

Thanks for your comments.
I too, share your views on manufacturer's lack of concerns for providing in-situ balancing facilities.

Have a good day,

MarkoLeo

Sometimes it's not manufacturers lack of "concern" that is the problem... it's their lack of knowledge. We assume they are knowledgeable about vibration, balancing, resonance, etc. and I think often that's just not the case.

As for balancing, I see fans quite often that will have a flat plate backplane with several inches of clearance between the backplane and the housing. How much trouble could it be to add a circular ring on the backside of the backplane so we could then attach clamp-on correction weights at any position desired? The answer is, of course, "very little"... but it would cost money. And the purchasing agent wouldn't care that it was there, he's just looking at the final cost.... so he buys the cheaper fan that has not provision for balancing, no access door on the housing, etc.

It's all driven by costs and the desire to make a profit.