Dynamic Balancing Issues 6

[mfgenggear]

General question: In regards to rotating shafts & gear or both.
at very extreme rpm's

How is the criteria for balancing decided?

What are the conditions required to make a decision that Dynamic Balancing is required?

If not balance what type failures can occur.

Thanks

 

[electricpete]

What are the conditions required to make a decision that Dynamic Balancing is required?

First I'd like to clarify, some few people use the term "dynamic balancing" to refer to balance by spinning up the rotor, as opposed to letting the heavy spot settle to bottom. Most people use the term "dynamic balancing" to mean 2-plane balance.

I'll assume you're asking about 2-plane balance and also considering the rotor to be rigid (operates sufficient margin below flexing rotor critical speed). Requirement for 2-plane vs 1-plane balance is often decided based on L/D of the rotor. For example perform 2-plane balance if L/D > 1/3

=====================================
(2B)+(2B)' ?

 

[mfgenggear]

electricpete

yes not static balance but by dynamic as "2 plane balance" ,
with parts balance at high rpm's.

L/D = length/Diameter? is greater than .3333?

how is the out balanced estimated that is required.
is it a function how much the bearings can take vibration?
or the gears?

is there a simple rule thumb or is it a complicated matter.

Thanks

 

[tbuelna]

mfgenggear,

There is an ISO 1940 standard for dynamic balance. Basically, it gives various quality grades that take into account speed, max imbalance force, etc. to establish a balance tolerance (ie. in-lbs or in/oz). It is generally best to select as low a balance grade as practical, otherwise it may not be possible to balance your part without excessive material removal.

http://www.plant-maintenance.com/articles/balancingqualitylimits.pdf

For dynamic balancing, you should specify the relevant process spec, the quality class, the max imbalance limit, the locations of the balance planes, where it is permissible to remove material to correct balance, what method is permissible for material removal (grinding, drilling, milling, etc.), and what dimensional/surface finish limits apply for material removal.

Sometimes, features are added to low stress areas in a part specifically for material removal during balancing, so that the material removal (the balancers usually prefer grinding) does not inadvertently create unwanted stress concentrations.

Good luck.
Terry

 

[geesamand]

My work with dynamically balanced components is not "at extreme RPMs". We do vertically hanging rotating assemblies and we dynamically balance when speed exceeds 200rpm.

The required balance quality depends on numerous factors included proximity to critical speeds, whether or not the component will accelerate through critical speeds and how long it will remain there, the amount of damping present in the system, the tolerance of the assembly for runout and/or vibration, to name a few. If the rotating member is an assembly, it is often wise to tighten tolerances on the components to reduce initial imbalance and to balance the subassemblies prior to final assembly.

There is a nice document in API 684 that covers rotordynamics. It's a relatively straightforward treatment of a complicated subject.

 

[mfgenggear]

Thanks guys

Iam on vacation so I am so to respond

Mfgenggear

 

[mfgenggear]

Tery ~ thanks for the info, will try to obtain the ISO spec.

geesamand where can I obtain the API684

Thanks

 

[geesamand]

We had to purchase API 684 from API.org or techstreet.com

David

 

[geesamand]

One thing to consider, because you mentioned gears: gears made from blanks of metal are turned/ground that creates an inherent, consistent balance quality. In my work (gears moving no faster than 1800rpm, industrial machinery) there is no need to balance the gears or the shafts within a gearbox as long as all surfaces of the gears/shafts are fully machined. I suggest first determining what balance quality is desired for the various components, then first calculate whether the as-machined quality will meet that balance spec. Usually you need to perform or check balancing of the gears and gearshafts in cases of very high speed gears/shafts, or in cases of imbalanced material (cast gears).

David

 

[drawoh]

mfgenggear,

A spinning device can generate straight centrifugal force, and it can generate a centrifugal moment. A static balance test will capture the centrifugal force component. The moment does not occur until the system spins.

A simple model is a shaft 1m long with a counterweight on top at 0.2m, and an identical counterweight on the bottom at 0.8m. The system is statically balanced in all planes. When you rotate, each load works mostly at its own end, and the thing vibrates.

JHG

 

[mfgenggear]

http://www.vtab.se/PHP-NBoard/html/images/materiali/Forum2/HTML/002533.html

I found this discussion on an other BBS

Mfgenggear

 

[Tmoose]

mfgenggear - note a participant in the discussion on that old version of the Reliability Magazine Boards is our very own Electricpete.

The OP's third post mentions something that doesn't seem to get much respect, even by pros. Clearances and runouts potentially change centering, and centering and "bAlAnCe" are siamese twins, sharing heart and brain, and wearing handcuffs.

The current board is here.

http://maintenanceforums.com/eve/forums/a/frm/f/3751089011

Lots of "discussions" about balance tolerance. Not sure if non-members can search.

Dan T

Dan T

 

[mfgenggear]

what type of failures are with Balancing issues?

can it cause tooth fatigue, or does cause the bearings to go first then the gears?

I have Interest because a customer that is having such issues.
How critical are planetary gears, what are the caveat's of high spinning assemblies. is it necessary to balance the planets. & the
sungear if it's a shaft?

Mfgenggear

 

[Tmoose]

Has some higher level failure analysis on the bearings to determine how they "went?"

The main result of unbalance is 1X radial load with continually varying direction.
"Good" balance typically creates radial force on the order of 10% of the component's weight.
It takes more than a vibration severity chart to determine Whether the bearings really care about that, compared to the 100% due to gravity (horizontal shaft), or side loads from belts, gears or the other process.

Then there is the issue of the frequency of vibration, and whether it is in the range of any of the system's natural frequencies, in which case the vibration may be amplified to greater than normal levels ( which may or may not result in greater bearing forces ).

Then there is whether auxilliary equipment or nearby humans may suffer or become whiney when exposed to the airborne or structureborne results.

I would think torsional vibration might contribute to tooth fatigue. But, unless there was an enormous amount of unbalance to require hugely increased torque on the upswing of a horizontal shaft machine I don't see unbalance effecting tooth loads much.

Has some higher level failure analysis on the bearings to determine how they "went?"

Yes, generally speaking, the planetary gear components must be "balanced." With their respective motions in mind.