where i must added trial mass?

[bloch2006]

hi friends:

when i want added trial mass to the equipment to do field balancing what is the best positions to put it on the equipment ?

any articles clarify this point will be gratefull

best regards

 

[electricpete]

1 - The simplest approach is to add the trial weight at random but limit the magnitude such that you won’t be concerned if it ends up adding to existing unbalance vector.

Thumbrules for trial weight magnitude
1A – twice the unbalance specification
or
2 B– enough mass to create a force equal to 5% of rotor mass (horizontal machines). One inch-ounce at 3600rpm creates approx 23 lbf. For other conditions, you can adjust this amount as directly proportional to unbalance (inch-ounce) and proportional to speed^2.

2 - a more complex approach would be to try to estimate where the residual unbalance is based on the phase of your measurements, your knowledge of phase measurement reference and machine response lag angle. If you have all this info then one of two choices:
2A – put your trial weight 180 opposite where you think the heavy spot is. If you’re really lucky the correction weight may end up being very close to trial weight.
2B – you might attempt to put your unbalance 90 degrees away from heavy spot to create a phase shift to provide a better determination of influence coefficient magntidue and phase. I think 2B is a better approach than 2A.

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[bloch2006]

hi friends:

thank u very much mr. electricpete for your valuable information .

but i want also know what is the parts suitable on any equipment to put tial mass ?

i suggest pulley what another part suitable?

 

[GregLocock]

Near your measurement plane. if you are multiplane balancing this makes things a lot easier.

If you are balancing a flexible rotor then I have no real experience of this, but you probably will also need a weight near the centre of the shaft.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Tmoose]

I try to pick balance correction planes that give me 360 degrees of easy correction. A symetrical bolt pattern of 6 or more holes at a decent radius makes me pretty happy.

When dealing with flexible rotors correction planes at the very ends don't usually get great results if speeds vary or if deflection/whip in the middle is a problem.

If a reasonable trial weight does not make the phase or amplitude change at least 20% or so a larger trial weight is needed. If the first few corrections don't "make sense" then I maket 2 or 3 start stops to prove repeatability.
Big motors should not be subjected to multiple start stops. The danger is long term damage, not so much instant failure.

 

[electricpete]

Another point to be mentioned is to attach is somewhere SAFE:

If you're using C-type clamps, prefer to attach them on the inner edge of rotating parts so centrifgual force does not tend to pull them off. Don't forget to manually rotate the rotor fully with trial weight installed to make sure you won't come close to contacting anything stationary

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[electricpete]

Also take a quick look to ensure you're not attaching to a part that might break under the extra force of the weight. We had a consultant attach a trial weight to a plastic motor fan blade on TEFC motor...didn't work too well.

What type of equipment are you balancing.

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[bloch2006]

hi friends:

thanks for every one help me .
electricpete my equipment is electric motor as sama as u described with plastic fan blade .

 

[electricpete]

On a TEFC motor:

If you have the luxury to disassemble the motor and balance the rotor on a balance machine attaching on rotor OD each end.

If you want to disassemble to the point of removing the endbells but not pulling the rotor, you can still access the same location but you have to disassemble/reassemble each time you change weights. Some people might say this can be a little bit traumatic on the bearings and might require bearing replacement.

Assuming you don't want to disassemble to the point of removing the endbells, your options are the outboard/drive end at the coupling or the inboard/non-drive end at the fan. With plastic fan, that isn't really an option unless you find some means of attachment to the shaft directly inboard of the fan or find a steel replacement fan that can replace the existing plastic fan. Depending on coupling type there may be easy locations to attach bolted weights at the coupling. Balancing at one end only may or may not be suitable.

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[electricpete]

My last post focused on where it is physically possible to attach weights.

In an ideal world if you could pick anyplace you want to add trial weights, you would consider which mode shapes are important at operating speed and possibly where the actual unbalance is (usually unknown). Attaching outboard of the bearings can have quite a different effect than inboard of the bearings. If the rotor acts rigid and everything responds linearly and your measurements/corrections are accurate, your influence coefficient will take these factors into account and you will still get a good balance using any 2 planes whatsover. But in the real world it may not always be the case.

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