How to calculate max permissible mass of one balancing weight 3

[Creator_87]

Hi everyone.

I am looking for a procedure on how to calculate the maximum permissible mass of one balancing weight?
For example, for a fan that is instaled on the rotor end.

 

[electricpete]

Some thoughts fwiw:

For a trial weight, I've heard people suggest limiting it so the resulting centrifugal force is 5% or 10% of rotor weight. If close to resonance, it's more important to avoid adding too much trial weight.

During balancing itself, if the balance program is telling you to add weight that seems more than the unbalance that could reasonably be present (or recommended weight amount/location changing in an unusual fashion) that's a clue to step back and think about what's going on. Also you obviously should not put on anything that cannot be safely held by the attachment location/mechanism.

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[Strong]

OP cannot think in terms of mass only. Unbalance force is related (by shaft speed) to mass times radial distance to center of rotation. A balancing mass/weight must also be at the correct angle to counter the existing unbalance vector.

Walt

 

[Creator_87]

As I understand. The maximum balancing weight is limited by two factors.

1. The increase in the entire mass of the rotor does not lead to resonance.
2. The strength of the fastening of the load to the fan at centrifugal force. So that the load does not fly out of the attachment point.
Is it correct?

Sorry for my English. Hope I made myself clear.

 

[drawoh]

Creator_87,

Are you dealing with an actual unbalanced fan? If so, do you know how far out of balance it is, i.e. do you have numbers?

If you do not have numbers numbers, you cannot do calculations. Also, if your fan is out of balance, you need the correct mass, not the maximum.

Can you statically balance your fan? Static balancing is not absolutely reliable, but it may be good enough for your purposes, and there are fairly easy, empirical processes for doing it.

--
JHG

 

[electricpete]

As I understand. The maximum balancing weight is limited by two factors.
1. The increase in the entire mass of the rotor does not lead to resonance.
2. The strength of the fastening of the load to the fan at centrifugal force. So that the load does not fly out of the attachment point.

#2 - I agree

#1 does not seem reasonable, the balance weight is typically such a small fraction of total weight that it isn't going to change proximity to resonance in any significant way.

I did mention resonance in the context of selecting a trial weight. You don't want to put on too much weight that the machine shakes violently and damages itself. The amount of weight required to make that happen is less when machine is operating near (balanced near) resonance... so it's more important to limit the amount of trial weight added when machine will be balanced at a speed near resonance. A thumbrule I've heard for general balancing (not necessarily near resonance where you'll want to be even more careful) is to use a trial weight which produces a centrifugal force which is less than 5% or 10% of the rotor weight. As I said before and Walt Strong re-emphasized, that 5% is not comparing trial weight to rotor weight, rather it's comparing trial weight centrifugal force (calculated as m*r*w^2 where w = 2*pi*Frotor) to rotor weight.

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[GregLocock]

On any system I've balanced the cost per run was small so the first time we balanced one we just started small and increased it until we got a good shift in signal. In practice we often already knew what sort of balance weights were typical, so just picked a small one of them.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[electricpete]

Greg's approach probably makes sense in a factory environment. For field balancing in plant environment there may be lockout / tagout changes required between every weight move and motor start depending on how rigorous (or anal, depending on your viewpoint) the safety program is. Also we try to keep total number of starts for large motors down, especially DOL start and when driving high inertia loads like fans. So if you start too light that you don't get a decent change in vibration angle or magnitude, you probably wasted a run... but even so it's still better to start with too little than too much.

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[GregLocock]

Wheels, driveshafts, crankshafts, prop shafts, none fail catastrpohically at typical balancing speeds, which typically are not at the resonaces.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Strong]

"#1 does not seem reasonable, the balance weight is typically such a small fraction of total weight that it isn't going to change proximity to resonance in any significant way."

Actually the total weight is not much of an issue unless the rotor is eccentric and the cause of unbalance. If the rotor operates at/close to resonance (especially rotor), then a relatively small weight placed at the same angle of the unbalance could cause high vibrations.

The OP question is rather vague, so more information might eliminate all of our guessing.

Walt

 

[electricpete]

As I understand. The maximum balancing weight is limited by two factors.
1. The increase in the entire mass of the rotor does not lead to resonance...

#1 does not seem reasonable, the balance weight is typically such a small fraction of total weight that it isn't going to change proximity to resonance in any significant way."

Actually the total weight is not much of an issue unless the rotor is eccentric and the cause of unbalance

I think you misunderstood my comment. I was responding to the op. Unless you think the mass associated with a balance weight is going to significantly change resonant speed, your disagreement ("actually" in response to my post rather than op's) seems out of place.

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