Balance tolerances 1

[dynaman]

Hi guys,

Just want to clarify something. I've been running single plane balancing of my small fan units and all is good. Only thing I'm uncertain is the allowable tolerance of the imbalance. I would like to use ISO 1940 as a guide. If I choose G2.5 as my standard, how can I use the "UCW" variable in the calculation below to determine if I meet the allowable imbalance?

Sorry for the double post, I didn't get a response to the previous one.

Vibration Variables: (displacement or veloctity or acceleration)
O = Original Vib (meausred)
O+T = Vib measured after adding trial weight (measured)
T = (O+T) – O = Computed vibration effect of trial weight

Unbalance Variables: (mass time radius)
UO = Original unbalance (unknown)
UT = Trial Weight unbalance (known)
UCW = Correction Weight unbalance (desired final answer)

(All of the above vibration and unbalance variables are 2-D vectors)

We want the correction weight unbalance to cancel out the original unbalance. Therefore we want:
UCW = -UO [Eq 1]

We assume vibration is proportional to unbalance. Therefore:
UO/UT = [O / T] = [O / (<O+T>-O) ]
UO = UT * [O / (<O+T>-O) ] [Eq 2]

Combine Eq1 and Eq2:
UCW = -U0 = - UT * [O / (<O+T>-O) ]
UCW = - UT * [O / (<O+T>-O) ] [Eq 3]

Reformat as ratio:
UCW/(-UT) = [O / (<O+T>-O) ] [Eq 4]

Equation 4 was only to demonstrate that the units of unbalance appear in a ratio and will cancel each other... same for units of vib.

Equation 3 shows how the spreadsheet calculates the required correction weight UCW.

thanks

Mark.

 

[electricpete]

You can convert ISO G2.5, you can convert that into an unbalance U based on machine speed and rotor weight.

Specifically
Uper = 9549 x G x W/N; g-mm
where: G = 2.5, W is weight in kg, N is speed in RPM, Uper is unbalance is g-mm.

Now, you can take that calculated Uper and convert it into a vibration level for your machine using our estimate that an unbalance of UT results in effect vector of (<O+T>-O).

i.e. Vmax = (Uper/Ut) * (<O+T>-O)

If final vibration is below that calculated level Vmax, you could make a case that you are balanced to that ISO level.


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

 

[electricpete]

Perhaps more conservative, if initial vibration level O is below that level and you make an add resulting in vibration decrease, you have even higher confidence.

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

 

[dynaman]

Thanks electricpete. I was thinking along the lines of,

1) Balance the rotor.
2) Run through a second balance and find UCW from the spreadsheet.

Since UCW is the residual unbalance after the second run and has units of g-mm I could then compare directly to Uper. If UCW < Uper then I'm within tolerance. Does this sound right?

cheers

Mark.

 

[electricpete]

Yes, that works for me.

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

 

[dynaman]

Thanks Pete,

Do you think UCW actually changes with frequency? I've noticed a small variation but could be due to experimental error.

cheers

Mark.