Categories: Machinery Diagnostics,Machinery Maintenance,Other Topics

Balancing How To #5 – How A Correction Weight and Location is Calculated

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By on October 9, 2019

In our last “How To” we talked about the trial run – using a trial weight and location, to calculate the amount and location of a correction weight, which should correct the unbalance.  Most modern-day balancing tools do the math for you.  Even though these tools calculate the correction weight and location, it is good to have some understanding of how to do the calculations yourself.

Remember that a correction weight is a counterbalance to a heavy spot, i.e. you want the correction weight to counter the unbalance forces.  And an easy way to explain it is by using a vector or polar graph.  Many of us “old timers” performed balancing corrections by using a polar graph.

Here’s how it’s done:

  1. We take a polar graph, and lay out our rotor. In our example, we’ll use an (8) blade fan.  Lay out 8 lines, 45 degrees apart, to denote the blades (shown in red).  Number them consecutively and note the direction of rotation.
  2. In our example, our original vibration is .36 inches per second at 18 degrees (shown in green). O=.36 ips @ 18°
  3. We draw a line from the center of the graph, in our case, each circle represents 0.05 ips. So we draw a line 7.2 units, or “circles” long, at 18 degrees.

4. We add a trial weight (TW). Ours is 50 grams at 180 degrees.

5. We restart the fan, and measure. Our vibration is now O+T=.26 ips @121 degrees.

6. We draw a line from O to O+T, label it “T”. We measure the length of Line “T” using the same scale as the circles.  In our case, T=10.4 units in length.

 

Here are our numbers:

Original vibration (O)=.36 @ 18° (7.2 units)

Trial Weight (TW)=50 gr. @ 180°

Original + Trial Weight (O+T)= .26 @ 121°

T= (10.4 units long)

 

To calculate our correction weight size, we use this formula:

Correction Weight = Trial Weight x (Original / Length of Line T) or CW=TW x (O/T)

CW=50 x (7.2 units/10.4 units)

Our Correction Weight is 34.6 grams.

Now to figure the location.  On our graph, going from Line “O” to “O+T” is counterclockwise, at an angular change of 106°.  We place our Correction Weight 106° in the opposite direction (180°-106°=74)

We remove our trial weight and add our Correction Weight of 34.6 grams @ 74°.

That’s the math.  We could also trim balance the fan even closer, by continuing this graphing process again.

But now, we have another problem.  There is no blade exactly at 74 degrees.  We would have to split the weight, so that two separate weights would counter the unbalance.  We have one blade at 45°, and one at 90°.  Fortunately, most balancing machines (including our Fixturlaser SMC and OneProd Falcon) can calculate the weight split.  There is a graphical method of doing this as well, called the Parallelogram method.

If you’ve made it this far, GREAT!  Be glad your balancing equipment calculates for you!

 

About the Author

Stan Riddle joined VibrAlign in 2008. He has over 35 years experience in aligning industrial machinery. Stan received his AAS Degree in Machinist Technology from Surry Community College in Dobson, NC, and also holds a diploma in Industrial Systems Technology from Forsyth Technical Community College in Winston-Salem, NC, where he was also an instructor in the program.

Stan began his maintenance career working as a machinist and millwright for companies such as Weyerhaeuser, R.J. Reynolds, and Tyco Electronics. He also has over 25 years experience in Predictive Technologies, such as vibration analysis, thermography, oil analysis, and ultrasonic inspection. He is a certified Level III Vibration Analyst with the Vibration Institute, and is a Past Chairman and Board Member of the Piedmont Chapter.

Stan and his wife live in Yadkinville, NC.

2 responses to “Balancing How To #5 – How A Correction Weight and Location is Calculated”

  1. Roberto says:

    Hi Stan,
    I would like to know how you have calculated the angular change of 106°.
    Thank you for the useful information of your blog.
    Roberto

  2. Stan Riddle says:

    Thanks Roberto. It appears I made a mistake. The shift from “O” to “O+T” (18 deg. to 121 deg.) is actually 103 degrees, not 106.

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