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Root Sum Square and Statistical Tolerancing help needed

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  #1  
Old 2nd May 2007, 03:28 PM
ScottK's Avatar
ScottK

 
 
Posts: 2,525
Please Help! Root Sum Square and Statistical Tolerancing help needed

A customer told us that they are accepting of RSS when developing tolerances.
I've never heard of it until today.
Googled, found some equations, found some papers on the internet, dug around here for some threads but I'm still not entirely sure how it works.

Does anyone have a real life example of RSS method?

thanks.

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  #2  
Old 2nd May 2007, 03:52 PM
Benjamin28

 
 
Posts: 379
Re: a little Root Sum Square and Statistical tolerancing help?

Haven't seen this applied myself but a quick search turned up this article, quite interesting I thought:

http://www.nasa.gov/offices/oce/llis/0713.html
Thanks to Benjamin28 for your informative Post and/or Attachment!
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  #3  
Old 2nd May 2007, 03:54 PM
Frank T.'s Avatar
Frank T.

 
 
Posts: 236
Re: a little Root Sum Square and Statistical tolerancing help?

Here is another article an Engineer told me about.

http://www.variation.com/techlib/ta-2full.html
Thanks to Frank T. for your informative Post and/or Attachment!
  #4  
Old 2nd May 2007, 04:02 PM
Steve Prevette's Avatar
Steve Prevette

 
 
Posts: 2,346
Re: a little Root Sum Square and Statistical tolerancing help?

Quote:
In Reply to Parent Post by Discordian View Post

A customer told us that they are accepting of RSS when developing tolerances.
I've never heard of it until today.
Googled, found some equations, found some papers on the internet, dug around here for some threads but I'm still not entirely sure how it works.

Does anyone have a real life example of RSS method?

thanks.
I have a training example I use for the red bead experiment. Split the paddle into two halves, and count each half as two different sources of variation. If you average 10 red beads for the whole paddle, each half will average 5 red beads. If we assume the number of red beads in the paddle is Poisson (or you can physically do this and calculate the standard deviations), the standard deviation for each half is the square root of 5.

Let's add the two numbers together. We should average 10 with a square root of 10 as the standard deviation. Indeed, if I add the square of the square root of 5 (which is 5) to the square of the square root of 5 (which also is 5), I get 5 + 5 = 10. I then take the square root of 10 and indeed get the expected answer.
Thank You to Steve Prevette for your informative Post and/or Attachment!
  #5  
Old 2nd May 2007, 10:18 PM
Paul F. Jackson

 
 
Posts: 164
Re: a little Root Sum Square and Statistical tolerancing help?

Linear tolerance stacks typically are performed to verify critical clearences/interferences using a RSS value of the stacked tolerances involved but that value is typically multiplied by 1.5 (Thanks to Mr. Bender).
He wrote an SAE paper in the 60's that advised a safety factor of 1.5*RSS to predict critical stacked tolerance values.

With the modern emphasis on +/-4 or +/-5 sigma statistical capability of the individual tolerance contributors to those stacks some have abandoned the traditional safety 1.5 factor which would set the stacked value at +/-4.5sigma.

Paul
  #6  
Old 19th November 2007, 05:22 PM
Chitchat

 
 
Posts: 20
Re: Root Sum Square and Statistical tolerancing help needed

Is RSS the correct method for combining the calibration tolerances of two similar pieces of equipment for comparison purposes?

For example, we use an IR camera to measure the temperature of the laser beam. In order to verify the correct operation of the IR camera, the laser is set to 200 degrees and a second IR camera is aimed in the path of the laser beam and the readings from both cameras compared. If the IR camera has a calibration accuracy of ±3 degrees, what is the acceptable error in order to verify the correct operation of the IR camera? Given that the RSS would be 4.24, is 200 ±4.24 correct?

Thanks!
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