![]() ![]() In the worst-case example above there was an extreme distribution of tolerances. ![]() Tolerance Analysis Stack-up Spreadsheet.Statistical Stack-up Tolerance Analysis.It is clear that it is beneficial to take the probability distribution of tolerances into account and perform a statistical analysis. In practice the probability of an extreme height is very low. Notice that this is an extreme distribution of tolerances with a low probability itself. If for instance only the highest stack is problematic than this probability is only 6.25%. The probability of an extreme thickness is 2/16 = 12.5%. With just 4 parts it is still doable to write down all possible combinations. Now assume that all the parts have a worst-case deviation and are either 9 ( part ‘9’) or 11 ( part ’11’) high (any dimension, mm, inch, meter, et cetera). The parts have a height specification of 1 0 +/-1. Suppose that you make a stack of 4 identical parts and you want to analyze the total height of the stack. Let’s take a look at the following example. A statistical tolerance stack-up analysis takes the probability of a tolerance value and the combination of tolerances into account. It turns out that the probability of a worst-case combination is negligible for already a small number of parts. Such an analysis assumes that all dimensions in the tolerance chain have worst-case deviations form their nominal value. In the article Worst-case Tolerance Stack-up Analysis you read about the worst-case or linear stack-up analysis. ![]()
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