Fit, Tolerance, and SOD!

Christopher asks “In engineering how can different tolerances give the same fit!!!!!”

In engineering there are two definitions of the word fit. There is the standard “to fit together”, as well as the acronym Failures in Time.

For the first definition, there is a fairly easy explanation. If two objects are of a similar dimension, let us say size for now, then their tolerances (the size by which the given value can vary, i.e. 15mm+/-0.5mm means that the true size of the object will lie anywhere within 14.5 and 15.5mm) may allow them to fit into the same sized hole. i.e. If two screws with marked size 2mm and 3mm have tolerances of 0.5mm, there is a chance (albeit a small chance) that they will both be exactly the same size, the 2mm one could be 1.5mm -> 2.5mm and the 3mm one could be 2.5 -> 3.5mm. Therefore with a 2.5mm hole with a tolerance of 0.5mm itself, around 50% of each type of screw will fit snugly within the hole. Although this has purely been related to size, it can relate to any dimension, capacity (of an electrical capacitor, or even volume of a bucket), time, voltage and current, weight and mass, anything with a measured value.

F.I.T. is defined as the probability of failure within a given time. So tolerance is related to this, but in a much more complex way. Due to the complex method in which failure happens in most systems, and the variability of such causes of these failures, tolerances are an integral part of the calculation.

As with any complex route, many of the variables may be quite different, but due to the tolerances, the final value of FIT could work out to be similar or the same in two different models due to the natural variation. This goes nowhere to explaining why at a critical moment, two completely separate, but important systems, both fail at the same time. Yes, I’m talking about Sod’s Law examples, like the power goes out in your house, and as you go to light your candle, the flint in your lighter decides it’s time it broke! Or the television breaks down, and when you go to switch on the radio for some entertainment, that blows a fuse also! But I assume nothing will explain those particularities! That’s just life

Thanks for asking

Triga@ask.me.anything