A tolerance stack up is the accumulation of manufacturing errors in an assembly or a part. If two parts have a tolerance of .005 inch, then the total tolerance would stack up to .010 inch in the assembly. There are many types of approaches to doing a tolerance stack. Some are more appropriate than others depending on the application.
Worst case analysis can be used which is very conservative. This type of analysis will tell you what the stack up is when every part is made at one end of the tolerance limit. So if you have a bunch of parts in a linear stack which are toleranced at +/-.005 inch, then a worst case analysis assumes that each one of those parts have been made at the either +.005 or -.005. Designing things can be difficult and cost prohibitive using this method of tolerancing. When designing large assemblies, it becomes less and less probable that every part will fall at one extreme or the other with more parts in play.
Sometimes a method called root sum square is used also. This technique assumes a normal distribution of tolerances though. This method is often scorned because of the fact that you are assuming that most of the parts will not be at either extreme. However, it can be used when many tolerances are being considered because at a certain point, the odds of having every part hit the extreme tolerance allowance is so low that it is acceptable.
Still other ways of analyzing tolerances exist which I will not take the time to explain. Do some research with these ideas in mind before you move forward.
By the way, what are you designing or analyzing that you need this for.
I am working for larsen & toubro, india. Am designing a collapsible drum for tyre manufacturing machinery. I have not been exposed to this concept thats why am asking....what do u do
I'm a Mechanical Engineer at a medical device company. I work with class 2 medical devices and have performed a few tolerance stack ups though it is not my specialty. Lately I have done a lot of research into the different methods of tolerance analysis and even posted a question on the subject recently. My question is whether the root sum square method is appropriate for medical devices and if there is some ISO standard which specifies what methods are not appropriate for the industry.
RSS should be calculated by the tolerance zone, then squre root these zone squred, then you get a value of zone, then you can divide it as your appropriate.
I am currently working on a new approach to stack for the engine design department within my company. In answer to how you add the stacks up very much depends on what the worst case is. If you are talking about a multiple flat sheets of materials and the worst case is the minimum or maximum thickness then you do just add them up. If you're looking into minimum thread engagement or bolt bottomin then you may have to look at a combination.
In the case of minimum thread engagement on a open threaded hole, you would require the maximum thickness of the sheets (to show the minimum amout of bolt coming through), you would then look at the shortest bolt and the minimum thickness of the part where the thread sits (smallest amount of thread).
The short answer is work out what the worst case is for your part, a simple spreadsheet and a logical approach should be enough for simple stacks. In my case I am looking into reducing tooling so the stackups then form the basis of governing the fasteners and surrounding geometry.
Tolerance Stackup Analysis
Re: Tolerance Stackup Analysis
