Tolerancestackup.pptVIP

Design Analysis Design analysis is used in place of test and inspection to find the expected part performance It basically means we are looking for the mean and sigma of the distribution before we build the parts Techniques used for design analysis include: 1. Worst Case Analysis 2. Sensitivity Analysis a) Monte Carlo Simulation b) Taylor Series Expansion c) Design of Experiments 1. Worst Case Analysis Worst case is used to determine what happens when all the parameters are at their limits Set every parameter to its spec limit Measure the response This is the same as adding tolerances If all the parameters represent 3 sigma limits then The number of parts bad for one parameter is 0.0026 Four parameters (0.0026) 4 = 0.00000000000286 (3 in a trillion) For 20 parameters we get 2 out of 10 58 bad! Analysis should limit probabilities to 6 sigma This is about 3 parameters all at their limits Tolerance Stack Up Worst Case Analysis Tough! Shaft 1 +-0.002 Hole 1 +.003, -.001 Shaft 2 +-0.002 Hole 2 +-.003 -.001 Shaft 3 +-.002 Hole 3 +-.003 -.001 Total Worst case is .002+.003+.002+.003+.002+.003 =+0.015, -.009 The SHAFT WON’T FIT IN THE LAST HOLE UNLESS ITS .009 SMALLER! Worst Case For the first shaft and hole the shaft can be +-.002 and the hole +.003, -.001. If the hole is small then -.001 and the shaft is large then +.002. The difference between the two must be 0.003 to make sure it fits. Add another set and the worst case says the second hole and shaft must be 0.006 different in size A third set would have to 0.009 etc. The problem with this is it makes for a too conservative design when you stack up tolerances 2. Sensitivity Analysis 2a) Monte Carlo Analysis Monte Carlo is the name of a gambling city on the Mediterranean Sea. With MC analysis you “roll the dice” and estimate the outcome many, many times If you randomize the result often enough you get a good idea of the outcome of a system You need to know the following An equation for what you are simulating Th