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That too-loose condition can still be assembled but could easily create a product performance failure mode, such as excessive side-to-side movement of the hinge as it rotates. Similarly you can check for a worst-case too-loose condition. If that calculated worst-case gap is too tight versus specifications, you have a potential failure mode. Here’s the math for this calculation: G min = (B min – C max) – A max determine the smallest possible gap (dimension G min) by subtracting the worst-case longest pin length from the worst-case shortest interior length.calculate the shortest possible length of the mounting bracket’s interior using the difference between the minimum total exterior length (dimension B min) and maximum thickness of the bracket support where the pin’s extension is mounted (dimension C max) and then.calculating the longest possible length of the large diameter part of the pin (dimension A max in the adjacent drawing).If the gap is too small, then the combined components will be too tight for correct assembly.Ī worst-case analysis to check for this too-tight failure condition will calculate the worst-case gap by: So, for example, for a hinge made up of a pin, with a smaller diameter extension or boss, that fits into bracket holes, there is a gap between where the smaller diameter begins and the inside surface of the bracket. Worst-case methods calculate the maximum deviation of geometric values that result from the sum of variations for each connected component and then use the results to determine if there is a potential failure condition. Worst-case methods calculate worst-case conditions, such as for the smallest possible gap (G min) between this hinge’s pin and interior of its mounting bracket. Additional stack-up calculation methods.Monte Carlo simulation stack-up calculations.The article also points to additional, less common, techniques to be aware of, including process tolerancing and inertial tolerancing. Software products for tolerance analysis, like Enventive Concept, build in all three of these methods so that designers can very quickly apply any combination of them to their decision making. This article overviews three of the most common methods: worst case, root sum squares (RSS), and Monte Carlo simulation. Which method, or combination of methods, to use for each direction will depend on factors such as how critical a stackup is to the fit and performance of the product, how many components there are in a stackup, or the complexity of geometries. Typically a designer needs to analyze stackups in multiple directions. The stackup calculations are integral to FMEAs. To help quantify the GD&T values needed to robustly deliver on those specifications, designers typically complete a Failure Modes and Effects Analysis, or FMEA, which identifies functional failure modes. These stackup calculations are key to ensuring that the manufactured products will reliably perform to customer specifications, such as for force levels or specific ranges of motion. Whether you are undertaking traditional fit-based analysis or functional tolerance analysis, there are several methods for calculating the sum of possible variances in any single direction within a stackup of assembled components so that a designer can make optimal decisions on tolerance values. ![]() Functional analysis is usually run before detailed design begins to determine robust GD&T values, and then is frequently run along with detailed CAD design. Traditional is typically run near the end of detailed design process within a CAD system. ![]() The designer can choose to apply traditional tolerance analysis, which focuses on ensuring fit for assembly, or functional tolerance analysis, which also ensures fit but additionally evaluates how robustly a product meets its functional performance requirements, such as for applied forces or specific motions. Getting the right GD&T values is important so that a product robustly meets its performance requirements for customers, can be properly assembled, and minimizes the costs of production. The specification of these values is also known as geometric dimensioning and tolerancing (GD&T). A mechanical engineer, design engineer, or possibly a product engineer determines the values for geometries and dimensions of an assembly and for each of its individual components - as well as how much they can vary (in other words their tolerance values). ![]() In manufacturing, tolerance analysis, also known as variational analysis, is a critical step in a design cycle for mechanical products. This article covers three of the most common methods for calculating variances of component stackups when completing a tolerance analysis.
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