2 Tolerancing Terminology Basic SizeThe theoretical size used as a starting point the application of tolerances.ToleranceThe acceptable dimensional variation that is permitted on a part.LimitsMaximum and minimum size shown by the toleranced value1.00 ±0.05Upper limit = 1.05Lower limit = 0.95AllowanceThe minimum clearance or maximum interference between two partsSmallest hole – Largest Shaft
3 Tolerancing Terminology Maximum Material Condition (MMC)The condition when the part contains the most amount of materialFor a shaft (external feature) this would be the upper limitFor a hole (internal feature) this would be the lower limitLeast Material Conditions (LMC)The condition when the part contains the least amount of materialFor a shaft (external feature) this would be the lower limitFor a hole (internal feature) this would be the upper limit
5 Tolerancing Terminology Fits between partsClearance Fit (running fit)A condition in which an internal part is smaller than the external part it fits into.Hole is always larger than the shaftInterference Fit (force fit)A condition in which an internal part is larger than the external part it fits into.Hole is always smaller than the shaftTransitional Fit (locational fit)The limits of size applied to a part can either result in a clearance fit or a transitional fit.
6 Fit Systems Selective Assembly Basic Hole System Basic Shaft System An assembly method where the parts are measured and grouped into small, medium and large parts, all within tolerance. The groups are then assembled with similar matching parts of small, medium and large part for a better than random fit.Basic Hole SystemThe basic hole system applies the basic size to the smallest hole.Most used as there are limited standard tools for making holesThe allowance is applied to the shaft which can typically be made any sizeBasic Shaft SystemUsed in the textile industry where many different parts fit on long shaftsShaft is made nominal and holes are machined to fit to achieve the desired clearance or interference fit
7 Basic Hole System Preferred Precision Fits Fit Classes English unit tolerance relationshipsANSI Standard B4.1Specifies a series of standard fits between cylindrical partsFit ClassesRunning and Sliding Fits (RC)Clearance Locational Fits (LC)Transitional Locational Fits (LT)Interference Locational Fits (LN)Force and Shrink Fits (FN)
8 Running and Sliding Clearance Fits There are nine classes of running and sliding clearance fits.Running and sliding clearance fits always provides a clearance between the shaft and the hole.Smaller classes of fit (RC1 or RC2) provide less clearance than larger classes of fit (RC8 or RC9).The amount of tolerance permitted by an RC fit is proportional to the size of the part.
11 Lecture ExampleDetermine the tolerances for a 3/4 in. shaft and hole using with RC3 class fit using the Basic Hole System.Determine the nominal size of the hole and shaft.Determine class of fit.Locate the Clearance and Standard Tolerance Limits on the appropriate Tolerance Table.Determine the maximum and minimum hole size by adding the tolerances to the nominal size of the hole.Determine the maximum and minimum shaft size by adding the tolerances to the nominal size of the shaft.
12 Lecture Example n 0.7500 Nominal Size RC3 Class Fit 0.71 – 1.19 Nominal Size Range in Tolerance TableMinimum Clearance =Maximum Clearance =Tolerance on hole+.0008-.0000Tolerance on the shaft-.0008-.0013