Tolerance And Fits - University Of Florida

s to decide on the basic size, thatdimension to which the tolerances are applied.The analysis for strength, deflection, orperformance of the part determines the nominalor minimum size required.Unless special conditions exist, the basic sizeshould be chosen from the lists of preferredbasic sizes in Table A2-1 for fractional-inch sizes,decimal-inch sizes, and metric sizes from the SI.5

Table A2-1.Mott, 2003, Machine Elements in Mechanical DesignClearance FitsWhen there must always be a clearancebetween mating parts, a clearance fit isspecified.The designation for standard clearancefits from ANSI Standard B4.1 formembers that must move together is therunning or sliding clearance fit (RC).Within this standard, there are 9 classes,RC1 through RC9, with RC1 providing thesmallest clearance, and RC9 the largest.Clearance Fits con’tRC1 (close sliding fit): Accurate locationof parts that must assemble withoutperceptible play.RC2 (sliding fit): Parts that will moveand turn easily but are not intended torun freely. Parts may seize with smalltemperature changes, especially in thelarger sizes.6

Clearance Fits con’tRC3 (precision running fit): Precisionparts operating at slow speeds and lightloads that must run freely. Changes intemperature may cause difficulties.RC4 (close running fit): Accuratelocation with minimum play for useunder moderate loads and speeds. Agood choice for accurate machinery.Clearance Fits con’tRC5 (medium running fit): Accuratemachine parts for higher speedsand/or loads than RC4.RC6 (medium running fit): Similar toRC5 for applications in which largerclearance is desired.Clearance Fits con’tRC7 (free running fit): Reliable relativemotion under wide temperaturevariations in applications whereaccuracy is not critical.RC8 (loose running fit): Permits largeclearances, allowing the use of partswith commercial, “as received”tolerances.7

Clearance Fits con’tRC9 (loose running fit): Similar to RC8,with approximately 50% largerclearances.Clearance Fits con’tThe complete standard ANSI B4.1 lists thetolerances on the mating parts and the resultinglimits of clearances for all 9 classes and for sizesfrom 0 to 200 in.The next table is abstracted from the standard.Let RC2 represent the precision fits (RC1, RC2,RC3); let RC5 represent the accurate, reliablerunning fits (RC4 to RC7); and let RC8 representthe loose fits (RC8, RC9).Clearance Fits con’tThe numbers on the table are inthousandths of an inch.A clearance of 2.8 from the table meansa difference in size between the insideand outside parts of 0.0028 in.The tolerances on the hole and the shaftare to be applied to the basic size todetermine the limits of size for thatdimension.8

.Mott, 2003, Machine Elements in Mechanical DesignClearance Fits con’tThe next figure shows a graphical display of thetolerances and fits for all nine RC classes whenapplied to a shaft/hole combination in which thebasic size is 2.000 in and the basic hole system isused.Note that such a diagram shows the total toleranceon both the shaft and the hole, as well as thedramatic range of clearances provided by the 9classes within the RC system.The tolerance for the hole always starts at the basicsize, while the shaft tolerance is offset below thebasic size to provide for the minimum clearance(smallest hole combined with the largest shaft).Mott, 2003, Machine Elements in Mechanical Design9

Clearance Fits con’tThe maximum clearance combines thelargest hole with the smallest shaft.The codes within the tolerance barsrefer to the tolerance grades.The capital H combined with a tolerancegrade number is used for the hole inthe basic hole system for which there isno fundamental deviation from thebasic size.Clearance Fits con’tThe lowercase letters in the shafttolerance bars indicates the minimumclearance between the basic hole sizeand the fundamental shaft size.Then the tolerance is added to thefundamental deviation.The size of the tolerance is indic achmember if sameEo modulus of elasticity ofouter memberEi modulus of elasticity of innermemberVo Poisson’s ratio of outermemberVi Poisson’s ratio of innermemberProcedure for Computing Stresses3. Compute the tensile stress in the outermember from:22 c b σo p 22 c b 4. Compute the compressive stress in theinner member from:22 b a σi p 2 2 b a Procedure for Computing Stresses5. If desired the increase in diameter ofthe inner and outer member due to thetensile stresses can be computed. Theequation for the outer member is given.δo 2bp c 2 b 2 c 2 b 2 υ o E o 16

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General Tolerancing MethodsThe tolerances must ensure that thecomponent fulfills its function.But it should also be as large aspractical to permit economicalmanufacture.This pair of conflicting principles mustbe dealt with.General Tolerancing MethodsSpecial attention should be paid to thefeatures of a component that mate withother components and with which theymust operate reliably or with which theymust be accurately loaded.The fit of the inner races of thebearings on the shafts is an example ofsuch features.General Tolerancing MethodsWhere no other component mates withcertain features of a given component,the tolerances should be as large aspractical so that they can be producedwith basic machining, molding, orcasting processes without the need forsubsequent finishing.18

General Tolerancing MethodsIt is often recommended that blankettolerances be given for such dimensionsand that the precision with which thebasic size is stated on the drawingimplies a certain tolerance.For decimal dimensions in USCustomary units, a note similar to thefollowing is given:.Mott, 2003, Machine Elements in Mechanical DesignGeneral Tolerancing MethodsFor example, if a given dimension has a basicsize of 2.5 inches, the dimension can be statedon the drawing in any of 4 ways with differentinterpretations.2.5 means 2.5 0.050 or limits of 2.550 to 2.450 in2.50 means 2.50 0.010 or limits of 2.510 to 2.490 in2.500 means 2.500 0.005 or limits of 2.505 to 2.495 in2.5000 means 2.5000 0.0005 or limits of 2.5005 to2.4995 inAny other desired tolerance must be specified on the dimension.19

.Mott, 2003, Machine Elements in Mechanical Design.Mott, 2003, Machine Elements in Mechanical Design20

Table A2-1 Mott, 2003, Machine Elements in Mechanical Design. Clearance Fits When there must always be a clearance between mating parts, a clearance fit is specified. The designation for standard clearance fits from ANSI Standard B4.1 for members that must move