Budynas-Nisbett:Shigley's I.Basics 1.Introduction to T©The McGraw--Hill Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design Introduction to Mechanical Engineering Design 19 In the reliability method of design,the designer's task is to make a judicious selec- tion of materials,processes,and geometry (size)so as to achieve a specific reliability goal.Thus,if the objective reliability is to be 99.4 percent,as above,what combination of materials,processing,and dimensions is needed to meet this goal? Analyses that lead to an assessment of reliability address uncertainties,or their estimates,in parameters that describe the situation.Stochastic variables such as stress,strength,load,or size are described in terms of their means,standard devia- tions,and distributions.If bearing balls are produced by a manufacturing process in which a diameter distribution is created,we can say upon choosing a ball that there is uncertainty as to size.If we wish to consider weight or moment of inertia in rolling, this size uncertainty can be considered to be propagated to our knowledge of weight or inertia.There are ways of estimating the statistical parameters describing weight and inertia from those describing size and density.These methods are variously called propagation of error propagation of uncertainty,or propagation of dispersion.These methods are integral parts of analysis or synthesis tasks when probability of failure is involved. It is important to note that good statistical data and estimates are essential to per- form an acceptable reliability analysis.This requires a good deal of testing and valida- tion of the data.In many cases,this is not practical and a deterministic approach to the design must be undertaken. 1-13 Dimensions and Tolerances The following terms are used generally in dimensioning: Nominal size.The size we use in speaking of an element.For example,we may spec- ify a 1-in pipe or a -in bolt.Either the theoretical size or the actual measured size may be quite different.The theoretical size of a 1-in pipe is 1.900 in for the outside diameter.And the diameter of the -in bolt,say,may actually measure 0.492 in. Limits.The stated maximum and minimum dimensions Tolerance.The difference between the two limits. Bilateral tolerance.The variation in both directions from the basic dimension.That is,the basic size is between the two limits,for example,1.005+0.002 in.The two parts of the tolerance need not be equal. Unilateral tolerance.The basic dimension is taken as one of the limits,and variation is permitted in only one direction,for example, 1.0058806in Clearance.A general term that refers to the mating of cylindrical parts such as a bolt and a hole.The word clearance is used only when the internal member is smaller than the external member.The diametral clearance is the measured difference in the two diameters.The radial clearance is the difference in the two radii. Interference.The opposite of clearance,for mating cylindrical parts in which the internal member is larger than the external member. Allowance.The minimum stated clearance or the maximum stated interference for mating parts. When several parts are assembled,the gap(or interference)depends on the dimen- sions and tolerances of the individual parts.Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design © The McGraw−Hill 25 Companies, 2008 In the reliability method of design, the designer’s task is to make a judicious selec￾tion of materials, processes, and geometry (size) so as to achieve a specific reliability goal. Thus, if the objective reliability is to be 99.4 percent, as above, what combination of materials, processing, and dimensions is needed to meet this goal? Analyses that lead to an assessment of reliability address uncertainties, or their estimates, in parameters that describe the situation. Stochastic variables such as stress, strength, load, or size are described in terms of their means, standard devia￾tions, and distributions. If bearing balls are produced by a manufacturing process in which a diameter distribution is created, we can say upon choosing a ball that there is uncertainty as to size. If we wish to consider weight or moment of inertia in rolling, this size uncertainty can be considered to be propagated to our knowledge of weight or inertia. There are ways of estimating the statistical parameters describing weight and inertia from those describing size and density. These methods are variously called propagation of error, propagation of uncertainty, or propagation of dispersion. These methods are integral parts of analysis or synthesis tasks when probability of failure is involved. It is important to note that good statistical data and estimates are essential to per￾form an acceptable reliability analysis. This requires a good deal of testing and valida￾tion of the data. In many cases, this is not practical and a deterministic approach to the design must be undertaken. 1–13 Dimensions and Tolerances The following terms are used generally in dimensioning: • Nominal size. The size we use in speaking of an element. For example, we may spec￾ify a 11 2 -in pipe or a 1 2 -in bolt. Either the theoretical size or the actual measured size may be quite different. The theoretical size of a 11 2 -in pipe is 1.900 in for the outside diameter. And the diameter of the 1 2 -in bolt, say, may actually measure 0.492 in. • Limits. The stated maximum and minimum dimensions. • Tolerance. The difference between the two limits. • Bilateral tolerance. The variation in both directions from the basic dimension. That is, the basic size is between the two limits, for example, 1.005 ± 0.002 in. The two parts of the tolerance need not be equal. • Unilateral tolerance. The basic dimension is taken as one of the limits, and variation is permitted in only one direction, for example, 1.005 +0.004 −0.000 in • Clearance. A general term that refers to the mating of cylindrical parts such as a bolt and a hole. The word clearance is used only when the internal member is smaller than the external member. The diametral clearance is the measured difference in the two diameters. The radial clearance is the difference in the two radii. • Interference. The opposite of clearance, for mating cylindrical parts in which the internal member is larger than the external member. • Allowance. The minimum stated clearance or the maximum stated interference for mating parts. When several parts are assembled, the gap (or interference) depends on the dimen￾sions and tolerances of the individual parts. Introduction to Mechanical Engineering Design 19