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中文8895字
并行
公差
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中文
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并行公差设计【中文8895字】,中文8895字,并行,公差,设计,中文
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ORIGINAL PAPER Concurrent tolerance design Wilma Polini1 Received: 19 September 2014/Revised: 5 August 2015/Accepted: 3 September 2015/Published online: 16 September 2015 ? Springer-Verlag London 2015 AbstractTechnical drawings are constituted by three components that are strongly correlated: the geometry showing the part shape, the dimensions defi ning the part volume and the tolerances establishing the variability of the two previously described components. A general method- ology to assign the tolerances, especially geometric toler- ances, to all the components of an assembly has not been clearly defi ned up to now. This is probably due to the complexity of the problem and to the existing gap between the existing standards and the industrial designers com- mon practices. In this work, a new general methodology to assign dimensional and geometric tolerances to all the components of an assembly with a concurrent design approach is proposed. It takes into consideration the rela- tionships between tolerances and a set of design principles that have been naturally extracted from the standards and literature and from a deep discussion with the Italian Association of Industrial Designers. In order to demon- strate and validate the proposed approach, the methodology has been applied to two real case studies: a volumetric gear pump and a pneumatic actuator. KeywordsGeometric tolerance ? Tolerance assignment ? Concurrent design ? Tolerance design 1 Introduction Geometric dimensioning and tolerancing (GD they are becoming strategically critical in order to better exploit the design resources where they are geographically placed: they may belong to dif- ferent fi rms or to different centres of the same fi rm that are placed all around the world. Tolerance design is a stage of product design. The research on tolerance design has defi ned an iterative method, starting from a fi rst tolerance assignment and ending with the defi nition of optimal values (Zeid 1991). Once all tolerances have been assigned to the part features that are critical from a functional point of view (tolerance assignment), tolerance analysis is performed. This stage aims to evaluate how the combined effect of all the pre- viously established tolerances infl uences the assembly characteristics in order to verify that all the design requirementsaremet.Subsequentlytheinteractions between the defi ned tolerances are analysed in order to verify that the parts work correctly together. Therefore, the critical functional requirements of the assembly are cal- culated as a function of the previously defi ned tolerances and are compared with the constraints due to assembly or use. Finally, feasible or economic aspects are considered onthebasisofbothavailableprocessesandcost evaluations. If the results are not satisfactory, one must modify the previous tolerance assignment. The whole tol- erance design is usually defi ned as tolerance synthesis. This work focuses on the tolerance assignment in the detailed design, which is the third step of the design pro- cess. In the literature, very few works dealing with this task exist. This is due to the peculiar nature of this topic: it presents as many different problems as there are design projects. The solution of these problems is entrusted to the designers experience. Lanzotti et al. (2000) assign the dimensional and geometrical tolerances on the basis of a feature classifi cation; they consider form and dimensional tolerances of internal or external features. The concept of relations between features is used to allow the extension of the product model from conceptual to detailed design stages in Bradley and Maropoulos (1998): feature relations are an integrated representation system for dimensioning, tolerancing and connectivity modelling. Ballu and Mathieu propose a model that allows one to express fully several points of view: functional, standards, inspection and man- ufacturing (Ballu and Mathieu 1995). A computer-aided tolerancing system is shown in Tsai and Wang (1999): it assists the designer, especially in evaluating the tolerance accumulation of an assembly. The method introduced in Cle ment et al. (1994), Salomons et al. (1996) and Weill (1997) generates tolerances from contact relations expres- sed as associations between surfaces (Technologically and Topologically Related Surfaces, TTRS). In Anselmetti (2001), geometric specifi cation on key component is gen- erated by rules involving the types of features and a user- defi ned priority order among them. Hu and Xiong (2005a, b) identify constraints to the relative motion among parts that they use to defi ne suitable geometric control on fea- tures. Mejbri et al. (2003, 2005) decompose a global geo- metricfunctionalrequirementofamechanisminto geometric specifi cations defi ned on key components. The tolerances are defi ned by the user as special geometric tolerances whose datums are all located on a single base part of the assembly. For each requirement, a chain of contacts is built from the toleranced part to the base part. This allows to transform the original, external datums into regular datums on the same part. More recently in Armil- lotta and Semeraro (2011), the methods available for the specifi cation of geometric tolerances, from common engi- neering practice to the development of computer-aided support tools, are described and compared. In Panneer and Sivaramakrishnan (2012), an integrated methodology is suggested for value specifi cation and for checking the coherence and completeness of position, symmetry and perpendicularity tolerances based on distribution of mini- mum allowance. In Saravanan et al. (2014), a nonlinear combinatorial optimization problem is framed based on assembly function requirement (AFR) in order to optimize 24Res Eng Design (2016) 27:2336 123 the tolerance values. In Lu et al. (2012), an optimization algorithm is proposed to achieve concurrent tolerance design with a game theoretic approach. All these approaches, shown in the literature, do not develop a framework, generally effective, supporting the reasoning about tolerance choices. Moreover, they are applied to assemblies constituted by few and simple-shape parts. This work shows a new general methodology to assign dimensional and geometric tolerances to all the components of an assembly with a concurrent design approach. It takes into consideration the relationships between tolerances and a set of design principles that have been naturally extracted from the standards and literature and from a deep discussion with the Italian Association of Industrial Designers (IAID). A collection of rules to assign tolerances on the basis of the pursued functionality is, therefore, defi ned. These rules enable also to choose the necessary datum reference frames and material modifi ers. Those rules help designer during the detailed design, where classically the tolerance approach is defi ned. In order to demonstrate and validate the proposed approach, the methodology has been applied to two real case studies: a volumetric gear pump and a pneumatic actuator. The paper is organized as follows. Firstly, the proposed methodology to tolerance design is presented together with some details on design rules. Then the application of the proposed method to a volumetric gear pump and to a pneumatic actuator is described in order to prove its effi - cacy and effi ciency. 2 Design methodology The proposed methodology to assign tolerances is a critical step of the functional product tolerance design. The pro- posed method is a set of decisions for carrying out a fi rst assignment of tolerances, as shown in Fig. 1. It involves eight sequential choices that pass from the product per- formance to a promising set of dimensional and geometric tolerances. It may be used for single parts or assemblies. The fi rst choice concerns the defi nition of the nominal geometry and the mechanical resistance, in terms of con- stitutive material, of a product, by taking into account the applied stresses, such as static or dynamic loads and tem- perature gradients. The product shape is defi ned together with its preliminary dimensions as a function of the chosen material in order to guarantee that the resistance require- ments are met. From the second to the eighth choice, there is the assignment of the geometric tolerances on the basis of the newly proposed approach. The second choice involves the identifi cation of all the technical functions that the product should perform, by analysing the product working. The third choice translates the functionality of the design assembly in all the couplings or kinematic con- straints among assembly components that are needed for the assembly to work correctly. Then, it defi nes the pairs of assembly components involved in each technical function. The following steps from the fourth to the eighth choice that reason on the need of dimensional and geometric controls are implemented for each pair of components belonging to each technical function. The features of each pair of components that are involved in each technical function are identifi ed and processed as follows. The fourth choice assigns the dimensional tolerances to each pair of features by evaluating whether the coupling should occur with an interference, with a clearance or in an uncertain way. The accuracy required by the design spec- ifi cation is also considered. Many standardized kinds of couplings exist in the literature (Straneo and Consorti 1991): for each of them, dimensional tolerances are fi xed. The following four choices (from fi fth to eighth) provide the geometric tolerances to characterize the functional aspects of each couple of features. They refl ect the sequence of geometric controls supplied by the different classes of geometric tolerances. The order of geometric controls is decreasing: this means that the tolerance class allowing the greatest control, i.e. location tolerance, is assigned fi rst. Run-out and profi le tolerances are assigned lastly, since they are scarcely used in practice, due to their peculiar nature. The fi fth choice evaluates whether it is needed to locate the features of each pair of components. Therefore, the necessity of a location tolerance is checked and, in the case of an affi rmative answer, the datum reference frame (D.R.F.), the need of the material modifi er (M or L) on the D.R.F., the tolerance kind (position, concentricity, or symmetry) and the need to use the material modifi er (M or L) or the projected tolerance zone (P) on the tolerance feature are completely defi ned. The term POSITION near the material modifi ers means that they may be applied on the datum reference frame or on the tolerance feature only for position tolerances. The sixth choice evaluated whether it is needed to orient the features of each pair of components. This means that if angular control of a feature greater than that due to a previously assigned position tolerance is needed, an ori- entation tolerance should be specifi ed too. At the same time, if no position tolerance has been previously assigned, this step allows control of the feature orientation. An ori- entation defi nition must provide a D.R.F., the need of the material modifi er (M or L) on the D.R.F., the tolerance type (parallel, orthogonal or inclination) and the need to use the Res Eng Design (2016) 27:233625 123 material modifi er (M or L) on the tolerance feature or to apply the tolerance to a tangent plane (T). The seventh choice specifi es a form tolerance if a form specifi cation greater than that due to position or orientation tolerances is needed. A form tolerance assignment involves the defi nition of tolerance type (straightness, fl atness, roundness or cylindricity) and the maximum material modifi er M that is typically used for straightness. The eighth choice specifi es run-out or profi le tolerances in peculiar applications. The assignment of a run-out or a profi le tolerance involves the defi nition of a D.R.F., the need of the material modifi er (M or L) on the D.R.F. and the defi nition of the tolerance type (circular or total run- out, line or surface profi le). All the steps of the design methodology are reported in Table 1, whereas the steps 48 should be repeated for each couple of assembly features involved in each pair of assembly components, steps 38 should be repeated for each pair of assembly components involved in each tech- nical function, and steps 28 should be repeated for each technical function. This geometric characterization of each pair of compo- nents is carried out by means of design criteria. Design criteria are rules, whose if-conditions make reasoning on the working of the analysed pair of parts and whose actions assign geometric tolerances to the part features. Examples of design criteria are the assignment of fl atness to planes that must guarantee an adhesion, or the attribution of a concentricity where a mass balance is required, or the defi nition of a projected tolerance zone where interferences among coupling parts must be avoided. Moreover, the choice of a datum reference frame may follow some simple and unequivocal rules, such as a datum should be easily accessible for verifi cation and it should be manufactured with the accuracy required by the design specifi cations. Some details on these design criteria are described in the following paragraph. Once a geometric tolerance is assigned to a pair of assembly components, it is verifi ed that this tolerance actually satisfi es the functional requirements identifi ed at the fi rst and the second steps of the proposed method: this comparison is shown as feedbacks in the fl ow chart Fig. 1 New methodology 26Res Eng Design (2016) 27:2336 123 shown in Fig. 1. If the functional requirements are not satisfi ed, refi ne the technical functions and reiterate the proposed methodology. Otherwise, the following step of the tolerance design process may be carried out, i.e. the tolerance analysis. Tolerance analysis aims to determine the variability of a design function that depends on the assembly, when the previously fi xed tolerances are var- ied. If all the tolerances assigned to the assembly com- ponents do not satisfy the design constraints, adjust the design by remaking the tolerance assignment. Otherwise, it is possible to carry out the last step of the design process. This stage takes into account the manufacturing or economic constraints related to the tolerances previ- ously assigned on the basis of functional reasoning. Reiterate the design process if the constraints are not satisfi ed. At the end of this process, a set of dimensional and geometrical tolerances, optimal in terms of func- tionality, manufacturing and cheapness, are obtained. 3 Design criteria Design criteria put functional requirements in touch with dimensional or geometric tolerances. The great diffi culty in defi ning design criteria is that there are so many different problems in mechanical design that it may not make any sense to defi ne general rules. At the same time, having a set of criteria may help young designers to approach the design process. This is fundamental, especially for geometric tolerances whose standards are often not well known and whose application may be very hard due to the unknown relationships between tolerances and functional perfor- mances. Therefore, the design criteria are a collection of rules that establish the tolerances to assign on the basis of the pursued functionality. Moreover, this set of rules includes some principles to choose both the datum refer- ence frame and the material modifi ers. These design cri- teria have been defi ned by collecting information of the Table 1 Design methodology NumberDescriptionExplanation 0Product dimensioning To defi ne the product shape together with its preliminary dimensions as a function of the chosen material in order to guarantee that the resistance requirements are met 1Products technical functions identifi cation To identify all the technical functions that the product should perform by analysing product working For each technical function 2 Identifi cation of the pairs of assembly components To identify the pairs of assembly components involved in each technical function that means whose assembly allows the product to explicate that function For each pair of assembly components 3 Identifi cation of the couples of assembly features To identify all the couples of features involved in assembly For each couple of assembly features 4Dimensional tolerance assignmentTo assign the dimensional tolerances by evaluating whether the coupling should occur with an interference, with a clearance or in an uncertain way 5Location tolerance assignment To check the necessity of a location tolerance. If yes, to defi ne datum reference frame (D.R.F.), material modifi er (M or L) on the D.R.F., tolerance kind (position, concentricity, or symmetry) and material modifi er (M or L) or projected tolerance zone (P) 6Orientation tolerance assignmentTo evaluate whether it is needed to orient the features. This means that if an angular control of a feature greater than that due to a previously assigned location tolerance is needed, an orientation tolerance should be specifi ed too. If no location tolerance has been previously assigned, this step controls orientation If yes, to defi ne datum reference frame (D.R.F.), material modifi er (M or L) on the D.R.F., tolerance kind (parallel, orthogonal, or inclination) and material modifi er (M or L) or tangent plane (T) 7Form tolerance assignment To evaluate whether it is need to assign a form specifi cation. This means that if a form control greater than that due to position or orientation tolerances is needed, a form tolerance should be specifi ed too If yes, to defi ne tolerance type (straightness, fl atness, roundness or cylindricity) and material modifi er M for straightness 8 Profi le or run-out
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