Shape Synthesisx


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SHAPE SYNTHESIS OF HIGH-PERFORMANCE MACHINE PARTS AND JOINTS By John M. Starkey Based on notes from Walter L. Starkey Written 1997 Updated Summer 2010
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2 SHAPE SYNTHESIS OF HIGH-PERFORMANCE MACHINE PARTS AND JOINTS Much of the activity that takes place during the design process focuses on analysis of existing parts and existing machinery. There is very little attention focused on the synthesis of parts and joints and certainly there is very little information available in the literature addressing the shape synthesis of parts and joints. The purpose of this document is to provide guidelines for the shape synthesis of high-performance machine parts and of joints. Although these rules represent good design practice for all machinery, they especially apply to high performance machines, which require high strength-to-weight ratios, and machines for which manufacturing cost is not an overriding consideration. Examples will be given throughout this document to illustrate this. Two terms which will be used are part and joint. Part refers to individual components manufactured from a single block of raw material or a single molding. The main body of the part transfers loads between two or more joint areas on the part. A joint is a location on a machine at which two or more parts are fastened together. 1.0 General Synthesis Goals Two primary principles which govern the shape synthesis of a part assert that (1) the size and shape should be chosen to induce a uniform stress or load distribution pattern over as much of the body as possible, and (2) the weight or volume of material used should be a minimum, consistent with cost, manufacturing processes, and other constraints. If the stresses (force per unit area) are indeed uniform throughout the part, and the material used is a minimum, then the stresses in the part will be at their maximum safe level which represents efficient design. In order to achieve these goals of uniform stress and minimum weight, it is necessary to consider the stress patterns that are present throughout the part as a result of applied forces and the geometry of the part. As external loads are applied molecular bonds within the material develop tensile and compressive and shear forces to transmit the load throughout the part. These internal load distributions are stress. For simple shapes and loading patterns, known stress patterns are introduced throughout the part. Because of this, it is useful to study stress patterns in simple-shaped parts because we can infer which of these stress patterns are inefficient, and, therefore, are more likely to cause failure in more complex parts. Strong stress patterns are those in which the large percentage of the material is stressed uniformly. Simple tension and compression are good examples of strong stress patterns, because the normal stresses are uniform across the cross section of a part loaded in tension and compression. Other examples of strong stress patterns include transverse shear stresses, torsional shear stresses in hollow tubes, and the normal stresses associated with bending of an I-beam cross-section. (See Figure 1.)
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