TABLE OF CONTENTS The reader may have detected that we have dealt extensively with assembly concepts in the last two chapters without talking much about parts in the usual sense. Most books on engineering design deal with parts such as shafts, gears, and bearings. The detailed shape of these parts is important to such studies. We have said virtually nothing about the shape of parts, and deliberately so, for the following reasons.
First, the concepts we need, such as location and orientation in space, and degree of constraint, can be described with mathematical precision and very few symbols (and correspondingly few bytes of memory) using a few numbers in a 4 4 matrix or a twist matrix. To capture the equivalent information, such as the location and orientation of a shaft axis, or the fact that one part can rotate with respect to another, using purely geometric data, would entail thousands or millions of bytes and could possibly be less precise. [23] A major point of the last two chapters is that the main information we need to define a kinematic assembly is not geometric. It amounts to coordinate frames and twist matrices. We can add the geometry later.
Second, we are dealing with only the geometric relationships between parts, not any forces, loads, or deformations that they might experience. We addressed force and deformation only when we showed why pure kinematic constraints consisting of sharp point contacts are not used in practice. A complete engineering design of an...
