The focus of this text is the design and analysis of machines and machine elements. Since machine elements carry loads, it follows that an analysis of loads is essential in machine element design. Proper selection of a machine element often is a simple matter of calculating the stresses or deformations expected in service and then choosing a proper size so that critical stresses or deformations are not exceeded. The first step in calculating the stress or deformation of a machine element is to accurately determine the load. Load, stress, and strain in all its forms are the foci of this chapter, and the information developed here is used throughout the text.