Design of Mechanical Elements

8Bolted Joints

8.1 Introduction

In this chapter, we will use the knowledge of the chapters on design for static and dynamic strength and apply it to the design of bolted joints. Bolted joints are connections between mechanical elements that we can assemble and disassemble in a nondestructive manner, using screw thread. This contrasts, e.g. welded joints, which once assembled, require destructive methods to disassemble.

We first recognize that to understand bolted joints, we must understand screw thread. Two categories of screw thread exist; screw thread that is intended to create motion between two mechanical elements and screw thread that is meant to fasten mechanical elements together. We refer to the former category as power screws (sometimes also referred to as lead screws or translation screws), and to the latter category as fasteners.

8.2 Power Screws

Power screws are a category of screw thread specifically designed to facilitate motion between mechanical elements. It is most often used to convert a rotary into a linear motion to displace a load. For instance, a car jack, a vise, or a lathe are examples of devices that use a power screw to convert rotary into linear motion and to displace a load.

8.2.1 Screw Thread Nomenclature and Geometry

Figure 8.1 illustrates the geometry of two common types of screw thread used in power screws. Figure 8.1a shows square screw thread, whereas Figure 8.1b shows trapezoidal screw thread, sometimes also referred to as ACME thread