Design of Mechanical Elements

7Shafts

7.1 Introduction

In this chapter, we will use the knowledge of Chapters 5 and 6, and apply it directly to the design of shafts. We define a shaft as a rotating mechanical element that transmits power or motion. This is different than an axle, which is a nonrotating member that does not transmit torque and only supports, e.g. rotating wheels or pulleys. Nothing is unique about the design of shafts. However, it is perhaps the most common machine element and, therefore, we pay extra attention to it.

7.1.1 Practical Considerations Related to Shaft Design

Shaft design involves both local stress and deflection calculations. Local stress in the shaft depends on the local geometry and dimensions of the shaft. In contrast, deflection of the shaft depends on the entire (global) geometry of the shaft. Hence, we first calculate the local stress in the shaft to determine the dimensions of the shaft geometry and, afterwards, we calculate the deflection of the shaft. We design the shaft dimensions to achieve the required design factor and account for the material we select through its yield stress. Alternatively, we select a material with yield stress that provides the required safety (design) factor for a given shaft geometry. Thus, the material choice primarily depends on the local stress in the shaft.

Deflection depends on the stiffness of the shaft, which in turn depends on the length of the shaft , the area moment of inertia of the cross‐section of the shaft (which might vary ...