This chapter deals with the effects of fatigue in rotating shafts subjected to elastic and plastic strains associated with bending stresses. It begins with a review of the basic approach to treating low-cycle fatigue in bending, explaining that the assumption that stress is proportional to strain is incorrect due to plastic flow, causing considerable discrepancy between measured and calculated stresses. Data plots of the axial and bending fatigue characteristics of a 4130 steel help illustrate the problem. A closed-form solution is then presented and used to analyze the effects of flexural bending on solid as well as hollow rectangular and round bars. The chapter also discusses the difference in the treatment of a rotating shaft in which all surface elements undergo the same stress and strain and a nonrotating shaft in which a few surface elements carry most of the load. The difference, as explained, is due to the volumetric effect of stress in fatigue.

This chapter describes an investigation that was conducted to determine why quill shafts were failing prematurely in gear boxes on aircraft engines. The investigation focused on the splines in a splined bore. Visual examination showed that the splines were heavily worn and covered with red powder on one end. Investigators also observed blueing, an effect of overheating. Based on these observations and the results of SEM imaging, it was concluded that the splines in the spline bore were wearing out for lack of lubrication.

The quill shaft in an aircraft engine was found in two pieces following a flameout. One piece was short, straight, and otherwise undamaged; the other piece was bent in several places as was the sleeve that covered it. The facture surface, as viewed under optical and scanning electron microscopes, was flat and shiny with deformation marks and dimples, typical of torsional overload, and signs of severe rubbing on the periphery. Based on their observations, investigators concluded that the quill shaft failed by torsional overload, the source of which could not be determined.

A cardon shaft operating in an aircraft engine failed and was taken out and analyzed to determine the cause. A photograph of the broken shaft in the as-received condition shows the location and orientation of the fracture. The fracture surface appeared smooth, indicating that a considerable amount of rubbing occurred after the shaft broke. SEM fractography revealed deformation marks and elongated dimples, typical of shear overloads, along with other details. Based on their analysis, investigators concluded that the cardan shaft failed under torsional overload. They also cited a need for a more detailed examination of the driven end of the shaft.

Induction heat treating is used in a wide range of applications. Typical uses, as described in this chapter, include the surface hardening of many types of shafts as well as gears and sprockets and the through-hardening of gripping teeth, cutting edges, and impact zones incorporated into various types of tools and track pins manufactured for off-highway equipment.

This chapter begins with a review of some of the terms used in the gear industry to describe the design of gears and gear geometries. It then discusses the types of gears that operate on parallel shafts, intersecting shafts, and nonparallel and nonintersecting shafts. Next, the processes involved in the selection of gear are discussed, followed by information on the basic stresses applied to a gear tooth, the strength of a gear tooth, and the most widely used gear materials. Further, the chapter briefly reviews gear manufacturing methods and the heat treating processing steps including prehardening processes, through hardening, and case hardening processes.

Gearbox vibrations are classified as synchronous or nonsynchronous depending on whether or not they are related to the rotational speed of an internal or connected component. This chapter presents a turbogenerator case study that demonstrates the source of nonsynchronous vibration. It then provides a detailed discussion on spiking vibrations and presents design improvement and reduction of spiking vibrations.