The successful design and manufacture of gears are influenced largely by design requirements, material selection, and proper heat treatment. This chapter addresses the cost factors and tradeoffs involved in selecting a material, design features, and a heat treating process to optimize gear performance for a particular application.

This chapter reviews the knowledge of the field of gear tribology and is intended for both gear designers and gear operators. Gear tooth failure modes are discussed with emphasis on lubrication-related failures. The chapter is concerned with gear tooth failures that are influenced by friction, lubrication, and wear. Equations for calculating lubricant film thickness, which determines whether the gears operate in the boundary, elastohydrodynamic, or full-film lubrication range, are given. Also, given is an equation for Blok's flash temperature, which is used for predicting the risk of scuffing. In addition, recommendations for lubricant selection, viscosity, and method of application are discussed. The chapter discusses in greater detail the applications of oil lubricant. Finally, a case history demonstrates how the tribological principles discussed in the chapter can be applied practically to avoid gear failure.

This chapter introduces the fundamental considerations involved in heat treating of gears and the primary processes used, namely, through-hardening, carburizing and hardening, nitriding, carbonitriding, and induction hardening.

Modern gears are made from a wide variety of materials. Of all these, steel has the outstanding characteristics of high strength per unit volume and low cost per pound. Although both plain carbon and alloy steels with equal hardness exhibit equal tensile strengths, alloy steels are preferred because of higher hardenability and the desired microstructures of the hardened case and core needed for the high fatigue strength of gears. This chapter provides an overview of the key considerations involved in the selection and application of heat treating processes for alloy steel gears and serves as an introduction to the subsequent chapters in this book.

This chapter provides a brief but practical overview of the case carburizing process. It discusses the benefits and challenges of the process and compares and contrasts it with other hardening methods. It explains how design allowables and safety factors compensate for unknowns and familiarizes readers with the steps involved in determining case depth and verifying that case carbon requirements have been met.

Powder metallurgy (P/M) is a flexible metalworking process for the production of gears. The P/M process is capable of producing close tolerance gears with strengths to 1240 MPa at economical prices in higher volume quantities. This chapter discusses the capabilities, limitations, process advantages, forms, tolerances, design, tooling, performance, quality control, and inspection of P/M gear manufacture. In addition, it presents examples that illustrate the versatility of the P/M process for gear manufacture.

Through-hardening heat treatment is generally used for gears that do not require high surface hardness. In through hardening, gears are first heated to a required temperature and then cooled either in the furnace or quenched in air, gas, or liquid. Four heat treatment methods are primarily used for through-hardened gears: annealing, normalizing and annealing, normalizing and tempering, and quenching and tempering. This chapter begins with a discussion of these through-hardening processes. This is followed by sections providing some factors affecting the design and hardness levels of through-hardened gears. Next, the chapter reviews the considerations related to distortion of through-hardened gears. It then discusses the applications of through-hardened gears. Finally, the chapter presents a case history of the design and manufacture of a through-hardened gear rack.

The through-hardening process is generally used for gears that do not require high surface hardness. Four different methods of heat treatment are primarily used for through-hardened gears. In ascending order of achievable hardness, these methods are annealing, normalizing and annealing, normalizing and tempering, and quenching and tempering. This chapter discusses the processes involved in the through-hardening of gears. It provides information on designing procedures, hardness, distortion, and applications of the through-hardened gears. The chapter presents a case history on the design and manufacture of a through-hardened gear rack.

This chapter summarizes the various kinds of gear wear and failure and how gear life in service is estimated and discusses the kinds of flaws in material that may lead to premature gear fatigue failure. The topics covered are alignment, gear tooth, surface durability and breakage of gear tooth, life determined by contact stress and bending stress, analysis of gear tooth failure by breakage after pitting, and metallurgical flaws that reduce the life of gears. The chapter briefly reviews some components in the design and structure of each gear and/or gear train that must be considered in conjunction with the teeth to enhance fatigue life.