Home Page

1.0 INTRODUCTION

This section presents a technical coverage of gear fundamentals. It is intended as a broad coverage written in a manner that is easy to follow and to understand by anyone interested in knowing how gear systems function. Since gearing involves specialty components it is expected that not all designers and engineers possess or have been exposed to all aspects of this subject However, for proper use of gear components and design of gear systems it is essential to have a minimum understanding of gear basics and a reference source for details.
       For those to whom this is their first encounter with gear components, it is suggested this section be read in the order presented so as to obtain a logical development of the subject. Subsequently, and for those already familiar with gears, this material can be used selectively in random access as a design reference.

2.0 BASIC GEOMETRY OF SPUR GEARS

The fundamentals of gearing are illustrated through the spur-gear tooth, both because it is the simplest, and hence most comprehensible, and because it is the form most widely used, particularly in instruments and control systems.

2.1 Basic Spur Gear Geometry

The basic geometry and nomenclature of a spur-gear mesh is shown in Figure 1.1. The essential features of a gear mesh are:

1. center distance
2. the pitch circle diameters (or pitch diameters)
3. size of teeth (or pitch)
4. number of teeth
5. pressure angle of the contacting involutes

Details of these items along with their interdependence and definitions are covered in subsequent paragraphs.

2.2 The Law of Gearing

A primary requirement of gears is the constancy of angular velocities or proportionality of position transmission, Precision instruments require positioning fidelity. High speed and/or high power gear trains also require transmission at constant angular velocities in order to avoid severe dynamic problems. 
       Constant velocity (i.e. constant ratio) motion transmission is defined as “conjugate action” of the gear tooth profiles. A geometric relationship can be derived (1,7)* for the form of the tooth profiles to provide cojugate action, which is summarized as the Law of Gearing as follows:
       “A common normal to the tooth profiles at their point of contact must, in all positions of the contacting teeth, pass through a fixed point on the line-of-centers called the pitch point.”
        Any two curves or profiles engaging each other and satisfying the law of gearing are conjugate Curves.
___________
*Numbers in parenthesis refer to references at end of text.

T25    Next Page