T-7
ELEMENTS OF METRIC GEAR TECHNOLOGY
Gears are some of the most important elements used in machinery. There are few mechanical
devices that do not have the need to transmit power and motion between rotating shafts. Gears not
only do this most satisfactorily, but can do so with uniform motion and reliability. In addition, they
span the entire range of applications from large to small. To summarize:
1. Gears offer positive transmission of power.
2. Gears range in size from small miniature instrument installations, that measure in
only several millimeters in diameter, to huge powerful gears in turbine drives that are
several meters in diameter.
3. Gears can provide position transmission with very high angular or linear accuracy;
such as used in servomechanisms and military equipment.
4. Gears can couple power and motion between shafts whose axes are parallel,
intersecting or skew.
5. Gear designs are standardized in accordance with size and shape which provides
for widespread interchangeability.
This technical manual is written as an aid for the designer who is a beginner or only superficially
knowledgeable about gearing. It provides fundamental theoretical and practical information.
Admittedly, it is not intended for experts.
Those who wish to obtain further information and special details should refer to the reference
list at the end of this text and other literature on mechanical machinery and components.
SECTION 1 INTRODUCTION TO METRIC GEARS
This technical section is dedicated to details of metric gearing because of its increasing
importance. Currently, much gearing in the United States is still based upon the inch system.
However, with most of the world metricated, the use of metric gearing in the United States is
definitely on the increase, and inevitably at some future date it will be the exclusive system.
It should be appreciated that in the United States there is a growing amount of metric gearing
due to increasing machinery and other equipment imports. This is particularly true of manufacturing
equipment, such as printing presses, paper machines and machine tools. Automobiles are another
major example, and one that impacts tens of millions of individuals. Further spread of metric
gearing is inevitable since the world that surrounds the United States is rapidly approaching complete
conformance. England and Canada, once bastions of the inch system, are well down the road of
metrication, leaving the United States as the only significant exception.
Thus, it becomes prudent for engineers and designers to not only become familiar with metric
gears, but also to incorporate them in their designs. Certainly, for export products it is imperative;
and for domestic products it is a serious consideration. The U.S. Government, and in particular the
military, is increasingly insisting upon metric based equipment designs.
Recognizing that most engineers and designers have been reared in an environment of heavy
use of the inch system and that the amount of literature about metric gears is limited, we are
offering this technical gear section as an aid to understanding and use of metric gears. In the
following pages, metric gear standards are introduced along with information about interchangeability
and noninterchangeability. Although gear theory is the same for both the inch and metric systems,
the formulae for metric gearing take on a different set of symbols. These equations are fully
defined in the metric system. The coverage is thorough and complete with the intention that this be
a source for all information about gearing with definition in a metric format.
