T-9
From the normalized metric rack, corresponding dimensions for any module are obtained by
multiplying each rack dimension by the value of the specific module m. The major tooth parameters
are defined by the standard, as:
Tooth Form:
Straight-sided full depth, forming the basis of a family of full depth
interchangeable gears.
Pressure Angle: A 20O pressure angle, which conforms to worldwide acceptance of
this as the most versatile pressure angle.
Addendum:
This is equal to the module m, which is similar to the inch value
that becomes 1/p.
Dedendum:
This is 1.25 m ; again similar to the inch rack value.
Root Radius:
The metric rack value is slightly greater than the American inch
rack value.
Tip Radius:
A maximum value is specified. This is a deviation from the American
inch rack which does not specify a rounding.
1.1.3 Comparison of Gear Calculation Equations
Most gear equations that are used for diametral pitch inch gears are equally applicable to
metric gears if the module m is substituted for diametral pitch. However, there are exceptions
when it is necessary to use dedicated metric equations. Thus, to avoid confusion and errors, it is
most effective to work entirely with and within the metric system.
1.2 Metric Standards Worldwide
1.2.1 ISO Standards
Metric standards have been coordinated and standardized by the International Standards
Organization (ISO). A listing of the most pertinent standards is given in Table 1-1.
1.2.2 Foreign Metric Standards
Most major industrialized countries have been using metric gears for a long time and
consequently had developed their own standards prior to the establishment of ISO and SI units. In
general, they are very similar to the ISO standards. The key foreign metric standards are listed in
Table 1-2 for reference.
1.3 Japanese Metric Standards In This Text
1.3.1 Application of JIS Standards
Japanese Industrial Standards (JIS) define numerous engineering subjects including gearing.
The originals are generated in Japanese, but they are translated and published in English by the
Japanese Standards Association.
Considering that many metric gears are produced in Japan, the JIS standards may apply.
These essentially conform to all aspects of the ISO standards.
