GEAR DESIGN - METRIC
19.0 GEARS, METRIC
19.1 Basic Definition.
Metric gearing is distinguished not only by different units of length,
but also by its own unique design standard. Historically, metric gears arose
as a result of a different approach to the standardization of tooth
proportions and this constitutes a major obstacle to the adoption of the
metric system by the American gear industry.
In the inch system diametral pitch was created as a convenient means far
relating pitch diameters to center distance. Thus, diametral pitch is
defined as:
From this relationship there are particular integer - values of diametral
pitch that yield integer values for center distance in inches. Thus 8, 16, 32
and 64 diametral pitches, to mention only some, can be associated with tooth
numbers which can result in center distances equal to an integral multiple of
one inch and/or convenient fractions of an inch.
In the metric system the module is analogous to pitch, and is defined as:
This defines the module as analogous to the reciprocal of diametral pitch.
However, the module is a dimension (length of pitch diameter per tooth);
whereas diametral pitch is the number of teeth to a unit length of pitch
diameter. Again convenient center distances in metric measure are obtained by
choosing integer module values and/or selected fractional values.
One consequence is that each system (inch diametral pitch and metric
module) has adopted preferred standard values which are non - interchangeable.
It should be noted that the term diametral pitch is associated with the
inch system. In the metric system the nearest analogue to pitch is termed
"module", and the word pitch is reserved for tooth spacing along the
pitch circle. In the inch system, the tooth spacing measure is more accurately
called "circular" pitch.
If the equations for diametral pitch and module are solved for pitch
diameter and these values equated by introducing the conversion factor 25.4,
we obtain:
Pd
* m=25.4
(64)
This shows that inch diametral pitch and the metric module are related by the decimal factor 25.4. It is obvious that conversion results in decimal values, often awkward numbers, for one or the other measure. It follows that convenient values in one system will not be convenient values in the other. For this reason each system (inch diametral pitch and metric module) has adopted preferred standard values which are non-interchangeable. Table 1.32 lists the commonly used pitches/modules of both systems, with preferred values in bold-face type. Corresponding equivalent values are given, but these are of no help since odd valued pitches and modules are usually not tooled for.
It becomes obvious, therefore, that direct replacement of conventional inch gearing with metric gearing is impossible. The best that can be done is to shift to the nearest standard module when converting from the inch system. One should keep in mind, however, the preferred module sizes which exist in different countries. The degree of non-correspondence between pitch and module is best measured by the circular pitch and the circular tooth thickness. These values are given in inches and millimeters in Table 1.32.
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