Why is the addendum of the gear kept equal to the module?

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Discussion Overview

The discussion revolves around the relationship between the addendum and dedendum of gears and their definitions in terms of the module. Participants explore the reasons behind these standard measurements and their implications in gear design, referencing both theoretical and practical aspects.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant notes that in gear design, the addendum is often defined as equal to the module or 0.8 times the module, while the dedendum is typically 1.25 times the module, questioning the rationale behind these definitions.
  • Another participant expresses difficulty in finding diagrammatic references that illustrate the module of a gear, suggesting that most diagrams reference the pitch circle instead.
  • A participant speculates whether the geometric differences between addendum and dedendum might explain the specific values of 0.8 for addendum and 1.25 for dedendum.
  • A later post references standards that have been established to reduce the variety of gears, detailing specific values for addendum and dedendum in standard tooth systems and stub tooth gears, while questioning whether these factors were derived from experimental results or analytical models.

Areas of Agreement / Disagreement

Participants express various uncertainties and questions regarding the definitions and standards of gear measurements, indicating that there is no consensus on how the specific factors were determined or their implications.

Contextual Notes

Limitations include a lack of clarity on the derivation of the factors for addendum and dedendum, as well as the dependence on specific definitions and standards that may vary across different contexts.

Who May Find This Useful

This discussion may be of interest to students and professionals in mechanical engineering, gear design, and those studying standards in machine design.

Divya Shyam Singh
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In general calculations of gear design, addendum is taken as a factor of the module of the gear such as equal to module or 0.8 times the module and dedendum is taken as 1.25 times the module. Why are both these defined in terms of module? How did we reach to this conclusion?
 
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I have yet to find a single diagrammatic reference to the module of a gear. Every diagram I look at shows the addendum and dedendum with reference to the pitch cicle.
 
Apropos of nothing, I wonder if this geometry explains why the discrepancy between addendum=0.8 and dedendum=1.25
FIG5.jpg
 
Section 3.2
"To reduce the varieties of gears to a manageable numbers, standards are evolved. Standard makes it easy for design, production, quality assurance, replacement etc. Three commonly used pressure angles are 14.5o , 20o and 25o pressure angle systems as shown in Fig. 3.3. In this, one can have full depth gears or stronger stub tooth gears. In Standard tooth system for metric gears, addendum: a =1m, dedendum: b= 1.25m where as the for the stub tooth gears, addendum a = 0.8m and dedendum: b= 1.0m. The shorter tooth makes it stronger and its load carrying capacity increases. It also helps in avoiding interference in certain cases"

Source: http://nptel.ac.in/courses/112106137/pdf/2_3.pdf

I have also read this in a number of machine design handbooks. But still i don't understand how did they reach that factor. Was it an experimental result? or perhaps some analytical model...?
 

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