National foreword

This British Standard is the UK implementation of ISO 21771:2007.

The UK participation in its preparation was entrusted to Technical Committee MCE/5, Gears.

A list of organizations represented on this committee can be obtained on request to its secretary.

This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application.

Compliance with a British Standard cannot confer immunity from legal obligations.

This British Standard was published under the authority of the Standards Policy and Strategy Committee on 28 September 2007

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 21771 was prepared by Technical Committee ISO/TC 60, Gears, Subcommittee SC 1, Nomenclature and wormgearing.

This first edition of ISO 21771 cancels and replaces ISO/TR 4467:1982, of which it constitutes a technical revision.

1 Scope

This International Standard specifies the geometric concepts and parameters for cylindrical gears with involute helicoid tooth flanks. Flank modifications are included.

It also covers the concepts and parameters for cylindrical gear pairs with parallel axes and a constant gear ratio, which consist of cylindrical gears according to it. Gear and mating gear in these gear pairs have the same basic rack tooth profile.

The equations given are not restricted to the pressure angle, α_p = 20°

The standard is structured as follows.

Listing of symbols and nomenclature for a unique description of gears and gear pairs (see Clause 3).
Equations and explanations of the relevant values for defining a cylindrical gear and its tooth system. The equations for determination of the nominal values for zero-deviation gear description parameters are stated for radial tooth dimensions (gear tooth heights), the distance between flanks of the same hand, the distance between flanks of opposite hand, as well as the tooth flank characterizing parameters (see Clause 4).
Equations and explanations of the relevant values for defining cylindrical gear pairs. The equations for the essential parameters characterizing the engagement conditions of the unloaded gear pair are listed (see Clause 5).
Equations and suggestions for desired flank modifications (see Clause 6).
Concepts and recommendations needed for a unique geometrical definition of the intended results from manufacture (Clause 7).
Equations for determination of the nominal values or the limiting values for the most used inspection methods for tooth thickness (see Annex A).

2 Normative references

3 Symbols, subscripts and units

3.1 Symbols

3.2 Subscripts

3.3 Units

4 Individual cylindrical gears

4.1 Concepts for an individual gear

4.2 Reference surfaces, datum lines and reference quantities

4.3 Involute helicoids

4.4 Angular pitch and pitches

4.5 Diameters of gear teeth

4.6 Gear tooth height

4.7 Tooth thickness, space width

5 Cylindrical gear pairs

5.1 Concepts for a gear pair

5.2 Mating quantities

5.3 Calculation of the sum of the profile shift coefficients

5.4 Tooth engagement

5.5 Backlash

5.6 Sliding conditions at the tooth flanks

6 Tooth flank modifications

6.1 Tooth flank modifications which restrict the usable flank

6.2 Transverse profile modifications

6.3 Flank line (helix) modifications

6.4 Flank face modifications

6.5 Descriptions of modifications by functions

7 Geometrical limits

7.1 Counterpart rack tooth profile

7.2 Machining allowance

7.3 Deviations in tooth thickness

7.4 Generating profile shift, generating profile shift coefficient

7.5 Generated root diameter

7.6 Usable area of the tooth flank, tip and root form diameter

7.7 Undercut

7.8 Overcut

7.9 Minimum tooth thickness at the tip circle of a gear

Bibliography

BS ISO 21771:2007

Gears. Cylindrical involute gears and gear pairs. Concepts and geometry

Published:

28 Sep 2007

Withdrawn:

29 Oct 2024