BS ISO 17485:2006 - Page 2

Bevel gears. ISO system of accuracy. | Status: Confirmed, Current

1 Scope This International Standard establishes a classification system that can be used to communicate geometrical accuracy specifications of unassembled bevel gears, hypoid gears, and gear pairs. It defines gear tooth accuracy terms and specifies the structure of the gear accuracy grade system and allowable values. This International Standard provides the gear manufacturer and the gear buyer with a mutually advantageous reference for uniform tolerances. Ten accuracy grades are defined, numbered 2 to 11 in order of decreasing precision. Equations for tolerances and their ranges of validity are provided in 5.4 for the defined accuracy of gearing. In general, these tolerances cover the following ranges: 1,0 mm ≤ mmn ≤ 50 mm 5 ≤ z ≤ 400 5 mm ≤ dT ≤ 2 500 mm where d T is the tolerance diameter; m mn is the mean normal module; z is the number of teeth. See Clause 6 for required and optional measuring methods. As tolerances are calculated from the actual dimensions of a bevel gear, tolerance tables are not provided. In order to provide an overview, example values of tolerances and graphs are given in Annex A. This International Standard does not apply to enclosed gear unit assemblies, including speed reducers or increasers, gear motors, shaft mounted reducers, high speed units, or other enclosed gear units manufactured for a given power, speed, ratio or application. Gear design is beyond the scope of this International Standard. The use of the accuracy grades for the determination of gear performance requires extensive experience with specific applications. Therefore, the users of this International Standard are cautioned against the direct application of tolerance values to a projected performance of unassembled (loose) gears when they are assembled. Tolerance values for gears outside the limits stated in this International Standard will need to be established by determining the specific application requirements. This could require the setting of a tolerance other than that calculated by the formulas in this International Standard.