BS ISO 21771:2007

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 flan...