DIN 21773:2014-08

The tooth thickness and its deviations are fundamental variables for the determination of the backlash of the wheel pair. DIN 3960:1987, 3.8 "Test dimensions for the tooth thickness" defined practical, indirect measuring methods the measured values of which represent the tooth thicknesses. These were included in informative Annex A of ISO 21771:2007, designated there as "nominal tooth thickness system" (Germany). This Annex also provides calculation equations in accordance to the "functional tooth thickness system" (USA). The tooth thickness test dimensions are calculated as a function of the measuring method applied and the allowable values for the individual deviations (runout, accumulated pitch, etcetera) of the gears. Thus, a direct correlation between the specified functional tooth thickness and the backlash shall be established. Due to the mixing of both systems only in the informative Annex of ISO 21777 it cannot replace the normative clause of DIN 3960. There is a serious risk to use wrong equations or quantities. Therefore, it is not recommended to use informative Annex A of DIN ISO 21771 in the present form. Instead of this annex, the present standard DIN 21773 includes the definitions of tooth thickness test dimensions in accordance with the nominal tooth thickness system. The thereby recommended approach basically corresponds to the specifications so far specified in DIN 3960:1987. For application of the functional tooth thickness system see English Annex A of ISO 21777:2007. Previous editions: DIN 3960:1987-03; 1980-07; 1976-10.