Tolerancing of Gears

Geometric deviations from the designed nominal contour of a gear due to tool wear and process variations during manufacturing can affect the tooth root and tooth flank load capacity. This influence must be considered in the design process with the aid of quantitative manufacturing tolerances to ensure that the manufactured gears meet the required specifications. Due to different geometric conditions and special features, a classic distinction is made between the areas of the tooth flank and the tooth root (Ref. 17).

ISO 21771:2007 provides the basis for tolerancing the tooth flank of involute gears (Ref. 33). It contains clear definitions and specifications of the various gear parameters and explains the geometric and mathematical relationships between them (Ref. 33). In addition to the macro geometric parameters of a gear, the various flank modifications are also described and explained (Ref. 33). Based on this, ISO 1328 1:2013 introduces an ISO tolerance classification system, which enables an objective evaluation of manufactured gears based on defined measured variables and permissible tolerance limits (Ref. 7). The gear quality is differentiated with the help of eleven tolerance classes IT1 to IT11, whereby IT1 denotes the lowest tolerance range and thus the highest quality and IT11 the lowest manufacturing quality with the largest possible tolerance range (Ref. 7). The application guidelines ISO/TR 10064 1:2019 and VDI/VDE 2612 supplement the IT classification system with information on measurement methods, measuring devices and the evaluation and interpretation of the measurement results [34,35]. The standardization of the flank tolerance creates a common basis for gear manufacturers and gear buyers, which enables a uniform conformity assessment of gearing in the area of the tooth flank (Ref. 7).

For the area of the tooth root, no standardized assessment basis or IT classification system has yet been created (Refs. 8,11). Due to the lack of a standard, only the tooth root diameter is usually tolerated in production drawings of gears and sometimes supplemented by the specification of a minimum tooth root radius (Refs. 8,11). On the one hand, this gives gear manufacturers great freedom in designing and optimizing the tooth root contour to increase the tooth bending strength (Ref. 8). However, this freedom also bears the risk that the tooth root contour is neglected during quality control and that unwanted tooth root fractures occur in the application due to manufacturing deviations (Ref. 8).