The significance of the bevel gear contact pattern
The majority of gears used in power transmission are designed based on general design guidelines and some standardized gear rating method (e.g. DIN, ISO, AGMA). This “conventional design approach” is widely applied, because it provides satisfactory results for more common applications, and involves little effort in the design stage. However, for spiral bevel gears this matter is much more significant, as the relations between tooth contact and the surrounding driveline are both sensitive and complex.
In the conservative design approach the operational behavior of the tooth contact is generally unknown. The design is essentially based on the assumption that the highest stresses occur in the middle of the tooth, and that the majority of the available flank area is well utilized without signs of edge contacts. In practice this may involve positioning the contact pattern ‘conservatively’ – in unloaded condition – at the center of the tooth and using relatively large flank reliefs to prevent hard edge contacts.
The risk of misaligned contact pattern
Properties of the surrounding driveline such as shaft bending, housing deformation and temperature differences often cause significant gear misalignments, which can have a major influence on gear meshing properties such as tooth stresses, noise excitation, lubrication and backlash. Of all gear types, spiral bevel gears are especially prone to misalignments.
Consequently, the tooth stresses are higher and the ability of the gears to withstand different kinds of variations (e.g. overloads) is not optimal. Other mesh properties, such as noise excitation and heat generation, are also typically negatively affected. One of the lesser-known aspects is tooth backlash, which can also be seriously affected by gear misalignments. Maintaining sufficient backlash in all loading conditions is extremely important for proper gear operation.
Application-specific bevel gear design
Significant improvements regarding the performance, reliability and robustness of a bevel gear drive can be achieved by designing the tooth micro-geometry (tooth flank topographies) so that a proper tooth contact is maintained in all loading conditions. This is a relatively simple task, if the behavior of the tooth contact in loaded conditions is known. Such data were previously only attainable through practical experience or costly prototype-testing – trial & error -but nowadays advanced simulation tools provide a reliable alternative.
ATA Gears apply a proven simulation-based design concept, in which the tooth geometry can be optimized as a part of a system-level analysis, taking into account the effects of the entire driveline environment. A complete model of the gearbox is used to predict the actual gear positions in various loading conditions. The misalignments are used as input for tooth mesh simulation based on precise 3D-tooth models. This process gives the designer valuable insight on important tooth mesh properties, such as tooth stress distribution, dynamic behavior, sliding speeds, flank temperatures and tooth backlash. The tooth geometry can then be modified on a case-by-case basis to ensure optimal tooth contact characteristics in all loading conditions.
The benefits of application-specific bevel gear design are obvious:
- Proper position and extent of tooth contact on both flanks
- Lower stresses, noise and heat generation
- Correct amount of operational backlash
The analysis/optimization concept applied has been verified extensively by full-scale experiments in special research projects, as well as in numerous successful customer applications with gear wheel size ranging from 150 mm to over 2000 mm.
No negative effects on costs and availability
Optimization of tooth contact does not increase gear manufacturing costs. Instead, the one-time effort applied in the design stage can reduce the overall costs in the long run, for example through improved reliability (e.g. elimination of a known gear problem), shorter product development time (‘right first time’) and faster assembly/testing processes, not to mention the value of customer satisfaction from improved operational characteristics (e.g. noise, efficiency). Optimization of tooth contact can also be easily applied to existing gear designs, since it usually does not require drawing revision, nor does it affect existing gear rating calculations.
It’s performance and competitiveness
These analysis/optimization methods are not confined to optimizing the location and size of the contact pattern (micro-geometry). Their full potential can be realized by applying them to optimizing the complete gear design, including the macro-geometry (e.g. number of teeth, spiral angle, profile shift etc.). With more accurate information on the tooth contact conditions, the actual precautions against various damage mechanisms can be assessed more precisely than with conventional design methods. If excessive/insufficient risks are observed, the designer can make effective changes to achieve a more “balanced” gear design. Furthermore, the gear design can be optimized to meet certain performance criteria, such as efficiency or noise excitation.