Compact Design of High-Contact-Ratio Spur Gears

This study presents a computer simulation for the dynamic design of compact high-contact-ratio spur gear transmissions.

Gears have been used in transmission systems of most vehicles and machinery for many years. The design of a gear set is a fairly difficult task, which involves the satisfaction of many design constraints. To achieve better transmission performance, gear geometry and dynamic load have been studied. To produce more reliable gears, gear fatigue, life criteria, and minimized wear have been analyzed. In general, the most desirable gear set is the smallest one that will perform the required job. However, there have been only a few studies that focus on optimizing gear design with smaller geometry and lighter weight. The objective of this study is to optimize the spur gear design by evaluating contact ratio.

High contact ratio gears have the potential to produce lower dynamic tooth loads and minimum root stress but they can be sensitive to tooth profile errors. The analysis presented in this work was performed by using the NASA gear dynamics code DANST (Dynamic Analysis of Spur Gear Transmissions). In the analysis, the addendum ratio (addendum/diametral pitch) was varied over the range 1.30 to 1.40 to obtain a contact ratio of 2.00 or higher. The constraints of bending stress limit and involute interference provide the main criteria for this investigation. Compact design of high-contact-ratio gears with different gear ratios and pressure angles was investigated. Comparison of compact design between low-contact-ratio and high-contact-ratio gears was conducted. Using the same operating parameters with the involute interference and bending tooth stress constraints, the high-contact-ratio gear sets appear to have much more compact than yhe low-contact-ratio gear sets. This is due to lower transmitted tooth load in the high contact ratio gears, which results in lower tooth bending stress. For compact design of high-contact-ratio gears, a diametral pitch of 6.00 appears to be the best choice for an optimal gear set, regardless of gear ratio and pressure angle variation. Note that the minimum center distance of optimal gear set increases when the gear ratio increases. This is due to the larger driven gear as required by the higher gear ratio. Also, the pressure angle has a significant effect on the optimal gear design. With a higher pressure angle, the optimal gear set tends to be smaller. This is due to the reduction of tooth bending moment when the pressure angle increases. The smaller tooth bending moment results in smaller tooth bending stress.