All components of modern vehicle drivetrains are highly optimized to minimize vehicle power consumption, weight, noise and vibration. As part of the drivetrain, the gear transmission is optimized as well. Gear transmission optimization includes defining the best gear arrangement, materials and heat treatment, the latest achievements in gear tribology, surface finish engineering and lubricants, etc. At the same time, gear transmission optimization is restricted by the gear tooth geometry definition method — based on a hundred-year-old gear rack generation concept — where the generating gear rack has a symmetric tooth shape and typically standardized tooth proportions.
Fig. 1. Helical asymmetric gear pair (Courtesy of Höganäs AB)
Just like practically any propulsion gear system, a drivetrain gear transmission is mostly unidirectional, i.e. the primary gear tooth flanks carry a much greater load for a much longer period of time than the opposite coast gear tooth flanks.
The design objective of asymmetric tooth gears (Fig.1.) is to improve the performance of the primary drive flank profiles at the expense of the opposite coast flank profiles, which are unloaded or lightly loaded during a relatively short work period. Asymmetric tooth flank profiles make it possible to simultaneously increase the contact ratio and operating pressure angle of the drive tooth flanks beyond the limits achievable by conventional symmetric tooth gears. The main advantage of asymmetric tooth gears is the drive flank contact stress reduction, which allows for a considerable amplification of power transmission density, an increase in load capacity, and reduced size and weight.
Asymmetric tooth gears are not bound by standards, and the application of Direct Gear Design  allows for optimized tooth geometry to maximize the gear drive performance. The asymmetry factor and drive flank contact ratio optimization maximizes load capacity and minimizes transmission error [1, 2]. The tooth root fillet optimization  guaranties minimal possible stress, providing even stress distribution along root fillet (Fig. 2).
Fig. 2. Root fillet optimization; a – stress isograms, b – stress chart along tooth profile; 1 – tension root stress, 2 – compression root stress
Though nonstandard, the asymmetric tooth gears can be rated by existing AGMA, ISO or other standards . Comparable symmetric tooth gears and conversion factors are defined by FEA. A significant reduction in flank contact and root bending stresses of asymmetric gears are to be traded for improvements in other gear drive performance characteristics: increased load capacity and life, reduced size and weight, higher efficiency and lower noise and Vibration.
The advantages of asymmetric tooth gears for unidirectional drives are analytically and experimentally confirmed when compared to the best traditionally designed symmetric tooth gears .
Articles about asymmetric tooth gears are available at http://www.akgears.com/publications.htm.
- A.L. Kapelevich, Direct Gear Design, CRC Press, 2013.
- A.L. Kapelevich, Y. V. Shekhtman. Analysis and Optimization of Contact Ratio of Asymmetric Gears, Gear Technology, March/April, 2017, 67-71.
- Kapelevich, A. L. and Y. V. Shekhtman. Rating of Asymmetric Tooth Gears, Power Transmission Engineering, April 2016, 40 - 45.
Dr. Alexander L. Kapelevich , is a gear design consultant at AKGears, LLC, (Shoreview, Minnesota, USA). He holds a Master’s Degree in Mechanical Engineering at Moscow Aviation Institute and a PhD Degree in Mechanical Engineering at Moscow State Technical University. Dr. Kapelevich has forty years of gear transmission research and design experience. He began his career working in the Russian Aviation Industry, where he was involved in R&D, software development, testing, and failure investigation for aerospace gear transmissions. Living in the USA since 1994, he developed the Direct Gear Design® methodology for custom gear transmissions, which have been implemented in various fields such as aerospace, automotive, agriculture, defense, robotics, racing, and many others. His specialty is the gear drive architecture, planetary systems, gear tooth geometry optimization, gears with asymmetric teeth, and gear transmission performance maximization. Dr. Kapelevich is an author of the book titled “Direct Gear Design” and many technical articles. He is a member of American Gear Manufacturers Association (AGMA).