expansion effects. Such gears are very expensive to manufacture. The medium accuracy grades common for automotive applications set tolerances within 10 to 20 microns. The adequate accuracy for gears used in electric vehicles is around ISO 1328 Grade 6, but high-speed gears rated for speeds over 20,000 rpm have higher quality requirements. The typical specification may list a few dozen parameters for each gear. There is always a limit on how accurate you want to go to stay economical.
initiation points for failures; gears manufactured of clean steels being able to carry higher loads.
Assuming that gears have been machined with the desired accuracy – which is normally accomplished by conventional grinding – additional surface finishing techniques can be applied in order to further optimise the surface roughness and waviness profiles. These include a variety of abrasive and non-abrasive processes, such as shaving, lapping, honing, abrasive flow machining, turbo-abrasive machining, stream finishing, accelerated surface finishing, electropolishing, burnishing. Recently developed mechanochemical surface finishing methods such as Triboconditioning® can be used as the final finishing operation bringing about a triad of effects: (i) surface roughness profile optimisation, (ii) compressive stress buildup, and (iii) tribofilm priming, which greatly improves the tribological and NVH behaviour of gears [4].
Figure 3: Common gear errors and their implications for the transmission performance [3].
A good fit of gears in the gearbox is equally important. The typical backlash, i.e. the gap in between meshed gear teeth, in the final assembly is usually around 0.1-0.2 mm. Specifying a lower backlash value improves noise characteristics but it puts higher requirements on gear and assembly quality and compromises adaptivity to distortions. Zero backlash cannot be achieved when using traditional gears. The so-called zero backlash precision reduction gearboxes use pre-loading to eliminate the backlash.
Some minor surface imperfections can be remedied during the running-in stage. By using gear oil with adequate viscosity and high content of sulfur, one can design workable solutions even for low quality gears. Sulphurised additives help prevent scuffing and provides chemical polishing. Unfortunately, such oils are not suitable for electrified gearboxes because of copper corrosivity. On the other hand, by investing into better gear quality, one can minimise dependency of anti-scuff additives.
The ability of gears to withstand desired loads is critical. Different gear rating methods are used to help engineers make the right choice: ISO6336, ISO10300, American Gear Manufacturers Association (AGMA) 2001, AGMA 6011, API 613. Material cleanliness is an important consideration, since inclusions can be the
Figure 4: Examples of mass-finishing platforms suitable for carrying out mechanochemical surface finishing.
www.tribonex.com
www.kth.se www.nuspec-oil.com
References: (1) S. Brauer, High Speed Electric Vehicle Transmission,
M.Sc. thesis, Karlstad University 2017.
(2) M. Jackson, Test Methods for Evaluating Electrified Vehicle Fluids, Lube-Tech No 141, 2022.
(3) V. Kharka, N.K. Jain, K. Gupta, Predictive modelling and parametric optimization of minimum quantity lubrication– assisted hobbing process. Int J Adv Manuf Technol 109, 1681–1694 (2020).
(4) Linus Everlid, Martin Bengtsson, Morteza Najjari, Florian Reinle, Andreas Storz, Boris Zhmud, Improving the Tribological and NHV Behavior of Gears by Mechanochemical Surface Finishing, VDI Int. Conf. Gears, Garching, September 12-14, 2022.
LUBE MAGAZINE NO.173 FEBRUARY 2023
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