Design of low noise micro geometries for helical gears on the basis of transmission error under load

Gearbox design must satisfy multiple demands that usually involve high power density, low energy consumption and favourable noise emission. Where the layout of loaded gears is concerned, the objectives comprise a high load carrying capacity, low gear losses and low noise mesh design. This has become more important of late since for example engine-generated noise levels that used to mask gear noise are reduced by increasingly electrified drive trains. In order to transmit high loads within the smallest possible space the tooth micro geometry must be modified from pure involute form using so-called standard modifications such as crownings or tip relieves to improve the pressure distribution on the flanks. However, these modifications do not generally grant favourable noise behaviour - it is more often found that these flank forms act to the detriment of the mesh excitation and lead to a conflict of objectives which makes compromises necessary. In this paper a new approach beyond the use of standard modifications is presented that relies on a detailed calculation of the transmission error under load (TE), which can be considered as a measurement of the parametric excitation source. The derived flank micro geometries compensate directly for the TE thus making it possible to ensure the load carrying capacity in a first step and then design a theoretically "optimal" low noise modification in a second subsequent step. Test results with ground specimens on a dynamics test rig show that both TE and acceleration levels are drastically reduced and lead to a superior noise and operating behaviour compared to the also investigated standard modifications. A few challenges associated with this kind of modification are mentioned such as complex manufacturability or compensation for higher mesh orders.