Evaluation of the gear noise reduction potential of geometrically uneven inequidistant gears

Randomizing the periodic excitation is a well-known noise reduction measure, for example in fans and tire patterns. Due to a randomized excitation the tonal components of the generated noise are reduced while broadband noise is increased in amplitude. This less tonal noise with a more broadband character is perceived less annoying. In the presented research this approach is applied to gears - called inequidistant gears. They are characterized by uneven tooth positions and uneven tooth thicknesses. The deviations from regular, perfectly even gears may be up to several millimeters in both position and thickness. In the same way as in fans and tire patterns, the uneven design of inequidistant gears leads to a less tonal and less annoying noise. In this paper the design of inequidistant gears is introduced. Equations are derived to define the uneven geometry of an inequidistant gear wheel and to exactly match a pair of meshing inequidistant gears. The presented approach is valid for spur gears, helical gears, and double helical gears, but only spur gears are considered in this paper. Furthermore, a method to calculate the gear mesh stiffness, which is the main excitation mechanism in gear noise, and a method to calculate the gear mesh forces are presented. Experimental investigations are carried out on a back-to-back gear test bench. The calculated gear mesh force and the experimentally determined sound pressure of a regular, perfectly even spur gear set are compared to those of an equivalent inequidistant spur gear set. The results reveal that inequidistant gears not only reduce the tonality but are - in contrast to uneven designs of fans and tire patterns - capable of reducing the total sound pressure level.