Experimental Approach for Solving Side Band Noise of Miniature Planetary Gearbox

Vibration and noise spectra from miniature planetary transmissions commonly exhibit components at distinct frequencies around the gear mesh (tooth passing) frequency and its higher harmonics, called modulation sidebands.

They constitute a significant portion of the vibration and noise generated by the transmission. Sidebands have influence on noise quality measures as well as having great potential to provide valuable clues regarding gear tooth combinations, phasing of planets, manufacturing errors like eccentricity, run out present in the gear set, planet pin position error, etc. First critical step in designing and controlling the sideband activity of a planetary transmission is to develop a compressive approach to analyze modulation sidebands as well as describing the mechanisms causing sidebands. The paper describes the investigation of side bands, correlation with test results and solution to minimize the noise.

INTRODUCTION

Today, motion solutions through miniature motor and gearbox are used in several applications like medical, industrial, robotics, aerospace, etc. Most of these applications use planetary gearboxes because of its high power-density ratio but increases the complexity in the system and may contributes the problems like noise. Some of the noise related issues arises because of the presence of side bands. Since these products are used by humans during their working environment, the critical aspects of noise and vibration needs to be addressed. Asymmetric sidebands are frequently observed in the planetary giving noise problems and are different from typical sidebands that are symmetrical around the gear mesh order (or mesh frequency). The asymmetric sidebands are found to be caused by phase differences between the different mesh points. The sideband’s locations can be calculated with knowledge of the gear set parameters. The positions of the meshing gears affect the gear noise in various ways. One of them is to generate sidebands (caused by the modulation) around the meshing frequencies or mesh orders, whenever the gear sets have run out, pitch errors, planet pin position error and/or imbalance [1,2].

There are three types of modulations: amplitude modulation (AM) frequency modulation (FM) or phase modulation (PM). Randall described AM and FM in gear noise measurement [3]. AM creates two symmetrical sidebands around the mesh order.

(centre frequency), one on each side, which are generally called as lower and upper sidebands. FM or PM creates an effectively infinite number of sidebands. When several pairs of gears are meshing at the same time, such as in planetary gear sets used widely in planetary transmissions, a phenomenon has frequently been observed: the sidebands are no longer symmetric, and they appear on only one side of the mesh order. Moreover, the noise amplitude at the mesh order is significantly lower than the sideband(s) or even close to zero. McFadden and Smith provided a mathematical explanation [4] suggesting that the different phase relations between gear meshing points cause the asymmetric sidebands. The different phase relations between gear meshing points cause the asymmetric sidebands. In a typical planetary gear set, every mesh point generates noise. Though each noise has the same frequency and amplitude, their phases are not identical due to the mesh timing difference between the mesh points.

In current paper an experimental approach to solve planetary side band noise is presented through NVH measurement as well as tear down analysis techniques. The paper highlights the Best of Best (BOB) and Worst of Worst (WOW) approach to compare 5 Best and 5 Worst samples by doing noise and vibration measurement. The noise and vibration signature highlight the presence of side band noise in WOW samples which are then taken forward for tear down analysis to identify the root cause of the side band noise.

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