Abstract
Planetary gears are widely used in automotive and aerospace applications. Due to demands for greater power density, these gearsets often operate at extremely high stress levels. This has made it imperative to account for system level influences in the design stage. One important system level influence is that of the pinion needle bearing support. The clearances in the bearings and their elastic deformation under load lead to shifts in the gear contact patterns. This has implications for durability of the gearsets as well as NVH characteristics of the transmission. In order to have a first-time capable design process, it is important to understand and account for these influences early in the design stage. That is the focus of this paper.
In single pinion planetary gearsets, there is one pinion in each parallel path (sun-pinion-ring). By contrast, in double pinion planetary gearsets there are two pinions in each parallel path (sun-inner pinion-outer pinion-ring). Double pinion gearsets are becoming common in automatic transmissions with more than four forward speeds. It will be shown that the influence of the pinion-bearing tilt is especially significant in double pinion transmissions. Its influence is to create off-centered loading at the sun-inner pinion and outer pinion-ring meshes. If uncorrected, double pinion transmissions will have poor contact pattern at these meshes and will consequently have high contact and root bending stresses.
In this paper, an analytical formulation will be presented that explains the physics behind the greater susceptibility of the double pinion gearset to off-centered loading. The analytical formulation will then be used to conduct sensitivity analysis and show the interactions between basic design variables. A numerical model of the entire gear system and support structure (GSAM model) will then be used to quantify the off-centered loading and calculate the resultant stresses on the gears. Various methodologies to correct for the off-centered loading will be discussed, and the GSAM model will be used to optimize the system. It will be shown that a substantial reduction in bending and contact stresses can be achieved. Finally, experimental contact patterns will be shown to validate the predictions.
The results reported in this paper are significant as they explain the physical reason causing the poor contact pattern and provide for corrections to be made early in the design stage. Also, by following the proposed correction strategies, the stresses are greatly reduced, and consequently the torque carrying capacity of the transmission is increased.
Keywords:Planetary transmission, double pinion planetary, pinion-bearing interaction, contact pattern, gearset durability