Planetary Gear Bearing Geometry for Lubricating Film Integrity
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Solution Overview
Problem
Planetary gears in geared turbofan engines face significant centrifugal forces and torques, leading to deformation and excessive lubricating film pressure, which can result in metal-to-metal contact between the planetary bearing pin and gear, causing functionality issues.
Innovation Solution
The design incorporates a recess on the axial end faces of the planet wheel and a crowned contact surface on the planetary plain bearing pins, allowing for flexible deformation and reducing lubricating film pressure, thereby preventing metal-to-metal contact and optimizing the plain bearing functionality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the planetary bearing pin is fixed at its ends in carrier plates, then the planetary gear unit provides stable support and torque transmission, but the planetary bearing pin is subjected to bending forces that cause excessive lubricating film pressure at the ends and risk of metal-to-metal contact
Solution Approach 1:
The planetary bearing pin is given a crowned shape with a convex curvature in the axial direction, causing the outer diameter to decrease towards the axial ends. This curved geometry redistributes the contact pressure along the bearing surface, preventing excessive pressure concentration at the ends and maintaining adequate lubricating film thickness throughout the bearing, thereby eliminating metal-to-metal contact while preserving structural stability
Solution Approach 2:
The outer diameter of the planetary bearing pin is varied along its axial length, being largest at the middle and decreasing towards the ends. This parameter change in the bearing pin geometry modifies the pressure distribution in the lubricating film, shifting the maximum pressure away from the axial ends to the middle region, thus preventing bearing failure while maintaining load-bearing capacity
2Strength
If the planet gear is made with sufficient mass and stiffness, then it can withstand high centrifugal forces and torques, but the bearing load increases and the risk of metal-to-metal contact persists due to rigid deformation behavior
Solution Approach 1:
The crowned contact surface of the planetary bearing pin creates a curved bearing interface that accommodates the deformation of the planet gear under load. The curvature allows the bearing surfaces to remain in contact while distributing pressure evenly, preventing the rigid gear from causing stress concentrations that would lead to metal-to-metal contact
Solution Approach 2:
By varying the outer diameter of the bearing pin along its axial length, the pressure distribution is optimized to match the deformation characteristics of the planet gear. This parameter modification ensures that the bearing can handle the flexible deformation of the gear without losing lubrication effectiveness
3Reliability
If a crowned contact surface is provided on the planetary bearing pin, then the lubricating film pressure is reduced and metal-to-metal contact is prevented, but the manufacturing complexity increases
Solution Approach 1:
The crowned shape of the planetary bearing pin, while geometrically more complex than a cylindrical form, can be manufactured using standard machining processes such as turning with a form tool or CNC programming. The curvature is simple and continuous, avoiding the need for complex multi-step manufacturing procedures, thus the increase in manufacturing complexity is minimal compared to the significant gain in bearing reliability
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design reduces the risk of metal-to-metal contact, maintains lubricating film thickness, and decreases the mass of the planet wheel, leading to a more efficient and reliable planetary gear system with improved load-bearing behavior.
Implementation Method 1
The planetary bearing pin and the planet gear form a lubricated plain bearing, wherein the adjacent contact surfaces of the plain bearing are separated from each other by a bearing gap
Implementation Method 2
Planetary gears are subjected to very high centrifugal forces and torques, which can deform the planetary bearing pin and the planet gear and affect the lubricating film
Data Source
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AI summary
The invention relates to a planetary gear (100) comprising: a sun gear (3); a plurality of planet gears (4); a ring gear (5) with which the plurality of planet gears (4) engage; and a plurality of planet bearing pins (6), wherein a planet bearing pin (6) is arranged in each planet gear (4) and the planet bearing pin (6) and the planet gear (4) form a lubricated bearing.It is provided that each planet gear (4) forms a recess (401) on its axially front end face (42) and/or on its axially rear end face (41), which extends from the end face (41, 42) within the planet gear (4), and that the planet sliding bearing pins (6) each form a crown on their contact surface (60) in the sense that their outer diameter (e) decreases from a maximum outer diameter (D) towards at least one axial end (65, 66) of the contact surface (60) and has a minimum at the axial end (65, 66).