Planetary Gearbox Journal Bearing Undercuts for Load Distribution
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Solution Overview
Problem
Aircraft engine gearboxes face challenges in distributing loads efficiently due to the increased length of bearings, which can lead to load concentration at the extremities, potentially causing lubrication film failure and distress under high torque conditions.
Innovation Solution
The design incorporates a planetary gear train with a sun gear, main gears, and ring gears, where the ring gears are symmetrically positioned to distribute the load and include undercuts in the journal bearing to enhance compliance and reduce load concentration, allowing for a more even distribution of torque and rotation speed ratios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If bearing length is increased to increase power to weight ratio, then power to weight ratio is improved, but load concentration at bearing extremities worsens
Solution Approach 1:
The bearing journal incorporates undercuts at its extremities, creating localized variations in the bearing surface geometry. This allows different regions of the bearing to have different functional characteristics - the undercut regions accommodate load concentration while the intermediate regions maintain normal bearing function, thereby resolving the load distribution problem while preserving the increased bearing length needed for high power to weight ratio
Solution Approach 2:
The bearing design changes the geometric parameters of the journal by introducing undercuts - local reductions in radial thickness at the extremities. This parameter modification allows the bearing to accommodate concentrated loads at the ends without compromising the overall structural integrity or requiring reduction in bearing length, thus maintaining the improved power to weight ratio
2Force
If bearing length is increased to handle higher torque, then torque capacity is improved, but lubrication film failure risk worsens
Solution Approach 1:
The undercuts create localized compliance zones at the bearing extremities where the lubrication film can better accommodate high contact pressures and transient load conditions. This local geometric modification ensures that the lubrication film remains stable even under the high torque conditions that the increased bearing length is designed to handle
3Strength
If ring gears are added to distribute load, then load distribution is improved, but device complexity worsens
Solution Approach 1:
The ring gears are integrated with the planet carrier assembly, forming a combined structure that distributes load across multiple contact points. By merging the load distribution function into the existing planetary gear architecture rather than adding completely separate components, the design achieves improved load distribution while minimizing the increase in device complexity
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 configuration reduces the number of moving parts, decreases the oil required for lubrication, minimizes heat generation, and allows for a more compact engine design while maintaining high torque and speed reduction ratios, enabling the gearbox to withstand higher torques and reducing the need for heavier rolling element bearings.
Implementation Method 1
allowing a radially inner face of a journal of the bearing to move toward a radially outer face of the journal by at least partially collapsing undercuts defined by the journal at opposed axial ends thereof
Data Source
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AI summary
An aircraft engine (10) has a gear train (30, 130) having a sun gear (32) and a plurality of planet gear assemblies (36). Each planet gear assembly (36) has a main gear (46) meshed with the sun gear (32), a fore lateral gear (48) and an aft lateral gear (48) disposed on opposite sides of the main gear (46) and rotating therewith. A diameter (52) of the main gear (46) is different than a diameter (56) of the fore and aft lateral gears (48). Each planet gear assembly (36) is rotatably mounted on a bearing (42) for rotation about an axis (A'). The bearing (42) includes a journal (62) defining two undercuts (64a, 64b). A planet carrier (40) rotatably supports the planet gear assemblies (36). Ring gears (54, 154) are meshed with the fore and aft lateral gears (48). One of the sun gear (32), the planet carrier (40), and the ring gears (54, 154) connected to an input, one is connected to an output, and rotation of a remaining one is limited.