Compliant Planetary Gearbox Bearings for Load Concentration Relief
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Solution Overview
Problem
Conventional aircraft engine gearboxes face challenges with load concentration at the extremities of bearings due to increased length, leading to potential lubrication film failure and distress under high torque conditions, which affects the durability and efficiency of the gearbox.
Innovation Solution
The gear train design incorporates a planetary gear train with two ring gears on opposite sides of the main gears to symmetrically distribute the load, using undercuts in the journal bearing to enhance compliance and reduce stiffness at the axial ends, allowing for the use of plain bearings and optimizing lubrication film distribution through a combination of geometric modifications and ElastoHydroDynamic analysis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the bearing length is increased to increase power to weight ratio, then the bearing can support higher loads, but the load concentrates on the extremities of the bearing causing lubrication film failure
Solution Approach 1:
The patent introduces undercuts at the axial ends of the journal bearing to create non-uniform compliance distribution. The undercuts reduce stiffness locally at the bearing extremities where load concentration occurs, while maintaining adequate stiffness in the central region. This localized modification allows the bearing to distribute load more evenly along its length, preventing lubrication film failure at the extremities while supporting higher overall loads.
2Strength
If the bearing length is increased to support higher torques, then the bearing capacity increases, but the stiffness at axial ends increases causing load concentration
Solution Approach 1:
The patent modifies the geometric parameters of the journal bearing by introducing undercuts at the axial ends. This changes the compliance parameter of the bearing, creating a compliant journal bearing with non-uniform stiffness distribution. The undercuts reduce the effective stiffness at the axial extremities, allowing the bearing to accommodate thermal expansion and manufacturing tolerances while distributing load more evenly, thereby reducing stress concentration despite increased bearing length.
3Force
If conventional journal bearings are used with increased length, then the bearing can handle higher loads, but the lubrication system becomes more complex to maintain film integrity
Solution Approach 1:
The compliant journal bearing with undercuts is designed to self-adjust under load conditions. The non-uniform compliance distribution allows the bearing to automatically accommodate thermal expansion, manufacturing tolerances, and load variations without requiring complex external control systems. This self-adjusting capability simplifies the lubrication system design while maintaining film integrity under high load conditions.
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 effectively reduces load concentration, enhances load sharing, and improves the gearbox's ability to withstand higher torques, enabling the use of plain bearings instead of heavier rolling element bearings, thus increasing the gearbox's durability and efficiency.
Implementation Method 1
a lubrication system (56, 60, 58) hydraulically connected to the gaps
Implementation Method 2
using undercuts in the journal bearing to enhance compliance and reduce stiffness at the axial ends
Implementation Method 3
optimizing lubrication film distribution through a combination of geometric modifications and ElastoHydroDynamic analysis
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.