Planet Gear Clearance Design for Takeoff Load Reliability
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Solution Overview
Problem
The existing gas turbine engines face challenges in managing loads on the power gear box to ensure safe and reliable operation, particularly during high-load conditions like takeoff, where unevenly distributed loads on journal bearings can lead to metal-to-metal contact and potential gearbox failure.
Innovation Solution
The implementation of an epicyclic gear train with a specific design that includes a sun gear, ring gear, carrier, and multiple planet gears, where the planet gear rim is designed with a bending stress neutral axis radius to maintain a minimum clearance between the planet gear and the bearing pin, even under high loads. This design utilizes a pin clearance parameter (PCP) to determine optimal sizing and clearance for the planet gear, ensuring minimal metal-to-metal contact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the planet gear is designed with standard clearance, then the gearbox structure is simple, but under high-load conditions metal-to-metal contact occurs between the planet gear and bearing pin
Solution Approach 1:
The patent applies parameter changes by modifying the planet gear design parameters, specifically introducing the bending stress neutral axis radius (rn) as a critical design parameter. The clearance is calculated using the formula cr = rn + 0.002 - (rp + 0.002), where rn is the bending stress neutral axis radius and rp is the bearing pin radius. This parameter-based approach ensures adequate clearance under high-load conditions while maintaining manufacturing feasibility.
Solution Approach 2:
The patent implements preliminary action by pre-calculating and designing the planet gear dimensions before operation, specifically determining the bending stress neutral axis radius and using it to establish the minimum clearance. This preliminary design approach prevents metal-to-metal contact during high-load operations like takeoff, rather than reacting to wear or contact after the fact.
2Object-affected harmful factors
If the clearance between planet gear and bearing pin is increased, then metal-to-metal contact is reduced, but the lubrication film stability deteriorates
Solution Approach 1:
The patent resolves this contradiction through precise parameter optimization. The clearance cr is calculated as cr = rn + 0.002 - (rp + 0.002), which ensures the clearance is sufficient to prevent metal-to-metal contact during high-load conditions while remaining small enough to maintain lubrication film stability. The inclusion of the 0.002 inch offset in the calculation accounts for manufacturing tolerances and deformation.
Solution Approach 2:
The patent substitutes the traditional approach of relying solely on fluid film lubrication with a hybrid approach that combines geometric clearance design (mechanical) with lubrication. By pre-establishing the clearance based on the bending stress neutral axis radius, the design ensures mechanical separation during high-load conditions while maintaining adequate space for lubrication film formation during normal operation.
3Strength
If the planet gear rim is designed to accommodate high loads, then the gear can handle takeoff conditions, but deformation under load causes clearance reduction and potential contact
Solution Approach 1:
The patent addresses this contradiction by introducing the bending stress neutral axis radius (rn) as the key design parameter. This parameter inherently accounts for the gear's structural response to load. The clearance formula cr = rn + 0.002 - (rp + 0.002) ensures that even when the gear deforms under high-load conditions, the clearance is sufficient to prevent metal-to-metal contact, while still allowing the gear to be designed for high load capacity.
Solution Approach 2:
The patent implements preliminary action by calculating and establishing the appropriate clearance based on the bending stress neutral axis radius before the gear undergoes operational loading. This pre-established clearance accounts for expected deformation under high-load conditions, ensuring that even when the gear rim deforms during takeoff or other high-stress operations, the clearance is maintained sufficiently to prevent contact.
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 maintains a minimum clearance during high-load conditions, reducing the likelihood of metal-to-metal contact and associated issues like friction and heat generation, thereby enhancing the reliability and safety of the power gear box operation.
Implementation Method 1
The fluid film provides lubrication to allow the gear rim to rotate around the stationary pin
Implementation Method 2
the planet gear rim deforms when a radial component force, a pinch component force, a tangential component force, and a centrifugal component force are applied to the planet gear
Data Source
AI summary
An epicyclic power gear box for a gas turbine engine delivers torque from a low speed shaft to the primary fan. The gear box includes a planet gear and a bearing pin for the planet gear, sun gear and ring gear. The planet gear and bearing pin are arranged so that a clearance between the two is maintained during a takeoff condition for the gas turbine engine.