Planet Gear Interference Fit Eliminates Weld Hardness Loss
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Solution Overview
Problem
In air turbine starter systems, the weld joint between the input and output gear portions of the planet gear can lead to a loss of hardness at the inner peripheral surface, compromising the performance and reliability of the gear.
Innovation Solution
A planet gear design featuring two separate parts with a significant interference fit, where the outer diameter of the shaft is larger than the inner diameter of the second part, eliminating the need for a weld joint and ensuring the parts do not rotate relative to each other, thereby maintaining the hardness of the carburized surface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the input and output gear portions are welded together, then the gear structure is formed, but the hardness at the inner peripheral surface is lost
Solution Approach 1:
The planet gear is divided into two separate parts: an input gear portion and an output gear portion. These parts are connected through an interference fit between a shaft extending from the input gear portion and a cylindrical portion of the output gear portion, eliminating the need for welding and preserving the carburized surface hardness.
Solution Approach 2:
The input gear portion and output gear portion are combined through a precise interference fit connection. The shaft of the input gear portion is interference-fitted into the cylindrical portion of the output gear portion, creating a unified functional assembly without welding, thus maintaining both structural integrity and surface hardness.
2Ease of manufacture
If a weld joint is used to connect gear portions, then the parts are joined, but the precision of the carburized surface is compromised
Solution Approach 1:
The gear is segmented into two separately manufactured portions that are precision-connected through interference fit. This allows the carburized surface on the output gear portion to be manufactured with high precision before assembly, without subsequent welding that would compromise the surface quality.
Solution Approach 2:
The carburized surface is formed on the output gear portion before the interference fit assembly is completed. This preliminary surface treatment ensures that the precision carburized layer is established prior to any joining operation, preventing any potential damage from welding or other post-assembly surface modifications.
3Ease of operation
If the outer diameter of the shaft is close to the inner diameter of the second part, then the fit is tight, but the parts may rotate relative to each other
Solution Approach 1:
The interference fit connection relies on precise control of dimensional parameters. The shaft outer diameter is made slightly larger than the cylindrical portion inner diameter within a specific range (ratio between 1.0005 and 1.0100), creating sufficient friction force to prevent relative rotation while maintaining ease of assembly and disassembly.
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 enhances the reliability and performance of the planet gear by preventing the loss of hardness and eliminating the need for a weld joint, ensuring a strong and durable gear operation without compromising the surface treatment.
Implementation Method 1
A second part is interference fit on the first part, with the second part having a second set of gear teeth. The second part is mounted on the shaft of the first part.
Data Source
AI summary
A planet gear for use in an air turbine starter is formed of a first part having a set of gear teeth at a first axial location. A shaft extends axially away from the first set of gear teeth. A second part is interference fit on the first part, with the second part having a second set of gear teeth. The second part is mounted on the shaft of the first part. An outer diameter of the shaft is selected to be significantly larger than an inner diameter of a cylindrical portion of the second part which is interference fit on the shaft. A ratio of the outer diameter to the inner diameter is between 1.0005 and 1.0100. A planetary gear system, an air turbine starter and a method of installing a planet gear are also disclosed.