Gas Turbine Bolt Thread Run-Out Fillet Radius

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Solution Overview

Problem

Bolts in rotating structures of gas turbine engines face high stresses due to weight, torque, and rotation speed, limiting their low-cycle fatigue life, especially in high-stress environments, where traditional alloy selection offers limited improvements.

Innovation Solution

A bolt design featuring a threaded portion with a smooth transition to a shank via a thread run-out fillet with a radius between two and six times the thread root radius, reducing peak stress concentrations and enhancing low-cycle fatigue life.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Weight of moving object

If lightweight alloys are used for the bolt, then the bolt weight is reduced, but the strength and stress resistance are compromised

Engineering Contradiction:

Improvebolt weightVSAvoidbolt strength

Core Design Contradiction:

Weight of moving objectVSStrength

Solution Approach 1:

The patent applies local quality by implementing a thread run-out fillet with a specific radius (2-6 times the thread root radius) at the critical transition zone between the threaded portion and shank. This localized geometric modification optimizes stress distribution precisely where bending stresses peak during rotation, allowing lightweight alloys to maintain adequate strength without requiring overall bolt redesign.

Inventive Principle:

Principle #3Local quality

2Ease of manufacture

If traditional bolt designs are used, then manufacturing is simple, but low-cycle fatigue life is limited under high stress

Engineering Contradiction:

Improvemanufacturing simplicityVSAvoidlow-cycle fatigue life

Core Design Contradiction:

Ease of manufactureVSDuration of action of moving object

Solution Approach 1:

The patent employs spheroidality by replacing the traditional sharp or minimal radius transition at the thread run-out with a fillet having a radius 2-6 times the thread root radius. This curved transition smooths stress concentration points, significantly improving low-cycle fatigue life under high bending stresses while remaining compatible with standard manufacturing processes.

Inventive Principle:

Principle #14Spheroidality (Curvature)

3Speed

If the bolt operates at high rotation speed, then power output increases, but bending stresses on the bolt increase significantly

Engineering Contradiction:

Improverotation speedVSAvoidbending stress

Core Design Contradiction:

SpeedVSStress or pressure

Solution Approach 1:

The patent applies preliminary action by pre-designing the thread run-out fillet geometry with an optimized radius (2-6 times the thread root radius) before the bolt enters service. This preliminary geometric configuration proactively reduces stress concentration at the most vulnerable location, preparing the bolt to withstand high bending stresses that will occur during high-speed rotation without requiring real-time adjustments.

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentUS9175713B2Bolt for gas turbine engine rotor

Publication Date: 2015.11.03 PRATT & WHITNEY CANADA CORP

AI summary

The bolt has, in sequence along a bolt axis: a threaded portion, a thread run-out portion, a shank, and a head, the threaded portion having a thread with a given root radius and a given depth, the thread run-out portion connected to the shank via a thread run-out fillet having a thread run-out fillet radius, the thread run-out fillet radius being between two and six times the thread root radius.