Gearbox Arrangement for Constant Speed Accessories
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Solution Overview
Problem
Gas turbine engine accessories experience significant speed variations due to the varying rotational speeds of the low and high speed spools, which can lead to inefficient operation and power fluctuations in accessories like hydraulic pumps and generators.
Innovation Solution
An accessory drive differential system, specifically an epicyclic gear reduction mechanism, is employed to stabilize the speed of accessories by using a combination of high and low speed input gears to drive an output gear, maintaining a consistent speed range within 10% across different engine power settings, ensuring that the sun gear speed remains within 110% of its idle speed at takeoff conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If accessories are driven directly by tower shafts connected to spools, then the structure is simple, but the accessory speed varies significantly with engine operation
Solution Approach 1:
An accessory drive differential is introduced as an intermediary mechanism between the variable-speed spools and the accessories. This differential receives inputs from both the high-speed spool tower shaft and low-speed spool tower shaft, and outputs a stabilized speed to the accessories, mediating the speed variation problem.
Solution Approach 2:
The system transitions from a static direct-drive configuration to a dynamic differential mechanism that automatically adjusts the accessory speed based on the relative speeds of the two spools. The differential's internal gear mechanism dynamically balances the speed inputs to maintain constant output.
2Device complexity
If a single spool drives the accessories, then the drive system is simple, but the accessory speed varies by significant amount over engine operation
Solution Approach 1:
The system merges the drive inputs from two separate spools (high-speed and low-speed) into a single accessory drive output. By combining the tower shafts through the accessory drive differential, the system achieves speed stability that neither spool could provide alone.
Solution Approach 2:
The system changes the speed parameter stabilization approach by using a differential mechanism that mathematically combines two varying speed inputs to produce a constant speed output. The differential transforms the variable speed parameters of the spools into a stable accessory speed parameter.
3Adaptability or versatility
If accessories operate at varying speeds, then the drive system adapts to engine conditions, but accessory efficiency and reliability decrease
Solution Approach 1:
The accessory drive differential acts as a mediator that decouples the accessory speed from direct spool speed variations. This intermediary mechanism allows the accessories to operate independently of engine power setting changes, maintaining reliable and efficient operation across all engine 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 solution provides near-constant speed to accessories, enhancing their efficiency and reliability by minimizing speed variations, allowing for optimized performance and reduced power consumption across varying engine power levels.
Implementation Method 1
the accessory drive differential is an epicyclic gear reduction. The low speed input gear drives a ring gear in the epicyclic gear reduction in a first direction. The high speed input gear drives a carrier in the epicyclic gear reduction in the first direction. The output gear is a sun gear driven to rotate in the first direction.
Data Source
AI summary
A gas turbine engine includes a high speed turbine and a high speed compressor connected by a high speed shaft to define a high speed spool. A low speed turbine and a low speed compressor are connected by a low speed shaft to define a low speed spool. A high speed tower shaft is driven by the high speed spool and a low speed tower shaft is driven by the low speed spool. The low speed tower shaft drives a low speed input gear in an accessory drive differential. The high speed tower shaft drives a high speed input gear in the accessory drive differential. One of the high speed input gear and the low speed input gear increases an output speed of an output gear, and the other decreases an output speed of the output gear. The output gear drives at least one accessory for the gas turbine engine.