Project Description

Gear train and analysis project completed for Mechanical Engineering Design class. The project involved a complete design and analysis of a jet engine accessory gearbox, driving four critical aircraft systems, using AGMA gear standards, fatigue analysis, and material selection for 10^8 cycle service life. 

The Problem

Modern jet engines contain a network of accessory systems to keep the aircraft flying. This project focuses on designing the gear train of the accessory gearbox mounted below the engine. The project requires designing gears to deliver the required speed and torque to each of the accessories. Optimizing weight saving and reliability. 

The Approach

This project was completed with a seven person team, collaborating early before diving up the different sections based on each member's strength. The process first involved gear and shaft sizing to find an arrangement of 14 gears across 10 shafts to fit within the gearbox housing. This was verified using a CAD model seen on the left. Using this configuration it was confirmed that all four accessories received the correct speed and torque required, done by going back and forth to change the gears pitch diameter. After being confirmed, the team split to focus on different sections of the project, including gear bending, contact stress analysis, bevel gear analysis, shaft loading, shear/moment diagrams, key set design, material selection, and fatigue life. Throughout the project, an excel based calculator was used to manage all the independent calculations, allowing team members to input their own values. The group collaborated often to compare results and ensure the overall system stayed below the 260lbf weight limit. 

My Contribution

I was specifically responsible for the material selection and fatigue life analysis. Analysis determined gear 5 on the hydraulic pump shaft to have the highest torque of 900.36 in lbf. Using the AGMA bending stress requirements and Brinell hardness targets, a grade 2 carburized and hardened steel was selected for the material due to its bending strength of 55000 psi, contact strength of 195000 psi, and elastic coefficient of 2300 psi. The drive shaft material was chosen to be 4140 steel, quenched and tempered at 400of, offering a yield and tensile strength of 238 ksi and 257 ksi respectively. Strong enough to handle the bending at torsional loads at critical locations, but keeping the total system weight in check.

The fatigue analysis was conducted on each accessory shaft at three critical locations, bearing shoulders, keyways, and grooves. At each location the endurance limit was calculated accounting for surface finish, size, reliability, and load modification factors. Three fatigue criteria where then applied, including DE Goodman, DE Gerber, and Von Mises. A minimum fatigue factor of safety of 1.1 was required for all locations, and verified to have been met with these calculations. Finally, an S-N curve was constructed for the drive shat at its critical location with a combined maximum bending moment of 1475in lbf. Ensuring fatigue life to the required 10^8 cycles with 98% reliability. 

What I learned

Overall this project proved valuable in learning the complexity in designing a gear box to aerospace standards. What struck me the most is how every decision cascades, whether it be changing a gear's pitch diameter or material, it causes changes across other analysis. Managing these interdependencies across 14 gears and 10 shafts proved very difficult and gave me a much deeper appreciation for systems-level mechanical design, and why parametric tools are essential for engineering workflows.