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Name of NGM Educator:

Alfred Gates, Ph.D., Engineering Department Head
Central Connecticut State University

Name of Host Company:

FuelCell Energy

Grade Level:

College Juniors and Seniors

Student Work Types:

1. Seminar and case study

Task Abstract:

Predict the reduction in strength, % relaxation, of springs over time at high temperatures subject to different initial loads using experimental material testing data. The results of the % relaxation for different materials and the material cost will be used to determine the cheapest and most effective spring combinations to provide a sustained force over a period of time at the operating temperature of a Solid Oxide Fuel Cell.

Task Objectives:

Students will learn how to use compression spring experiential data, material fatigue rupture data and curve fitting techniques to estimate the % reduction in spring material strength over time based on temperature and initial corrected shear stress for springs with a spring index close to 6. Using this data along with equations students will be able to design springs for high temperature applications.

Esssential Understandings/Questions:

• Solve the maximum shear stress equation for compression spring.
• What is the spring index.
• What is nested springs.
• How is % relaxation curve fit.
• How is % relaxation subject to different initial stress estimated using material rupture data.
• Develop a spring spread sheet that provides the spring force, stiffness, and deflection as a function of corrected shear stress, number of coils, wire diameter, outer diameter, material, and temperature.
• Used the estimated % relaxation to solve an initial stress followed by designing a spring with a minimum deflection of ½ inch.

Task Description:

Students will;

• Develop a spread sheet to design a spring.
• Curve fit experimental data for compression spring for % relaxation.
• Develop a multiplying factor using material rupture data to provide final % relaxation where compression spring data is not available.
• Use the final % relaxation compression spring data to determine the maximum corrected shear stress for a given operating temperature.
• Using the operating temperature, maximum corrected shear stress design a spring to provide a required force and deflection.
• Perform this task for different materials to determine the lowest cost solution.

Resources Required:

• Microsoft spread sheet software
• Spring test data sheet
• Material tensile test data sheets

Prior Learning Required:

• Machine Design with perquisite of Strength of Materials
• Material Analysis 

classroom

Individual or Group Work: 

Individual

Educator Comments:

Designing springs for applications with Solid Oxide Fuel Cells provided a learning experience dealing with limited information and making engineering judgments with experimental data. Students will gain from my experience an unpublished method of designing springs for high temperature applications. They will understand that material loses a percentage of strength over time, % relaxation or % reduction in strength, when subject to high temperatures. The students will also understand that some strength reduction is acceptable but that there will be a limit on the maximum allowed reduction in strength. Students will learn how to set up a spread sheet using compression spring data provided in a Machine Design text book providing a particular desired output. Finally the students will be able to apply this to different materials to determine the lowest cost springs for a particular application. This procedure can be applied to designing other structural load bearing members. The requirements of applying a similar procedure are having the following information; engineering equations that represent a maximum stress state, experimental data from structural test and material tensile test data.