Development and Evolution of a New Mechanical Design Laboratory Course

A new sophomore-level mechanical design laboratory course was developed two years ago at Johns Hopkins University to support a required lecture course entitled Mechanics Based Design. The laboratory course was created because students requested more instruction about machine components and they desired additional hands-on design experiences. The laboratory course is structured into three components which build on each other throughout the semester. Lectures, given every one to two weeks, introduce the students to various machine components, terminology, standards, and design tools and methodologies. Laboratories, also scheduled every one to two weeks, provide the students with opportunities to apply the lecture material to real machine components and systems and to develop practical skills in design and machining. Each laboratory includes three separate activities for the students to perform, and almost all of the laboratory equipment was designed and constructed in-house specifically for the course. The third component of the course is a design project, which is assigned at the beginning of the semester and requires the students to integrate what they learn from the weekly lectures and laboratories, in addition to the material they learn in the Mechanics Based Design lecture course. The students work in small teams to design and build a mechanical device to meet a set of performance and budget specifications. The devices are tested at the end of the semester and the students submit a design report for evaluation. The mechanical design laboratory course was added to the curriculum in the Spring of 2012 and included five lecture topics and five laboratories. The subjects of these lectures and laboratories included screws and threaded fasteners, bearings, gears, pressure vessels, fits and tolerances, finite element modeling, and mechanical failure. Additional lectures and laboratories were added in 2013, including two laboratories at the start of the semester in which the students learn how to operate a milling machine and a lathe. Subsequent laboratories require the students to use one of these machines to fabricate a component of their design for a specific activity. Additional changes were made for the 2014 course offering, including a new laboratory on belt and chain drives, and a new design project. Background A new one-credit mechanical design laboratory course was developed two years ago at Johns Hopkins University. Prior to this, all sophomore-level mechanical engineering students were required to take a four-credit course during the Spring semester entitled Mechanics Based Design, which included a laboratory component. The students taking this course had some strength-of-materials knowledge from a previous required course. This knowledge included stresses in beams and in circular cylinders under torsion. The Mechanics Based Design course built upon and extended this knowledge with coverage of deflections, three-dimensional stresses and strains, stress concentrations, elastic constitutive laws, yielding, column buckling, basic fracture mechanics, and fatigue failure. The laboratory component, however, involved mostly theoretical modeling and mechanical testing of structures, with little “hands-on” experience for the students. Further, the amount of material that had to be covered in the fourteen-week semester meant that the students had very little exposure to machine components, and no actual design experience. This deficiency, as articulated by the students in their course evaluations and senior exitinterviews and recognized by the mechanical engineering faculty, was addressed in 2012 by splitting the original 4-credit Mechanics Based Design course into one 3-credit Mechanics Based Design lecture course and one, new and separate, 1-credit Mechanics Based Design Laboratory course. The laboratory course was taught by a separate instructor and was designed to support, but be independent of, the lecture course. The new Mechanics Based Design Laboratory course is located in a 1000 ft 2 room next to several other mechanical engineering undergraduate laboratories on campus. A working, but antiquated, hydraulically-driven tension/compression testing machine (MTS Systems Corporation) was already available in the room, but it had a very poor user interface and no dataacquisition system. Despite these shortcomings, the machine was used in 2012 and 2013 for several laboratory activities. In addition to the laboratory space, a startup budget of $30k was provided by the Johns Hopkins University Whiting School of Engineering to develop the laboratory facilities. The laboratory course is composed of three separate components which build on each other throughout the semester: lectures, guided laboratory activities, and a design project. Changes were made to each of these components when the course was taught for the second time in 2013, and additional changes were made for the 2014 course. The lecture topics, laboratory activities, and design projects will be described shortly. It should be noted that the average class size for the course is 60 students, and that most of the students have had no prior machining training (e.g. how to use a milling machine) and have not yet gained any computer-aided design (CAD) skills. It should also be noted that Nagurka and Anton recently published a similar paper on their experiences developing a new junior-level machine design laboratory at Marquette University. 1 It is hoped that the work presented here will provide additional insights for others who may be developing similar laboratories. Lecture topics Lectures are given every one or two weeks and cover the background material needed for the upcoming laboratory activities and for the design project. Each 50-minute lecture includes a mixture of conceptual (e.g. stresses on a gear tooth) and practical (e.g. how to specify a thread size on a drawing) knowledge. Many of the lecture topics build upon material presented in the 3credit lecture course, but it is not critical that the two courses be synchronized. In addition to basic course and safety information, the following topics are covered. Screws and Threaded Fasteners, including: • Terminology • Thread identification conventions • Tap and clearance hole sizes • Relationship between tightening torque and developed axial force • Bolt preload • Thread stripping • Design considerations for threaded fasteners • Shear loading of bolts • Power screws Bearings, including: • Lubrication regimes • Sleeve bearing selection • Journal bearings • Roller element bearings – terminology, kinds, and performance differences • Bearing failure and life calculations • Design considerations for bearings Gears, including: • Types of gears • Gear terminology and definitions • Backlash and interference • Contact ratio • Torques and forces on spur and worm gears • Gear failure • American Gear Manufacturing Association analysis of bending stress, contact stress, and fatigue strength of spur gear teeth. • Gear train design • Worm gear efficiency Belt and chain drives, including: • Standard styles of belts and drive chains • Comparison of belt, chain, and gear drives • Friction drive equations (initial tension, transmitted torque, etc.) • Standard methods of maintaining belt tension • Chain chordal action • Design considerations for belt and chain drives • Friction losses in belt drives Fits and tolerances, including: • Classifications of fits • Hole basis vs. shaft basis • Calculating and indicating tolerances • Relationship between press-fit interference and holding force/torque Shaft whirl, including: • Derivation of inherent critical shaft speed • Effect of added loads on critical shaft speed A lecture is also presented to introduce the students to the finite element method. This lecture does not support any particular lab activity, but is it believed that all mechanical engineering students should be aware of this powerful, and too often improperly used, tool. Possible topics for future lectures include hydraulic and pneumatic systems, linkages, and digital image correlation (DIC) for measuring strain. Guided laboratory activities Laboratories are scheduled every one to two weeks and provide the students with opportunities to apply the lecture material to real machine components and systems and to develop practical skills in design and machining. Five separate student lab sections were offered to students in 2012 and 2013. Six sections were offered in 2014 in order to limit the number of students in each section to no more than 12. Each lab section was scheduled for 1 hour and 20 minutes in 2012. This amount of time was found to be too short to complete all of the activities planned for each lab, so the lab times were increased to 1 hour and 50 minutes in 2013. Students receive a handout at the start of each laboratory which guides them through the various activities they must perform. Each laboratory session includes three activities, and the students rotate through them in groups of 3-4 students. As they work through each activity, they must fill in sections of the handout with measurements, calculations, drawings, short answers, and comments. The completed handouts are returned at the end of the lab to be graded. There are no lab reports, and no homework is assigned. The guided laboratory activities are designed to provide the students with several deliverables, including: • familiarity with various common machine components through hands-on experiments • practical applications of the material presented in the Mechanics Based Design lecture course • appreciation of the limitations of theory • preparation for the senior-level capstone design project course • experiences in decision making, design, and basic machining In order to better provide for the last bullet-point above, a new miniature mill and miniature lathe were added to the laboratory in 2013. The mill is a MicroLux High Precision Heavy Duty