Review of Project-Based Learning in a Junior Level Mechanical

Engineering Course

by Hodge Jenkins

Mercer University School of Engineering

“Elements of Machine Design” Course

• Junior level, 2nd semester• Last required solid mechanics course. • Transition course for professional practice

in machine design • Prepare for senior design project and

working as a graduate engineer• Uses project work as a real an open-

ended design experience

Elements of Machine Design Course Topics

Topics:• Stress analysis for evaluation of static and

fatigue failure of mechanical design elements.

• Design/selection: structures, shafts, springs, power transmission (gearing, belts, chains), keys, couplings, brakes, fasteners, and bearings, etc.

Project Work Component

• Open-ended design project• Significant part of the course grade 20% • Make as real an experience as possible• Changing from 1 semester long project to

two smaller, focused projects

Project Work Learning Outcomes

• Students become accustomed to using industrial sources for common mechanical hardware.

• Students learn to how to make good assumptions in the design process.

• Students decide which parts or components are most critical, and how to analyze them

Typical/Past Projects

• Semester long design single project

• Design limited elements of machine, not complete the entire design

• Submitted as a formal written report at end of semester.

Goals for the Project

• Define/Determine Design Requirements & Specifications (including imposed forces, HP requirements, etc.)

• General assembly type drawing of machine • Design & analyze specific/critical elements, not entire

machine (minimum factor of safety)• Use standard industrial hardware when possible• Summarize, Review, Critique Results• Submit as a formal written report

Design & Selection

• Overall look and size of machine

• Determine : Motor & mounting

• Stress Analysis: Major frame at 2 critical locations,

consider maximum and fatigue loading

• Select fasteners with preload (2 locations) ,

• Springs, Shafts with couplings and keys

• Bearings and drive chain/gears components

• Optionally: Flywheel, Clutch, Welds, Brakes, etc.

Typical projects

Baseball Pitching Machine Specifications:

Be able to project a standard major league baseball between 10 and 90 MPH

from 40-60 feet away, between a height of 15 inches and 65 inches off of the

ground.

Provide a ball reservoir with release mechanism. Machine must have 1 power

source, use 1 or more shafts, bearings, keys, gears, couplings, springs and

fasteners.

Electric powered, 120 Vac.

Go-cart Specifications:

Go from 0-30 mph in under 1 minute with a person weighing 250 pounds.

Safely and securly transport a person weighing 250 pounds.

Be able to maintain speed up a 10% gradient.

Electric powered or gasoline powered

Curb Weight max. 500 lbs.

Maximum Speed (Approx.) 25 mph

Go up a 25% grade at 10 miles per hour

Survive a 6” jump (Suspension system required)

Typical projects

Self-Propelled Lawn Mower

1. Must be self-propelled, with clutch or other disengagement

2. 24” wide cut of 3” high (above blade) Bermuda grass or fescue grass

3. May be electric powered or gasoline powered

4. Must move at 3 mph while going up a 15% incline, while cutting grass

Robot

Use articulating, SCARA, or Cartesian design.

Use electric power, remote to joints (gear, cables, chains or belts).

Maximum payload of 10 pounds.

Min. Work space (L x W x H) of 3ft x 3ft x 3 ft

Design for a maximum speed of 5 inches per second.

Typical projects

Adult Torsion Catapult (or trebuchet) Class (18 and older)

1. Pumpkins must weigh between 8 & 10 pounds.

2. Pumpkin must leave the machine intact.

3. No part of the machine shall cross the firing line.

4. No "wadding" (including bean chaff, straw, foam, metal, or any other object, or

foreign matter).

5. Machine shall consist of torsion springs, or cords that once wrapped around

its axle (pivot point) will create a stored energy.

6. Motorized winches and/ or other cranking devices should cock the machine.

Electric Wheel Chair/Disability Scooter

Transport a 350 pound person

Arm supports

Suspension system required

Minimum of three wheels

Electric powered

Go up a 15% grade at 10 miles per hour

Typical projects

Wheelchair+person stair lift

1. Must be electric powered (115 Vac or 230 Vac)

2. Lift a 250 lb person + wheel chair

4. Must climb up conventional straight house stair case

Outboard motor

Transport a 350 pound person in an average bass-boat

Electric powered or gasoline powered

Maximum Speed (Approx.) 20 mph

Cardboard Compactor or Aluminum Can Crusher

Crush a volume of 24”x24”x12”(LxWxH) of loose flat cardboard or aluminum cans

Electric powered or hand powered

Typical projects

3-Wheeled Vehicle

Safely and securely transport a person weighing 250 pounds.

Accelerate from 0-30 mph in under 30-seconds with a person weighing 250 pounds.

Top speed of at least 60 mph

Be able to maintain speed up a 5% gradient.

Electric (battery) or gasoline powered.

Operate on secondary roads; consider potholes and bumps.

8-year life

Automatic Potato Launcher

Design a mechanical launcher for potatoes, up to 2 lbs

The machine must automatically load and launch at least 5 spuds (in succession, one at

a time)

Range should be a minimum of 50 yards.

Hand crank, electric, or gasoline powered

No chemicals or explosions.

Group or Individual Projects: Class Size Matters

• It is desirable for each student to have a complete and

personal design project experience.

• Beyond ~20 students individual, open-ended projects

become unwieldy for the student and instructor.

• At 40 students or larger class size, it is impractical to

implement as an instructor

Two Short Duration Projects

Short Duration: 2-3 weeks

Focused, Limited Content in each project:

Retain some open-endedness

Cover same topical areas

Similar format

Less time to grade

1. Support design and fastener specification for lawn

mower seat

2. Gear box for a wind turbine

Mower Seat Support Project

1. Design a Lawn Mower Seat Mounting Bracket to mount the seat to a ¼-inch 1020

steel decking platform of a riding mower.

2. Select an appropriate seat height based on anthropomorphic data

3. Design for a person weighing 250 pounds will be sitting in the seat.

4. Design for 10 years use, or infinite life, with minimum safety factor of 2

5. Seat must be bolted to the bracket.

6. Keep the weight of the bracket as low as possible.

7. Select the attachment bolts and specify the bolt preload.

8. Consider the maximum acceleration of the mower to be +/-25 ft/s2.

9. Consider incline lawns of +/-15-degrees

10.Develop mean and alternating forces that the seat with a person will transmit.

Mower Seat Support Project

1) Specifications defined into loadings

2) Create solid models and support drawings

3) General drawing of seat and support showing overall dimensions

4) Determine loading (mean and alternating forces and moments).

5) Free body diagrams of bracket. Two 2-D FBDs.

6) Stress Analysis of frame 2 most critical locations using FEA, consider maximum

and fatigue loading.

7) Select fasteners (2 different locations/loadings)

8) Specify preloads, determine factors of safety for bracket, bolts, and joint

separation

Wind Tubine Gear Box Project

• Design a gearbox for a small (~10 kW) wind turbine .

• Specify the bearings, spur gears and shafts required to

transmit power from the shaft of the turbine blades

(rotating slowly) to an electromechanical generator

(requiring faster rotation). gear ratio of 14:1 to 15:1

• Determine the size (LxWxH) of the gearbox housing

and shaft positions

• Additional specifications provided

Wind Tubine Gear Box Project

• Using the same report format students will:

• Create overall assembly drawing that shows

each component in place

• Gearing Specification & Selection

• Determine the forces on the gears to find the bearing loads

• Specify commercial bearing & shafting for a 10 year life

Comparisons of Student Work

Student performances remain similar, with consistent

student perform (2016 first year of 2 small projects).

Average of All Project Work and Average of Final Project Submission

Conclusions

• Data are preliminary and may not be statistically significant.

• Two smaller projects found to be more complete and correct,

compared to a single larger project

• Students claimed less overwhelmed with the project work, yet

completed very similar design tasks to a single larger project.

• The two projects were also more easily graded, less variation

• First project provided feedback for the second project.

• Anticipate this approach will continue to be implemented

Questions?

Thank you for listening.