SECME Mousetrap Car Originally prepared by: Brad Nunn BSIE Purdue University Program Manager - Citrix 10/1/2005 UPDATED: September 2011

Today's Topics SECME Overview SECME Overview Performance rules and scoring Performance rules and scoring Component design Component design Construction techniques Construction techniques Prototyping Prototyping Levers and pulleys Levers and pulleys Gears and gear trains Gears and gear trains Calculations Calculations Drawing and Technical report Drawing and Technical report

Performance Rules Refer to Mousetrap Car Construction and Operation Rules Refer to Mousetrap Car Construction and Operation Rules Bail Bail don’t cut or remove or add to itdon’t cut or remove or add to it OK to straightenOK to straighten

Performance Scoring – Do the Math N = w D D * + * + W L T W L T F = N * 100 * 100 N L N L Consider tradeoffs (easy math) W....is the total mass of the completed car in grams. W....is the total mass of the completed car in grams. L....is the car’s longest measurement along one of the three basic dimensions—length, width, or height—in centimeters, measured with the bail extended or retracted, whichever is greater.* L....is the car’s longest measurement along one of the three basic dimensions—length, width, or height—in centimeters, measured with the bail extended or retracted, whichever is greater.* T....is the total time in seconds that the car travels from the starting point to the stopping point. T....is the total time in seconds that the car travels from the starting point to the stopping point. F is Final Performance Score F is Final Performance Score ( middle and high: a normalized score, i.e., best score gets 100 and the other scores are relative( middle and high: a normalized score, i.e., best score gets 100 and the other scores are relative Max team score is 200: Max team score is 200: Performance (100), Design Drawing (50), Technical Report (50) Performance (100), Design Drawing (50), Technical Report (50)

Terminology Potential to kinetic energy transfer Potential to kinetic energy transfer Torque Torque Acceleration Acceleration Speed Speed Momentum Momentum Friction Friction

Desired Outcomes A small car that travels 2500 cm quickly and doesn’t weigh much A small car that travels 2500 cm quickly and doesn’t weigh much A gradual transfer of energy that has just enough torque to establish motion A gradual transfer of energy that has just enough torque to establish motion A sustained transfer of energy that delivers sufficient momentum to cover the distance A sustained transfer of energy that delivers sufficient momentum to cover the distance

Wheel Design Wheel diameter Wheel diameter

Wheel Design Wheel Construction Wheel Construction Rubber bands around wheels for traction Rubber bands around wheels for traction

Axle Design Axle diameter and mechanical advantage Axle diameter and mechanical advantage Simple ratio of diameters Simple ratio of diameters For distance cars use the smallest axle that provides sufficient mechanical advantage to drive a large wheel For distance cars use the smallest axle that provides sufficient mechanical advantage to drive a large wheel Glue at least one drive wheel to axle Glue at least one drive wheel to axle

Two Step Axle At start, use the thick part of the axle for increased torque At start, use the thick part of the axle for increased torque Once rolling, use the thin part of the axle for more distance Once rolling, use the thin part of the axle for more distance

Wheel and Axle Design Minimize friction loss Minimize friction loss Lubrication – silicone or graphite powder – WD-40 not recommended Lubrication – silicone or graphite powder – WD-40 not recommended

Construction Techniques Releasing the drive string from an axle to allow coasting Releasing the drive string from an axle to allow coasting Being able to disconnect drive strings on either end might make it easier to wind a car with a more complex pulley or gear drive Being able to disconnect drive strings on either end might make it easier to wind a car with a more complex pulley or gear drive

Construction Techniques Creating an axle hook on a solid shaft Creating an axle hook on a solid shaft

Construction Techniques Simple, easy to tie knots Simple, easy to tie knots Surgeon’s Loop – useful for making a loop at the end of a stringSurgeon’s Loop – useful for making a loop at the end of a string

Another Axle Hook Plastic wire tiePlastic wire tie

Prepping the Trap Parts of the trap that are OK to removeParts of the trap that are OK to remove Don’t cut the bail!Don’t cut the bail!

Super Glue – Gel Control Safety first! (immediate clean up with soap and water, goof-off, nail polish remover)Safety first! (immediate clean up with soap and water, goof-off, nail polish remover) Gel Control formula isn’t runny – a little goes a long way (and dries faster)Gel Control formula isn’t runny – a little goes a long way (and dries faster)

Making the Frame Align the axle holesAlign the axle holes Not the ends of the side railsNot the ends of the side rails

Prototyping What problems were encountered? What problems were encountered? What solutions were effective? What solutions were effective? What can be done for further improvement? What can be done for further improvement?

Maximizing Axle Rotations Options to control torque, acceleration, speed, and number of rotations Options to control torque, acceleration, speed, and number of rotations LeversLevers PulleysPulleys GearsGears

Use of Levers Length of lever vs. torqueLength of lever vs. torque

Use of Levers Where do they go?Where do they go? Locate the pivot point of the lever as far as possible from the axle to maximize the string that is pulled from the axle Locate the pivot point of the lever as far as possible from the axle to maximize the string that is pulled from the axle Additional lever length that extends beyond the axle reduces torque and only pulls marginally more string Additional lever length that extends beyond the axle reduces torque and only pulls marginally more string Only 50% of the additional lever length will translate into pulling more stringOnly 50% of the additional lever length will translate into pulling more string The additional lever length will potentially extend the overall length of the car by 100% of the additional lever lengthThe additional lever length will potentially extend the overall length of the car by 100% of the additional lever length

Use of Levers Position of lever arm for max torque at startupPosition of lever arm for max torque at startup

Use of Levers Torque (and acceleration) due to use of a leverTorque (and acceleration) due to use of a lever A simple demonstration of levers and torqueA simple demonstration of levers and torque

Use of Levers A good distance carA good distance car

Cars with Levers

Pitsco Doc Fizzix Kits Good Lever based car Good Lever based car Good instructions Good instructions Light weight wood, wheels, axles Light weight wood, wheels, axles Rubber CD/DVD mounts / bushings Rubber CD/DVD mounts / bushings Axle hook Axle hook Axle bushings Axle bushings Kevlar string Kevlar string Doesn’t follow SECME guidelines for cutting the bail – straighten only! Doesn’t follow SECME guidelines for cutting the bail – straighten only!

Car with Pulleys

A simple pulley demonstration…A simple pulley demonstration…

Putting Levers and Pulleys together Design calculationsDesign calculations How big are the wheels? How big are the wheels? How many rotations are needed? How many rotations are needed? What benefit is derived from the pulley? What benefit is derived from the pulley? What size lever to use? What size lever to use?

How Big, How Many? Target 2500 cm = 82 feet (note that the minimum to even record a score is 20 feet)Target 2500 cm = 82 feet (note that the minimum to even record a score is 20 feet) For a 4” wheel, the circumference = 1’For a 4” wheel, the circumference = 1’ need 82 rotations need 82 rotations For a ” axle diameter loaded up with string there is a 0.125” to.25” effective diameter that has a max circumference of.79”For a ” axle diameter loaded up with string there is a 0.125” to.25” effective diameter that has a max circumference of.79” need to pull 82*0.79 = 65” inches of string need to pull 82*0.79 = 65” inches of string 80% Design Margin80% Design Margin 100 rotations from 80 inches of string 100 rotations from 80 inches of string

Levers and pulleys? Target = 80 inches of stringTarget = 80 inches of string 40 inch lever? 40 inch lever? Bigger wheels and smaller lever? Bigger wheels and smaller lever? Add a pulley? Add a pulley? For a 1” diameter pulley, C=3.14”For a 1” diameter pulley, C=3.14” Need 80/3.14 = 25 rotations Need 80/3.14 = 25 rotations For a ” axle dia. with string (0.125” eff. dia.), C=.4”For a ” axle dia. with string (0.125” eff. dia.), C=.4” Need to pull 25*.4 = 10” Need to pull 25*.4 = 10” Consider 80% design marginConsider 80% design margin Mount a 6” lever and locate the pivot point 6“ away from the pulley shaft to pull 12” of string Mount a 6” lever and locate the pivot point 6“ away from the pulley shaft to pull 12” of string

Use of Gears Why are gears generally used?Why are gears generally used? Transmit torque from one shaft to another Transmit torque from one shaft to another Increase or decrease the speed of rotation Increase or decrease the speed of rotation Reverse the direction of rotation Reverse the direction of rotation Why are gears useful in this applicationWhy are gears useful in this application Small Small Lightweight Lightweight Significant multiplications possible Significant multiplications possible Enables unmodified mousetrap bail Enables unmodified mousetrap bail

Gears Gears Gears A simple gear demonstration… A simple gear demonstration…

Typical Spur Gear Nomenclature Nomenclature Spur gear with 40 teeth = 40t gearSpur gear with 40 teeth = 40t gear Having the same size teeth and the same spacing of the teeth allows the gears to mesh properly Having the same size teeth and the same spacing of the teeth allows the gears to mesh properly Ratio of the radii is equal to the ratio of the number of teeth Ratio of the radii is equal to the ratio of the number of teeth

Calculating Gear Ratios For a 8T gear driving a 24T gear, for a movement of one tooth, the 8T gear rotates 1/8 revolutions and the 24T gear rotates 1/24 revolutions For a 8T gear driving a 24T gear, for a movement of one tooth, the 8T gear rotates 1/8 revolutions and the 24T gear rotates 1/24 revolutions Gear ratio Gear ratio 1/8:1/24 = 24:8 = 3:11/8:1/24 = 24:8 = 3:1 What would it be if the 24T gear drives the 8T gear?What would it be if the 24T gear drives the 8T gear? Quick calc: Gear ratio is the inverse of the ratio of the number of gear teeth Quick calc: Gear ratio is the inverse of the ratio of the number of gear teeth 12T drives 6T then ratio is 6:12 = 1:212T drives 6T then ratio is 6:12 = 1:2 Model for classroom demonstration Model for classroom demonstration

Gear Trains Compound gear trains using double spur gears Compound gear trains using double spur gears A simple gear train demonstration… A simple gear train demonstration…

Calculating Gear Train Ratio Multiplying a series of gear ratios Multiplying a series of gear ratios Pair 1 – 8T drives 40T therefore ratio is 40:8 = 5:1 Pair 2 – 8 T drives 24T = 24:8 = 3:1 Note that pair 2 8T is on the same axle as pair 1 40T (output axle 1 is input axle 2) Gear ratio for entire compound train Gear ratio for entire compound train Multiply gear ratios 5:1 * 3:1 = 5*3:1*1 = 15:1Multiply gear ratios 5:1 * 3:1 = 5*3:1*1 = 15:1 Input axle makes 15 revolutions for the output axle to make 1Input axle makes 15 revolutions for the output axle to make 1

Readily Available Gear Trains 2-in-1 Gearbox 2-in-1 Gearbox Electronix ExpressElectronix Express $5.25$ om/kit_1130.htmhttp:// om/kit_1130.htmhttp:// om/kit_1130.htmhttp:// om/kit_1130.htm

Readily Available Gear Trains Tamiya Tamiya Ten different models availableTen different models available tech.com/gearboxe s.htmlhttp:// tech.com/gearboxe s.htmlhttp:// tech.com/gearboxe s.htmlhttp:// tech.com/gearboxe s.html

Readily Available Gear Trains Universal Gearbox Universal Gearbox KelvinKelvin $9.45$ om/Merchant2/mer chant.mv?Screen=P ROD&Store_Code= K&Product_Code= http:// om/Merchant2/mer chant.mv?Screen=P ROD&Store_Code= K&Product_Code= http:// om/Merchant2/mer chant.mv?Screen=P ROD&Store_Code= K&Product_Code= http:// om/Merchant2/mer chant.mv?Screen=P ROD&Store_Code= K&Product_Code=

Readily Available Gear Trains Motor and Gearbox Motor and Gearbox KelvinKelvin $12.95$ om/Merchant2/mer chant.mv?Screen=P ROD&Store_Code= K&Product_Code= http:// om/Merchant2/mer chant.mv?Screen=P ROD&Store_Code= K&Product_Code= http:// om/Merchant2/mer chant.mv?Screen=P ROD&Store_Code= K&Product_Code= http:// om/Merchant2/mer chant.mv?Screen=P ROD&Store_Code= K&Product_Code=

Cars with Gears

Discuss limitations What are the limitations with the use of a lever? What are the limitations with the use of a lever? What are the limitations with the use of pulleys? What are the limitations with the use of pulleys? What are the limitations with the use of gears? What are the limitations with the use of gears?

Iterative Design Approach Prototype Prototype Calculate performance score Calculate performance score Tweak the design (farther, shorter, lighter) Tweak the design (farther, shorter, lighter) Iterate (repeat steps 1-3) Iterate (repeat steps 1-3) Replicate (repeatable results?) Replicate (repeatable results?) Calculate (goal N > 35,000) Calculate (goal N > 35,000) Celebrate Celebrate

Optimization Use rubber bands on the wheel surface for traction? Use rubber bands on the wheel surface for traction? Reduce size and weight? Reduce size and weight? Use axle bushings to reduce friction? Use axle bushings to reduce friction? Use guides/bushings for string alignment? Use guides/bushings for string alignment? Maintain alignment of axles and wheels? Maintain alignment of axles and wheels? Maintain alignment of shafts/pulleys/gears? Maintain alignment of shafts/pulleys/gears? Use the space between the wheels? Use the space between the wheels? Use the space above and below the trap? Use the space above and below the trap? Figure out a faster way to wind it up? Figure out a faster way to wind it up? Lube the axles with powdered graphite? Lube the axles with powdered graphite?

Review Rules Refer to Competition Guidelines for Mousetrap Car Drawing Refer to Competition Guidelines for Mousetrap Car Drawing

Drawing Example

Drawing Guidelines Views – Front, Top, Side (RH rule) Views – Front, Top, Side (RH rule) Scale Scale Hidden lines Hidden lines Center lines Center lines Dimension lines Dimension lines Identify components Identify components Title Block Title Block Engineering paper = Vellum Engineering paper = Vellum

Review Rules Refer to Competition Guidelines for Mousetrap Car Written Technical Report Refer to Competition Guidelines for Mousetrap Car Written Technical Report

Review National Rules Adds a team interview with judges worth 50 points for a total of 250 pts. Adds a team interview with judges worth 50 points for a total of 250 pts. M-DCPS SECME Olympiad and Festival Competition M-DCPS SECME Olympiad and Festival Competition Team Registration – December (submitted online) Team Registration – December (submitted online) Project check-in – Friday, January 20, 2012 Project check-in – Friday, January 20, 2012 Competition– January 21, 2012 Competition– January 21, 2012 Location – Miami Dade College North CampusLocation – Miami Dade College North Campus

Most important Have Fun! Have Fun!