SECME Mousetrap Car

SECME Mousetrap Car

Originally prepared by: Brad NunnOriginally prepared by: Brad Nunn

BSIE Purdue University Program Manager - CitrixBSIE Purdue University Program Manager - Citrix

10/1/200510/1/2005

UPDATED: September 2011UPDATED: September 2011

Today's TopicsToday's Topics

SECME OverviewSECME Overview Performance rules and scoringPerformance rules and scoring Component designComponent design Construction techniquesConstruction techniques Prototyping Prototyping Levers and pulleysLevers and pulleys Gears and gear trains Gears and gear trains CalculationsCalculations Drawing and Technical reportDrawing and Technical report

Performance RulesPerformance Rules Refer to Mousetrap Car Refer to Mousetrap Car

Construction and Construction and Operation RulesOperation Rules

BailBail• dondon’’t cut or remove or t cut or remove or

add to itadd to it• OK to straightenOK to straighten

Performance Scoring – Do the Performance Scoring – Do the MathMath

N = N = ww DD DD * + * + W L TW L T

F = F = NN * 100 * 100

NNLL Consider tradeoffs (easy math)Consider tradeoffs (easy math) WW....is the total mass of the completed car in grams. ....is the total mass of the completed car in grams. LL....is the car....is the car’’s longest measurement along one of the three basic dimensions—s longest measurement along one of the three basic dimensions—

length, width, or height—in centimeters, measured with the bail extended or length, width, or height—in centimeters, measured with the bail extended or retracted, whichever is greater.*retracted, whichever is greater.*

TT....is the total time in seconds that the car travels from the starting point to the ....is the total time in seconds that the car travels from the starting point to the stopping point. stopping point.

FF is Final Performance Score is Final Performance Score • ((middle and high: a normalized score, i.e., best score gets 100 and the other scores are middle and high: a normalized score, i.e., best score gets 100 and the other scores are

relativerelative 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)

TerminologyTerminology

Potential to kinetic Potential to kinetic energy transferenergy transfer

TorqueTorque AccelerationAcceleration SpeedSpeed MomentumMomentum FrictionFriction

DesiredDesired OutcomesOutcomes

A small car that travels 2500 cm A small car that travels 2500 cm quickly and doesnquickly and doesn’’t weigh mucht weigh much

A gradual transfer of energy that has A gradual transfer of energy that has just enough torque to establish just enough torque to establish motionmotion

A sustained transfer of energy that A sustained transfer of energy that delivers sufficient momentum to delivers sufficient momentum to cover the distancecover the distance

Wheel DesignWheel Design

Wheel diameterWheel diameter

Wheel DesignWheel Design

Wheel ConstructionWheel Construction

Rubber bands around wheels for tractionRubber bands around wheels for traction

Axle DesignAxle Design Axle diameter and mechanical advantageAxle diameter and mechanical advantage

Simple ratio of diameters Simple ratio of diameters For distance cars use the smallest axle For distance cars use the smallest axle

that provides sufficient mechanical that provides sufficient mechanical advantage to drive a large wheeladvantage to drive a large wheel

Glue at least one drive wheel to axle Glue at least one drive wheel to axle

Two Step AxleTwo Step Axle At start, use the thick part of the axle for At start, use the thick part of the axle for

increased torqueincreased torque Once rolling, use the thin part of the axle Once rolling, use the thin part of the axle

for more distance for more distance

Wheel and Axle DesignWheel and Axle Design Minimize friction lossMinimize friction loss

Lubrication – silicone or graphite Lubrication – silicone or graphite powder – WD-40 not recommendedpowder – WD-40 not recommended

Construction TechniquesConstruction Techniques Releasing the drive string from an axle to allow Releasing the drive string from an axle to allow

coastingcoasting

Being able to disconnect drive strings on either Being able to disconnect drive strings on either end might make it easier to wind a car with a end might make it easier to wind a car with a more complex pulley or gear drivemore complex pulley or gear drive

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

Construction TechniquesConstruction Techniques

Simple, easy to Simple, easy to tie knotstie knots• SurgeonSurgeon’’s Loop – s Loop –

useful for useful for making a loop at making a loop at the end of a the end of a stringstring

Another Axle HookAnother Axle Hook

• Plastic wire tiePlastic wire tie

Prepping the TrapPrepping the Trap

• Parts of the trap that are OK to removeParts of the trap that are OK to remove

• DonDon’’t cut the bail!t cut the bail!

Super Glue – Gel ControlSuper Glue – Gel Control

• Safety first! (immediate clean up with Safety first! (immediate clean up with soap and water, goof-off, nail polish soap and water, goof-off, nail polish remover)remover)

• Gel Control formula isnGel Control formula isn’’t runny – a little t runny – a little goes a long way (and dries faster)goes a long way (and dries faster)

Making the FrameMaking the Frame

• Align the axle holesAlign the axle holes

• Not the ends of the side railsNot the ends of the side rails

PrototypingPrototyping

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

improvement?improvement?

Maximizing Axle Maximizing Axle RotationsRotations

Options to control torque, acceleration, Options to control torque, acceleration, speed, and number of rotationsspeed, and number of rotations• LeversLevers• PulleysPulleys• GearsGears

Use of LeversUse of Levers

• Length of lever vs. torqueLength of lever vs. torque


• Position of lever arm for max torque at Position of lever arm for max torque at startupstartup

Use of LeversUse 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


• A good distance carA good distance car

Cars with LeversCars with Levers



Pitsco Doc Fizzix KitsPitsco Doc Fizzix Kits

Good Lever based carGood Lever based car Good instructionsGood instructions Light weight wood, wheels, axlesLight weight wood, wheels, axles Rubber CD/DVD mounts / bushingsRubber CD/DVD mounts / bushings Axle hookAxle hook Axle bushingsAxle bushings Kevlar stringKevlar string DoesnDoesn’’t follow SECME guidelines for t follow SECME guidelines for

cutting the bail – straighten only!cutting the bail – straighten only!

Car with PulleysCar with Pulleys

Car with PulleysCar with Pulleys

• A simple pulley demonstration…A simple pulley demonstration…

Putting Levers and Pulleys Putting Levers and Pulleys togethertogether

• 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?How Big, How Many?• Target 2500 cm = 82 feet (note that the Target 2500 cm = 82 feet (note that the

minimum to even record a score is 20 feet)minimum to even record a score is 20 feet)• For a 4For a 4”” wheel, the circumference = 1 wheel, the circumference = 1’’

need 82 rotationsneed 82 rotations

• For a 0.0625For a 0.0625”” axle diameter loaded up with axle diameter loaded up with string there is a 0.125string there is a 0.125”” to .25 to .25”” effective effective diameter that has a max circumference of .79diameter that has a max circumference of .79””

need to pull 82*0.79 = 65need to pull 82*0.79 = 65”” inches of string inches of string

• 80% Design Margin 80% Design Margin 100 rotations from 80 inches of string100 rotations from 80 inches of string

Levers and pulleys?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 1For a 1”” diameter pulley, C=3.14 diameter pulley, C=3.14”” Need 80/3.14 = 25 rotationsNeed 80/3.14 = 25 rotations

• For a 0.0625For a 0.0625”” axle dia. with string (0.125 axle dia. with string (0.125”” eff. eff. dia.), C= .4dia.), C= .4””

Need to pull 25*.4 = 10Need to pull 25*.4 = 10””

• Consider 80% design marginConsider 80% design margin Mount a 6Mount a 6”” lever and locate the pivot point 6 lever and locate the pivot point 6““ away away

from the pulley shaft to pull 12from the pulley shaft to pull 12”” of string of string

Use of GearsUse of Gears• Why are gears generally used?Why are gears generally used?

Transmit torque from one shaft to anotherTransmit torque from one shaft to another Increase or decrease the speed of rotationIncrease or decrease the speed of rotation Reverse the direction of rotationReverse the direction of rotation

• Why are gears useful in this applicationWhy are gears useful in this application SmallSmall LightweightLightweight Significant multiplications possibleSignificant multiplications possible Enables unmodified mousetrap bailEnables unmodified mousetrap bail

GearsGears GearsGears

A simple gear demonstration…A simple gear demonstration… gear-animation[1].swf

Typical Spur GearTypical Spur Gear

NomenclatureNomenclature• Spur gear with 40 teeth = 40t gearSpur gear with 40 teeth = 40t gear

Having the same size teeth and the Having the same size teeth and the same spacing of the teeth allows the same spacing of the teeth allows the gears to mesh properlygears to mesh properly

Ratio of the radii is equal to the ratio Ratio of the radii is equal to the ratio of the number of teethof the number of teeth

Calculating Gear RatiosCalculating Gear Ratios For a 8T gear driving a 24T gear, for a movement For a 8T gear driving a 24T gear, for a movement

of one tooth, the 8T gear rotates 1/8 revolutions of one tooth, the 8T gear rotates 1/8 revolutions and the 24T gear rotates 1/24 revolutionsand the 24T gear rotates 1/24 revolutions

Gear ratioGear 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 Quick calc: Gear ratio is the inverse of the ratio of the number of gear teeththe 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 demonstrationModel for classroom demonstration• http://sciencekit.com/category.asp_Q_c_E_433769

Gear TrainsGear Trains

Compound gear trains using double Compound gear trains using double spur gearsspur gears

A simple gear train demonstration…A simple gear train demonstration… geartrain-animation[1].swf

Calculating Gear Train Calculating Gear Train RatioRatio

Multiplying a series of gear Multiplying a series of gear ratiosratios

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 Gear ratio for entire compound traincompound train• Multiply gear ratios 5:1 * 3:1 Multiply gear ratios 5:1 * 3:1

= 5*3:1*1 = 15:1 = 5*3:1*1 = 15:1 • Input axle makes 15 Input axle makes 15

revolutions for the output axle revolutions for the output axle to make 1to make 1

gears4An[1].mpeg

Readily Available Gear Readily Available Gear TrainsTrains

2-in-1 Gearbox 2-in-1 Gearbox • Electronix ExpressElectronix Express• $5.25$5.25• http://www.elexp.co

m/kit_1130.htm


TamiyaTamiya• Ten different Ten different

models availablemodels available• http://www.e-clec-te

ch.com/gearboxes.html


Universal GearboxUniversal Gearbox• KelvinKelvin• $9.45$9.45• http://www.kelvin.c

om/Merchant2/merchant.mv?Screen=PROD&Store_Code=K&Product_Code=281740


Motor and GearboxMotor and Gearbox• KelvinKelvin• $12.95$12.95• http://www.kelvin.c

om/Merchant2/merchant.mv?Screen=PROD&Store_Code=K&Product_Code=280411

Cars with GearsCars with Gears






DiscussDiscuss limitationslimitations

What are the limitations with the use of a What are the limitations with the use of a lever?lever?

What are the limitations with the use of What are the limitations with the use of pulleys?pulleys?

What are the limitations with the use of What are the limitations with the use of gears?gears?

Iterative Design Iterative Design ApproachApproach

PrototypePrototype Calculate performance scoreCalculate performance score Tweak the design (farther, shorter, Tweak the design (farther, shorter,

lighter)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) CelebrateCelebrate

OptimizationOptimization Use rubber bands on the wheel surface for Use rubber bands on the wheel surface for

traction?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 RulesReview Rules

Refer to Competition Guidelines for Refer to Competition Guidelines for Mousetrap Car DrawingMousetrap Car Drawing

Microsoft Word Document

Drawing ExampleDrawing Example

Drawing GuidelinesDrawing Guidelines

Views – Front, Top, Side (RH rule)Views – Front, Top, Side (RH rule) ScaleScale Hidden linesHidden lines Center linesCenter lines Dimension linesDimension lines Identify componentsIdentify components Title BlockTitle Block Engineering paper = VellumEngineering paper = Vellum

Review RulesReview Rules

Refer to Competition Guidelines for Refer to Competition Guidelines for Mousetrap Car Written Technical Mousetrap Car Written Technical ReportReport

Microsoft Word Document

Review National RulesReview National Rules Adds a team interview with judges worth Adds a team interview with judges worth

50 points for a total of 250 pts.50 points for a total of 250 pts. M-DCPS SECME Olympiad and Festival M-DCPS SECME Olympiad and Festival

CompetitionCompetition Team Registration – December (submitted Team Registration – December (submitted

online)online) Project check-in – Friday, January 20, 2012Project 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 importantMost important

Have Fun!Have Fun!

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SECME Mousetrap Car