Brad Nunn BSIE Purdue University Program Manager - Citrix 10/1/2005 SECME Mousetrap Car Brad Nunn BSIE Purdue University Program Manager - Citrix 10/1/2005

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

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

Performance Scoring N = w D 2 * W L F = N * 100 NL Consider tradeoffs * 100 NL Consider tradeoffs D and L squared F is a normalized score National - best score gets 100 and the other scores are relative District – different formula resulting in scores between 50 - 100 based on performance rank Max team score is 200: Performance (100), Design Drawing (50), Technical Report (50)

Terminology Potential to kinetic energy transfer Torque Acceleration Speed Momentum Friction

Desired Outcomes A small car that travels 2500 cm and doesn’t weigh much 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

Wheel Design Wheel diameter

Wheel Design Wheel Construction Rubber bands around wheels for traction

Axle Design Axle diameter and mechanical advantage Simple ratio of diameters 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

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

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

Construction Techniques 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

Construction Techniques Creating an axle hook on a solid shaft

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

Another Axle Hook Plastic wire tie

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

Super Glue – Gel Control 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)

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

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

Maximizing Axle Rotations Options to control torque, acceleration, speed, and number of rotations Levers Pulleys Gears

Use of Levers Length of lever vs. torque

Use of Levers 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 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 string The 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 startup

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

Use of Levers A good distance car

Cars with Levers

Cars with Levers

Cars with Levers

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

Car with Pulleys

Car with Pulleys A simple pulley demonstration…

Putting Levers and Pulleys together Design calculations How big are the wheels? How many rotations are needed? What benefit is derived from the pulley? 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) For a 4” wheel, the circumference = 1’ need 82 rotations For a 0.0625” 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 80% Design Margin 100 rotations from 80 inches of string

Levers and pulleys? Target = 80 inches of string 40 inch lever? Bigger wheels and smaller lever? Add a pulley? For a 1” diameter pulley, C=3.14” Need 80/3.14 = 25 rotations For a 0.0625” axle dia. with string (0.125” eff. dia.), C= .4” Need to pull 25*.4 = 10” Consider 80% design margin 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? Transmit torque from one shaft to another Increase or decrease the speed of rotation Reverse the direction of rotation Why are gears useful in this application Small Lightweight Significant multiplications possible Enables unmodified mousetrap bail

Gears Gears A simple gear demonstration…

Typical Spur Gear Nomenclature Spur gear with 40 teeth = 40t gear 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

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 Gear ratio 1/8:1/24 = 24:8 = 3:1 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 12T drives 6T then ratio is 6:12 = 1:2 Model for classroom demonstration http://sciencekit.com/category.asp_Q_c_E_433769

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

Calculating Gear Train Ratio 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 Multiply gear ratios 5:1 * 3:1 = 5*3:1*1 = 15:1 Input axle makes 15 revolutions for the output axle to make 1

Readily Available Gear Trains 2-in-1 Gearbox Electronix Express $4.75 http://www.elexp.com/kit_1130.htm

Readily Available Gear Trains Tamiya Ten different models available http://www.e-clec-tech.com/gearboxes.html

Readily Available Gear Trains Universal Gearbox Kelvin $5.45 http://www.kelvin.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=K&Product_Code=281740

Readily Available Gear Trains Motor and Gearbox Kelvin $4.95 http://www.kelvin.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=K&Product_Code=280411

Cars with Gears

Cars with Gears

Cars with Gears

Cars with Gears

Cars with Gears

Cars with Gears

Discuss limitations 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 gears?

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

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

Review Rules Refer to Competition Guidelines for Mousetrap Car Drawing

Drawing Example

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

Review Rules 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 an Essay and Poster Theme refer to www.SECME.org for guidelines These are individual events

Most important Have Fun!