SECME Mousetrap Car

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

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

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

4. Performance Scoring – Do the Math Middle and Senior High Car Scoring Elementary Car Scoring F = D DL N = wDD * + W L T F = N * 100 NL Consider tradeoffs (easy math) • 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.* • T....is the total time in seconds that the car travels from the starting point to the stopping point. • F is Final Performance Score • (middle and high: a normalized score, i.e., best score gets 100 and the other scores are relative • Max team score is 200: • Performance (100), Design Drawing (50), Technical Report (50)

5. Terminology • Potential to kinetic energy transfer • Torque • Acceleration • Speed • Momentum • Friction

6. DesiredOutcomes • 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 sustained transfer of energy that delivers sufficient momentum to cover the distance

7. Wheel Design • Wheel diameter

8. Wheel Design • Wheel Construction • Rubber bands around wheels for traction

9. 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

10. 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

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

12. 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

13. Construction Techniques • Creating an axle hook on a solid shaft

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

15. Another Axle Hook • Plastic wire tie

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

17. 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)

18. Making the Frame • Align the axle holes • Not the ends of the side rails

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

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

21. Use of Levers • Length of lever vs. torque

22. Use of Levers • Position of lever arm for max torque at startup

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

24. Use of Levers • A good distance car

25. Cars with Levers

26. Cars with Levers

27. Cars with Levers

28. 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!

29. Car with Pulleys

30. Car with Pulleys • A simple pulley demonstration…

31. 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?

32. 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

33. 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

34. 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

35. Gears • Gears • A simple gear demonstration…

36. 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

37. 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

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

39. 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

40. Readily Available Gear Trains • 2-in-1 Gearbox • Electronix Express • $5.25 • http://www.elexp.com/kit_1130.htm

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

42. Readily Available Gear Trains • Universal Gearbox • Kelvin • $9.45 • http://www.kelvin.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=K&Product_Code=281740

43. Readily Available Gear Trains • Motor and Gearbox • Kelvin • $12.95 • http://www.kelvin.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=K&Product_Code=280411

44. Cars with Gears

45. Cars with Gears

46. Cars with Gears

47. Cars with Gears

48. Cars with Gears

49. Cars with Gears

50. Discusslimitations • 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?