1. 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

2. Today's Topics 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 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 based on performance rank
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. 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

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 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

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

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

25. Use of Levers
A good distance car

26. Cars with Levers

27. Cars with Levers

28. Cars with Levers

29. 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!

30. Car with Pulleys

31. Car with Pulleys
A simple pulley demonstration…

32. 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?

33. 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 ” 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

34. 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 ” 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

35. 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

36. Gears
Gears
A simple gear demonstration…

37. 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

38. 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

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

40. 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

41. Readily Available Gear Trains
2-in-1 Gearbox
Electronix Express
$4.75

42. Readily Available Gear Trains
Tamiya
Ten different models available

43. Readily Available Gear Trains
Universal Gearbox
Kelvin
$5.45

44. Readily Available Gear Trains
Motor and Gearbox
Kelvin
$4.95

45. Cars with Gears

46. Cars with Gears

47. Cars with Gears

48. Cars with Gears

49. Cars with Gears

50. Cars with Gears

51. 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?

52. 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

53. 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?

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

55. Drawing Example

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

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

58. 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 for guidelines
These are individual events

59. Most important
Have Fun!