SECME Mousetrap Car

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SECME Mousetrap Car

• Brad Nunn
• BSIE Purdue University Program Manager - Citrix
• 10/1/2005

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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
• dont 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
• 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)

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Terminology

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

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Desired Outcomes

• A small car that travels 2500 cm and doesnt
  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

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Wheel Design

• Wheel diameter

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Wheel Design

• Wheel Construction
• Rubber bands around wheels for traction

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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

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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

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Wheel and Axle Design

• Minimize friction loss
• Lubrication silicone or graphite powder WD-40
  not recommended

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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

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Construction Techniques

• Creating an axle hook on a solid shaft

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Construction Techniques

• Simple, easy to tie knots
• Surgeons Loop useful for making a loop at the
  end of a string

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Another Axle Hook

• Plastic wire tie

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Prepping the Trap

• Parts of the trap that are OK to remove

• Dont cut the bail!

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Super Glue Gel Control

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

• Gel Control formula isnt 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?

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Maximizing Axle Rotations

• Options to control torque, acceleration, speed,
  and number of rotations
• Levers
• Pulleys
• Gears

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Use of Levers

• Length of lever vs. torque

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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

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Use of Levers

• Position of lever arm for max torque at startup

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Use of Levers

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

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Use of Levers

• A good distance car

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Cars with Levers
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Cars with Levers
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Cars with Levers
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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
• Doesnt follow SECME guidelines for cutting the
  bail straighten only!

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Car with Pulleys
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Car with Pulleys

• A simple pulley demonstration

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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?

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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 820.79 65 inches of string
• 80 Design Margin
• 100 rotations from 80 inches of string

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Levers and pulleys?

• Target 80 inches of string
• 40 inch lever?
• Bigger wheels and smaller lever?
• Add a pulley?
• For a 1 diameter pulley, C3.14
• 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

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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

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Gears

• Gears
• A simple gear demonstration

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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

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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
• 1/81/24 248 31
• 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 612 12
• Model for classroom demonstration
• http//sciencekit.com/category.asp_Q_c_E_433769

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Gear Trains

• Compound gear trains using double spur gears
• A simple gear train demonstration

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Calculating Gear Train Ratio

• Multiplying a series of gear ratios
• Pair 1 8T drives 40T therefore ratio is 408
  51
• Pair 2 8 T drives 24T 248 31
• 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 51 31 5311 151
• Input axle makes 15 revolutions for the output
  axle to make 1

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Readily Available Gear Trains

• 2-in-1 Gearbox
• Electronix Express
• 4.75
• http//www.elexp.com/kit_1130.htm

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Readily Available Gear Trains

• Tamiya
• Ten different models available
• http//www.e-clec-tech.com/gearboxes.html

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Readily Available Gear Trains

• Universal Gearbox
• Kelvin
• 5.45
• http//www.kelvin.com/Merchant2/merchant.mv?Screen
  PRODStore_CodeKProduct_Code281740

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Readily Available Gear Trains

• Motor and Gearbox
• Kelvin
• 4.95
• http//www.kelvin.com/Merchant2/merchant.mv?Screen
  PRODStore_CodeKProduct_Code280411

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Cars with Gears
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Cars with Gears
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Cars with Gears
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Cars with Gears
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Cars with Gears
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Cars with Gears
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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?

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Iterative Design Approach

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

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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?

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Review Rules

• Refer to Competition Guidelines for Mousetrap Car
  Drawing

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Drawing Example
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Drawing Guidelines

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

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Review Rules

• Refer to Competition Guidelines for Mousetrap Car
  Written Technical Report

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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

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Most important

• Have Fun!