Presented by University of Miami 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

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

Terminology Potential to kinetic energy transfer Torque Acceleration Speed Momentum Friction

Desired Outcomes 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

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

Car with Pulleys

Car with Pulleys A simple pulley demonstration…

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…

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

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?

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?

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

Most important Have Fun!