How To

Power Transmission

  • Common problem: how to transmit power between a shaft and something attached to it
    • Gear
    • Sprocket
    • Wheel
  • High stresses, so easy to break
  • Often hard to fix

Power Transmission Set Screws

  • "Set screws inhale audibly"...
  • Weak
  • Prone to coming loose
  • Sometimes the only choice for small shafts
  • Use a dimple on the shaft
  • Use Loctite or similar to slow down loosening
  • Check/tighten every match

Power Transmission Pins

  • Stronger than set screws
  • Fairly easy to use
    • Can be tricky to get hole centred
  • Weakens shaft
  • Hard to remove
    • Tapered pins can be used, but they're hard to find and tricky to use

Power Transmission Keys

  • Very strong
  • Does not significantly weaken shaft
  • Keying shaft requires special cutter
    • Can buy standard pre-keyed shaft
  • Still need some way to hold gear and key from sliding along shaft
  • Often a good idea to have set screw hold key in place
  • Can have some backlash if not done properly

Power Transmission Bolts

  • Often underrated
  • Very simple and easy to build
  • Very strong
  • Easily disassembled
  • Most often used to attach sprocket/gear to wheel
  • Can also be used between sprockets/gears


  • Strength
    • Switch to 3/8"?
    • Power argument: P = Fv (power = force × velocity)
    • Power P is a constant
    • We want to reduce the tensile force F
    • So...?
  • Tensioning
    • Sprung tensioner or simple block/roller
    • Reduce backlash
    • Reduce probability of chain becoming derailed

Motor Curves

Multiple Motors

  • Only way to increase actual power in drivetrain
  • "But won't the motors fight each other?"
    • As long as each motor is not running faster than free speed, it is contributing power
    • Some motors might contribute more power than others
    • Need to (roughly) match motor speeds if using different motors

Computer Aided Design

  • Usually used for complex and/or high-precision parts
  • Not just a drafting tool - use for design as well!
    • See how parts go together
    • Check clearances between components
    • Calculate mass of subassemblies
    • Visualize mechanism motion
    • Edit, restructure, and redesign parts and assemblies
  • Get familiar with a CAD program before the build season
    • Inventor
    • SolidWorks
    • Pro/ENGINEER
  • Think about "design intent"
    • Make sure you can modify the design easily (because you will)
  • Use version control for CAD files
    • Peace of mind
    • Allow sharing of data
    • Subversion, Autodesk Vault