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13Motion

Linkages & Cams

Turn rotation into controlled motion with four-bar linkages and shaped cams - the mechanisms behind launchers, lifts, and grippers.

Intermediate ~14 min

Four-Bar Linkage - the simplest mechanism that converts rotation into controlled swinging motion.

Cam & Follower - a rotating cam with a harmonic profile drives a flat follower up and down.

1Experience
2Reflect
3Theorize
4Apply

Linkages - Rotation into Controlled Motion

A linkage is a set of rigid bars connected by pin joints. The most famous one is the four-bar linkage: four bars, four pins, one repeating motion path.

Why Linkages Matter
  • Predictable path: The end of the linkage sweeps a known arc - great for pick-and-place arms.
  • Force multiplier: A short input bar driving a longer output bar multiplies force like a lever.
  • No gears needed: Simple, cheap, and easy to build from laser-cut plates.
Four-Bar

Classic rocker or crank. Used on folding arms, windshield wipers, and lifting tables.

Slider-Crank

Turns rotation into straight-line motion - the mechanism behind a piston engine.

Scissor Lift

Crossed pairs of bars stacked up - short extension in, big lift out.

Cams & Followers

A cam is a shaped wheel that pushes against a follower as it spins. The cam's shape decides the motion pattern - smooth rise, quick drop, or a dwell.

Three Cam Profiles
  • Eccentric (offset circle): Produces smooth, wave-like motion.
  • Heart-shape: Constant-velocity rise, flat return - used in sewing machines.
  • Snail: Gradual rise then a sharp drop - a catapult mechanism.
Design tip: Keep the pressure angle below 30° on rises - steeper than that and the follower jams.
Pause and Reflect
✓ Your reflections are saved automatically
Apply What You Learned

Design a launcher for a ball-shooting robot.

  • Decide whether a four-bar linkage or a cam best matches the motion you want.
  • Sketch the mechanism showing input rotation and output motion.
  • Label the bar lengths (for a linkage) or the cam profile (for a cam).
  • Identify one thing that could fail and how you would protect against it.
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