🚧 Parking Barrier Gate: step-by-step build

A long striped barrier that lifts with a tiny turn — thanks to a heavy counterweight.

At a car park the barrier arm swings up to let cars in and down to stop them. The arm is long and heavy — so how can a small motor lift it? The trick is a counterweight: a heavy weight on the short end, on the other side of the pivot. It balances the long arm like a friend on the other end of a see-saw, so lifting takes almost no effort. Build it, then try it with and without the counterweight!

  • Level 2 of 5 (easy)
  • About 20 minutes
  • 9 steps, 25 parts
  • Powered by: Hand-crank
  • Series: Mechanical Wonders

What you will learn

  • Counterweight: A weight on the other side of a pivot that balances a load.
  • Levers: A bar on a pivot multiplies your push.
  • Centre of Gravity: Keep the heavy stuff low and in the middle and your model won't tip.
  • Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
  • Frames: A frame is a ready-made strong rectangle.

Build these first

  • ⚖️ Balance See-Saw: A see-saw that is also a scale. Can one gear beat two? (level 1 of 5, about 12 minutes, 10 steps)
  • ⚙️ Gear Friends: Three gears holding hands — spin one and they all dance. (level 1 of 5, about 8 minutes, 6 steps)

Parts you need

  • 1 × Open frame 5×7
  • 4 × Black pin (grippy)
  • 2 × Beam 7
  • 3 × Blue long pin
  • 1 × Axle 5
  • 5 × Half bush
  • 1 × Medium gear (24 teeth)
  • 1 × Bush
  • 1 × Beam 15
  • 1 × Beam 9
  • 2 × Axle 4
  • 1 × Tiny gear (8 teeth)
  • 1 × Hand crank
  • 1 × Big tyre wheel

Steps

  1. Stand the 5×7 frame up on its short end. This is the gate post.
  2. Pin a 7-hole foot to the back of the post with two black pins.
  3. Push two blue long pins through the front and add the front foot.
  4. Push a 5-long axle through the top middle hole. Add a half bush behind, and a 24-tooth gear and a bush in front.
  5. Push a blue long pin into the gear, then slide the 15-hole boom on by its 5th hole, onto the pin. Lock with a half bush.
  6. Make the boom longer: push two black pins into the last two holes and press a white 9-hole beam onto them.
  7. Crank shaft: push a 4-long axle through the top corner hole with a tiny 8-tooth gear and a half bush in front and a half bush behind.
  8. Put the red crank on the back end of the crank shaft.
  9. Counterweight: push a 4-long axle into the first hole of the boom's short end, lock it behind the boom with a half bush and add the big wheel on the front.

Play time

Crank the barrier up and down to let toy cars through. Take the big wheel off and crank again — can you feel the difference? Try the small wheel as a counterweight instead: is it enough?

Real-world inspiration

Boom barriers at car parks and level crossings — https://en.wikipedia.org/wiki/Boom_barrier