🧗 Worm-Drive Crawler: step-by-step build

Super slow, super strong: worm gears drive both axles and it never rolls back.

This crawler is the slowest car you will ever build — and one of the strongest! A long shaft runs from the front to the back, with a worm gear over each axle. Every turn of the shaft moves each 24 gear by just ONE tooth: 24 : 1! Because both axles are driven, all four wheels push (four-wheel drive). And worm gears lock: the wheels cannot turn the worm, so on a steep book ramp the crawler just stops and holds when you switch off.

• Level 4 of 5 (tricky)
• About 50 minutes
• 20 steps, 62 parts
• Powered by: Motor
• Series: Gearbox & Drivetrain Cars

What you will learn

• Worm Gear: The Lock: A worm gear turns a gear super slowly — and never slips back.
• Four-Wheel Drive: Power goes to both axles so all four wheels push.
• Torque vs Speed: Slower gears are stronger gears.
• Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
• Mounting a Motor: A motor must be pinned tight, or it spins itself instead of your wheels.
• Plugging In: Power Functions: Battery box → (switch or receiver) → motor. Plugs stack like bricks.
• Triangles Are Strong: A triangle cannot be squished — a square can.

Build these first

• 🏗️ Worm-Gear Crane: Crank the worm gear to lift the boom — then let go. It stays up by itself! (level 3 of 5, about 30 minutes, 15 steps)
• 🔋 Motor Buggy: Your first car that drives by itself — with a forwards/backwards switch and headlights! (level 3 of 5, about 30 minutes, 20 steps)

Parts you need

• 2 × Beam 15
• 14 × Black pin (grippy)
• 2 × L-beam 3×5
• 5 × Bush
• 2 × Axle 11
• 5 × Axle joiner
• 4 × Medium gear (24 teeth)
• 1 × Double cross block
• 2 × Axle 5
• 1 × AA battery box
• 4 × Beam 9
• 1 × L motor
• 4 × Blue long pin
• 1 × Axle 2
• 5 × Half bush
• 4 × Big tyre wheel
• 2 × Worm gear
• 1 × Axle 8
• 1 × Axle 9
• 1 × LED lights

Steps

1. Stand a 15-hole beam on its edge — the right rail. Push black pins into holes 4 and 6 (inside) and press an L-beam 3×5 onto them, long arm standing up at hole 4.
2. Push an 11-long axle through the 2nd hole, with a grey bush on its outside end. On the inside slide an axle joiner and a 24 gear.
3. Another axle joiner after the gear, then slide the left rail onto the axle and push black pins into its holes 4 and 6 (inside).
4. Press the second L-beam onto the pins. Hold a double cross block standing up between the L-beams and push 5-long axles through the L-beams' 2nd and 4th holes and the block — the driveshaft bearing. Now the rails are held at three places.
5. Lock the front axle with a bush outside the left rail.
6. Battery pack: take the AA battery box.
7. Black pins into the 2 lowest holes of the front ear, on both sides.
8. The same in the back ear.
9. Press a 9-hole post onto each pair of pins, hanging down.
10. Lower the L motor between the L-beams, output facing backwards. Push blue long pins through holes 5 of both rails and the L-beams into its front side holes.
11. Lower the pack over the motor so the 4 posts stand inside the rails at holes 1 and 11. Black pins through the rails into the front posts; blue long pins through the rails and the back posts into the motor's back side holes.
12. A 24 gear on the motor's output, held by a 2-long axle, and a half bush on the end.
13. Rear axle: hold an axle joiner, a 24 gear and another axle joiner in a row between the rails and push an 11-long axle through the last holes and all three.
14. A bush on each end of the rear axle, outside the rails.
15. …and the four big wheels.
16. Driveshaft: hold a half bush and a worm gear right above the front 24 gear and push an 8-long axle through the middle hole of the double cross block, the half bush and the worm. Another half bush on the front end.
17. An axle joiner on the back end of the shaft — it spins with the shaft.
18. Hold a 24 gear right above the motor's 24, a bush and a half bush behind it, and push a 9-long axle from the back through them into the joiner.
19. Slide the second worm onto the back end of the shaft, right above the back 24 gear, and lock it with a half bush.
20. Headlights: push the two LED lights into the round holes on the front of the battery box.

Play time

Build a mountain out of books and cushions. Can the crawler climb it? Stop half-way up and switch off — it does not roll back. Race it against a snail… it might be close!

Real-world inspiration

Worm-driven axles on old trucks and trial crawlers; Sariel's compact pendular steered suspension. https://en.wikipedia.org/wiki/Worm_drive · https://sariel.pl/2008/12/compact-pendular-steered-suspension/