🦾 Crank Excavator Arm: step-by-step build

Crank the worm gears to swing the digger round and lift the boom — then dig!

Real excavators move their huge arms with hydraulic cylinders full of oil. Ours uses worm gears instead — they are slow, strong, and they never let the arm drop. One handle turns a worm that bites the teeth of the turntable, so the whole top of the digger swings round. A crank turns a second worm that lifts the long boom. The stick has a stiff joint you set by hand, and the bucket swings on smooth pins so you can tip it to scoop and dump. Make a sandpit out of bushes and pins and start digging!

Level 4 of 5 (tricky)
About 50 minutes
22 steps, 69 parts
Powered by: Hand-crank
Series: Mechanical Wonders

What you will learn

Worm Gear: The Lock: A worm gear turns a gear super slowly — and never slips back.
Turntables: A turntable lets a whole model spin round.
Linkages: Beams joined by pins change one motion into another.
Triangles Are Strong: A triangle cannot be squished — a square can.
Torque vs Speed: Slower gears are stronger gears.

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)
📡 Spinning Radar: A motor-driven radar dish that sweeps round and round on a turntable — with a blinking light. (level 3 of 5, about 25 minutes, 15 steps)

Parts you need

1 × Open frame 5×11
1 × Turntable
13 × Red axle-pin
3 × Double cross block
5 × Bush
2 × Worm gear
1 × Axle 12
1 × Axle joiner
2 × Half bush
1 × Beam 7
1 × Axle 3
4 × Black pin (grippy)
1 × Open frame 5×7
9 × Blue long pin
8 × Split pin block
2 × Beam 5
2 × Beam 3
1 × Axle 11
2 × Beam 11
1 × Medium gear (24 teeth)
2 × Beam 9
2 × L-beam 3×5
2 × Grey pin (smooth)
1 × Axle 7
1 × Hand crank

Steps

Lay the white 5×11 frame flat. This is the digger's undercarriage.
Put the turntable in the middle.
Push two red axle-pins up into the left end of the frame, in the 3rd and 5th holes, and press a double cross block down onto them.
Do the same at the right end. The middle round holes of the two blocks hold the slewing axle.
Hold a bush, the worm gear and another bush in a row between the blocks, the worm touching the turntable's teeth, and push the long axle through both round holes and all three. The bushes keep the worm in the middle.
Push a yellow axle joiner onto the right end of the slewing axle as a twist handle, and a half bush on the left end. Joiner and half bush hug the two blocks, so the axle cannot slide. (A crank would hit the table: this axle is only 1½ holes high.)
Lay a 7-hole beam flat across the turntable and push a 3-long axle down through its middle hole into the turntable. Push a red axle-pin into the beam two holes along, and slide a double cross block down onto the axle and the axle-pin.
Push black pins up into the beam's end holes and press the 5×7 frame flat onto them, around the block: the upper deck.
Boom posts: push blue long pins up into the 5th and 7th holes of the deck's left long side and slide two split pin blocks down onto them, one on top of the other.
Do the same on the right long side.
Push red axle pins into the blocks, pointing out, and stand a 5-hole post on each pair, just outside the deck.
Press the two 5-hole posts on.
Worm bearings: push blue long pins up into the 2nd and 4th holes of the deck's back end and slide two split pin blocks onto them.
The same on the front end of the deck.
Push red axle pins into the blocks, pointing out to the back and to the front.
Press a 3-hole beam, standing up, onto each pair of red pins. Their top holes hold the worm axle.
Hold a bush and the second worm in a row between the posts and push an 11-long axle from the back through the back bearing, the bush, the worm and the front bearing. Push a half bush onto the tip sticking out at the front.
Boom: hold an 11-hole beam slanting up, push a blue long pin into its 2nd hole and slide the 24-tooth gear onto the pin by its small hole.
Press the second boom beam onto the other end of the blue pin.
Stick: push a black friction pin into the boom tip on each side, pointing out, and press a 9-hole beam onto each, pointing down.
Pin the two L-beam bucket halves to the inside of the stick end, each with a smooth grey pin so it can swing — that is the bucket joint.
Lower the boom between the posts from above, gear on top of the worm, and push the pivot axle through post, bush, boom, gear, boom, bush, post. Put a crank on the back end of the worm axle.

Play time

Build a pile of bushes and pins. Swing the digger round, lower the boom, scoop with the bucket, lift and swing to dump it in a box. Can you move the whole pile?

Real-world inspiration

Hydraulic excavator — https://en.wikipedia.org/wiki/Excavator