The crane and its two-arch bridge on an oak desk, a white coffee mug beside the base and a notebook in front

Roman Bridge Crane

A Roman treadwheel crane that raises its own two-arch stone bridge, one stone at a time.

One keystone, wheel to seat

Turn the wheel

Press a fingertip on the rim and the treadwheel turns, winding the cord onto the drum on its axle. Every turn lifts the stone 28.3 mm.

Hear it click

A gravity pawl drops into the ratchet once per tooth, 16 clicks a turn, 1.77 mm each. Let go and the stone stays where it is.

Tongs that tighten

Scissor tongs take the stone by its two lifting holes, a Roman mason's trick. The lift pulls the arms shut, so the pins hold it rather than friction.

Swing and roll

The crane slews on its turntable and rolls along the bank on its rail, carrying the keystone 95 mm out over the crown.

Keystone last

Flip the pawl up, lower the red keystone into the last gap and open the tongs by hand. Then the walls, the deck and the parapets go on.

Strike the centering

Tap the drift with the mallet: each folding wedge slides 10 mm, the centering drops 2 mm and pulls out, and both arches stand alone.

Nine stones, five shapes

Each arch is 9 stones. Every one is drilled with its pair of lifting holes straight above its centre of weight as it sits in the arch, so the holes wander from stone to stone, and each stone leaves the tongs already tilted to its seat.

One arch's nine stones drawn apart along their radii, front face on: the lifting hole sits at a different height on each kind

Up close

The tongs' pins in the keystone's holes, the red stone hanging below the bail over the half-built arch
Pins in both holes, and the keystone hangs at the angle of its gap.
The treadwheel's spokes and the ratchet wheel beside it, arch stones stacked behind
16 teeth on the ratchet. You hear every one.
Mid-swing: the keystone rides on the tongs over the first arch while the second waits on its centering
Halfway over. The other arch is still on its timber.

Two crews, one clock

Built for two players or two teams around one table: race to raise an arch, then strike the centering together and watch the bridge hold itself up.

  1. Set all 9 stones of one arch in the pallet. Start the clock.
  2. Raise them in order: springers at both ends, then the voussoirs, the keystone last.
  3. The clock stops when the keystone seats. Hand the stones back; the centering stays up, so a reset takes under 3 minutes.
  4. Swap crews and race again. Fastest arch wins.
  5. After the last round, strike the centering and let both arches stand.

99 pieces in the box

29
for the crane: treadwheel, ratchet, turntable, jib and pins
10
for the scissor tongs
18
arch stones, 9 to each arch, two of them red
19
for the bridge: pier, abutments, walls, deck, embankments, parapets
10
timber centerings and folding wedges
10
for the site: base, river, rail, stone racks and pallets
3
tools: a mallet, a drift and its guide
The complete site from the crane's side, stones in place and the centering still under both arches
Size
11.8 × 7.5 × 8.1 in, crane standing
Weight
2.0 oz
Material
PLA
Scale
1:60: each arch is a 4.8 m Roman span
Arch span
3.1 in, bridge 8.3 in long
Hoist
28.3 mm per turn of the wheel
Reach
95 mm from the turntable
Joints
10 pins; no glue, screws or springs

Good to know

Does the crane really lift the stones?

Yes. A fingertip turns the treadwheel, the cord winds onto its drum and the tongs rise 28.3 mm per turn. The pawl holds the load at every click, so you can let go mid-lift.

Is there anything else I need?

A length of thin cord, about 1.0 mm thick, for the hoist. It is not one of the 99 pieces. Nothing else: no glue, screws or springs.

Will the arches really stand without the centering?

Once the keystone is in, the ring locks. Drive the folding wedges and the centering drops 2 mm and slides out from under; the stones carry each other, as they did on Roman bridges.

How big is it?

11.8 × 7.5 × 8.1 in with the crane standing, and 2.0 oz. Each arch spans 3.1 in, a 4.8 m Roman span at 1:60.

Can we race again after the strike?

Yes. With the centering out, slide each lower wedge back by hand, set the centering on its upper wedges and load the pallet again.