Cardboard Tower Crane with Pulley

You will need

• Cardboard tubesAt least two: one tall (wrapping paper roll), one shorter (paper towel roll) for the jib
• Small cardboard boxesTwo: one for the base, one for the counterweight on the far jib end

  Use boxes that held food or household goods, not cleaning chemicals; wipe down and do not allow chewing; recycle when done.

• Thread spool or jar lidThe pulley wheel; a wooden spool is ideal; a jar lid with a tape-wrapped groove works
• String or twineAbout 3 feet (90 cm); threads through the pulley and hangs the load
• Masking tape and packing tapeBoth; masking for positioning, packing for final structural joints
• Scissors

  Adult nearby for any thick multi-layer cardboard cuts that require real force.

• Washable markerFor marking cut lines and labeling jib sections before cutting
• Paper cupThe crane hook bucket for the test load
• A few coins or small stonesTest load for the cup; add one at a time to find the tipping point
• Large flat piece of cardboardBase plate to tape the tower foot to; prevents tipping

Steps

1. 1

   Sketch the design first

   Before touching cardboard, ask the child to draw their crane: where is the tower, where does the jib arm go, where does the counterweight sit, how does the string travel. Say: 'Real engineers draw before they build — find one problem in your drawing before you start.' Give them five minutes.

   Tip: If they want to skip the sketch, ask: 'Where will the string go?' — they usually realize they haven't thought it through yet.

2. 2

   Build and brace the tower base

   Tape the base box flat to the large cardboard base plate, then tape or slide the tall tube vertically into or against the base box, holding it plumb (straight up). Reinforce the tube-to-box joint with at least four strips of packing tape, pressing hard. Say: 'This joint takes all the stress — what would make it stronger?'

   Tip: Wedge crumpled paper inside the tube base to stiffen it before taping; it prevents the tube from crushing.

3. 3

   Attach the jib arm

   Tape the shorter tube horizontally near the top of the tower, extending out over one side. The child's sketch should tell them how far. This joint will droop — when it does, ask: 'How would you stop a shelf from sagging?' and let them cut a diagonal cardboard brace (triangle gusset) to run from the tower to the underside of the jib.

   Tip: A triangle cut from a cereal box and taped at both ends adds huge rigidity.

4. 4

   Mount the pulley and run the string

   Tape the spool or jar-lid pulley to the outer end of the jib so it can spin or at least rotate a little. Thread the string over the pulley groove, bring one end down to where the load will hang and tie on the paper cup. The other end is the hauling line — the child pulls it to hoist. Test the path by pulling slowly.

   Tip: If the spool won't spin freely, a pencil through its center hole, taped across the jib end, makes a better axle.

5. 5

   Add the counterweight

   Tape the second small box to the short end of the jib, opposite the load side. Fill it with a few stones or coins to balance. Say: 'A counterweight stops the crane from toppling toward the load — adjust it until the jib sits level without holding it.' This is genuinely satisfying to get right.

   Tip: Add weight a little at a time; too much counterweight tips the tower backward instead.

6. 6

   Run the load test

   Put one coin in the paper cup and hoist it by pulling the hauling line. Add a coin at a time and observe what happens to the tower base, the jib joint, and the pulley. Ask the child to record (on paper or just verbally) at what load each joint begins to move or fail. Reinforce the weakest joint and retest.

   Tip: Failure is useful data — say 'now we know where it needs to be stronger' rather than rescuing the structure immediately.

7. 7

   Redesign one joint

   After the first full test, ask the child to pick the weakest point and fix only that one thing — a gusset, extra tape wraps, a second tube alongside the tower. Retest with the same load sequence. The comparison between test one and test two is the real learning.

   Tip: If time allows, a third iteration with a heavier load sets a personal record that is hard to walk away from.

Take it to the table

Materials checklist, the steps and the variations on one Letter page.