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ELI5 Yahaaa

A see-saw with a concrete end

Why Doesn't a Crane Tip Over?

A tower crane is balanced about its mast, and a block of concrete on the short arm holds up whatever hangs on the long one.

A long arm carrying theloadA short arm carryingconcreteThe mast is the pivotbetween themDistance matters asmuch as weight
A crane is a see-saw stood on a tower.

  1. Step 1 of 5

    It is a balance, not a strong arm.

    The whole craneLoad on the long armCounterweight on the short arm

    The mast is a pivot. Everything on one side tries to tip the crane that way, and everything on the other side tries to tip it back.

  2. Step 2 of 5

    Weight far out counts for more.

    Heavy, held close A modest turning effectLight, held far out The same effect, frommuch less weight

    What matters is weight multiplied by distance from the mast. A small load right at the tip can out-tip a much bigger one held close in.

  3. Step 3 of 5

    So the counterweight sits close and is very heavy.

    Load: 2 t at 40 mthe turningeffect to beatCounterweight: 16 t at 5 mthe same, frommuch closer in

    There is no room for a long back arm on a building site, so the balance is made up with mass instead: slabs of concrete on a short stub.

  4. Step 4 of 5

    Driving the trolley in and out changes everything.

    1 Trolley closeto the mast2 Runs outwardalong the arm3 Safe load fallsas it goes

    The hook hangs from a trolley that runs along the arm. Moving it outward increases the tipping effect without adding a gram, so the safe load shrinks as it travels.

  5. Step 5 of 5

    Every crane carries a chart of its limits.

    Sensors read loadand radius It refuses to lift

    The load chart lists the heaviest weight allowed at each distance. A computer watches the same numbers and stops the crane before the chart is broken.

    A tower crane is not bolted to the ground so much as sat on it: the concrete base under a big one can weigh several hundred tonnes.

The short version

A crane does not tip because weight times distance on one side of the mast is matched by concrete on the other — and moving the hook outward eats the margin.

Try it yourself

Balance a ruler on a pencil and put a coin near one end. See how much closer to the pivot the second coin has to sit to hold it.

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