Oil does the lifting
Nearly every big machine on a SITE works by hydraulics. A pump forces oil into a cylinder at enormous pressure — hundreds of times the pressure in a car tyre — and because a liquid cannot be squashed, that pressure shoves a piston out with tremendous force. The wider the cylinder, the more it lifts, because the push depends on the pressure multiplied by the area it acts on.
That is how a fairly small EXCAVATOR can curl its arm and BUCKET around several tonnes of earth. It also explains the way these machines move: smooth, deliberate, and quite impossible to shove back once they have started.
How a tower crane grows
A tower CRANE puts itself up. Once the base is bolted down, the crane lifts a fresh section of mast into place, a climbing frame jacks the entire top of the crane upwards, and the new piece slots into the gap that opens. Do it again and again, and the crane climbs alongside the building it is making.
Balance decides everything it can do. A heavy concrete counterweight sits on the short arm, the load hangs from the long one, and the further out the load travels the less it may weigh — so operators work from a chart rather than one simple maximum. Taking the crane down is the awkward part: a smaller crane is often assembled on the finished roof purely to dismantle the big one.
Concrete does not dry
The MIXER turns constantly, but not to stop the CONCRETE drying out. Concrete never dries. It sets through a chemical reaction between water and CEMENT that grows tiny crystals, locking the water in and knitting the sand and GRAVEL into artificial stone. It will harden perfectly well underwater, which is how bridge foundations get poured in a river.
That reaction is also why the trucks are always in a hurry. Once mixing starts the clock is running — usually a couple of hours between the plant and the pour — and a load that goes hard inside the drum is an extremely expensive mistake.
Why a forklift steers from the back
A FORKLIFT is a see-saw. The load sits out in front of the front wheels, and a heavy counterweight behind the driver holds the whole thing down — so how much it can lift depends on how far out the weight sits, not just how heavy it is. Put a load too far forward and the back wheels lift off the ground.
Those back wheels do the steering, which is what lets a forklift spin round almost on the spot in a narrow aisle, and also why it feels alarming to drive at first: the tail swings out wide. Everyone nearby wears a HARDHAT, a habit that started on big American dam and bridge projects in the 1930s, where hard hats were first made compulsory.
Machines the size of buildings
The largest vehicles that move on land are mining excavators. One bucket-wheel machine built for German coal mines stands around 96 metres tall and stretches about 225 metres from end to end, weighs over 14,000 tonnes and creeps along on caterpillar tracks at roughly a tenth of walking pace. It eats into the ground with a great wheel of buckets, each big enough to park a small car inside.
Tracks are what stop such a thing sinking into the earth it is digging. They spread the weight over a huge area of ground — the same trick a BULLDOZER uses to shove soil around a soft SITE without burying itself up to the WHEELS it does not have.