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ELI5 Yahaaa

Oil that refuses to be squashed

How Does a Digger Lift So Much?

A digger's arm is worked by oil, and because oil cannot be compressed, a small push at one end becomes a huge one at the other.

Three joints, threeramsEach ram is a piston ina tubeOil is pumped in behinditOil will not squash, sothe arm must move
Every joint on a digger is a tube full of oil.

  1. Step 1 of 5

    Squeeze air and it shrinks. Squeeze oil and it does not.

    Air in a tube Squashes down — thepush is absorbedOil in a tube Nowhere to shrink — thepush travels

    Air molecules have gaps between them to close up. In a liquid they are already touching, so pushing on oil moves it rather than shrinking it.

  2. Step 2 of 5

    Pressure spreads out equally everywhere.

    A small pistonpushes The whole systemfeels it at once

    Push on the oil anywhere in a sealed system and every square centimetre of every wall feels the same extra pressure at the same instant.

  3. Step 3 of 5

    A bigger piston has more area to push on.

    Small piston, small areamoves far,pushes littleBig piston, ten times the areamoves slowly,pushes ten timesharder

    Same pressure, ten times the area, ten times the force. The cost is distance: the big piston creeps while the small one races.

  4. Step 4 of 5

    The arm is a chain of levers on top of that.

    1 A ram extends alittle2 It pivots theboom3 The bucketsweeps a longarc

    Each ram pushes near a joint, so the far end of the boom swings a long way. The digger stacks hydraulics and levers into one movement.

  5. Step 5 of 5

    Valves decide which ram gets the oil.

    Engine drives the pump Levers routethe oil The chosen rammoves

    One pump serves every joint. The levers in the cab open and close valves, sending the oil where the driver wants it and holding it there when they stop.

    Digger hydraulics run at around 250 bar — about 250 times the pressure of the air around you, and enough for a 10 cm ram to push some 20 tonnes.

The short version

A digger is strong because oil cannot be squashed: pressure from a small piston reaches a much bigger one, which pushes proportionally harder.

Try it yourself

Fill a syringe with water, block the end, and push. Now try it full of air. The water one barely moves — that is a hydraulic arm.

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