A steel tube, a piston and a few litres of oil. Pump the oil in and the piston pushes with tonnes of force: that is how an excavator lifts, digs and holds its load.
The boom cylinder is cut open and the body is see-through. Push the lever to raise the boom and watch the oil fill the cap end; centre it to hold the load. Drag to turn the machine, scroll or pinch to zoom, and click a part to use it.
Try itSlide the lever to Raise. The pump sends oil into the cap end, behind the piston, and the rod slides out.
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Red: under pressure, from the pump
Blue: low pressure, to and from the tank
Amber: trapped, holding the load
This browser can’t show the 3D model, so here is a still picture.
Meet the parts
Thirteen parts, one push
The boom cylinder taken apart and laid out, with the valve that controls it. Tap any part of the picture or pick a name below: the barrel at the back, the rod and piston, the rings that seal it, and the valve in front.
Hover or tap the picture. Gold rings mark every part.
Parts diagram
Cut down the middle
The boom cylinder sawn in half along its length, pushing out: red oil under pressure fills the cap end behind the piston, and blue oil leaves the rod end for the tank. The numbers run from the cap end to the rod end.
How it works, step by step
Thirteen steps, one idea each
The same steps as the video. Each picture is a moment from it; press Show me in 3D to set the model up for that step.
Step 1 of 13
The strength behind the arm
Demo · force and speed
Push, pull and how fast
Force is the oil’s pressure times the area it pushes on. Speed is the flow of oil divided by that same area. Change the cylinder and the oil, then lift a load.
On the rod side the rod covers part of the piston, so the oil pushes on a ring instead of a full circle. That is why a cylinder pulls with less force than it pushes, and pulls faster for the same flow of oil.
Push out: pressure × the whole piston face.
Pull in: pressure × the ring round the rod.
–push, kN (tonnes)
–pull, kN (tonnes)
–pushing out, cm/s
–pulling in, cm/s
Demo · the control valve
Where the oil goes
The operator’s lever slides a spool inside the valve. Its two lands open and shut the ports: the pump to one end of the cylinder and the other end to the tank, or, in the middle, nothing at all. Drag the lever, or use the buttons.
0.0spool moved, mm
–cap-end pressure, bar
0rod speed, cm/s
–rod out, mm
Demo · overload
When the load is too much
Add load until the oil can’t lift it. The pressure climbs to the relief valve’s setting, the valve opens, and the pump’s oil goes straight back to the tank.
Oil can hardly be squeezed, so with nowhere to go its pressure would shoot up in a fraction of a second. The relief valve is the safety vent: it caps the pressure, so the pump, the hoses and the cylinder are never pushed past their limit.
Its spring holds the valve shut up to the setting: 250 bar here.
Held open, it turns all the pump’s power into heat in the oil.
0pressure, bar
–most it can lift, tonnes
0through the relief valve, L/min
0turned into heat, kW
Demo · the cushion
A soft landing
Send the piston home at full speed, with and without the cushion. Without it, steel slams into steel. With it, a little trapped oil brakes the piston in the last 30 mm.
As the spear slides into the cushion bore, the oil behind the piston can only leave through a thin gap round the spear and a small adjustable hole. Its pressure rises and slows the piston before it reaches the end cap.
The cushion screw sets the size of the hole: smaller is softer, but slower.
Without a cushion, all the moving energy goes into one bang.
–speed at the end cap, m/s
–energy in the bang, J
–highest cushion pressure, bar
–time for the last 30 mm, s
More to know
Kinds, history and care
Six kinds of cylinder
Double-actingThe kind on this page: oil can go into either end, so it pushes and pulls under power. Almost every cylinder on an excavator is one.
Single-actingOil goes into one end only. The weight of the load or a spring brings it back. Car jacks and forklift masts work this way.
TelescopicSeveral tubes nested inside each other slide out one after another, so a short body gives a very long stroke. Tipper trucks use them.
Ram (plunger)No separate piston: the thick rod itself is pushed out by the oil. Simple and strong, for presses and lifts.
Tie-rod or weldedFactory cylinders often hold their end caps on with long bolts, so they come apart easily. Welded ones, like this, are tougher and more compact for machines that work outdoors.
Double-rodA rod comes out of both ends, so the area is the same both ways: equal force and equal speed. Used for steering and machine tools.
From a barrel of water to the building site
Blaise Pascal shows that pressure in a still liquid pushes equally in every direction.
Joseph Bramah patents the hydraulic press, which turns a small push on a small piston into a huge one on a big piston.
William Armstrong builds a crane worked by water pressure on the quayside at Newcastle upon Tyne.
London starts piping water at about 50 bar under its streets to work lifts, cranes and theatre stages.
Tower Bridge opens. Its two halves are raised by hydraulic engines, fed with water stored under pressure.
Oil takes over from water: it lubricates the seals, doesn’t rust the parts and doesn’t freeze.
The first excavators worked entirely by hydraulic cylinders appear in Europe. By the 1970s they have replaced most diggers worked by cables.
Excavators, cranes, aircraft landing gear, car brakes, lifts and presses all rely on them.
Keeping one healthy
Keep the rods clean and the wipers in good shape. Grit carried in past the wiper cuts the rod seal and scores the rod.
A wet, oily rod or drips at the gland mean the rod seal is wearing out.
If a raised load creeps down with the lever in the middle, oil is leaking away: past a worn spool, or past the cylinder’s own seals and valves.
Jerky or spongy movement often means air in the oil. Bleed it out and check the suction side for loose fittings.
Don’t hold a cylinder against its end or a stuck load with the lever on. The relief valve turns all that power into heat.
Change the oil and its filters on time. Dirty or overheated oil wears out seals, pumps and valves.
Never feel for a leak with your hand. A pinhole jet at 200 bar can pierce skin: use a piece of card instead.
Things worth knowing
Almost solidSqueeze hydraulic oil at 200 bar and it shrinks by only about 1 %. Air at the same pressure would shrink to a two-hundredth of its size.
Ten newtons a bar1 bar is 10 newtons on every square centimetre. At 200 bar, every square centimetre of the piston carries about the weight of three people.
Two kilometres down200 bar is the pressure of the sea about 2 km below the surface.
One in your carCar brakes are hydraulic: your foot pushes a small piston, and the brake fluid pushes bigger pistons at the wheels with much more force.
Thinner than a hairThe rod’s hard chrome is only about 25 micrometres thick, about a third of a hair, polished to a mirror finish so the seals glide.
Push 157, pull 110At 200 bar this cylinder pushes with 157 kN but pulls with 110 kN, because the rod covers part of the piston. For the same oil it pulls in 1.4 times faster.
Every Mechanism
