How does oil lift a bucket of earth?
The excavator’s steel arms do not grow longer. Hydraulic cylinders between the joints extend and retract, turning the arms and lifting the bucket.
The engine drives a pump. Oil enters one side of a cylinder and pushes its piston, while oil from the other side returns to the tank. Oil carries force; the engine supplies the energy.
More oil each second generally means faster movement. Supporting a heavier load calls for higher pressure. Follow the extending rod to its joint, then separate speed from pushing force.
Change one condition at a time
- Follow the extending rod. Change the boom pose slowly in the illustration and watch the joint turn.
- Compare separately. At the same pose, add load. Then keep the load and change oil flow. Which changes affect pressure and movement speed?
Connecting pressure, area, line of action and pivot
Look closer: the full process and its parts
Watch the extending rod on the boom. Oil enters its cylinder, pushes the rod outward and lifts the steel arm. The engine supplies energy; the oil carries force to the joint.
Start with the whole machine. The bucket curls, the stick draws in and the boom rises: three joints turn in sequence, yet none of the yellow steel members changes length. What extends and retracts are the three cylinders bridging the joints. One cylinder pushes out, one joint turns. This is where the force comes in.
Cut one boom cylinder open along its axis and you find a rod with a piston at its end. The piston divides the barrel into two chambers: the cap end at the base, and the rod end around the rod. The pump sends oil into the cap end; as oil accumulates the piston has nowhere to go but outward, and the rod lifts the boom. Oil in the other chamber is squeezed out and returns to the tank.
Where the oil comes from: one closed circuit
The engine drives the main pump, which draws oil from the tank and keeps sending it out. A pump provides flow; it does not decide the pressure. The oil first reaches the main control valve. Each cylinder has its own valve section, and the spool inside slides left or right to decide which chamber the oil enters. With the spool centred, both cylinder ports A and B are blocked, the oil is trapped and the boom stays put. Move the spool and the pump port P connects to the cap end A while the rod end B connects to the return port T: supply pushes the piston out and the displaced oil returns to the tank. Nothing is consumed; the oil circulates in a closed loop.
Pressure answers “how hard”
Pull the lever a little and the cylinder is connected to the pump. The gauge now depends on the weight in the bucket: about 6.5 MPa with 300 kg, rising to 9.5 MPa at 1,800 kg. The pump does not “set” the pressure; it only delivers oil, and pressure rises just far enough to support the load.
Pascal’s principle says that in closed, still, connected oil the pressure is nearly the same everywhere (ignoring height differences and flow losses). Picture a manual hydraulic sketch built on it: a 20 mm small piston and the 120 mm boom piston feel the same pressure, so 36 times the area means 36 times the force — at 9.5 MPa about 3 kN on the small piston and about 108 kN on the boom piston. That is how hydraulics turns a small push into a large one. A real excavator’s pump, though, is not a hand-pressed piston: driven by the engine it supplies oil continuously, the pressure settles wherever the load demands it (as the last paragraph showed), and the flow sets how fast the motion is. Pressure times area is the force per cylinder; the piston face is fixed at 113 cm².
The lever answers “how fast”
The lever is not linked to the cylinder; it moves the spool (through pilot pressure on modern machines). Pull a little and the port opens a crack, so little oil gets through: 40 litres a minute, about 3 cm/s at the piston. Pull all the way and the port is wide open: 100 litres a minute, over 7 cm/s. Area and pressure barely change; only the volume arriving each second does. In the ISO control pattern the right lever runs the boom fore-and-aft (back lifts, forward lowers) and the bucket side-to-side; the left lever runs the stick fore-and-aft and the swing side-to-side.
Detailed controls
- Keep the pose and drag Bucket load. Watch cap-end pressure and piston force follow the load.
- Drag the Right joystick. The further it leaves neutral, the further the spool in the circuit panel travels and the wider the port opens; flow and piston speed scale together while pressure stays put. Push forward and P connects to the rod end, so the boom lowers.
- Drag the Boom angle and read the lever arm d and the required pressure: the same load, but a different gravity moment and a different lever arm.
- Turn on Teeth under a boulder and pull the joystick back. Pressure climbs to the 37.3 MPa relief setting, oil returns to tank through the relief valve, and the boom stops rising. Return to neutral and the pressure falls to the weight alone.
- Drag Bucket curl and follow the bucket cylinder rod: the joint is pushed toward the tip, the link drives the bucket ear, and the bucket curls about its pin.
The pose sliders compare static conditions; they do not send commands to the hydraulic system. Reaching a pose does not mean the machine could lift an arbitrary load there.
For a single-rod double-acting cylinder, taking extension as positive and neglecting friction, both chamber pressures set the net force:
Here A is the piston area, A_rod the rod cross-section, and both pressures are gauge values. The rod end has less effective area, so at equal pressure the cylinder pushes harder than it pulls, and at equal flow it retracts faster than it extends.
This scene takes two identical boom cylinders extending together with zero return pressure. With Q the total flow delivered to both, they share one speed:
The cylinder pushes along its own axis while the boom turns about the boom foot pin; the perpendicular distance d from that pin to the line of action is the lever arm. The pin coordinates on screen also define that line. The gravity moments of the boom, stick, bucket, cylinders and payload together set the required pressure:
where x is each centre of mass’s horizontal distance from the boom pivot. As the boom rises, the earth moves closer to the pivot and the moment shrinks, while the lever arm shortens too; the reading follows both. The bucket-linkage joint is solved as the intersection of the rocker circle and the link circle, so both short links keep their lengths. A linkage changes how force and motion are traded; it never adds energy.
Relief and holding
The main relief valve limits the system’s maximum working pressure. With the teeth caught under something immovable, the pump keeps delivering while the piston cannot advance; once pressure reaches the relief setting the surplus oil returns to tank and the boom stops rising. That is how the system protects itself. Releasing the lever closes the valve, traps the oil and lets the pressure fall to the weight alone. Real boom cylinders also carry dedicated load-holding and hose-burst protection valves that meet the lowering requirements of ISO 8643; the ideal trapped oil here simplifies those devices.
The circuit panel is a teaching view of the boom section of the main control valve: full lever travel in the exploration corresponds to a teaching flow of 130 L/min, while the film’s joystick chapter peaks at 100 L/min and holds about 40 L/min in its first half (the narration quotes those two demo conditions), metered linearly with travel. Real machines shift the spool with pilot pressure, share the 439 L/min maximum pump flow among all actuators, and add load sensing, flow combining and regeneration circuits, all omitted here. The 20 mm pump piston is a teaching size for Pascal’s principle, not a specific pump’s dimension.
This is a planar rigid linkage with a quasi-static hydraulic model. It omits inertia, friction, digging resistance, oil compressibility, leakage, valve dynamics, swing, travel, structural stress and whole-machine stability. Cylinder bores and the main relief setting follow the Komatsu PC200-8 product brochure (boom 120/85 mm, arm 135/95 mm, bucket 115/80 mm, implement relief 37.3 MPa, maximum pump flow 439 L/min). Member lengths, barrel lengths, component masses and body dimensions are teaching values for this scene, not a reconstruction of that machine and not a lifting chart.