Principles in motion.
Eighteen animations.

Hydraulics is what happens between the piston and the spool. These animations, from our training archive, show the mechanism without words: one idea per clip.

Pumps5

Single-piston pump, end to endSuction, delivery, check valves and the crank: the smallest complete pump there is.

Swash-plate axial piston pumpNine pistons against one tilted plate. The plate angle is the displacement, and it is what the regulator moves.

Vane pump, in sectionVanes seal against a cam ring. Compare with the piston group to see why piston pumps survive pressure.

External gear pumpTwo gears, one sealed pocket. Simple and cheap, which is why gear pumps stay off the main circuit of an excavator.

Double-acting reciprocating pumpBoth sides of the piston deliver on every stroke: the ancestor of every positive-displacement pump.

Valves2

Pilot-operated check valveHolds a load without leaking, opens only when pilot pressure releases it. The reason the boom stays up.

Servo valveA small signal moves a big flow. The same idea the regulator uses to tilt the swash plate.

Circuits8

Three-stage pressure circuitRelief valves in series: each one decides a different working pressure. This is how pressure modes are switched.

Two-stage pressure circuitThe simplest version: one relief set, the second introduced by opening a valve.

Two-pump speed circuitTwo pumps into one function for speed, one pump for control. Tandem pumps exist for this reason.

Accumulator holding circuitStores energy and holds pressure with the pump idle: what a circuit does when it must hold a load for minutes.

Lock circuitTrapped oil held by check valves: the excavator version of a parking brake.

Directional control circuitThe spool decides where the oil goes. Everything the operator feels starts here.

Flushing valve circuitFlushes the low-pressure side and cools the circuit: why dirty oil kills a closed circuit.

Sequence by limit switch and solenoidOne function finishes, the next begins: the logic behind almost every automatic cycle on a machine.

Theory3

Leakage through an annular gapLeakage grows with the third power of the gap and falls with viscosity. Hot thin oil loses pressure.

Leakage through a flat gapThe same equation that explains a worn valve plate: a few microns is the whole game.

Absolute and gauge pressureWhy a gauge reads zero at atmospheric pressure, and why a suction leak is measured in negative numbers.

Principles in motion.Eighteen animations.

Pumps5

Valves2

Circuits8

Theory3

Principles in motion.
Eighteen animations.