Hydraulic and Pneumatic Systems
Principles
Pascal's law: pressure applied to a confined fluid transmits equally in all directions. Force = pressure × area, and cylinder speed = flow ÷ area. The pump creates flow, not pressure - pressure comes from resistance. The relief valve limits system pressure and must be set per the schematic; an individual circuit may have its own.
Components include gear, vane, and piston pumps; directional control valves (open vs closed center); flow controls for speed; check valves; accumulators for stored energy and shock absorption; filters rated in microns and ISO cleanliness codes; and coolers.
Pneumatics
Air is compressible, so actuators are springy and speed is controlled with flow controls (meter-out is typical for cylinders). Air preparation is FRL - filter, regulator, lubricator. Water is the enemy: drain traps and dryers. Shop air runs far lower than hydraulic pressure, typically 80–120 psi.
Safety
Depressurize and verify zero energy before breaking a line. Accumulators hold pressure after the pump stops - bleed them per procedure. Never search for a hydraulic leak with your hand; pinhole injection injuries are surgical emergencies. Respect hose pressure ratings, use whip checks, and secure routing.
Traps
Assuming relief valve setting equals working pressure; dead-heading a pump to check flow; using air to test a hydraulic component; confusing a flow control with a pressure control; ignoring viscosity and oil temperature.
Rule of thumb
Force = pressure × area. A 2 in bore at 1000 psi gives 3.14 in² × 1000 = 3,140 lbf extending; retraction is less because the rod reduces effective area (π/4 × (D² − d²)).