Pneumatics, Hydraulics & Electro-Fluid Control

Force and motion from pressurised fluids. Learn pneumatic and hydraulic system components (compressors/pumps, FRL units, valves, cylinders, motors, accumulators), ISO 1219 symbols, cylinder force and speed, flow and power, directional/pressure/flow control valves, solenoid valves and electro-pneumatic circuits with PLCs, proportional and servo valves, sequencing (A+B+A−B−), and safety — with Python sizing hydraulic and pneumatic cylinders.

Fluid power delivers very high forces from compact actuators: a hand-sized hydraulic cylinder can push tonnes; a simple pneumatic cylinder cycles millions of times in packaging and assembly machines. Excavators, presses, aircraft flight controls and factory automation all rely on it.

Pneumatics (compressed air, ~6 bar) is clean, cheap and fast but compressible and hard to position precisely; hydraulics (oil, 100–350 bar) gives huge force and stiffness. Electro-fluid systems combine them with solenoid, proportional and servo valves controlled by PLCs and motion controllers.

An analogy

A hydraulic system is like a syringe pushing another syringe: push a small piston a long way and a big piston moves a short way with much more force — Pascal's law multiplies force exactly as a lever does.

System components

A typical fluid-power circuit:

Circuits and control

ISO 1219 symbols describe circuits; valve naming 'ports/positions' (a 5/2 valve has 5 ports, 2 positions). Speed control by meter-out throttles gives stable motion; relief valves protect the system; pressure-reducing valves limit force. Electro-pneumatics uses solenoid valves driven by PLC outputs and reed/inductive sensors as inputs — sequences like A+ B+ A− B− are programmed as state machines (or with cascade/step methods).

Pneumatics vs hydraulics vs electric

Pneumatics: cheap, fast, clean, safe in explosive areas, overload-tolerant; low stiffness and energy efficiency (~10–20% from electricity). Hydraulics: highest force density and stiffness, good for heavy loads; leaks, fire risk, noise, maintenance. Electric actuators: precise, efficient, programmable; increasingly replace both in light-to-medium duties.

Proportional and servo-hydraulics

Proportional valves meter flow in proportion to a solenoid current; servo valves (torque motor + spool) have high bandwidth. Closed-loop position control of hydraulic cylinders reaches micrometre precision in presses, test rigs and flight-simulator motion platforms. Load-sensing pumps and variable-displacement pumps save energy; electro-hydrostatic actuators (EHA) integrate motor, pump and cylinder. Safety: pressure ratings, burst factors, lockout of stored energy, ISO 4413/4414.

Formulas

Worked example: Hydraulic force

Given: 150 bar, 50 mm bore

Find: Extend force.

Answer: 29.5 kN

Worked example: Cylinder speed

Given: 20 L/min into a 50 mm bore

Find: Extend speed.

Answer: 0.17 m/s

Worked example: Air consumption

Given: 40 mm bore, 100 mm stroke, 6 bar(g), double-acting, 30 cycles/min

Find: Free-air flow (ignore rod).

Answer: ≈ 53 L/min free air

Sizing hydraulic and pneumatic cylinders

import math
# Hydraulic and pneumatic cylinder sizing
p_hyd, bore, rod = 150e5, 0.050, 0.028        # Pa (150 bar), m
A_cap = math.pi * bore**2 / 4
A_rod = A_cap - math.pi * rod**2 / 4
Q = 20 / 60000                                # 20 L/min in m^3/s
print(f"hydraulic: extend force {p_hyd*A_cap/1000:.1f} kN, retract {p_hyd*A_rod/1000:.1f} kN")
print(f"extend speed {Q/A_cap:.3f} m/s, retract speed {Q/A_rod:.3f} m/s")
eta = 0.85
print(f"pump power {p_hyd*Q/eta/1000:.2f} kW")
# Pneumatic cylinder at 6 bar gauge, 40 mm bore, 90% efficiency
F_pn = 6e5 * math.pi * 0.040**2 / 4 * 0.9
print(f"pneumatic 40 mm @ 6 bar: {F_pn:.0f} N")
# hydraulic: extend force 29.5 kN, retract 20.2 kN
# extend speed 0.170 m/s, retract speed 0.247 m/s
# pump power 5.88 kW
# pneumatic 40 mm @ 6 bar: 679 N

A 50 mm hydraulic cylinder pushes 3 tonnes; a pneumatic one of similar size only ~68 kg-force — but pneumatics is simpler, cleaner and faster. Retraction is faster and weaker than extension because the rod reduces the effective area.

Common mistakes

Applications

Key takeaways

Going further