Hydraulics & Pneumatics Guide
A complete guide to hydraulics and pneumatics: Pascal's principle and formulas, circuit components, valve types, oil viscosity and cleanliness, pneumatic systems, accumulators and filters, and interactive calculators for cylinder force, power, buckling, and accumulator sizing.
๐ง
Hydraulics & Pneumatics
Principles ยท Circuit ยท Valves ยท Oils ยท Pneumatics ยท Components ยท Calculators ยท Standards
๐ Pascal's Principle and Basic Formulas
Core Formulas
// Pascal's Principle โ pressure in a closed fluid is transmitted equally in every direction
F = P ร A // Force [N]. P=[Pa], A=[mยฒ]
P = F / A // Pressure [Pa=N/mยฒ]
// Convenient pressure conversions
1 bar = 100,000 Pa = 10.2 kgf/cmยฒ = 14.5 PSI
1 MPa = 10 bar = 145 PSI
// Flow rate and velocity
Q = A ร v // [mยณ/s] โ ร60000 = [L/min]
v = Q / A // Piston speed from flow rate
// Hydraulic power
P_kW = p[bar] ร Q[L/min] / 600
// Piston areas (rod-side cylinder)
A_ext = ฯรDยฒ/4 // Extend area (full)
A_ret = ฯ(Dยฒโdยฒ)/4 // Retract area (minus rod)
โธ Retract area is always smaller than extend area โ faster retract speed at the same flow rate, but lower force!
โธ Typical area ratio (Regeneration): 1.6:1 to 2:1
โธ Typical area ratio (Regeneration): 1.6:1 to 2:1
Typical Operating Pressures
| System | Pressure (bar) |
|---|---|
| Industrial pneumatics | 4-8 |
| Light mobile hydraulics | 150-180 |
| Mobile hydraulics (excavators) | 250-350 |
| Industrial hydraulics | 140-210 |
| Hydraulic presses | 300-700 |
| Aerospace systems | 210-280 bar |
System Efficiency โ Loss Sources
Pump85-95% (Gear low, Piston high)
Valvesฮp loss at every pass โ heat!
PipingFlow friction โ depends on velocityยฒ
Fixed SystemTypically 60-70% โ excess returns to Relief
Load-Sensing System80-90% โ significant savings
๐ Hydraulic Circuit โ Core Components
Pumps
| Type | Max Pressure | Efficiency | Use |
|---|---|---|---|
| Gear | 250 bar | 85-90% | Cheap, general |
| Vane | 175 bar | 85-92% | Quiet, medium |
| Axial Piston Fixed | 350 bar | 92-95% | Fixed flow, heavy duty |
| Axial Piston Variable | 420 bar | 90-94% | Load-Sensing, efficient |
| Radial Piston | 700 bar | 92-96% | Extreme pressure |
โธ Variable Displacement = adjusts volume on demand โ the basis for energy savings
โธ Piston is always more expensive but more efficient and durable than Gear
โถ YouTubeโธ Piston is always more expensive but more efficient and durable than Gear
Hydraulic Motors
PrincipleThe reverse of a pump โ flow creates rotation
Gerotor/OrbitHigh torque, low RPM โ wheel drive
Axial PistonHigh RPM, excellent efficiency โ hydrostatic
TorqueT[Nm] = p[bar]รV[cc/rev]/(20รฯ)
โถ YouTubeHydraulic Cylinder โ Construction
TubeHoned bore Ra 0.1-0.4ฮผm
PistonSeal + Wear Ring โ side bearing
RodHard Chrome plated 20-40ฮผm
GlandRod Seal + Wiper + Bushing
CushionBuilt-in end deceleration (see Seals tab)
โถ YouTube
Reservoir โ Oil Tank
Volume3-5ร pump flow rate per minute [liters]
FunctionCooling, dirt settling, air release
Baffle PlateSeparates suction from return โ prevents short-flow
BreatherAir filter at inlet/outlet as level changes
Basic Circuit Diagram โ Component Sequence
Reservoir โ Suction filter โ Pump โ Relief valve (protection) โ Directional control valve (DCV) โ Cylinder/motor โ Flow control valve (speed) โ Return filter โ Reservoir
โธ Relief always right after the pump โ the circuit's first line of protection
โธ Return filter is no less critical than suction โ it catches component wear debris
โธ Return filter is no less critical than suction โ it catches component wear debris
๐ง Hydraulic and Pneumatic Valves
Full Valve Table
| Valve | Function | Note |
|---|---|---|
| Relief Valve | Limits maximum pressure | First line of protection โ mandatory in every circuit |
| Check Valve | One-direction flow | Cracking 0.3-3 bar |
| Pilot Operated Check | Load Holding lock | Prevents piston drift |
| Directional Valve 4/3 | Piston direction โ 4 ports 3 positions | Closed/Open/Float/Tandem |
| Flow Control | Flow/speed regulation | Meter-In / Meter-Out |
| Pressure Reducing | Reduces pressure for a secondary circuit | Normally open |
| Sequence Valve | Triggers action B after A | By pressure threshold |
| Counterbalance Valve | Supports a hanging load | Prevents free fall |
| Proportional Valve | Continuous electrical control | ยฑ5% hysteresis |
| Servo Valve | High precision with feedback | ยฑ0.1% โ simulators |
Meter-In vs Meter-Out
Meter-InRestricts incoming flow โ positive (raising) load
Meter-OutRestricts outgoing flow โ negative (lowering/falling) load
โธ Meter-Out prevents cavitation with an overrunning load โ the safe choice for lifts and cranes
โถ YouTubeDCV โ Center Position Coding
Closed CenterAll ports blocked โ load locked
Open CenterP-T open โ pump idles at zero pressure
Float CenterA,B to T โ piston free to move
Tandem CenterP-T open, A,B closed โ common in forklifts
โถ YouTube๐ข๏ธ Oils, Lubrication, and Cleanliness
Viscosity Grades โ ISO VG
| ISO VG | cSt @40ยฐC | Hydraulics | Gear |
|---|---|---|---|
| VG 10 | 10 | Cold systems | โ |
| VG 22 | 22 | Light-duty tools | โ |
| VG 32 โ | 32 | Standard hydraulics | โ |
| VG 46 โ | 46 | Medium hydraulics (most common) | โ |
| VG 68 | 68 | Heavy-duty hydraulics | โ |
| VG 100 | 100 | Lathes | Light gear |
| VG 150 | 150 | โ | Medium gear |
| VG 220 | 220 | โ | Heavy gear (automotive) |
| VG 320 | 320 | โ | Industrial gear |
| VG 680 | 680 | โ | Gearbox โ mill drive |
โธ Choose VG so viscosity at operating temperature is 16-36cSt at the pump
โธ High Viscosity Index (VI) = less sensitive to temperature changes
โธ High Viscosity Index (VI) = less sensitive to temperature changes
Oil Cleanliness โ ISO 4406
CodeParticles at 4/6/14 micron (e.g. 18/16/13)
General Hydraulics20/18/15
Proportional18/16/13
Servo16/14/11
โธ Every lower number = half the particles! 80% of hydraulic failures are caused by contamination
Oil Additives
Anti-Wear (AW)Zinc โ high-pressure contact protection
Anti-FoamPrevents foam that causes cavitation
Anti-OxidantExtends oil life โ prevents oxidation
Anti-RustPrevents corrosion on metal
DemulsifierSeparates water from oil quickly
Temperature Control
Operating Range30-60ยฐC ideal
Above 70ยฐCAccelerated oxidation, seal wear
Below 10ยฐCHigh viscosity โ cavitation at suction
Oil CoolerAir/Water Cooler โ required above 15kW loss
๐จ Pneumatics โ Principles and Components
Comparison to Hydraulics
Operating Pressure4-8 bar (vs. 150-350 in hydraulics)
Efficiency~40% (hydraulics ~90%) โ compressed air
ForceF=PรA โ same formula, lower pressure
AdvantagesClean, safe (no fluid burst risk), fast, cheap
DisadvantagesLow force, noisy, imprecise (air compressibility)
FRL โ Air Preparation Unit
FilterMoisture and dirt filter โ 5-40 micron
RegulatorStabilizes a constant working pressure
LubricatorAdds an oil mist (older components only!)
โธ FRL always at the inlet of every pneumatic system โ without it, fast failure follows
โถ YouTubeAir Dryers
RefrigeratedDew Point 3-10ยฐC โ general purpose
DesiccantDew Point down to โ40ยฐC โ critical/outdoor
MembraneCompact, no electricity โ small lines
โถ YouTubeActuation Components
CylinderSingle/Double Acting โ like hydraulic
Rotary ActuatorRotation up to 360ยฐ โ Rack/Vane
Air MotorRotating tools โ safety (explosive zones)
GrippersPneumatic clamps โ robotics
โ๏ธ System Accessories
Accumulator
BladderRubber bladder โ most common, fast response
PistonLarge volumes, durable
DiaphragmSmall volumes, compact
Pre-Charge80-90% of minimum operating pressure
โธ Uses: emergency backup (brakes!), pulse absorption, peak flow supplementation
โธ PED standard for large volumes โ periodic inspection mandatory
โถ YouTubeโธ PED standard for large volumes โ periodic inspection mandatory
Filters โ Filtration
| Location | Rating (ฮผm) | Function |
|---|---|---|
| Suction | 100-150 | Pump protection from debris |
| Pressure | 10-25 | Protects sensitive valves |
| Return | 10-25 | Catches component wear debris |
| Off-Line/Kidney | 3-6 | Continuous cleaning โ critical systems |
Piping and Flow
Suction LineMaximum velocity 1.2 m/s
Pressure LineVelocity 4-6 m/s
Return LineVelocity 2-3 m/s
โธ Too fast = heat and pressure losses. Too slow = expensive, unnecessary piping
Cylinder โ End-of-Stroke Deceleration
CushionA built-in plunger decelerates before stroke end
AdjustmentA screw on both sides โ separate per direction
Without CushionHydraulic shock, anchor breakage
โถ YouTube๐งฎ Hydraulics Calculators
Piston Force and Speed
Calculate force, speed, and required flow rate for a cylinder
โ
Required Flow and Power
Calculate required motor power for a pump given pressure and flow
โ
Rod Buckling Check
Check if the cylinder rod is stable against buckling (Euler)
โ
Accumulator Sizing
Calculate required accumulator volume using gas law (isothermal)
โ
๐ Standards โ Hydraulics and Pneumatics
Piping and Component Standards
ISO 4413Safety of hydraulic systems
ISO 4414Safety of pneumatic systems
ISO 4406Oil cleanliness code
ISO 6149O-Ring Boss (ORB) fittings
SAE J514JIC 37ยฐ fittings
Oil and Filtration Standards
ISO VG 3448Industrial viscosity grades
ISO 11171Particle counter calibration
NAS 1638Older cleanliness standard (still in use)
Accumulator Standards
PED 2014/68/EUEuropean pressure equipment โ mandatory above a threshold volume
ASME VIIIAmerican pressure vessels
ISO 10771Accumulator fatigue testing