Q = V_disp · n (V_disp in cm³/rev or in³/rev, n in rpm)
Pressure
p
bar, MPa, psi
1 bar = 14.504 psi = 0.1 MPa
Hydraulic power
P_h
kW
P_h = Q·p / 600 (Q in L/min, p in bar)
Cylinder force (extend)
F
N
F = p · A_bore
Cylinder force (retract)
F
N
F = p · (A_bore − A_rod)
Cylinder speed
v
m/s
v = Q / A (Q in m³/s, A in m²)
Motor torque
T
Nm
T = (V_disp · Δp) / (2π · η_mech), V_disp in m³/rev
Motor speed
n
rpm
n = Q / V_disp · η_vol
Bulk modulus (mineral oil)
β
MPa
~1500 MPa (at 50°C, 100 bar) — governs system stiffness
Reynolds number (pipe)
Re
—
Re = ρ·v·D / μ; transition ~2300
ISO viscosity grade (ISO 3448) = kinematic viscosity in cSt at 40°C. Hydraulic fluids: ISO VG 22, 32, 46, 68, 100 most common.
2. Hydraulic Pumps — Gear
2.1 External Gear
OEM
Country
Flagship
Displacement
p_max
Service
Bosch Rexroth
DE
AZP series (Silence Plus), 0510/0511
0.25-200 cm³/rev
210-330 bar
Industrial, mobile
Parker
US
PGP/PGM (Commercial Intertech heritage) 300/600 series
1-220 cm³/rev
230-310 bar
Mobile + industrial
Casappa
IT
Polaris, KP/KM/PLP
0.25-200 cm³/rev
250-300 bar
Mobile
Marzocchi
IT
ALP, GHP, ELI, 1P/2P/3P
0.25-200 cm³/rev
230-300 bar
Mobile + industrial
Eaton (now Danfoss)
US
26 / V20 vane heritage; 740 series gear
1-90 cm³/rev
210-300 bar
Mobile
Tuthill
US
M / 2C / 4C industrial
0.5-150 cm³/rev
207 bar
Transfer pumps
Haldex / Concentric
SE
LP / LF (lube oil)
0.16-99 cm³/rev
180-250 bar
Lube + truck PTO
Roquet
ES
T / 1P / 2L
1-110 cm³/rev
250-300 bar
Mobile
Salami
IT
1PE / 2PE / 3PE
1-200 cm³/rev
250-300 bar
Mobile
Hydac
DE
PGE (external gear)
1-150 cm³/rev
250 bar
Industrial
Hydreco (Duplomatic)
UK
gear pumps
1-180 cm³/rev
280 bar
Mobile + industrial
2.2 Internal Gear (Gerotor / Crescent)
OEM
Flagship
p_max
Bosch Rexroth
PGH / PGF (silent internal gear)
250-330 bar
Eckerle / Bosch
EIPH (internal gear high-pressure)
330 bar
Voith Turbo
IPV (inverse + variable internal gear)
250-320 bar
Truninger / Parker
QT / QX
250-280 bar
Settima ContinuumX
three-screw heritage; also internal gear
—
Maag
gear pumps for high-viscosity polymer + chemical
70-700 bar special
Viking (IDEX)
gerotor + internal gear chemical
low pressure transfer
3. Hydraulic Pumps — Vane
OEM
Flagship
Displacement
p_max
Type
Eaton/Vickers (Danfoss)
V10 / V20 / V104 (fixed); 25 V / 35 V / 45 V (industrial); VVA / VVB / VVT (variable)
1-280 cm³/rev
175-210 bar
Industrial workhorse
Parker Denison
T6 / T7 (fixed), PVV / PVR variable
1-300 cm³/rev
250-280 bar
Industrial
Bosch Rexroth
PV7 (variable), PVV (fixed)
up to 200 cm³/rev
210 bar
Industrial
Yuken
PV2R fixed, AR / SVPF variable
6-237 cm³/rev
250 bar
Industrial (machine tool)
Continental Hydraulics
PowrFlow variable vane
100-450 cm³/rev
250 bar
Machine tool
Tokyo Keiki / TOKIMEC
SQP / VPV
8-200 cm³/rev
210-280 bar
Industrial JP
Atos
PFE (vane fixed), PVPC (variable)
5-250 cm³/rev
280 bar
Industrial EU
Pressure-compensated variable vane preferred for hold-and-clamp industrial machinery: pump destrokes to zero displacement at set pressure → minimal heat input vs throttling losses.
Servo valve = high-bandwidth nozzle-flapper or jet-pipe pilot stage + spool main stage with continuous position feedback (LVDT); used in injection molding, flight simulators, structural test (MTS, Instron).
7. Pressure Control Valves
Type
Function
Example product
Direct-acting relief
Limits system pressure (small flow)
Sun RDDA, Rexroth DB10
Pilot-operated relief
High-flow system protection
Sun RPGC, Rexroth DBW
Reducing
Maintains reduced downstream pressure
Sun PBDB, Rexroth DR
Sequence
Opens at set pressure to feed secondary circuit
Sun RBAA, Rexroth DZ
Counterbalance
Holds load against gravity, controlled lowering
Sun CB series (4-port), Rexroth VW
Brake (motor brake)
Counterbalance for hydraulic motor
Sun CB, Hydac DBMA
Unloading
Diverts pump flow to tank above set pressure
Sun DSAE, Rexroth DA
Pressure switch
Electrical signal at set pressure
Hydac EDS series, Sun PSAB
8. Flow Control + Check Valves
Type
Function
OEM example
Throttle (needle)
Adjustable orifice
Rexroth DV, Parker NV
Pressure-compensated flow control
Maintains set flow regardless of Δp
Rexroth 2FRM, Sun FXEA
Priority flow control
Steering priority over implement (mobile)
Sauer-Danfoss OSP/OSPC, Rexroth LFA
Bypass flow control
Bypasses excess
Sun FXED
Check (one-way)
Reverse flow blocked
Sun CXBA, Rexroth Z2S
Pilot-operated check
Releases blocked flow with pilot
Sun CKED / CKCB, Rexroth SV / SL
Counterbalance check
Holds load — see “Counterbalance” above
—
9. Cartridge Valves — ISO 7368 (DIN 24342)
Slip-in two-way (2/2) cartridges in standard cavities sized NG16-NG100. Main-stage cartridge + pilot valve = high-flow logic element (used for hydraulic press, shear, large mobile).
Standard cavity sizes: 16, 25, 32, 40, 50, 63, 80, 100 mm bore.
OEMs (slip-in 2/2): Bosch Rexroth, Parker (Vickers), Eaton Vickers, Hydac, Atos, Argo-Hytos, Sun Hydraulics (screw-in cartridges with Sun cavity), HydraForce.
OEMs (screw-in cartridges, common cavities): Sun Hydraulics (Helios), HydraForce, Comatrol (Eaton), Hawe (CMV), Bucher (Caproni), Rexroth, Parker.
10. Hydraulic Cylinders
10.1 Industrial Tie-Rod (NFPA T3.6.7 / ISO 6020-1 / ISO 6022)
SAB / SAOB / FXG / FXP / FMP series + Bag-Gripper FXCB
NBR, NK, HT1, FDA-silicone, EPDM
Piab (Patrick)
BX / Bellows / FlatCups / piGRIP
Polyurethane, silicone, NBR
Festo
ESS / ESH / VAS / VAD / OGVM
NBR, FKM
SMC
ZP / ZPT / ZPY
NBR, silicone
Vuototecnica
flat, bellows, oval
many
Coval
VPC / VPF / VPVL
NBR, PU
Anver
F / FB / B series
NBR, silicone
Bimba
—
various
Festo
OGVM (Octo) — flexible 4-fold cup, ContacTouch
—
20. Pneumatic Tubing
Material
Pressure
Temperature
Use
Polyurethane (PU)
10 bar
-10 to +60 °C
General pneumatic (most common)
Polyamide PA11 / PA12
27 bar
-40 to +100 °C
Higher-temp, higher-pressure
PTFE
14 bar
-200 to +260 °C
Chemical, hot air
FEP
20 bar
-65 to +200 °C
Chemical, cleanroom
Nylon braided
up to 14 bar
varies
Truck air brake (DOT-approved)
Push-to-connect tubing
per fitting rating
—
Modular pneumatic
Common OD sizes: 4, 6, 8, 10, 12, 16 mm (metric); 1/8, 1/4, 5/16, 3/8, 1/2 inch (imperial).
OEMs (tubing): Festo PUN / PAN / PFAN, SMC TUS / TIUB, Parker Parflex, Camozzi 1080, John Guest Speedfit (water), Norgren Fluidfit.
21. Common Pitfalls
Sizing hydraulic pump from continuous flow instead of peak: pump runs at relief most of cycle; specify load-sensing or pressure-compensated variable pump
Specifying high-pressure cylinder seal kit for low-pressure service: high-friction NBR/PUR seals cause stick-slip; use low-pressure low-friction seal (PTFE-bronze) below 10 bar
Open-center directional valve with pressure compensator: pump destrokes but spool open-center wastes energy; use closed-center with PC pump
Servo valve installed with insufficient fluid cleanliness: spool spool-bore tolerance ~2 μm; ISO 4406 17/15/12 or cleaner required, else seizure
Accumulator pre-charge too high vs system minimum pressure: bladder bottoms on poppet at low pressure → bladder destruction within hours
Bladder accumulator mounted horizontally: bladder folds at gas-fluid interface and fails; mount vertically gas-up
Counterbalance valve pilot ratio wrong (3:1 vs 4.5:1 vs 10:1): instability or load creep; 3:1 stable but high system pressure; 10:1 fastest but oscillation-prone
Vacuum cup selected by lifting force alone: factor 4 dynamic, 8 vertical-pull safety; verify against acceleration + cup-orientation
Compressor sized for instantaneous peak: cyclic loads benefit from receiver tank sized 6-10× cfm/min; VSD compressor for stable demand
Polyurethane tubing in compressed-air system above 60°C (downstream of unsized aftercooler): tubing softens, fails; specify PA12 or check air temp at point-of-use
Cylinder cushioning under-set for fast extending stroke: end-of-stroke impact damages rod-end; adjust pneumatic cushion screws or specify hydraulic shock absorber (Ace Controls, SMC RB, Festo DYEF)
ORFS vs JIC mismatch in same machine: visually similar; mixing causes leaks; specify ORFS exclusively for high-vibration mobile
Hose routing with twist > 5°: rapid pressure failure; bend in single plane, allow service loop
Skydrol phosphate ester in seals not rated for it: NBR/SBR swell + dissolve; use Viton, EPDM, butyl per spec
Water-glycol HFC fluid in pump with leaded bronze: zinc + lead leach; specify FKM seals + non-leaded bronze
HEES biodegradable in cold ambient with viscosity index too low: high startup pressure / hose burst; use HEES with VI improver
Underspec’d return-line filter on system with large reservoir: contamination builds up in tank; specify β₁₂ ≥ 1000 minimum on critical servo
Pneumatic cylinder used for precision positioning without stop block: air compressibility yields variable end position; use electromechanical actuator or hydraulic with servo valve