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The pressure principle

The power required to drive oil through a restrictor or valve is directly related to the pressure drop across and flowrate through it. To assist in system design the following formulae can be used to calculate the power required. This may help to perhaps specify the rating for a prime mover or consider the power absorbing.braking capacity of a valve.


 Hydraulic power (kW) =

incrementp (bar) x flowrate (litre/min)
where incrementp = pressure drop
also Hydraulic power (HP) = incrementp (psi) x flowrate (USgpm)  


The temperature rise in oil flowing through a restriction is caused by fluid pressure energy being converted into heat energy as the fluid experiences a drop in pressure.

The following formula can be used to give the approximate temperature rise across a restriction commonly used hydraulic mineral oil in the ISO viscosity range VG 10 - 100.

  incrementT (°C) = 5.7 x incrementp (bar) where incrementT = temperature rise
    100   incrementp = pressure drop
also incrementT (°F) = 7 x incrementp (psi)    


Regenerative flow between the rod end and full bore end of an actuator is normally used to give a higher speed of extension compared to that due to pump flow acting alone.

The following formulae can be used to calculate the total input flowrate to the actuator and the equivalent effective pressure available to do work. The flow values can then be used to calculate the speed or time of actuator extension and enable suitably sized pipelines or components to be selected.

  Q (regen) = Q (pump) x (D² - d²)/d² where: d = actuator piston rod diameter
      D = actuator full bore diameter
  Q (total) = Q (pump) + Q (regen)   Q (regen) = regenerative flowrate from the rod end
      Q (pump) = pump supply flowrate
also Q (total) = Q (pump) + Q (regen)   Q (total) = extension flowrate when regenerating

Note: The maximum force available from an actuator during regenerative operation is reduced by the opposing pressure in the rod end of the actuator. Discounting any additional pressure losses due to the higher flowrate, the reduced regenerative force and equivalent regenerative pressure become:

  regenerative force = supply pressure x rd cross sectional area (p d²/4)

  equivalent regenerative pressure = supply pressure x (d/D)²



Current Ampere (A) base unit
Potential Volt (V) V = W/A
Power Watt (W) W = J/s = VA
Resistance Ohm (ohm) ohm = V/A
Capacitance Farad (F) F = As/V = C/V
Inductance Henry (H) H = Vs/A
Magnetic Influx Wieber (Wb) Wb = Vs
Energy Joule (J) J = Nm = Ws
Charge Coulomb (C) C = As


PRESSURE or STRESS Pascal (Pa) [N/m²] millibar (mbar) 1 mbar = 100 Pa
kiloPascal (kPa) bar 1 bar = 100 kPa - 14.5 psi
psi (lbf/in²) 1 psi = 6.897 kPa
MegaPascal (MPa) [N/mm²] bar 1 MPa = 10 bar
psi (lbf/in²) 1 MPa = 145 psi
tonf/in² 1 tonf/in² = 15.45 MPa
FLOWRATE litre/min (l/min) UKgal/min 1 UKgal/min = 4.546 l/min
USgal/min 1 USgal/min = 3.785 l/min
TEMPERATURE degrees Celsius (°C) Farenhiet (°F) 1°F = 1.8°C + 32
AREA square millimetre (mm²) square inch (in²) 1 in² = 645.16 mm²
square metre (m²) square feet (ft²) 1 m² = 10.76 ft²
hectare (ha) [10,000m²] acre (acre) 1 ha = 2.471 acre
VOLUME millimetre (ml) [10-³l] cubic inch (in³) 1 in³ = 16.387 ml
millimetre (ml) [10-³l] drops of ISO (VG 46) oil 1 ml = 17 drops (appox)
litre (l) [10-³m³] UK gallon (UKgal) 1 UKgal = 4.546 l
MASS litre (l) [10-³m³] US gallon (USgal) 1USgal = 3.785 l
gram (g) ounce (oz) 1 oz = 28.35 g
kilogram (kg) [1000 g] pound (1lb) [16 oz] 1 kg = 2.205 lb
tonne (t) [1000kg] imp ton (ton) [2240 lb] 1 ton = 1.016 t



Newton (N) [kgm/s²]

micron(µm) [10-6m]

kilogram force (kgf) 1 kgf = 9.807 N
pound force (lbf) [lb/s²] 1 lbf = 4.448 N
thousanths of inch (thou) 1 thou = 25.4 µm
LENGTH millimetre (mm) [10-³m] inch (in) 1 in = 25.4 mm
centimetre (cm) [10 mm] foot (ft) [12 in] 1 ft = 30.48 cm
metre (m) yard (yd) [3 ft] 1 m = 1.0936 yd
kilometre (km) [1000 m] mile (mile) [1760 yd] 1 mile = 1.609 km
TIME milliseconds (ms) no alternative 1 second = 1000 ms
seconds no alternative base unit of time
minute (min) no alternative 1 min = 60 s
hour (h) no alternative 1 h = 60 min
day (d) no alternative 1 d = 24 h
TORQUE Newton metre (Nm) pound force.feet (lbf.ft) 1 lbf.ft = 1.356 Nm
POWER kiloWatt (kw) [1000 Nm/s] horsepower (hp) 1 kW = 1.341 hp
metric horsepower (CV) 1 kW = 1.36 CV
ANGLES & CYCLES pi (pi) constant no alternative circle circumference / diameter
radian (rad) revolution (rev) 1 rev = 2 pi rad
degree (°) revolution (rev) 1 rev = 360°
frequency (Hz) [cycles/s] no alternative not applicable
rotational speed (rad/s) rpm (rev/min) 1 rad/s = 9.55 rev/min