| TECHNICAL NOTES
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) = |
 p
(bar) x flowrate (litre/min) |
where p
= pressure drop |
| |
|
600 |
|
| |
|
|
|
| also |
Hydraulic power (HP) = |
p
(psi) x flowrate (USgpm) |
|
| |
|
1714 |
|
HEATING EFFECTS
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.
| |
T
(°C) = |
5.7 x p
(bar) |
where |
T
= temperature rise |
| |
|
100 |
|
p
= pressure drop |
| |
|
|
|
|
| also |
T
(°F) = |
7 x p
(psi) |
|
|
| |
|
1000 |
|
|
REGENERATIVE FLOW IN AN ACTUATOR
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
USEFUL FORMULAE
| QUANTITY |
SI UNIT |
DERIVED FORMULAE |
| Current |
Ampere (A) |
base unit |
| Potential |
Volt (V) |
V = W/A |
| Power |
Watt (W) |
W = J/s = VA |
| Resistance |
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 |
| QUANTITY |
SI UNITS |
ALTERNATIVE UNITS |
CONVERSION FACTORS |
| 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 |
| QUANTITY |
SI UNITS |
ALTERNATIVE UNITS |
CONVERSION FACTORS |
| FORCE |
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 ( )
constant |
no alternative |
circle circumference / diameter |
| radian (rad) |
revolution (rev) |
1 rev = 2
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 |
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