Torque and horsepower relations
T = HP x 5252 ÷ RPM
HP = T x RPM ÷ 5252
Torque values are in foot pounds
Hydraulic (fluid power) horsepower
HP = PSI x GPM ÷ 1714
PSI is gauge pressure in pounds per square inch; GPM is oil flow in gallons per minute
Velocity of oil flow in pipe
V = GPM x 0.3208 ÷ A
V is oil velocity in feet per second; GPM is flow in gallons per minute; A is inside area of pipe in square inches
Charles' Law for behaviour of gases
P1V1 = P2V2 or T1P2 = T2P1
T1, P1 and V1 are initial temperature, pressure, and
volume, and T2, P2, and V2 are final conditions
Boyles' Law for behaviour of gases
P1V1 = P2V2
P1 and V1 are initial pressure and volume; P2 and V2 are final conditions
Hydraulic cyl. piston travel speed
S = CIM ÷ A
S is piston travel speed, inches per minute; CIM is oil flow into cylinder, cubic inches per minute; A is piston area in square inches
Force or thrust of any cylinder
F = A x PSI
F is force or thrust, in pounds; A is piston net area in square inches; PSI is gauge pressure
Circle formulas
Area = πr2, or πD2 ÷ 4
Circumference = 2πr, or πD
r is radius; D is diameter; π = 3.14


Force for piercing or shearing sheet metal
F = P x T x PSI
F is force required, in pounds; P is perimeter
around area to be sheared, in inches; T is sheet
thickness in inches; PSI is the shear strength
rating of the material in pounds per square inch
Side load on pump or motor shaft
F = (HP x 63024) ÷ (RPM x R)
F is the side load, in pounds, against shaft;
R is the pitch radius, in inches, of sheave on
pump shaft; HP is driving power applied to shaft
Effective force of a cylinder working at an angle to direction of the load travel
F = T x sinA
T is the total cylinder force, in pounds; F
is the part of the force which is effective,
in pounds; A is the least angle, in degrees,
between cylinder axis and load direction
Burst pressure of pipe or tubing
P = 2t x S ÷ O
P is burst pressure in PSI; t is wall thickness,
in inches; S is tensile strength of material in
PSI; O is outside diameter, in inches
Relationship between displacement and torque of a hydraulic motor
T = D x PSI ÷ 24π
T is torque in footlbs; D is displacement in cubic
inches per revolution; PSI is pressure difference across motor; π = 3.14
Heat radiating capacity of a steel reservoir
HP = 0.001 x A x TD
HP is the power radiating capacity expressed in horsepower;
A is surface area, in square feet; TD is temperature
difference in degrees F between oil and surrounding air


Rules–of–thumb
Horsepower for driving a pump
For every 1 HP of drive, the equivalent of 1 GPM @ 1500 PSI can be produced.
Horsepower for idling a pump
To idle a pump when it is unloaded will require about 5% of its full rated horsepower.
Compressibility of hydraulic oil
Volume reduction is approximately 1/2% for every 1000 PSI of fluid pressure.
Compressibility of water
Volume reduction is about 1/3% for every 1000 PSI pressure.
Wattage for heating hydraulic oil
Each Watt will raise the temperature of 1 gallon of oil by 1°F per hour.
Flow velocity in hydraulic lines
Pump suction lines 2 to 4 feet per second; pressure lines up
to 500 PSI, 10 to 15 feet per sec; pressure lines 500 to 3000
PSI, 15 to 20 feet per sec; pressure lines over 3000 PSI, 25
feet per sec; all oil lines in airoveroil system, 4 feet per sec.


