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Fluid Power Formulas
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Rules-of-Thumb
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Torque and horsepower relations:
T = HP x 5252 ÷ RPM
HP = T x RPM ÷ 5252
RPM = HP x 5252 ÷ T
Torque values are in foot pounds.
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Horsepower for driving a pump:
For every 1 HP
of drive, the equivalent of 1 GPM @ 1500 PSI can be produced.
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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.
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Horsepower for idling a pump:
To idle a pump
when it is unloaded will require about 5% of its full rated
horsepower.
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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.
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Compressibility of hydraulic
oil:
Volume reduction
is approximately 1/2% for every 1000 PSI of fluid pressure.
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Charles' Law for behavior
of gases:
T1 V2 = T2 V1, or T1 P2
= T2 P1
T1, P1. and V1 are initial temperature, pressure, and volume, and T2,
P2, and V2 are final
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Compressibility of water:
Volume reduction
is about 1/3% for every 1000 PSI pressure.
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Boyle's Law for behavior
of gases:
P1 V1 = P2 V2
P1 and V1 are initial pressure and volume; P2 and V2 are final conditions.
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Wattage for heating hydraulic
oil:
Each watt will
raise the temperature of 1 gallon of oil by 10F per hour.
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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.
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Flow velocity in hydraulic
lines:
Pump suction lines 2
to 4 feet per second; pressure lines up to sure to 500 PSI,
10 to 15 feet/sec.; Pressure lines 500 to 3000 PSI, 15 to 20
feet/sec.; pressure lines over 3000 PSI, 25 ft/sec; all oil
lines in air-over-oil system, 4 ft/sec.
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Heat equivalent of fluid
power:
BTU per hour = PSI x
GPM ÷ 2
Thrust or force of any
cylinder:
T = A x PSI
T is thrust or force, in pounds, A is piston net area in square inches,
PSI is gauge pressure.
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