U.S. Customary Units Compared to ISO International Standard Units

FLUID POWER - Design Data Sheet 21

Preliminary work was started on an international system of units prior to 1954, and some European countries have adopted their own version of a metric system which could be used internationally. However, there are many duplicate metric units which could be used for the same function. At this writing the basic units which we believe will be adopted are shown in the second column of the chart, and will form the SI (International Standard) set of units which will eventually be used all over the world. Some of the units (like the meter) are unhandy for fluid power use and a variation (like the centimeter) may be used instead. These are shown in the third column of the chart. Correct abbreviations for each unit are shown in the table under the chart. There is still controversy over the use of the "bar" for pressure.

Function

Basic
SI Unit

Suggested
Variation,
Fluid Power

Other Metric
Equivalents

Conversion To/
From U.S. Units

Length m cm 1 m = 10 dm = 100 cm = 1,000 mm 1 m = 39.37 in; or, 1 in = 2.54 cm
Area m2 cm2 1 m2 = 100 dm2 = 10,000 cm2 1 m2 = 1,550 in2 = 10.76 ft2
Volume m3 dm3 1 m3 = 1,000 dm3 = 1,000 liters 1 m3 = 61,023 in3 = 35.3 ft3
Accel. of Gravity m/s2 Same Actual value  = 9.806 m/s2 Actual value = 32.17 ft/s2
Mass kg Same 1 kg = 9.807N (joules/meter) 1 kg = 2.2046 lbs (mass) = 70.92 poundals
Force (Weight) N Same 1N = 0.102 kg = 1 × 105 dynes 1N = 0.225 lbs
Torque Nm Same - - - - 1 Nm = 8.85 in. lb = 0.74 ft. lb
Stress N/m2 Same 1 N/m2 = 1 × 10-5 bar 1 N/m2 = 1.45 × 10-4 lbs/in2 = 0.021 lbs/ft2
Pressure (Fluid) N/m2 bar 1 bar = 100,000 N/m2 1 bar = 14.5 PSI; or, 1 PSI = 0.069 bar
Power w kw 1 kw = 1,000w 1 kw = 1.34 HP; or, 1 HP = 746w = 0.746 kw
Velocity m/s dm/min 1 m/s = 600 dm/min 1 dm/min = 3.937 in/min

 

cm = centimeter  32clear in. lb = inch pound  32clear min = minute
dm = decimeter in/min = inches per minute m/s = meters per second
dm/min = decimeters per minute kg = kilogram m/s2 = meters per sec. per sec.
ft = foot kw = kilowatt N = newton
ft. lb = foot pound lb = pound Nm = newton meters
ft/s2 = feet per second per second lb/in2 = pound per square inch N/m2 = newtons per square meter
g = gram lb/ft2 = pounds per square foot PSI = pounds per square inch
HP = horsepower m = meter s = second
in = inch mm = millimeter w = watt

 

 

Famiiar Formula in Customary U.S. Units  32clear Same Formula in the New SI Standard Units
 32clear
Torque, Power, and Speed Relations in Hydraulic Pumps & Motors
 32clear
T = HP × 5,252 ÷ RPM  32clear T = kw × 9,543 ÷ RPM
HP = T × RPM ÷ 5,252 kw = T × RPM ÷ 9,543
RPM = HP × 5,252 ÷ T RPM = kw × 9,543 ÷ T
 32clear
T is torque in foot lbs.  32clear T is torque in Nm (newton meters)
RPM is speed in revolutions per minute RPM is speed in revolutions per minute
HP is horsepower (33,000 ft. lbs/min.) kw is power in kilowatts
 32clear
Hydraulic Power Flowing in the System
 32clear
HP = PSI × GPM ÷ 1,714  32clear Kw = Bars × dm3 ÷ min ÷ 600
 32clear
HP is horsepower (33,000 ft. lbs. per min.)  32clear Kw is system power in kilowatts
PSI is gauge pressure, lbs. per square inch Bars is system pressure
GPM is flow in gallons per minute dm3/min is flow in cubiic decimeters per minute
 32clear
Force Developed by Air or Hydraulic Cylinder
 32clear
T = A × PSI  32clear N = A × Bars × 10
 32clear
T is force or thrust, in pounds  32clear N is cylinder force or thrust in newtons
A is piston area in square inches A is piston area in square centimeters
PSI is gauge pressure, lbs. per sq. in. Bars is gauge pressure
 32clear
Travel Speed of a Hydraulic Cylinder
 32clear
S = V ÷ A  32clear S = V ÷ 6A
 32clear
S is travel speed in inches per minute  32clear S is travel speed in meters per second
V is volume of oil into cylinder, cu. ins. per min. V is oil flow in cubic decimeters per minute
A is piston area in square inches A is piston area in square centimeters
 32clear
Barlow's Formula for Calculating Burst Pressure of Tubing and Pipe
 32clear
P = 2t × S ÷ O  32clear P = 2t × S ÷ O
 32clear
P is burst pressure in PSI  32clear P is burst pressure in bars
t is wall thickness of pipe in inches t is wall thickness of pipe in millimeters
S is tensile strength of pipe material in PSI S is tensile strength of pipe material in bars
O is outside diameter of pipe in inches O is outside diameter of pipe in millimeters
 32clear
Velocity of Oil flow in Hydraulic Lines
 32clear
V = GPM × 0.3208 ÷ A  32clear V = dm3/min ÷ 6A
 32clear
V is velocity in feet per second  32clear V is oil velocity in meters per second
GPM is oil flow in gallons per minute dm3/min is flow in cubic decimeters per second
A is inside area of pipe in square inches A is inside area of pipe in square centimeters
 32clear
Recommended Maximum Oil Velocity in Hydraulic Lines
 32clear
Pump suction lines - 2 to 4 per sec.  32clear Pump suction lines - 0.6 to 1.2 m/s (meters/sec)
Pressure lines up to 500 PSI - 10 to 15 ft/sec. Pressure lines up to 35 bar - 3.0 to 4.5 m/s
Pressure lines 500 to 3,000 PSI - 15 to 20 ft/sec. Pressure lines 35 to 200 bar - 4.5 to 6.0 m/s
Pressure lines over 3,000 PSI - 25 ft/sec. Pressure lines over 200 bar - 7.5 m/s
Oil lines in air/oil system - 4 feet per sec. Oil lines in air/oil system - 1.2 m/s
 32clear
See Design Data Sheet 25 for many other conversions between U.S. customary and ISO units

 

CLAMPING DELICATE PARTS
When an air cylinder is used for clamping delicate parts, the parts may be distorted, crushed, or suffer other damage for either or both of two possible reasons:

  1. Too high a travel speed of the clamp cylinder may cause damage from impact, especially when the clamp is carrying a significant weight (mass) attached to its piston rod. This type of damage can be avoided by using a speed control and reducing travel speed. A flow control valve, 1, connected in a meter-out circuit will solve this problem.
  2. Even though speed is reduced, the part can be crushed from too much steady force after the clamp has closed. When the clamp stalls against the part to be clamped, it will, if given a second or so, come up to full force developed by system PSI working against full piston area. A speed control system, by itself, cannot prevent full force from being exerted against the clamped part. To reduce the force, a pressure regulator, 2, should be installed in the air line to the clamp, and by-passed with a check valve so the clamp can be released fast and retracted under full pressure.

To adjust this circuit, first set the pressure regulator to a pressure which will not damage the part when clamping force is constantly applied. Then adjust the speed control to a speed which will not produce a damaging impact.

 Illustration 21_1

Both a speed and a pressure control may be required to avoid a damaging a delicate part which is being clamped.

 

Download a PDF of Fluid Power Design Data Sheet 21 - U.S. Customary Units Compared to ISO International Standard Units.

© 1990 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.

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