Building Blocks

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Building Blocks

• Variables and Units Units Units

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Fluid System Parameters

• In electrical systems we deal with Current and
  Voltage.
• In hydraulic systems we deal with Flow rate and
  Pressure
• You know the units for EE systems. What are the
  relevant units for Fluid (hydraulic) systems?

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Pressure in Hydraulic Systems

• Pressure is defined as a force applied over an
  area
• Common expressions of pressure in hydraulics
  include
• PSI (pounds per square inch)
• Pa (Pascals)
• MPa (MegaPascals)
• Bar

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Pressure Units

• PSI - this is fairly intuitive to us. One
  pound of force applied to one square inch
• Pascal - the force of one Newton applied to the
  area of one square meter.
• Try to develop your own sense of scale. It can
  keep you from making a simple mistake.
• What can you relate to a lb and a Newton

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1 PSI (lb/ in2)?
1 Pascal (N/m2)?
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Units Conversions

• 1 PSI 6895 Pa
• 1 PSI 6.895 kPa
• 1 PSI 0.006895 MPa
• 1 bar 14.504 PSI
• Keep these handy as you will need them in
  homework and on exams

• 1 Pa 0.00014504 PSI
• 1 kPa 0.14504 PSI
• 1 MPa 145.04 PSI
• 1 bar 100 kPa 0.1 MPa

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Atmospheric Pressure and Vacuum
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Gage and Absolute Pressures

• Gage pressure is what we read on a pressure dial,
  or measure with a transducer
• It is measured ABOVE atmospheric pressure
• Atmospheric pressure (standard) is 14.7 psi
  absolute (or psia) or 101 kPa.
• 0 psi gage 14.7 psia

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Pressure Scales
Pressures normally measured in hydraulic systems
0 psi gage
0 psi vacuum
14.7 psi abs
Vacuum Pressure
14.7 psi vacuum
0 psi abs
Vacuum of space, or in a vacuum chamber no gas
pressure
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Pressure Head
?gh
Density x gravity x depth
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Pressure and Flow

• Pressure results from resistance to flow
• We generate FLOW with a pump
• We only have pressure at the pump outlet if there
  is some resistance to the flow

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Volume Measures Conversions

• 1 Gallon 231 in3 3.785 Liters
• 1 Foot3 1728 in3 7.48 Gallons
• 1 Liter 1000 cm3 0.264 Gal 61 in3
• 1 m3 1000 Liters 264 Gal

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Flow Rate Units Conversions

• Gallons per minute (GPM)
• 1 GPM 231 in3/min 3.785 L/min
• 1 GPM 3.85 in3/sec
• 1 Liter / min 0.264 GPM
• 1 Liter/Min 16.67 cm3/sec
• I recommend the following downloadable program
  for managing units
• http//joshmadison.com/software/convert/

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Areas, Volumes, and Velocities

• We will frequently calculate areas, volumes flow
  rates and velocities
• Many components in fluid power systems have
  circular cross sections
• Linear actuator (cylinder) piston area
• Cylinder rod cross section
• Pump piston area
• Pump rotor area
• Conductor (line) cross sectional area
• Area of a circle is p r2, or p D2/4 Remember
  this!

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Some Examples

• What is the volume, in Liters, of a hydraulic
  hose 24 feet in length and having an inside
  diameter (I.D.) of 5/8?
• First calculate the hose cross sectional area
  Note diameter 5/8 0.625
• Area p D2/4 p (0.625)2/4 0.307 in2
• Now calculate the volume in cubic inches Note
  the volume of a cylinde is Area x Length
• Vol Length x Area L 24 ft 288 in
• Then Vol 288 in x 0.307 in2 88.4 in3

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Some Examples

• Next convert volume to Gallons
• 88.4 in3 / 231 in3/gal 0.383 gal
• Next convert Gallons to Liters
• 0.383 Gal x 3.785 Liters / Gal 1.45 liters
• Finally, check to make sure you have answered
  what was ASKED. (Read the problem again before
  going on)

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Another Example

• A double acting hydraulic cylinder has a piston
  bore of 4.5 inches and a rod diameter of 1.75
  inches. If I supply fluid at 22 GPM (Gal / min),
  to the cap end of the cylinder (end opposite
  the rod end), how fast will the cylinder extend?
  (in/sec)
• Some assumptions No leakage anywhere
• Fluid is incompressible

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Cylinder Speed Flow Example

• Draw and label a sketch . It helps
• This is a problem of volumes and rates
• Convert the incoming flow rate to cubic
  inches/sec
• 22 Gal/min x 231 in3/Gal x 1min / 60 sec 84.7
  in3/sec
• 84.7 in3/sec must equal piston area x piston speed

Extension Speed
Piston Area
84.7 in3/sec
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Cylinder Speed Flow Example

• Calculate the Area
• Area p D2/4 p (4.5)2 / 4 15.90 in2
• Solve for the Speed
• Speed Flow (in3/sec) / Area (in2) in/sec
• Calculate extension speed
• Speed 84.7 in3/sec/ 15.90 in2 5.3 in/sec

15.9 in2
5.3 inches/sec
Extension Speed
Piston Area
84.7 in3/sec
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Cylinder Example Part II

• What is the rate at which fluid is being forced
  OUT of the cylinder?
• This is a very similar problem to Part I
• We will need to know the annular area on the rod
  end of the cylinder

15.9 in2
5.3 inches/sec
Extension Speed
Piston Area
84.7 in3/sec
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Cylinder Example Part II

• Determine annular area of the piston minus the
  rod
• Piston area (15.9 in2) Rod area (p (1.75)2 / 4)
• Same as p (D2piston- d2rod) /4 p (4.52
  1.752)/4 13.5 in2
• Rate of outflow Area x speed
• 13.5 in2 x 5.3 in/s 71.5 in3/s
• Convert to GPM 71.5 in3/s x 1gal/231 in3 x 60
  s/min

15.9 in2
5.3 inches/sec
Extension Speed
Annular area 13.5 in2
18.6 GPM Out
84.7 in3/sec
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A Pressure Example

• You are specifying components for a system. The
  pump catalog data indicate a maximum pump
  pressure of 30 MPa. Your hose vendor works in
  English units (psi). If your hoses must
  withstand at least twice the pump outlet
  pressure, what must the minimum hose rating be in
  psi?

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A Pressure Example

• Pump rating 30 MPa
• Min Hose rating 2 x 30 MPa 60 MPa
• 1 MPa 145.04 psi
• 60 MPa 8,702 psi
• Your hose should be selected to tolerate at least
  9000 psi