fluid power fundamentals

Fluid Power Fundamentals

A Hydraulic Machine

A larger hydraulic machine

O&K RH120 in Paraburdoo

Oil versus air (discussion)OIL• Needs pumps• Accurate & precise• Large force• Low temperature• Leaks not tolerable• Expensive• Elaborate

AIR• Needs compressors• Inaccurate & imprecise• Small force• OK at high temperatures• Leaks no problem• Inexpensive• Quick to set up

Relevant Physical Properties

Density

Density (kg/m3)• Water : 1000• Oil : 900• Air : 1.21

Specific Gravity• Water : 1• Oil : 0.9• Air : 0.00121

Bulk Modulus

P, V

pv V

β∆= −

∆

P+∆p, V-∆v

ViscosityViscosity is the measure of the internal friction of a fluid. This friction becomes apparent when a layer of fluid is made to move in relation to another layer. The greater the friction, the greater the amount of force required to cause this movement, which is called "shear.”

Shearing occurs whenever the fluid is physically moved or distributed, as in pouring, spreading, spraying, mixing, etc. Highly viscous fluids, therefore, require more force to move than less viscous materials.

ViscosityV2

V1

Force x

Newton defined viscosity by using the above model:

2 1V V dvFx dx

µ µ−= =

Force per unit Area

Viscosity is a measure of speed

Saybolt Viscosimeter

Measures viscosity in SUS

or Saybolt Universal Seconds

Metric units for viscosity

Absolute Viscosity

Centipoise or cP

1cP = 0.001 N-s/m2

Kinematic viscosity, νµρ

=ν

CentiStokes or cS

1cS = 10-6 m2/s

SUS - cS Relation

180[ ] 0.220 100

135[ ] 0.220 100

cS SUS SUSSUS

cS SUS SUSSUS

= × − ≤

= × − >

ν

ν

ExerciseOil with a specific gravity of 0.9 has an absolute viscosity of 25 cP. Calculate the kinematic viscosity in centistokes

3225 10 25/

900 0.9m s cSµ

ρ

−×= = =ν

In short,µνγ

=

Electro-hydraulic systems

Design Questions• the system pressure? • the piston area? • the piston velocity? • the pump flow rate? • the pump power requirements?

Typically 7 - 15 MPa

< 1 m/s when p≈0< 6 m/s at high p

???

???

???

Bonus Point Question

V < 1 m/s in low pressure regions

V < 6 m/s in high pressure regions

WHY ??

Basic Formulae

FAp

=2 [N][mm ] [MPa]

Piston Area

Q V A=3

2m m[ ] [ ] [m ]s sPump Flow Rate

W Q p=3m[Watt] [ ] [Pa]s

!Pump power

W Q p=l[kW] [ ] [MPa]s

!or

Laminar or Turbulent Flow

Re d u d uρµ ν

= =m2/s

m/sm

Units ?

Hydraulic Pumps

• Piston Pumps/Motors (Swash-Plate Pumps) - high pressure, high-grade applications

• Vane Pumps/Motors - medium pressure applications

• Gear Pumps/Motors - low pressure applications

Gear Pump

Vane Pump

Swash Plate Pump

Pump Efficiencies

Control Valves•Binary valves or directional control valves affect only the direction of the flow or turn it off completely •Proportional valves or flow control valves vary the flow by varying the spool position •Servovalves are similar to proportional valves but provide very high control precision •Relief Valves divert the flow back to the reservoir if the pressure exceed a set value •Check valves allow flow in one direction only

Directional Control ValvesThese are referred to as n/p valves where

n : Number of ports

p : Number of positions these ports may have

4/2 Valve2

1 3

4/2 DCV NC

4 1 From Pump

3 Back to tank

2,4 Output Ports

Symbols

Solenoid with one winding Double Solenoid Solenoid with spring return Check valve Spring-loaded check valve

2 2 2

1 1 13

2

1 3 3

2/2 DCV 3/2 DCV NO 3/2 DCV NC 4/2 DCV NC

4

Single-acting cylinder Double-acting cylinder Pressure Regulator Filter

Fixed-Displacement Pump

Double-acting cylinder

Pushing rightPushing left

Single-acting cylinder

Blocked

Regenerative Circuit - 1

The valve iscentered. The cylinder is not moving.

P

Blocked

Regenerative Circuit - 2

V

Retraction at normal speed

P

Regenerative Circuit - 3

Blocked

P

Rapid Extension

V

Proportional Valves

Spool position is continuously controlled by the solenoid current

Proportional Valves

Spool position is continuously controlled by the solenoid current

THE END