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