HYDRAULIC POWER
Hydraulic Power
•Hydraulic power
•Hydraulics vs.
pneumatics
•Early hydraulic uses
•Hydrodynamic
systems
•Hydrostatic systems
•Liquid flow
•Mechanical advantage
•Bernoulli's principle
•Viscosity
•Common hydraulic
system components
•Emerging hydraulic
application example
Hydraulic Power
Hydraulics
– The use of a liquid flowing under pressure to
transmit power from one location to another
Liquid in a hydraulic system behaves like a solid
since it compresses very little
Hydraulic Power
Hydraulics vs. Pneumatics
Hydraulic Systems . . .
•
•
•
•
•
Use a relatively incompressible liquid
Have a slower, smoother motion
Are generally more precise
Lubricate naturally
Are not as clean as pneumatics when leakage
occurs
• Often operate at pressures of 500 - 5000 psi
• Generally produce more power
Early Hydraulic Uses
Water Wheels
• Create rotational
motion
• Descriptions exist as
early as 1st century
BC
• Several examples in
ancient China
• Grist mill is pictured
Early Hydraulic Uses
Roman Aqueducts
• Delivered water to
buildings, to agricultural
fields, and to fountains
• Used gravity to create
flow
• Fountains were
decorative and used by
people to collect water for
practical use
Hydrodynamic Systems
Fluid is in motion
Fluid is not pressurized
Force / energy is created by flow
Water Turbine Propeller
Hydrostatic Systems
Fluid is at rest
Fluid is pressurized
Pressure creates force and energy
Most common in industrial settings
National Fluid Power Association & Fluid Power Distributors Association
Hydrostatic Systems
Pascal’s Law Pressure exerted by a confined fluid acts
undiminished equally in all directions
F
p
A
Click the arrows to activate the hydraulic press.
Liquid Flow
Flow Rate
The volume of fluid that moves through a system in a given
period of time
Flow Velocity
The distance the fluid travels through a system in a given
period of time
Symbol
Definition
Q
Flow Rate
v
Flow Velocity
A
Area
Example Units
gpm or gal/min
in.3 / min
(gallons per minute)
fps or ft/s
(feet per second)
in.2
in. / min
Q  v(A )
Liquid Flow Example
A flow meter attached to the main line in
a hydraulic system measures the flow
rate at 15 gpm. The line has an inside
diameter of 2 in. What is the flow velocity
in the meter?
Q = 15 gal/min
d = 2 in.
v=?
Float
Convert 15 gal/min to in.3 /min
1 gal = 231 in.3
3
15 gal
231in.


min
1gal
3465 gal in.3
1min gal

in.3
3500
min
Reprinted with permission from
Introduction to Fluid Power, by
James L. Johnson. Copyright ©
2002 Thomson Delmar
Learning.
Liquid Flow Example
A flow meter attached to the main line in a hydraulic system measures
the flow rate at 15 gpm. The line has an inside diameter of 2 in. What is
the flow velocity in the meter?
Q = 3465 in.3/min d = 2 in.
Formula
A  r 2
Sub / Solve A  1
( )2
Final
Formula
Sub / Solve
A  3.14 in.2
Final
v=?
Q  v(A )
in.3
3465
 v(3.14 in. )2
min
in.3  in.
3465
min
v 
3.14 in.2
in.
v  1,100
min
Mechanical Advantage
Force at the output
Mechanical Advantage 
Force at the input
Fout
MA 
Fin
National Fluid Power Association & Fluid Power Distributors Association
Mechanical Advantage Example
A force of 100. lb is applied to the input cylinder of the
hydraulic press seen below. What is the pressure in the
system? How much force can the output cylinder lift? What
is the mechanical advantage of the system?
Fin = 100. lb
din = 4.0
in.
Fin = 100. lb
Fout = ?
din = 4.0 in.
dout = 12.0 in.
Ain = ?
Aout = ?
p=?
MA = ?
dout = 12.0 in.
Mechanical Advantage Example
Find the area of each cylinder.
Fin=100. lb
Ain=?
Fout=?
Aout=?
Formula
Rin=2.0 in.
p=? MA=?
A  r 2
Formula
Sub / Solve A in  (2.0 in2 )
Final
Rout =6.00 in.
2
A in  13 in.
A  r 2
Sub / Solve A out  (6.0 in2 )
Final
A out  110 in.2
Mechanical Advantage Example
Find the pressure in the system.
Fin=100. lb
Fout=?
Ain=12.57 in.2
Formula
Aout=113.10 in.2
F
p
A
Sub / Solve p 
Final
Rin=2.0 in.
100. lb
12.57 in.2
lb
p  8.0 2
in.
Rout=6.00 in.
p=?
MA=?
Mechanical Advantage Example
Find the force that the output cylinder can lift.
Fin=100. lb
Fout=?
Rin=2.0 in.
Ain=12.57 in.2 Aout=113.10 in.2
Formula
p
Rout =6.00 in.
p=7.955 lb/in.2
MA=?
F
A
Fout
lb
Sub / Solve 7.955 2 
in.
113.10 in.2
Fout
lb
 7.955 21
( 13.10 in. )2
in.
Fout  9.0  10
2
lb in.2
in.2
Final
Fout  9.0  102 lb
Mechanical Advantage Example
Find the mechanical advantage of the system.
Fin=100. lb
Fout=900.28 lb
Ain=12.57 in.2 Aout=113.10 in.2
Formula
Sub / Solve
Final
Rin=2.0 in.
p=7.955 lb/in.2
Fout
MA 
Fin
900.28 lb
MA 
100. lb
MA  9.0
Rout =6.00 in.
MA=?
Bernoulli’s Principle
An increase in velocity results in a decrease in pressure.
Likewise, a decrease in velocity results in an increase in
pressure.
Viscosity
The measure of a fluid’s thickness or
resistance to flow
Crucial for lubricating a system
Measured in slugs/ft 3 (US) or centistokes
(metric)
– Hydraulic oil is usually around 1.4 slugs/ft 3
Decreases as temperature increases
Common Hydraulic System
Components
Cylinder
Transmission
Lines
Directional
Control Valve
Filter
Pump
Reservoir
National Fluid Power Association & Fluid Power Distributors Association
Common Hydraulic System
Components
Click the lever on the valve to
extend and retract the cylinder.
Cylinder
Valve
Reservoir
Pump
Emerging Hydraulic Application
Example
Hydraulic Hybrid Vehicles
Braking provides stored energy that is used to propel the vehicle
forward.
UPS™ expects 60-70% better fuel economy and 40% reduction in CO2
emissions.
UPS
Image Resources
National Fluid Power Association. (2008). What is fluid power. Retrieved
February 15, 2008, from
http://www.nfpa.com/OurIndustry/OurInd_AboutFP_WhatIsFluidPower.asp
Johnson, J.L. (2002). Introduction to fluid power. United States: Thomson
Learning, Inc.
National Fluid Power Association & Fluid Power Distributors Association. (n.d.).
Fluid power: The active partner in motion control technology. [Brochure].
Milwaukee, WI: Author.
Microsoft, Inc. (2008). Clip Art. Retrieved January 10, 2008, from
http://office.microsoft.com/en-us/clipart/default.aspx
UPS Press Room (n.d.) Fact sheets. Retrieved March 12, 2008, from
http://www.pressroom.ups.com/mediakits/factsheet/0,2305,1315,00.html