Understanding Principles
of Fluid Power Transmission
Objectives
• Define “hydraulics.”
• Describe the advantages and disadvantages of
hydraulics as a method of power transmission.
• Describe basic hydraulic system components and
functions.
• Explain Pascal’s Law and its relation to hydraulics.
• Describe the relationship between force, pressure and
area in a hydraulic system.
• Explain how Conservation of Energy applies to a
hydraulic system.
• Calculate the “cycle time” for a hydraulic application.
• Determine the mechanical and fluid horsepower output
of a hydraulic system.
Hydraulics
• Science and technology dealing with the
mechanical properties and practical applications of
liquids in motion.
– Hydrodynamics – Use fluid at high flow, low
pressure.
– Hydrostatics – Use fluids at high pressure, low flow.
Advantages and Disadvantages
• Advantages:
– Increased design
flexibility.
– Infinitely variable
speed.
– Reduced component
wear.
– Easy reversibility.
– Multiplication of
force.
• Disadvantages:
– Use of fluids at high
pressure:
• Safety hazards
• Need for cleanliness
• Expense
Hydraulic System Components
E lec tr ic
M o to r
H yd rau l ic
P ressu re
P um p
R e l ie fV a lve
M
F i lte r
R ese rvo ir
D irec t o
i n a l H yd rau l ic
C o n tro lV a lve C y l n
i de r
Basic Hydraulic System Operation
Pump
Pressure
Relief
Valve
Reservoir
Cylinder
Directional
Control
Valve
Pascal’s Law
10-lb
1 in2
10 psi
Area = 10 in2
Force = 100-lbs.
• Pressure applied to a confined
fluid is:
– transmitted undiminished
in all directions,
– acts with equal force on
equal areas, and
– acts at a right angle to the
walls of the container.
F,P, A Relationships
F
(lbs.)
P
(psi)
A
2
(in )
Force, Pressure, Area
Relationships in a Hydraulic
System
100-lbs
10-in2.
100 in2
1000lbs
10 psi
Determine force (lbs.) and pressure (psi) in this system.
Cycle Time
Fluidfrom
to reservoir
Fluid
pump
to reservoir
Fluid from
pump
•Total time required for hydraulic cylinder to complete one
cycle.
•Extend
•Retract
•Depends on volume (cylinder size) and flow (pump GPM).
Fluid Horsepower
• Fhp = P x Q
1714
•
•
•
•
Where,
P = Pressure (psi)
Q = Flow rate (gpm)
1714 = a constant
A tractor has a hydraulic system with a maximum working
pressure of 2500 psi and a flow rate of 11.5 gpm. What is the
maximum fluid horsepower output?
Mechanical vs. Fluid Power
Mechanical HP
Fluid HP
•Force (lbs)
•Pressure (psi)
•Distance (ft.)
•Flow (GPM)
•Time (min.)
Horsepower =
Horsepower =
Pressure x Flow
1714
FxD
t (min) x 33,000
Mechanical vs. Fluid HP Example
• Determine the
Fluid HP and the
Mechanical HP
in the situation
described:
– 4.0-in. diameter
ram
– 24-in stroke
– 50,000 lb load
– 10-gpm pump