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Technical
Service Bulletin 96-1
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Hydraulic Systems
As
hydraulic driven equipment becomes more sophisticated, the need increases
for a better understanding of its operation and maintenance. Hydraulic
systems can be either simple or complex. They can operate at high temperatures
(e.g. 60°C, 140°F), pressures and rapid cycle times. We will divide
the broad subject matter of hydraulics into smaller topics and cover each
in separate bulletins. This will provide you with a better understanding
of each subject.
This opening bulletin will give you some basic understanding of hydraulics.
Future subjects to be covered will include, sources of contamination,
filter performance indicators, hydraulic fluid additives, recycling and
preventive maintenance.
To begin, the basic law of hydraulics is stated by Pascal, "pressure
at any point in a static liquid is the same in every direction and exerts
an equal force on all equal areas." (See Figure 1) Fluids are virtually
incompressible, mechanical forces may be directed and controlled by means
of fluids under pressure.
Most
hydraulic circuits contain five basic mechanical components: a reservoir,
a filter, a pump, flow control valves, and a cylinder or actuator (see
Figure 2). There is also the hydraulic fluid to consider. No matter how
sophisticated the system gets, the hydraulic fluid in the system performs
four simple functions:
- transmit power
- lubricate the pump, valves and seal
- protect the system by removing contaminants
o moisture
o dirt
o heat
o air- sealing internal components
The pressure applied
to the fluid will give the fluid the power necessary to transmit a force
within the system. As the sophistication of the system increases, the
work required of the fluid will also increase. The fluid transmits power
while it lubricates the components which it flows through. The hydraulic
fluid, as a lubricant, reduces friction in the components by producing
a barrier or film which separates the surfaces that will roll or slide
past each other.
Viscosity is a measure
of a fluid's resistance to flow. A fluid that has a high resistance to
flow (high viscosity) is like cold molasses or SAE 140 weight gear oil.
A fluid that has a low resistance to flow (low viscosity) is like water
or SAE 10 weight hydraulic oil. Viscosity of the fluid is directly related
to its ability to lubricate. A high viscosity fluid generates greater
film thickness between lubricated surfaces because it has a greater resistance
to being squeezed out from between the lubricated surfaces. Viscosity
of the fluid will change with the temperature of the fluid. Raising the
temperature of the fluid will the temperature will increase its viscosity.
Conversely, lowering of the temperature will increase its viscosity.
In many instances,
the fluid is the only seal against pressure inside a hydraulic component
where no seal ring exists between the valve spool and body to minimize
leakage from high to low pressure areas. The close mechanical fit and
oil viscosity determines leakage rate.
To keep friction and
wear of the system to a minimum, the proper filtration must be specified
and you must use a fluid of the correct viscosity while operating the
system within the proper design parameters.
For further information
about hydraulics refer to other FMC Technical Service Bulletins and also
the NFPA (National Fluid Power Association) Publications Catalog.
For additional information, contact:
Filter Manufacturers
Council
P.O. Box 13966
Research Triangle Park, NC 27709-3966
Phone: 919/406-8817 Fax: 919/406-1306
www.filtercouncil.org
Administered by Motor & Equipment
Manufacturers Association
Created April 1996
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