TECHNOLOGY
FEATURE
HYDRAULICS
Hydraulic oils- the basics
H
ydraulic oil is a fluid lubricant used in
hydraulic systems for transmitting power.
Hydraulic fluids, also called hydraulic
liquids, are the medium by which power
is transferred in hydraulic machinery.
Common hydraulic fluids are based on mineral oil
or water. Examples of equipment that might use
hydraulic fluids include excavators and backhoes,
By Denis Mwai
brakes, power steering systems, transmissions,
Denis Mwai is a
Lubricants Sales garbage trucks, aircraft flight control systems, lifts, and
Manager at NOCK industrial machinery.
There are two major types:
Strainers- Mainly used to protect the pump
and is found in the reservoir pick-up line to
prevent large particles from being sucked
up. Generally, use of coarse wire mesh type
filters is common.
In-line filters – Mainly used to protect
valves and actuators. They are fine fibrous
filters to extract small particles.
• Control valves:
Various types of valves work either mechanically or electrically. There are various types
that can be found in hydraulic systems:
Directional valves - Used to control direction of flow
Servo valves - controls the amount as well
as direction of flow
Pressure relief valves - Controls system
pressures
Volume/Flow control valves - Regulates the
speed
• Actuators or Pistons:
This is the output device that moves the load
and generally consists of a piston and cylinder. It varies depending on the job, conditions
and power requirements.
• Pipes, hoses and seals:
Schematic diagram of a simple hydraulic system
Common Hydraulic system
A common hydraulic system consists of the
following components:
• Oil tank or reservoir:
An oil reservoir serves several functions
like, storing the bulk of the fluid for reserve,
settling and separations and for drainage of
contaminants.
• Hydraulic pump:
The hydraulic pump is the heart of the system.
There are different types of pumps:
Gear pumps – These are the simplest,
compact and with low maintenance cost.
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They run at low pressures between 3,500
and 21,000 kPa. They are noisy with high
contamination tolerances
Vane Pumps - These pumps have a good
efficiency, compact and low cost. Run at
moderate pressures 7,000 to 21,000 kPa.
Cleanliness of the oil is critical.
Radial and axial pumps – These pumps
have a good efficiency, they are costly and
run at moderately high pressures between
21,000 to 70,000 kPa
• Oil filter:
Oil filters are essentially for wear protection.
Pipes are usually a mixture of metallic and
rubber hoses which need to withstand high
pressures. Pipes and seals form the weakest
part in the hydraulic system as they have the
potential to leakages, can be hazardous due to
high oil pressure and hence should always be
regularly checked and maintained.
The following characteristics and properties
are important for hydraulic oils:
• Low temperature sensitivity of viscosity;
• Thermal and chemical stability;
• Low compressibility;
• Good lubrication (anti-wear and anti-stick
properties, low coefficient of friction);
• Hydrolitic stability (ability to retain properties in the high humidity environment);
• Low pour point (the lowest temperature, at
which the oil may flow);
• Water emulsifying ability;
• Filterability;
Lubezine Magazine | January-March 2012