KNOW ALL ABOUT BEARINGS
We provide technical consultancy for all areas of thermal power plants.
Objectives of this article :
- To achieve
maximum bearing performance.
- To reduce
costly down time.
- To minimize labor
requirements.
- To ensure that
the equipment, in which the bearings are installed, runs smoothly /
effectively.
Reasons for bearing failure :
|
Reasons
|
Counter measures
|
1.
|
Bearing Quality
|
-
Avoid installing spurious bearings
|
2.
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Operating Environment
|
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Proper alignment.
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Protection from temperature, moisture & contamination.
|
3.
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Proper installation
|
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Knowledge of proper installation.
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Knowledge of use of proper tools.
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4.
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Proper Maintenance
|
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Lubrication.
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Maintenance schedules.
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Monitoring operating conditions
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Selection of bearing :
Each
bearing type is designed to meet the requirement of specific bearing
applications.
Whenever
a bearing fails prematurely or repeatedly, it is very important to establish
the cause of the damage, so that preventive action can be taken and proper
replacement bearing can be selected.
Rolling
bearings include radial and thrust bearings for radial and axial loads and some
bearings are designed for continued radial and axial loads.
Generally
ball bearings are recommended for light and moderate board roller bearings are
recommended for heavy loads.
Ideal condition :
1)
The actual load is less than designed
load.
2)
When the rolling surfaces are
effectively separated by a lubricant film.
3)
When surface damage caused by
contamination is limited.
Under
ideal condition bearing can attain a much longer life / infinite life.
Failure
is initiated by fatigue, wear, corrosion contamination, seal failure, etc.
Bearing
life is defined as the number of revolutions or the number of operating hours
at a given constant speed which a bearing is capable of enduring before the
first sign of fatigue occurs on one of its rings or rolling elements.
Of the
bearings which do fail, approximately one third “die” from old age, i.e.
fatigue of the bearings surfaces.Another third fail because of poor lubrication
and rest fail mainly as a result of contaminations entering the bearings or
because of handling damage or faulty mounting.
However,
pattern varies between different industrial segments.
How bearing failure starts
Fatigue is the result of shear
stresses cyclically appearing immediately below the load carrying surface. After a time these stresses cause cracks
which gradually extend up to the surface.
As the rolling elements pass over the cracks fragments of material break
away and this is known as sapling or flaking.
The flaking progressively increases in extent and eventually makes the
bearing un-serviceable.
This type of bearing damage is a relatively
long drawn out process, making its presence known by increasing noise and
vibration. Therefore, it gives user
adequate time to replace the bearing before it fails completely.
When cracks forms on the surface and grow
into the material, surface distress is at hand.
All surface contains roughness.
If the oil film has a proper thickness in relation to the surface
roughness, the probability for surface distress is very low.
If however the load is above the fatigue load
limit, normal material fatigue may sooner or later occur.
Replace with care :
Bearings are calculated to run under very
specific operating conditions. Too
often, changes involving the use of a different lubricant, higher machine
speeds, higher loads, and change in lubrication system,are made without
anticipating possible negative effects.
Therefore, when replacing a bearing no other
changes should be made, which would adverse affect bearing operation.
What to look for during
operation ?
Checking the condition of a machine in
operation and planning for its necessary overload is becoming an issue of
increasing importance.
Bearings constitute a vital factor in all
machines with rotating parts. Condition
monitoring represents a rapidly growing activity in the field of preventive
maintenance. Early indications of
bearing damage allow bearings to be replaced during regularly scheduled
maintenance and thus avoid unscheduled equipment downtime due to bearing
failure.
Bearings in critical machinery or in severe
environment should be checked frequently.
Many systems and instruments are available for monitoring bearings.
However, for practical reasons not all
machines or machine functions are monitored using advanced instrument. In these instances the machine operator or
the maintenance engineer must remain alert for bearing trouble signs, such as
noise, increase in temperature and vibration.
Listen,
Feel and Look are three important factors.
Listen
:
ü A
common way of identifying an irregularity in running is by listening. With the help of an electronic stethoscope,
abnormal noise can be detected and even traced to an individual machine
component by an experienced operator.
ü Bearings
in good operating condition produce a soft purring noise.
ü Grinding
squeaking and other irregular sounds usually point to bearings in poor
operating conditions. They are:
- A
squeaking noise may be caused by inadequate lubrication.
- Insufficient
bearing clearance can produce a metallic tone.
- Indentations
in the rear ring raceway of a bearing can cause vibration resulting in a
smooth, clear tone.
- Ring
damage caused by blows during mounting or scoring leads to sounds, which
vary according to the operating speed of the bearings.
- Intermittent
noises may indicate damage to a rolling element. The sound is generated when the damage
surface is rolled over.
- Dirt
in a bearing often causes a grinding sound.
- Severally
damaged bearing produce irregular and loud noises.
Bearing
damage can certainly be detected by listening, but the damage will often reach
such an extent that an immediate bearing change has to be carried out.
Using
of Electronic Condition Monitoring devices are much safer and more accurate
prediction than the early method of placing one end of a wooden strake or a
screwdriver against the bearing housing and putting one ear to the other end.
Feel
:
ü High
temperature often indicates that the bearing is acting abnormally. Overheating is attributed to the bearing
lubricant. Running for extended period
at temperatures in excess of 125°C can reduce bearing life.
ü Causes
of high bearing temperature include insufficient or excessive lubrication,
impurities in the lubricant, overloading, bearing damage, insufficient
clearance, pinching and high friction in the seals.
Any
change in temperature can be an indication of malfunction, if the running
conditions have not been altered.
Critical
bearings which cause equipment downtime when they fail should ideally be
equipped with a heat probe.
Look
:
ü Properly
lubricated bearings that are adequately protected from dirt and moisture should
not exhibit wear.
ü However,
it is advisable to visually check a bearing when it is uncovered and to
periodically check the seals.
ü Check
the condition of seals near the bearings to ensure that they do not permit hot
or corrosive liquids and gases to penetrate along the shaft.
ü Keep
protective collars and labyrinth seals filled with grease to ensure maximum
protection. Replace worn felt and rubber
seals as soon as possible.
ü In
addition to preventing the entry of dirt seals, it is also important to keep
lubricant in the bearing housing.
Lubricant leaks at seal locations should be inspected immediately for
worn seals, seals defects and loose plugs.
Leaks can also result from slacking of the joint between the matching
surfaces of the bearing housing or from the grease breaking down by churning
and releasing free oil.
ü In
order that the automatic lubricating systems perform properly, fill them with
oil or grease and ensure that the right amount of lubricant is released.
ü Also
take a good look at the lubricants’ discoloration or darkening because these are
usually signs that the lubricant contains impurities.
We provide technical consultancy for all areas of thermal power plants.
