Bearing WEAR and its Causes.
In normal cases there is
no appreciable wear in rolling bearings. Wear may, however, occur as a result of the
ingress of foreign particles into the bearing or when, the lubrication is unsatisfactory.
Vibration in bearings which are not running also gives rise to wear.
Wear caused by abrasive
particles, such as grit or swarf that have entered the bearing by some means or other,
cause wear of raceways, rolling elements and cage. The surfaces become dull to a degree
that varies according to the coarseness and nature of the abrasive particles. Sometimes
worn particles from brass cages become verdigrised and then give light-coloured grease a
The quantity of abrasive
particles gradually increases as material is worn away from the running surfaces and cage.
Therefore the wear becomes an accelerating process and in the end the surfaces become worn
to such an extent as to render the bearing unserviceable. However, it is not necessary to
scrap bearings that are only slightly worn. They can be used again after cleaning.
The abrasive particles
may have entered the bearing because the sealing arrangement was not sufficiently
effective for the operating conditions involved. They may also have entered with
contaminated lubricant or during the mounting operation.
indentations around the raceways and rolling elements. Dull, worn surfaces. Grease
cleanliness before and during mounting operation. Ineffective seals. Lubricant
contaminated by worn particles from brass cage.
unpack bearing until just before it is to be mounted. Keep workshop clean and use clean
tools. Check and possibly improve the sealing. Always use fresh, clean lubricant. Wipe the
grease nipples. Filter the oil.
Figure 1 -
The outer race of a spherical roller bearing with raceways that have been worn by abrasive
particles. It is easy to feel where the dividing line goes between worn and unworn
Wear caused by
If there is not
sufticient lubricant, or if the lubricant has lost its lubricating properties, it is not
possible for an oil film with sufficent carrying capacity to form. Metal to metal contact
occurs between rolling elements and raceways. In its initial phase, the resultant wear has
roughly the same effect as lapping. The peaks of the microscopic asperities, that remain
after the production processes, are torn off and, at the same time, a certain rolling-out
effect is obtained. This gives the surfaces concerned a varying degree of mirror-like
At this stage surface distress can also arise. If the lubricant is completely used up, the
temperature will rise rapidly. The hardened material then softens and the surfaces take on
blue to brown hues. The temperature may even become so high as to cause the bearing to
frequently mirror-like, surfaces; at a later stage blue to brown discolouration.
has gradually been used up or has lost its lubricating properties.
the lubricant reaches the bearing. More frequent relubrication.
Figure 2 -
The outer ring of a spherical roller bearing that has not been adequately lubricated. The
raceways have a mirror finish
When a bearing is not
running, there is no lubricant film between the rolling elements and the raceways. The
absence of lubricant film gives metal to metal contact and the vibrations produce small
relative movements of rolling elements and rings. As a result of these movements, small
particles break away from the surfaces and this leads to the formation of depressions in
the raceways. This damage is known as false brinelling, sometimes also referred to as
washboarding. Balls produce sphered cavities while rollers produce fluting.
In many cases, it is possible to discern red rust at the bottom of the depressions. This
is caused by oxidation of the detached particles, which have a large area in relation to
their volume, as a result of their exposure to air. There is never any visible damage to
the rolling elements. The greater the energy of vibration, the more severe the damage. The
period of time and the magnitude of the bearing internal clearance also influence
developments, but the frequency of the vibrations does not appear to have any significant
effect. Roller bearings have proved to be more susceptible to this type of damage than
This is considered to be because the balls can roll in every direction. Rollers, on the
other hand, only roll in one direction; movement in the remaining directions takes the
form of sliding. Cylindrical roller bearings are the most susceptible. The fluting
resulting from vibrations sometimes closely resembles the fluting produced by the passage
of electric current. However, in the latter case the bottom of the depression is dark in
colour, not bright or corroded. The damage caused by electric current is also
distinguishable by the fact that the rolling elements are marked as well as the raceways.
Bearings with vibration damage are usually found in machines that are not in operation and
are situated close to machinery producing vibrations. Examples that can be cited are
transformer fans, stand-by generators and ships' auxiliary machinery. Bearings in machines
transported by rail, road or sea may be subject to vibration damage too.
in the raceways. These depressions are rectangular in roller bearings and circular in ball
bearings. The bottom of these depressions may be bright or dull and oxidised.
has been exposed to vibration while stationary.
bearing during transport by radial preloading. Provide a vibration-damping base. Where
possible, use ball bearings instead of roller bearings. Employ oil bath lubrication, where
Where machines subject to
constant vibration are concerned, it is essential that the risk of damage to the bearings
be taken into consideration at the design stage. Consequently, where possible, ball
bearings should be selected instead of roller bearings. The ability of ball bearings to
withstand vibrations without being damaged can also be considerably improved by applying
axial preloading with the aid of springs, see fig 25. An oil bath, in which all rolling
elements in the load zone are immersed in the oil, has also proved to provide satisfactory
protection. A vibration-damping base helps to prevent damage too.
The bearings in machines that are to be transported can be protected by locking the shaft,
thus preventing the small movements that have such a damaging effect on the bearings.
Figure 3 -
The inner and outer ring of a cylindrical roller bearing that has exposed to vibration.
The inner ring has changed position.
Figure 4 -
The outer ring of a self aligning ball bearing damaged by vibration. The bearing has not
rotated at all.
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