Bearings: how to avoid failures
Bearing failures are almost invariably the result of external factors
which, with correct bearing selection and handling, can be significantly
reduced if not eliminated altogether.
The most important piece of advice that can be given to you when
considering the problem of bearing failure and how to avoid it, is to
discuss your application with specialist bearing engineers who will point
you towards the best bearing solution, the best mounting methods and
suitable lubricants. But in the meantime, here are the six main
categories of bearing failure, together with their solutions, to get you
started.
Fatigue This can occur if the bearing material is not of the highest
quality, and inclusions in the steel cause sub-surface cracking and
eventual raceway failure. It can also be caused by surface cracks from
foreign particles entering the bearings and causing indentations in the
raceway surface - thus creating small areas with very high Hertzian
pressures.
Solution -Select high quality bearings, keep them in a clean environment
prior to, and during fitting, and make sure they are not contaminated.
Greases should be clean (do not leave tubs open in dirty areas) and,
where practical, oils should be filtered.
Wear - There are two types - abrasive wear and adhesive wear. The former
is usually due to lack of, or poor quality lubrication, while the latter
occurs when too high a loading on the bearing results in a breach of the
lubricant film. Under these conditions, the bearing surfaces weld
together and then tear apart.
Solution - Select an appropriate lubricant for the relevant operating
conditions and ensure that sufficient lubricant reaches the bearing.
Corrosion - Caused by aggressive media entering the bearing or
contaminating the lubricant. The corrosion causes pitting which will
break down the bearing raceway leading to early failure.
Solution - Store bearings in conditions where they are not exposed to
significant temperature changes and consequent condensation formation. An
appropriate grease should be used to lubricate the bearing if
condensation is a possibility. Alternatively, the bearings should be
coated with a protective layer.
Electrical erosion - High voltages or high current flows through the
bearings cause components to weld together. Raceways can take on the
appearance of a washboard and will increase noise and cause early failure.
Solution - where a current flow through a bearing is likely, the bearing
should be insulated. In-built insulation such as a ceramic coating on the
inner or outer ring is available with some bearings. Alternatively a
non-conducting material should be used for the housing.
Plastic deformation - This can be caused by high static loading, which
causes indentations in the raceway surface at the pitch of the rolling
elements.
Solution - Always use the correct type of assembly tooling supplied by
major bearing manufacturers. Take into account the static loads of the
bearing and select a bearing with sufficient static capacity to withstand
these loads.
Fracture cracking - Where the static loads exceed the plastic limit of
the material and result in a crack or fracture. Fracture cracks can also
appear when rapid temperature changes take place, namely heating and
cooling within a very short time period.
Solution - take the advice of a reputable bearing manufacturer before
finalising your selection when these conditions are unavoidable.
Two of the world's leading bearing companies, Schaeffler Group members,
INA and FAG, have joined forces to offer one of the world's most
comprehensive ranges of rolling bearing products. With access to over
40,000 standard products, the UK based operation offers designers the
best technical advice and applications support in the business, and is
capable of developing innovative value-engineered bearing solutions to
meet the exacting requirements of any application, in any industry.
INA Bearing Company Ltd
Forge Lane, Minworth, Sutton Coldfield, West Midlands B76 1AP
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