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PRESS RELEASE – 3rd September 2009
New bearing surface coating and ceramic rolling
elements reduce premature bearing failure in large
electric motors
A new surface coating for rolling element bearings has been
developed by Schaeffler that insulates the bearings from damage
caused by the passage of electrical current in large AC or DC
motors.
Schaeffler’s Isotect® A coating is an insulating layer for rolling
element bearings that can be applied to the outer or inner ring
external surfaces. Alternatively, depending on the application, the
bearings can be assembled with ceramic rolling elements.
In the past, bearings used in large AC or DC motors with only a
few pairs of poles were prone to damage from the passage of
electrical current. Today, due to the increasing use of frequency
converters, this potential danger is growing. Around 3 per cent of
all electrical machines manufactured worldwide are controlled by
a frequency converter. This figure is expected to rise to 10 per
cent over the next couple of years.
Isotect® A acts like a resistor and capacitor connected in parallel.
With DC voltage and low frequency AC voltage, the choice
depends on the ohmic resistance of the bearing. With higher
frequency AC voltage, which is often found in frequency
converters, it depends on the capacitive reactance of the bearing.
To ensure good insulation, the ohmic resistance should be high
and the capacitance should be as low as possible.
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Depending on the operating temperature, Isotect® A has a high
resistance ranging from several Ohm up to 10Gohm. To allow for
the capacitive resistance of bearings used with a frequency
converter, Schaeffler has selected not only the appropriate
material for the insulating layer, but also the most suitable layer
thickness, which both play a key role in ensuring optimum
protection.
As capacitance is derived from the surface area of the protective
layer and its thickness, the coating is applied to either the inner or
outer ring external surfaces, depending on the application. If
operating conditions require even higher protection, Schaeffler
offers rolling element bearings assembled with ceramic rolling
elements across most of its bearing range.
Types of bearing damage
There are typically two types of bearing damage caused by the
passage of electrical current: ‘fluting’ and ‘welding beads’ (or
craters).
Fluting is relatively easy to identify and runs transverse to the
raceway. The washboard appearance of the resulting pattern
represents the final stage of the destruction of the raceway in a
bearing exposed to the passage of electrical current. Fluting can
be caused by either a continuous flow of electrical current, EDM
(electro-discharge machining) or high frequency electrical current.
Welding beads or craters represent the early onset of raceway
destruction and the initial stages of fluting. Identifying craters and
welding beads requires detailed examination of the structural
changes that have taken place in the bearing rolling surfaces.
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This is greatly simplified by the use of a scanning electron
microscope.
Current-related bearing raceway damage is typically caused by
‘classic bearing current’, in other words, the asymmetrical
distribution of magnetic flux in mains-operated AC or DC
machines, as well as to converter-induced bearing current (which
occurs in the form of circular current, EDM current and rotor earth
current).
Classic and converter-induced bearing current can severely
damage the raceway structure and the lubricating grease, either
through continuous current flow or in the form of voltage flashover
and discharge. Capacitive bearing current found in converter
operation is considered as harmless.
In electrical terms, a bearing can be thought of as simply a
connection between resistance and capacitance of the lubricant.
When the elasto-hydrodynamic (EHD) condition (i.e. the complete
separation of the rolling elements by the lubricant) is achieved
during operation, the bearing behaviour is capacitive. In this case,
the lubricant acts as an isolator, preventing the passage of
electrical currents up to a specific threshold value.
However, under conditions of mixed or boundary friction, the
electrical behaviour of the bearing changes and is considered as
an ohmic resistor. Here, the lubricant does not completely
separate the rolling elements, resulting in roughness peaks and
metal-to-metal contact. This can cause a continuous flow of
electrical current through the bearing. During such periods of
contact, the rolling elements fuse with the rolling surfaces for a
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short time and immediately break apart again due to the dynamics
of the bearing. During this process, extremely fine metal particles
are torn away from the bearing raceway and the rolling element
surfaces, mixing with and contaminating the lubricant. These
metallic particles are then rolled into the raceway, forming a new
layer that does not have the same hardness as the original
surface. The problem is exacerbated by system-related vibration
and fluting.
Discharge flashes
Even though a protective lubricant layer may prevent a
continuous flow of current through the rolling elements, converter
operation can cause electrical discharges that penetrate the
lubricant film. These ‘discharge flashes’ blast fine metal particles
out of the rolling surfaces, producing ‘craters’ with welding beads.
The resulting metallic particles add to lubricant contamination.
When the number of metal particles reaches a certain level, new
contacts and bridges can be formed in the contact zones.
Therefore, the process takes the same course as continuous
current flow, resulting in fluting.
Experience gained by Schaeffler using its own and independent
research has established that current densities of less than
0.1 A/mm2 do not represent a danger to bearings. However,
current densities at or above 1 A/mm2 often lead to eventual
raceway damage.
Not every bearing in an electrical machine will need to be fitted
with one that has current insulating properties. With mainsoperated machines, it is often sufficient to replace only one
bearing with a current-insulated bearing in order to avoid possible
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current damage. However, when frequency converters are used,
it is normally necessary to fit current-insulated bearings in several
bearing locations.
For more information on Schaeffler’s Isotect® A coated bearings
or ceramic rolling element bearings, please visit
www.schaeffler.co.uk or contact the marketing department on
0121 313 5870 or [email protected] .
ENDS [989 words]
Note to editors:
With a total of 66,000 employees at over 180 locations around the globe
and group sales of 8.9 billion euros (fiscal year 2008), the Schaeffler
Group is one of the world's leading rolling bearing manufacturers and
automotive component suppliers. The corporate group includes the INA
Group with headquarters in Herzogenaurach, the FAG Group based in
Schweinfurt as well as the LuK Group, with headquarters in Bühl.
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Schaeffler (UK) Ltd,
Forge Lane
Minworth
Sutton Coldfield
West Midlands B76 1AP
Tel: 0121 351 3833 Fax: 0121 351 7686
e-mail: [email protected]
www.schaeffler.co.uk
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