Hybrid ceramic bearings have proved so effective in
challenging applications that ranges have expanded to
serve ever-more demanding environments, as Phil Burge,
country communication manager at SKF, explains

It was the need to provide bearing
solutions for high performance gas
turbines that kicked-off the
development of ceramic rolling
elements. However, as their
combination of strength and
lightness has become more
widely appreciated, ceramic
rolling bearings have been
applied to more common
applications, such as
machine tool spindles,
electric motors and
generators.

One reason behind the
success of ceramic ball
bearings is that they offer a
higher speed capability than
standard steel bearings and
typically provide a longer
service life than all-steel
bearings. They also offer a high
degree of resistance to high
temperatures; the operating
conditions for the main shaft bearings
within a jet engine include shaft speeds in
excess of 30,000 r/min and predicted bearing
temperatures higher than 650°C.

Ceramic ball bearings are proven to
withstand these conditions above
steel bearings and, indeed, where gas
temperatures rise above 1100°C,
only high-performance ceramic
materials are considered.

However, there is a
further quality to consider
that has made ceramic
rolling bearings the
component of choice in
many applications, and
that is the capability
to insulate against
electrical
current. When
stray currents
pass across
bearings, it
leaves behind a
fluting pattern
and a darkened
appearance on the
raceways. The excessive
noise that results from this damage causes
premature aging of the lubricant and early
bearing replacement. In contrast, the
natural insulating properties of ceramic
material eliminate this type of damage.

One cause of stray currents is an
increasingly popular device used to enhance
efficiency: the variable speed drive. In motor
installations, bearings can provide the path
of least resistance for electrical currents. To
high frequency currents, the lubricant film in
a rolling element bearing acts as a capacitor.

A voltage difference builds up between the
bearing casing and the rolling elements until
it becomes strong enough to overcome the
insulation provided by the lubricant. At that
point, discharge occurs. As a result, heat
generated by the discharge causes local
melting of the bearing metal surface.

However, bearings with ceramic rolling
elements eliminate the electrical pathway
through the motor bearings, providing
efficient electrical insulation between stator
and rotor.

More recently, hybrid rolling bearings,
comprising ceramic balls and a split inner
bearing ring, have been introduced to an
expanding range of applications, including
rail and renewable energy. Despite the fact
that the cost of a hybrid bearing is typically
higher than that of a standard steel bearing
the difference is soon recovered
in maintenance savings, which would
otherwise be caused by problems such as
stray electrical current damage.

Hybrid ceramic bearings have proved
effective in extra-large bore sizes to combat
the dangers of stray electrical currents in a
demanding application: wind turbines. There
is always pressure to minimise maintenance
visits to remote and hard-to-access wind
turbines, where premature bearing failure,
generator breakdown and unplanned turbine
downtime can prove extremely costly.

This pressure is relieved by hybrid ceramic
ball bearings, which are now available in
sizes specifically designed to fit these
turbines. These robust hybrid bearings can
measurably increase machine lifespan, while
providing a highly efficient performance.

Without regular and correctly measured
lubrication, standard bearings wear and fail
prematurely, but hybrids require less
lubricant and therefore the margin of error is
wider. Hybrid bearings run cooler and can
operate with thinner lubricant films, so there
is less aging of the grease and required relubrication
intervals are longer.

With so many benefits to offer, ceramic
rolling bearings and hybrid rolling bearings
are enabling operators in an increasing range
of applications to decrease downtime and
increase profitability.