Modern plant and machinery greatly depends on the
reliability of rolling bearings. Nick Dowding of The Barden
Corporation, discusses the importance of lubrication,
including correct selection, application and maintenance of
rolling bearing lubricants and greases

Bearings operate on very thin films of
lubricant, which have to be
maintained in order to ensure that
the bearing’s design life is achieved. Three
key considerations are: Selection of the
correct lubricant, correct application, and
maintaining lubricant in a clean condition.

Correct selection of a lubricant can:
Reduce friction and wear by providing an
elasto hydrodynamic film of sufficient
strength and thickness to support the load
and separate the balls from the raceways,
preventing metal-to-metal contact.

Minimise cage wear by reducing sliding
friction in cage pockets and land surfaces.

Prevent oxidation/corrosion of the bearing
rolling elements.

Act as a barrier to contaminants.

Serve as a heat transfer agent. In some cases,
conducting heat away from the bearing.

Bearing lubricants fall into three main
categories: oils, greases, and solid dry film
lubricants, which are usually limited to
moderate speed and very light loading
conditions. Greases are the most widely used
of the three, and have been the focus of much
development over the last decade.

The selection of a particular type of bearing
lubricant is generally governed by the
operating conditions and limitations of a
bearing system. Three significant factors are:
Viscosity at operating temperature.

Maximum and minimum allowable
operating temperatures.

Speed at which the bearing will operate.

Grease considerations The primary advantage of grease over oil is
that bearings can be pre-lubricated,
eliminating the need for an external
lubrication system. Grease lubrication
requires less maintenance and has less
stringent sealing requirements than oil
systems. Grease tends to remain in proximity
to bearing components, metering its oil
content to operating surfaces as required.

The drawbacks are that it does not conduct
heat away from a bearing as efficiently as oil.

Also, it can increase the initial torque within
a bearing and cause running torque to be
slightly higher. The speed limits for greases
(expressed as a ndm value, with ndm being
the rpm multiplied by the bearing PCD in
mm) are generally lower than for oils due to
the plastic nature of grease that tends to
cause overheating at high speeds.

In applications where, for example, the
bearings are subjected to harsh operating
conditions such as in a vacuum
environment, the design of the bearing and
selection of a suitable grease become
challenging. It is critical that the bearing
supplier has the knowledge and experience
to suggest a grease that ensures reliability of
the bearings over long operating periods
without re-lubrication. Current ‘greased-forlife’
bearing technology can consistently give
30,000+ hours of life at 700,000ndm.

Oil considerations While grease lubrication is inherently simpler
than lubrication with oil, there are applications
where oil is a better choice. In high-speed
spindle and turbine applications, for
example, oil is supplied continuously and
provides cooling as well as lubrication. A
further example is instrument bearings with
low values of starting and running torque.

These require only a minimal, one-time
lubrication, each bearing receiving just a few
milligrammes of oil ? a single drop or less.

Limiting speeds for oil-lubricated bearing
are governed by the size of the bearing and the
design of the cage, rather than by the lubricant
itself. To illustrate this, petroleum or diesterbased
oils can accommodate bearing speeds to
1,500,000ndm or higher. In the case of siliconebased
oils, the maximum speed rating drops to
350,000ndm. Similarly, when calculating life
for bearings lubricated with silicone-based oils,
the Basic Load Rating (C) should be reduced
by two-thirds (C/3). In addition, to ensure
long life at high speeds, the lubrication system
should provide for retention, circulation,
filtration and possibly cooling of the oil.

In harsh operating environments such as
dry pump bearings, often oil selection is predefined
to a certain extent by the end user. In
dry pumps, the challenge for the bearing
supplier is to optimise the design of the
bearings in order to make the best use of
relatively poor lubrication.

Solid soft film lubricants Solid soft films are primarily used to provide
solid lubrication for bearings in extreme
applications where traditional fluid lubricants
would be rendered ineffective. Their friction is
independent of temperature (from cryogenic
to extreme high temperature applications), and
they do not evaporate or creep in terrestrial
vacuum or space environments. The solid soft
film lubricant can be applied directly to the
surface or transferred by rubbing contact
from a sacrificial source such as a selflubricating
bearing cage.

Examples of the two processes include the
application of Teer Coatings’ physical
vapour-deposited MoSTTM and Barden’s
PTFE-based ‘TB’ polymeric cage material.

The processes are complementary and have
been used successfully in a variety of extreme
aerospace applications.