Rolling bearings: Lubrication matters

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.