Nick Dowding, the Applications Engineering Manager at The Barden Corporation, discusses how to select the correct grease, oil or solid dry film lubricant for rolling element bearings.
To a large extent the performance of modern plant and machinery depends on the reliability of rolling bearings. One of the major contributing factors to achieving high reliability is proper lubrication.
Bearings operate on very thin films of lubricant that have to be maintained to ensure that the bearing's design life is achieved. The three primary principles of maximising bearing life are:
- selecting the correct lubricant
- applying the lubricant properly
- maintaining the lubricant in a clean condition
Neglect or failure in any of these three areas will seriously increase the risk of premature bearing failures and will interfere with the trouble-free running.
Increased speeds and higher temperatures, combined with the demands placed on bearings for improved accuracy and reliability, mean that the process of selecting a suitable bearing lubricant today is more critical than ever.
Correct selection of a lubricant will result in the following benefits:
- Reduces 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, thereby preventing metal-to-metal contact.
- Minimises cage wear by reducing sliding friction in cage pockets and land surfaces.
- Prevents oxidation/corrosion of the bearing rolling elements.
- Acts as a barrier to contaminants.
- Serves 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, because of their convenience, are by far 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 of the most significant factors in selecting a lubricant are:
- Viscosity of the lubricant at operating temperature.
- Maximum and minimum allowable operating temperatures.
- Speed at which the bearing will operate.
Compared with oil, the primary advantage of grease is that bearings can be prelubricated, which eliminates the need for – and the cost of – an external lubrication system. Besides simplicity, grease lubrication also 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 of using grease are that it does not conduct heat away from a bearing as efficiently as oil. Grease can also increase the initial torque within a bearing and cause running torque to be slightly higher. The speed limits for greases (expressed as an 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 certain applications where, for example, the bearings are subjected to harsh operating conditions such as in a vacuum environment, the design of the bearing and the selection of a suitable grease become very challenging. Here, it is critical that the bearing supplier has the knowledge and experience to suggest a grease that ensures maximum reliability of the bearings over long operating periods without re-lubrication. Current greased-for-life bearing technology can consistently give 30,000+ hours of life at 700,000 ndm.
Oil lubrication
While grease lubrication is inherently simpler than lubrication with oil, there are still applications where oil is the 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 extremely 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.
The limiting speeds for oil-lubricated bearings 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 diester-based oils can accommodate bearing speeds of up to 1,500,000 ndm or higher. In the case of silicone-based oils, the maximum speed rating drops to 350,000 ndm.
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 extremely 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.
Dry 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. They offer the advantages that 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 either be applied directly to the surface or transferred by rubbing contact from a sacrificial source such as a self-lubricating bearing cage. Examples of these 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.
Comprehensive details regarding bearing lubricant selection are contained in Barden's Precision Bearing catalogue, which can be downloaded at www.bardenbearings.co.uk.
