Feature Seals & bearings Bearing lubrication boosts reliability
Nick Dowding, applications engineering manager at The Barden Corporation, discusses the importance of bearing lubrication, including the correct selection, application and maintenance of rolling bearing lubricants and greases
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he performance of modern plant and machinery depends to a large extent upon the reliability of rolling bearings. One of the major contributing factors to achieving high reliability is proper lubrication. The three primary principles of maximising bearing life are: selecting the correct lubricant, b) applying the lubricant properly, and c) maintaining the lubricant in a clean condition. Neglect or failure in any of these areas will increase risk of premature bearing failures and interfere with the trouble- free running, now of such importance in modern plant and machinery. Correct selection of a lubricant will bring the following benefits: 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; minimise cage wear by reducing sliding friction in cage pock- ets and land surface; prevent oxida- tion/corrosion of the bearing rolling elements; act as a barrier to contami- nants; serve as a heat transfer agent. Bearing lubricants fall into three cat- egories: oils, greases and solid dry film lubricants.
Grease considerations
The advantage of grease over oil is that bearings can be pre-lubricated, elimi- nating the need for and cost of an external lubrication system. It also requires less maintenance and has less stringent sealing requirements. Grease tends to remain in proximity to bear- ing components, metering its oil con- tent to operating surfaces as required. The drawbacks are that it does not
conduct heat away from a bearing as efficiently as oil. Also, grease 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 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 suit- able grease become very challenging. Here, it is critical that the bearing sup- plier has the knowledge and experi- ence to suggest a grease that ensures maximum reliability over long operat- ing periods without re-lubrication. Current ‘greased-for-life’ bearing tech- nology can consistently give 30,000+ hours of life at 700,000 ndm.
Oil Considerations
While grease lubrication is inherently simpler, there are still applications where oil is the better choice. In high- speed spindle and turbine applica- tions, 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 single drop or less. The limiting speeds for oil-lubri- cated bearings are governed by the size of the bearing and the design of the cage, rather than by the lubricant itself.
are said to reduce levels of friction by up to 55 per cent, compared with conventional shaft seals. The new seals feature a low friction elastomer lip design based on the wafer technology used for PTFE shaft seals. The new seals also use a specially formulated FKM material with improved compression set and relaxation properties to compensate for the lack of a spring, normally found in shaft seals. The result is said to be extremely low friction, outstanding sealing security and long service life. The SKF Low Friction Seal ensures robust sealing and high durability, even at high speed. They
Low friction seals lower C02 emissions A
n innovative new range of low friction seals manufactured by SKF can lower CO2 emissions and improve fuel economy. Designed to fit on rotating engine and transmission shafts, the new seals
SKF
provide minimal shaft wear and have excellent dirt exclusion. With no additional sealants or lubricants necessary, it is simple to install. Using their Vehicle Environmental Performance Simulator, SKF has calculated the CO2 reduction for a four-cylinder petrol engine passenger car at over 1g per km.
Factory Equipment NOVEMBER/DECEMBER 2011
T: 01582 496433
www.skf.co.uk Enter 319
The performance of modern plant and machinery depends on the reliability of rolling bearings
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 cal- culating 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 environments such as dry pump bearings, often oil selection is pre-defined to an extent by the end user. In dry pumps, the bearing supplier’s challenge 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 tradi- tional fluid lubricants would be inef- fective. Their friction is independent of temperature (from cryogenic to extreme high temperatures), 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-lubri- cating 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 mater- ial. The processes are complementary and have been used successfully in extreme aerospace applications. Download Barden’s Precision Bearing catalogue from the company’s website for more details.
The Barden Corporation T: 01752 725 531
www.bardenbearings.co.uk
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