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One of the major contributing factors to achieving high reliability in machines is proper lubrication.
Bearings operate on thin films of lubricant,which have to be maintained in order to ensure that the bearings design life is acheived. Lubrication reduces friction and wear by creating 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.
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 increase the risk of premature failure and will trouble free running.
The increased speeds and higher temperatures at which modern bearings often operate, combined with the demands placed upon them for improved accuracy and reliability mean that the process of selecting a suitable bearing lubricant is a significant decision.
Correct lubricant selection
- Reduces friction and wear by providing an elasto-hydrodynamic film that prevents metal to metal contact.
- Minimises cage wear by reducing sliding friction in cage pockets and land surfaces.
- Prevents oxidisation/corrosion of the bearing rolling elements.
- Acts as a barrier to contaminents.
- Serves as a heat transfer agent.
Bearing lubricants fall into three main categories – oils, greases and solid dry film.
The selection of a particular type of bearing lubricant is generally goverened by the operating conditions and limitations of a bearing system.
- The viscoisity of the lubricant at operating temperature.
- The maximum and minimum allowable operating temperatures.
- The speed at which the bearing will operate.
The primary advantage of grease over oil is that bearings can be pre-lubricated, eliminating the need and cost for an external lubrication system. Besides simplicity, grease lubrication also requires less maintenance and has less stringent sealing requirements than oil systems. The drawbacks of using grease are that it does not conduct heat away from a bearing as efficiently as oil as it tends to remain in proximity to bearing components. Grease can also increase the initial torque within a bearing and cause running torque to be slighlty higher. The speed limits for greases are generally lower than for oils due to the plastic nature of grease that tends to cause overheating at high speeds.
In certain applications the design of the bearing and 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 relaibility of the bearings over long operating periods without re-lubrication. Current ‘greased-for-life’ bearing technology can consistently give 30,000+ hours of life.
- CASTROL NUCLEOL G 110 is a radiation resistant grease. Moderate to high speeds and temperatures up to 150centigrade, where the grease may be exposed to radiation doses of up to 109 rads
- Apezion Vacuum Grease
- Leadscrews can use a light consistency lithium-based grease, NLG1 aer0-grade
While grease lubrication is 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 exremely low values of starting and running torque. These require only a minimal, on-time lubrication, each bearing receiving just a few milligrams of oil.
The limiting speeds for oil-lubricant bearings are governed by the size of the bearing and the design of the cage, rather than by the lubricant itself.
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, the 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 traditioanl fluid lubricants would be rendered ineffective. They offer the advantages that their friction is independent of temperatures and they do not evaporate or creep in terrestial vacuum or space environments.
The solid soft film lubricant can either be applied directly to the surface or transferred by rubbing contact from a sacrifical source such as a self-lubricating bearing cage.
Self lubricating materials
The linear tables, rotary tables and elevator tables we design and manufacture at LG Motion Ltd all have their specific lubrication equirements to ensure maximum life in a given environment. With such a variety of applications from the Film Industry to Nuclear to Formula One, we have a wide scientific and industrial experience with motion technology and the importance of understanding lubrication requirements.