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Minimising friction and preventing bearing wear
There are many things to consider when selecting a lubricant such as temperature, loads, speed, environment, and desired life just to name a few. The experts at AST Bearings offer some tips and advice.
Bearing Lubrication plays a vital role in the performance and life of rolling element bearings. The most important task of the lubricant is to separate parts moving relative to one another (balls or rollers and raceways) in order to minimise friction and prevent wear. A lubricant that is designed for specific operating conditions will provide a load bearing wear protective film. The ideal condition is when the friction surfaces are separated by this film.
In addition to providing this load bearing film, the lubricant should also allow for the dissipation of frictional heat, thus preventing overheating of the bearing and deterioration of the lubricant and provide protection from corrosion, moisture, and the ingress of contaminants. Lubricants used in rolling element bearings should maintain a stable viscosity over a broad range of temperatures, provide good film strength that can support loads, and offer a stable structure that provides for long service life. The lubricant should also be non-corrosive and compatible with adjacent components, and provide a barrier against contaminant and moisture, yet not leak out of the bearing
Bearing lubricant types
The primary types of bearing lubricants are oil, grease and solid films, each with their own benefits and applications.
Oil: Both petroleum-based and synthetic oils are available. Examples of synthetic oils are silicone, diesters, PAOs and fluorinated compounds. Bearings lubricated with oil will exhibit less start up and running torque and have higher speed capability. Oils are subject to evaporative losses so there service life in a bearing is less than grease.
Miniature and instrument bearings are often only lubricated once for the life of the bearing, making the choice of lubricant critical. Larger bearings are subject to re-lubrication as part of the machinery maintenance cycle. These bearings are often lubricated via oil recirculation systems that are designed into the machinery or equipment. Temperature range, viscosity, evaporative rate are key characteristics to consider when selecting an oil.
Grease: Grease consists of a base oil with a thickener added. These thickeners consist primarily of metal soaps (lithium, sodium, aluminium, and calcium), organic compounds (ureas) or inorganic compounds. While these thickeners greatly influence the characteristics of the grease, the lubricating properties of the grease are attributable to its base oil. In addition, grease can contain additives that improve its performance. Additive types include antioxidant, anti-corrosion, anti-wear, fillers, fortifiers, and extreme pressure fortifiers. Temperature range, base oil viscosity, and stiffness or penetration level are key characteristics to consider.
Solid Films: These are non-fluid coatings applied to the frictions surfaces to prevent wear. They are used in extreme situations where an oil or grease cannot survive and are typically selected as a last resort or option. These include harsh environments such as extreme temperatures, vacuum, or radiation. These coatings include graphite, MoS2, silver, gold, or PTFE. Hard coatings include TiC or chrome.
The lubricant (and amount) selected also impacts the maximum operating speed and torque (both starting and running). In miniature bearings the lubricant can impact the noise level. Filtered greases and oils are recommended for use with miniature or instrument bearings.
Bearing lubrication methods
Grease is normally applied with special lubricating equipment with a head that deposits the grease between the balls, forcing it into, and around, the ball (or roller) raceway interface. Upon rotation the grease is distributed within the bearing. The amount of grease is typically specified as a percentage, such as 30% fill. The percentage represents the actual grease volume compared to the free internal space within the bearing. In other words, if the internal space in the bearing is completely filled with grease with no voids, it would be 100% full. Manufacturers of bearings have different amounts that they would consider standard. This typically ranges from 20% to 40%. In small or miniature bearings grease fill amounts can be as little as 10%.
Oil is applied by the manufacturer with special equipment as well. The amount is not normally specified. In torque sensitive applications excess oil can be removed via centrifuging. This is more common with miniature or instrument type bearings.
The grease plating process starts with carefully mixing the candidate grease with a volatile solvent. The consistency of the grease is thinned considerably. This mixture can be controlled to achieve different coating thicknesses. The mixture is then injected into the bearing coating all of the internal surfaces.
A dipping process is also used in some cases. The solvent is then baked off at low temperature. This method is often used in torque sensitive applications and where lubricant migration associated with oil lubrication is a undesirable.
Shelf life of bearing lubricants
Synthetic oils are inherently stable materials. Generally, they are not expected to oxidise, polymerise or volatilise at room temperature for ten years or more. Ester oils, where the ester linkage may be subject to a minute degree of hydrolysis in the presence of moisture, could become more acidic if moisture is present. Fluorinated oils and silicones are not likely to be affected by simple aging.
Greases can age in more complicated ways. Grease quality could be affected by a change in the gel structure. If the gel contracts, significant oil bleed would be evident and the remaining grease would stiffen. The gel structure may also become softer over a period of time.
Shelf life is the period following the lubricant’s manufacture during which it is deemed suitable for use without re-testing its physical characteristics. In addition, the manufacturers state that the shelf life applies only if oils and greases are properly stored in their original, unopened containers.
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