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Bearing Lubrication

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 minimize 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 have the following characteristics:

  • Maintains a stable viscosity over a broad range of temperatures
  • Good film strength that can support loads
  • Stable structure that provides for long service life
  • Non-corrosive and compatible with adjacent components
  • Provides a barrier against contaminant and moisture, yet does not leak out of the bearing

Types of Bearing Lubricants

Oil: Both petroleum based and synthetic oils are available. Examples of synthetic oils are silicone, diesters, PAO’s, 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, aluminum, and calcium), organic (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, anticorrosion, anti-wear, fillers, fortifiers, and extreme pressure fortifiers. Temperature range, base oil viscosity, and stiffness or penetration level are key characteristics to consider when selecting a grease. Most greases used in rolling element bearings are NLGI grade 2

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. Solid films are engineered on a specific application by application basis.

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. Miniature and instrument series bearings are typically lubricated in a clean room environment.

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.

Grease Plating (MGGP) - The MGGP 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 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 oxidize, polymerize or volatilize at room temperature for 10 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.

The lubricants that are in the bearings supplied by AST Bearings are high quality lubricants. Many of these lubricants are also qualified to military or other specifications. The type and quantity of lubricant is in many cases specified by the customer, and in those cases where the customer has not made a particular selection, the bearings are lubricated to conform to industry standards.

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.

AST purchases the required lubricants from approved suppliers. Our purchasing documents require a Certificate of Analysis to be supplied with the lubricant, and that 75% of the manufacturer’s shelf life is remaining upon receipt. With respect to the shelf life of any lubricant, AST must rely on the shelf life specifications of that particular lubricant’s manufacturer. In addition, AST has established internal lubricant control procedures that include proper storage of lubricants and applying “use by” dates that are consistent with the lubricant manufacturer’s specifications. AST does not “certify” the shelf life of the bearings we lubricate for the following reasons:

  1. We are not the lubricant manufacturer.
  2. Once the lubricant has been applied to the bearing, or other device, factors, which AST can neither predict, or control will govern the lubricant life.

Therefore, the end user should establish their own database or guidelines regarding storage life or functional life of lubricated bearings. AST routinely marks the date of lubrication on our labels and packaging so that customers can apply their internal procedures for age sensitive material.

There are many things to consider when selecting a lubricant such as temperature, loads, speed, environment, and desired life just to name a few. In addition, aside from those already mentioned, there are a variety of characteristics of greases and oils that should be considered such as oil separation, evaporative loss, dropping point, oxidation stability, channeling capability/stiffness, and others. Lubrication is one of the most critical specifications for the designer to consider.

The following Tables list common bearing lubricants and some of their properties. AST stocks hundreds of others.

Table I - Grease Lubricants

Rykon Premium #2   -10/200 Mineral Arylurea Reddish
Rykon Premium #3   -20/250 Mineral Arylurea Pink
Supermil ASU31052 MILG25013 -100/450 Silicone Arylurea Lavender
Supermil ASU72832 MILG23827A -100/250 Diester Lithium Amber
Braycote 627S MILG23827 -100/300 Ester Organic Lt. Brown
Braycote 637S MILG25537 -65/260 Mineral Calcium Soap Lt. Brown
601EF   -100/390 Polyether Tetrafluor Off White
BRB-2 MILG3545C -20/350 Mineral Polyurea Blue/Green
OHT   +20/300 Mineral Sodium Greenish
NRRG335   -65/300 Synthetic/Aeromatic Sodium Maroon
Poly FM #2 USDA H1 0/-320 White Oil Polyurea Apricot
SRI-2 MILG3545G -20/350 Mineral Polyurea Blue/Green
Molykote BR2 Plus   -20/300 Mineral Lithium Black
Molykote 33   -100/400 Silicone Lithium Gray
Molykote 41   -0/550 Silicone Lithium Black
Molykote 44 MILG46886A -100/400 Silicone Lithium Dark Amber
Molykote 55M MILG4343 -65/350 Silicone Lithium Tan
Krytox 240AA MILG27617 -30/450 Fluor Carbon Vidax White
Krytox 240AB MILG27617 -30/450 Fluor Carbon Vidax White
Krytox 240AC MILG27617A -30/550 Fluor Carbon Vidax White
Krytox 240AZ MILG27617 -65/300 Fluor Carbon Vidax White
Krytox 283AC MILG27617 -30/550 Perfluor Tetrafluor White
Andok B MILG18709A -20/250 Mineral Sodium Brown
Andok C   -20/250 Mineral Sodium Brown
Andok 260 MILG3545C -20/250 Mineral Sodium Amber
Beacon 325   -65/250 Diester Lithium Lt. Tan
Cosmolube 615 MILG4343 -65/375 Silicone Lithium Lt. Brown
Asonic GLY 32   -58/284 Ester/PAO Lithium White/Beige
Asonic GHY 72   -40/356 Ester Polyurea Beige
Barrierta L 55/2   -31/482 PFPE PTFE Creamy White
Isoflex Super LDS 18 MILG23827 -76/266 Mineral/Ester Lithium Yellow
Isoflex LDS 18 Special MILG23827 -76/266 Mineral/Ester Lithium Yellow
PS #2   -60/230 Diester Lithium White
SRL   -40/300 Ester Lithium Tan
BRB #23 MILL7711 -0/250 Petroleum Sodium Tan
Mobil 24 MILG25013 -100/550 Silicone Organic Reddish
Mobil 27 MILG23827 -65/325 Carbon Non Soap Tan
Mobil 28 MILG81322A -65/350 Hydro Carbon Non Soap Dark Red
MINAPURE MILG81937 -65/250 Diester Lithium Lt. Tan
703A   -30/250 Mineral Sodium Tan
716B   -60/300 Polyol Ester Lithium Tan
Rheolube 703A   -30/250 Mineral Sodium Tan
Instrument Grease 706E   -65/300 Polyol Ester Lithium Light Brown
Rheolube 716B   -60/300 Polyol Ester Lithium Tan
Nyogel 781 D Replaces GE
Versilube G-300
-95/390 Silicone Lithium Off White
Fluoroether 899 RP   -130/480 PTFE PTFE White
Rheolube 2000   -60/260 Hydrocarbon Organic Red
Rheotemp 500 MILG3278A -65/350 Diester Sodium Blue
Rheoplex 6000HT   -40/302 Ester Sodium Light Brown
Aeroshell #5 MILG3545C -20/300 Petroleum Microgel Dark Brown
Aeroshell #6 MILG24139 -40/250 Mineral Microgel Amber
Aeroshell #7 MILG23827A -100/300 Diester Microgel Amber
Aeroshell #14 MILG23827 -65/250 Mineral Calcium Soap Tan
Aeroshell #17 MILG21164 -100/300 Diester Microgel Dark Gray
Aeroshell #22 MILG81322A -80/350 Hydrocarbon Microgel Dark Gray
Alvania #2 MILG18709 -20/275 Mineral Lithium Amber
Alvania #3 MILG81322C -30/275 Mineral Lithium Amber
Cyprina #3 MILG18709 -0/250 Mineral Lithium Lt. Tan
Dolium R #2   -30/300 Mineral Ashless Amber
Darina MILG18709 -0/300 Mineral Microgel Amber
Royco 13D MILG25013 -100/450 Silicone PTFE Lavender
Royco 21 MILG7421 -100/250 Diester Lithium Brownish
Royco 22MS MILG81827 -80/360 Diester Clay Black
Royco 27A MILG23827 -100/275 Diester Lithium Brownish
Royco 37 MILG25537 -65/250 Mineral Calcium Soap Tan
Royco 64C MILG21164 -65/250 Diester Lithium Black
Anderol 753A   -40/300 Diester Lithium Lt. Brown
Anderol 757   -40/300 Diester Lithium Lt. Brown
Anderol 761   -40/400 Diester Silica Lt. Brown
Anderol 793A   -65/300 Diester Lithium Lt. Amber
Anderol 794   -65/250 Diester Lithium Lt. Amber
Anderol 795   -65/300 Diester Lithium Off White
Premium RB   -30/325 Mineral Lithium Orange
Low Temp EP MILG23827 -65/250 Synthetic Material Lithium Purplish Brown
Regal AFB #2 MILG18709 -40/250 Paraffin Lithium Green
Unitemp 500   -65/350  Diester Sodium Blue
Table II - Oil Lubricants
CS +75°F/
L245X MILL6085A -70/350 Diester -75 420 20/3.5
LS252 MIL17353A -65/250 Diester -75 340 7.6/1.9
P10 MILL6085A -70/350 Diester -80 420  23.4/3.8
NPT3A   -65/175 Diester -90 400 19/3.5
885 MILL6085 -50/400 Diester -85 410 1875/9
NPT9   -30/350 Ester -50 495 710/55
DC200 VVL1078 -40/550 Silicone -50 600 Various
DC510 MILL27694 -70/500 Silicone -80 600 Various
DC550   -40/450 Silicone -50 600 125/20
FS1265   -50/300 Silicone -30 500 Various
Krytox 143 AB   -45/450 Perflour -45 500 85/10.3
Krytox 143 AC   -30/550 Ester -35 550 270/26
P15A MILL7808 -65/300 Diester -75 450 22/3.5
Aviation Inst. Oil MILL7870 -65/290 Petroleum -70 300 17/2.6
Univis P12 MILL6085A -75/300 Diester -90 410 30/3.6
Univis P38 MILL6085 -65/300 Diester -70 415 72/37
Versilube F44   -100/500 Silicone -100 550 70/15
Versilube F50 MILS81087 -100/400 Silicone -100 550 75/22
Versilube SF81   -40/400 Silicone -55 600 Various
Versilube SF96   -40/400 Silicone -50 600 40/16.5
Synthetic Fluid #6   -50/275 Mineral -90 295 3200/12
Cosmolube 270A MILL6085A -65/250 Diester -70 365 15/3.5
SHC824   -50/350 Synthetic -65 455 100/6.5
XRL743A   -50/350 Synthetic -65 520 100/6.5
M0119   -30/250 Synthetic -80 455 119 @ 100°F
Aeroshell #3 MILL7870 -70/240 Petroleum -75 275 16.5/2.3
Aeroshell #12 MILL6085A -70/300 Diester -70 365 21.5/3.5
Aeroshell #4 MILH5606 -70/500 Petroleum -85 215 859/10.4
Anderol L401D MILL6085A -75/260 Diester -80 430 19.7/3.4 
Anderol L423   -80/350 Synthetic -100 370 200/5.1

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