Viscosity is the most important property of the lubricant. That phrase is widely used by lubrication experts around the world. And it is no coincidence.
Viscosity has a very important impact on the lubrication of industrial mechanisms. The formation of the lubricating film between two friction surfaces depends on the viscosity of the lubricant and this relationship will be explored in detail later. In any case, once the viscosity is well selected, the main benefits observed are:
The viscosity of the lubricant impacts the formation of the lubricating film and therefore the wear of the bearing. The better the separation of the roughness in contact, the less wear observed in the bearing. Therefore, a lubricated bearing with a very low viscosity and not suitable for its operating conditions will have a shorter service life.
In ISO 281:2007, a methodology for calculating bearing life is presented. One of the factors that is taken into consideration is the kappa factor, which can be understood as the thickness or quality of the lubricating film. According to the standard, when the viscosity of the lubricant is 4 times higher than the recommended minimum, the bearing reaches its maximum service life. Below the minimum recommended viscosity, the bearing’s service life is severely affected.
To give an example, considering two cases exactly the same in all bearing life parameters, just with the difference in lubricant viscosity from 0.4 to 4 times the minimum viscosity (K=0.4 to K=4), the bearing life modification factor (aiso) can go up from 0.8 to 50, respectively (Image 1). A considerable increase in bearing life.
Image 1. Life Modification Factor, aiso, for Radial Ball Bearing.
Source: ISO 281:2007.
And of course, a bearing that has a longer lifespan has a direct impact on:
If increasing viscosity increases bearing life, one might think: let’s use as much viscosity as possible!
That thought has two implications. First, according to the ISO standard, above 4 times the minimum viscosity, there is no longer an increase in bearing life. Second, viscosity is the internal resistance of a fluid to flow. The more viscous, the greater that resistance. So, when applied to a bearing, the higher the viscosity, the greater the resistance offered to the rotation of the bearing, that is, the greater the fluid friction.
In practice, what happens is that this increase in fluid friction means increased energy losses. It takes more effort to turn the bearings. In other words, the electric motor is going to need more energy to execute the same operation, since the losses in the bearings increased.
And energy is an extremely relevant issue today, not only because of the price increases in its supply, but also considering the TCO (total cost of ownership) of an industrial machine. According to a publication by SKF (Sporer and Doyer, 2007), over a period of 10 to 15 years, between 95 and 99% of the TCO of an electric motor is expected to be relative to its energy consumption.
Holmberg, K. and Erdemir, A. (2017) also present the importance of tribology in global energy consumption and that the potential for savings in 15 years can reach 40%. Of course, that potential is not entirely related to bearings operating with high fluid friction, but it does have its share.
Finally, selecting viscosity is looking for the optimal balance between bearing life and energy consumption. That balance is normally sought through related costs. However, the environmental impact is something that cannot be left out.
According to SKF, 12.2 kg of CO2e (https://www.skf.com/mx/products/rolling-bearings/roller-bearings/spherical-roller-bearings/productid-22218%20E) are emitted to produce the spherical roller bearing code 22218 E. According to the International Energy Agency (IEA), to produce 1 kWh of electrical energy, 475 g of CO₂ are emitted, considering a world average (https://www.iea.org/reports/global-energy-co2-status-report-2019/emissions).
That means that making a good selection of lubricant viscosity, with the impacts already presented in terms of reducing bearing consumption and reducing energy consumption, in addition to the financial side, also leaves a positive footprint on the environmental impact. Reducing consumption means reducing waste generated and the need for raw material extraction, production inputs and logistics. All of this can be summed up in reduction of greenhouse gas emissions.
In a study by Dr. Yulia Sosa, published in STLE in 2024, the carbon footprint of a grease-lubricated bearing system was presented in 14 days and 2 years. In 14 days, emissions due to rolling bearing consumption account for more than 71%, due to friction (energy consumption) around 26% and due to grease as little as 2%. Now considering a period of 2 years, which would be an average life for a bearing like that, the impact of emissions due to friction increases enormously, reaching more than 90% of the total (Image 2).
Image 2: Carbon footprint in a grease-lubricated bearing.
Source: adapted from Yulia Sosa, 2024.
Reducing environmental impact is something that is increasingly sought after by companies today. And, through lubrication, that can be achieved with small investments and the correct selection of the viscosity of the base oil.
The goal of lubrication is to generate a lubricating film between the two parts that are in contact and relative motion. Despite seeming simple, it is not always easy to get a film to form that completely separates the contact between surfaces, especially when there is fluid lubrication.
Any solid surface, no matter how polished, has a surface roughness. That is, it has peaks and valleys of different sizes. Even a glass, if it is placed under a microscope, we’re going to see something similar to Image 3 on its surface.
Image 3: surface roughness of solids.
That means that when two surfaces are in contact and relative motion, those roughness touches and generates resistance to motion – friction. This solid contact also leads to damage to surfaces – wear. This is where lubrication comes in: forming a film between the roughnesses, assuming that the resistance to movement of this film is less than the resistance of solid contact. For this reason, most of the lubrication is done with fluids, since these can offer less resistance to movement, that is, less friction and wear.
The problem is that in order to form that miraculous film, many factors must be taken into account. Relative speed, load and viscosity of the oil are some of the most important variables. It is possible to do a parallel with driving a vehicle on the road on a rainy day. In this scenario, the aquaplaning effect can occur, which is characterized by the formation of a film of water between the vehicle’s tires and the asphalt. Yes, a micrometric film is capable of supporting the weight of a vehicle due to the hydrodynamic effect. That causes the vehicle to lose traction and handling control. Question: When would be the easiest scenario for this aquaplaning effect to happen?
In case 2 of the light car at high speed, it will be much easier for this effect to happen. And the same can be applied to bearings. Of course, in vehicles traction is wanted and not film formation. On the other hand, in rolling bearings, it needs film to be formed. Therefore, bearings at high speed and low load are more susceptible to film formation than bearings at low speed and high load.
In addition to load and speed, another important factor is the viscosity of the fluid. The higher the viscosity, the greater the ease of generating the hydrodynamic effect mentioned above and the greater the thickness of the film. Therefore, in low-speed bearings, i.e. more difficult to achieve film formation, more viscous oils are used.
Importantly, the thickness of the film must be greater than the roughness of the surfaces in contact so that it can fully generate the effect of reducing friction and wear. As a reference, in bearings, film thicknesses of less than 1 μm are formed, invisible to the naked eye.
There is an equation that is used conceptually to understand the different lubrication regimes that can happen in fluid lubrication. The equation is:
Where h is the thickness of the film and Ra is the mean roughness.
By making a quick analysis of the possible values of λ, some conclusions can be drawn. First, if there is a λ less than 1, that means that h is lower than Ra, that is, that the film thickness is lower than the roughness. In that condition, contact between surfaces is going to occur. Second, if there is a λ greater than 1, that means that the thickness of the film h is greater than the roughness Ra. In other words, there is enough thickness to separate the contact between the surfaces – but not completely. Some points may still have contact, and the entire film emerges approximately when the thickness is at least 3 times greater than the roughness: λ > 3 (Wu, S., & Cheng, 1991; Hohn et al., 2009).
Therefore, in a conceptual way, the lubrication regimes can be defined according to their film (Image 4): boundary (λ < 1), mixed (1 < λ < 3) and hydrodynamic (λ > 3). The boundary film is when there is no formation of a complete film, allowing a high degree of contact between solid surfaces. A mixed film is when there is a thicker film being formed, with fewer points of contact between the surfaces than the boundary film. The hydrodynamic film is already when there is a very thick film, which does not allow any contact between the surface roughness.
Image 4: lubrication regimes.
The hydrodynamic film arises only in the contact between conforming surfaces, coinciding geometries, such as, for example: a shaft in a plain bearing. When there is non-conforming surfaces, with non-matching geometries, a film called elastohydrodynamic (EHL or EHD) arises. This happens in bearings and gears, for example, and it is so named because very high pressures (500,000 psi) are generated that elastically deform the solid surfaces in contact (Image 5).
Image 5: Elastohydrodynamic film regime.
After understanding the different types of lubricating films, it is essential to analyze how friction and wear behave as a function of operating conditions. This behavior is represented by the Stribeck curve, an essential tool in tribology. The curve allows you to visualize how the system transits between the different lubrication regimes and how it impacts friction and wear, when varying the speed, viscosity of the lubricant or the applied load.
Image 6 shows the Stribeck curve with friction, wear, and film thickness on the vertical axis and load, speed, and viscosity on the horizontal axis. Here it is also done a conceptual analysis, but it helps a lot to understand what happens in the different regimes and how to act from the point of view of lubrication.
Image 6: Stribeck curve.
Source: Noria Media (https://www.machinerylubrication.com/tribology-31340)
In the first quadrants, on the left of the image, it is the boundary lubrication regime. This is where the thickness of the film is the least and therefore friction and wear are very high. Also, it’s the point where extreme pressure (EP) and anti-wear (AW) additives are most effective. The contact between the roughness and the temperature activates these additives that form a protective layer. That is why both wear and friction can be better controlled, as seen in the graph.
Boundary lubrication usually occurs under the following variable conditions: low viscosity; low speed; high load. In addition to that, machine starts and stops, changes in direction of movement, high vibrations and impacts are conditions that favor contact between roughness and boundary lubrication regime.
Here it is already going to the right side on the graph, increasing the speed or increasing the viscosity or reducing the load. There is not yet complete separation of roughness, but with the thickness increase in the lubricating film, friction and wear are reduced. In mixed lubrication, wear is controlled by both viscosity of the lubricant and EP and AW additives.
When the point of minimum friction and wear is reached, if one continues to play with the variables in the same way, i.e. increasing the speed or increasing the viscosity or reducing the load, the thickness of the film will continue to grow. Wear is not impacted by this increase, since it continues without solid contact of the surfaces, so it remains at its minimum. Then, the wear is controlled only by the viscosity of the lubricant.
On the other hand, in this situation, friction is impacted. Continuing to grow the film thickness means that there is a very viscous oil for the speed and load presented. As mentioned earlier, high viscosity means greater resistance to movement, i.e. greater fluid friction. Therefore, friction rises again in the hydrodynamic regime.
These conceptual analyses are important and help to some extent. In laboratory and under controlled conditions, these parameters can be analyzed in a very deep way. However, measuring film thickness or coefficient of friction in the field is not feasible, so finding the optimal point of friction and wear in a mechanical component becomes complex.
For rolling bearings, based on many experimental tests, the theory of the kappa factor was created. Based on real data on the operation and dimensions of the bearing and lubricant in use, this theory helps us to identify in which lubrication regime the bearing is operating.
But before getting into the theory presented in ISO 281:2007, it’s important to understand the relationship between rotation and tangential velocity in a bearing. This is because one usually has the rotation data, but depending on the size of the bearing, the speed at the working point of the lubricant may be different. And the speed factor helps to understand that effect a little better.
The speed factor (velocity factor, ndm or dn) is a measurement that relates the rotation of a bearing (rpm) to its average diameter, thus representing the linear speed in the place where the lubricant is subjected to shear. Unlike angular velocity (rotation) which only indicates how many times the bearing rotates per minute, the speed factor reflects the energy applied to the lubricant, which is crucial for defining the proper viscosity of the base oil and ensuring efficient lubricating film formation. In its most common form, it is calculated as the multiplication between the rotation in rpm and the average bearing diameter in millimeters.
In addition to being able to calculate it, this factor is also present in the technical sheet of some lubricants. It determines the channeling and film-forming capacity of lubricating greases. Greases with low viscosity base oils typically withstand higher speed factors as they flow better and allow effective lubrication at higher speeds. On the other hand, lubricants with high viscosity oils have lower channeling capacity and form thicker films, suitable for low-speed applications. Thus, each lubricant has a speed factor limit normally reported by the manufacturers.
Considering two bearings of different diameters, but both with the same rotation, exemplified here as 400 rpm, it is known that the linear speed of each will be different at the point where the lubricant will be placed. In other words, the linear velocity at the point of average diameter will be greater at the bearing of the largest diameter.
In this example, the smallest bearing has an average diameter of 90 mm, while the largest bearing has an average diameter of 550 mm. The result of the speed factor calculation will be 36,000 and 220,000 mm/min for the smallest bearing and the largest bearing, respectively.
Therefore, there are bearings of the same rotation that work the lubricant at different velocity and possibly need to use different lubricants. When selecting the viscosity of the base oil for bearing, one should not rely solely on rotation. That is, 400 rpm means little without knowing the size of the bearing.
A more complete information about the speed factor can be found in: https://www.machinerylubrication.com/Read/32997/all-about-the-rolling-bearing-speed-factor
The theory of the kappa factor uses the speed factor indirectly and goes even further. Comparing the calculated speed factor with the speed factor presented in the lubricant data sheet disregards a very important variable: the operating temperature. For the calculation of the kappa, the viscosity of the oil at the operating temperature of the bearing is used and therefore it is a more accurate tool for selecting viscosity for bearings.
The kappa factor is simply a relationship between viscosities and helps to define the optimal viscosity for the application in study. Mathematically speaking, the kappa factor is the ratio of the viscosity of the oil at the operating temperature of the bearing (vop) divided by the minimum viscosity required by the rolling bearing, according to ISO 281:2007 (vref). The equation presented by the standard considers the dimensional aspects of the bearing and the rotational speed, and then a minimum viscosity required for that bearing is defined to ensure the formation of a lubricating film.
So, if the operating viscosity (selected by the lubrication engineer) is equal to the minimum viscosity required, the division will result in 1. That is, the minimum required is 10 cSt and the engineer is recommending a lubricant that at the operating temperature has also 10 cSt. Ten divided by ten equals 1. Therefore, it will form a minimum required film. If the viscosity one recommends is less than 10, the result of the division will be less than 1 and will not form a proper film. Similarly, above 10, the result will be greater than 1 and there will be film formation.
It is important to understand that the kappa factor is simply a statistical relationship that considers viscosity and velocity for film formation. Therefore, the thickener, the additives, the type of oil do not fall into this theory of the kappa factor. Obviously, these are factors that influence lubrication, but here it is in isolation that the optimal viscosity is selected. The restrictions of the standard are:
And as it was already explored, here it is considered the rotation and dimensions of the bearing, that is, the velocity at the point where the lubricant is. High speed facilitates film formation, so less viscous oils will be recommended in this condition. While the low speed makes it more difficult to form a film and then it is necessary to increase the viscosity of the base oil.
Based on a bearing life study, the ideal range of kappa factor for lubricant viscosity selection was defined. Tests were carried out on lubricated bearings in various kappa factor ranges and the service life of these bearings was measured. Here on the graph (Image 7), on the vertical axis is the a23 factor (or aiso) which impacts the life of the bearing and on the horizontal axis is the kappa factor. Therefore, it is observed that as the kappa factor increases, the bearing life increases, until a maximum limit is reached, where the bearing life no longer increases significantly with the increase in the kappa factor.
Image 7: Graph of the kappa factor by the life modification factor aiso.
Here it is also possible to see that, below 1, the service life has a considerable drop, which can be mitigated with the use of lubricants with EP additives. Below 1, it is almost certain to be in the boundary lubrication regime, where no film is formed and there is metal-to-metal contact, i.e. a situation in which EP additives will be more efficient.
And there near 4 is where the bearing reaches its maximum life, with little increase after this range.
So, in summary:
Kappa < 1: boundary lubrication regime, contact between roughness, increased wear and friction, activation zone of EP additives. Recommendation: increase the viscosity of the oil.
Kappa between 1 and 4: mixed film regime, range considered ideal, the closer to 4, the longer the rolling bearing life.
Kappa 4: elastohydrodynamic film regime, optimal kappa, where the bearing life is maximum and without too much increase in fluid friction.
Kappa between 5 and 10: elastohydrodynamic film regime, maximum bearing life zone, however, with increased fluid friction, temperature and energy loss the closer it is to 10.
Kappa > 10: extremely high fluid friction, possible excessive temperature increases and energy losses. Recommendation: decrease the viscosity of the base oil.
Example with step-by-step calculation
Input data: bearing code 23228 operating with 1000 rpm rotation and temperature of 60°C. Lubricant to be calculated will be a grease with base oil viscosity at 40°C of 32 cSt and viscosity index of 145. With the bearing code and a simple search on the internet, you can find the outer and inner diameters, respectively, 250 and 140 mm. Calculating the speed factor, the result is 195,000 mm/min.
The first step is to calculate the reference viscosity (vref), i.e. the minimum viscosity needed to reach the mixed film. To do this, first it is calculated the average diameter of the bearing, that is, the outer diameter plus the inner diameter divided by 2. The result is the average diameter which in this example gives 195 mm.
This graph is the one that is generated from the equations presented in ISO 281 to estimate the minimum viscosity. The horizontal axis receives the average diameter of the bearing, a line is raised vertically until it touches the diagonal line of rotation, which in this case is 1000 rpm, and it is moved to the left horizontally till it touches the Y axis (Image 8).
Image 8: Graph used to find the reference viscosity.
Source: ISO 281:2007
It is found approximately vref = 10.19 cSt.
Therefore, this value of 10.19 cSt is the minimum viscosity required to form a mixed lubricating film under these operating conditions. That is, remembering that the kappa should be between 1 and 4, one can already think about how much viscosity should recommended. It should be roughly between 10 and 40 cSt, correct? This, of course, at the operating temperature.
And the second step is precisely this. Calculate the viscosity of the recommended lubricant at the operating temperature.
It is known that the operating temperature in this example is 60°C and it is also given the lubricant data, so it is just a matter of using the viscosity index graph (Image 9), i.e. the variation in viscosity with temperature. Put the temperature on the X-axis, go up vertically until you find the curve and throw it to the left towards the Y-axis to find out the viscosity, which in this case is giving 16.22 cSt. Thus, this lubricant at a temperature of 60°C has a viscosity of 16.22 cSt.
Image 9: Viscosity curve by temperature (°C) of the lubricant in the example.
Observation 1: The graph was generated from the https://interlub.com/herramientas-de-lubricacion/graficar-viscosidades-aceites/ calculator, using viscosity data at 40°C and VI of this oil. Not suitable for other oils of other viscosities and other VIs.
Observation 2: A parallel, just to clarify the graph. If setting the temperature at 40°C, what would be the resulting viscosity using the graph? It would be 32 cSt, which is the viscosity at 40°C of this lubricant.
And finally, to calculate the kappa factor, what is missing?
Simply divide the two viscosities found: the viscosity at the operating temperature that was recommended and the reference viscosity.
Therefore, 16.22 divided by 10.19, which gives a result of kappa = 1.59.
Well, 1.59 is within the range that was defined as ideal from 1 to 4. It is possible to increase the viscosity of the lubricant a little with the aim of getting closer to 4, but it is already at a kappa considered good.
Remembering that, it is a theory for viscosity, that is, it does not tell anything about the thickener or the additive that the lubricant has. It just says that the viscosity of 32 cSt at 40°C is suitable to form a mixed film with 1.59 kappa in this 23228 rolling bearing running at 1000 rpm and 60°C.
For these examples, a calculator available online will be used to facilitate all the work. This calculator is based on ISO 281:2007 and is available on the page: https://interlub.com/en/lubrication-tools/kappa-value-bearings/
Image 10: Detail of an electric motor
Input data:
This is a simpler case for which calculations will be done for two products: a common multipurpose grease in the market and another special grease with low viscosity oil.
Multipurpose grease:
Special grease:
With the help of the calculator:
Image 11: Results of the kappa factor calculations for example 1
Comments: A common multipurpose grease on the market is not suitable for lubrication of high-speed electric motors. It normally has a very viscous oil for that operating condition, which is clear with the kappa being so high at 16.28. That could increase the engine’s operating temperature, power consumption, and impact premature failures. In such manner, a grease with 32 cSt base oil viscosity is much more suitable and the engine will be able to operate more energy-efficiently.
Image 12: Detail of a scrap metal shredder
Input data:
High viscosity grease 1:
Medium viscosity grease 2:
With the help of the calculator:
Image 13: Results of the kappa factor calculations for example 2
Comments: In a quick evaluation of this case, many might already assume that it is a low-speed case, since the rotation is only 450 rpm. With that in mind, the recommendation would be a high-viscosity oil. However, that is a paradigm, since, as presented, you must consider the dimensions of the bearing to get the velocity where the lubricant is working. And in this case, it is a large bearing. So the lower viscosity lubricating oil is more suitable for this machine.
Image 14: Detail of a ball mill
Input data:
High viscosity grease 1:
Medium viscosity grease 2:
With the help of the calculator:
Image 15: Results of the kappa factor calculations for example 3
Comments: Another case that at first you could judge it as low speed and recommend grease with high viscosity base oil. But because of the size of the bearing, the velocity at the point of average diameter is medium to high. So, once again, the oil with a lower viscosity can offer a more reliable and energy efficient operation.
Image 16: Detail of a steel turret
Input data:
High viscosity grease 1:
Ultra-high viscosity grease 2:
With the help of the calculator:
Image 17: Results of the kappa factor calculations for example 4
Comments: In this case, despite a large bearing, the rotation is extremely low and incomplete turns are used during its operation. Therefore, this is a case that is practically impossible to get out of the condition of a boundary film regime. With a base oil grease of 3000 cSt at 40°C it is almost reached kappa of 1, but it already makes an enormously heavy grease that could be a problem to apply it or other issues for example at winter seasons. Therefore, the lubricant recommendation must consider that this bearing will operate under the boundary regime and include extreme pressure additives and/or solid lubricants.
ISO 281:2007. Rolling bearings — Dynamic load ratings and rating life. Geneva: International Organization for Standardization, 2007.
SPORER, R. y DOYER, A.-L. Bearings that save energy. SKF Automotive Division, Schweinfurt, Germany and SKF Industrial Division, St-Cyr-sur-Loire, France, 2007. Available in: https://evolution.skf.com/bearings-that/. Accessed on November 11, 2025.
HOLMBERG, K. y ERDEMIR, A. Influence of tribology on global energy consumption, costs and emissions. Friction, 2017, 5 (3), pp. 263–284. DOI: 10.1007/s40544-017-0183-5.
SKF Group. Spherical roller bearing 22218 E – Product page. Available in: https://www.skf.com/mx/products/rolling-bearings/roller-bearings/spherical-roller-bearings/productid-22218%20E. Accessed on November 11, 2025.
INTERNATIONAL ENERGY AGENCY (IEA). Global Energy & CO₂ Status Report 2019 – Emissions. Paris: IEA, 2019. Available in: https://www.iea.org/reports/global-energy-co2-status-report-2019/emissions. Accessed on November 11, 2025.
SOSA, YULIA. How to develop grease that helps save energy. STLE Feature Article – March 2024 | Tribology & Lubrication Technology (TLT). Available in: https://www.stle.org/files/TLTArchives/2024/03_March/Feature.aspx
WU, S. y CHENG, H. S. A friction model of partial-EHL contacts and its application to power loss in spur gears. Tribology Transactions, 1991, 34 (3), pp. 398–407. DOI: 10.1080/10402009108982050.
HÖHN, B. R., MICHAELIS, K. y OTTO, H. P. Minimised gear lubrication by a minimum oil/air flow rate. Wear, 2009, 266 (3–4), pp. 461–467. DOI: 10.1016/j.wear.2008.04.037.
NORIA MEDIA. Tribology Basics and Resources. Machinery Lubrication. Available in: https://www.machinerylubrication.com/tribology-31340. Accessed on November 11, 2025.
NORIA MEDIA. All about the rolling bearing speed factor. Available in: https://www.machinerylubrication.com/Read/32997/all-about-the-rolling-bearing-speed-factor. Accessed on November, 11, 2025.
INTERLUB. Kappa value – Bearings | Lubrication Tools. Available in: https://interlub.com/en/lubrication-tools/kappa-value-bearings/. Accessed on November 11, 2025.
