Lubrication of Bearings

Lubrication Basics

General Information

The selection of the lubrication method and the lubricant itself must be an essential part of the dimensioning, to ensure a failure-free operation. A statistical assessment of all roller bearing failures finds the highest failure rate of over 80% is attributed to the lubrication. The lubricant usually is contaminated, inappropriate or obsolete. The following information prime an overview over the lubrication group of themes. It is advisable, concerning roller bearing lubrication to question the lubricant or roller bearing manufacturer.

The main task of the lubrication is to reduce or prevent the metallic contact of the roll and gliding surface, and therefore generate a friction minimisation and reduction wear in the roller bearing. Besides the separation of the metallic surface, the lubrication has further functions:

• Corrosion prevention

• Heat dissipation (oil lubrications)

• Contamination removal from the bearing inside (oil lubrication)

• Sealing the bearing inside (grease collar and oil-air-lubrication)

Chemical and Physical Basics

Bearing lubrication is distinct in two (physical and chemical) categories. During a physical lubrication the lubricant is being brought into the contact areas of the roller bearing. By the adhesion on the separated surfaces, a lubricating film is building up and the surfaces "swim" upon this lubricating film thus prevent metallic contact. The chemical lubrication is applied, when no full-strength lubricating film is built up and the surfaces are not completely separated. In this case, additive that develop a layer by a tribomechanical resp. chemical reaction, which separates the surfaces, are applied.

A separation layer is not solely built up by additive, but fat thickener and lubricant additive e.g. sold lubricants as well. In special cases, a lubrication with solid lubricants is also possible.

Lubrication State

The life time calculation according DIN ISO 281 demands an enough separation of the metallic surfaces. Thereby the operational life, wear and friction basically correlate with the lubrication state. A lubricating film thickness of a few micrometre is enough to develop a sustaining and separating lubricating film. Three lubrication states are distinguished by the roller bearing engineering that are displayed in a Stribeck curve:

A boundary lubrication does not build up a lubricating film, and the metallic surfaces grind each other. Roller bearings usually run in a boundary lubrication at:

• to small lubrication amount

• insufficient operation viscosity

• unwanted relative movements between the metallic surfaces.

If a boundary lubrication cannot be avoided, enough lubricants with appropriate additive must be used. The additive must build up a carrying layer with the aid of reaction products.

If a lubricating film is developed whose height is not enough to separate the metallic surfaces completely, it is called mixed lubrication.

A heavy lubrication is a lubricating film which height is enough to separate the metallic surfaces completely. For a safe continuous operation, heavy lubrication must always be achieved.

Lubrication Theories

Two theories are used within the roller bearing engineering, for the physical description of a lubricating film build-up. For lightly loaded contact points, for example between the pocket surface and roller body or between the roller body front face and bord, the hydrodynamic theory (HD theory) is used. The theory describes the lift of metallic surfaces by a lubricating film. The elastohydrodynamic theory (EHD theory) is based on the HD theory and considers the elastic deformation of the metallic partner additionally. The EHD theory is used for the rolling contacts lubrication state description.

Lubricant Parameters for the Rolling Bearing Calculation

Lubricant parameters are required for the calculation of the above-mentioned theories and the lubrication film thickness. Besides the concentration, the viscosity hereby is very important. Therefore, it must be pointed out that parameters depend of temperature and pressure.

Viscosity identifies the inverse value of a fluids flowability and is a measure for the inner friction. A high viscosity describes a viscous oil. A low viscosity describes a mobile oil. Viscosity is the highest parameter for lubricating oils. For grease, it always describes the viscosity of the base oil.

In the lubrication theories, the dynamic viscosity η is used. It describes the relation of shear stress and the maximal speed change over the lubrication film thickness vertical to the laminar flow direction and declared in millipascal-seconds (mPa∙s).

The kinematic viscosity (v) is used for the roller bearing calculation. It is declared in square millimetre per second (mm²/s) or centistokes (cSt). The kinematic viscosity can be converted to the dynamic viscosity with the aid of the density (ρ in kg/dm³).

Calculation:        η = ν ∙ ρ

Note that kinematic viscosity as well as density are temperature dependent. The density depends of the lubricating oil and its refining degree. Synthetic oils mostly have a higher density than mineral oils.

Due to temperature-dependence, lubricant viscosities are declared by standardised temperatures. The higher the temperature the lower the viscosity. It concerns viscosities around 40°C, the so-called nominal viscosity, and around 100°C. With those parameters, the viscosity index (VI) can be determined. The VI states the operation temperature span of the lubricant. The higher the viscosity index the broader the temperature scope of the lubricant.

Lubrication oils were standardised in the DIN ISO 3448 in ISO VG classes on behalf their nominal viscosity.

ISO VG classes according DIN ISO 3448 for industrial oils:

Besides the temperature dependence, the viscosity also is pressure dependent. At increasing pressure, the viscosity increases in general as well. This must be considered especially for the EHD theory and high pressures (> 500 bar).

Besides the nominal viscosity, the rated viscosity is required for the roller bearing life time calculation. It is speed-dependent and is determined with the aid of the mean diameter of the rolling bearing and the shafts rotational frequency:

Besides the rated viscosity, the operational viscosity is required by the roller bearing calculation. The operational viscosity describes the viscosity of the lubricant at operation temperature. This viscosity is being determined through the viscosity-temperature diagram (VT diagram). The diagram axes are logarithmised. The x-axis shows the operational viscosity, the y-axis shows the operation temperature. With the aid of the nominal viscosity and the viscosity at 100 °C, the diagram shows a straight line. The operational viscosity can only be determined by the operation temperature.

The viscosity relation κ is a measure for the lubrication film thickness within the lubrication gap. It is calculated with the help of rated and operational viscosity.

The above figure indicates that values less than 1 describe the mixed friction condition, whereas values greater 1 describe the complete separation of the metallic surfaces. κ =1 describes the border case. A lubrication film is built up which is not fully sustaining.

The relation has a direct influence of the life adjustment factor calculation. This calculation can be found in DIN ISO 281.

A further parameter for the determination of the lubricant is the rolling bearing rotational frequency value. This value is calculated with the aid of the diameter (d_m) and the rotational frequency.

Lubrication Method

The determination of a suitable lubrication method must be carried out early after choosing the right roller bearing. A lubrication method change during a progressed design status is very time and cost consuming and should be avoided.
 

Grease Lubrication

The grease lubrication is with 90% share of the roller bearing lubrication the commonly used lubrication method.

In the most use cases, roller bearings are lubricated for life, this means that the bearings are one-time filled with an application-related type of grease and amount during assembly. It is not necessary to regrease the bearing at normal operation and environmental conditions. A regrease is necessary because of extreme environmental influences, for example high temperature fluctuation, frequent speed changes or a nonavoidable grease aging. Therefore, lubrication feed and discharge channels must be provided. During the design of the channels the pumping of grease must be considered.

The above-mentioned influences have effects auf the regrease interval and the grease amount. The calculation of the regrease interval and regrease amount can be found in the GfT-sheet 3 (Gesellschaft für Tribologie). The following regulations for the grease lubrication must be heeded:

• Increasing temperatures quicken the grease oxidation

• At 70°C the grease life span is halved every 10 K

• At increasing temperatures grease tend to be more fluid, a leak of the housing then is possible

• In a dirty environment the roller bearings must be regreased more often, to avoid contamination.

Oil Lubrication

Oil lubrication is appropriate if other machine elements are already supplied with oil, e.g. gears in a gearbox.

The following oil lubrication methods are distinguished:

• oil sump lubrication

• circulating oil lubrication

• oil injection lubrication

• oil impulse lubrication

• oil-air lubrication

Oil lubrication is often associated with a high level of design complexity and usually includes auxiliary units such as circulation pump and oil cooler. With the above-mentioned oil lubrication methods, the design complexity and speed suitability are increasing. Oil sump lubrication is easier to implement than oil-air lubrication. It requires a high design effort but offers a higher speed suitability due to the minimum quantity lubrication and the resulting reduced splash losses.

Solid Lubrication

A special case of lubrication is solid lubrication. In this process, a supporting lubricating film is created with bonded coatings, pastes and powders in slowly rotating rolling bearings. Commonly used lubricants are graphite, molybdenum sulphide or PTFE compounds. The solid material can even serve as a component of the rolling bearing. In some applications the solid material replaces the cage of the rolling bearing. Since this is lifetime lubrication, the lubricant is completely used up at the end of the bearing life. A typical application example is the axis bearing of kiln cars.

Selection of a Suitable Lubrication Method

The selection of the lubrication method depends on:

• the operating conditions of the rolling bearing

• the desired running behaviour, running noise, frictional and temperature behaviour of the rolling bearing

• the requirements for safety against premature failure

• the costs for installation and maintenance of the lubrication system

The requirement for safety against premature failure has the highest priority when choosing the lubrication method. It must be ensured that the lubricant reaches every functional surface contact and is sufficiently available. The presence of lubricant is not guaranteed in the same way for all lubrication methods. For example, with oil sump lubrication, the oil level must be checked at regular intervals. In contrast, the presence of lubricant in a grease-lubricated rolling bearing is always sufficient as long as the grease operating life or relubrication intervals are not exceeded.

Common lubrication methods with detailed instructions

Lubricants

Greases

The basic properties of a grease are determined by the base oil type and viscosity, the thickener and the additives. Mineral and synthetic oils are usually used as base oil. Metal soaps or metal complex soaps are usually used as thickeners. In addition to the base oil and thickener, additives are becoming increasingly important. They are divided into two types:

• acting on the base oil, e.g. viscosity improver, antioxidation

• acting on the bearing or metallic surfaces, e.g. corrosion protection and wear protection

A further classification of greases is possible according to their consistency. They are divided into the so-called NLGI classes (National Lubrication Grease Institute). These are standardised according to DIN 51818. The higher the NGLI class the "harder" the grease is. NLGI classes 1, 2 and 3 are frequently used in rolling bearings.

Greases are also subdivided according to their main components thickener and base oil.

Overview of lubrication greases¹⁾

1) Cf. [Brändlein, Johannes (Hg.) (1998): Die Wälzlagerpraxis. Handbuch für die Berechnung und Gestaltung von Lagerungen (korrig. Nachdr. der 3. Aufl. Mainz: Vereinigte Fachverl.]

Oils

Mineral and synthetic oils are basically suitable for the lubrication of rolling bearings. Mineral oils are used most frequently. These must at least meet the requirements of DIN 51517.

Synthetic oils are rarer. They are often used under special and extreme operating conditions, e.g. high temperatures. Synthetic oils are artificially produced in refineries at great expense by splitting hydrocarbons from mineral oils. They therefore have a high degree of purity.

The viscosity ratio has already been mentioned above (κ). A value of 2 to 4 is targeted for lubricating oils.

High viscosity oils have a high liquid friction. At low and normal temperatures, oil can accumulate in the supply and discharge lines.

It is not possible to achieve the desired operating viscosity in every application because:

• the oil choice is determined by other machine components

• the circulation lubrication requires a sufficiently flowable oil

• strongly varying operating conditions do not allow a clear choice

In the cases mentioned above, a different viscosity is permissible; if the oil is provided with the appropriate additive (EP additives), it can also be permitted with a lower viscosity. Otherwise, signs of wear and a reduction of the bearing service life must be expected.

The following table shows a choice guide according to operating conditions for lubrication oils. However, it is advisable to consult the lubricant supplier.

Additives

Additives are usually optimised for the respective function. They represent an "alloy" of the oil and enhance oil characteristics, such as

• corrosion protection

• oxidation stability

• wear protection

or gives "new" non-existent characteristics, such as

• neutralization capacity

• dispersing capacity

• emulsifying capacity

Miscibility of Fats and Oils

The mixing of lubricants involves high risks. On one side, some oils and thickeners are not compatible. On the other side, the effect of additives and the performance of lubricant mixtures can only be estimated to a limited extent. Only suitable tests can provide a concrete statement on the miscibility and compatibility of two lubricants.

Before mixing, always check with the lubricant supplier whether mixing is possible.

The mixing of fats should be avoided. If mixing is unavoidable, the following parameters must be observed for both mixing partners:

• same oil base

• matching thickener type

• similar base oil viscosities (max one ISO VG class)

• same consistency class (NLGI class)

*Source: Oil Doc GmbH, Brannenburg

*Source: Oil Doc GmbH, Brannenburg

If the grease type is changed, the used grease must be completely flushed out of the bearing and housing. A mixture of incompatible greases leads to a hardening of the lubricant or to a strong softening of the grease. Both can cause premature rolling bearing failure.