Ressource KRW Product Bearing basics

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:

Lubrication State

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 is unavoidable, enough lubricants with appropriate additive must be used. The additive must build up a carrying layer by the aid of reaction products.

Speaking of mixed friction, a lubricating film is developed whose height is not enough to separate the metallic surfaces completely.

A heavy lubrication is identifying 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 States

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).

Dynamic Viscosity, h= Lubricating Film Thickness

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:

 

 

ISO viscosity classViscosity at 40°C in mm²/smaximal deviation

min.

at 40°C in mm²/s

max.

at 40°C in mm²/s

2

2,2

1,98

2,42

3

3,2

2,88

6,52

5

4,6

4,14

5,06

7

6,8

6,12

7,48

10

10

9,00

11,0

15

15

13,5

16,5

22

22

19,8

24,2

32

32

28,8

35,2

46

46

41,4

50,6

68

68

61,2

74,8

100

100

90,0

110

150

150

135

165

220

220

198

242

320

320

288

352

460

460

414

506

680

680

612

748

1000

1000

900

1100

1500

1500

1350

1650

 

 

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:

 

Diagram for the determination of the operation viscosity

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.

 

Diagram for the determination of the reference kinematic viscosity

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.

 

Lubrication States

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 (dm) and the rotational frequency.

DOuter diameter[mm]
dInner diameter[mm]
nRotational frequency in the operating point[min-1]

 

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.

Advantages of a grease lubricationDisadvantages of a grease lubrication

Low design effort for the sealing of the bearing position

Increased friction and strong heating by increased grease amount

Simple design of the relubrication equipment resp. maintenance-free lubrication due to a lifetime lubrication

Wear particles are not removed

Applies for a broad speed range

No heat dissipation through the lubricant

Low friction torque

 

 

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.

Advantages of oil lubricationDisadvantages of oil lubrication

heat dissipation via the lubricant

very elaborate design

adjustability of the operating temperature

sealing against oil leakage

low start-up friction

maintenance-intensive due to oil and filter change

 

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.

 

Choice of a Suitable Lubrication Method

The choice of 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

Lubricant

Lubrication Method

Equipment for the Lubrication Method

Constructive Measures

Achievable rotation speed characteristic value [min-1  mm]

Solid Lubricant

lifetime lubrication

-

-

≈ 1.500

Grease

lifetime lubrication

-

-

≈ 0,5 ∙ 106

relubrication

handpress

grease pump

automatic relubrication systems

feed hole

grease quantity regulator

collecting chamber for used grease

for suitable special greases and bearings:

≈ 2,6 ∙ 106

Oil-

larger amounts

oil sump lubrication

dipstick

standpipe

level control

housing with sufficient oil volume

overflow hole

connection for control devices

≈ 0,5 ∙ 106

circulating oil lubrication

ciculating lubrication systems

sufficiently large holes for oil inlet and oil outlet

≈ 1,5 ∙ 106

oil injection lubrication

circulating lubrication system with spray nozzles

oil inlet through directed nozzles

oil outlet through sufficiently large holes

tested up to:
≈ 4,0 ∙ 106

Oil-

minimum amounts

oil impulse lubrication

consumption lubrication system

drip oiler

oil spray lubrication systems

outlet holes

≈ 1,5 ∙ 106
depending on bearing design, oil viscosity, oil quantity, design

oil mist lubrication

oil mist system or oil separator

possible suction devices

oil-air lubrication

oil-air lubrication system

possible suction devices

 

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)

Grease TypeCharacteristics
ThickenerBaseoilTemperature Range (°C)Droppingpoint (°C)Water ResistancePressure ResistanceSuitability for Rolling BearingsSpecific Instructions
Lithium soapMineral oil

-30 bis +130

+170 bis 220

+++

+

+++

multipurpose grease

PAO

-60 bis +150

+170 bis 220

+++

++

+++

for lower and higher temperatures

for high speeds

Ester

-60 bis +130

+190

++

+

+++

for lower temperatures

for high speeds

Aluminium complexMineral oil

-30 bis +160

+260

+++

+

+

multipurpose grease

Barium complexMineral oil

-30 bis +140

+220

++

++

+++

multipurpose grease
steam resistant

Calcium complexMineral oil

-30 bis +140

+240

++

++

+++

multipurpose grease
tends to harden

Lithium complexMineral oil

-30 bis +150

+240

++

++

+++

multipurpose grease

Aluminium complexPAO (Poly-alfaoelinfine)

-60 bis +160

+260

+++

++

+

wide temperature range,
highly exploitable

Barium complexPAO (Poly-alfaoelinfine)

-40 bis +140

+220

+++

+++

+++

high rotation speeds

Calcium complexPAO (Poly-alfaoelinfine)

-60 bis +160

+240

+++

+++

+++

for lower and higher temperatures

for high speeds

Lithium complexPAO (Poly-alfaoelinfine)

-40 bis +180

+240

++

+++

+++

wide temperature range

Barium complexEster

-40 bis +130

+200

++

++

+++

for high rotation speeds

for moderate load

 

Calcium complexEster

-40 bis +130

+200

+++

++

+++

Lithium complexEster

-40 bis +180

+240

++

+

+++

particularly wide temperature range

 Silicone

-40 bis +180

+240

++

-

++

for light loads only

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

 

 

Guide values for filling the bearing clearance according to the rotation speed characteristic value (rotating inner ring):

 

Movement Type

Rotation Speed Characteristic Value n∙dm

[min-1 ∙ mm]

Degree of fulfillment

[% of free bearing space]

rotating inner ring

< 50.000

up to 100

50.000 - 100.000

up to 30

> 100.000

20-30

> 400.000

up to 20

 

 

Guide values for filling the bearing clearance according to the rotation speed characteristic value (rotating outer ring):

Movement Type

Rotation Speed Characteristic Valuen∙dm

[min-1 ∙ mm]

Degree of fulfillment

[% of free bearing space]

rotating outer ring

< 100.000

up to 90

100.000 - 400.000

up to 50

> 400.000

1-30

 

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.

Base oils and their typical characteristics:

Base OilSpecial FeaturesOperating Temperatures [°C]
lowerupper

Mineral oil (Min)

most commonly used type of base oil

In case of unavoidable water ingress only limited use, water is bound as an emulsion, therefore limitation of service life is possible

Under unfavourable conditions, solid residues (oil carbon) can form and thus impair lubrication

-20

+120

Polyalphaolefin (PAO)

widely used synthetic oil type

can be found in lubricants for food approval

better oxidation resistance compared to mineral oils

are mixable with mineral oils

-40

+150

Polyglycol (PG)

critical for aluminium contact

mostly not mixable with mineral oils, esters and PAO

are for the most part not water-soluble

poor water separation ability

-40

+150

Ester (E)

thermally stable

also as mixture with PAO and mineral oils

partly biodegradable

low volatility

suitable for high rotation speed characteristic values and high temperature

reaction with water different, saponification and splitting of the ester possible

-60

+180

Silicone oil (Si)

load capacity (P/C ≤ 0,03) and their wear protection capacity is low

thermally very stable

extremely low surface tension

steel-steel contacts tend to fretting

-60

+200

Alkoxy fluorine oil (PFAE, PFPE)

not mixable with other oils

oxidation and water resistant

rolling bearings must be free from hydrocarbons

very expensive compared to mineral oil

-30

+250

 

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.

 

Choice guide for lubrication oils:

Criteria for the choice of oilCharacteristics of the oil to be chosen

normal conditions:

temperature in oil sump up to 100 °C / oil circulation up to 150 °C

load ratio P/C < 0,1

rotation speed below the reference rotation speed

use of unalloyed oils alternatively with additives e.g. corrosion or ageing protection

code letter L according to DIN 51502

if the viscosity recommendation cannot be implemented, wear protection additives must be provided

high rotation speed characteristic values

n∙dm > 500,000 min-1 ∙ mm

oxidation stable oil with low foaming tendency

good viscosity-temperature behaviour

recommended are synthetic oils such as PAO and Ester

high loads

P/C > 0.1

or

operating viscosity ν lower than related viscosity ν1

use of oils with high-pressure additives (EP additives)

code letter P according to DIN 51502

suitability of wear protection additives is strongly dependent on temperature

high temperatures

in addition to the operating temperature limits, the viscosity-temperature behaviour must be observed

the choice of oil should be made on the basis of oil characteristics

 

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

Lubricant additives and their effect:

Additive TypeAdditivationTask

extreme pressure additive

EP

improvement of the pressure absorption behaviour

wear reduction through reaction layer formation

friction modifier

FM

altered friction in mixed and boundary friction

wear protection

AW

reduction of mild adhesion-abrasion wear with mixed friction

corrosion inhibitors

KI

protection of metal surfaces against corrosion

ageing inhibitors

OI

delay of the oxidative decomposition of the lubricant

adhesive additive

improved adhesion of the lubricant to the surface

detergents and dispersant

improvement of the dirt dissolving and dirt carrying behaviour of the lubricant

viscosity improver

improvement of the viscosity-temperature dependence

foam inhibitors

avoidance of stable foaming

pourpoint-lower

decreasing the pour point

 

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)

 

Miscibility of oils: *

 Mineral oilPAOEster oilPGSilicone oilAlkoxy fluorine oil
Mineral oil

mixture generally not critical

mixture generally not critical

mixture generally not critical

mixture not permitted

miscible in individual cases, testing recommended

mixture not permitted

PAO

mixture generally not critical

mixture generally not critical

mixture generally not critical

mixture not permitted

miscible in individual cases, testing recommended

mixture not permitted

Ester oil

mixture generally not critical

mixture generally not critical

mixture generally not critical

miscible in individual cases, testing recommended

mixture not permitted

mixture not permitted

PG

mixture not permitted

mixture not permitted

miscible in individual cases, testing recommended

mixture generally not critical

mixture not permitted

mixture not permitted

Silicone oil

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

mixture not permitted

mixture not permitted

mixture generally not critical

mixture not permitted

Alkoxy fluorine oil

mixture not permitted

mixture not permitted

mixture not permitted

mixture not permitted

mixture not permitted

mixture generally not critical

*Source: Oil Doc GmbH, Brannenburg

 

 

 

Compatibility of different thickener types: *

 Lithium soapLithium complexSodium complexCalcium complexAluminium complexBarium complex
Lithium soap

mixture generally not critical

mixture generally not critical

mixture not permitted

mixture generally not critical

mixture not permitted

mixture generally not critical

Lithium complex

mixture generally not critical

mixture generally not critical

miscible in individual cases, testing recommended

mixture generally not critical

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

Sodium complex

mixture not permitted

miscible in individual cases, testing recommended

mixture generally not critical

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

Calcium complex

mixture generally not critical

mixture generally not critical

miscible in individual cases, testing recommended

mixture generally not critical

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

Aluminium complex

mixture not permitted

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

mixture generally not critical

miscible in individual cases, testing recommended

Barium complex

mixture generally not critical

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

miscible in individual cases, testing recommended

mixture generally not critical

*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.