Lubrication and maintenance are important for the reliable operation and long operating life of rolling bearings.

The lubricant should, Figure 1:

• form a lubricant film on the contact surfaces that is sufficiently capable of supporting loads and thus preventing wear and premature fatigue
• dissipate heat in the case of oil lubrication
• give additional sealing of the bearing, in the case of grease lubrication, against the entry of both solid and fluid contaminants
• reduce the running noise
• protect the bearing against corrosion .

It should be determined as early as possible in the design process whether bearings should be lubricated using grease or oil.

The following factors are decisive in determining the type of lubrication and quantity of lubricant:

• the operating conditions
• the type and size of the bearing
• the adjacent construction
• the lubricant feed.

In the case of grease lubrication, the following criteria must be considered:

• very little design work required
• the sealing action
• the reservoir effect
• long operating life with little maintenance work (lifetime lubrication possible in certain circumstances)
• if relubrication is required, it may be necessary to provide collection areas for old grease and feed ducts
• no heat dissipation by the lubricant
• no rinsing out of wear debris and other particles.

In the case of oil lubrication, the following criteria must be considered:

• good lubricant distribution and supply to contact areas
• dissipation of heat possible from the bearing (significant principally at high speeds and/or loads)
• rinsing out of wear debris
• very low friction losses with minimal quantity lubrication
• more work required on feed and sealing.

Under extreme operating conditions (such as very high temperatures, vacuum, aggressive media), it may be possible to use special lubrication methods such as solid lubricants in consultation with the engineering service.

The feed lines and lubrication holes in the housings and shafts, Figure 2 and Figure 3:

• should lead directly to the lubrication hole in the rolling bearing
• should be as short as possible
• must be provided individually for each bearing.

Ensure that the feeds are filled, Figure 2; the feed line should be bled if necessary.

Follow the instructions provided by the lubrication device manufacturer.

Greases can be differentiated in terms of their thickeners and base oils. The base oils of greases are covered by the information in the section link.

Conventional greases have metal soaps as thickeners and a mineral base oil. They also contain additives. These have a specific influence on, for example, the characteristics in relation to wear prevention, corrosion prevention or resistance to ageing. These combinations of additives are not, however, fully effective across every temperature and load range.

Greases exhibit widely varying behaviour in response to environmental influences such as temperature and moisture.

Lubricants must always be checked for their compatibility with:

• other lubricants
• anti-corrosion agents
• thermoplastics, thermosets and elastomers
• light and non-ferrous metals
• coatings
• colouring agents and paints
• and the environment.
  When considering compatibility with the environment, attention must be paid to toxicity, biodegradability and water pollution class.

The characteristics of a grease are dependent on:

• the base oil
• the viscosity of the base oil (this is important for the speed range)
• the thickener (the shear strength is significant for the speed range)
• the additives.

Greases are subdivided into consistency classes
(NLGI classes to DIN 51 818).

For rolling bearings, classes 1, 2, 3 should be used in preference, Figure 5.

Rolling bearing greases K to DIN 51 825 are suitable.

Greases should be selected in accordance with the operating conditions of the bearing:

• temperature
• pressure conditions, see link
• speed, see link
• the presence of water and moisture, see link.

The operating temperature range of the grease must correspond to the range of possible operating temperatures in the rolling bearing.

Grease manufacturers indicate an operating temperature range for their rolling bearing greases K to DIN 51 825.

The upper value is determined in accordance with DIN 51 821 by means of testing on the FAG rolling bearing grease test rig FE 9. At the upper operating temperature, a 50% failure probability rate (F₅₀) of at least 100 hours must be achieved in this test.

The lower value is defined in accordance with DIN 51 825 by means of flow pressure. The flow pressure of a grease is the pressure required to press a stream of grease through a defined nozzle. For greases of type K, the flow pressure at the lower operating temperature must be less than 1 400 mbar.

The use of flow pressure in determining the lower operating temperature only indicates, however, whether the grease can be moved at this temperature. This cannot be used to make any statement about its suitability for use in rolling bearings at low temperatures.

In addition to the lower operating temperature of a grease, therefore, the low temperature frictional torque is also determined in accordance with ASTM D 1478 or IP 186/93. At the lower operating temperature, the starting torque must not exceed 1 000 Nmm and the running torque must not exceed 100 Nmm.

Schaeffler Group Industrial recommends that greases should be used such that the normally occurring bearing temperature falls within the standard operating range, in order to achieve a reliable lubricating action and an acceptable grease operating life, Figure 6.

At low temperatures, greases release very little base oil. This can result in lubricant starvation. Schaeffler Group Industrial therefore recommends that greases are not used below the ­lower continuous limit temperature T_lowerlimit, Figure 6. This is approx. 20 K above the lower operating temperature of the grease as stated by the grease manufacturer.

The upper continuous limit temperature T_upperlimit must not be exceeded if a temperature-induced reduction in grease operating life is to be avoided; see section Grease operating life, link.

At consistently low temperatures (for example in cold store applications), it must be ensured that the grease releases sufficient oil in relation to the bearing type.

The viscosity at operating temperature must be sufficiently high for the formation of a lubricant film capable of supporting loads. At high loads, greases with EP characteristics (“extreme pressure”) and high base oil viscosity should be used (KP grease to DIN 51 825). Such greases should also be used for bearings with substantial sliding or line contact.

Silicone greases should only be used at low loads (P ≦ 3% C).

Greases with solid lubricants should preferably be used for applications with mixed or boundary friction conditions. The solid lubricant particle size must not exceed 5 μm.

Greases should be selected in accordance with the speed parameter n · d_M for grease, see table:

• For rolling bearings running at high speeds or with a low starting torque, greases with a high speed parameter should be used
• For bearings running at low speeds, greases with a low speed parameter should be used.

Under centrifugal accelerations ＞ 500 g, separation (of the thickener and base oil) may occur. In this case, please consult the lubricant manufacturer.

The consistency of polycarbamide greases can be altered by shear stresses to a greater extent than that of metal soap greases.

Water in the grease has a highly detrimental effect on the operating life of the bearing:

• the static behaviour of greases in the presence of water is assessed in accordance with DIN 51 807, see Figure 7
• the anti-corrosion characteristics can be tested according to DIN 51 802 (Emcor test) – information is given in the grease manufacturer’s data sheets.

Many of the rolling bearings supplied by Schaeffler Group Industrial are prefilled with grease. The greases used have proved particularly suitable for the applications in mechanical-dynamic tests, see table.

For users who wish to charge their rolling bearings with grease themselves, there is a range of particularly suitable Arcanol rolling bearing greases.

The greases in the range are graded in terms of their performance capability such that they can be used to cover almost all areas of application.

The grease operating life t_fG applies where this is below the calculated bearing life and the bearings are not lubricated.

A guide value can be determined in approximate terms as follows:

Where a grease operating life ＞ 3 years is required, this should be agreed in consultation with the lubricant manufacturer.

Observe the guidelines on calculating the grease operating life on link.

This applies under the preconditions in the table.

The basic grease operating life t_f is dependent on the bearing-­specific speed parameter k_f · n · d_M and is calculated using Figure 8.

The radial and axial bearing components must be calculated separately; the decisive value is the shorter grease operating life.

If the outer ring rotates, there may be a reduction in the grease operating life.

In the case of yoke and stud type track rollers:

• the angular misalignment must be zero
• the effect of the rotating outer ring on the grease operating life is taken into consideration in the bearing type factor k_f.

The grease operating life cannot be determined using the above method:

• if the grease can leave the bearing arrangement
• • there is excessive evaporation of the base oil
  • in bearing positions without seals
  • in axial bearings with a horizontal axis of rotation

• if air is sucked into the rolling bearing during operation
• • this can cause the grease to oxidise
• in combined rotary and linear motion
• • the grease is distributed over the whole stroke length
• if contamination, water or other fluids enter the bearings
• for spindle bearings
• for drawn cup roller clutches
• for screw drive bearings
• for high precision bearings for combined loads
• for high precision cylindrical roller bearings NN30.

The additional guidelines on lubrication in the product sections must be observed.

If the bearing temperature is higher than the continuous limit temperature T_upperlimit, K_T must be determined from the diagram, Figure 9.

The diagram should not be used if the bearing temperature is higher than the upper operating temperature of the grease used, see table Greases, link. If necessary, a different grease should be selected or contact should be made with the Schaeffler engineering service.

The factor K_P is dependent on the bearing and describes the reduction at higher load (this places greater strain on the grease), see Figure 10 and table.

The factor K_R applies for an angle of oscillation φ ＜ 180°, Figure 11 and Figure 12. Oscillating motion places a greater strain on the grease than does rotating motion.

In order to reduce fretting corrosion, the lubrication interval should be reduced.

If the rolling elements do not undergo complete rotation, please contact the Schaeffler engineering service.

The factor K_U takes account of the influences of moisture, shaking forces, slight vibration (leading to fretting corrosion) and shocks, see table.

It does not take account of extreme environmental influences such as water, aggressive media, contamination, radiation and extreme vibrations such as those occurring in vibratory machines.

In relation to contamination, the influence of contamination on rating life calculation must also be noted, see link.

If increased escape of grease is expected, for example in the case of radial bearings with a vertical axis of rotation, the factor K_S according to the table must be taken into consideration.

Where rolling bearings are relubricated, attention must be paid to the lubrication interval in order to ensure reliable function of the bearings.

The precise lubrication interval should be determined by tests conducted under application conditions. To do this:

• sufficiently long observation periods must be used
• the condition of the grease must be checked at regular intervals.

For reasons of operational reliability, relubrication intervals of ＞ 1 year are not recommended.

Experience shows that a guide value for most applications is:

The grease used for relubrication must be the same as that used in initial greasing.

If different greases are used, their miscibility and compatibility must be checked; see link.

Due to the compact construction of the bearings, relubrication should be carried out using 50% to 80% of the initial greasing quantity (recommendation).

If feed lines filled with air are present, the filling volume of the feed lines should be included in calculation of the relubrication quantity.

Relubrication should always be carried out as follows:

• with the bearing still warm from operation and rotating if safe to do so
• before the bearing comes to rest if safe to do so
• before extended breaks in operation.

Relubrication should continue until a fresh collar of grease appears at the seal gaps. Old grease must be allowed to leave the bearing unhindered.

The initial greasing quantity is between 30% and 100% of the available volume in the bearing, dependent on the bearing type and operating conditions.

A grease reservoir can extend the grease operating life. The grease in the reservoir must be in constant contact with the grease on the raceway. The grease operating life does not increase proportionally with the size of the grease reservoir.

The volume of the grease reservoir should correspond to the volume in the bearing between the inner and outer ring (not taking account of the cage and rolling elements), Figure 13 and Figure 14.

Evaporation of the base oil should be prevented by design measures, for example by sealing shields, Figure 13 and Figure 14.

Mixtures of greases should be avoided if at all possible.

If they are unavoidable, the following preconditions must be fulfilled:

• the base oil must be the same
• the thickener types must match
• the base oil viscosities must be similar
  (they must not differ by more than one ISO-VG  class)
• the consistency must be identical (NLGI class).

Miscibility of greases must always be agreed in consultation with the lubricant manufacturer.

Even when these preconditions are fulfilled, impairment of the performance capability of the mixed grease cannot be ruled out.

If a decision is taken to change to a different grease grade, the grease should be rinsed out if this is possible. Further relubrication should be carried out after a shortened period.

If incompatible greases are mixed, this can leading to considerable structural changes. Substantial softening of the grease mixture may also occur.

Definite statements on miscibility can only be obtained by means of suitable tests.

In general, the greases used can be stored for 3 years.

The preconditions are:

• a closed room or store
• temperatures between 0 °C and +40 °C
• relative humidity no more than 65%
• no influence of chemical agents
  (vapours, gases, fluids)
• the rolling bearings are sealed.

Lubricants age due to environmental influences. The information provided by lubricant manufacturers must always be observed.

After long periods of storage, the start-up frictional torque of greased bearings can be temporarily higher than normal. The lubricity of the grease may also have deteriorated.

Since the lubrication characteristics of greases vary and different raw materials may be used for greases of the same name, we cannot offer any guarantees either for the lubricants used by customers for relubrication or for their characteristics.

For the lubrication of rolling bearings, mineral oils and synthetic oils are essentially suitable.

Oils with a mineral oil base are used most frequently. They must fulfil at least the requirements according to DIN 51 517 or DIN 51 524.

Special oils, often synthetic oils, are used under extreme operating conditions or where there are special requirements relating to oil resistance.

In these cases, please consult the lubricant manufacturer or the Schaeffler engineering service.

The information provided by the lubricant manufacturer should be taken as authoritative.

The achievable bearing life and security against wear are higher with better separation of the contact surfaces by a lubricant film, Figure 15 and section link.

The guide value for ν₁ is dependent on the mean bearing diameter d_M and the speed n. It takes account of the EHD theory of lubricant film formation and practical experience.

Depending on the operating speed, the oil at operating temperature must have at least the reference viscosity ν₁, Figure 16.

The reference viscosity ν₁ is determined as follows:

• Assign ν₁ to a nominal viscosity with ISO-VG  between 10 and 1 500 (mid-point viscosity to DIN 51 519)
• Round intermediate values to the nearest ISO-VG  (due to the steps between groups).

This method cannot be used for synthetic oils, since these have different V/P (viscosity/pressure) and V/T (viscosity/temperature) characteristics.

In these cases, please consult the Schaeffler engineering service.

As the temperature increases, the viscosity of the oil decreases. This temperature-dependent change in the viscosity is described using the viscosity index VI. For mineral oils, the VI index should be at least 95.

When selecting the viscosity, the lower operating temperature must be taken into consideration, since the increasing viscosity will reduce the flowability of the lubricant. As a result, the level of power losses may increase.

Very long life can be achieved with a viscosity ratio κ = ν/ν₁ = 3 to 4 (ν = operating viscosity). Highly viscous oils do not, however, bring only advantages. In addition to the power losses arising from lubricant friction, there may be problems with the feed and removal of oil at low or even at normal temperatures.

The oil selected must be sufficiently viscous that it gives the highest possible fatigue life. It must also be ensured that the bearings are always supplied with adequate quantities of oil.

If the bearings are subjected to high loads or if the operating viscosity ν is less than the reference viscosity ν₁, oils with anti-wear additives (type P to DIN 51 502) should be used.

Such oils are also necessary for rolling bearings with a substantial proportion of sliding contact (for example bearings with line contact).

These additives form boundary layers to reduce the harmful effects of metallic contact occurring at various areas (wear).

The suitability of these additives varies and is normally heavily dependent on temperature. Their effectiveness can only be assessed by means of testing in the rolling bearing (for example on our test rig FE8 to DIN 51 819).

Silicone oils should only be used for low loads (P ≦ 0,03 · C).

Before an oil is used, its behaviour must be checked in relation to plastics, seal materials (elastomers) and light and non-ferrous metals.

This must always be checked under dynamic loading and at operating temperature.

Synthetic oils must always be checked for their compatibility. The lubricant manufacturer must be consulted on this at the same time.

The mixing of different oils should be avoided wherever possible. In particular, the presence of different additive packages may lead to undesirable interactions.

In general, oils with a mineral oil base and the same classification are miscible, for example type HLP with type HLP. The viscosities should vary by no more than one ISO-VG  class.

Synthetic oils must always be checked for their compatibility. The lubricant manufacturer must be consulted on this at the same time.

Miscibility must be checked in advance for each individual case.

The cleanliness of the oil influences the rating life of bearings, see also section link.

The Schaeffler Group therefore recommends that an oil filter should be provided; attention must be paid to the filtration rate. The filter mesh should be ＜ 25 μm.

The essential lubrication methods are:

• drip feed oil lubrication
• pneumatic oil lubrication
  (in order to protect the environment, this should be used as a substitute for oil mist lubrication)
• oil bath lubrication
  (immersion or sump lubrication)
• recirculating oil lubrication.

This is suitable for bearings running at high speeds, Figure 17.

The oil quantity required is dependent on the type and size of bearing, the operating speed and the load.

The guide value is between 3 drops/min and 50 drops/min for each rolling element raceway (one drop weighs approx. 0,025 g).

Excess oil must be allowed to flow out of the bearing arrangement.

This method is particularly suitable for radial bearings running at high speeds and under low loads (n · d_M = 800 000 to 3 000 000 min^–1 · mm), Figure 18.

Clean compressed air free from moisture feeds oil to the bearing. This generates an excess pressure. This prevents contaminants from entering the bearing.

With a pneumatic oil lubrication system designed for minimal quantity lubrication, low frictional torque and a low operating temperature can be achieved.

Parameters for design of the lubrication system should be requested from the equipment manufacturers.

Pneumatic oil lubrication of axial bearings should be avoided if possible.

The oil quantity required for adequate supply is dependent on the bearing type.

Pneumatic oil lubrication has little cooling effect.

Follow the instructions provided by the manufacturers of the lubrication systems.

The oil level should reach the centre line of the lowest rolling element, Figure 19. If the oil level is higher than this, the bearing temperature may increase at high circumferential speeds and losses due to splashing may occur. Furthermore, foaming of the oil may occur.

In general, it is suitable for speeds up to n · d_M = 300 000 min^–1 · mm. At n · d_M ＜  150 000 min^–1 · mm, the bearing may be completely immersed.

In bearings with an asymmetrical cross-section, oil return ducts must be provided due to the pumping effect so that recirculation can be achieved.

In axial bearings, the oil level must cover the inside diameter of the axial cage.

The oil quantity in the housing must be adequately proportioned, otherwise very short oil change intervals will be necessary.

In recirculating oil lubrication, the oil is subjected to additional cooling, Figure 20. The oil can therefore dissipate heat from the bearing. The quantity of oil required for heat dissipation is dependent on the cooling conditions, see section link.

The oil quantities are matched to the operating conditions, Figure 21. The diagram indicates oil quantities that can be fed through the bearing without pressure with a side feed arrangement and banking up to the lower edge of the shaft.

For bearings with an asymmetrical cross-section (such as angular contact ball bearings, tapered roller bearings, axial spherical roller bearings), larger throughput quantities are permissible due to the pumping effect than for bearings with a symmetrical cross-section. Large quantities can used to dissipate wear debris or heat.

The lubrication holes in the housing and shaft must align with those in the rolling bearings. Adequate cross-sections must be provided for annular slots, pockets, etc.

The oil must be able to flow out without pressure (this prevents oil build-up and additional heating of the oil).

In axial bearings, the oil must always be fed from the inside to the outside.

The cross-section of the oil outlet hole should be significantly larger than that of the inlet, Figure 22.

The cross-section A_rab is dependent on the oil quantity and the viscosity:

In bearings running at high speeds, the oil is injected into the gap between the cage and bearing ring, Figure 23. Injection lubrication using large recirculation quantities is associated with high power loss.

Heating of the bearings can only be held within limits with a considerable amount of effort. The appropriate upper limit for the speed parameter n · d_M = 1 000 000 min^–1 · mm for recirculating lubrication with suitable bearings (for example spindle bearings) can be exceeded to a considerable degree when using injection lubrication.

Oil can dissipate frictional heat from the bearing. It is possible to calculate the heat flow _L that is dissipated by the lubricant and the necessary lubricant volume flow _L.

If these values cannot be calculated, the guide values according to Figure 24 apply for a temperature difference of Δϑ_L = 10 K.

At temperatures in the bearing of less than +50 °C and with only slight contamination, an oil change once per year is generally sufficient.

Guide values for oil change intervals are given in Figure 25.

The precise oil change intervals should be agreed in consultation with the oil manufacturer.

Under severe conditions, the oil should be changed more frequently. This applies, for example, in the case of higher temperatures and low oil quantities with a high circulation index.

The circulation index indicates how often the entire oil volume available is recirculated and pumped per hour: