Bearing Oil lubrication is mandatory when greasing limits are exceeded.

Several different oil lubrication systems are used, the most common for machine tool being:

  • oil injection
  • oil mist
  • air-oil

Oil Injection

Oil injection is preferred for bearings having to operate at very high speed and high load, that is in conditions that do not allow “oil mist” lubrication because of the need to cool the bearings.

Oil is injected into the bearings through nozzles placed so as to lubricate the ball to race contacts with minimum oil churning. Drainage channels must be provided to prevent oil from stagnating and churning and hence generating heat.

Besides ensuring proper lubrication, the amount of oil crossing the bearings also removes the heat generated by bearing operation and maintains overall temperature at acceptable levels.

The assembly should also include oil filtering, a heat exchanger to dissipate the heat removed from the bearings and an adequate oil reserve. A suitable size reservoir facilitates heat dispersion and the settling out of debris and avoids early lubricant ageing.

This type of lubrication obviously requires accurate and proper analysis. Precise rules for calculating oil flow take account of the bearing type to be lubricated (series and pitch diameter) and assembly composition. Oil viscosity is an injection system is according to ISO standards VG10 or VG15.

Viscosity Grade ISO Kinematic Viscosity at 40¡ deg C Kinematic viscosity limits at 40¡ deg C
Minimum Maximum
VG 2 2.2 1.98 2.42
VG 3 3.2 2.88 3.52
VG 5 4.6 4.14 5.06
VG 7 6.8 6.12 7.48
VG 10 10.0 9.00 11.00
VG 15 15.0 13.50 16.40
VG 22 22.0 19.80 24.20
VG 32 32.0 28.80 35.20
VG 46 46.0 41.40 50.60
VG 68 68.0 61.20 74.80
VG 100 100.0 90.00 110.00
VG 150 150.0 135.00 165.00


Required oil quantity for oil jet lubrication

Bore (mm) 50 50 120 120 280
Quantity of oil (l/h) 2…24 15,,,120 60..300


Bearing Oil Mist

Oil mist lubrication is widely used, especially in high speed applications, as it offers the following advantages:

  • satisfactory efficiency even with complex bearing arrangements
  • low temperature, reduced power absorption
  • low cost assembly
  • simple construction (channels, spacers, etc.)
  • good protection against outside contamination (pressurized environment)

Oil mist lubrication systems must be designed to precise standards taking account of the features and speed of the bearings to be lubricated. (Oil mist equipment manufacturers are generally in a position to supply the required specific data).

Oil recommended for use in oil mist lubrication systems is ISO VG32.

Air – Oil

An important peculiarity of this system is the use of a high viscosity synthetic oil (generally ISO VG68) which, even when applied in small amounts, ensures the presence of a resistant film between rolling parts and bearing races.

This means simultaneous reduced ball rolling resistance and excellent bearing behavior even under high stress. The system is moderately polluting for the following reasons:

  • low oil consumption
  • reduced misting effect

The conveyor element air and the oil reach the bearing via side nozzles (Figure 5) or through holes in the bearing outer ring without being mixes during their passage.


Air – oil lubrication has led to outstanding results in high frequency, high power electrospindles.

Series VEB and VEX high speed bearings in the air-oil lubricated version, with ceramic material balls and outer ring with radial lubrication holes (H1/NS execution) can reach high speeds of over 2,500,000 ndm.
Oil quantity (Q) required can be calculated, roughly, with the following formula:

Q = 1.2 x dm mm3/h (for each bearing)

*where dm is the bearing pitch diameter in millimeters

Air-oil flow to the bearings must be homogeneous and without losses along its path. Individual air-oil supply to each bearing is therefore strongly recommended, even if a more elaborate supply system may be required in certain instances.

Minimum Oil

Any stems capable of supplying a bearing with the amount of oil required for proper operation can be considered as minimum.

This type of lubrication can be used for high speed bearings too, by direct injection of small amounts of oil directly into the bearings, through the outside ring (H1 version bearings).

Oil type and dosage are monitored by a central control and a circuit ensuring continuous pressure and flow.

Nozzle Position

Maximum efficiency is achieved for all oil lubrication systems when the lubrication flow reaches the bearing contact areas with minimum turbulence.
Nozzle positioning, as indicated in the table below, is therefore strongly recommended.
“P” and “S” values of nozzle position

Lubricant Contamination Effect on Bearing Life and Behavior

Cleanliness affects bearing life and efficiency. It is necessary to achieve an application where bearings operated in free of external contamination.
It is important, therefore, to keep bearings clean during greasing and to provide good sealing during operation.
Addition precautions must be taken for oil lubricated bearing to monitor lubrication contamination levels. In the case of oil re-circulation systems in particular, this depends on the rate of contamination, the filtration standard and the size of the filter.
Contamination by the ingress of coolants, cutting oils, etc., alter lubricant properties thereby effecting bearing life. This should be eliminated at the design stage by providing adequate sealing.
Contaminating particle classifications are available, which specify limit sizes and amounts per unit volume of 100 cm3.
With reference to ISO standard 4406 (Figure 6) for high precision applications as, for instance, high performance electrospindles, contamination should be restricted to level 10/7, for new spindles and to level 13/10, after long use. In any case, the maximum particle size should not exceed 5 micrometers. Figure 6 shows the area with acceptable pollution levels.