Types of Bearing Lubricants

Bearing Lubricants

The correct choice of lubricant and method of lubrication is just as important for the proper operation of the bearing as is the selection of the bearing and design of the associated components. Maximum speeds and maximum service life of the bearing are based on the formation of a friction-reducing lubricant film between rolling and sliding bearing parts. This lubrication film between the bearing balls/rollers and the bearing races is usually no more than a few microns thick.

Other objectives of lubrication include:

  • Reduction of structure noise and vibration
  • Thermal dissipation
  • Sealing effects
  • Corrosion protection
Lubrication Grease Oil
Should be used if…
  • Maintenance-free operation over a long period of time is desired
  • Max speed of the bearing is less than
    the speed factor (n *dm) of the grease
  • The heat generated is almost uniformly dissipated
    tby the environment – application specifics – of the application
  • High speeds do not permit the use of greases
  • The lubricant is required to simultaneously
    cool the bearing(s)

Lubrication Type – Selection

The criteria table below demonstrates the performance capability of grease versus oil lubrication.

Lubricant Grease Oil
General Speed suitability in n * dm +++
~2 MM
+++++
~3 - 4 MM
Service life +++ +++++
Design / system costs +++++ +
Operating costs +++++ +
Heat dissipation ++ ++++
Running in procedure ++ +++
Pollution +++++ +

*{+++++: very good; +: poor}

Grease Lubrication

The selection of a suitable grease for high precision bearings is based on the specific bearing applications maximum operating speed and temperature. The bearing industry has created a Reference Speed Factor to more accurately choose lubrication types. The reference speed factor, n * dm, accommodates the operating rotational speed of the center line of the bearing balls diameter, and indicates the maximum speed capacity of the lubricant being specified. This Reference Speed Factor helps compare applications for what the grease is seeing versus a general shaft or housing RPM.

Reference speed factor : n * dm
  • n * dm bearing = n * [(D+d) / 2]
  • n = bearing operating speed [1/min]
  • d = bearing bore diameter [mm]
  • D = bearing outer diameter [mm]
  • *Many bearing companies provide an exact dm in their catalogs

Consideration of the reference speed factor helps ensure you select a lubricant that will not prematurely degrade the bearing performance.

Oil Lubrication

In comparison to grease-lubricated bearings, oil lubrication enables long-term reliable operation at maximum speeds. There are three main oil lubrication methods:

  • Oil-mist lubrication
  • Oil-Air / Air-Oil lubrication (minimized lubrication consumption)
  • Oil injection lubrication (greater cooling effects)
Oil-Mist Oil-Air Oil-Jet
  • Oil mist is produced in an atomizer & conveyed to the bearing(s) by an air current
  • Air current also serves to cool bearings
  • Slightly higher pressure prevents contamination from penetration
  • Oil is conveyed to the bearing(s) in droplets by compressed air
  • Droplet size & intervals between droplets are controlled
  • Considerable amounts of oil are carried through the bearing by injection
  • Frictional heat generated in the bearing(s) is dissipated
  • The cooling of the oil is achieved with an oil-to-air heat exchanger or other methods

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