Babbitt Bearing Lubrication Types and Systems

Babbitt Bearings

Made from special soft alloys, Babbitt bearings are rotary bearings consisting of a thin layer of Babbitt material, also known as white metal, deposited on a plain bearing housing. While the plain or journal bearing supports the rotating component, the Babbitt alloy layer mainly functions as a wear component that reduces friction between the journal and the bearing. In addition, this layer also helps in cushioning shells from hard rotating shafts. Babbitt materials usually comprise of tin, copper, antimony, or lead, combined in varied proportions, to create an alloy consisting of hard crystals dispersed in a matrix of soft alloys. Some applications of the Babbitt bearing include high-speed rotating equipment such as compressors, chillers, turbines, electric motors, gear drives, locomotives, marine, or pumps. There are several types of Babbitt bearings available, such as self-equalizing, sleeve, thrust, tapered, as well as tilting shoe bearings. Typically, this type of bearing requires a lubricant, such as spindle oil between SAE 10 to 70, supplied by a lubrication system.

Babbitt Bearing Lubrication Systems:

The types of lubrication systems can be categorized into three groups:

  • Bearings that require external application of lubricants
  • Self-lubricating bearings
  • Bearings made of materials that serve as a lubricant

While most of the bearings that require external lubricant application have a plain inner surface, some of them are grooved to help the lubricant enter the space between the bearing and journal. On the other hand, self-lubricating bearings have lubricants that are contained within the bearing walls. Most common sintered metal bearings have porous walls that draw oil by capillary action and the oil is released by pressure or application of heat. The last type of lubrication system consists of a solid one-piece metal bushing with eight-grooved channels, filled with graphite, on the ID.

Lubricant Properties:

The selection of a lubricant is primarily based on its properties. A good lubricant should ensure low friction, adhere to the bearing surface, and reduce wear. In addition, the lubricant must have good thermal stability and durability while removing excessive heat generated by friction. Antifoaming ability, corrosion protection, and a good cleaning effect on the bearing surface are some other desired properties of a good lubricant. Compatibility with seal materials is also desired.

Babbitt Bearing Lubrication Types:

Journal bearings can be lubricated by various methods and common classification of journal bearing lubrication types are:

  • Hydrodynamic or full-film condition
  • Boundary
  • Hydrostatic

Hydrodynamic Lubrication:

Hydrodynamic lubrication, which is also referred to as full film or thick film lubrication, involves the creation of a wedge-shaped zone between the journal bearing and the rotating shaft. The wedge-shaped formation generally occurs at high velocity, and consequently leads to the creation of a lubricant film between the bearing and journal. This film pushes the shaft up and supports its weight, creating a full-film condition that occurs when it carries the shaft load in entirety. The oil film is maintained by the rotation of the journal. As the journal rotates, heat is generated on the bearing surface by the shearing action of the oil film. To prevent thermal damage, cooling is improved by increasing the oil pressure and forcing greater amounts of oil into the bearing. In this type of lubrication, there is no contact between the two moving surfaces. Hydrodynamic lubrication can be performed by using oil or grease.

Boundary Lubrication:

Boundary lubrication is typically used when full-film lubrication is not possible. In boundary or mixed lubrication conditions, the shaft load is partially supported by the bearing, while the oil film between the two carries the remaining load. This type of lubrication occurs when not all the conditions for full-film lubrication are satisfied. The occasional contact between the moving surface results in bearing wear, which can be reduced by the inclusion of additives to the lubricant. In addition, surface treating the bearings also helps reduce wear.

Some of the conditions to be satisfied for both full-film and boundary lubricants include:

  • The condition in which a bearing runs is dependent on operating conditions, load, relative surface speed, clearance within bearing, quality and quantity of lubricant, as well as temperature.
  • If the bearing is not designed to run in the dry or boundary condition, it will wear out resulting in high coefficient of friction.

Hydrostatic Lubrication:

Hydrostatic lubrication is the process of using lubricating oil under pressure to create a separation between the two contact surfaces. This type of lubrication is typically used in applications where heavy loads are to be supported at speeds that are too low to use hydrodynamic lubrication. The bearings are lubricated by an external pump, which always maintains static amount of pressure. While hydrodynamic lubrication can use both grease and oil, this lubrication type can only use oil. An example of hydrostatic bearing is a heavily loaded bearing that is found in automobile engines, which usually feeds oil through a hole in the bearing. Hydrostatic lubrication has low deflections in particular load ranges, making it ideal for high precision machine tools. However, it is comparatively more complex and expensive than hydrodynamic lubrication systems.

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