Fluid bearing

Fluid bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fluid bearings to have lower friction, wear and vibration than many other types of bearings.

They can be broadly classified into two types: fluid dynamic bearings (also known as hydrodynamic bearings) and hydrostatic bearings. Hydrostatic bearings are externally pressurized fluid bearings, where the fluid is usually oil, water or air, and the pressurization is done by a pump. Hydrodynamic bearings rely on the high speed of the (the part of the shaft resting on the fluid) to pressurize the fluid in a wedge between the faces. Fluid bearings are frequently used in high load, high speed or high precision applications where ordinary ball bearings would have short life or cause high noise and vibration. They are also used increasingly to reduce cost. For example, hard disk drive motor fluid bearings are both quieter and cheaper than the ball bearings they replace.

The fluid bearing may have been invented by French civil engineer L. D. Girard, who in 1852 proposed a system of railway propulsion incorporating water-fed hydraulic bearings.

Fluid bearings are noncontact bearings that use a thin layer of rapidly moving pressurized liquid or gas fluid between the moving bearing faces, typically sealed around or under the rotating shaft. The moving parts do not come into contact, so there is no sliding friction; the load force is supported solely by the pressure of the moving fluid. There are two principal ways of getting the fluid into the bearing:

Hydrostatic bearings rely on an external pump. The power required by that pump contributes to system energy loss, just as bearing friction otherwise would. Better seals can reduce leak rates and pumping power, but may increase friction.

Hydrodynamic bearings rely on bearing motion to suck fluid into the bearing, and may have high friction and short life at speeds lower than design, or during starts and stops. An external pump or secondary bearing may be used for startup and shutdown to prevent damage to the hydrodynamic bearing. A secondary bearing may have high friction and short operating life, but good overall service life if bearing starts and stops are infrequent.

Hydrodynamic (HD) lubrication, also known as fluid film lubrication has essential elements:

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