They can be broadly classified as fluid dynamic bearings, hydrostatic or gas bearings. They are frequently used in high load, high speed or high precision applications where ordinary ball bearings have short life or 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.
Fluid bearings use a thin layer of liquid or gas fluid between the bearing faces, typically sealed around or under the rotating shaft.
There are two principal ways of getting the fluid in to the bearing.
In gas bearings and hydrostatic bearings, the fluid is pumped in through an orifice or through a porous material.
In fluid-dynamic bearings, the bearing rotation sucks the fluid on to the inner surface of the bearing, forming a lubricating wedge under or around the shaft.
Hydrostatic bearings rely on an external pump. The power for that pump is arguably part of overall bearing friction. 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 low speeds or during starts and stops. A 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.
Fluid bearings can be relatively cheap compared to other bearings with a similar load rating. The bearing can be as simple as two smooth surfaces with seals to keep in the working fluid. In contrast, a conventional rolling-element bearing may require many high-precision rollers with complicated shapes. Hydrostatic and gas bearings do have the complication and expense of external pumps.
Most fluid bearings require little or no maintenance, and have almost unlimited life. Conventional rolling-element bearings usually have shorter life and require regular maintenance. Pumped hydrostatic and aerostatic (gas) bearing designs retain low friction down to zero speed and need not suffer start/stop wear, provided the pump does not fail.
Fluid bearings generally have very low friction -- far better than mechanical bearings. One source of friction in a fluid bearing is the viscosity of the fluid. Hydrostatic gas bearings are among the lowest friction bearings. However, lower fluid viscosity also typically means fluid leaks faster from the bearing surfaces, thus requiring increased power for pumps or seals.
Since no rigid mechanical element supports load, it may seem fluid bearings can give only low precision. In practice, fluid bearings have clearances that change less under load (are "stiffer") than mechanical bearings. It might seem that bearing stiffness, as with maximum design load, would be a simple function of average fluid pressure and the bearing surface area. In practice, when bearing surfaces are pressed together, the fluid outflow is greatly constricted. This significantly increases the pressure of the fluid between the bearing faces. As fluid bearing faces are comparatively large areas, even small fluid pressure differences cause large restoring forces, maintaining the gap.
It is also very difficult to make a mechanical bearing which is atomically smooth and round; and mechanical bearings deform in high-speed operation due to centripetal force. In contrast, fluid bearings self-correct for minor imperfections.
Fluid bearings are typically quieter and smoother (more consistent friction) than rolling-element bearings. For example, hard disks manufactured with fluid bearings have noise ratings for bearings/motors on the order of 20-24 dB, which is a little more than the background noise of a quiet room. Drives based on rolling-element bearings are typically at least 4 dB noisier.
Tilting pad bearings are used as radial bearings for supporting and locating shafts in compressors.
Foil bearings are a type of fluid dynamic air bearing that was introduced in high speed turbine applications in the 1960s by Garrett AiResearch. They use a gas as the working fluid, usually air and require no external pressurisation system.
Pressure-oiled journal bearings appear to be plain bearings but are arguably fluid bearings. For example, journal bearings in gasoline (petrol) and diesel engines pump oil at low pressure in to a large-gap area of the bearing. As the bearing rotates, oil is carried in to the working part of the bearing, where it is compressed, with oil viscosity preventing the oil's escape. As a result, the bearing "hydroplanes" on a layer of oil, rather than on metal-on-metal contact as it may appear.
This is an example of a hydrodynamic bearing which does not use a secondary bearing for start/stop. In this application, a large part of the bearing wear occurs during startup and shutdown, though in engine use, substantial wear is also caused by hard combustion contaminants that bridge the oil film.
Unlike contact-roller bearings, air bearings utilize a thin film of pressurized air to provide an exceedingly low friction load-bearing interface between surfaces. The two surfaces don't touch. Being non-contact, air bearings avoid the traditional bearing-related problems of friction, wear, particulates, and lubricant handling, and offer distinct advantages in precision positioning and high-speed applications.
The fluid film of the bearing is air that flows through the bearing itself to the bearing surface. The design of the air bearing is such that, although the air constantly escapes from the bearing gap, the continual flow of pressurized air through the bearing is enough to support the working loads.
Another example of a fluid bearing is ice skating. Ice skates form a hydrodynamic fluid bearing where the skate and ice are separated by a layer of water caused by entropy (formerly thought to be caused by pressure-induced melting; see ice skating for details.)
Kingsbury/Michell dynamic tilting-pad fluid bearings were invented independently and almost simultaneously by both the American tribologist Albert Kingsbury, and a British-born Australian, Anthony George Maldon Michell.
The bearing has "shoes" or "pads" on pivots. When the bearing is in operation, the rotating part of the bearing carries fresh oil in to the pad area. Fluid pressure causes the pad to tilt slightly, building a wedge of pressurised fluid between the shoe and the other bearing surface. The pad tilt adaptively changes with bearing load and speed. Various design details ensure continued replenishment of the oil to avoid overheating and pad damage.
Kingsbury/Michell fluid bearings are used in a wider variety of heavy-duty rotating equipment, including in hydroelectric plants to support turbines and generators weighing hundreds of tons. They are also used in very heavy machinery, such as submarine propeller shafts.
The first tilting pad bearing in service was probably that built under A.G.M. Michell's guidance by George Weymoth (Pty) Ltd, for a centrifugal pump at Cohuna on the Murray River, Victoria, Australia, in 1907, just two years after Michell had published and patented his three-dimensional solution to Reynold's equation. By 1913, the great merits of the tilting-pad bearing had been recognised for marine applications. The first English ship to be fitted out with the bearing was the cross-channel steamboat the Paris, but many naval vessels were similarly equipped during the First World War. The practical results were spectacular - the troublesome thrust block became dramatically smaller and lighter, significantly more efficient, and remarkably free from maintenance troubles. It was estimated that the Royal Navy saved coal to a value of £500,000 in 1918 alone as a result of fitting Michell's tilting-pad bearings.
According to the ASME (see reference link), the first Kingsbury/Michell fluid bearing in the USA was installed in the Holtwood Hydroelectric Power Plant (on the Susquehanna River, near Lancaster, Pennsylvania, USA) in 1912. The 2.25-tonne bearing supports a water turbine and electric generator with a rotating mass of about 165 tonnes and water turbine pressure adding another 40 tonnes. The bearing has been in nearly continuous service since 1912, with no parts replaced. The ASME reported it was still in service as of 2000. As of 2002, the manufacturer estimated the bearings at Holtwood should have a maintenance-free life of about 1,300 years.
Currently, fluid film bearings are extensively modeled with finite element computer codes developed at the University of Virginia in the Rotating Machinery and Controls Industrial Research Program. This includes fixed pad radial bearings, tilting pad radial bearings, and thrust bearings. The analysis includes pressure effects, thermal effects, and deformation effects in the bearings. The codes are widely used by industrial firms.