Fluid bearing

From Academic Kids

Fluid bearings, also called fluid dynamic bearings or hydrostatic or gas bearings, are bearings which are frequently used in high speed or high precision applications where ordinary ball bearings may cause noise or vibration. Noise and vibration from ball bearings occur either from the difficulty of making perfectly round bearings to a tight tolerance, or in high speed applications where the speed of the bearing will cause deformation in the ball bearings, and the resulting possibility of noise and vibration; fluid bearings produce little if any noise. Fluid bearings are also used where loads are very high and building regular bearings is difficult or expensive.

Fluid Bearings use a thin layer of liquid or gas fluid between the bearing faces, typically sealed around or under the rotating shaft.

This layer of fluid is either, in gas bearings or hydrostatic bearings, pumped in through an orifice or porous material, or else with hydrodynamic bearings, is swept in by the rotation of the shaft; forming a lubricating wedge under or around the shaft. Foil bearings are a type of hydrodynamic bearing that was introduced in turbine applications in the 1960s.

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 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 more or less unlimited life. Conventional mechanical ball bearings have a fixed life and require regular maintenance. Pumped hydrostatic and aerostatic (gas) bearing designs retain low friction right down to zero speed and need not suffer wear, provided the pump does not fail.

Hard disks manufactured with fluid bearings have noise ratings for bearings/motors on the order of 20-24 dB, where the threshold for human hearing is around 25 dB. Drives based on rolling-element bearings are typically at least 4 dB noisier.


Pumped bearings

Gas bearings have very low friction due to the low viscosity of gas. However, some designs may have friction from seals. Seals which give less friction leak more, but require more pump power to maintain bearing pressure.

Kingsbury/Michelle Tilting-Pad Fluid Bearings

Kingsbury/Michelle dynamic tilting-pad fluid bearings were invented independently and almost simultaneously by both the American tribologist Albert Kingsbury, and an Australian, A. G. Michelle.

The bearing has "shoes" or "pads" with pivots. When the bearing is in operation, its friction with the fluid causes it to tilt very slightly, building a wedge of pressurised fluid between the shoe and the other bearing surface. A critical, poorly understood feature is that the shoe's surface has to be babbitted and "scraped" or roughened slightly, or the bearing overheats and fails within a short time.

Kingsbury/Michelle fluid bearings are used in hydroelectric plants to support turbines and generators weighing hundreds of tons. They are also used in heavy machinery of all types, such as submarine propellor shafts.

According to the ASME (see reference link), the first Kingsbury/Michelle fluid bearing 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.

In addition to their long life, Kingsbury/Michelle 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 bearings are among the lowest friction bearings.

Since no rigid mechanical element supports load, it might seem that fluid bearings will 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 bearings faces are comparatively large areas, even small fluid pressure differences cause large restoring forces, maintaining the gap.

A disadvantage of dynamic fluid bearings is that although they have infinite life when running continuously, they perform poorly and may wear at low speed or when the machinery is started and stopped.

See also



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