Also called Fluid Dynamic Bearings, Fluid Bearings are used in high speed or high precision applications where ordinary ball bearings may cause noise, vibration or accoustics. 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 are also used where loads are very high and building regular bearingss is difficult or expensive.
Fluid Bearings use a thin layer of liquid or gas fluid sealed around the rotating shaft. The speed of rotation of the shaft causes hydrostatic effects that tend to keep the shaft centered in the casing and smooth vibration and noise (accoustics).
Fluid bearings can have long service life. According to the ASME, a 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 Holtwood bearings should have a maintenance-free life of about 1,300 years.
Fluid bearings can also have low friction. One source of friction in a fluid bearing is "churning" or continual motion of the fluid. Another is the tendency of the fluid to leak past the seals; seals which do a better job tend to have more friction, while seals which leak require energy to pump in fluid in to maintain bearing pressure.
Fluid bearings can be relatively cheap compared to other bearingss with a similar load rating. The bearing can be as simple as two smooth surfaces plus seals to keep in the working fluid. Another design uses ramped surfaces to self-pump the fluid. In contrast, a conventional bearing may require many high-precision rollers with complicated shapes.
A disadvantage of fluid bearings is they depend on hydrodynamic effects to carry load, and so they perform poorly at low speed or when the machinery is stopped.
Hard disk drives 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 much noisier.
Fluid bearings are also used in hydroelectric plants to support turbines and generators weighing hundreds of tons. Some use high-pressure water for both turbine power and for the bearing fluid. Where bearing fluid pressure is not provided by bearing rotation, the bearing may support heavy loads when stopped. However, for service and for safety in the event of a loss of pressure, a mechanical bearing is also used. The "safety" bearing may have high friction and a short service life (hours instead of years) and may thus be relatively cheap.
