A fluid bearing is a type of bearing in which the load is supported by a thin layer of fast-moving pressurized fluid or gas between the bearing surfaces. Because there is no contact between the moving parts, there is no sliding friction, giving fluid bearings lower friction, wear and vibration than many other types of bearings. As a result, some fluid bearings may have near-zero wear if operated correctly.
They can be broadly classified into two categories. Hydrodynamic bearings and Hydrostatic bearings.
Fluid bearings can be relatively inexpensive compared to other bearings with similar load ratings. Bearings can be as simple as two smooth surfaces with seals to keep in the working fluid. In contrast, conventional rolling bearings may require many complexly shaped, high-precision rollers. Hydrostatic bearings and many gas bearings do have the complexity and expense of external pumps.
Most fluid bearings require little maintenance and have a nearly unlimited life. Conventional rolling bearings usually have a short life and require periodic maintenance. Pumped hydrostatic and aerostatic (gas) bearings are designed to maintain low friction to zero speed and do not suffer start/stop wear, provided the pump does not fail.
Fluid bearings typically have very low friction – much better than mechanical bearings. One source of friction in fluid bearings is the viscosity of the fluid, causing dynamic friction to increase with speed, but static friction is usually negligible. Even at very high speeds, hydrostatic gas bearings are among the lowest friction bearings. However, lower fluid viscosity also usually means that fluid leaks from the bearing surface more quickly, thus requiring increased pump power or seal friction.
♦ Bearings must maintain pressure to prevent wear, and hydrostatic bearings may not move at all when depressurized.
♦ Overall power consumption is usually higher compared to ball bearings.
♦ Power consumption and stiffness or damping vary greatly with temperature, which complicates the design and operation of fluid bearings over a wide temperature range.
♦ Many types of fluid bearings can seize catastrophically in shock situations or with unexpected loss of supply pressure. Ball bearings gradually deteriorate and provide acoustic symptoms.
♦ As with cage frequency vibration in ball bearings, half-frequency eddy motion is a bearing instability that produces eccentric feed, resulting in poor performance and shortened life.
♦ Fluid leaks; keeping fluids in bearings can be a challenge for fluid types and in some cases may require vacuum recovery and filtration.
♦ In environments where oil leaks can be destructive or uneconomical to maintain, oil bearings are impractical.
♦ Fluid bearing “shingles” must often be used in pairs or triples to avoid tilting the bearing and losing fluid from one side.