Fluid Bearing - Characteristics and Principles of Operation

Characteristics and Principles of Operation

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 many 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 friction from seals.

When a roller or ball is heavily loaded, 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 constricted. This significantly increases the pressure of the fluid between the bearing faces. As fluid bearing faces can be comparatively larger than rolling surfaces, even small fluid pressure differences cause large restoring forces, maintaining the gap.

However, in lightly loaded bearings, such as disk drives, the typical ball bearing stiffnesses are ~10^7 MN/m. Comparable fluid bearings have stiffness of ~10^6 MN/m. Because of this, some fluid bearings, particularly hydrostatic bearings, are deliberately designed to pre-load the bearing to increase the stiffness.

Fluid bearings often inherently add significant damping. This helps attenuate resonances at the gyroscopic frequencies of journal bearings (sometimes called conical or rocking modes).

It is 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.

Fluid bearings can be made with a lower NRRO (non repeatable run out) than a ball or rolling element bearing. This can be critical in modern hard disk drive and ultra precision spindles.

Tilting pad bearings are used as radial bearings for supporting and locating shafts in compressors.