Fluid Bearing

Fluid bearings are noncontact bearings that use a thin layer of rapidly moving pressurized liquid or gas fluid between the moving bearing faces, typically sealed around or under the rotating shaft. The moving parts do not come into contact, so there is no sliding friction; the load force is supported solely by the pressure of the moving fluid. There are two principal ways of getting the fluid into the bearing:

1. In fluid static, hydrostatic and many gas or air bearings, the fluid is pumped in through an orifice or through a porous material. Such bearings should be equipped with the shaft position control system, which adjusts the fluid pressure and consumption according to the rotation speed and shaft load.
2. 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 required by that pump contributes to system energy loss, just as bearing friction otherwise would. 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 speeds lower than design, or during starts and stops. An 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.

Hydrodynamic (HD) lubrication, also known as fluid-film lubrication has essential elements:

1. A lubricant, which must be a viscous
2. Hydrodynamic flow behavior of fluid between bearing and journal.
3. The surfaces between which the fluid films move must be convergent.

Hydrodynamic (full film) lubrication is obtained when two mating surfaces are completely separated by a cohesive film of lubricant.

The thickness of the film thus exceeds the combined roughness of the surfaces. The coefficient of friction is lower than with boundary-layer lubrication. Hydrodynamic lubrication prevents wear in moving parts, and metal to metal contact is prevented.

Hydrodynamic lubrication requires thin, converging fluid films. These fluids can be liquid or gas, so long as they exhibit viscosity. In computer fan and spinning device, like a hard disk drive, heads are supported by hydrodynamic lubrication in which the fluid film is the atmosphere.

The scale of these films is on the order of micrometers. Their convergence creates pressures normal to the surfaces they contact, forcing them apart.

Three types of bearings include:

1. Self-acting: Film exists due to relative motion. e.g. spiral groove bearings.
2. Squeeze film: Film exists due to relative normal motion.
3. Externally-pressurized: Film exists due to external pressurization.

Conceptually the bearings can be thought of as two major geometric classes: bearing-journal (anti-friction), and plane-slider (friction).

The Reynolds equations can be used to derive the governing principles for the fluids. Note that when gases are used, their derivation is much more involved.

The thin films can be thought to have pressure and viscous forces acting on them. Because there is a difference in velocity there will be a difference in the surface traction vectors. Because of mass conservation we can also assume an increase in pressure, making the body forces different.

• Hydrodynamic lubrication – characteristics:
  1. Fluid film at the point of minimum thickness decreases in thickness as the load increases
  2. Pressure within the fluid mass increases as the film thickness decreases due to load
  3. Pressure within the fluid mass is greatest at some point approaching minimum clearance and lowest at the point of maximum clearance (due to divergence)
  4. Viscosity increases as pressure increases (more resistance to shear)
  5. Film thickness at the point of minimum clearance increases with the use of more viscous fluids
  6. With same load, the pressure increases as the viscosity of fluid increases
  7. With a given load and fluid, the thickness of the film will increase as speed is increased
  8. Fluid friction increases as the viscosity of the lubricant becomes greater

 

• Hydrodynamic condition – Fluid velocity:
  1. Fluid velocity depends on velocity of the journal or rider
  2. Increase in relative velocity tends towards a decrease in eccentricity of journal bearing centers
  3. This is accompanied by greater minimum film thickness

 

• Hydrodynamic condition – load:
  1. Increase in load decreases minimum film thickness
  2. Also increases pressure within the film mass to provide a counteracting force
  3. Pressure acts in all directions, hence it tends to squeeze the oil out of the ends of the bearing
  4. Increase in pressure increases fluid viscosity

Bearing characteristic number: Since viscosity, velocity, and load determine the characteristics of a hydrodynamic condition, a bearing characteristic number was developed based on the effects of these on film thickness.

Therefore,

Viscosity × velocity/unit load = a dimensionless number = C

C is known as the bearing characteristic number.

The value of C, to some extent, gives an indication of whether there will be hydrodynamic lubrication or not