Question

Please Using Microsoft Word Program

Report In General About;

HydroDynamic Journal Bearing

Report Contents:

1- Abstract 2- Introduction

3- Body of Report 4- Conclusions

5- References

Answer 1

Definition

A journal bearing obtains its capacity to support an applied load through the resulting eccentric position of
the shaft when subjected to rotation. A journal, under load, shifts its location to an eccentric position inside the bearing (sleeve) that houses the shaft. The pressure field created through eccentric rotation of the journal balances the change in momentum in the convergent-divergent sections of the fluid film hosted in the clearance between the journal and the sleeve. This pressure, when integrated in the axial and circumferential directions, yields the load-carrying capacity that the journal is capable of. In a journal bearing the only source of pressure generation in the film is the eccentric rotation of the journal, hence the synonym self-acting bearing. To enhance the action of the shaft rotation in pressure generation the surface of the sleeve is sometimes shaped as a Rayleigh step, a wavy surface, or other momentum-enhancing geometries.

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 lubrication

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

1. A lubricant, which must be a viscous fluid.
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.

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.

• Increase in velocity increases min. film thickness
• Increase in viscosity increases min. film thickness
• Increase in load decreases min. film thickness

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

C is known as the bearing characteristic number.

Characteristics 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 leading to dynamic friction that increases with speed, but static friction is typically negligible. Hydrostatic gas bearings are among the lowest friction bearings even at very high speeds. 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.[citation needed] 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 and slight deformations.

Fluid bearings are typically quieter and smoother (more consistent friction) than rolling-element bearings. For example, hard disk drives 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.

References

• R.O. Boswell, The Theory of Film Lubrication (Longmans Green, London,
  1928)
• V. Castelli, J. Pirvics, Review on numerical methods in gas bearing film
  analysis. J. Lubr. Technol. Trans. ASME 90, 777–792 (1968)
• D. Deng, M.J. Braun, Taylor vortices induced instability (transition flow)
  versus “turbulence” in concentric cylinders separated by microscale
  clearances. Tribol. Trans. 50(4), 573–584 (2007)
• D. Deng, M.J. Braun, A new model for transition flow of thin films in long
  journal bearings. Tribol. Trans. 51(1), 1–11 (2008)