Hydrodynamic thrust bearings

Bearings are machine components used to guide rotating shafts. There are two main types of bearing: plain bearings and rolling bearings. In the former, the shaft rests on a bearing shell and is separated from it by a lubricant film; in the latter, rolling bodies (balls or rollers) separate the rotating ring from the outer bore. Depending on the direction of the load in relation to the rotating shaft, a distinction is made between radial load-carrying bearings, generally referred to as pillow blocks, and axial load-carrying thrust bearings. We will only discuss fluid bearings, and more specifically hydrodynamic bearings and thrust bearings, in which a thin film of fluid separates the surfaces in relative motion.

The behavior, service life and durability of hydrodynamic bearings depend on a number of parameters, including geometry (bearing size and shape), kinematics and dynamics (rotational speed and applied load), lubricant characteristics (mainly viscosity and, in some cases, density) and the nature of the bearing materials.

Hydrodynamic bearings are therefore determined not only by lubrication theory, but also by a set of conditions related to the mechanism environment. However, we can briefly outline their main operating characteristics. First of all, the minimum thickness of the lubricant film must always be significantly greater than the sum of the surface roughness heights, otherwise rapid wear of the bearing will be due either to abrasion, or even more rapidly to seizure of the surfaces. Of course, this minimum thickness depends on the dynamic aspect of the system, and in particular on the vibrations of the rotating shaft. As the hydrodynamic bearing itself can be a source of vibration, the dynamic aspects need to be examined in detail. They are analyzed very differently for shaft bearings (relatively constant loads) than for reciprocating engine or compressor bearings (modulus and direction loads vary greatly with time).

Moreover, under the effect of these dynamic loads, cavitation phenomena in the bearing lubricant film can lead to the destruction of the bearing by fatigue.

Finally, the power dissipated by shear in the lubricating fluid in the bearing leads to a rise in the temperature of the mechanism. This rise in temperature may be responsible for the melting or creep of the soft, low-melting regule material that generally covers the bearing surface.

The maximum temperature of the bearing can be approximated using a global heat balance of the bearing; it can also be calculated with excellent accuracy by performing a fine analysis of the thermoelastohydrodynamic problem of the entire bearing.

Since most of the heat is dissipated by the fluid, it's important...