There are four essential elements in hydrodynamic lubrication, the first two are obvious, a liquid (hydro-) and relative motion (-dynamic). The other two are the viscous properties of the liquid, and the geometry of the surfaces between which the convergent wedge of fluid is produced. Hydrodynamic lubrication is an excellent method of lubrication since it is possible to achieve coefficients of friction as low as 0.001 (m=0.001), and there is no wear between the moving parts. Special attention must be paid to the heating of the lubricant by the frictional force since viscosity is temperature dependent. One method of accomplishing this is to cycle the lubricant through a cooling reservoir in order to maintain the desired viscosity of the fluid. Another way of handling the heat dissipation is to use commercially available additives to decrease the viscosity’s temperature dependence.
Journal bearings, Herringbone groove bearings, Lubrication in Textured surfaces
Unique research activities are going on in our laboratory on the topic. Extensive numerical modelling of the micro-textured lubrication problem has been carried out in last five years. These numerical simulations have addressed several fundamental issues like cavitation, fluid inertia effects, velocity slip at the interface etc. The role of shapes and sizes of the extruded micro-textures have also been investigated, numerically. The challenge now is to experimentally validate and optimize the textures sizes, shapes, and orientations for better performances. Moreover, some ideas are being developed towards using these micro-textured pads as sealing devices. A method has been devised for generating textures on the bearing surface and a customized experimental set up has been developed in house.
Now–a-days textured surfaces play a vital role in improving the tribological performance between two mating pairs in hydrodynamic lubrication. Textured surface maintains lubrication film due to the development of hydrodynamic pressure which reduces friction and enhances the load bearing capacity. The micro dimples can act as fluid reservoirs which help to retain a thin film of lubricant between two mating pairs.
The thin film lubrication in bearings and seals having smooth surfaces is generally obtained from the solution of Reynolds equation. However, its use in textured surfaces is not convincible, especially while operating under moderate and/or high Reynolds numbers. The most accurate outcome is expected with the solution of full form Navier-Stokes equation, which is highly complicated and sometimes a commercial software package is needed. Therefore, it will be more meaningful to modify the conventional Reynolds equation by dropping some of the assumptions of the thin film lubrication. Surface textures are classified into two types: Positive and Negative textures.
Positive textures are helpful for preferential end flow while negative textures always maintain retention of the fluid film between the two mating surfaces. A number of sophisticated techniques such as laser surface texturing, LIGA process, photolithography process, reactive ion etching, abrasive jet machining, vibrorolling, milling and chemical etching method, etc are developed to fabricate textures. But the chemical etching method is cost effective and produces burr-free parts in intricate and complex designs and shapes with close tolerances. Fabrication of surface texture in this method is a two-stage process whereby photographic emulsion method is used to pattern the microstructure, and conventional chemical etching method is employed to fabricate the patterns on the surface of the thrust pad. The height of the textures obtained in chemical etching process depends on time for which the disc is kept inside chemical solution.
