Study Of TEHL Point Contact Lubricated With Two-phase Lubricants

The elastohydrodynamic lubrication regime (EHL) is found in many machine elements,such as rolling element bearings, gears, cam/tappet, where a combination of hydrodynamiceffect, elastic deformation of the surfaces and an increase of the lubricant’s viscosity withpressure create a continuous lubricant film which is capable of supporting pressures of theorder of tens of thousands of atmospheres. One of the most important features of these filmsis their film thickness, as this determines whether the bounding surfaces are completelyseparated, thus avoiding premature wear and failure of the contact. Consequently for manyyears scientists were interested in improving lubricants properties to obtain a better filmthickness and predicting their behavior under operating conditions couple with findingmethods for measuring the lubricant film thickness in elastohydrodynamic conditions.The lubricants under study were a#30SAE mechanical oil and two-phase lubricantformed from the ordinary#30mechanical oil which is Newtonian with (Polytetraflouretene,PTFE and Molybdenum disulfide, MoS2) solid lubricants. Based oil was use as standard, itdynamic viscosity is0.205Pas at21oC. The dynamic viscosities of the liquid lubricants withsolid lubricants are0.255Pas and0.251Pas for PTFE added to#30SAE oil and MoS2addedto#30SAE oil respectively, both were measured at21oC. The difference between thebehaviors of the base lubricant (#30mechanical oil) and the lubricant with solid lubricant(Polytetraflouretene, PTFE and MoS2) were studied. Much emphasis was given to theinfluence of the PTFE and MoS2solid lubricant into liquid.One of the most versatile and widely used techniques for measuring lubricant filmthickness in EHL contacts is the optical interferometry method which is used in this researchwork.The setups of optical interferometry method used for this research work consisted of asapphire plate disc driven by a variable speed motor. A steel ball of25.4mm diameter wasloaded with forces from1N to2.6N in contacts with the disk at a radius of60mm. The bottomof the ball dips into a test lubricants reservoir cap, ensuring fully flooded EHL contact to beachieved. The contact area is viewed by an optical microscope. Light of600nm wavelength,which was reflected from the contacting area, gave clear interference fringes. The disk’sangular speed (rev/min) was determined by the tachometer and fringe order is recorded foreach dark and bright fringe. Then, the lubricant film thickness was measured by computerprogram imaging for the base lubricant (nm) and the lubricant with solid lubricant (nm). Thetemperatures of the film thickness were measured by noncontact infrared thermometer for each selected loads and speeds. The temperatures were used to evaluate the effect oftemperature on the film thickness.The novelty of this thesis is to use the micropolar fluid theory to analyze the non-Newtonian behavior of the two-phase (Liquid-Solid) lubricant. The hydrodynamic lubricationgoverning equations for micro-polar fluids were presented focusing on the theory of micro-polar fluids. The lubricant compressibility and thermal effect were taken into account. TheElastohydrodynamic (EHL) regime is used as the bases of the study with emphasis on thethermal effect on the lubricants under two-phase model and this was critically investigated.Full numerical analysis for two-phase solid liquid lubricants is obtained. The impact of thePTFE and MoS2solid lubricants in the two-phase model on the film, temperature andpressure distribution of the lubricants were studied.The goal is to study the relationship among the relative speed of two surfaces, thelubricant film viscosity between them, the influence of temperature on the film thickness, theinfluence of load, the effect of PTFE and MoS2properties on the pressure, temperaturedistribution, and film thickness.The procedure was, various dynamic intensity interferogram were captured at varyingspeed and loads with an optical technique at the atmospheric temperature (21oC). Theexperimental film thickness was measured by the help of a computer program and comparedto the lubricant thickness calculated numerically by Hamrock and Dawson’s Equation. TEHLanalyses were done with a novel-FORTRAN program by Prof. Huang Ping to study the filmthickness, pressure distribution and temperature distribution. Finally, the micropolar fluidtheory base on the Reynolds theory was used to also study the effect of the solid lubricants inthe liquid on the film and pressure distribution.The results, in summary shows that, the regime was not purely a classical EHL regimebut instead a mixture of the classical elastohydrodynamic lubrication and hydrodynamiclubrication. Meaning, it is a transition between elastohydrodynamic to hydrodynamic. Theobtained and calculated results that are shown graphically shows that both the base lubricant(SAE#30) and the lubricant with additive were function of load applied to the contact point.A plot of the measured and calculated film thickness and the rolling speeds show muchdifference between the Newtonian and the non-Newtonian fluids. An expected result is thatthe lubricants with additive lose very little percentage of its film thickness and viscosity as therolling speed is increased. The non-Newtonian effects could not have appeared much clearbecause the rolling speed and the loads were never high enough.The further work propose in this research is to include taking measurements at higher rolling speed, loads and increasing the percentage of the PTFE and MoS2additives. More so,rheological study into these formulations should be given a prime focus. This will enable theimpact of all the parameters to be felt and proper analysis and firm conclusion to be arrived.