Thomas J. Kasun

Elastohydrodynamic Film Thickness and Tractions for Oil-in-Water Emulsions

Emulsions, consisting of a small volume of oil dispersed in water in the form of small particles, are popular lubricants for metal rolling and some machine design applications. A number of mechanisms have been suggested for the lubricating behavior of emulsions, among which plate-out, starvation, and dynamic concentration are of particular interest here. At low speeds, the emulsion provides essentially the same lubricating ability as neat oil for a point contact, consistent with plate-out. At some critical speed, the emulsion behavior departs from the neat oil, associated with starvation of the inlet zone. At a second critical speed, dynamic concentration becomes the important mechanism. This article measures the film thickness and traction coefficients of oil-in-water emulsions in the different regimes of behavior and compares the results to existing theoretical understanding. The effect of droplet size is isolated as a causative element in fluid film formation.

Numerical Simulation of an Oil Particle in Emulsion Lubrication

The analogy of lateral particle migration in shear flows has been used to investigate the entrainment of oil particles in the inlet region where a Poiseuille flow is assumed. Previous researchers have studied the particle segregation positions and the particle size effect through two-dimensional simulations. In this paper, both two-dimensional and three-dimensional simulations were conducted. The two-dimensional analysis showed that there are two stable off-center segregation positions and one neutrally stable center segregation position for the small particles, while only one stable center segregation position was found for the large particles. The results are similar in three-dimensional simulations except that the center segregation position is more stable than in the two-dimensional case. It should be noted that all the segregation positions are within the backflow region, which means all the oil particles will be rejected from the contact region if the interparticle collisions are ignored. This is supported by experimental observations.

Lubrication of High Rolling Speed Ceramic Contact with Two Percent Slip at 815°C (1500°F)

A selection of liquid lubricants was vaporized and deposited on a hot, 705°C (1300°F), ceramic surface. Friction was then evaluated in low speed, 7 MPa (1000 psi), sliding contact. The ceramic materials included aluminum oxide, silicon aluminum oxynitride (SiAlON), silicon nitride, and silicon carbide. Each specimen was mated with an equally hot specimen of the same material. Deposits reduced friction significantly compared to the unlubricated contact. Two superior lubricants were then evaluated in continuous lubrication of silicon nitride using a high speed ball-on-disk contact. Rolling speed was 2.5 m/sec at a Hertzian stress of 2 GPa (300 ksi). Operating temperatures were varied between 621°C (1150°F) and 815°C (1500°F). With the slip level maintained at 2%, the traction coefficient recorded at 815°C was 0.05, comparable to current metallic bearings in aerospace applications. Wear volume was reduced by an order of magnitude compared to the unlubricated case. Temporarily stopping the lubricant flow resulted in a steady rise of the traction coefficient, while resuming the lubricant flow reduced the traction to the original level.