Experimental Support for the Dynamic Concentration Theory of Forming an Oil Reservoir at the Inlet of the Roll Bite by Measuring the Onset Speed of Starvation as a Function of Oil Concentration and Droplet Size

Abstract

The dynamic concentration theory describes a mechanism of how an oil reservoir forms at the inlet of the roll bite when an emulsion is used as the lubricant. This theory assumes that oil droplets larger than the thickness of the lubricant film in the deformation zone become trapped when they come into contact by both the surface of the sheet and the surface of the work roll. As the droplets move toward the deformation zone, water is left behind or squeezed out by the flattening of the oil droplets. At some point, the surfaces of the trapped oil droplets come into contact with each other, resulting in an inversion where oil becomes the continuous phase. The dynamic concentration theory predicts that the height of the meniscus where this inversion occurs is a function of the percent oil concentration and the diameter of the oil droplets. To test the dynamic concentration theory, the onset speed of starvation on a laboratory rolling mill was determined by measuring the speed in which load increases relative to the load measured for the same process but where the lubricant film is fully flooded. Both the concentration of oil and the size of the monodisperse oil droplets were varied to change the height of the meniscus and consequently the onset speed of starvation. Solid support for the dynamic concentration theory was obtained when starvation occurred at the same speed for emulsions with different oil droplet sizes in which their concentrations were specifically adjusted so that their meniscus heights theoretically were the same. Finally, an unexpected consequence of the dynamic concentration theory was that the thickness of a starved lubricant film is independent of rolling speed, but this is only true at high speeds.