Thin film rheology and ferrohydrodynamic lubrication of magnetic fluids

Resumen

Using the Reynolds lubrication theory we studied the theoretical performance of a lubricated contact operating in the hydrodynamic lubrication regime. This contact consisted of two tilted flat circular rigid surfaces, separated by a thin film of a Newtonian fluid. One of the surfaces was made to rotate while the other was kept fixed, transmitting a torsional shearing force on the fluid film under the assumption of no slip at the surfaces. The shearing force resulted in the fluid passing periodically from a converging wedge-like zone to a diverging one in circular laminar flow inside the contact. This mechanism produced hydrodynamic forces that pressurized the fluid film. The pressure film increased in the convergent zone while in the divergent one, it decreased until cavitation was induced. The area of the cavitated region depended on the operating conditions of the contact. A magnetorheological plate-plate rheometer was used to study the performance of a contact lubricated with a magnetic nanofluid. This geometry corresponds to rotating contact between two slightly tilted surfaces described previously. Thus, in agreement to the theoretical results hydrodynamic forces appeared in the fluid due to the periodically passage of the fluid through converging and diverging wedge-like zones formed by the tilted surfaces. In addition, magnetic body forces were introduced in the fluid film due to a non-uniform magnetic field distribution applied inside the contact. In addition to experimental results, a theoretical description of the problem was carried out by solving the modified Reynolds theory for magnetic nanofluids. A contact works in the soft elasto-hydrodynamic lubrication when the pressure in the fluid film separating completely its surfaces is high enough to deform at least one of the contact surfaces but not to cause changes in the fluid viscosity at the inlet of the contact. This regime of lubrication was studied experimentally in a contact composed of a soft elastomeric ball loaded and rotated against a flat surface of a rigid rotating disc in the presence of a limited amount of a magnetic nanofluid. A set of circular magnets arranged in a tower like structure was placed under the disc, centered with the contact. This induced a non-uniform magnetic field radial dependent distribution which confined the fluid around the contact. Due to the small amount of lubricant at the contact, starvation occurred in the contact at high speeds. This was observed in an abrupt change of the slope in the curves where the friction was represented as function of the relative surfaces speed. The friction coefficient increases more rapidly in a starved regime than in a fully flooded one. The starvation occurs because the rupture of the natural replenishment mechanism of lubricant to the inlet contact. In agreement with the observed the higher the viscosity, the lower the speed associated to the onset of starvation. The load capacity of the contact had no significant influence on this. Experiments showed that the transition between the fully and starved lubrication regimes can be controlled by modifying the magnetic field distribution. The magnetic forces appear to act like an extra lubricant replenishment mechanism, externally manipulated by the applied magnetic field. The higher the viscosity, the higher the magnetic field strength required to prevent starvation.