The feasibility of drop-on-demand printing as a novel method for oil supply in elastohydrodynamic lubrication was tested for a single ball-on-disk contact to allow precise lubrication control at a minimal level of lubricant. It is shown that it is feasible to jet droplets of 3.3 pL for an oil of 46 mPa s viscosity at a controlled frequency which, for the conditions considered, results in droplets on the surface with a radius significantly smaller than the Hertzian contact radius. Experimental results of the film thickness evolution over time are presented from dry contact to fully flooded contact with different supply frequencies and a total oil usage of approximately 40 nL. By means of numerical simulation, the details of droplet ingestion into the contact are illustrated. It is shown that the central film thickness over time can accurately be predicted. The theoretical and experimental results provide a very strong incentive for further investigation of drop-on-demand printing as method to achieve optimal (re)lubrication at minimum level in the realistic setting of a rolling bearing.

This paper describes interferometry measurement of the film profiles of sliding elastohydrodynamic lubrication (EHL) contacts with spin. In the custom-made EHL test rig, spin motion is introduced through adjusting the center offset of the ball-on-disc contact with respect to the disc rotation axis. A parameter, spin ratio S sp , is employed to represent the spin level, which is defined as the ratio of the Hertzian contact radius to the center offset. Experimental results show that with spin the film shape is obviously skewed, and the film thicknesses at the two side lobes are no longer the same; therefore, the symmetry of the classical horseshoe film shape is lost. The film thickness dependences on entrainment speeds are significantly influenced by the spin ratio S sp , and high spin ratios induce high speed indices. At a fixed spin ratio S sp , with increasing sliding speeds the film thickness difference between the two side lobes becomes large, and the horseshoe film shape is more distorted. When applied loads are raised, more spin is introduced, film thickness decreases, and film shape is obviously twisted.

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A well-recognized phenomenon of typical traction tests of elastohydrodynamic lubrication (EHL) contacts is finite maximum traction at increasing speeds, which led to the postulation that the limiting shear stress of liquid lubricants, a high-pressure rheological property, existed. If slippage occurs at the oil-solid boundary, the limiting traction measured is not necessarily an intrinsic property of the lubricant but rather a function of interfacial properties between the bounding solid surface and the lubricant. A recent report presented experimental evidence of boundary slippage at EHL contacts using a simple methodology based on differences in the speed of oil entrapment and the apparent entrainment. The reported experiments were carried out under pure sliding conditions. The phenomenon may also be explained by internal slippage in the bulk fluid film because of the limiting shear stress of the lubricant. To clarify this, similar experiments were repeated under zero entrainment velocity (ZEV) conditions. Evidence of the highly pressurized lubricant at the center of the oil entrapment sliding against the solid bounding surface was obtained. The purpose of this article is to discuss whether the slippage is attributed to the limiting shear stress of the oil or the critical shear stress of the oil/solid interfaces, and how to differentiate the magnitudes of the critical shear stress of the two bounding surfaces in a conventional optical EHL test rig.