Drop on Demand lubrication: Feasibility, Achievements and challenges for bearing application

Prof. Dr. ir. Kees Venner

Summary

The need for sustainability and minimization of energy losses directly translates into requirements for lubrication. Ensuring surface separation and reliable motion at low friction is as urgent as ever. At the same time the challenges for realization are larger. As a result of higher stresses (less material use), larger accelerations and speeds in novel applications, e.g. in electric vehicles, and trends to thinner (green) and less lubricants, contacts need to operate at much lower film levels. In feasibility studies using single elastoydrodynamically lubricated (EHL) contacts in model and machine element setups , e.g. ball-on-disc apparatus, twin-disc tribometer, back-to-back gear test rig, and in a thrust bearing setup, Drop on Demand (DoD) lubrication, has shown great potential as a versatile approach to control lubrication performance and achieve low friction. With DoD picoliter order droplets are ejected towards the raceway or gear circumference, at a controlled (low) frequency, from a piezo-elastically driven nozzle . Theoretical predictions for single ball-on-disc contact have shown very efficient use of lubricant droplets to generate a separating film once they are on the raceway, as in starved contacts almost all lubricant is directly used for film formation. This was confirmed by experimental results. However, there are als also significant challenges towards realistic applications. The first is to generate sufficiently small droplets in continuous operation without nozzle clogging. The second is to determine the appropriate droplet size, and ejection speed for droplets to pass through the windage field and arrive at the contact inlet. Subsequently, droplet spreading and film formation near and in the contact, and the generated pressure, as well as the induced subsurface stress modulations need to be understood. Experimental and theoretical results are presented demonstrating achievements and challenges towards future bearing applications.