Experimental measurement of the friction torque on hydrodynamic plain journal bearings during start-up

Due to the increasing interest in energy savings, the start-up friction coefficient is a very useful parameter for engineers, who design hydrodynamic bearings. It is most often based on an approximation. Several studies can be found in the literature on this topic, but most of them are concerned with air bearings or are just numerical models. Some studies are more general and deal with the transient thermal behavior in both journal bearings and thrust bearings. Other studies deal with the friction coefficient during running, given a constant rotational speed.In order to understand the mechanisms occurring during the transition between mixed lubrication and hydrodynamic lubrication, several studies [1], [2], [3] and [4] have been conducted over the past three years. The aim of this paper is to provide an overview of the results of these studies and also to draw several general conclusions on the behavior of a plain journal bearing during start-up. The first part of the results focuses on the measurement of bearing torque during start-up with varying specific pressure (load divided by the projected area). Thus, the friction coefficient at start-up (deduced from measurements) is obtained for four different bearings, varying feeding conditions, radial clearance and length. The second part of the results consists of observations relating to the stick–slip phenomenon which occurs during this transient period. Finally, a study dealing with bearing characteristics influencing the friction coefficient (surface roughness and bearing materials) is presented.The main focus of this work is then to provide useful experimental data for bearing users and designers.

Research highlights
► The friction coefficient at start-up increases with increasing surface roughness, except when surface has a high roughness.
► The friction coefficient remains roughly constant for bronze bearings at a value of between 0.16 and 0.20.
► It depends on the oil supply temperature.
► And is directly correlated to the surface finish and the materials involved in the contact.
► Mixed regime occurs during the very first revolution of the shaft, inducing stick-slip.