The influence of oscillating motion on grease lubrication performance in ball bearings

verfasst von

Muyuan Liu

betreut von

Gerhard Poll

Abstract

Bearings are critical components in industrial applications and are used in a wide variety of operating conditions. To meet the lubrication demands of different operating conditions, it is essential to study wear and lubrication mechanisms to select appropriate lubrication methods and lubricants. With the advancement of automation industries and clean energy technologies, reciprocating bearings are increasingly utilized in applications such as robotics and wind power systems, where grease lubrication is commonly employed. However, compared to the in-depth studied rotational bearings, the lubrication mechanisms and lubricant selection for oscillating bearings remain insufficiently investigated. This study primarily investigates the grease replenishment mechanisms in oscillating bearings through a combination of bearing tests and elastohydrodynamic (EHL) model experiments. In the bearing tests, 6008 deep groove ball bearings were used to evaluate the anti-wear performance of various greases under a range of operating conditions. Performance was assessed through measurements of frictional torque and rolling surface wear. Parallel to this, the model experiments focused on measuring the film thickness within the contact zone of tribo-pairs, simulating corresponding bearing conditions to gain insight into the lubrication mechanisms at play. To explore the role of the bearing cage, both experimental setups were modified: the bearing cage was redesigned, and the traditional ball-on-disc configuration was adapted into a ball–cage–disc system. These modifications enabled a focused investigation of how the cage influences grease replenishment. In the bearing experiments, a total of three cage designs were evaluated. The results confirmed that, under certain amplitude conditions, the presence of a cage significantly enhances lubrication performance. However, even in the absence of cage-assisted replenishment, lubrication performance was observed to improve with increasing oscillation amplitude. This phenomenon is likely attributed to the translational movement of the rolling elements between the inner and outer raceways. As the rolling trajectory spans both rings at larger amplitudes, it facilitates grease exchange across the raceways, contributing to improved lubrication. Complementing these findings, the model experiments led to the development of a thickener starvation theory, which identifies the critical boundary conditions under which the thickener can or cannot enter the contact region in a ball-on-disc configuration. According to this theory, under starvation conditions, the distribution of grease differs between the inner and outer raceways. When the rolling element transitions from one raceway to the other, this movement disrupts the development of a starvation state by redistributing thickener into the contact zone, thereby enhancing lubrication. By integrating findings from both experimental approaches, the study identifies three key lubrication mechanisms in grease-lubricated reciprocating bearings: oil replenishment from surrounding grease reservoirs, grease replenishment facilitated by the cage, and grease exchange between the inner and outer raceways. The study also discusses the characteristics of each mechanism and provides recommendations for cage design.

Organisationseinheit(en)

Institut für Maschinenkonstruktion und Tribologie

Typ

Dissertation

Anzahl der Seiten

129

Publikationsdatum

14.10.2025

Publikationsstatus

Veröffentlicht

Elektronische Version(en)

https://doi.org/10.15488/19775 (Zugang: Offen)