An Experimental Study on Starved Grease Lubricated Contacts

Abstract

The film thickness of a ball-on-disc contact lubricated with four greases of different formulations was measured under different operating conditions until starvation. Two polymer-thickened greases and two lithium-thickened greases, formulated with base oils of different nature and/or viscosity, were tested. The central film thickness was measured under constant operating conditions (load, temperature, slide-to-roll ratio) varying only the entrainment speed. In a separate test, the film thickness was measured over time with all operating conditions set to constant. Pictures of the film thickness profile across the contact area were also registered. The results were compared with the fully flooded results. The coefficient of friction (COF) was measured in a ball-on-disc contact under equal operating conditions and the results were correlated with the film thickness findings. The different grease formulations and the influence of the operating conditions on the film thickness and COF were discussed. The polymer thickened the greases, promoting lower COF and higher film thickness, especially when there is thickener material crossing the contact which happens quite often for these greases.

Effect of Over Rolling Frequency on the Film Formation in Grease Lubricated EHD Contacts under Starved Conditions

“…The measurement results show that during the first 500 disc revolutions, the film thickness increases and then decreases, due to the grease distribution on the disc. This initial behavior is characteristic for grease measurements, and is commonly referred to as a churning phase [21,32,34,[36][37][38][39][40]. As soon as the grease is distributed uniformly on the disc, the film thickness reaches a constant level after 1000 disc revolutions.…”

Aspects of Self-Organization of Tribological Stressed Lubricating Greases

“…A number of previous experimental works [7][8][9] show the influence of shear rate, stress, temperature, type of base oil, type of solid (thickener) and thickener content on the curve behaviour (for example τ vs.γ). Papers from [9][10][11][12][13][14] are focused on the structure and present the system behaviour, as influenced by the observed grease structure.…”

Improving the lubrication and anti-corrosion performance of polyurea grease via ingredient optimization

“…At this moment, the base oil is released from the network of thickener, replenishing the contact area for providing the lubrication function [15,16]. Once the grease cannot bleed enough base oil, it is considered to be at starved conditions [17][18][19]. The yield stress and oil-releasing mechanism of grease strongly rely on the structure of thickener [20,21].…”

“…Meanwhile, the severe wear leads to the increasing temperature, which causes the base oil to bleed from the thickener and increases the mobility of base oil. The increasing temperature accelerates the base oil return to the track and replenishes the contact area, thereby recovering the starved conditions [35]. The lubrication of PUGs with high consistency is continually changed from the full flood condition to the starved condition, namely the dynamic behavior of lubrication, while PUGs with better mobility is at the steady-state [36].…”