The performance of grease under starved conditions was investigated by researchers from Kluber. Two lithium complex semi-synthetic greases were tested which differed only in their base oil viscosity. The greases were compared with their base oils. Testing was carried out using an optical EHL rig manufactured by PCS Instruments.
Two primary testing protocols were used in this study. The first was used to record a film thickness-velocity curve, run the contact in for two hours and then measure a second film thickness-velocity curve. The second protocol was to run a contact at a constant speed of 1 m/s with the test being stopped at intervals to allow grease to flow back into the contact.
As velocity increased a grease in starved conditions went through several stages (from low to high velocity):
- Film thickness drop with increasing velocity at very low speeds. Sometimes this stage did not occur because the velocity range did not extend low enough. The inverse behavior is due to a thickener layer
- Increasing film thickness with velocity until a starvation point was reached. The behavior is due to increasing oil entrainment.
- Film thickness decreases with velocity due to starvation.
- Increasing or flat film thickness. This usually occurred around 1 m/s. It must be due to grease reflow into the contact but the reasons for it are unknown.
The investigators found that starvation, the transition from phase 2 to 3, was controlled by the term (velocity x viscosity / load) and temperature. However, the critical uv/W level can vary along with the film thickness it occurs at based on many factors. Some of these factors include running time, lubricant viscosity, lubricant volume, temperature and other test conditions.
The base oils exhibited same behavior as the greases. Even though oil can reflow more easily, it can also be flung out of the contact with less effort. Interestingly, fully flooded greases experienced the same behavior as starved greases after having been run-in for a period of time.
The researchers also looked at the effect of the thickener and additive system on film thickness by calculating the ratio of the grease film thickness to base oil film thickness. The thickener and additive systems increased film thickness at low speeds but were often detrimental at higher speeds.
It was postulated that the film thickness consists of three different layers:
- Solid-like layer of deposited thickener. This was small in this study because of the non-reactive glass disk.
- Adsorbed layer of thickener and additive molecules
- Residual grease layer of thickener-rich high viscous material
Citation
- Balasubramaniam Vengudusamy, Marius Kuhn, Michael Rankl & Reiner Spallek (2016) Film Forming Behavior of Greases Under Starved and Fully Flooded EHL Conditions, Tribology Transactions, 59:1, 62-71, DOI: 10.1080/10402004.2015.1071450
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