Effect of Preshearing on the Coefficient of Friction

During the experiments on the two-disk test bench, the effect of preshearing on the coefficient of friction was investigated. To research this behavior, the coefficient of friction was measured using two different states of the examined grease. In the first state, the grease was not conditioned beforehand and was directly applied to the two discs. For the second state, the grease was sheared using a kneader for grease according to DIN 2137. 

The determined traction curves for the two different states of the examined grease are shown in Figure 3, this time showing the results of an alternative grease in contrast to Figure 1. In this diagram there is again the difference between the values for positive and negative SRR. But in contrast to that the difference between the normal and the kneaded state of the grease is very small, especially in the area of positive SRR.

Film Thickness in the Tooth Contact
To determine the share between fluid and boundary friction, the lubricating film thickness is a key indicator. To describe the film thickness of the grease more precisely than the approximation with only the properties of the base oil, both additional methods presented in the state of the art will be investigated in this work. The investigation will take an eye on the applicability of these calculation methods on the examined grease.

Determination of Lubricant Parameters
Both examined calculation methods require the dynamic viscosity of an oil from the grease, either from the base oil or from the bleed oil. Since these viscosities are not provided by the manufacturer of the lubricant, the viscosities of both oils were measured using a high-pressure viscometer.

Extraction of the Bleed Oil
Bleed oil refers to the oil in a grease that is “bled” out under load. To determine the properties of this oil, it must first be extracted using a filter method based on the principle in Ref. 12. Therefore, the grease was placed in a filter and then subjected to a static load for a longer period of time. A pleated filter made of cellulose with a retention capacity of 7–9 µm was used and placed in a plastic funnel. The grease was then distributed along the walls of the funnel, and a second funnel was used to apply the load evenly. The load was applied using a disk with a weight of 1 kg, which was placed in the second funnel. The entire combination of funnels, grease, and weight was then placed on a beaker to collect the bleed oil. To speed up the extraction process, the entire assembly was then placed in an oven at a constant temperature of 70°C. In this way, approx. 15 ml of bleed oil per funnel assembly could be obtained over a period of 5 days.