Rob Bosman

On a new method to determine the yield stress in lubricating grease

An experimental study using both a controlled stress and a controlled strain rheometer has been undertaken to characterize lubricating grease in shear, creep, stress relaxation, and oscillatory flow, with a main focus on determining the yield stress. The yield stress was examined using a cone–plate and parallel-plate system with smooth and rough surfaces. Clear discrepancies were observed in the yield stress values obtained using different techniques where oscillatory strain sweep measurements seem to be the best choice. This technique is less sensitive to wall slip, shows good reproducibility, and is relatively easy to perform. The method also shows that the yield stress is a function of the imposed frequency and therefore of the time domain. At lower values of shear—that is, in the linear viscoelastic regime—there is no structural breakdown and the rheology of the grease can be described by the Maxwell model where the stress and the strain are almost proportional to each other. Based on this observation, a novel method to determine the yield stress is proposed: “The yield stress can be determined from the point where this linearity no longer applies.” This method is compared to those that are commonly used. The yield stress was found to depend exponentially on temperature and linearly on frequency.

Transient Thermal Effects and Heat Partition in Sliding Contacts

In tribological applications, calculating the contact temperature between contacting surfaces makes it possible to estimate lubricant failure and effectiveness, material failure, and other phenomena. The contact temperature can be divided into two scales: the macroscopic and the microscopic scales. In this article, a semi-analytical transient temperature model is presented, which can be used at both scales. The general theory is presented here and used to calculate the contact temperatures of single micro- and macrocontacts. For the steady state situation, the results obtained are in good agreement with those found in literature. Further, it is shown that the simplification of modeling a microcontact as an equivalent square uniform heat source to simplify the calculation of the maximum temperature is justified in the fully plastic regime. The partition is calculated by setting a continuity condition on the temperature field over the contact. From the results, it can be concluded that at low sliding velocities the steady state assumption, which is often used for microcontacts, is correct. However, at higher sliding velocities, the microcontact is not in the steady state and transient calculation methods are advised.

Impact of Water on the Rheology of Lubricating Greases

ABSTRACT The operational life of bearings is often determined by the performance of the lubricating grease. The consistency of the grease prevents it from leaking out of the bearing and provides good sealing properties. The possible ingress of water into the bearing will have a considerable impact not only on this consistency but also on the lubricating ability of the grease. There are numerous applications where water ingress may occur, such as in the steel, food, pulp, and paper industries. Some greases are less sensitive to water than others. No specific guidelines are available to select the proper grease for bearings subjected to water ingress. The goal of the article is to contribute to the development of such guidelines for greases subjected to water ingress by studying the impact of water on grease rheology. Fully formulated, commercially available greases with the most common thickeners and base oils are used as model greases. It will be shown that water strongly influences rheological properties such as zero-shear viscosity, yield stress, and storage modulus. Calcium sulfonate greases were found to become stiffer after absorbing a considerable amount of water, leading to an increase in zero-shear viscosity and yield stress. However, lithium, lithium complex, and polyurea greases were found to soften, with appreciable changes in measured rheological properties.

Impact of Water on EHL Film Thickness of Lubricating Greases in Rolling Point Contacts

Abstract This paper investigates the influence of water on the EHL film thickness of six commercial lubricating greases under fully flooded and starved conditions. Although grease can absorb large quantities of water, separation occurs due to pressure and shear, leading to free water. This does not have an impact on the film thickness under fully flooded conditions. However, water does have an effect on the film thickness under starved conditions where the differences are related to the change in oil bleed. In the presence of water, an increase in oil bleed was found for lithium, lithium complex and polyurea grease. These greases showed a reduction in the levels of starvation and, therefore, thicker films. Water contamination led to lower oil bleed for calcium sulfonate complex greases, which led to an increase in starvation, and therefore, thinner films compared to their uncontaminated counterparts.

A stress-criterion-based model for the prediction of the size of wear particles in boundary lubricated contacts

In this paper, the formulation and validation of a model for the prediction of the wear particles size in boundary lubrication is described. An efficient numerical model based on a well-established BEM formulation combined with a mechanical wear criterion was applied. The behavior of the model and particularly the influence of the initial surface roughness and load was explored. The model was validated using measurements of the wear particles formed in steel–steel and steel–brass contacts. In the case of steel–steel contact, a reasonable quantitative agreement was observed. In the case of steel–brass contact, formation of the brass transfer layer dominates the particles generation process. To include this effect, a layered material model was introduced.

The Microstructure of Calcium Sulfonate Complex Lubricating Grease and Its Change in the Presence of Water

ABSTRACT Calcium sulfonate complex grease is often selected for rolling bearing lubrication if there is a risk for water ingress, because it is reported throughout literature that it can absorb large quantities of water. In this article, atomic force microscopy (AFM) measurements have been performed on a commercially available grease with and without water to investigate the microstructural change under the influence of water. From the results it is clear that the formation of large (inverted) micelles, which can be up to a few micrometers in diameter, is the main mechanism responsible for the absorption water. These results can be used to explain the earlier reported effect of water on the rheological and lubricating properties of these grease types. It is shown here that the size distribution of the micelle structure is a function of the amount of water mixed into the bulk grease.

A Master Curve for the Shear Degradation of Lubricating Greases with a Fibrous Structure

Abstract In this article, the mechanical aging behavior of lubricating greases with a fibrous structure is studied by aging fresh samples at controlled shear rates and temperatures in an in-house-made Couette aging machine. The rheological properties of fresh and aged samples were evaluated in a plate–plate rheometer. In the absence of oxygen, no chemical reactions occurred. The results showed that shear degradation is accelerated by increasing the temperature. This thermal effect can be described by an Arrhenius law. A grease aging master curve was created to describe the influence of shear and temperature on the mechanical aging of fibrous structured grease. Next, the model was validated using a conventional grease worker test. Finally, the model was applied to full ball bearings using an R0F + bearing test rig. The master curve forms an important contribution to the development of grease life models.

Erratum to: Analysis of Wear Particles Formed in Boundary-Lubricated Sliding Contacts

Due to unfortunate misinterpretation of dynamic light scattering (DLS) measurement results, Figures 10–12 and 17 contain errors. This also affects the discussion based on the comparison of atomic force microscopy (AFM) and DLS results (Fig. 17). The DLS measurements indicate smaller particles compared to AFM (Figs. 10, 11, 12). The calculation of the number of particles per sliding distance and per unit load was based on the DLS data and therefore is incorrect. The correct numbers are: 1200–6000 particles mm N for steel and approximately 12,800–15,200 particles mm N for brass. These corrections do not change the final conclusions. Load, N 2 4 6 8 10 R ad iu s, n m