Influence of the thickener on wear behavior and service life in oscillating linear actuators

Two greases of the same NLGI grade, identical base oil, and identical additive package were tested on the screw-driver linear actuator test rig under continuously increasing normal load until system failure. The only variable was the thickener system: barium soap (grease 6) versus lithium soap (grease 5). Grease 6 achieved a runtime of over 60,000 seconds with stable friction coefficient hysteresis. Grease 5 failed after approximately 30,000 seconds, accompanied by immediate wear particle formation, an unstable friction coefficient profile, and increasingly deficient lubricant film supply. The results demonstrate that the thickener system, independent of the additive package, exerts a dominant influence on tribological performance in oscillating linear contacts.

Greases consist essentially of three components: a base oil (typically 70–90 wt.%), a thickener (5–20 wt.%), and performance-enhancing additives. While the base oil provides the primary lubrication function, the thickener determines texture, mechanical stability, oil bleeding behavior, and overall re-lubrication capability of the grease—precisely those properties that govern adequate or deficient lubricant film supply in oscillating contacts. It also contributes directly to lubricant film formation within the contact.

Oscillating systems, such as linear screw drives, impose special demands on the lubricant: unlike in rotating bearings, the grease is not evenly redistributed at the reversal points but is repeatedly transported toward the ends of the stroke.

The objective of this study was to demonstrate the isolated influence of the thickener system on wear behavior and service life under realistic operating conditions. The screw-driver test rig at the Competence Center for Tribology at Mannheim University of Applied Sciences enables systematic tribological characterization of lubricants in linear screw drives. The spindle nut oscillates between the left and right end positions, while the normal load is continuously increased in defined load steps. With each load step, contact pressure, frictional work, and local temperature increase.

Test rig and measurement technology

The test rig records the following parameters synchronously:

• Friction coefficient hysteresis: coefficient of friction over the stroke, resolved for both directions of motion
• Normal force and sliding speed: basis for a Stribeck-based evaluation
• Non-contact IR temperature measurement: measurement of local temperature directly at the spindle nut
• Camera-based lubricant monitoring: at the reversal points, grease quantity, color, and morphology of the grease bead are documented as a direct indicator of particle contamination and grease degradation

Observations and Results

For grease 5, the freshly applied grease initially appears semi-transparent, grey-beige at the start of the test. After only the first few strokes, the grease bead forming at the reversal points shows a distinct brown discoloration. This immediate discoloration is a direct indicator of metallic wear debris from the spindle nut and its rapid incorporation into the lubricant. The brown coloration further indicates that no load-carrying lubricant film could be established from the very beginning of the test. As the test progresses, a progressively increasing stick-slip behavior of the spindle becomes visible in the time-lapse recordings. This represents a mechanical indication of increasing clearance due to wear-related material removal at the spindle nut, as the positioning accuracy of the drive visibly deteriorates.

For grease 6 (bluish-white in its fresh state), the grease bead shows a significantly delayed discoloration, and the spindle motion remains uniform over a longer period.

The friction behavior is also notable in the comparison of the greases. For grease 5, elevated coefficients of friction > 0.15 initially occur only at the reversal points, later extending over increasingly larger portions of the stroke, until in the final quarter of the test the overall level rises toward 0.15. This indicates that the lubricant film is neither able to form stably nor to be maintained. In contrast, grease 6 exhibits coefficients of friction < 0.1 over most of the test, with reversal peaks only occasionally approaching 0.15. Only toward the end of the test does the overall level increase. In both tests, system failure is indicated by the friction-force shutdown, which is preceded by characteristic trends in the friction signal.

Why does the thickener make the difference?

Since base oil, additive package, and NLGI grade are identical for both greases, the observed performance difference must be attributed to the thickener system. In oscillating linear contacts, the thickener influences performance through several tribologically relevant mechanisms:

The thickener is not merely a passive carrier of the base oil. Studies on the shear strength of thickener structures show that a more shear-stable thickener can maintain more intact structures in the inlet zone of the contact, contributing to a measurable increase in effective lubricant film thickness¹. If the thickener cannot withstand mechanical stress, this effect is lost. Lithium 12-hydroxystearate, for example, forms a fibrous network of fibrils that fragments under shear in the contact inlet².

Lithium soap greases therefore undergo a two-phase aging process under mechanical stress: an initial rapid degradation phase followed by a slower deterioration phase². This degradation impairs oil transport within the grease network and reduces the re-lubrication capability, i.e., the ability of the grease to supply base oil from the reservoir back into the contact.

Barium complex soap greases, in contrast, exhibit higher mechanical stability under shear and are considered particularly suitable for heavily loaded applications with high contact pressures. In addition to the more shear-stable thickener structure, this is also attributed to better compatibility with EP/AW additives³.

Conclusion

The present experimental results demonstrate that the thickener system of a grease exerts a dominant influence on service life, friction behavior, and wear intensity in highly shear-stressed oscillating systems, independent of base oil, NLGI grade, and additive package. The observed halving of service life (grease B compared to grease A) is therefore not statistical scatter but a systematic effect attributable to the differing tribological properties of the thickener systems.

For the selection of greases in loaded systems, application-specific testing under realistic operating conditions is always recommended. Datasheet values such as viscosity, NLGI grade, and additives are not sufficient to reliably predict tribological behavior (for example, in spindle contacts). Camera-based discoloration analysis of the grease bead has proven to be a fast, non-invasive indicator of system failure and enables early assessment already during the running-in phase of the test.

[1] Cousseau, T., Björling, M., Graça, B., & Campos, A. (2012). Film thickness in a ball-on-disc contact lubricated with greases, bleed oils and base oils. Tribology International, 53, 53–60. https://doi.org/10.1016/j.triboint.2012.04.018

[2] Meijer, R. J., Osara, J. A., & Lugt, P. M. (2024). On the Required Energy to Break Down the Thickener Structure of Lubricating Greases. Tribology Transactions, 67(1), 123–128. https://doi.org/10.1080/10402004.2023.2297041

[3] Wang, Z., Xia, Y. & Liu, Z. The rheological and tribological properties of calcium sulfonate complex greases. Friction 3, 28–35 (2015). https://doi.org/10.1007/s40544-014-0063-1