Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


Test configuration: rolling contact under electrical load


The MPR test geometry features a 12 mm diameter roller pressed against three rotating rings with a much larger diameter of 54.15 mm. The roller is chamfered so that only a 1 mm-wide cylindrical track contacts the rings. This creates a focused, repeatable contact path ideal for simulating the highly stressed regions of a bearing raceway. A load of 500 N is applied through the top ring, resulting in a maximum Hertzian contact pressure of 1.9 GPa, representing what’s seen in the wind turbine main shaft or intermediate bearings.


Unlike conventional bearing tests that rely on rolling alone, the MPR introduces controlled sliding via independently driven rings and roller shafts. For this experiment, the test used a slide/roll ratio of 0.3, with the roller and rings rotating at linear speeds of 1.495 m/s and 1.105 m/s, respectively. This moderate sliding ratio promotes micro-slip and surface interactions critical to lubricant film breakdown, hydrogen release, and tribofilm formation.


The contact components were made from AISI 52100 bearing steel—a standard choice for bearing applications—with the rings hardened to 750 Hv and the roller slightly softer at 650 Hv. All surfaces were polished to 0.15 µm Ra to ensure consistent boundary and mixed lubrication regimes. Under the test conditions, the estimated elastohydrodynamic (EHD) film thickness was 150 nm, resulting in a Lambda ratio close to 1, meaning the contact was operating in mixed lubrication, where surface asperities interact intermittently, allowing wear mechanisms to manifest.


To replicate thermal conditions in gearbox bearings, the test cell was heated to a bulk temperature of 100 °C, controlled precisely throughout the test duration. This elevated temperature increases lubricant degradation rates and accelerates hydrogen release, making it essential for simulating WEC-prone environments.


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Electrical modification: Simulating stray currents To simulate stray currents seen in wind turbines and e-motor driven machinery, the test introduces a continuous DC electrical current through the tribological contact.


A custom slip ring assembly was integrated into the MPR rig, allowing the team to apply a stable 250 mA DC current directly across the roller and rings during operation. This current level corresponds to a current density of approximately 750 mA/mm²—a threshold known from previous literature to induce electrically assisted WEC damage reliably.


Maintaining electrical insulation between rotating and stationary components was critical. On the roller side, an inline torque meter was connected through a Mercotac slip ring and ceramic bearing assembly to ensure electrical isolation while allowing current flow through the wear track. On the ringside, a spring-loaded carbon brush applied the current to the shaft, with force finely adjusted via a tension screw to maintain a stable and consistent electrical contact. Voltage and current were monitored continuously throughout the test.


The Lubricants: Formulated for comparison The test was conducted using two ISO VG 320 lubricants with matched base oil viscosity and application intent—designed for wind turbine gearboxes—but with distinctly different additive chemistries: • Lubricant A was a commercially available, OEM-approved synthetic gear oil, formulated with conventional ashless antiwear and antioxidant packages. It served as the control, representing current industry-standard oils used in high-reliability applications.


• Lubricant B was a prototype formulation developed for this study, featuring an additive package that included an oil-soluble organic copper salt, along with an antioxidant, a dispersant, and an antifoam


LUBE MAGAZINE NO.188 AUGUST 2025 33


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