Executive Summary
When it comes to electrical currents in tribological contacts, readers report using a variety of strategies to monitor possible damage. Some use tribotesting, microscopy and even visual inspection. Measuring electrical conductivity is also a frequently used strategy. Choosing the correct lubricant for the application is also important. While some readers report that they have transitioned to using biodegradable lubricants, slightly more say they have not, citing costs, availability and performance concerns as the reason.

Q.1. Please describe the strategies and techniques employed to evaluate and address damages resulting from electric currents in tribological contacts, specifically in bearings or gears utilized within electric motors and vehicles, wind turbines and similar demanding applications.

Tests on electrified tribometer and surface analysis.

Grounding through electromechanical devices.

Tribological testing using a mini traction machine or electrification of other tribometers (ball-on-disc, four ball, etc.).

I have not made any tests in full machine applications. In tribometers, I have tested the impact of the use of an ionic liquid made as an additive for a base oil in electrified contact with an alternative movement. To evaluate the damage on the samples we used optical profilometry, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS) and Raman spectroscopy.

Insulating and conductive strategies on the e-drive motor axis. Specifically insulating the non-drive end bearing (hybrid ceramic) and conducting on the drive end (carbon brush) to direct electrical discharge through a non-damaging path.

Have you implemented insulation measures, such as ceramic coatings or insulating materials, to mitigate damages caused by electric currents in tribological contacts?
Yes 47%
No 53%
Based on an informal poll sent to 15,000 TLT readers.


 
I guess that the main strategy is to avoid difference in the electric potential among the surfaces, connecting both surfaces to an “earth” potential. In this line a lubricant with high conductivity would be best than an isolant one.

Controlling copper corrosion seems to be manageable, and there are several bench tests to screen for the problem.

In e-motor bearings, there is a specific discoloration that occurs when electrification takes place. I do not have experience with e-vehicles.

Depending on the actual design, sometimes you need a lubricant with conductive properties to assist with the grounding. While in other designs, you might need an insulative lubricant to break the electric currents.
 
Measurement of the electrical properties of the lubricants under application oriented conditions. Simulation of the individual electrical loads on each tribo-contact.

In my opinion, I would like to say that in all electric and tribology applications, damages and technique can be better dealt with biodegradable lubricants. For biodegradable lubricant cases, tribology can be of multiple types as per formulation.

Optical and SEM microscopy, and visual inspection.

Sounds and vibrations, electrical measurements, oil analysis, ocular inspections, microscope analysis.

Electrical damage in tribological contacts is usually identified through vibration, current and surface analysis that reveal fluting or discharge marks. Engineers trace the root cause often as inverter induced bearing currents or poor grounding using shaft voltage probes or simulations. To prevent recurrence, insulated or hybrid bearings, shaft-grounding rings and conductive lubricants are applied.

We have run tests with our own design electrical discharge machining (EDM) circuit, in which a capacitor is mounted in parallel with the tribo-contact and a split ring commutator. The capacitor charges during the non-conducting part of the cycle and discharges, depending on the tribo-contact resistance, during the conducting part of the cycle. Applied voltages are typically in the range 6 to 24 volts. The number of discharge pulses are recorded. This arrangement is used for modeling the damage caused by common mode generated electrical discharges in motor bearings. We also have our own design low contact resistance measurement (LCR) circuit, which incorporates a constant current source, producing a voltage output proportional to the resistance of the tribo-contact. Although schematically a very simple circuit, practical implementation is somewhat more complicated. Finally, we also have a programable continuous AC/DC current source circuit. It allows a current of up to 1 amp to be passed through the tribo-contact. This is the sort of continuous current that might be generated in large rotating electrical machinery such as wind turbines.

Which of the following methods have you used to monitor and assess damages from electric currents in tribological contacts?
Thermographic imaging         17%
Vibration analysis         31%
Electrical conductivity measurements 69%
Visual inspection         83%
Other         28%
Based on an informal poll sent to 15,000 TLT readers. Total exceeds 100% because respondents were allowed to choose more than one answer.


To address damages resulting from electric currents, the high-performance, low sulfated ash, phosphorus and sulfur (SAPS) lubricant was formulated by synthesizing ionic liquid as oil additives. The formulated oils possess high thermal stability properties to resist the effects of Joule heating and prevent oxidation and deterioration of the lubricating oil. In addition, the formulated oils possess acceptable electrical conductivity or dielectric strength values. These additives interact synergistically with the effects of electrification to promote the formation of a wear-resistant tribofilm. Frictional tests of formulated oils were conducted using an SRV-IV tribometer in a ball-on-disc mode. The dynamic behavior of the friction coefficient and electrical contact resistance signals was analyzed during the friction process. This technique showed the dynamic interaction between electrified tribological contacts and offered information about the evolving states of the wear-resistant tribofilm. Additionally, the surface characterization techniques (SEM, EDS, 2D/3D profiling, focused ion beam-transmission electron microscope [FIB-TEM] and X-ray photoelectron spectroscopy [XPS]) were applied to explore the tribofilm mechanism under electrification conditions.

Not an expert in this area so I’ll shoot off the hip: acoustic, visual, X-ray, direct electrostatic measurement with specialized equipment.

Electric current damage evaluation can be done by electrical monitoring (or voltage measurement) to identify electric discharges, or by acoustic emission or electrochemical impedance during tribological or bearing testings. Physical inspection (SEM, X-Ray diffraction [XRD]) and surface analysis (profilometry) can help to identify microstructural changes, defects, pits, fluting and melting patterns. Lubricant analysis (dielectric and Fourier-transform [FT] spectroscopy) monitors performance changes. Mitigation of these damages can be solved by using conductive greases, insulated bearings (ceramic coated or hybrid ceramic) and couplings, dielectric lubricants for reducing discharge or contact metalization. Advanced lubricant design can help to a better film strength to tolerate micro-arcing. Advanced surface coating with diamond-like carbon (DLC) or TiN coatings reduce electrical conductivity and improve wear resistance. Use common-mode chokes, EMI filters or dv/dt filters to limit high-frequency current coupling. Ensure symmetrical motor windings and shaft alignment to minimize induced voltages.

1.) Visual inspection of the componentry, photographing. 2) Section through the wear scar looking for subsurface defects and cracking/crack initiation/material phase changes/white etching cracks (WEC).

Tribometer customized validation and analysis, component level customized validation and analysis, dyno level customized validation and analysis.

Q.2. Please describe the key considerations and challenges involved in the selection, implementation and performance assessment of biodegradable oils or greases in tribological applications, particularly in environments where environmental sustainability is a priority.

Biodegradable oils are a challenge when current is applied to the tribological contacts because these oils are more likely to experience early degradation, which is a main thing to avoid if you have a system where it is difficult or expensive to replenish lubricant.

1. Selection. Base oils: Vegetable oils and synthetic esters are most common; good lubricity and biodegradability but limited oxidation and thermal stability. Additives: Must be non-toxic and biodegradable; conventional additives (like ZDDP) often unsuitable. Compatibility: Check seal, metal and paint compatibility. Standards: Must meet Organisation for Economic Co-operation and Development (OECD) biodegradability and eco-toxicity tests (e.g., OECD 301B). 2. Implementation. Oxidation and hydrolysis: Main weaknesses; need antioxidants and moisture control. Greases: Require stable thickeners (e.g., calcium complex, biopolymer) for water and temperature resistance. System changes: Clean systems carefully before switching from mineral oils.

In our case, we have been working on the obtention of transesterified bio-oils from microalgae. The main considerations in the biomass selection were its content of lipids, as a higher content of lipids tends to help in the obtention of a proper lipidic profile. The performance of the transesterification reaction is basic to determine the economical viability. The need (or not) for an epoxidation process is determined by the unsaturation degree of the resulting oil, and the performance of the epoxidation reaction affects again the viability of the process. Finally, once we have the modified bio-oil, its stability, physico-chemical properties and tribological properties are the final goal.

Most of my applications where lubricants are not already decided are greased bearings, where performance parameters are key (longevity, wear resistance, water resistance, corrosion protection).
 
The challenge, I guess again, is to have a stable compound, that keeps the viscosity and superficial tension but is easily consumed by biological agents. Another point is to protect the compound while it is in use, maybe the use of a retarder to keep the biological agent away during the service time.

There are relatively few choices, limited production, and the cost is usually significantly higher than for conventional products. Problems are difficult to solve limiting these oils’ use. Industry bodies are also not helping when it comes to engines oils and the hurdles needed to gain approvals.

Key considerations for selection of biodegradable lubricant comprise feedstock features, performance gaps, chemical and biochemical modifications and application-related needs. Implementation challenges for such lubricants include costs, feedstock availability and hazards and performance variability. Performance assessment challenges for these lubricants comprise their oxidative stability (also in various aggressive environments), cold-flow behavior, tribological properties and biodegradation evaluation methods.

The main problem that needs to be solved is how to make something that is both biodegradable and long lasting in a formulation.

Like any application, you have to consider the operating conditions to see if a synthetic is required or if a non-synthetic will work. The method of application will also dictate the product in use. Then the potential for contamination into the environment and the potential for environmental damage.

Overall total cost of implementation is always the most important consideration. This is followed by the oil drain interval and the usage life of the lubricants.

As according to the suitable instances, such as viscosity, lubricity and oiliness, such formulation can be useful.

Key challenges are durability/stability, availability and price.
 
Key challenges include balancing biodegradability with oxidation stability, wear protection and low-temperature performance. Natural esters and synthetic esters offer good lubricity but can suffer from hydrolysis and limited thermal stability. Successful implementation requires additive optimization, compatibility testing with seals and metals and regular monitoring of degradation and performance in service.

Have you transitioned to using biodegradable lubricants in your tribological applications to improve environmental sustainability?
Yes 47%
No 53%
Based on an informal poll sent to 15,000 TLT readers.


The key considerations and challenges are balancing electrical, viscosity and antiwear properties in lubricant formulation. Thermal oxidative stability is a critical issue at high temperatures because it prevents sludge formation and viscosity changes. Compatibility with the rubber materials is a crucial point that requires attention. Biobased oils are expensive due to raw material costs. Furthermore, standard tribological tests are insufficient for evaluating the lubricants and materials.

I would imagine they would have to meet the same performance criteria in terms of load carrying capacity and lubricating properties as the non-biodegradable lubricants do.

Main challenges are to achieve at least comparable performance at comparable costs/price to conventional products. Besides, biodegradability has to be considered throughout the process (selection, formulation, performance assessment and implementation). Mostly, toxicity and bioaccumulation are ignored, although being as important as biodegradability. Maintenance programs may require adaptation due to reduced stability, increased water uptake due to higher lubricant polarity, change in material compatibilities, etc. Legal requirements for lubricant application in sensible environments simplifies the implementation of biodegradable oils/greases. Summarizing, the replacement of conventional lubricants by biodegradable ones requires a sound procedure.

Lubricants are mature products and in core well understood. The overlay of amperage and voltage are game changers in formulating lubricants for electrified tribosystems. They increase the size of testing matrix and complexity. The impact of current and voltage on the reactions of additives is unexplored, especially on the tribofilm. By chance were found formulations, where their wear protection is more or less unaffected by amperage. More delicate is the impact on fatigue resistance (rolling, slip-rolling).

Selecting and implementing biodegradable oils or greases in tribological systems, especially where environmental sustainability is a priority (e.g., marine, agricultural, forestry, hydropower or eco-sensitive industrial zones), requires balancing environmental performance with technical functionality. Key considerations are biodegradability and environmental impact, base oil type selection and performance requirements. Formulations considerations, implementation challenge and monitoring tools would help with formulating or selecting one for a particular application (e.g., wind turbine yaw gears, agricultural chains, hydropower turbines).

Biodegradable oils do not behave like most other fluids, and so consideration of this like microbial attack, oxidation stability, elastomer compatibility and operating temperature all need to be considered.

Biodegradable oils and greases have a higher price due to the availability, amount and regulatory requirements. Customers are not always ready to invest in them unless they are obliged by the law. Performance assessment is done with the current ASTM, ISO and DIN standards, and it is expected that they perform at the same level as their petrochemical equivalents. Lubricity is not always the same, solubility of antiwear/extreme pressure additives is not always given and hydrolitical stability is a challenge since in the tribological contact could lead to corrosion.

Friction level at all tribological conditions, life, cost.

Editor’s Note: Sounding Board is based on an informal poll sent to 15,000 TLT readers. Views expressed are those of the respondents and do not reflect the opinions of the Society of Tribologists and Lubrication Engineers. STLE does not vouch for the technical accuracy of opinions expressed in Sounding Board, nor does inclusion of a comment represent an endorsement of the technology by STLE.