TLT: How did you get into tribology?
Grillenberger: My main requirement for a job after earning my doctorate was to have interesting research and development tasks. I also wanted to work in a larger company, as I had recognized the benefits of such a working environment during my tenure working for a major company in the semiconductor industry. These benefits included the depth of the scientific work I would be undertaking as well as the potential for continuous professional development, leading to a high knowledge standard. Schaeffler offered me exactly such an opportunity within the company’s fundamentals department, and I have never regretted the decision in accepting their offer. So, tribology happened to be the vehicle for my curiosity to discover new things. In doing so, I switched my research interests from silicon crystals to bearings.

TLT: What are the key challenges in the rolling bearing industry?
Grillenberger: From my point of view, the strongest trends in drivetrain technology are standardization, increased power density/downsizing as well as enhanced energy efficiency. As Schaeffler is always working in close cooperation with our customers, we are extremely knowledgeable about the latest developments within these trends for the different applications, which allows us to generate both technical and economic added value. The trend toward energy-efficient solutions is based, on the one hand, on the global climate accord (Paris Agreement). On the other hand, electrical motors make up 70% of the energy consumption in industrial applications. Consequently, the energy efficiency of electric drivetrains needs to be improved, which is often accomplished using variable speeds, frequency converters and downsizing. This might, however, induce electric currents in bearings, which leads directly to another big challenge for the bearing industry.

TLT: Any feedback on the current state of the art in this area, particularly what are the new materials which are being tried for applications in the bearing industry?
Grillenberger: If electricity inside a bearing is an issue, Schaeffler offers a variety of solutions to manage current, depending on the current intensity, voltage and frequency. Because the effects induced in a bearing (or in the complete system) can vary significantly, the solutions—which are typically developed together with the customer—also depend on the application. For example, the induced effects of electrical current could be white etching cracks (WECs), remolded raceway zones or pre-aged lubricants. The countermeasures, therefore, also can range from insulating coatings or hybrid bearings to different sealing or lubrication concepts.

TLT: How are you incorporating sustainability in the industrial design of bearings?
Grillenberger: Within any mechanical system, rolling bearings play a key role in friction reduction and energy efficiency. Reducing friction inside bearings using innovative technologies, such as coatings or other surface treatments, can significantly help to reduce carbon emissions.

Inventing and developing solutions for renewable energies like wind turbines or hydrogen technologies are essential to achieving sustainability. Schaeffler is a key cooperation partner in these technology fields.

TLT: What do you think are the key challenges for tribologists in rolling element bearing technology?
Grillenberger: There are numerous interesting challenges for tribologists with regard to bearings. For example, the interaction between new coatings and lubricants—including their additives—is a broad and interesting field. Schaeffler has a worldwide leading surface technology department that optimizes coatings for both automotive and industrial applications.

Another field of interest, which reaches into simulation, is bearing optimization. Schaeffler’s proprietary bearing design software Bearinx¹ has been continuously developed and improved over the past 25 years. Bearinx is used by Schaeffler engineers, customers and others to design thousands of bearings every day. It offers analysis of single contacts, bearings, shaft systems and complete gearboxes, while taking into account performance criteria such as friction, life rating or noise. Due to the huge demand for calculations, we are constantly developing and validating this tool with regard to functionality as well as automatization and complexity, which makes it possible to not only analyze bearings but also enable holistic system analysis. Since 2015, an optimizer called OptiKit is included and developed within Bearinx, which can be used to get the best solution for a given application. By utilizing it for parameter studies and generating reduced order models, OptiKit opens the door to digital services and a wide field of applications. Transforming the simulations to cloud services is one of the future tasks that Schaeffler has already started to work on in order to enhance the availability of Schaeffler’s knowledge and expertise for our customers and partners.

When talking about the core knowledge of a tribologist—the contact—it also is important to think about the system boundaries. This “component” interacts with the complete thermal, electrical and mechanical system that surrounds it. In wind turbines, for instance, we consider the interaction along the entire path—from wind flow profiles all the way to the rolling element contact, and vice versa. A tribologist can accept such interesting challenges to further improve existing methods—as well as develop new ones—to describe the contact performance in a system simulation.

TLT: Why is simulation in tribology so important?
Grillenberger: Simulation might reach areas that cannot be accessed by testing only—be it because of the quantity or size of samples or operation points, unmeasurable variables or lack of specified test samples, to name just a few examples. In particular, simulation might help to understand underlying principles and run parameter variations and optimizations. For example, rolling bearing noise simulations enable us to optimize the value we can offer our customers with respect to the noise generated by our bearings. These optimizations would not be possible by testing only.

Of course, simulations are basically models. However, experiments are models, too, as they are typically run on specified test rigs with a defined operation state. To get the maximum benefit, it is essential to utilize testing as well as simulation, and to combine the inherent benefits offered by both methodologies.

TLT: What has been your most interesting project?
Grillenberger: It is really hard to pick just one. However, I would like to introduce my top two projects.

The first one pertains to the development of a bearing noise simulation feature within Bearinx. It should be noted here that the mere task of simulating rolling bearing noise is something that not many companies can do. Implementing this dynamic phenomenon into a quasi-static tool such as Bearinx adds a huge challenge on top of an already-difficult undertaking. Solving this challenge combines all the interesting tasks an engineering scientist at a company can hope for: creativity, programming, simulations, validation with other software and testing methods, as well as direct interaction with the users and customers. Plus, solving this challenge offers a huge benefit to the company. Thanks to our work, this noise simulation method now can be used by many engineers, instead of just dynamic simulation experts. The results of our research should already have been published; due to the COVID-19 pandemic, however, this will have to wait for the upcoming technical conferences.

Another fascinating topic is cage dynamics, especially with a focus on elastic cage instability (see Figure 1). Although the instability phenomenon has been discussed in publications since the 1990s, a study focusing on elastic vibrations in the bearing’s cage during the instability phase had not been previously published. I had the opportunity to publish my results at the STLE Annual Meeting in 2015. This elastic phenomenon is subject to many influencing factors. Consequently, it is a challenging task to measure these very high frequency vibrations, simulate them and develop validated countermeasures. Very few companies/research entities can do this.


Figure 1. Elastic cage instability during testing (left) and simulation (right). The green arrows show the elastic deformation speed of the cage.²

TLT: How did you get involved in STLE?
Grillenberger: My first contact with STLE was at the 2015 Annual Meeting in Dallas, Texas. I had been invited to give a presentation on cage dynamics and instabilities induced by friction contacts. This gave me the opportunity to meet a lot of interesting and supportive people, who have since influenced me in both my personal and professional development. For example, STLE gave me the opportunity to implement a new discussion roundtable for the annual meeting, and I am quite pleased that this roundtable format has been utilized each year since 2016. I like to think that this little success inspired STLE member Manfred Jungk to ask me to assist him in organizing the first Tribology Exchange Workshop that was co-hosted by ELGI and STLE. This engaging workshop encompassed a mix of presentations by leading scientists from both academia and industry, which were followed by lively discussions. I hope there will be many more interesting chapters in my STLE story.

TLT: What are your personal benefits and for your company?
Grillenberger: I think these new opportunities and the experience gained through coordinating these important events also have inspired much of my work at Schaeffler.

On the one hand, the scientific information taken from these conferences, along with the rapidly expanding and passionate network, is an inspiration for my projects. Additionally, being able to meet in person and interact with the authors of these acclaimed technical papers helps to not only understand the subject matter but also the presenter’s motivation. I also have gained access to an invaluable network of high-caliber personalities. Moreover, this network supports customer and university partnerships, which bolster Schaeffler’s reputation as a technology leader within the bearing community. This, in turn, makes Schaeffler an attractive partner for universities and customers seeking high-level technology.

On the other hand, the experience gained in organizing the various technical sessions, discussion roundtables and workshops has improved my skillset when it comes to coordinating similar events and activities at Schaeffler. This really helps our group realize synergies and enhance our potential.

TLT: What does the future look like for young professionals who want to start their career in this industry?
Grillenberger: Stay curious and open-minded. There is still so much to discover and learn within any industry, even one that is built around a seemingly “mature” mechanical device like the rolling bearing. And this holds true not only for technical innovation, but also for collaborating, networking and, I am sure, for people in general. Keep this attitude toward life, and there will always be an inspiring and interesting job that will benefit your employer and, of course, yourself.

REFERENCES
1. Available here.
2. Schwarz, S., Grillenberger, H., Tremmel, S. (2019), “Investigations on Cage Dynamics in Rolling Bearings by Test and Simulation,” Presented at the 74th STLE Annual Meeting & Exhibition, Nashville, Tenn.

You can reach Dr. Hannes Grillenberger at hannes.grillenberger@schaeffler.com.