Show notes
Cory Mittleider of Malloy Wind and Loren Walton of NSK on main bearing failures, why the industry is pulling DLC coatings, and the material changes replacing them.
Sign up now for Uptime Tech News, our weekly newsletter on all things wind technology. This episode is sponsored by Weather Guard Lightning Tech. Learn more about Weather Guard’s StrikeTape Wind Turbine LPS retrofit. Follow the show on YouTube, Linkedin and visit Weather Guard on the web. And subscribe to Rosemary’s “Engineering with Rosie” YouTube channel here. Have a question we can answer on the show? Email us!
Allen Hall: Cory and Loren, welcome back to the podcast.
Cory Mittleider: Thanks for having us.
Allen Hall: So we’ve got two bearing experts in one location, and this is the point where we start asking all of our bearing questions. Cory, you’re with Malloy Wind, and we’ve had you on the podcast two or three different times. Loren’s with NSK — we’ve had Loren on at least once before.
Loren Walton: Once, yes.
Allen Hall: Yeah, and that was good.
Loren Walton: I appreciate that. It was fun.
Allen Hall: There are a lot of bearing issues happening in the States at the moment, but also globally. Whatever happens in the States, you can pretty much find in Australia, Canada, Singapore, Mexico, South America, Brazil — everywhere. We’re hearing a lot about main bearings, and there’s a variety of things that I think you two know from being on the inside that we on the outside haven’t heard yet. I want to get some of those stories out and understand what’s going on, because operators are trying to keep their assets running, and bearings are a big issue. Let’s talk main bearings. What are you seeing in the field right now? What kinds of problems are happening?
Cory Mittleider: It seems like operators are coming to us and asking us to supply bearings that no longer have DLC. That’s a bit of a phenomenon lately. For a little over a decade we spent our time supplying bearings with DLC on the rollers to address problems found fifteen years ago.
Allen Hall: DLC is diamond-like coating.
Cory Mittleider: Correct.
Allen Hall: Which is a really hard specialty coating applied to the bearing surfaces to provide hardness and durability — or it’s supposed to provide durability.
Cory Mittleider: That’s a good point. It’s a coating that’s one to two microns thick — one to two thousandths of a millimeter — and a very hard material. The big feature was that it’s a dissimilar material to the steel. So when we break through the mixed and boundary lubrication regimes and those asperities touch each other, that dissimilar material prevents the welding and tearing that leads to the peeling damage we saw fifteen years ago. That peeling damage eventually turned into spalling, cracking, and other failures. So it made a lot of sense at the time to turn to something like this to mitigate the peeling.
Allen Hall: So the peeling damage was one of those issues where you basically had some sliding happening. In my electrical world, and from looking at these on the ground, you see things moving relative to one another instead of rolling relative to one another.
Loren Walton: It’s more of a welding and shearing of the contacts. I used a finger analogy last time: think of your asperities as fingers — one set is the roller, one set is the outer raceway. They weld under high load and high pressure, then they shear, leaving behind debris. That’s what creates the beginning of the peeling damage, and then it continues to create more debris, and the bearing starts to basically eat itself alive.
Allen Hall: The start of that process, though — is that a lack of lubrication, or a finish or hardness issue on the bearing?
Loren Walton: I love that question, because this is the crux of the whole thing, and I think it’s the part that gets missed. People immediately want to throw the whole thing out and start over with something different. Fundamentally, when we fixed the surface issue by adding the coating, the problems pretty much went away. We went from one-to-five years of life to ten-plus years, depending on the application — without changing the construction, the bearing type, or the contact angle. Just by adding the coating, we increased life significantly. The root of what you’re asking is that the bearing would operate better if it had the proper amount of separation. It’s not a fatigue issue and it’s not a loading issue. At its heart, the bearing isn’t able to create that separation. There isn’t enough speed, and there isn’t enough of a gap created by the lubricant.
Allen Hall: So ideally you have this almost molecular-scale film of lubricant between the two surfaces. If it isn’t designed properly, or you have an issue, that lubricant gets squeezed out of the space, and at that point you have trouble. That’s some of what I’m hearing on main bearings — especially when turbines have been curtailed and aren’t turning. Is that partly just the fact that there’s so much load?
Cory Mittleider: I think that’s a fundamental difficulty of the main shaft bearing. You’ve got extremely variable loads, from full load to idle, and a wide range of operating conditions — from northern North Dakota in the winter to Texas in the heat this week. High load, heavy load, incredibly slow speed, and even slower if it’s idling. It’s hard to reliably build that film. It’s not necessarily that there isn’t enough lubrication; it’s that the film isn’t building properly where it needs to be to separate the metal and the rolling elements.
Allen Hall: So the diamond-like coating was meant to solve that welding problem — you put the coated bearing in, and it worked okay until more recently, when all of a sudden we started having other issues. To me those aren’t related to the coating itself, but to other things happening up in the nacelle.
Loren Walton: If we recall some of your previous episodes, you were on the forefront of understanding and talking about DLC starting to become an accelerant to failure. I know you talked about it with Cory. Those episodes have aged very well. A lot of people now are recognizing what we were saying years ago and changing their strategy toward removing DLC — whether on bearings for newer turbines, typically two megawatts and greater, or in some cases going backwards and removing DLC as they do additional replacements, and looking for another solution, because there’s potential for additional issues you weren’t expecting by adding the coating.
Allen Hall: The coating is non-conductive, which is part of the issue, because you wouldn’t think bearings are conducting electricity. But as turbines got some of these uptower and downtower converters and inverters connected to the generator, we started seeing current levels — according to Motor Doc, where people like Howard Penrose have gone out and measured currents in the nacelles — of well over a hundred amps running through ground straps and the like, into bearings. That’s a lot of current. If you’re shoving that into a bearing that has DLC on it, you’re going to break it down and create these really hard steel bits stuck inside the bearing, which wear it like pouring sand inside a bearing. That’s what eventually happens, and it has nothing to do with the bearing. It has more to do with the electrical and control systems we stuck up top and didn’t pay much attention to, but probably should have. We created an electrical situation, and now all the upkeep comes to people like you to deal with. You haven’t seen a lot of work to eliminate it, although there are a couple of good attempts happening. The reality is: okay, we have to have a bearing, and I’ve got this current going around from the nacelle. How do I put those together in a way that removes the DLC?
Cory Mittleider: That’s what we’ve spent the last ten-plus years on. As a bearing supplier, we can’t change the whole system. We have to do the best we can to accommodate what’s happening in your system. We would absolutely encourage you, if you can identify and remove the electricity, please do that.
Allen Hall: They should. And there are a lot of people who do.
Cory Mittleider: There’s a pursuit of that, absolutely. But the turbine still needs to run.
Loren Walton: We work very closely with an owner-operator that did a lot of that work. To your point from before, it does sound like, from what they’ve investigated, the current has been there for a while. It’s been there in different models and different turbines. Maybe the way it presented, or its impact, wasn’t to the same extent as what we’re seeing now. That’s where I’d say there’s more to it than just the current. I think I said last time it’s not just a smoking gun. The bearing is sitting in front of a firing squad. You put it all together and now we’re in a tough position. But to Cory’s point, we get brought the application, we get brought the environment, and we get told, “Here, make it work.”
Allen Hall: And you don’t actually see everything that’s happened. You get all the mechanical loads, but they don’t tell you, “Hey, we’re running a hundred amps through this nacelle.”
Loren Walton: No, I don’t remember hearing that.
Cory Mittleider: No, that’s not usually disclosed.
Allen Hall: No one’s ever said that. So that’s a real troubling thing happening in the industry — we’re assigning blame to mechanical components when really it’s an electrical mistake. When you dig into it, what you find is that currents have been running up top for years, but what’s changed now is that with more focus on emissions from inverters, they’ve pushed things into higher frequencies. Higher frequency bands are harder to ground out and get rid of. When things were in the kilohertz range, we could partly ground them and they’d go away. Now we’re working at ten kilohertz and up, and that energy distributes into a lot of places, including the bearings, where it wasn’t before. That’s really hard to deal with. Some electrical designer sitting in a remote location, probably in Germany, designs the circuit, and now you bearing gurus have to go fix it.
Cory Mittleider: And that system’s probably well optimized for that particular package.
Allen Hall: For that particular package, right. It meets all the requirements and does everything they wanted — except for the effect on the bearings.
Loren Walton: You solve one problem and move it to another. That’s ultimately how it works.
Allen Hall: If you’re an electrical engineer, you’d never have thought you were destroying the bearings. The industry has moved quite quickly, though. Everybody started noticing this problem with DLC. They went out to check and figure out what the problem was, and, more importantly, to find a solution. Those solutions are unique, because the reason DLC went on in the first place was to extend lifetime. So if you’re taking the DLC out of the equation, can you still get to those lifetime numbers without it?
Loren Walton: Yeah, and that’s where our message has been that adjusting the material will get you the difference you’re looking for. I want to be very clear: I’m not saying DLC as a solution is bad. When it was applied in the right space — turbines with a lighter duty — it worked great. But once you add in additional factors, it becomes an accelerant to failure at certain points. So it definitely still has its place. But once you move away from DLC, you’re going to be right back where you started — regardless of construction — with the life that was always aided by DLC. Once you’ve removed it, you have to know for sure you’re not going right back to the peeling layers and the spalling you were seeing. From what we’ve investigated, the material changes are where you get that. Having a harder surface combats it, and having a better way to combat any additional debris introduced into the system helps.
Allen Hall: And reducing the possibility of generating that debris.
Loren Walton: Correct.
Allen Hall: So what does that mean in terms of bearing design — different alloys, different heat treats, different coatings?
Loren Walton: The first two, not the third. From the recipe of the steel, adjusting some of the alloying elements, there’s a lot you can do. A lot of people think of engineering mostly through the mechanics of it, but one part of mechanical engineering that doesn’t get talked about is material science. That’s the part we dive into extremely deeply, and it gives you the biggest bang for your buck when you’re moving away from a coating as your — I don’t want to call it a crutch, but as the thing helping you get by — toward changing the bearing from the inside so it lasts better once the coating is gone.
Cory Mittleider: I like describing it as being baked into the cake. It’s not a nice thing added afterward like a coating that’s one to two microns thick. It is the bearing.
Allen Hall: It’s hard to think about steel and a lot of the metals used in the bearing industry as unique chemistries, but they are. There are a lot of varieties of steel, just like there are a lot of varieties of copper or aluminum.
Loren Walton: Yes.
Allen Hall: You’d think steel is just steel — we make cars out of it, airplanes, whatever.
Loren Walton: I was talking to someone who’s more into gears, and even when I spoke of a carbon-nitride version of a bearing versus a carbon-nitride version of a gear, it’s not exactly the same. For all intents and purposes it’s easier for everyone to consider it as steel — one word, means the same thing. But once you get into how much chromium is in it, how much molybdenum, how much manganese —
Allen Hall: It comes down to that, and it can be very small percentages of the total.
Loren Walton: It can make a huge difference. And then you get into the heat treat — your time, your soaking, what you do for quenching. It all matters, and everyone does it differently, so you get different results.
Allen Hall: That’s the kicker. You see a lot of discussions where it’s just, “Oh, it’s been heat treated.” As an electrical engineer I used to see it that way too. But there’s heat treatment and there’s heat treatment. It depends on what you’re doing and what the result needs to be, because you’re changing the whole crystalline structure of the steel. The way you do it and the way you quench it all matters. It’s not one size fits all.
Loren Walton: That’s the part that gets glossed over so quickly, because everyone’s eyes go to what they can see. You change an angle here or there, or the bearing type, and you can see that. It’s different when you don’t have X-ray vision to tell you where all the alloying elements are and in what percentages, and then whether you carburized it, through-hardened it, or carbonitrided it. There’s so much to it that I can see people’s heads start to spin. That’s where we say there are a lot of experts out here — you two are among them, and there are others. Engage in conversations. Ask questions.
Allen Hall: That’s a great call to action — “Cory, help me understand what’s going on.” There’s a variety of bearings out there. Loren’s with NSK, a great bearing company with tremendous history. Those are a couple you can trust. But operators can feel inundated by the guy down the street trying to sell them a bearing, and you don’t know if that’s the right solution for your two-million-dollar wind turbine.
Cory Mittleider: These are critical infrastructure assets. Let’s make sure we understand what we’re doing and why. To Loren’s point, you can open three boxes and they all look the same, but what’s inside is what really matters.
Allen Hall: It’s a tremendously difficult business. With as many main bearings getting swapped out today, over the last couple of years there have been a lot of decisions made on the fly — some correct, some really wrong.
Loren Walton: I’d hesitate to say wrong, because I think people are doing the best they can. It’s not because they’re not trying.
Allen Hall: It’s because they don’t have the knowledge in front of them, or maybe they haven’t made the call to Malloy or NSK yet to get the ground truth.
Loren Walton: What you mentioned a second ago is pivotal. There’s been enough selling that we’ve kind of gotten away from the engineering. People hear “sales engineer” and they cut off at “sales.” If we can get back to the engineering, a lot more people will improve their assets. And it doesn’t have to be just listening to Cory and me — poll the audience. There are a lot of us out here. Everybody has a different background; we all know a little about this or a lot about that. Take the opportunity to learn. I’d liken it to your personal life: you wouldn’t buy a new vehicle or a stereo system without doing your own research. You wouldn’t just listen to the salesperson and buy the first thing you see. It’s the same here. If you’re making decisions without engaging at least the top three to five people in this space, you’re doing yourself a disservice.
Allen Hall: And that’s what happens a lot, because people get pushed. There’s a timeline, especially now with the repower situation — “I’ve got to put something on now.”
Cory Mittleider: Right. And new platforms — the next-generation three, four, five, six megawatt platforms, and offshore — are having their first failures. We need to learn from it. That’s where we’ve worked with operators to participate in the teardown and collect the sample. We get clues, we mark it up, and we do a lot of the investigation — metallurgy, metrology, raceway traces — to inform us on what the problem is on that specific platform.
Allen Hall: As we get to these bigger turbines, some data is coming back on O&M costs relative to a one or two megawatt machine, and it doesn’t scale linearly. It goes almost exponentially, because everything is more expensive. Replacing a bearing on a six megawatt machine is a much more expensive ordeal than on a two megawatt machine. What should we be paying attention to and monitoring more closely on these larger machines? The new shiny turbine is great, but that doesn’t mean you don’t have to monitor and maintain it.
Loren Walton: I’d start with verifying all your original fits and clearances. We’ve had cases with a four-point mount main shaft — two main bearings — where one side wasn’t installed properly from the beginning, so it didn’t actually float. It’s supposed to be a fixed side and a floating side; now you’ve got one side that’s not floating, and you get overload. So make sure you’re set from the start. A lot of machines now come already outfitted with instrumentation — vibration monitoring, oil monitoring, different ways to start trending from the beginning. Back when we got started, that wasn’t the case. You got your new turbine and in a lot of cases it had nothing on it — you were flying blind. Now that it’s there, use it.
Cory Mittleider: That’s a good point. Specifically to bearings, something earlier versions didn’t have, and newer ones mostly do, is auto-lubers.
Allen Hall: I see more of those lately.
Cory Mittleider: That’s great from a lubrication-delivery and reliability point of view, but it’s its own little machine. We’ve heard of cases where the auto-luber failed, or ran when it shouldn’t have, or for whatever reason had very large output. So you need regular assessment of the entire system, including uptower.
Allen Hall: You’ve got to monitor everything that’s uptower.
Cory Mittleider: It’s its own little machine. It requires its own maintenance. If you’re relying on it, you’ve got to check it.
Allen Hall: As we move into these larger machines and see more of them deployed, what are the useful things you should be doing in that first year to make sure your bearing is working optimally? Is it just checking vibration levels? Is it getting uptower and doing a quick sweep to confirm the grease isn’t oozing out where it shouldn’t be? Is it that simple?
Loren Walton: Having a regular maintenance interval definitely helps. Even getting grease sampling to understand your baseline levels after the first six months and the first year. In a lot of cases the turbines are under a couple-year warranty, so maybe you don’t have as much access. But as much as you can, getting a baseline is huge, because you’re going to want to compare later. You’ll want to say, “Okay, I took this grease sample — what does it mean? Does it normally run that high or not?” Same for vibration, getting the trending. For main bearings in general, more grease is better than less, because you can never quite get it all out when you’re regreasing. So a lot of that first year or two is about getting a good baseline so you know what you’re actually expecting, and what it means when you take a reading in year two or three.
Allen Hall: What does a grease sample look like in terms of the response you get back? I take a sample, send it to a lab, and it comes back with — what? Is it “good or bad,” or a bunch of chemical numbers about composition and dirt? I’ve never seen one.
Cory Mittleider: It’s a matrix. You can request different versions, but probably ten or fifteen different elements they give you numbers on, in parts per million. Iron and brass will be up there.
Allen Hall: So if you see something floating in the grease —
Cory Mittleider: Silicon, phosphorus, water.
Allen Hall: Water would not be great.
Cory Mittleider: No.
Allen Hall: So those reports come back, and I assume there’s more knowledge needed to interpret the results. What do you do?
Loren Walton: We have some guidelines we share with our partners and customers. If you see a certain amount of parts per million of copper, ferrous material, or the like, we can say, “That’s worth monitoring for a while,” or “You should probably purge it, try to get it out, and see if it stabilizes.” We get those questions and respond in kind. There’s definitely help available. If we work together, we typically have a lot more success. A lot of people right now feel like they’re trying to work in their own silos, and you don’t have to do that. You don’t have to be the subject-matter expert for lubricants, gears, bearings, and everything else. You can reach out to experts who can help, and hopefully that frees up your time to assess and work on other things.
Allen Hall: The turbines are so complex today. It used to be you could have one person on site who knew most of what was going wrong, because they’d made thousands of these things — there was a legacy. When you get to six megawatt machines, where you don’t have a lot of history, particularly in the United States, there’s really no one to ask. You’d better find somebody who knows what they’re talking about.
Cory Mittleider: And the operators are responsible for multiple systems — six or seven or eight systems they’re looking at. We can help with bearings; we’re niche and focused on that. If we can take that off your plate, now instead of six systems you’ve got five to worry about.
Allen Hall: That’s key. There are experts out there, and one thing the podcast is trying to do is give those experts a chance to talk so you know who to ask. Your phones should be ringing right about now, because it’s repower time, and it’s main-bearing repair and replace time, pitch-bearing repair and replace time. There’s a lot of bearing activity going on. I always say call Malloy Wind if you need somebody who really knows their stuff, the technology, and what’s going on internally. How do people get ahold of you two if they have questions? What’s the easiest way?
Loren Walton: I try to be at most of the industry events. We usually hold a booth. And my email, my phone number — I’m on LinkedIn, so reach out there. After our last discussion I had a few folks reach out, actually mostly from other countries. It was interesting; we heard about a few issues before they even hit the US. Some folks were having problems with the larger turbines, and we were able to get our teams in Brazil and Spain involved right away. Then once it started cropping up in the US, I could say, “Yeah, I already solved that.” We can put my email in the show notes.
Allen Hall: We’ll put it in the show notes for sure. And Cory, how do people get ahold of you?
Cory Mittleider: I’m pretty active at the events — ACP, and the Drivetrain Reliability Collaborative is another one we had a couple of months ago. Email, phone, and I’m pretty active on LinkedIn. I’ve had similar experiences to Loren, getting contacted from other countries across the globe. It’s fun to investigate problems and share results in the technical articles on our website, and have people send me a picture of an article I wrote and say, “Hey, let’s talk about this.”
Allen Hall: Your articles are great. Check out malloywind.com — just Google it and it’ll come right to the top. If you have bearing questions or something you’ve seen, that website is a great first place to get some answers. It’s very helpful. Well, Loren and Cory, I love having you on the podcast. We need to have you on more, because there’s a lot going on in the bearing world.
Loren Walton: There are things we didn’t even touch on today.
Allen Hall: You’re always welcome back.
Loren Walton: Awesome. Appreciate it.
Allen Hall: Thank you.