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Malloy Wind and NSK on Main Bearing Failures

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.

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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...