The problem with pitch bearings is that they don’t move much, just back and forth in a narrow angle. This leads to metal-to-metal contact between rolling elements and races. It is a frequent possibility that leads to false brinelling. It’s a kind of damage from fretting and appears as wavy, chipped, or corroded damage on metal contact surfaces of power-transmission devices. It’s a problem in assemblies with mechanical subsections that move slowly and are subject to high vibration.
“False brinelling often develops on wind-turbine parts where bearings in pitch-control mechanisms can remain in a set position for long periods,” says Charles E Harris chief engineer with Plymouth Machine Integration. “In these situations, the lubricant film is gradually displaced, allowing metal-to-metal contact. The vibration in the joint causes this contact to produce physical damage. Purposely inducing small amounts of relative motion can be helpful, but the benefit is limited because the lubricant can be pushed aside in either direction,” he adds.
Consider the blade pitch control, suggests Harris. “Blade-pitch angle is set to match aerodynamic forces with wind speed and load demand. Changes to blade pitch are infrequent and small, leading to damage to the bearings supporting the blade. Intentional motion can be imposed on the system to promote lubricant flow,” says Harris. This motion is called dither. The minimum motion required is a function of the spacing of rolling elements within the bearing. A greater range-of-motion decreases the turbine efficiency, while too little range-of-motion in dithering does not reintroducing lubricant between the loaded surfaces.
Harris suggests a solution to false brinelling. His company has made a provisional patent application for a design that uses two concentric one-way bearings, installed so that their actions are opposed. With a modest amount of oscillatory blade motion, it induces large movements of the intermediate race — and in both sets of rollers. That action evenly distributes the lubricant.
It works like this: The motion in the middle race is from a one-way clutch in one of the two bearings that holds that race motionless with respect to first the inner, then the outer race, depending on the direction of rotation. In this way, the design avoids false brinelling by maintaining a lubricant film and preventing adhesion of surface asperities. The video at the link shows how a one-way clutch works: http://goo.gl/YtL4ff
Harris points out that the design doesn’t resolve issues from assemblies that lie motionless for substantial periods between use. The design needs oscillatory motion of a magnitude large enough to engage the one-way clutch, oscillatory motion that’s either natural to the operation of the machine or imposed by the controls.
Readers may also be interested in the classic resource on the topics here at: tinyurl.com/NREL-akronu
A LITTLE BACKGROUND: THE DIFFERENCE BETWEEN FALSE VERSUS TRUE BRINELLING
False-brinelling damage leaves series of divots that resemble those of true brinelling, which is permanent indentation of a component’s hard surfaces. However, true brinell damage is permanent material deformation (without loss of material) and occurs during one load event. In contrast, false brinelling is material wear or removal that happens over time because of vibration and light loads.
More specifically, true brinelling, named after the Brinell hardness scale, leaves repetitive series of indents on the working surfaces of mechanical parts. Most common on hydraulic pistons and bearings, it happens when a material surface failure caused by Hertz contact stress that exceeds the material limit — as when a heavy load impacts a small surface area, for example. Ultimately, brinelling causes chattering, vibration, and other forms of wear.
In contrast, false brinelling happens when a bearing design only redistributes lubricant during large rotations of all bearing raceway surfaces. When such bearings only move a little bit, small oscillations or vibrations can squeeze lubricant out of loaded spaces. Then wear begins and only accelerates with move vibrations. What’s more, sometimes small bits of material break off the raceway and oxidize. This material further abrades the damaged surface and accelerates wear.
For further information, visit plymouthmachineintegration.com.
Also download the classic resource, Tribological Challenges in Wind Turbine Technology
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