At Sentient Science we computationally test models of rotating components to understand components useful life through a material science based approach. With this we create Digital Life models of wind turbines to predict health and remaining useful life of the turbines major system.
Our approach allows us to understand prior to a failure where a crack is occurring and to provide our *supplier and operator* customers with asset actions to extend the life of our machines. We also are in different industries, we started in the Aerospace Industry with money from the US Government and we have expanded into wind with currently 22,000 assets being monitored. We have also been making headway into the Rail industry with the millions of kilometers of railways that need smart maintenance.
In this presentation before we get into the how main bearings fail let’s go briefly over the why we should investigate this.
Main bearings have a high cost ranking from $150k to $300k and the maintenance is usually overlooked during annual services. Most operators put a higher focus on gearbox maintenance, but there are a number of actions that can be done uptower to prevent damage to the main bearing like hot oil flushes, measuring and correcting misalignment and thrust load control.
Moreover damage to the main bearing can spread to the gearbox as the bearing transfers thrust load instead of supporting it as it should. If you know the main bearing is failing ahead of time you can prevent the damage spreading to the gearbox.
If you know the the main bearing is failing, you can also plan ahead to obtain the best quality components at the best price while eliminating unplanned downtime. Sentient Science allows the operator to select the best components because we look at depth at the design and materials of each component and of course determine which component will have the longest life..
We want to avoid the firefight that starts once the SCADA temperature alarms are going off and it is too late.
Now that we briefly talked about the why let’s talk about the *why* let’s talk about the *how* do Main bearing fail prematurely in 6-10 years as we have observed.
Spherical Roller Bearings are the bearing of choice for most main bearings because the high load capacity and their ability to accommodate misalignments. However the ratio of radial to axial or thust load needs to be high. Thrust load should not be more than 20%--30% of the radial load, but we observe in the field that the thrust load the main bearings observe is as high as 60%. Thrust loads lead to loss of lubrication condition and uneven load distribution that in turn leads to low lambda or a low film thickness to surface roughness ratio and high contact pressure. The downwind side is overloaded with less lubrication.
Also Spherical Roller Bearings lend are inherently prone to high sliding velocities, this compounded with the loss of lubrication conditions can also lead to low lambda situations.
Moreover Spherical Roller Bearing see low RPM conditions at which there is not enough velocity for a lubrication film to develop to separate the raceways of the rollers, depending on the lubrican viscosity and the surface roughness asperity contacts may readily happen.
The most common failure progression in a main bearing is found to be wear on the inner race. Due to high sliding, uneven loads and low lambda conditions, asperity contacts occur and adhesive wear or surface fatigue wear takes place. This cause narrow and shallow wear or plastic deformation bands on the inner race.
When these wear bands occur the geometry at the microscale is no longer optimal and high pressure along the vicinity causes micropiting. High cyclic shear stresses in the near subsurface are the primary cause for this type of failure. Micropitting eventually leads to macropitting and abrasive damage due to generated debris. At this stage the failure accelerates and leads to roller and cage cracks.
The most common failure progression in main bearing is found to be wear on inner race. Due to high sliding, uneven loads and low lambda conditions, asperity contacts occur and adhesive wear or surface fatigue wear takes place. This causes narrow and shallow wear bands on the inner race.
There are however exceptions to this common failure mode. White structure flaking is observed as well as spalling on outer race. However, the cause of these failures are associated with the same mechanisms we mentioned like initial sliding, poor lubrication conditions and higher loads.
Now that we talked about the why investigate main bearing failures and how main bearing fail let me speak briefly about the Sentient Approach. We divide our work into three main buckets.
For the wind load model we take the scada data and other factors to determining the moments and forces that act on a bearing. Then we develop a bearing dynamics model to go from the macro loads to the micro loads like local contact forces and sliding velocities. Then we take these inputs from the system and bearing model, with out material characterization to determine the wear rates at various locations.
We have talked about individual bearings. However Sentient has spent a large amount of time and resources in solving the problem of running these models for the large fleets we serve. Hence we can look at the wear problem from a fleet perspective and find the turbines the components that need to be checked to apply life extension actions like comparing suppliers, , hot oil flushes, measure and correct alignmnent, and advance planning.
Why Investigate Main Bearings?
Main bearing failure often
transfers thrust loads to the
gearbox, which can lead to
catastrophic damage in the
Main Bearing Replacement
ranges from $150K - $300K. In
extreme cases can cause
upwards of $500K.
Alerted of main bearing failure after
SCADA temperature alarms are
Supply Chain Control:
Lack of control in supply chain due
to no lead time for main bearing
failures, causing unexpected
Premature Failure in Main Bearings
Main Bearing Wear Model
Three Point Configuration
Spherical Roller Bearing
INNER RACE, OUTER RACE,
CAGE, GUIDE RINGS
High Radial load capacity
Require relatively high ratio of
radial to axial load
Increased sliding due to
Loss of Lubrication
Uneven Load Distribution
Failure Modes in Main Bearings
Main Bearing Wear Model
Wear on Inner
Abrasive Damage Roller, Cage Crack
FAILURE PROGRESSION IN MAIN BEARINGS (manifests as early as 6 TO 10 YEARS)
Sentient’s Main Bearing Failure model identifies early manifestation of damage in Main Bearings and provides
recommendations to slow down damage progression, such that the Remaining Useful Life of the bearing can be extended
Asperity Plastic Deformation
Adhesive wear, Surface Fatigue
Wear Bands on Inner Race
Incubates in the vicinity of wear
Cyclic shear stresses at
shallow depths below the
Loss of geometry due to
High contact stresses, edge
Debris dents the race and
Rapidly evolves and
Main Bearing Wear Model
DigitalClone Live Main Bearing Model Strategy
WIND LOAD MODEL BEARING DYNAMICS MODEL WEAR MODEL
Loads on Main shaft
Bearing Configuration Geometry
Material Elastic modulus
Main shaft RPM
Material : Hardness, Elastic Modulus, Ultimate
Surface topography : 𝑅 𝑞, 𝑅 𝑠𝑘, 𝑅 𝑘𝑢, 𝛽 𝑥, 𝛽 𝑦
Lubricant: Viscosity, PV Coeff
Main Bearing Risk Ranking [MTOD]
Life Extension Action
Local Contact Forces
Thrust, Radial Load