Blade Bearing Cracking Rings Part 1: What to look for
By Cory Mittleider on 9/28/21
Background
Cracking rings on blade bearings is an important topic with many facets. This article will be broken into 2 parts. The first part (this article) will show failures, discuss the importance of early identification and appropriate response to this failure mode. Part 2 of this topic will expand on the root causes of these failures and solutions to them. Figure 1 shows a cracked ring typical of many we have seen this year.
Blade bearing with crack in outer ring near lifting plate.
Components of a blade bearing.
Current Situation
Multiple platforms are experiencing cracking outer rings. Some models are seeing first failure after 12 or more years, but some are experiencing this problem as early as only 3 years online. Replacement of blade bearings is a large task including downtime, crane and labor support scheduling, replacement bearings and ancillary parts such as new hardware. It is critical to understand the cause of failure and make sure that the replacement bearing is well suited to solve them.
For the turbines that are having their first failure past mid-life, installation a similar bearing with targeted upgrades should be sufficient. For the turbines that are having these failures with very short operating life it is clear replacement with the same bearing will result in multiple expensive replacements in the life of the turbine.
From the field we have seen cracks in multiple places in the bearing. Below are some examples.
Cracks through filling slots:
The filling slots are required to assemble this type of bearing. These filling slots are sometimes aligned and sometimes staggered. We have seen cracking through both types as shown in figures 3 and 4.
Cracking through aligned filling slots
Cracking through offset filling slots
Cracks next to lifting plates:
Some turbines use lifting plates attached to the bearing and others use stiffening plates. It is common on some platforms to see a crack just next to these plates as shown in figures 4.
Crack next to lifting plate
Cracks in other bolt holes:
Some applications are also having cracks in other locations with less obvious reasons for initiation of crack. Figure 6 shows a crack originating in a bolt hole with less externally obvious reason for initiation. The crack has even propagated through the nut.
Ring crack also with cracked hardware
Consequences
Cracking rings is the most dangerous form of failure on blade bearings. The result will eventually be the bearing splitting and the blade falling off the tower or even being thrown. There are many risks if this happens including both personal safety and asset damage.
Some turbines are known to be more prone to cracking rings than others. These turbine types should be monitored regularly to ensure if a crack forms it is found as soon as possible. If a crack is identified on a blade bearing the turbine should be immediately shutdown until the bearing is replaced.
Crack initiation and identification
For reasons explained in part 2 these cracks typically form within the outer ring bolt hole behind the blade side raceway as shown in Figure 7.
Typical crack initiation point
It is very difficult to identify a crack initiated in a bearing until it propagates to the surface. The first place you will be able to see this crack externally will be between the bolt hole and the seal on the blade side face as shown in figure 8. It is important to recognize that when the crack is visible here it is has already grown quite a bit from initiation and the turbine should be shut down. Also the speed of crack growth is non-linear, so as the crack grows the growth rate will accelerate because the material cross-section has been reduced.
Secondary failures
As the crack progresses through the outer ring of the bearing there will be additional loads placed on other components leading to secondary failures. Figure 9 shows a secondary failure of cracking hardware. This one is very common.
Cracking hardware
Figure 10 shows a second crack that occurred in a bearing we disassembled. This crack was about 165 degrees away from the primary crack and this turbine was very close to have the outer ring split completely in half which would have certainly led to the blade falling from the turbine. This discovery illustrates the risk of operating a turbine with a known cracked blade bearing.
Second crack found on bearing with 1 crack already opened
Risks
There are many risks associated with this cracking ring failure mode and why we must recommend that a turbine be shut down immediately upon discovery of a crack. The largest is the risk of personal safety of the technicians or even the general public depending on the location of the turbine.
There are also additional risks to the turbine itself, such as additional load on other systems, cracking the hub structure, or the blade striking the tower sections or nacelle.
Next steps
The first thing to do after finding a cracked blade bearing is to take the turbine offline eliminating the risks described above.
Replacement
The next step is scheduling replacement, but it is important not to simply reinstall the same part and expect a different result. Understanding the root cause of failure and selecting an upgraded bearing that is targeted at solving that root cause is critical to long term confidence that you will not need to revisit the same turbine again.
For replacement bearings please contact Malloy. We have already investigated the root cause of these failures on multiple platforms and integrated solutions to them in the blade bearings we stock. If the failure is from a turbine model we have not already investigated, we would like to work with you to identify the root cause and apply the appropriate countermeasures.
Articles about other blade bearing failure modes can be found below:
Cage failure --> **READ HERE**
Ring cracking Part 2 --> **READ HERE**
Raceway edge wear and fracture
False brinelling
For more information on this topic or to discuss failing bearings in your fleet please contact me at 605-357-1076or email atcmittleider@malloyelectric.com
-Cory Mittleider