Failure mode ISO 15243: 5.4.2 Subsurface initiated fatigue


Failure mode ISO 15243: 5.4.2 Subsurface initiated fatigue

Is this normal Fatigue Failure, how many of you get to see a bearing actually fail from normal fatigue? Usually we come across bearing failures/damage due to secondary factors such as misalignment, over or under lubrication, imbalance, resonance and poor installation……

This is also a great example of how important knowing the asset you are monitoring is as to know when to remove the asset from service, ensuring that the client has got the maximum life out of the asset for the associated risks.

# You can’t analyse what you don’t know or understand #

 

This Case Study Application:

This is a DC motor that is direct coupled driving a gearbox.

 

 

Summary:

We have been monitoring this motor since 2006 and in May 2017 a subtle change in the PeakVue level was noticed, closer monitoring was initiated and a bearing inner raceway frequency was found. Next in June 2018 there was a further step change that prompted the decision to remove from service as we felt the risk of failure was too high.The motor was overhauled at the next opportunity, this was in July 2018.

 

 

Vibration Data:

Figure 1 is the Velocity spectrum, there are no indications of any defect in this data.

Fig 1:

 

Figure 2 is the Peak Acceleration 10 KHz FMax trend from October 2017 until change out in July 2018, this displays an increasing trend.

Fig 2:

 

Figure 3 is the PeakVue Max Peak Acceleration trend from October 2017 until change out, this also shows the increasing trend.

Fig 3:

 

Figure 4 is the PeakVue spectrum. This shows the running speed activity and a beautiful text book bearing ball pass inner raceway defect frequency with harmonics and sidebands at 1 Order.

Fig 4:

 

Figure 5 is the PeakVue time waveform, this shows a distinct periodic impactive activity.

Fig 5:

 

Figure 6 is the Auto correlation of the PeakVue time waveform. Auto correlation is great tool for distinguishing periodic activity within a time signal. This data shows us that there is a defect that is modulating by 1 Order. Therefore a component on the motor shaft, rotating with the motor shaft has a defect.

Fig 6:

 

Figure 7 is a zoom in on the Auto correlation of the PeakVue time waveform. From this we can see that the 1 Order activity is side banded by the inner raceway defect frequency.

Fig 7:

 

 

Images of the bearing defect

Image 1 is of the bearing inner raceway. This shows the track of the rolling element in the race way, due to the DC drive, also within the arrows there is the defect.

Image 1:

 

Image 2 is a microscopic image of the defect. Has anyone else pulled a bearing with this type of defect?

Image 2:

 

 

Failure Mode:

Suspected failure mode is ISO 15243: 5.4.2 Subsurface initiated fatigue,

The images show that this bearing had reached its end of life, the cyclic stress changes occurring beneath the contact surfaces had initiated subsurface micro cracks, and this would have been in part of the bearing at the maximum shear stress. We are at the point where the crack has propagated to the surface and spalling has started to occur.

 

 

Final Note:

A special thanks to James Pearce for the data and working with me on the analysis.

 

 

A reliable plant is a safe plant

…..an environmentally sound plant

….. a profitable plant

……a cost-effective plant

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