Let’s look at five case studies involving bearing faults in oil-free compressors. The bearings were measured using a combination of HD technologies and a DuoTech transducer. The first four cases are from the same compressor.
We were called to the plant in March, April, and July 2019 to perform portable measurements. In August, an online system was mounted.
Figure 1: Portable versus online measurements
When the readings became high, they decided to stop the compressor, so the compressor was not in operation. In December, they replaced all parts of the compressor, so the measurements at the end are of new elements.
The spectrum shows a clear BPFO representing a fault on the outer race. And the bearing was found to be damaged.
Figure 2: Spectrum showing BPFO, damaged bearing
The next case also included portable and online measurements, but of the low-pressure element on the synchronization side.
Figure 3: Portable versus online measurements
The spectrum showed a clear BPFI with harmonics, indicating faults on the inner race.
Figure 4: Spectrum showing inner-race fault
Next, we move to the high-pressure element on the drive side of the same compressor.
Figure 5: Portable versus online measurements
The spectrum showed BPFO signals, and the outer race of the bearing was found to be damaged.
Figure 6: Outer race bearing damage
The readings were very high on the synchronization side of the same compressor.
Figure 7: Portable versus online measurements
The spectrum showed BPFO signals, and the bearing was found to have outer-race damage.
Figure 8: Outer race bearing damage
To summarize, the readings on this compressor were high at the beginning of 2019, and the machine was stopped in September, meaning there were eight or nine months of warning time. On all of the time signals above, you can see that the readings extended up into the red zone at their highest. The readings for the new bearings were in the green zone.
The next case comes from Scania, which is a large truck manufacturer in Sweden. They installed a large oil-injected compressor, and after a short time, they saw high readings on the low-pressure element on the drive side. This was in September 2019.
Figure 9: High readings on the low-pressure element on the drive side
They kept the compressor running and saw an increase in the readings one year later. At the beginning of 2021, they replaced the element, and the readings dropped dramatically.
Note the position of the readings in relation to the green, yellow, and red indicators to the sides of the time signal.
The frequency domain shows a clear outer race fault.
Figure 10: Frequency domain with outer race fault
At the beginning, we could only see ball spin frequencies.
Figure 11: Color spectrum overview showing BSF
But by the beginning of August 2020, we could also see signals generated by the outer race.
Figure 12: Color spectrum overview showing BPFO with harmonics
By the end, there were signals all over the place. The divide between red and blue at the end shows the time at which the element was replaced.
So the warning time was quite long. The company saved about $120,000 by replacing the bearing before it completely failed.
Figure 13: Damaged bearing
