Let’s talk about the mechanical process of bearing damage, starting with the generation of elastic waves. The energy emitted from a bearing surface will move from a higher frequency to a lower frequency as the damage develops.
When a subsurface crack network has been developed and the rolling element passes over it, very weak elastic waves are emitted. The emission of elastic waves is actually caused by small surface irregularities colliding with loose parts.
Figure 1: Small surface irregularities collide with loose parts
These waves typically travel at the speed of 20,000 feet per second through steel, which is 18 times slower than the speed of sound. The waves are normally a mix of shock and vibration. Picture the plates of the earth when they collide and cause earthquakes.
Figure 2 shows Stage 1 bearing failure. These signals are extremely weak and usually masked by the forcing frequencies, such as those for unbalance, gear mesh, and misalignment.
Figure 2: Stage 1 bearing failure
Figure 3 shows more severe bearing damage. In this case, the rollers are colliding with the sharp corners of the spall. The energy is now far greater than it was before, and the waves are significantly stronger. The energy in the waves is strong enough at this point to excite the bearing in the natural frequency, when you start to see a haystack in your spectrum.
Figure 3: A spall with sharp corners has formed
In the final stages of failure, spalls start to wear down, and there is no collision of the rolling element in the raceway. The rollers simply follow the shape of the damage.
Figure 4: The rolling elements follow the shape of the damage
In places with cold winters, the streets often get potholes. When the potholes are brand new after a freeze and a thaw, the edges are very sharp. If you hit one with your car, the car’s shock bottoms out and you experience a big jolt. By spring, after a number of heavy vehicles have rolled over the pothole with their loads, the sharp corners have rounded off. When you hit that pothole with your car, you don’t bottom out the shock or cause a jolt; the car just bounces up and down a bit. There is a difference in energy.
When a crack network starts to form in a bearing, you get a series of high-frequency shocks, shown in red in Figure 5. They are stress waves that typically range from 15 to 40 kHz, and it is broadband, not a single frequency. Every time the rolling element passes the bearing damage, it will create a stress wave, which creates a broadband of energy.
If you watch that red floor noise rise, you can find the BPFO, for example, 2.7 times per second.
Figure 5: A crack network forms
We start to see bearing natural frequencies as parts start coming loose.
Figure 6: Loose parts
When the rounding of the corners of the spall starts to happen, we start to see random frequencies. When the damage is pronounced enough, we will start to see actual bearing fault frequencies.
Figure 7: Spall
Based on the frequency at which we are seeing the damage, we are able to assess what stage of failure or damage that the bearing is in.
