Bearing vibration signal modulation

What is vibration signal modulation?
Modulation occurs if one signal modifies another. We call the modifying signal a modulator. The modified signal a carrier. Modulation changes a certain characteristic of the carrier signal, like frequency or amplitude.
In this video, we will focus on amplitude modulation. The carrier and the modulator waves are not simply added together if amplitude modulation occurs. Instead, the modulator envelops the carrier wave.
Amplitude modulation in vibrations
Amplitude modulation is very common in vibration analysis. For example, it often happens to bearing fault frequencies.
Let’s examine a ball bearing with 7 balls. The inner race rotates at the speed frequency of 25 Hz. The outer race is stationary. Each time a ball passes a fault on the inner race or the outer race, it generates a shock.
Modulation of shocks
What must happen in a bearing for modulation to happen? It could be due to unbalance of the shaft. This unbalance acts on the bearing elements with a centrifugal force Fc. As the shaft rotates, it pushes more in the direction of the force and less in the opposite direction. Each time a ball passes a fault on the outer race, the force creating a shock is different.
Shaft weight
There is another influence which may modulate the amplitude of shocks, weight. Because of gravity, the weight of the horizontal shaft acts with a downward force Fg. It puts more pressure on the bearing balls under it than above. Shocks produced there are stronger. This is true for both races. In this case, only inner race shocks are modulated. The outer race shocks are not modulated. It is because the position of each outer race fault relative to Fg is constant.
Unbalance and shaft weight
If there is unbalance and a force due to the shaft’s weight, they add together. Modulation happens to the outer race shocks and to the inner race shocks. It is because the sum of the forces Fs changes as the shaft rotates. Therefore, forces acting on faults on both races are variable.
We must note that this almost never happens. The centrifugal force from unbalance is usually much larger than the downward force of gravity. When examining bearing faults, unbalance is usually behind modulation. Effect of the shaft’s weight is negligible compared to the effect of unbalance.