Amplitude Demodulation Applied to Bearing Analysis

If a rolling element bearing has a defect such as a crack in one of the races, there will be an impact created every time a rolling element passes over it. These impulses cause the bearing race to “ring” at its natural frequency, just as a bell will ring when tapped. The race is not free to vibrate very much because it is held in the bearing housing, so its ringing is very highly damped. This gives rise to a series of very, short “pings” which occur at the rate of the ball passing, as illustrated below:

ThThe resulting waveform is actually an example of amplitude modulation, with the ringing frequency corresponding to the carrier and the envelope of the pings is the modulating signal. If the signal is passed through a spectrum analyzer, there will be almost no energy at the ball pass frequency in the spectrum, but there will be a component at the natural “ringing” frequency and there will be sidebands around it spaced apart by the ball pass frequency. In practice, it is usually very difficult to see these sidebands in a machine spectrum, most due to extraneous noise components that mask them. Also, the ringing frequency is usually quite high, sometimes over 10 kHz, and these frequencies are difficult to pick up reliably.

If the signal is demodulated by being rectified, the result is a series of impulses spaced apart by the ball pass frequency, as shown below. The rectification turns the negative-going parts if the waveform to become positive:

Rectified Ping Signature

The signal is then passed through a low-pass filter to remove the fluctuations due to the ringing frequency, and smoothing the pulses. These pulses, when subjected to frequency analysis will produce a strong component at the ball pass frequency, along with harmonics of it. This is because they have more area under them, contributing more energy at their fundamental repetition rate that is the outer race ball pass frequency of the bearing.

Smoothed Rectified Bearing Pings

In order to separate the high frequency ringing of the bearing race from the rest of the vibration signature of the machine, the accelerometer signal is passed through a high-pass filter tuned to about 2.5 kHz. This filter removes all the lower frequency components due to rotation rates and their harmonics, and effectively isolates the modulated natural frequencies. This results in a very great increase in signal to noise ratio, and is one of the main reasons for the sensitivity of demodulation in detecting small bearing defects. This is the most important benefit of amplitude demodulation as a machine diagnostic tool. The block diagram of an effective amplitude demodulation scheme is shown below.