Machinery Lubrication

Reliable Plant | www . reliableplant.com Machinery Lubrication | www . machinerylubrication.com 2025 Annual 25 random signal. Signals with a higher kurtosis value have more peaks that are greater than three times the signal's root mean square (RMS) value. In vibration analysis, kurtosis is used to monitor fatigue development in rolling bearings with a simple instrument. • Signal averaging: Since signals change with time, signal averaging is important in spectrum analysis because it determines the level of the signal at each frequency. It's particularly important for low-frequency measurements because they need a longer averaging time to get a statically accurate estimate of the spectrum. Signal averaging is often used in the monitoring of a gear in relation to its rotational speed. In this example, signal averaging will show you the cyclic action of each tooth in the gear. If a tooth has a large crack, it would be detected due to its increased flexibility. • Cepstrum analysis: Originally invented to characterize seismic echoes produced by earthquakes and bomb explosions, cepstrum is used to look at the repeated patterns in a spectrum. Repeated patterns in the spec- trum are sensed as one or two components in the cepstrum with multiple sets of sidebands, which can be confusing. e cepstrum separates those sidebands like the way the spectrum sepa- rates repetitive time patterns in the waveform. Cepstrum analysis is often utilized to look at interactions between the rotational frequency of bladed rotors and the blade passing frequency. Another example is examining gear tooth meshing frequencies and gear rotational speeds. VIBRATION ANALYSIS MEASUREMENT PARAMETERS All of these vibration analysis techniques help to identify three major parameters: accel- eration, velocity (RMS) and displacement. Each of these parameters emphasizes certain frequency ranges in their own way and can be analyzed together to diagnose issues. Let's take a look at each parameter. • Acceleration: Acceleration places greater importance on high frequencies. An acceleration signal is not exclusive, however. e acceleration signal can be converted to velocity or displacement. • Displacement: Just like acceleration places greater importance on high frequen- cies, displacement looks at low frequencies. Displacement measurements are generally only used when examining the broad picture of mechanical vibrations. You might use displacement to discover unbalance in a rotating part due to a significant amount of displacement at the rotational frequencies of the machine's shaft. • Velocity: Velocity is related to the destructive force of vibration, making it the most important parameter. It places equal importance on both high and low frequencies. Usually, the RMS value of velocity (measured in the range of 10 to 10,000 Hz) shows the best sign of vibration severity. RMS is calculated by multiplying peak amplitude by 0.707. Above is an example of what accelera- tion, displacement and velocity look like on the same signal. You can see some peaks at the same frequencies, but each has different amplitudes. is is a good visual of how each parameter assigns different importance to frequency ranges.

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