Vibration wave Motion

We have seen earlier, under the topic nature of vibrations, that displacement, velocity and acceleration of a spring-mass system in motion can be represented by sine and cosine waves. The waveform is a visual representation (or graph) of the instantaneous value of the motion plotted against time.

The motion can be harmonic, periodic, and/or random. All harmonic motion is periodic. However, all periodic motion is not harmonic. Random motion means the machine is moving in an
unpredictable manner.

Harmonic Motion
Harmonic motion is characteristically a sinusoidal or some distorted version, depending upon the harmonic content. All harmonic motion is periodic, meaning it repeats at some point in time. In a linear system, imbalance in rotating equipment could generate harmonic motion. However, with many variables such as gear problems,looseness, bearing defects, misalignment, etc., such sinusoidal are not often found. It is important to understand that a sine wave is simply a plot of a circle against time. All harmonic motion is repeatable and is just one form of periodic motion .

Periodic Motion
Periodic motion is all motion that repeats periodically. This includes harmonic motion,pulses, etc. Periodic motion is any motion that repeats itself in equal time periods. For example, a misaligned motor coupling that is loose could have a bump once per revolution of the shaft. Although this motion is not harmonic, it is periodic. The time signal will have one pulse every x seconds .

Random Motion
Random motion occurs in an erratic manner and contains all frequencies in a particular frequency band. Random motion is any motion that is not repeatable. Popcorn in a popper, rain hitting a roof, and bowling pins being knocked over are examples. Random motion is also called noise. When random noise is generated by a machine, a recording of the noise played back ten times faster than it was recorded can sound like a TV set after the station has signed off the air. A time signal of random noise will contain all frequencies in a given range. The frequency spectra from such time signals will be up off the baseline , random motion in a machine is caused by severe looseness.