Vibration has been defined as a form of wasted energy. Generally, the greater the vibration, the greater the cost of energy, usually measured as motor current, or amperage. Equipment with reduced vibration is not only quieter, but it lasts longer and costs less to operate.
Vibration is measured by relating amplitude(amount) with time(frequency), or how frequently it moves. Vibration is measured in 3 ways: displacement, velocity, and acceleration. Displacement is a measure of the distance the piece of equipment is physically moving. We use inches for convenience, and go to three decimal places. [.001" = 1 mil]
Velocity is a measure of how fast the equipment is moving, and is measured in in/sec. [As compared to miles/hr for larger distances in an auto]. Acceleration measures the change in velocity over time. It is measured in g's/sec, and is very useful in detecting the amount of high frequency "noise" generally associated with bearing wear. Displacement is used most often to indicate the amount of imbalance in a rotating assembly. The physical movement can then be offset by adding counterweights. It is important to point out that when someone tells you the displacement measure of vibration, such as .004"(4 mils), you cannot determine the severity unless you know how fast the equipment is rotating. Four mils on a 600 rpm motor is normal, while 4 mils on a 3600 rpm motor is cause for shutdown.
Velocity is the generally accepted industry standard for vibration measurement of the overall condition of rotating equipment. The amplitude of .24 in/sec is the same severity for any speed. The only variations are for different classes of equipment. There is an ISO spec #10816, that specify values for good, fair, and poor condition.
Good vibration monitoring equipment measures data in all 3 ways. A well trained analyst not only needs to look at all 3 measures of amplitude, but to review the vibration spectrums, time waveforms, and other monitoring data. The measurements are collected in all 3 planes: vertical, horizontal, and axial. Trending the amplitude data tells you the vibration is changing, while analyzing the data spectrums tells the trained analyst why it is changing.
Knowing why will allow for corrections to be made(such as a bearing change), before a catastrophic failure results in expensive repairs and downtime. Monitoring the health of your equipment can be compared to monitoring the health of your body. Checkups are always justified, especially for the older equipment!
Once again, investing in PM, will save you money!
