SELECTION OF UNITS

This area usually leads to a lot of confusion, since there are many hard headed individuals out there that believe one unit is the best for every measurement.

The use of displacement is best for low speed machinery. If I am checking a fan with a speed of 300 RPM and the displacement is 5 mils I know that this warrants a closer investigation. However, in velocity that would be 0.078 IN/S and acceleration would be 0.006g, hardly worth a second look.

The use of velocity is a good vibration indicator at mid-range speeds. If a motor with a speed of 3600 RPM has a velocity of 0.5 IN/S I look closer at it. But with a reading of 2.6 mils or 0.488g the amplitude might considered normal.

So what good is acceleration other than "inversion testing of accelerometers", which is yet to come. Well, if a gear with 150 teeth is rotating at 3600 RPM that gives us a gear-mesh frequency of 540,000 CPM. So if an amplitude of 0.068 IN/S or 0.002 Mil is recorded what's to worry? Of course, if you use the formulas you will understand the severity of the situation (you might want to grab your calculator).

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Last updated: November 24, 2002