In this amusing case a hydroplant operator claim that the incoming water pipe is about to shake to pieces. One frequency dominate at 40mm/s! Turbine is a odd version that have angle gear in the pipe with the turbine and a generator 90 deg to the pipe vertical. Free end gen. bearing is completely bad and in theory BPFO is pretty close to the freq. found at the pipe but not that hi at the bearing but existing. Some other bearing freq´s from the other brg´s in the line is also close or even closer in theory. So anyway step 1, swap definately bad generator bearing and alakazam, machine works like a dream and pipe is as silent as the other identical machine. Who needs bearing monitoring, just tune a pipe so it gets fuzzy when the bearing is bad and everybody will know :-). You can then put some animal on top that have tickly feet, like a horse. Who would have bet more than a cup of coffe on that bearing being the source of 40mm/s a couple of meters away. Talk about perfect tuning. Olov

Oli,
We have a DC motor driving a pressure roll and the tach was showing 4.0 in/sec at the first order of BPFO. The bearing housing was only .03 in/sec at the first order of BPFO. When they pulled the motor we cut the bearing open and there was a nice spall in the outter race.
Before pulling the motor they changed the tach and made sure everything was tight but the new tach was still vibrating as bad as the one they pulled. We do pay close attention to this tach now.

Ronnie

Vibration of "attachments" can be crazy at times. Was called in to look at an extruder gearbox because it would "rattle your teeth" when you stood next to it. This is a large motor/gearbox/extruder unit, mounted on a common frame, and all sitting on a bunch of large (about 8" diameter), rubber isolators around the outside of the frame. The "rattle" is at a piece of checkerplate, that is simply laid over the top of the frame. The plate has a primary vertical resonance at 1140 cpm, and a secondary resonance at 1260 cpm. The motor runs in a range from 800 to 1265 cpm. I did impact tests (2-channel with a force hammer), impacting the motor frame (horizontal, at shaft centerline) and measuring the motor, frame, and deckplate response. The motor/frame has a primary resonance at 960 cpm.

I like to use Impedance values because you have relatively large, whole numbers that are easy for folks to grasp and do mental math with (as opposed to Mobility where you have small numbers with 4 or 5 deciamal places).

The deckplate has an impedance of 68 lbf/ips at 1140 cpm, whereas the motor impedance is 1760 lbf/ips at 1140 cpm. In theory, 68 lbs of input force will cause a 1 in/sec vibration of the deckplate (at 1140 cpm), whereas, it takes 1760 lbs to produce a motor vibration of 1 in/sec. Again, in theory, if the motor has a substantial vibration of 0.4 ips (not unusual with a resonance), then the input is 700 lbs. which produces a 10 ips vibration of the deckplate. It doesn't actually vibrate this bad because the plate is 'fixed' at the edges which resists physical movement, but it tries to.

The motor is resonant at 990 cpm, so when the unit runs at 1050 rpm, it is within 7.5% of the motor resonance AND 7.5% of the deckplate resonance, and the deckplate just goes nuts.

The motor resonance is really a "system" resonance since everything (motor, gearbox, extruder, frame) move together and in phase, so it's not a real problem. But the deckplate vibration is a huge "apparent" problem, but only when you happen to be standing on it with the machine at just the right speed.

If you think this is a little hard to follow, just imagine me trying to explain this to a manager who has no idea whatsoever what I'm talking about!

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