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Vibration Case Study No. 17 ­ Axial Screw Compressor Cracked Case at JournalBearing

In vibration analysis sometimes it's not so much knowingwhat the problem is, but rather that the signature justdoesn't look right.

This first article is dedicated to all those new vibrationanalysts out there that are taking vibration readings forthe first time and come across their first situation wherethe famous words are uttered: "What is that? I don't knowbut it just doesn't look right." This one's for you...

The picture to the left shows the inside of a rotary lobescrew compressor from the "suction side" of thecompressor, and looking toward the "discharge end" ofthe compressor. There are two shafts which fit inside,each one shaped like a screw, and both messingtogether; the way in which they mess conveys the gasstream from the suction side to the discharge end andincreases the pressure of the gas stream along the way.The higher pressure gas stream discharges through thebottom middle hole in this picture. The top two holes oneither side are where the discharge end journal bearingsare located for the two shafts.

This rotary lobe screw compressor had just been overhauled for its second time after having been taken out ofservice due to a very high increase in vibration (see Figure 2). The overhaul went smoothly without a hitch.When the unit was started back up and put on­line the vibration dropped back down to a normal level. Byvibration level standards, the problem had been solved because the vibration was back down to its historicallylow and typical level. However, the vibration frequency spectrum was all wrong, and the waveform pattern wasall wrong (see Figure 3). It's moments such as these that makes one wonder whether the data is right or whetherthe vibration analyzer is wrong. What was intimidating to say the least was the thought of telling themaintenance manager that something was not right in a machine that was painstakingly overhauled; and thennot even being able to tell him what that "something" was. So at this moment of truth I remembered a phrasethat a man by the name of Art Crawford once said at a conference presentation, he said "The little black boxnever lies." The little black box being the vibration analyzer.

This is the time known as "the leap of faith," because you're all alone; and there will be hell to pay if you'rewrong. It is at a point such as this that one had to step back and really look at the data. The first question asked

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was: "What's not wrong with the compressor?" We knew what a bad journal bearing looked like, what a rublooked like, what a thrust bearing going bad looked like; none of these things were happening. So if they weren'tgoing bad, then what was? Well......." I DON'T KNOW, BUT IT JUST AIN'T RIGHT!" So the compressor wastaken off­line and disassembled.The journal bearings looked good, thethrust bearings looked good, the rotorlobes looked good, the balance drumslooked good; everything looked good.Well of course they all looked good,none of that was showing up on thevibration diagnostics! It was reallylooking like that "little black box" finallyblew the call. The compressor bodywas placed into the parts washer priorto reassembling the whole thing overagain, it looked as though crow wasgoing to be served for dinner. When thecompressor came out of the partswasher and the mechanics were gettingready to insert the journal bearings,they noticed two small cracks. Thecracks were located at the thin sectionof the compressor body (see figure).This small detail was almost overlookedbecause dirt and grease had hidden thedamage. The cracked sections of thecompressor body eliminated the pressfit which held the journal bearings inplace, and therefore the bearings were actually loose and not providing any support to the two rotary screws. Ifthe unit had been reassembled and placed back into service it is very likely that the same problem would havepersisted.

Atypical

vibration signature for the male drive rotary lobe is shown at the left. This signature is for a good runningcompressor, and is unique in that the four lobes create multiple harmonics throughout the frequency range. Thevibration software easily highlights where all of the harmonics are located with the red squares. The timewaveform is also unique in that there is basically four sinusoidal waves per revolution. The black vertical lines inthe lower graph indicate each revolution of the drive lobe.

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The

compressor had been recently overhauled and when it was put back on­line there was some difficulty with waterand dirt contamination of the lubrication system during start­up. It was obvious that the compressor rebuild hadbeen ruined and that it would have to be overhauled a second time. A follow up failure analysis had revealed thata critical filter had ruptured and allowed all sorts of debris into the system; subsequent flushing of the systemhad only introduced additional contaminates and water . The journal bearings had been wiped. The red square inthe top graph indicates the 4X fundamental frequency of the male drive screw, note the significant difference toFigure 1. The waveform has completely changed too.The compressor was rebuilt and put on­line for a second time. Although the vibration level had returned tonormal levels the signature did not look right. Note how the harmonics of four times running speed (4X) aregone. The dominating frequency in the spectrum at the top is two times running speed (2X). The waveform atthe bottom is also dramatically different, showing two sinusoidal waves per revolution of the male drive lobe.Armchair quarterback vibration analysts will of course say that these signatures are signs of "classiclooseness," but it wasn't quite that obvious under the circumstances. There are times when you just aren't goingto believe what you see, and you're just going to say something like "it just doesn't look right." And that remark,although scary, is good enough.

When the compressor was opened up for inspection it revealed that the two discharge end journal bearings wereloose in the compressor. The compressor body had cracked where the journal bearings were pressed in; thusallowing the two bearings, one on either rotary lobe, to literally rattle inside the compressor (see above close upfigure of discharge end of compressor, click on where you think the cracks are located, and if you're correct aclose up of the crack will appear).

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A new

compressor body was used, and the compressor was rebuilt for the third time. This time the vibration signaturereturned to its normal configuration.

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