18.125X (21750 CPM) SPIKE ON CAT G398 ENGINE

[Yats]

Routine vibraton readings were taken on a caterpillar G398 engine generator set. This unit runs at a constant speed of 1200 rpm. An increase of over 100% in the overall amplitude of vibration was detected for a reading on the outboard axial (non drive end radiator/fan side) of the engine. Normally the overall amplitude is 0.11 ips. Our last measurement is .25 ips. A mild increasing trend was observed. Analysis of the spectrum indicated a spike at 18.125X at o.14 ips. Also 1X spike at 0.17 ips was detected. Sidebands at 16.625X, 18.625X, 19.125X and 20.375X were detected also. The 18X spikes first appeared in Jan 2003. Two other similar engines do not show this behaviour in the spectrum. The outboard axial reading is taken at crankshaft level. The probe is positioned axially in the area of the water pump.The spectrum for a horizontal reading taken at this point show the 18.125X spike present. The spike is not evident on the inboard. I have no doubt a problem has developed at the front of the engine but I would like to know some possible sources of the 18.125X spike and its sidebands. Can anyone help?

 

[MachineryWatch]

Yats,

Are there any rolling element bearings at this end of the engine? It may be a bearing defect frequency (or harmonic of a fundamental bearing defect frequency) with the typical sidebands that typically surround the defect frequencies when they appear. However, the sidebands seem to be spaced at 0.5X RPM rather than 1X RPM so could the bearing be on an accessory drive that runs at 1/2 the crankshaft RPM (or camshaft?)?

I am unfamiliar with this particular engine so you may have already eliminated these possibilities.

Skip Hartman

http://www.machinerywatch.com

 

[Yats]

As far as I know there are no rolling elements closeby here.The accessory gears etc run on sleeve bearings.I agree with the sideband at .5x. I have to look at the drawing to determine which ones turn at .5X the crankshaft speed. Drawings are difficult to source here.

 

[GregLocock]

How many teeth does the cam gear and drive gear have? ditto idler gears. Less likely, how about the oil pump. How many cylinders does this thing have, and what is the configuration?

Cheers

Greg Locock

 

[Yats]

Thank u all for the input and concern. The cam gear has 96 teeth. the crankshaft gear 48. main idler 71 teeth. oil pump gear 32 teeth. water pump gear 25 teeth. the oil pump is gear type with 12 teeth. There are two balancer gears but we are not sure how many teeth (suspect may be approx. 40 and 35 respectively). Also the radiator fan was changed from a 16 blade fan to a 12 blade on this engine. The fan assembly is located at the front of the engine where we take the readings. Also, I have noticed the 18X frequency on a similar engine but the amplitude is not as high. there are no rolling element bearings close to the point where the reading is taken. The fan drive assembly does have two pillow blocks but these checked out ok.

 

[Yats]

the engine has 12 cylinders. it is a V12 engine.

 

[enginesrus]

Is the engine generator assembly attached to a skid? If it is a frame work such as what the radiator and the fan assembly is mounted to. Then it may be some sympathetic vibration harmonic due to something like a mounting (weld) comming loose. It may not be the engine at all.
You mentioned that this is an axial frequency.
How many hours are on this engine?
Have you eliminated a chance that it could be something to do with the generator? Like thrust bearings, how does that vibration compair with what ever frequency the generator is putting out? Have you tried varying the engine speed? What happens?

 

[enginesrus]

So did you find anything?