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i would like to get your input on this
i'm monitoring a gearbox with an input shaft of 7525 RPM
output shaft 1800 rpm with a 4,181 gear ratio
22/92 teeth

last time i have run analysis on this gear box i got some reading on the FFT for input shaft of 251 Hz,502 Hz,751Hz,
i got almost the same reading on the potput shaft but with low amplitude.
i would like your comments on this thankyou very much


Your problem is interesting.  You have even harmonics of the input shaft speed on a reduction gearbox.  In the time domain, a saw-tooth signal would produce the even harmonics, and since the problem is occurring at the input shaft frequency, then the problem should be on the input shaft.

However there are a few questions to be answered:

1) Check for sideband frequencies of the input shaft speed (125 Hertz) around the gear mesh frequency (2760 Hertz) and on the driver machine bearing (is it a turbine?).

2) Is the amplitude at input shaft speed high?

My guess based on the limited information available is the coupling on the input shaft or the input shaft itself.  Check for a locked/fixed coupling, check the input bearing clearances and check the input shaft to see if it is bent and/or cracked.

C. Hugh (www.Hatch.ca)


1)cheking the the rest of the information today
I got this, the first side band at the input shaft
a)2636 Hz,2880Hz both at the same amplitude

b)5390Hz, 5645Hz again both at the same amp

and the third SB
d)yes the driver machine is a turbine

2) yes the AMPLITUDE of the input shaft is high is about
20 db above bckground noise level..

knowing that fairly high amplitude sidebands around GMF suggest gear eccentricity,backlash or non parallel shfts.
also knowing that high ampl= gear probl..
i know i have a problem but i just want to look at it deeply before getting to the conclusion .
again any idea  help will be very appreciate it
thankyou once again.

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