Hi all although I've been reading your posts for years, i've decided to enter your holy sanctum and ask the "higher powers" out there to look at some data taken on an speed increasing gearbox. This gearbox have rolling element bearings(23036cc c3) on the input and sleeve bearings on the output.The gearmesh frequency amplitude is very low but the spectrum is dominated by an frequency lower than the gearmesh showing sidebands at the bullgear(input shaft) running speed frequency. Its not an fractional frequency and did not show in the old gearbox. This is a new gearbox the old one after 25 years service started to show geartooth wear.
Input speed 1495 Rpm
Output 6300Rpm
bullgear teeth 152
Pinion teeth 36

Vibration_Posts.12-01-2006.doc (178 Kb, 58 downloads)

I assume you have already dismissed a possibility that the dominant peak has to do with roller bearing fault. If so, there is a possibility of gear or other element resonance frequently modulated by running speed. I'd extend Fmax three folds to make sure there is no harmonics of this peak. I'd also examine TWF in velocity units.

I would first question the bearing selection and having a C3 fit. Graph out your machine through the sleeve bearings and see where your AF bearing clearances take you within the sleeve fit tolerance. Most every machine I've seen fitted with both have a precision ABEC# bearing with tight tolerances throughout (bearing and journal). Forgive me; this stood out and I haven't looked at your data yet.

Some of the measurement also shows very low frequency Hunting tooth frequencies. When the spectrum is “zoomed” one can see that there are sidebands (marked with 1) around the centre frequency (143905 cpm)
not only at the Bull Gear running speed frequency but also at 18778 cpm which calculates as 3x the blower running speed frequency


And Sam says - and from your comments: plot out your clearances and see where you physically are and where the wobble goes. Make all projections to every bearing point on both ends of the sleeve and to the gears as well and see if you run-out of air gap.

Thank you for he responses, the gearbox intially arrived with c clearance bearings from startup the Bull bearing temps ran high compared with the "old box". The client decided to open the box together with the supplier and the bearings were found to be "carbonised" therefore they went back to the "old" c3 clearance bearings.This machine is responsible for 2/3rds of the plant's production so the client is VERY nervous about this.

The plant is currently down for maintenance, i will be updating the data shortly.