After close inspection, I conclude that my harmonics are 1/3 gear mesh. No bearing numbers come close. So, 21/75 teeth, common factor of 3 contributing? Similar box does not have this. Possible bearing loosness or high spots? Levels are low but does the fact these exist make it a concern? I would like to see them do a visual. Any ideas.

1_3gm.doc (42 Kb, 45 downloads) 1/3 gear mesh harmonics

I don't believe a 1/3rd gear mesh exists. That's like saying I push up and down on one side of a see-saw and the other end moves up and down at 1/3rd that frequency.

Can you provide the time waveform for the spectrum you've attached? You may have amplitude modulation and associated sidebanding.

tm

1/3 gear mesh can exist and this instance it is as yiou expected beacuse of the commmon shared prime factor of 3. This link will take you to an article that I've written about gear generated components.

http://www.bently.com/articles/691winter.asp

The attached shows the extreme of what can happen. In this case there is a shared common factor of 5. The situation was also aggravated by some externally applied thrust to the pinion. Double helical gearsets don't like external thrust as it can upset the 50/50 torque split on the helices.

assembly_phase.doc (256 Kb, 36 downloads) assembly phase

John,

You wouldn't happen to have spectra from that gear available would you?

Regards,

The enclosed waveform shows impacting at the 1/3 frequency. I would like to have them visually inspect the gears.

1_3gmwave.doc (41 Kb, 17 downloads)

jbalko,

If you didn't pick up on the full content of my article let me caution you on any disassembly that you might do in the future. Make sure you mark the teeth and put things back together with the same indexing, otherwise the gears have to develop a new wear pattern and during that phase the casing vibration and airborne noise level can be quite high. Hopefully you haven't already taken the box apart and randomly put it back together. You would have a 1 in 3 chance of getting it back together in the same assembly phase.

The down side of putting things back together in a different phase is it is possible the gears will never wear in again. The tooth flanks get work hardened in their initial operation and this can slow or stop future wearing in. In that instance you pretty much have to have the gears recut to get back to original and then that has to be studied carefully or you might intorduce excessive backlash and introduce other issues.

I'm suspucious that with a 21/75 toth combination you are dealing with bevel gears. If that is the case you will often find markings on the ends of the teeth that show the indexing from the factory. In addition they show the factory backlash setting. Whenever the gearbox is disassembled they are to be indexed back to the factory settings using the original backlash as a guideline to setting the bevels.

John

Danny, I don't have spectra from that specific gearset. That picture came off the cover of the VI magazine from vintage 1998. I use it in teaching a course on gear vibration and failures.

Depending on what you need it for I have some very limited spectral data (like one plot) on another gearset that happens to have a common factor of 5.

eMail me if you want to discuss more.

John dot Winterton@ge dot com

Very interesting article John, thanks for sharing. I already had a fair understanding of this issue, but your paper sums it up well.