I have a relatively small gearbox with 20 teeth on the input and 56 on the output. It is running 29.6 HZ on the input, and 10.54 on the output. I have impacting at 14.5 HZ in the time domain and a 1/2 gearmesh.The 14.5 is 1/2 input speed.I know that 20 and 56 teeth is not a good ratio however what could be causing the impacts every other rotation of the input shaft. It's not a beat it is a sharp impact every other rotation looks just like a broke tooth. Reguardless were going to change it. Whatever is causing it is not good. Any ideas why i get this impact.

Thanks

This is Gear Assembly Phase frequency. Check the attached graphic for a great visualization.

This message has been edited. Last edited by: Bill Kilbey-Mobius,

Go to http://www.bently.com/articles/691winter.asp where you'll find an article I wrote titled "Component Identification of Gear-generated Spectra."

John

Hi Partners!!!

In this case yuo have 20 and 56 teeth
20/56= 5*4/14*4.
This relatios has a common factor 4 then In an spectrum It can apear frecuencies 1/4*Geramesh , 2/4*Gearmesh, 3/4*Gearmesh, etc.
In your case can apear the 2/4*Gearmesh.

Can you send me an spectrum?

Best regards
Guillermo
guillermo.chiappero@ar.abb.com

Hi mike,

Whatz the status of this equipment now? Did u solve the problem? If so what were your findings?

1/2 Gearmesh can be there if one of your gears are eccentric/loose since you have a common factor of 4.. Can you post the spectrum & waveform?

Have a nice time!!

Guys, the common prime factor is what counts and that is 2, hence the 1/2 gear mesh.

Hi John,

I read somewhere that common factor also matter... Like what Guillermo describes, If you have a common factor of 4 and the gears are eccentric, then you can expect 1/4,1/2 or 3/4 GMF.

What others are thinking? Is it COMMON FACTOR or COMMON PRIME FACTOR?

Have a nice day!!

This message has been edited. Last edited by: Jenish,

It is common prime factor...go to the link I previously posted. http://www.bently.com/articles/691winter.asp

John

Hi John,

I checked Taylor's Vibration Handbook..Itz common factor!!

Have a nice day!!

Hi Vibe fellows,
I’d like to jump in the conversation if you don’t mind. I see two trends in this conversation. First is the expression of a gear train common prime factor. This will affect the gear train during its life and tend to reduce gear’s life expectancy by introducing a mating wear on the gears. Second, fractional gear mesh, which result from a defect from the reduction arrangement as thrust load, tooth defect. I would react more strongly to the second since it may be symptomatic of a serious problem.

Best regard, Marcel

Hi guys,

Let me quote what Taylor describes:

The common factor identifies the number of teeth from one bad tooth to the next; the uncommon factor identifies the number of bad teeth on each gear.


1/4 GMF = 5 * Pinion Speed (29.6Hz)
= 14 * Gear Speed (10.54Hz)
= 148Hz

So if the gears have a common factor that is not a prime number, such as 4,6,8,9,or 12, then fractional gearmesh frequency could occur at the reciprocal of the common factor or any of the prime factors of the common factor, depending upon the eccentricity in the gear.

Also if you have a common factor of 4, the % of life expected would be 25%.

Have a nice day!

This message has been edited. Last edited by: Jenish,

My God, do I love those topics!
I would like to share a gear mesh phenomenon that I experience a couple of time on herringbone gearing. I observe x/3 gear mesh on pinion that was off center. Those pinion had strong axial thrust and for some reason generate x/3 gear mesh frequencies either they have common factor or not! We had a major breakdown on speed reducer that stopped the production for one day where the pinion brg lost its entire rolling element into the mesh of the other reduction, you can imagine the mess! From this experience I inspected a similar gearbox that show similar x/3 gear mesh spectrum to find out that the pinion’s brg cage was cut off and ready to spit out its rolling elements. I monitor as well nine ball mills fitted with herringbone and they all show x/3 gear mesh even with gear ratio of 23:264 or 25:264???? I feel confident in the diagnostic but I would like to understand how it is possible to generate such frequency. Didn’t find the answer in Taylor’s book I joint the picture of the cut off cage. You will see it in the inspection mirror and free onto the shaft.

Best regard, Marcel

Marcel,

Your case is definitely interesting. One possibility that may explain x/3 is shaft looseness due to improper cage location rather then something related to a common factor. I can make a quess that there are also harmonics of x/s. Seeing the spectrum would be helpful.

David

Hi Dave,
In both cases the defect developed after installation of wrap flex coupling. The couplings were not in cause but rather improper installation of them. Herringbone gears are floating except for the input shaft, this allows the gears to perfectly align at all temp. But in our case we didn't leave enough gaps between the couplings. As he motor's shaft expand, it pushed the input shaft axially to the point where it exceed the brg spec and run the gear off center. The Ball mills generate x/3 gear mesh as well. In that case I believe that the floating of the brg are suspect due to the inertia of the pinion arrangement friction. They generate similar x/3 gear mesh spectrum but at much lower values.

Best regard, Marcel

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