Hi
I would like to start a discussion on right angle gearboxes set up as agitator drives in a tank.
I have in the input shaft measurement point a peak in the acceleration spectrum @ 4x the 2>3 gearmesh freq. It seems quite common over all 80 gearboxes with the amplitude varying in each box. Most occasions the afundamental frequency is very small in amplitude compared to the 4x. Does anyone have any idea what the 4x harmonic of a gearmesh could be eluding too.
It can be seen @ the second shaft measurement point where you would think it would be, but it is not as predominant as in the input.



Thanks in advance for any advice.
Matt W
Original Post
The tooth combination is desirable and also it would be good to know your method of measurement (velocity coil, accelerometer, magnet mount, stud mount, etc.).

Also, if I ask "is the bevel gear (as opposed to the bevel pinion) mounted to a spider with a four hole bolt pattern" do you know what I mean by the spider? Sometimes, instead of the bolted arrangment, the bevel gear is mounted to the shaft with a keyseat. Can you attach a cross section of the gearbox
Hi John
The Bevel gear is definatley shaft mounted with no spider arrangement.
Since writing this post it seems that I should be looking at the alignment of the shafts. Being that it is at right angles it is pretty hard to get the bearing positions 100% correct and is possibly why it is a common peak through out the gearboxes with different amplitudes.
Would you agree to that explaination.
I have attached the exel spreedsheet that I use for fault freq with an idea of the gear train setup.

Attachments

Hi Guys
The measurement method is magnet mounted accelerometer using acceleration.
Reduction Ratios 47.7
1>2 36 81
2>3 14 47
3>4 19 120
Shaft Speeds
RPM Hz
Shaft 1 1485 24.75
Shaft 2 660.00 11.00
Shaft 3 196.60 3.28
Shaft 4 31.13 0.52
Gear Mesh Frequencies
GearSet RPM Hz
1>2 53460 891.0
2>3 9240 154.0
3>4 3735 62.3
Blades RPM Hz
3 31.13 1.56
quote:
Originally posted by Mattweldon:
I have in the input shaft measurement point a peak in the acceleration spectrum @ 4x the 2>3 gearmesh freq. It seems quite common over all 80 gearboxes with the amplitude varying in each box. Most occasions the afundamental frequency is very small in amplitude compared to the 4x. Does anyone have any idea what the 4x harmonic of a gearmesh could be eluding too.
It can be seen @ the second shaft measurement point where you would think it would be, but it is not as predominant as in the input.


I am in agreement with all the assorted frequencies you've calculated and provided. My spreadsheet outputs are attached FYI. Can you supply some actual spectrums for us to review?

By your statement "4x the 2>3 gearmesh freq" I assume you mean 4*154 = 616 Hz or 36960 CPM being a frequency dominant in all the gearboxes. I don't see anything inherent in the tooth combinations that would generate such a frequency. Having said that there is one pecularity in that the frequency of interest of 36960 CPM is an exact integer of the 1st reduction gear speed/2nd reduction reduction pinion which likely are mounted on the same shaft; in other words 36960 CPM/660 RPM = 56. Have you checked the anticipated bearing frequencies?

Just as a point of interest that 1st reduction has 9 assembly phases hence the reason why the spreadsheet has the yellow shaded lines. Some cautions are in order if the gearboxes are ever disassembled in that the timing relationship between the 1st reduction pinion/gear should be maintained.

Attachments

Files (1)
At least in the two plots you've presented I don't see anything in the way of amplitudes that would cause me concern. Having said that I can offer a few ideas as to the source of the particular frequencies of interest. But before I do that I would suggest care in attempting to identify elusive (meaning small in amplitude) components in a gearbox spectrum. That practice will drive you nuts! The perfect gear does not exist so when we mesh a pair we have a great device to create not only the anticipated fundamental frequencies but sum and difference frequencies. Couple that with the bearing related frequencies and the spectrum can be quite a mess. Many years ago I wrote the following as an introduction to a tutorial on gearing that related the aspects of design to dynamics.

quote:
The term "odontics" may be selected for that limited but important branch of kinematics that is concerned with the transmission of continuous motion from one body to another by means of projecting teeth. It is a field that for several hundred years has received the interest of both mathematician and engineer; it can be educational, frustrating and entertaining. Thousands of pages have been written on the kinematics of gear teeth, however, little has been exhausted except for the writers and readers.


OK, now to some thoughts as to the source.

Observation 1...As I previously mentioned there is the pecularity in that the frequency of interest of 36960 CPM is an exact integer of the 1st reduction gear speed/2nd reduction reduction pinion which likely are mounted on the same shaft rotating at 660 RPM; in other words 36960 CPM/660 RPM = 56 exactly. So what might generate the 56th order of the 2nd reduction pinion which by coincidence has 14 teeth. 56/14 = 4, where does that leads us? I'm not sure, just the numbers game I suspect.

Observation 2...again, back to the frequency of interest of 36960 CPM being the 56th order of the 2nd reduction pinion shaft. Couple that thought with your statement "It seems quite common over all 80 gearboxes with the amplitude varying in each box" and I become suspicious of what is called a ghost frequency. As you may know the machines that make gears are also full of gears. Hopefully all those gears are as near perfect as possible because any error in them gets "impressed" into the final product. Usually, if this happens, it happens in a final tooth cutting or finishing process. If that 81 tooth gear or that 14 tooth pinion was machined using a 56 tooth index wheel and that index wheel had an error, it would be seen in the final gearbox as a 56th order. You would have to determine with the manufacturer the exact method of manufacturer and the teeth involved in the actual gear cutting machines. This can be an exhaustive process and the gear manufacturer, depending on their level of expertise may think you are crazy. My personal first encounter of "ghost frequency" was in a paper written by Bruel & Kjaer that dates to about 1980. I've attached it here for your use but later found it at http://www.bksv.com/doc/233-80.pdf. I doubt B&K would mind due to its age and not to my knowledge being on the web. Download it and look at the content on pages 10 & 11. My 2nd encounter with ghost frequency was when the famous Charlie Jackson called me while commissioning a gearbox made by a manufacturer in Texas. He was concerned about a frequency in the spectrum, quite large actually, that was not identifiable by the tooth count. I determined that it was an exact integer (292X) of rotating speed. Some further work using time synchronous averaging determined the exact rotor that had the fault. Consultation with the gearbox manufacturer initially confirmed I was potentially nuts but Charlie prevailed and on investigation it was determined there were 292 teeth on the index wheel of the hobbing machine used to generate the rotor. I am aware of other instances of this issue but in my 17 to 18 years with a gear manufacturer they number about five cases. So it is very rare. Wherever the source the primary concern is amplitude because that represents energy. In the case of the gearbox being commisssioned the amplitude at the ghost frequency of 521,000 cpm was in excess of 20 g's peak while gearmesh was about 2 g's peak. Obviously something here was wrong. The gearset was reground (on a totally different machine) and the frequency was no longer present at any measurable amplitude.

So, if you have a good relationship with the gear manufacturer you might investigate ghost frequency. Do you have an ability to simulate a keyphasor on the shaft with the 81 x 14 tooth combination and perform time synchronous averageing? Those techniques can help identify the actual gear with the fault. But if I was in your shoes, and I say that only having seen the two plots showing low amplitudes, I would chalk this up to academic interest only and a probable ghost frequency.

Attachments

Some additional info:

Ghost frequency as I describe it above tracks speed. If you do a web search you will find other information about ghost frequencies that do not track speed. One in particular is the identification of some ghost frequencies using a CSI 2140 to be found at http://www2.emersonprocess.com...p_GhostFrequency.pdf. The key take away from that paper is in the conclusions it is mentioned that one goal is to identify the source of the component. A key characteristic of the 1900 Hz component being termed a ghost frequency is the constant frequency during coastdown. This is not the same form of a ghost frequency as I discuss above although a gearbox is involved. I would almost suspect another nearby machine or perhaps some artifact of the signal processing. The 1900 Hz peak is measured as 3 microns pk-pk which converts to about 21 g's peak, yet the paper states the component is not measurable on the casing. That seems unusual for a possible mechanical fault.
I understand that you would not normally consider such a low amplitude in the vibration spectra, however my concern for the peaks and their amplitudes evolve from the the fact that the measurement points for this shaft is not in a particularly ideal spot. The vibration has to transmit through around about 150mm of flange before reaching the accelerometer so I find that all of my vibration amplitudes are very low in comparison to the amount of damage I find in the gearboxes.
I will take into consideration the discussion on ghost freq when I next have a discussion with my colleagues, it was a very interesting talk so thanks for that.
I think I will also take into consideration a remark from Beatnik in regards to natural frequencies as I have noted in the past but never experimented with them.
No doubt I have further studies to conduct to verfy the peak in question but I will stick at LOL.
Thanks for the talk much appreciated.
quote:
Originally posted by Mattweldon:
The vibration has to transmit through around about 150mm of flange before reaching the accelerometer so I find that all of my vibration amplitudes are very low in comparison to the amount of damage I find in the gearboxes.


Until now you haven't mentioned damage. Can you supply some pictures or a description of the damage you encounter? Early on I mentioned that the 1st reduction had 9 assembly phases. Are you aware of the ramifications of that statement and the potential for gear distress?

Also, the 150mm flange, can you show a picture of that with the transducer mounted? Depending on the signal path there may not be much attenuation.

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