Gear Mesh Vibration in Coal Pulverizer

[gesweind]

I have a pulverizer that was vibrating at 1 ips measured at various bearings within the gearbox. This vibration appeared after the gearbox was overhauled. It is a double reduction gearbox. %99 of the vibration is occuring at 332 Hz, which corresponds to the gearmesh frequency of the first reduction. The 1x pinion speed sidebands are almost invisible.

Last night the machine was brought offline and all of the gears were realigned. The vibration is now at .8 ips. My question is: should I really be concerned? Are the gears "wearing in to each other"?

 

[EnglishMuffin]

What exactly did the "overhaul" involve ? Were any of the gears replaced/reground/remachined etc ? Are they spur or helical ? Are they hardened and ground gears, or just cut from heat treated steel ? Any idea what the vibration level was before ? When you say "realigned", what exactly do you mean by that? Sorry to ask so many questions - just trying to get a handle on it - mught help others too.

 

[gesweind]

During the overhaul the intermediate gearset assembly was replaced (bearings, shaft, and gears). The high speed pinion gear driven by the motor was replaced. The bullgear that turns the ball mill was not changed. The coupling to the motor is new. The herringbone gears are made of hardened steel. The vibration appeared after this work was done.

Then, the gears were aligned to try to eliminate the vibration. I don't fully understand the alignment procedure. I believe the bullgear and small intermediate gear had some angular misalignment, as indicated by the new wear pattern. The lash was also checked and adjusted. The high speed pinion was then realigned to the large intermediate gear. This gearmesh is the one that corresponds to the high vibration. The wear patterns did not indicate anything wrong with this one the first time. The vibration is now at 1.1 in/s @ 332 Hz. I don't know whether we should run it until our next opportunity to take it down (probably this weekend), or shut it down before permanent damage occurs.

 

[EnglishMuffin]

So if I understand you correctly, it sounds like both the gears in the suspect mesh are new, but the known angular misalignment is on another mesh. Nine times out of ten, gear noise is due to inaccurately cut teeth, so if you do take it apart I would have the profiles of all the gears checked if possible. Sometimes, gear noise and vibration can be misleading - the things you see an the analyzer are not always the main culprits. You haven't actually mentioned noise - just vibration, but I assume it is noisy.

 

[gesweind]

It doesn't sound any noisier (acoustically) than the other two mills on this unit which are identical. I probably should have mentioned this before ... this gear mesh frequency does not show up at all on the other machines, and never has before on this machine. These mills have a lot of noise and vibration due to the large size and the several tons of steel balls tumbling in the drum. Again, there are no prominent sidebands to indicate eccentricity or a tooth defect. There are 2nd and 3rd harmonics of the gearmesh, but they are small (amplitudes are %10 and %3 of fundamental respectively).

If the teeth are cut inaccurately, then the inaccurate cut is consistent with all of the teeth. The vibration is not modulated in any way.

 

[EnglishMuffin]

If the teeth are cut inaccurately, I would not be surprised to find them all to be equally inaccurate.
Another thing you could check is the lowest point of single tooth contact -its possible that you are digging into the root for some reason.
Were the new gears made to the original drawings, or did someone come up with what they thought the original gears were supposed to be ?
How does it compare to the others when you don't have any noise sources other than the gears themselves? (Just curious).
You probably need to get production out of it, so your options may be limited. If you are seeing the tooth pass frequency, I can't think of anything else to check other than the tooth profiles - maybe someone else has an idea.

 

[PaulJuhnke]

EnglishMuffin is correct! You definitely have gear problems that will create a lot of trash metal in your lube oil, possibly damaging other bearings & gears. You should pull oil samples on a regular basis and filter the oil to somewhere between 2 and 10 micron depending on your oil. Or you could change the oil frequently, until you can get the gear problem corrected. A thicker lube oil, or high tech oil with solid lubricants could help cushion the damage while you craft a solution.

Sounds like you’re on top of the problem doing your vibration homework very thoroughly, and properly. Most plants would not know about the problem, due to all the other equipment noise involved. Pre-mature gearbox failure would be their first indication, and then the cause of the problem would be very hard to diagnose (without the data you’re collecting), after a catastrophic gearbox failure.

Gear quality control is most likely a serious issue in your case as EnglishMuffin has defined. I know of a very good AGMA gear expert you could contact. I’ve used him in Portland cement plants, and my brother has used him in food plants, to deal with improperly manufactured OEM and replacement gears. He’s a nice guy, very helpful, and relatively inexpensive since he’s an independent consultant.

If you need more information, call me at 641-423-9363 with more details about your application. Good Luck, Paul Juhnke, Maintenance Engineering Services.