I currently have a double reduction Falk gearbox (driven by a direct coupled motor and driving a head pulley of a belt conveyor with a HTD belt) that, for the second time in a row immediately after a rebuild, showed very high high-speed gearmesh (1 & 2X of about equal amplitudes) w/sidebands of both the high speed / input and intermediate shafts.

Both the 1 & 2X increased over several months, as did the howling of the box, and then, suddenly dropped as low speed gearmesh suddenly popped up w/severall harmonics and sidebands of the output speed AND BPFO of one of the Timkens on the output shaft.

My question is, is this possibly a symptom of misaligned gearing, and, if it is, how could the shafts/gears be misaligned? I had a hard time explaining this to the maintenance leader who asked me, and my only "guesses" were that one or more bearing housings within the box might be worn and allowing the shaft to move/misalign.

Thanks for any ideas / education here!

Tony

The Timken bearings can sometimes spin on the shaft. The shaft material is usually what starts to wear away. Even with the shaft being damaged the cup cone and rollers can still look like new. Removing shaft material allows the shaft centerline to move, which results in misalignment. The gearbox will start to howl as the teeth are no longer meshing properly. This can be very noisy, but unlike your case this usually doesn’t get better.
Also, howling or GMF can alter drastically depending on load.

Sounds to me like the backlash was set too tight! This would account for the howling to start with, as you can actually bow or bend the shaft if set too tight. You usually would have gotten a bearing temp problem also! Then the gears wear in and the noise drops! What is the speed of the box, input & output, along with drive HP?

Both Waylon and Ken are correct. I would add that the bearing bore will also be opened up if the outer race has turned in the shaft, but like Waylon said, it usually doesn't get better.

What type of gearbox is it? Sometimes when their Y unit parallel shaft reducers are used as drives on vacuum pumps, I have seen resonance in the parallel extended plates that are used for the lifting lugs. This can be extremely loud, but once again doesn;t go away. I used a pipe clamp as a temporary fix on one of these once, then had a piece of angle bolted on to stiffen it.

It doesn't sound like you can do much more with vibration analysis and I would suspect that Ken is on the right track. I would do oil analysis and look for wear particles and check with the rebuild shop regarding the inspection/assembly procedures. I have seen everything from proper procedures to people peening the shafts and bearing bores used in rebuilding gearboxes, so nothing would surprise me.

Not that this is the case here, but I have been given wrong bearing and gear data directly from Falk/RExnord-twice just yesterday- so be careful what you believe there too.
Good Luck,

Danny

Hi guys! The gearbox in question is a Falk Mod 1002-8EM2-46B3 MO# 7-437844 Ratio: 6.211:1 and consists of three parallel shafts running at 1792/950.3/288.5 RPM. I'm fairly confident in the data Falk/Rexnord supplied in this case, but I have gotten some incorrect info in the past, too.

Thanks for the input!! Tony

HI Tony.
Let me tell you a story. There is a machine here call a pug mill, which is a mixer unit that mix dry ore with slurry. It is build with two parallels horizontals shafts fitted with paddles and driven by a single entry, double output, parallel shaft double reduction gearbox. The gearbox had a mtbf from few months to a year at best. The maintenance leader tried every thing with seldom result. Got rid of the belt drive, precision alignment was performed, high tech synthetic oil was used and it came down to change every gearbox with a new custom design replacement unit. At that point, I decided to jump in, I just could accept to spend that much money without any more investigation. The failure pattern was always the same, Input shaft BPFO that turn into high gear mesh. First thing first, I retrieve one of the failed brg and had a look at it. I had never seen such a defect on a brg; fortunately, the Timken guy came along and explains me that the defect was due to excessive brg clearance. I work with the Timken fellow and the mechanical shop and after few try we find the perfect clearance for that application. We when from 16 gearbox per year to 2!!! What a success! Cone brg has to be fitted depending on the application, your gearbox may run a little hotter or colder than normal for various reasons either safety ratio, production increase or … The brg clearance goes from few thou to preload depending on casing and shaft materials, running temperature and or speed.

In conclusion, my advice is to get one of those failed brg and analyse, who knows?

Best regard, Marcel

Good advice from proven experience! To get bearings here I have to ride the process pretty hard, but in this case (w/repeat failures), sounds like it'll be worth it. Thank you!

Tony

How's the pattern? Same from end-to-end and same depth with same back-lash everywhere?

I have used a soft foot to create case distortation to align the gears in an instance.

How's the oil analyses/ferrography?

Hi Sam!

If by "pattern" you're referring to how the actual gears / teeth look, they rebuilt the last before I could look at it. You can bet the current one will be looked at pretty closely! Oil analysis has shown increasing levels of both ferrous and "other" contaminants (keeping in mind that our oil "lab" has been having some reliability issues lately).

Tony

Sam makes a good point. Bluing the contact pattern would be a quick way to look at the gear set if not already damaged. Improper shimming can greatly change the contact pattern and it does not take very much to twist a gearbox. Just how much affect a few thousand plus or minus on a mounting pad will create might surprise you. I think Lufkin Gears, in Lufkin Texas, still has a nice two day seminar on gearbox installation and reliability. I would suggest it to anyone who’s interested. The instructors perform such test as contact pattern reading while making shimming adjustment to show the importance of proper installation practices and procedures. The results were a real eye opener for me.

I can attest to the quality of the shop at Lufkin. They run a first class operation.

No connection, just seen their work.

Gearbox initial setup is very important. Nice to shoot the shim pack in the same plane prior to setting the G/B. Or shoot the split (best with top off). Pattern and go from there. Generally the G/B is the anchor point so setting the shim pack to an exact elevation removes the necessity of check soft soft. Also, G/B's generally have a mass that overcomes the stiffness factor making the soft foot check questionable. As Waylon mentioned it is unbelieveable how much flexibililty there is in a G/B.

On another note: I like to get an oil sample from the middle and bottom when I can and/or from all points depending on the system. I have five G/B's on one system that is a nightmare.

A great rebuild doesn't have a chance with a faulty installation. Blueing is a good idea. Also, I have found a good check to see if the gearbox is sprung as it would be with an improper installation.

SPLITLINE CHECK

Remove the bolting on the gear top, clean the splitline and set the lid back on. With the lid just resting on the splitline check for gaps between the lid and the box at the splitline. Anything above .002 or .003" is excessive and suggests that the box is distorted (most of the time because it isn't sitting flat.

This check works because the base and the splitline are machined parallel. The lid usually does not distort perminently and is a good flat surface from which to do a feeler gauge check.

If the box is twisted, then the gear alignment will not be proper. Incidentally, this is a good check to do immediately after installing a gearbox for obvious reasons.

Good Luck

quote:

Originally posted by TonyW:
for the second time in a row immediately after a rebuild, showed very high high-speed gearmesh (1 & 2X of about equal amplitudes) w/sidebands of both the high speed / input and intermediate shafts.

Both the 1 & 2X increased over several months, as did the howling of the box, and then, suddenly dropped as low speed gearmesh suddenly popped up w/severall harmonics and sidebands of the output speed AND BPFO of one of the Timkens on the output shaft.

My question is, is this possibly a symptom of misaligned gearing, and, if it is, how could the shafts/gears be misaligned?
Thanks for any ideas / education here!

Tony

Tony, is the rebuild shop giving you guys the specs on the bearing bores or are you able to measure them yourself ?
Could the boxes be warped from previous failures ? Rebuild shop should be able to set up and indicate on bores and determine if the bores are all in spec and alignment.

I am thinking the "howling" noise right after rebuild came from the clearance in the bearings instead of a pre-load condition.
You mentioned the sidebands surrounding the gearmesh were equal to "BOTH" the input and intermediate shaft speeds.

I would luv to get a look at the spectra showing both sets of sidebands.

Out of curiosity, what level of g's are you getting from the box after rebuild ?

I trend a 3000hz waveform ( g's ) if the motor runs 1800 rpm. I have a few gearboxes that run higher than I like, but the rebuild shop builds them to specs with axial clearance set to .008

From the get go the gears "howl" and the trend levels are higher ( 20g's ) than I like but it runs so for this box I just adjust my level of acceptance and watch for higher levels.

We have lowered vibration levels and noise on several drive reducers from the lineshaft to the dryer sections by adjusting backlash and timken bearing clearances. When bearing clearances become excessive the gearmesh levels and noise increases.

Axial clearance in a timken bearing means there is radial clearance also.

last thought....... could there have been a lubrication issue in this box before the failure, increasing the temperature that made the shafts grow, reducing bearing clearances before problems showed up on the output shaft ?

still learning and listening but thought I'd throw in my 2 cents.

Wish you the best in solving the problem.

Mike

Hi all--

Attached is a Word doc. w/the last four month's worth of spectral & waveform data from both my high speed and low speed measurement points. The box was rebuilt in Jan, but the March data is pretty similar.

Tony

Chip_Conv__2_vibe_data.doc (88 Kb, 20 downloads) Chip Conv #2 Vibe data