I wonder if somebody can share their experience with vibration pattern associated with wear of a flexible gear type of coupling. This situation is similar to that of a 3 jaw coupling type which has a known vibration pattern when wear, misalignment or both are present.

Thanks in advance for your input.
David

One thing you may see is coupling lockup. Depending on the machine, that can cause axial vibration which changes with temperature and changes after shut-down startup.

We had a gear coupling completely fail a few years back from excessive wear. I'll have to go back and refresh my memory on the patterns we saw before failure. Won't have a chance to do that until next week, though (getting ready to evacuate ahead of Rita).

I have seen large clunking at running speed in low speed applications using a floating shaft arrangement of geartooth couplings withe one badly worn and possibly locked up. It showed very clearly in the PeakVue timewaveform as I recall.

I'll check tomorrow (late) and post some data.

Good Luck,

I measure airborne ultrasound emissions from coupling instead of vibrations.

Interesting idea Walt... I've been fooled by gear couplings before, thinking it was "looseness" in a bearing. Ultrasound might help.

I've seen a gear coupling lock up, and it resulted in a large 2x. I've also seen multiple harmonics of 1x which I thought was looseness in a bearing, only to have the coupling "strip out" a few days later.

Will look up the spectra on these two cases and post them.

Thanks to all.

Danny, how were you able to relate abnormal PV readings to coupling wear? Just a guess?

Walt, did you have to establish a baseline for ultrasound reading on each coupling or it is pretty much absolute when a coupling is worn? Does it emmit ultrasound even with sufficient lubrication?

Pete, Rusty, a locked coupling is almost a catastrophic failure and if the unit is still running, is could be relatively easy to diagnose, although sometimes mistaken for something else. I guess, in this case one has to utilize elimination method.

It would be nice to see some plots.

quote:

This situation is similar to that of a 3 jaw coupling type which has a known vibration pattern when wear, misalignment or both are present.

Are you speaking here of your situation at present or as a general rule of a worn gear coupling being the same as a 3 jaw type coupling's pattern?

Are you close enough to the Texas coast for Rita to blow you away? Hope not.....

David,

An ultrasound baseline is not necessary, although it can be helpful to have previous survey data or a comparison to asimilar machine. The combination of the meter reading (dB) and audible sound characteristics from headphones allows detection of both friction and impacting. Even a properly lubricated gear coupling can have excessive friction if it is worn or misaligned. A very worn or dry gear coupling will often have impacting. I have made many correct diagnostic calls on couplings with ultrasound, including a feedwater pump at a nuclear plant.

I have been unable to reliably detect coupling faults with vibration measurements, so I am most interested in comments by others who claim success.

Walt

A case history in March 2005 Vibrations magazine reports that lack of lubrication caused rise in 2x in axial direction. No word of wear though. I wonder if this is typical.

Danny, it sounds logical to expect PV rise in case of wear or lube problems in a gear coupling. I'm just curious to know if there is a way to attribute such an increase to the coupling rather then to anything else. In this regard aiborne ultrasound, as Walt says, may point out to the source more accurately.

David,

Sorry that I couldn't reply earlier, but I have been on the road.

It was not a guess so much as a process of elimination which means lots of wrong calls before you get to the right one.

It was a sudden onset problem and I suppose that it began when the lubricant had finally all leaked out through the seals and collected in the guard. The guard was a solid tube and the pile of nasty coupling grease couldn't be seen. The entire process probably took years.

Good Luck,

Danny

Walt,
I have a gear coupling that has suddenly developed a high 3x (went from normal low level 1x only, to high 1x and 3x). The fan is center hung and the axial 3x is highest. My question is, what distinct sound do you listen for with ultrasound?

Sid,

First of all, your vibration data may or may not indicate a coupling fault, because the vibrations were measured on the bearing housings. So all you can do is rule out bearing faults, and that leaves the coupling as possible suspect.

Ultrasound can pinpoint whether the coupling has a fault, and it can also indicate which, if any, bearing has a fault. Locate microphone close to coupling split-joint as possible (1" to 2" is ideal). Listen for a clicking/popping sound to indicate poor lube or significant wear. Measure dB value and determine if high, based on trend over time, comparison to another coupling at same speed, or to severity criteria. A high dB value without a distinctive audible sound indicates excessive friction. If coupling has a spool piece, then measure and listen at each coupling half (split-joint). Make ultrasound measurements at each bearing with microphone or contact probe to separate coupling fault from bearing fault. What ultrasound meter do you have available?

Walt

David,
I'm seeing lots of harmonics of running speed and a high 3x in the axial direction. I can't find any looseness or fan shaft displacement in relation to the bearing housings. The bearings are sleeve type and held in the cradle by a hold down bolt. All seems to be tight. There is a elevated 10x in the spectrum (#of blades). I'm leaning more toward running speed sidebands off of vanepass rather than looseness as an indicator of gear coupling wear. Most all points are in phase with the highest difference being on the motor backend bearing at 135 degrees from vertical to horizontal.
Walt, space is limited but I do hear soft sounding impact with ultrasound (SDT 170) on the motor side of the coupling. The noise does not sound like a "tick" but more like a "puff". I do not hear nearly as much on the fan side. I'm going to record the noise today. The sister fan has no noise at all. With a strobe and mirror I can see the fan coupling hub sticking through the cover about 3/8" and on the motor side the edge of the cover is even with the edge of the hub. In the position of the cover, engagement of the coupling gears on the motor side may only be about 60%.

I went back and looked at our records for a gear coupling complete failure that occurred on 12/11/00. At that time the coupling failure caused the pump to stop rotating and lose discharge pressure, resultiong in a start of the standby pump. Investigation revealed that the coupling was not effectively greased (grease had left the coupling by centrifugal force... 3600rpm requires special grease).

We had taken vibration on 11/27/00 and there was no indication of a problem. I have attached velocity waterfalls for all motor/pump positions in attached powerpoint. The horizontal line indicates the last test before failure The only thing you might notice is an increase in 4x harmonic just before the failure. Blade pass is 6x. By and large there is no significant change in spectral pattern before failure or after repair.

I don’t have any good records of TWF data or demod data for this machine during that timeframe. It is possibly they might have shown something going on.

GC12_GearCouplingFailure.ppt (434 Kb, 85 downloads)

Pete,

Thanks a lot for sharing this info. It looks like there was indeed no advance warning at least in the spectra other then some inrease in 4x, but who would make a coupling call based on that?

How did the coupling physically fail? Were all teeth shaved off, or ...?

It appears that a 3 jaw coupling with a plastic/rubber insert produces a more clearly recognizable pattern in the spectrum as the insert wears out then that of a gear type.

May be a locked gear coupling will exibit this condition very clear in a spectrum but it will likely fail soon after locking up and, therefore, most of the time will be missed by vibration survey.

Possibly, the ultrasonic method is the best one so far as an advanced tool.

Sid,

As a side note, what kind of recorder do you use? I mean type and frequency range.

Pete,

Do you have any time waveforms?

David,
I use a small Sony IC recorder that I insert into the headphones of the ultrasound unit. Crude, but it allows me to record a sound! I have a few failure spectrums for gear type couplings (one being two feet in diameter and only 300rpm), but haven't been able to post because of file size (and I think permission). Most of the failures that I've seen with jaw and gear type couplings have been exhibited by harmonics of 1x or sidebands off of the pertinent driven frequency (blade pass, gearmesh, etc..). Obviously, from only the spectrum, I can't tell if looseness exists or a coupling problem is present. A lot of gear pumps that I monitor use Lovejoy type couplings and when I see running speed begin to connect with normal gmf, by harmonics, I begin with a spider inspection. Thanks to Walt, I have recorded a sound that other gear coupling driven equipment doesn't generate (for the moment). Unfortunately, my experience using the untrasound method for coupling problem detection is limited. In my present case, I see an elevated 3x axially and harmonics (or 1x sidebands around 10x).

The typical increase in 4X is classical. Also you'll increase the 1X component as well but with axial slippage look for 6X.

All vibrations to some degree or precentage manifest themselves into running speed. That number may be very small or large depending on the source. The offset on the coupling from a fault generally isn't large but is also affected by the amount of misalignment. With good alignment and no coupling lube the 4X is not as pronounced.

As Pete mentioned theirs was lack of lube. Extreme care must be taken on coupling gaskets and O-rings else the centrifugal force will sling out the lubricant.

The coupling failed by tooth wear to the point that internal teeth slipped by each other and failed to transmit torque.

Powerpoint below. You can really see the difference between teeth of new hub and old hub.

Unfortunately, we don't have any time waveforms or other data for that time period.

gc12coupling.ppt (272 Kb, 71 downloads)

quote:

Originally posted by Sam Pickens:
The typical increase in 4X is classical. Also you'll increase the 1X component as well but with axial slippage look for 6X.

Sam,
I'm not questionning validity of this statement. Apparently it comes from real life experience. I'm just trying to understand as to why worn or partially shaved off teeth will produce 4x ONLY (well, may be little increase in 1x as well)? How much time do you think passed from the moment 4x was observed first time to the point when it failed catastrophically?

Pete,

Your vibration data does not show any symptoms of a locked coupling, does it? Does it mean the coupling did not lock up before it failed or the snow ball process was so fast that data was not acquired during this time?

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