Looking for some input on the following gearbox/pump.

Background:
Four pole motor driving a positive-displacement pump through a two-stage speed-reducing gearbox.

Falk Gearbox
Input Pinion: 24 teeth
Intermediate Gear: 53 teeth
Intermediate Pinion: 21 teeth
Output Gear: 86 teeth

Motor Speed: 1787 rpm
Intermediate Shaft: 809 rpm
Output Shaft (Pump): 197.6 rpm

Problem:
There is a peak at 6525 cpm present throughout the gearbox, that I can't correlate to any bearing/shaft frequencies (table in attached word document).

Also, there is a family of peaks between 10500 -12500 cpm spaced at the running speed of the pump that is growing in amplitude (waterfall plot in attached word document).

I don't see anything in the time-waveform or spike-energy data that suggests a bearing problem.

The historical data from an identical gearbox/pump shows a drop in the 6525 peak following the replacement of a damaged motor/gearbox coupling. At this time, we are seeing an uptrend on another unit at 6525 cpm.

Any thoughts?

Steve

Gearbox_Vibration.doc (48 Kb, 59 downloads)

You might try time synchronous averaging, triggering off each shaft to see if it's an integral multiple of shaft speed. Some quick calculations show it to be pretty close to 8x the intermediate shaft.

I was asked about a chiller that reported had 136 teeth on the gear, but a peak at exactly 100 x rotation rate. Obviously, the gear info was wrong. Inspecting the gear showed that the count was indeed 136, and that there weren't any oil pump drives or anything like that to yield a tone at 100x.

We used TSA to verify that the tone was shaft-rated. After ruling out every possibility, the only thing left was hobbing error of the gear. When threatened with a lawsuit, the manufacturer finally admitted that the index gear on the hobbing machine had 100 teeth. They wouldn't admit that a hobbing error would cause any long-term problems but they did agree to a really long warranty period during which the gears would be replaced if there was a failure.

A brief overview of TSA: TSA uses a trigger on the shaft to collect samples of data at the same point in the rotation of the shaft, and averages the WAVEFORM before computing the FFT. Imbalance, a clunky tooth, vane rate and so on happen exactly the same during every revolution, so these signals aren't diminished by averaging. Bearing tones and signals not synchronous to the shaft change phase during each revoluting, so these signals decease with averaging. If a peak is reduced by averaging, you know the source is not synchronous with the shaft. Often, a hundred or more samples are taken.

Jon
Spintelligent Labs

What kind of PD pump? 6525 CPM is close to 33x of the output shaft (33.02155, per the supplied data). I routinely see 11x and harmonics in Viking PD pumps from rotor / idler mesh. I would think it would show up more on the pump, tho.

Steve,

I have also been given incorrect data on Falk gearboxes, though not frequently. Most of the mistakes have been from their parts program that they give out to distributors. If you check with the factory using the MO number, you may get a different answer.

What type of drive is this? I have seen resonances in the extended plates that are used as the lifting lug in the bigger type Y parallel shaft drives. This has been expecially prevalent in vacuum pump drives.

I would also suggest looking at time waveforms for the drive. I don't see anything to suggest a cracked or broked tooth, but you probably wouldn't be able to see it in the spectrum anyway.

Steve,

I think it is also important to identify the peak in the middle of the cluster. Something is being heavily modulated by the PD pump. It could be some kind of resonance in the GB. Do you also see this cluster on the pump?
Is motor current also modulated by the pump RPM?

Also I'd have switched to the log scale and take a closer look at possible bearing fault peaks.

Thanks for the input. I'll try to answer the various questions that were posed.

Jon - The intermediate shaft is not exposed, so I won't be able to put a trigger on it for TSA. I suppose I could do the input and output shafts, to see if the peaks remain.

Michael - This is a Gaulin PD pump, 3 plungers. Both the 6525 cpm peak and the family of peaks 10500 - 12500 are present on the pump bearings but the amplitudes are low and don't appear to be increasing.

Danny - This is a Falk 2070Y2. The tooth count and speeds match well. I have the bearing information from the Falk parts guide. The time waveform data looks good, 3 g's max - no impacting.

Dave_man - I'm going to collect some high-resolution data to try to identify the source. The cluster is seen throughout the gearbox and pump. Why/How could a resonance be modulated by the pump running speed?
The only nearby bearing defects are the Motor BPIR and 2xBSF on the input shaft bearing.

Thanks,
Steve

Steve,

If a resonance is excited every rev of the pump then the TWF will have a typical impact pattern and will be repeatable with every rev. In the spectrum it will translate into a center peak, which is the resonance frequency, and SBs spaced at modulation frequency.

If possible post the TWF, although it may be obstracted by some other higher freqeuncies. I'd collect data in g's with Fmax=12000 CPM (cluster range) in order to see the modulation.

You also may have speed variation within one revolution. I suggest collecting very low speed data.

Dave

Steve, it isn't very likely that you have a ghost frequency in the 1st reduction; speeds are just too high unless this is just a hobbed gearset. With the lower speeds at the 2nd reduction a ghost frequnecy is a possibility. I would pursue doing the time synchronous average until you find the shaft where the component persists and then investigate further. I wouldn't want to bet that Falk can trace the manufacturing cycle as to number of teeth on a hob but it is info like that you should seek.

As far as generating a Kph on the various shafts don't you have a TK-16/TK-15 combo to use?

John

Steve,

How does the oil look? I just had a gearbox where I picked up several frequencies I had never seen before on and of the similar 28 gearboxes. I am fairly certain they are bearing frequencies but have not finalized this. The oil sample we took right after I found this has a large amount of water in it.

I may just be ringing the gears and getting some natural frequencies that are normally not there. These new frequencies are not related to any of the shaft speeds so that only leaves me with bearings and natural frequencies.

The attachment has the latest timewaveform data from the gearbox. All plots are scaled at -4 to +4 g's.

Ron - How much water are you talking about? I'm told that we draw off water fairly frequently, so the data we have isn't really trendable. We have an oil sample scheduled in a couple of weeks.

2MP192_Gearbox_Timewaveform_Plots.doc (224 Kb, 24 downloads)

Steve,

In a 4 oz sample bottle we had about 1/4 inch of it settle out as water. The oil came out very milky colored. Besides breaking down the oil properties the water also leads to corrosion in the gearbox.

It is hard to tell but it looks like the intermediate shaft has something going on at around 600 RPM or at about every .1 seconds. This could be at the shaft speed of 809 RPM, it is hard to be accurate with the images.

From your first post it is the family of peaks that seem to suggust a bearing problem. If the 6525 cpm frequency is there on all of the similar machines then the fact that it is there would not concern me. It would just be nice to know what is generating it. The fact that is changes in amplitude slightly just may be a response to system load.

Ron - Thanks for the reply. Your description of the amount of water is what I expected. I've been told that we have a lot less water in these gearboxes.

I believe the 600 cpm signal on the intermediate shaft is due to the 3 plungers on the pump (3 x 200 cpm = 600 cpm).

The amplitude of the 6525 cpm peak on this gearbox isn't too bad and is only changing slightly. It doesn't really concern me except not knowing the source. However, I'm seeing a much larger (~0.1 ips) 6525 cpm peak on one of the motors driving the same type of gearbox. The work history / vibration data shows a decrease in this peak after a damaged gear-coupling was replaced.