I have an odd problem with a gearbox. It is a Farrel SR158A, 2.72:1 ratio (106T/39T), single reduction, herringbone gears. It is driving a calendar roll (via a drive shaft) for our paper machine. The gear box is not old: about 2 months since installation. It is rebuilt, to what extent, I don’t know, but the gears look new. It has been making a sharp knocking noise at the output shaft frequency since the rebuild (output shaft = 533 RPM). The highest amplitude is vertical on the input shaft (1449 RPM). We don’t see any gear fault frequencies on either shaft. The Gear Mesh and 1st harmonic is modest (< 20% of overall).
We were sure there was damage on the output gear (there seems to be a rash of them lately). So we pulled the inspection cover off of it, and examined the teeth. We slowly turned the gear around and did not see any significant wear or damage on the teeth, just a few very small dinks, and no uneven wear. We took an oil sample and it had no visible metal or crud in it (it’s still out for analysis). We pried on it radially and axially to see if it had play: none. We have run out of time on this shut down, but next chance we get we’ll put a dial indicator on the gear to see if it’s cocked on the shaft, though I don’t think that’s the problem.
Any other ideas on where we should look? the attached document has some plots and a picture of the gears.

Knocking_on_a_gearbox_at_output_RPM.doc (4,243 KB, 131 downloads) Word Doc

Your post suggests and audible knock at output shaft speed. You may want to confirm this. If the sound is distinct enough, I have often had success by using a microphone to obtain a spectrum of the sound. If you have ultrasound available, you might use the headphone output, assuming you hear the knock in ultrasound, to obtain an ultrasound spectrum.

Have you tried collecting peak view data on the gearbox?

Carson,

"We pried on it radially and axially to see if it had play: none. "

There should be some play, especially with herringbone gears. One gear shaft should be fixed within limits and the other shaft should float, the position being held by the herringbone. Plus there should be backlash in the gearmesh.

Regards
Joe Mc Cormack

Carson,

Can you relate, what appears to be a 1500 HZ peak, to anything in this gearbox or it is just a resonance frequency excited by this impact? You may want to impact various parts in the GB to find the resonating one if it is indeed a resonance.

It also appears that one of the gears is bowed by looking at relatively high SBs. Could bowed shaft be related to the impact?

Also Fmax has to be extended to see at least one more harmonic of Gmesh.

The earlier comment about allowing axial shaft float is correct. Speed reducers with double-helical gearing have a slightly different installation procedure, not difficult, just different. Can you comment on the types of bearings in this unit (i.e. tapered roller, spherical roller, etc.) and which shaft they are located on? Tapered bearings would indicate a "held" shaft, and typically the held shaft would be on the low speed, but not always. These older gear drive designs all seem to be a little different. At any rate one of the shafts needs to "float" during operation to ensure proper gear teeth contact.
Additionally, you may be picking up something from your low speed drive shaft. Is it a floating shaft gear coupling arrangement or a U-joint shaft arrangement? Has the assembly been disassembled and inspected for wear recently?

I have just recently completed a report on a similar condition - herringbone gear box being replaced and the problem was associated with the gearbox output shaft - 129 RPM. It was not an easy answer - if you would like to discuss the approach I took please email me your contact information. We could not really hear it but it was very visible on the demodulation spectras.
William
info@diagnalysis.com

Carson - I believe I have seen this myself, and what I have found in our gearboxes is that when they are rebuilt in-house, most of the guys just go back with original shims, which in most cases is alright as long as you remember where they go, but if your mill is anything like ours we sometimes do not have all the spare parts on hand,so we end up tearing down one day and putting back together several days or weeks latter. Anyway my point is: It appears to me as though the pre-load on the output bearings is set to high or the jack-shaft my be locked up and not allowing any float. Does this happen no matter what speed the paper machine is running? I know that sometimes coming off of a long outage the operators sometimes forget how to set the draws correctlly if you know what I mean. You may want to check there also, I know with our machine if the draws are not set right it will make our gearboxes knock pretty badly because of the push and pull affect of the paper. Hope I've been helpful, Happy Holidays

One more idea - I'm assuming this gear drive is driven by a variable speed DC motor. If this is the case, have a good electrician check the signal to each pole of your DC motor. You may find the signal to be weak or non-existent at one or more of the poles. When the motor is turning the gear drive under loaded operation, and one of the poles is "mis-firing", the motor rpm slows slightly. The calender wants to keep going as an over-running load. Essentially what happens is the gear drive is slapping back and forth through it's own backlash. The gear drive needs backlash for thermal growth and lube pickup. I've had this happen about four times, where almost everyone thinks it's a gear problem. The "slapping" takes on the sound of rapid-fire knocking, and can be very loud within these older cast-iron gear drive housings.

Although a visual inspection is likely to show some defects, a defect can be quite small and still cause a knocking sound. You might want to get a keyphasor on that output shaft and take some time domain data, and see if you can isolate the knock to a specific physical area. The next step would be to clean the teeth well and spray with a light coat of Dykem layout blue. Rerun and see if anything then becomes evident.

John from PA

This message has been edited. Last edited by: John from PA,

Carson,

I have encountered several "knocking" gearboxes over the years including:
1) Broken gear teeth
2) Broken shaft inside gear hub (hidden from view)
3) Metal debris on tooth faces
4) Chatter from torsional natural frequency
5) Broken lube oil pump drive shaft
6) Shaft coupling fault
7) Worn quill shaft

There were no incidences caused by fauly bearings, athough I suppose it could occur. Here are a few methods that I have used to detect "knocking":

1) Observations -- touch and audible
2) Conventional waveform and spectrum analysis (sometimes ineffective)
3) Demodulated vibrations including PeakVue
4) Motor current spectrum analysis
5) Ultrasound -- contact measurements
6) Audible sound pressure
7) Torsional vibration

Knocking symptoms on a gearbox can indicate a fault or abnormal condition ranging from minimal to severe severity. I suggest that the cause be identified rather than waiting for it to get worse. If someone wants to wait for it to get worse, then ask if they can accept the down-time and have a spare gearbox on hand to deal with the worst-case scenario.

Walt

Carson,
Herringbone gearings are extremely trust sensitive so axial load will produce odd things. It generally produces fractional gear mesh frequency (2/3 or 5/3) with side band at either or both shaft’s speed. You may not see the beginning wear but depending on type, brg maybe rapidly damage most likely at the cage if it is of double spherical roller on a slide fit. Waveform on G4V shows strong vertical impact at 8 g’s PtoP???? Could it be the knocking sound? Drive shaft with universal joint may produce impact if the joints are not 90° apart from each other.
Good luck, Marcel

Do you have a drive shaft connected to the output of the gearbox that has a spline that allows you to separate the two ends of the drive shaft? I have seen this same problem were the mechanics separated the drive shaft and did not put the spline back into the right position which causes the drive shaft to knock at each revolution. The knuckles on each end should line up with each other so you have one at each end going through the load zone of the offset at the same time.