Dear gentlemen,

There is a peculiar case of gearbox vibration in one of our plants. We have a centrifugal overhung blower connected to a gearbox and motor. The issue is high vibration (current value 138 micron and trip at 140) in gearbox DE bearing (in both X & Y probes). The vibration levels in the remaining gearbox bearings is normal at 20 micron. Motor vibration is also normal at 20 micron and blower vibration is also normal at 15 micron.

1. The vibration is high only at Gearbox DE bearing. The spectrum indicates purely 1X component.

2. The value is varying with every startup. Sometimes the vibration is 35 micron after start up and sometimes it is 90 micron and presently it is 138 micron. But whatever the value is after each start up it will remain same till its stoppage. However, there is no change in casing vibration. It is always less with maximum value of 1.5 mm/s

3. All instruments probes, cables, proximeters and complete loop check done and no abnormality found

4. Gearbox bearing inspected and clearance checked. Found within limits

5. Vibration is increasing with increasing load. Load is increased with IGV opening

6. Lube oil pressure to bearing was checked and is ok

We don't have a forward path as both instrumentation and mechanical have made their checks and found no abnormality. Also this different vibration value with each startup is baffling. Issue taken up with blower vendor Howden and they too don't have an answer at the moment and referred to the gearbox vendor Renk.

Any ideas?

Regards,

Nagesh

Original Post

Not sure about the last inspection date. But the gearbox to motor doesn't have a coupling spacer. Pin bush arrangement directly with gearbox & motor flanges. But in that case shouldn't the motor vibration be also high in case of damaged coupling? Motor vibration is around 20 micron

Becar posted:

Bearing type on motor, gearbox, blower.....Any photo for a better picture?

Adding to Becar's request, what fit (shrink or loose) is maintained of the respective coupling halves on both the motor and gearbox?  Single or double keys?

If the vibration is high, and power is switched off to the motor, do you know the behavior of the vibration?  Sudden decrease or does it hang up a bit as the system coasts down?

I think that the arrangement & section drawing is better.

I agree with John that the coupling hub should be considered. Do you have any trend of vibration? could you take the trend of vibration at shutdown time (direct and 1X) to see its behavior.  How the axial displacement value? is it changed with radial vib? How about bearing temp?

We are planning for a coast down plot whenever operation is ready for a shutdown. Right now running at 132 micron steadily. Trip at 140 micron.

1. I shall see the vibration trend during the coastdown and report it back here

2. Cannot provide any picture as of now but will try to give as much info as possible. All the bearings are plain bore journal bearings. Double helical gear type.

3. There is a direct correlation of axial position and the radial vibration. When there is a High axial position then there is high radial vibration. May be this can give an idea to check surface run out next time we open the machine

4. Haven't checked electrical run out yet. But will try next time. This is also in my radar. But does the electrical run out value be erratic or steady?

5. Bearing temperatures are normal and steady at 69-70 deg

Yasaswy posted:

 

2. Cannot provide any picture as of now but will try to give as much info as possible. All the bearings are plain bore journal bearings. Double helical gear type.

3. There is a direct correlation of axial position and the radial vibration. When there is a High axial position then there is high radial vibration. May be this can give an idea to check surface run out next time we open the machine

4. Haven't checked electrical run out yet. But will try next time. This is also in my radar. But does the electrical run out value be erratic or steady?

Your comments, quoted above would make me do a loop test or gap check on the probes to be absolutely sure your are dealing with the DE probes of the gearbox low speed shaft.  I have, on rare occasions admittedly, found some probes mis-wired causing people to search for an incorrect solution.

In a double helical with the speeds as you mention there likely will be some form of thrust bearing on the low speed shaft, so axial motion should be minimal.  Axial clearance is typically about 20 mils tops, and more likely should be down around 10 to 15 mils.  The pinion will not have a thrust bearing, and is free to float relative to the gear within the backlash of the gearset.  The axial float of the pinion relative to the gear can be high, perhaps 50 mils as it is a function of the backlash and helix angle.

Runout should really not vary much with axial position change, especially on the gear.  The pinion however, due to its increased amount of axial motion, may show a change in turnout as the probe target area can change.  Sometimes there is an undercut near the probe area used as a stress relief.

A loop test is accomplished by disabling all trips, and then disconnecting the probe you wish to verify.  If it is disconnect there should be significant gap voltage change at the rack or in your monitoring system.   A gap check is done by again disabling all trips and checking the DC gap at the proximitor, then again checking at the rack.  Usually there is sufficient variance in gaps that this method will help verity probe identification.

 

 

As you said the axial displacement and radial vibration is correlative. I think that I should check to verify if this change happens whenever stop and start again. If so, the situation could be as John said that the vibration is from change of gap voltage due to axial motion and I should conduct the loop check as John's recommendation.

Besides, when shutdown, the checking and correcting the position of radial probe and right alignment (axial displacement, right contact, backlash) setting should be done. Flange to flange distance should be checked.

Hi John,

I am not exactly sure I understand the loop check and the gap check you are mentioning. However, I will list out the activities done by instrumentation team.

1. They have swapped the probes at the junction box to check for any issue in probe or the cable. No change observed in the gearbox vibration

2. They swapped the proximeter of the gearbox with the motor and again no change observed in the gearbox vibration

3. They performed a continuity check for both the probes and found no issue

4. They have replaced the card in the 3500 rack which has these 2 probes connected. No change in vibration observed

5. They made extensive testing of the probe range according to its 'curve' and it was well within its range

6. One more thing I think I missed to mention is, we have measured the AC component of the axial probe (through buffer output from the 3500 rack) located at the low speed shaft (where the vibration is high) and it was found to be around 66 micron as compared to axial vibration on high speed shaft which is 20 micron. Does this say something?

I shall speak with the instrumentation team meanwhile to get more details on the loop check

A loop check simply insures that a probe, say what we think is the low speed shaft Horizontal at the machine is identified as such on the monitor.  For example, say the machine is a 100 meters from the monitor.  That is a lot of wiring to check so we physically determine which is the low speed shaft horizontal probe at the machine and disconnect the probe.  Then we verify that at the monitor, the response is what we would expect for a disconnected probe for the channel identified as the low speed shaft horizontal.

It sounds as if you are certain of the point-to point wiring and that the probes are identified properly.

As far as the dynamic axial vibration being higher on the low speed shaft than the pinion, it just brings me back to wondering about the condition of the coupling on the low speed shaft.  But it could also be something as simple as some physical damage on the shaft end, excessive runout, ...

By the way, what type coupling is used at the high speed shaft?

Do you have correlation between casing and proximity probe?  In other words, right now the proximity is very high, 138 micron.  What is the casing in that same physical area?  Is it relatively high?

You've stated that "Sometimes the vibration is 35 micron"; do you have similar casing measurements when the proximity is 35 microns?  Is it now relatively low?

 

I want to tell a couple of things about the casing vibration.

1. When vibration in May was 95 micron, the maximum casing vibration was observed to be 1.3 mm/s. After the next start-up when shaft vibration was 35 micron, casing vibration was 1 mm/s and now when vibration is 135 micron, casing vibration is 1.6 mm/s. All of these are in the horizontal direction. Vertical and axial values are less than 1 mm/s.

2. In a different train but same blower, we had a low speed coupling bush failure in the past and the shaft vibration shot up from normal 15 micron to 65 micron on the gearbox and 8 micron to 11 micron on the motor. And during that time the casing vibration on gearbox was increased from 1.5 mm/s to 6.5 mm/s. and on motor it increased from 1.5 mm/s to 4.5 mm/s. 

I am not sure if I can say an increased of 0.5 mm/s now is a big increase compared to the increase in shaft vibration

Yasaswy posted:

 

I am not sure if I can say an increased of 0.5 mm/s now is a big increase compared to the increase in shaft vibration

I would agree.  I guess I'm back to the coupling being the issue.

Are you at liberty to share the make and model of the gearbox of a cutaway drawing?  The best would be what is termed a mass-elastic drawing that gives some overall dimensions and hand of assembly.

Add Reply

Post

HACKATHON in association with IMC-2019

×
×
×
×