Conveyor head pulley bearing demonstrating short MTBF's. 5

[Zylinderkopf]

This question pertains to a bearing performance issue, however, it may be better suited for either one of the "Structural / other topics" or the "Materials Handling Equipment" forums. If that should prove to be the case, then please accept my apologies in advance and know that I will gladly re-post in the forum that I will be first directed to.

I am a relative new-comer to the mining industry and have been employed at a surface copper mine (for approximately four months) that is using the mine-for-leach strategy.

Part of the belt conveyor system utilized to transport ore, from the primary crushers to the intermediate ore stockpile, crosses over a state highway. I have been informed that approximately 16 months ago, the truss that extended over this thoroughfare and the part of the conveyor that it supports, rotated about its foundations and the entire structure came to a woeful rest on top of the state highway.

The conveyor head pulley is located at the discharge end of this section of belt conveyor, and is located on the conveyor structure after it has immediately crossed-over the state highway. Here, the carried ore is slung out on top of the intermediate ore stockpile.

The left side bearing on the head pulley reportedly went bad and was replaced in 10/2005. The head pulley and the bearings, one each on the pulley's sides, were reported in bad order and replaced again in 12/2005.

During this week (during 2/2006), the subject head pulley left side bearing race was reported by the mine “reliability centered maintenance” group as indicating that the left side bearing was, yet again, going bad. I have been informed that this group uses the shock-pulse method of investigating bearing performance issues.

It was recently announced that an independent contract engineering concern was going to evaluate the conveyor structure. Management elected to take this step, reportedly, because an SKF bearing company representative was present at the installation of the head pulley and both of its bearings in 12/2005. This same installation was reportedly verified as being acceptable by the mine “reliability centered maintenance” group.

My hope is that at least one of you good folks could provide me with your informed judgment on this matter. I have no prior experience with bearings that repetitively fail in such a short period of time. It seems that there was a period of a ten month dormancy that followed the structural mishap, and then the bearing problems appeared, and with a vengence.

What would, in view of the history that I have presented, I want to check, or perhaps, re-check to begin to narrow-down the etiology of this problem?
When a group comes to look at the supporting structure, is there a preferred method of investigation, in way of bearing failures, which should be employed? There is nothing that appears to be out-of-sorts with the structure at this time.
Is there a list of “standard” information that I should be attempting to illicit at this juncture from the investigating company?
I can only repeat that the maintenance management feels positive that because the SKF representative blessed the latest bearing installation, then the problem is more structural than mechanical. Is that assumption correct, in your view?

Any guidance that you will provide me with will be most greatly appreciated. Thank you for your kind consideration.

 

[Skogsgurra]

I think it would help to know what size and type the bearings are. Also the power and type of motor, coupling etc.

I would check for alignment, shock loads (unexpectedly big ore parts), belt tension, grease, dirt, temperature, high-pressure/steam cleaning, electric discharge in the bearings. The SKF guy may have checked the more obvious things but forgotten some.

Gunnar Englund

http://www.gke.org

 

[daveboy]

Have you sent your failed bearings back to SKF and asked them to identify the cause of failure. SKF a very good at making an assesment of probably caase of failure from looking at broken bearing parts.

Are the loads as per design, has anything changed?

The SKF guy and probably any bearing installation guy is usually there to ensure the bearing is fitted correctly and is unlikley to verify that the loadings on the bearing are as designed unless this is a specific requirement in the contract.

It is amazing how quickly bearings will fail if overloaded - smaller bearings can catastophically fail in hours if seriously overloaded.

 

[diskullman]

Sometimes a bearing failure can be caused by other failed components further down the line. Check your idler bearing temperatures with an infra-red thermometer. Look for frozen idlers, clear all accumulated debris from conveyor path.

Provide more details. What is the average life of the bearings, size, speed? What has changed?

Russell Giuliano

 

[Zylinderkopf]

Thank you all, very much, for the excellent direction.

Mr. Gunnar Englund:
Sir, thank you for your time.
The SKF bearing is a spherical design with cylindrical bore (for our application, a 14-inch bore). The SKF part number is 23276, and the particulars are found at:

http://www.skf.com/files/151311.pdf

I will utilize your list and immediately begin surveying to determine what items on this list, if any, have already been addressed. Although the items on your list I "sense" may have already been completed, I know with certainty that issues pertaining to electric discharge have not been reviewed. Thanks again.

"daveboy":
Sir, thank you for your time.
The loads and other design issues that you allude to have been reported as not changing, and, from what has been reported in the historical trends by the operations group, this reported lack of change is substantiated. Thank you for your advice pertaining to the primary focus of attention in way of the bearing manufacturer's representative visit to the job site, and I will certainly follow-through on this advice that you have provided me with.

Mr. Russell Giuliano:
Sir, thank you for your time.
The conveyor was deisgned to run at 7500 tonnes per hour ("tph"), and has been running, historically, at 5000tph.
I have been told that on isolated occaisions, when power costs have been high, this conveyor has been shut down in way of "peak consumption hours" in the day and then run at "off-peak" hours at night. So that the daily tonnage averages could be kept at a consistent level in way of these "peak-off-peak" periods, the belt was reportedly operating at 7000tph.
I will follow-through on the advice contained in the first sentence of your response...I am reasonably confident, at this time, that these issues have been addressed (but, maybe at least one of these issues may have escaped someone, so I will check).
The SKF bearing is a spherical design with cylindrical bore (for our application, a 14-inch bore). The SKF part number is 23276, and the particulars are found at:

http://www.skf.com/files/151311.pdf

Thanks again for your time.

 

[Skogsgurra]

I had a look at this bearing's data sheet. Since it is a spherical bearing, it should tolerate quite lot of misalignment. I think that misalignment can be removed from the list (if it isn't extremely misaligned - check if anything gets bent during those peak load nights). Is life of belt normal? Or does it wear abnormally, too? That could be a sign that things are overutilized. Have you discussed the CARB type of bearing in this application? Takes less misalignment but can take more radial load.

Gunnar Englund

http://www.gke.org

 

[Zylinderkopf]

Greetings, Mr. Englund:

As I know it was a bit of a hunt through the web page material, I find myself becoming more deeply in debt for your time and kind advice.
As of this moment, the "update" is that the issues that you had previously directed my attention to have all been checked and reported as having a satisfactory condition, EXCEPT the issue of damage resulting from electrical transients. Count this issue as one that would not have occurred to me without your excellent advice.
The belt wear has been reported to me as "normal", but that means to me that it is simply characteristic of the wear that they have traditionally experienced with this belt. As such, I can tell you that I honestly do not know if the belt presents signs of a premature wear.
It is with some degree of embarassment that I must tell you that I do not know what a "CARB"-type bearing is...intuitively I feel that a definition is easily had on either the internet or other reference materials and, as with your previously-shared guidance, I feel that I am making great progess in way of at least learning more about bearings, which is more than half of the fun!
Thank you again for your advice.
Best Regards,
Pete

 

[Skogsgurra]

Cylinder Head,

You may want to read this:

http://www.engineeringtalk.com/news/skf/skf120.html

Gunnar Englund

http://www.gke.org

 

[Tmoose]

If I understand the symptoms described so far, it <might> just be time to re-grease/lubricate the bearings.

I would re-lube anyway, while gathering More info.
---------------------------------------

How many hours did the conveyor run before it hit the highway? Were the bearings part of the structure that fell?

How long have each of the bearings really "lasted," total?

Was the Feb-2006 bad bearing verdict based on 1 measurement, or a trend? Was the bearing greased and re-shock-pulsed? Has traditional vibration analysis been used to evaluate the suspect bearings? Ask the shock-pulse tech representative if he can detect a rubbing labyrinth seal (if that is the style your bearings use).

Spherical rollers laugh at significant amounts of static misalignment, like varying pedestal heights. Looking at the SKF info you linked it appears your brgs may have lip seals that can handle about 1/2 degree of misalignment. However Dynamic misalignment (bent shaft) requires them to skid to some degree, and their skiddy-screechy complaints probably would alert and upset the Shock pulse meter. As would bearings whining and complaining about old grease, water contaminated grease/oil, or other-wise inadequate lubrication. If the complaints have only just begun, and the lube condition is corrected the actual bearing damage is sometimes minimal.

 

[Zylinderkopf]

Herr Englund:
Thank you, again, very much for your time and kind advice.

"Tmoose":
Sir, thank you for the time that you had taken to respond to my request.

The bearings were attached to, and were part of, the structure that fell. The other excellent questions that you have tabled have, I regret, not yet been answered.

Having received your valuable advice, I have sent requests to the "RCM" group at the plant to determine, precisely, the answers to your questions pertaining to the lubrication and shock-pulse measurement issues. It is my hope to receive useful information by the middle of this coming week.

Thank you again for your time and consideration.

 

[unclesyd]

Throwing something out there that be a contributor to you problem.
You state that the bearing is dormant for periods of time.

During this time is this bearing subject to vibrations of any kind?

I have seen several failures of stationary or unloaded bearings fail immediately upon being started. A majority of the failures were due to relative motion of the ball and race resulting in false brinelling with rapid failure upon being loaded.

 

[Zylinderkopf]

"unclesyd":
Sir, thank you for your time.

There certainly is a possibility that the bearing is exposed to some vibration during periods of both dormancy and activity; a railroad spur is within 75 metres of the foundation that supports the frame for the subject head pulley. I will investigate this issue today.

Thank you, again, for your time and kind advice.

 

[Philrock]

Contamination control and lubrication are particularly important for heavily loaded bearings - especially at low speeds. At low speeds it is difficult to maintain a good elastohydrodynamic film between the rollers and races.

If the head shaft is driven by a chain or belt, this drive could be greatly increasing the load on the bearing near it.

Would it be possible for you to post a sketch of the drive somewhere? The following information would be helpful:

Motor nameplate power, rpm, and current.
Any data you may have on motor current while running under load.
Any strain gage or load cell data you may have relating to the problem.
Hours of operation per day.
Frequency of starting/stopping.
Head shaft rpm.
Head pulley diameter.
If the head shaft is driven be a chain or belt, the diameter of the drive sprocket or pulley on the head shaft.