Re: Material Bunching
Hi Zlatni,
from the foto that you attached I would conclude the following (please comment):
- the supports for the idlers are a rigid (?) structure bolted to the side support beams of the conveyors upper strand and having a continuous structural beam to the SIDE of the idler station, but not BELOW the idler station
- the support plates of the idlers may have suffered some deformation (idler support in the front plane of your foto)
- the bunching forms under way, but not already close to the loading area
My opinion is it that you should check the alignment of the center rollers of your idler stations in the upper strand / the angle of incline of the side rollers. Maybe you can identify the appr. area where this effect starts to form these bunchings and do some measuring, once your conveyor is down for maintenance.
I think there are some misaligned stations / rollers that are higher (for center rollers) or narrower (for side rollers) than the line. Did you have some "early failure" rollers lately, somewhere on this conveyor? Then there's also the possibility of misalignment of the whole idler station, being not perpendicular to the centerline of the conveyor. This may even happen due to deformations after assembly of the conveyor line!
Roland ■
Re: Material Bunching
Hi Roland,
Yes, idlers do have rigid support structure, but they are not damaged. Plate in the picture is bent by manufacturer to give it some stiffnes. There was no reported failures of carry idlers.
Bunching starts (visibly) around 200m after loading point and gets worse along the way.
Unfortunatelly, I am not placed on the site and do not have ready access to the conveyor, so I can't check the alignament of idlers and support structure. I will keep it in mind to check during next site visit, but I would be fairly confident to say that idlers were aligned within acceptable tolerances.
Thanks, Zlatni ■
Volcanoes On Belts
Yes, yes and no.
Where are you located Zlatni? Who do you work for?
A likely problem is the natural frequency of the idler support structure is tuned to the idler spin frequency and maybe the natural vibration frequency of the belt at the given spacing, speed, rpm et al.
Fix is to break the cadence and by stiffening the support structure. There maybe components of the support structure that provide higher than acceptable compliance. The trick is to identify these highly compliant support components and increase their rigidity. Even with these modifications, you need to know if the belt's natural modal frequency is also synchronized with the idler spin frequency.
Not many belts the travel at 7.6 m/s. Western Oz? RTZ? If so, I can give you some input that may help. ■
Re: Material Bunching
There is a test rig in Western Australia that can demonstrate the event.
Material coalesces on the belt from vibration. It tends to gather from the edges around the mounds, flow under the mound toward the center of each mound and flow upward simulating a volcanic action. As you noted, it is tuned to the idler spacing.
The volcanoes also come in wavelets and can transfer from conveyor to conveyor. The problem is more prevalent in higher speed systems. Most engineers are not aware of the potential problem until they enter the higher speed conveyor design realm.
There are analytic methods we use to keep from operating in these damaging modes. Curragh North, transports 2500 t/h, operates at 7.5 m/s, and has preset discrete operating speeds that are selected to avoid the volcanoes. There are no such damaging conditions over the 20 km length.
Curragh has other features such as offset idler supports, variable roll diameters, special stiffening and other implementations we use to avoid these warts. ■
Re: Material Bunching
Hallo Mr. Nordell,
that one should be aware of dynamical behaviour of the conveyor, this I met also with stacker boom conveyors, the boom being something of easy going into vibrational pattern.
But then: 2.5 km of overland conveyor, to be dynamically analyzed and then UPGRADED to meet the necessities. Isn't this a truly major scale scope of work, the more with a running system that earns? Could you please give me some hint as to which timeframe you'd estimate such business? Do you have an experience to which approximate downtime an upgrade amounts?
Concerning this 2.5 km-conveyor, I'd really have thought: the smoother the run of the belt, the lesser the coalescences. But then, if vibration (or even resonance pattern) plays a role, must this not be felt as co-vibration of the substructure, premature wear on the rollers, something else?
Best regards
Roland ■
Conveyor Vibration & Volcanoes
Dear Mr. Heilmann,
At CDI we analyze belt, idlers, and their supporting structures as a coupled dynamic system. We do this, in many ways, as our normal course of design. We have practiced this discipline for many years. So far the genii is still in the bottle.
We argue it is a necessity. The client must be sure their investment is protected. The cost for this added procedure is small in respect to the total investment.
I do not understand your last sentence references and the term "co-vibration"?. ■
Re: Material Bunching
Mr. Nordell, Thank you for your information.
With co-vibration I wanted to express my thought, that once the problem is of a vibrational / oscillating nature and the stiffness of the substructure is of importance, then the substrucure should kind of "swing along" and vibrate with some answering pattern or natural frequency.
Then I would also conclude, once the substructure vibrates, these vibrations should pass through the bearings of the idlers and could maybe lessen their lifecycle.
Only I imagine it being rather lengthy to do dynamical analysis of long conveyors and find a solution through implementation of structural improvements. The more, when it comes to welding on site + conveyor downtime... one should really take care of this point in advance.
You mentioned occurence on "higher speed systems". Is there some rule of thumbs in order to decide, whether specialist support is necessary?
Best regards
Roland ■
Belt & Structural Vibration
The volcanoes, mounds, bunchings, generated by sympathetic vibrations produce localized impulses on idlers which does dramatically reduce their bearing life, life of idler weldment supports, and may reduce the fatigue limit of structural supporting members.
BELTSTAT Professional version does have a feature that gives insight to some of the sympathetic vibration coupling issues between belt and idlers. ■
Re: Material Bunching
Just out of interest, I have seen a similar bunching phenomenon occur on a steeply inclined conveyor where the transfer chute did not provide enough forward momentum to the ore.
The transfer chute in effect set itself up as an oscillator, due to internal recirculation.
Installation of a kick plate in the loading boot solved the problem. ■
Re: Material Bunching
Apron feeders can also cause variations in crossectional loading due to the pan articulations. I do not believe these become the mounds or hills that we are discussing and that are naturally pitched at the idler spacing. ■
Re: Material Bunching
Hello Zlanti..
The old 100 plus kilometre Krupp overland conveyor system carrying phosphate through the Sahara desert apparently suffered from this phenomenon. They fed it uniformly, but got blobs out the other end. Unfortunately the system was withdrawn from service due to too much sabotage seemingly before the effect could be cured. It took much longer to manifest than for your one though.
If you are getting the capacity through with the mounds would it be correct to assume that you could slow it down to a more normal speed that does not cause mounds?
I was asked to review and "tune" an eleven km coal overland which has a variable speed up to 6.5m/sec. I found it to be like a car with dubious wheel balancing. Anything over 5m/sec, and it misbehaved. At 5m/sec however we got the desired capacity and had a very nice smooth uniform operation.
Also, try plastic rolls of the machined variety. They run far more smoothly and could solve your problem completely.
Cheers
LSL Tekpro ■
Re: Material Bunching
Thank you all for your contributions and sorry for not replying earlier. I was slightly concerned and distracted by the prospect of not having job at all this week. Wall Street crash can be felt now in Western Australia and could make our conveyor problems (and my job) redundant.
But, I think we all love solving problems (even if we are not paid for it) so, back to the original one with some extra explanation.
Conveyer is running at lower speeds, but even initial design allowed for increase to 7.4m/s and 6000t/h. When tested for higher speeds (it is variable speed drive), it was discovered that above 6.5m/s and rates of 4700t/h, mounds starts to form. At discharge end there is a number of 25m long trusses and, with burden formed in mounds, they were shaking quite violently. Later research found that trusses natural frequency of 3.5Hz coincided with mounds travel frequency (7m/s divided by 2m idler and mounds spacing).
The first remedy was to install additional support trestle for the worst affected truss. Secondly, in the first quarter of conveyor length, additional idlers (only centre roll, to make it easier) were installed, at uneven spacing.
Those measures pushed the start of bunching above 6.8m/s and 5600t/h. At higher speeds up to 7.6m/s and feeds up to 6300t/h, bunching did occur and it seemed to be at the same spacing as before (2m). It is starting approximately 250m from feed point and after about 10 minutes running.
At the present feed rate demands, it is possible to run conveyor without bunching, but we are aiming at 6200t/h. We are prepared to stiffen existing low level modules and install some additional idlers (preferably 3-roll, full trough). I am quite confident that would fix the problem, but I am still not sure what is causing the bunching and how can we predict it in the future.
This does not seem to be very common problem. Beside couple of iron ore conveyors in WA, I was not aware of any others (Krupp’s in Africa is now mentioned). It does not seem to be many articles on this topic and that is why I placed the question to the wider on-line community. I would like to get from you as many cases as possible, to see if we can see some pattern.
Nordell mentioned 8 factors that may contribute to bunching appearance and I agree with all of them. I could even add one more, internal material friction factor. Problem is that so many factors make predictions at design stage too hard, if not impossible. Original design was done by experienced conveyor designers and reviewed by reputable international consultancy and we still got the problem. I would like to be able to identify just a couple of main contributors if possible, otherwise it all becomes a “black magic”, too hard to solve.
So, please, any other examples and all suggestions are welcome. I promise to let you know what I learned if the project goes on (and if I am still present to see it).
I hope this answers the most of questions placed to me. Regarding concern for the cost of upgrading the conveyor structure it is obvious that is going to be high, but the option we have is to make a second conveyor. That one turned out to be much more expensive. As for suggestions that mounds might be caused at loading point, I can decline it as burden is uniform after loading. Bunching appears later, down the length.
Regards,
Zlatni ■
Re: Material Bunching
For the record, I quote from the Guinness Book of Records 1985 page 153:
"The worlds longest multi-flight conveyor is one of 100km (62 miles) between the phosphate mine at Bucraa and the port of El Aaiun Morrocco, built by Krupps and completed in 1972. It has 11 flights of between 9 and 11km and was driven at 4.5m/sec, but was closed down due to Polisario Front guerrilla activity."
Cheers
LSL Tekpro ■
Re: Material Bunching
Krupp overland is 96 km.
The sabotage is ancient and had been fixed many years ago. Phosbucraa spent over 30 million to rebelt the system. I believe it is still in operation.
The problem may not be as simple as finding a new belt speed. It depends on:
1. Coupling between the structural stiffness and resonance of belt and idler spin, 2. Take-up control and tension variations
3. Resonance of return side and carry side
4. Material loading
5. Idler diameters and construction tolerances
6. Idler transom rigidity
7. Idler stringer rigidity
8. Suspended steel natural frequency
We once warned of a potential vibration on a short region of the carry strand structure of a 1.2 km downhill conveyor. The small region went into resonance that then caused a steel bridge to sympathetically oscillate as well as about 300m of steel ground supports with a 1st mode belt flap. Applying support stiffness and a few extra idlers in the initiating region stopped the fluctuating episode. ■
Material Bunching
I am not sure if terminology is correct, but that is how we call it. That term is trying to describe congregation (or segregation) of material on top of the belt, when uniformly loaded material shifts and forms heaps (mounds) after some traveling. Mounds are spaced approximately at idler spacing. Bunching is bad as it reduces conveyor capacity and can cause significant vibration of the structures.
It is clearly caused by material trampling over the idlers. There was some research done and it was observed that conveyor speed, material loading, belt tension, idler spacing and rigidity of structure play the role.
Despite all of that it seems that no one came with full understanding what is happening and how, meaning having a model to predict in what conditions is bunching going to start.
My questions to the Conveying comunity are:
1. Have you experianced such problems?
2. Have you solved the problem and how?
3. Can you point me to some reading on bunching problems?
Conveyor in question is 2.5km long, using 1050 wide ST-2000 belt, 3x7" roll 35° carry idlers, 2x7" roll 10° return idlers, transporting up to 5500t/h secondary crushed iron ore at variable speed (up to 7.6m/s).
Thanks!
Attachments
bunching (JPG)
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