James, the trapping of rock usually causes belt damage called entrapment point damage. The belt sag between the idlers allows the rocks to become entrapped, damaging the skirting and the belt. Depending on the amount of material in the hopper, the pressure on the belt can be huge. Even when the idlers are placed as close as possible. belt sag can still occur.

To solve this problem, the belt sag in the hopper area must be eliminated. This can be done with belt support systems. These can be custom made to match the picking idler configuation. Depending on the solution, this may increase the horsepower requirements of the conveyor.

The style and installation of wear liners inside the chutes is aslo critical once the belt sag problem has been solved.Martin Engineering would be happy to work with you on solving this problem. Let me know where you are located so we can contact you. ■

James,

I come across this problem frquently. I have written an unpublished paper on the conceptual design of skirt systems, if you email me I will send a copy by return.

Col Benjamin



colin.benjamin@gcsm.com.au ■

James,

The photos are typical of transfers where this problem occurs. It is not about widening the contact area but designing the skirts so you cannot trap material.

Col ■

Dear James,

The photo reveals an entrapment zone where the down stream skirt liner is proud of the up stream skirt liner on its under side.

One alternative, as a partial correction, is to replace the picking rolls with a sealing slider bar, or at least support it between rolls to inhibit deflection of the belt between rolls.

As Larry says, increasing the take-up belt tension will further reduce the sag. This would need to be reviewed in the light of equipment duty ratings.

It is sad to see the initial design is flawed. That makes the correction more difficult, having spent the money.

I have a solution, but wish to make the methods private for the moment. A number of corrective measures need to be implemented together.

Do you need a business plan to justify it? ■

Dear James,

I do read your insight. I believe I am outside of your box of ideas. Thus, I believe that I can fix your problem. Can we discuss it outside the forum for the moment? ■

James,

I had almost forgot to comment on the Palabora South Africa underground jaw crushed copper sized ( 800 x 0 mm) mine installation. This uses a 1050 mm wide belt. The product has a vast size range, which in part, causes the grief.

The small intermediate rocks, as you note, are where the cheese meets the bread.

Here CDI successfully designed a special skirt and idler seal concept in 1995. This used a axial oriented slide bar under the skirt. It is now a concept that others are applying. This is on of the two principles that I have in mind. THe other we can discuss.

I am curious about the slag size range. Its size is not an issue for my solution. ■

Dear Mr. R J Morrish,

You have mentioned 8m long feed zone length. This means you must be having very stiff belt. Such stiff belt when used for picking type idler demands specific investigation whether the belt bending radius at idler kink is extending under the skirt rubber or is it remaining clear of the skirt rubber. If the bending radius, when the belt is empty, is extending under skirt board then you cannot have proper sealing between belt and the skirt rubber. This will create material leakage and material abrasion in the gap.

You need to have a very close spaced idler, but at the same time one has to maintain tension in the belt so as to limit the sag numerical value within acceptable limits. Often such requirement of tension is not taken care. Such tension should be imparted and maintained by calibrated means. Possibly you may be using such take-up device.

The expansion type of skirt board being used by you is correct in view of long feed zone length.

It is difficult to make more specific statement without knowing the hopper arrangement, material pressure at hopper outlet, stability or steadiness of feed, etc.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Belt sag limitation becomes a bit redundant when you apply a axial support under belt at the skirt seal.

CDi designed a very long skirt seal (>100m from memeory) when we had to load primary crushed copper ore at four spaced remote jaw crusher station.

I have one new addition to further reduce the possibility of rock entrapment. ■