Hi Saithish..

You should only really use skirts on a belt feeder to contain the material conveyed on initial filling of the hopper/silo, when you get the initial unrestricted flood coming through.

This does not give much capacity for a flat belt feeder when the conveyed material is not in contact with the skirts, so wing rolls are indicated.

As soon as you have a 3-roll configuration however, this must flatten out in the transition, and this is where you will need the skirts again.

As such, hte skirts must extend well into the discharge chute to prevent the material flowing sideways onto the pulley shaft.

Cheers

LSL Tekpro ■

One of the issues affecting the design and set up of the skirting system for belt feeders is belt tracking. Best idea is have a flat belt to avoid transitions and to use on the tail drum a crowned pulley to control tracking. The design of the tensioning device is then important, make sure you can tension it uniformely by applying the tension through a single action tensioning device centrally positioned behind the tail pulley. The skirt system can then be like any normal conveyor system and frankly should be. It is only if you try to then overload the system that the skirts systems can become damaged. I wrote a paper on the set up of skirts part of which is now in our book. Let me know if you want a copy of the paper.

Cheers

Colin Benjamin

Gulf Conveyor Systems P/L

colin.benjamin@gcsm.com.au

www.conveyorsystemstechnology.com ■

There are two schools of thought. First, the straight, non-trough is applied with wide and high powered feeder belts, with belt tensions exceeding ST-800 N/mm. However the pulleys are not crowned. The fabric construction, multi-ply design, is substantial to take the reclaim tensions. The belts are often continuous loops constructed at the belt factory.

Second, some apply picking rolls (small length outer 20 degree wing) to contain the material as the skirt relief becomes significant, over longer slot lengths, and to aid in tracking. Again, no crowned pulleys are used.

Crowned pulleys might be used in medium or low powered belt feeders that do not require significant belt tensions. Below 500 PIW or ST-600 N/mm might benefit from a tail crowned pulley. I would propose a picking idler over crowning in most cases to control tracking.

Many large belt feeders use individual hydraulic actuation of the tail take-up. The hydraulic action is momentary to adjust the tracking and belt extension range. The hydraulic actuator is not in the working circuit. The take-up design is set with fixed blocking shear bars and the individual sides are adjustable with a special screw and nut arrangement to differentially set the two pulley bearing sides.

I am sure there are many variants of these concepts that can be argued for. I would caution on the use of crowned pulleys beyond the ST-600 N/mm range due to the potential for failing the center of the multi-ply fabric carcass. Multiple carcass fabrics are used to overcome the high impact loads and stockpile pressures. This comment does not apply to lighter loaded belt construction. ■

Dear Shri Sathish,

Widely used present practice appears to be skirt board height below the feed zone in following range :

Skirt board height = net width of the skirt board i.e. 100% of skirt board net width.

Skirt board height = 0.875 x (net width of the skirt board) i.e. 87.5% of skirt board net width.

Skirt board height = 0.75 x (net width of the skirt board) i.e. 75% of skirt board net width.

The material layer in the skirt board, directly under feed zone will have average velocity across its cross section less than belt speed. When this material moves out of the feed zone, there is a uniform common velocity (equal to belt speed) across its cross section, because there is no drag at the top layer. This results in to reduction in height of material layer beyond feed zone. Considering the velocity gradient the approximate height of material layer beyond feed zone for above three cases will be around 71% of skirt board width, 65% of skirt board width and 58% of skirt board width respectively.

The choice of skirt board height below the feed zone depends upon abrasiveness of the material and other application conditions.

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.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: parimul@pn2.vsnl.net.in ■

I am aware of many belt feeders with the exit opening having a shear height equal to the shear width opening. This facilitates a better mass flow reclaim performance. If the height is too restrictive compared to the slot length, the material will reclaim from the frontal opening causing stagnation of the rear material zone. This leads to much higher belt wear.

Higher skirt height can lead to higher demand power. The more significant power influence comes from the slot length and whether the material reclaims from front, full length or rear zone.

Mr. Mulani's rating ratio of exit height to width is OK, but conservative. Nothing is noted above slot length.

Material leaving the exit shear plane, will neck down depending on the particle size distribution and frequency of larger particles relative to exit plane. ■

Long slot lengths sometimes have vertical partions that ratio the reclaim height from each zone. The vertical partion zone closest to the back plate is closest to the belt. Each succeeding partion must open in proportion to the desired degree of mass flow desired.

Note, the lowest rear vertical opening must easily pass the largest lumps without blockage.

This configuration has been referred to as a "Mexican Hat" feeder. I do not know the origin of the labeling. ■

In my sheltered existance, I have done many many many (many) belt feeders.

Big heavy duty jobs

The only one I battled with is the one that they omitted to crown the two terminal pulleys.

The crowning is crucial and must only be at the edges of the pulleys, as the bit in the middle would stuff up the belt and has no good training effect.

LSL Tekpro ■

Originally Posted by nordell

Long slot lengths sometimes have vertical partions that ratio the reclaim height from each zone. The vertical partion zone closest to the back plate is closest to the belt. Each succeeding partion must open in proportion to the desired degree of mass flow desired.

Note, the lowest rear vertical opening must easily pass the largest lumps without blockage.

This configuration has been referred to as a "Mexican Hat" feeder. I do not know the origin of the labeling.

Interestingly we have just solved a problem on a front end loader fed inlet hopper to a belt conveyor for waste material by just this method. The customer on site laughed when our site guys cut a couple of plates and welded them into the hopper, telling us we'd have to fit something a lot more complicated and expensive than a couple of plates. But we're the ones with the smiles now

Of course, dividing up a long inlet to an en-masse chain conveyor to even out the draw down from a hopper goes back in time far beyond me. ■

There are 2 issues here that are creating some confusion. Firstly when designing a belt feeder you have to allow for the flow dynamics of what you are feeding it with be it a bin, hopper, etc. The issues raised re material stagnation etc are all valid. Based on this and the tonnage required we set the width and speed of the belt. The skirting system comes after this. If we try to use a skirting system to "bulk" up the capacity of the conveyor belt then it could affect the flow from the feed source as has been explained. So skirting systems on belt feeders are not about maximising belt capacity but centralising the material such that outside the feed area the material will remain on the belt. Setting up additional skirts along the length can be done as suggested. As to what is the maximum height, it gets back to design but generally they should be similar to a normal troughed belt at the load point. Another point already made is that using a skirt system to bulk up the belt capacity does lead to excess pressure on the belt along the skirt line and this invariably leads to premature belt wear in the same area. Many a belt on a feeder is changed out with plenty of rubber in the centre and gouges right through the plies along the skirt area.

Concluding, skirt systems cannot be used to solve feeder belt capacity issues, if they are they lead to other consequences

Cheers

Colin Benjamin

Gulf Conveyor Systems P/L

www.conveyorsystemstechnology.com ■

Hi all..

We have been using Mexican Hats for as long as I can remember, and they are great.

If you imagine a short pipe say 450mm diameter x 450mm long, and put a flange on one end. It looks like a hat.

Now imagine a hopper with for example a 400mm discharge pipe hanging down above a conveyor. Slide the upside down hat over the discharge pipe. Material in the hopper flows out of the pipe, and the bed depth of material on the conveyor is controlled by the level of the Mexican hat.

There is a great example of this at Rossing Uranium in Namibia. There they have a large bunker of ore, which is served by lots and lots of conveyors under the bunker, but right angles to it. Each conveyor has lots and lots of Mexican Hats, each hat being progressively higher towards the head end, thus effectively giving the bunker a "live bottom".

Cheers

LSL Tekpro ■

As a rule, to determine belt width and speed at a flat feeder, we try to limit the bed depth (beyond the loading area) to about 40% of the skirt width. For a self feeding troughed belt conveyor, where the skirts end just beyond the feed area, the calculations are like any conveyor considering the normal edge distance and the surcharge angle. The speed must be slow like any feeder that draws the material by shearing it from a material head across the throat. Too high of a speed will result in early wear of the belt covers.

Joe Dos Santos ■

This is now an old thread but I hope I can get the attention of all participants. I will try to address each of you with individual posts that will follow.

As you can see I made the last post which basically said that:

1. - At a true belt feeder, skirted from loading to discharge I try to limit the material height (beyond the shear gate) to about 0.40 x Skirt Width

2. - For a self feeding conveyor, one where the skirts end and it continues on as a troughed belt conveyor, the cross-section filling rules are according to the CEMA rules for filling cross-section, based on CEMA edge distance, troughing angle, material surcharge angle, and a recommended percentage of that area, say 85% or more.

In a type 1. feeder (true feeder) we have backed into a problem and I want to be wrong on my rule of thumb material depth of 0.40 x Skirt Width. I must admit that I have long used this rule but, from memory, I don't know where this rule came from. It may be, to some extent, hedging for cases such as this.

After designing to our rules we find ourselves needing to deliver, in very infrequent and short duration cases, a much higher (quadrupled) design rate with the same basic feeder. Without revisiting our material bed depth of 0.40 x Skirt Width this would required quadrupling of the feeder speed. This is pretty much unacceptable. On the positive side I do note that our skirt height under the feed throat is about equal to the skirt width. ■

Dr. Mulani,

You mention three different levels of skirt height 1.0xSW, 0.875xSW, 0.75xSW, with corresponding draw down material bed depths of 0.71xSW, 0.65xSW, 0.58xSW. What are the rules to determine these selections? Instinctively I always thought the material bed depth limitation is to ensure that the material does indeed move with the belt rather than having the belt undercut that material which overly drags on the skirts. Can you please elaborate on the rules and offer some comforting assurances that the material will indeed move with the belt in all cases.

Joe Dos Santos ■

Mr. Lawrence Nordell,

Your comments indicate that you agree with Dr. Mulani, citing that there are many cases with the skirt depth equal to the skirt width and generally agreeing with the draw down ratios. Can you offer comforting assurances in light of my comments in the previous post?

Joe Dos Santos ■

Mr. Colin Benjamin,

You mention that skirt width and height rules should be the same for a belt feeder as for any conveyor. This generalization tends to be ambiguous since the loading rules at any belt conveyor are dependent on the material characteristics and the conveyor's troughng configuration beyond the skirts, that is; material free edge distance, troughing angle, surcharge angle. A true feeder from end to end does not have the cited controlling parameters. This leads me to believe that you are describing the type 2 feeder, what I call a self-feeding conveyor.

I welcome your comments on this matter.

Joe Dos Santos ■

Originally Posted by Joseph A. Dos Santos

Mr. Lawrence Nordell,

Your comments indicate that you agree with Dr. Mulani, citing that there are many cases with the skirt depth equal to the skirt width and generally agreeing with the draw down ratios. Can you offer comforting assurances in light of my comments in the previous post?

Joe Dos Santos

Dear Joe:

CDI has a Design Criteria for setting Shear Bar height = Skirt Height that have other factors that influence our selection:

1. Mass flow or no mass flow reclaim criteria

2. Minimizing power consumption

3. Minimizing liner wear

4. Minimizing belt wear

5. Meeting higher flow capacity such as you are suggesting

6. Belt speed limiting condition

7. Lumps size and its influence

8. Feeder Capacity may override some conditions noted above

9. Shape of slot feeding belt feeder

10. Belt width selection from above vs. accepting a given width

ROCKY DEM can give you the necessary flow features, power, wear, mass flow behavior, et al. I claim it is possible to have a Shear height to skirt width ratio > 1.0:1.

I believe Ajax also have published something on their recommendations on the ratios. Lynn Bates has an expertise/experience in this area. ■

Larry,

Thank you. That is very reassuring. Our intention is to take the material bed depth up to 0.70 SW, up from our original of 0.40 SW. At the new 0.70 SW, rate will be handled by VFD speed control.

I do welcome comments from AJAX engineers and from Mr. Lynn Bates. If you have a link to their particular articles that deal with this issue it will be greatly appreciated.

Joe Dos Santos ■

Hello,

Referring to my earlier reply and information; it is clarified that designer is free to choose any height of opening at feed zone exit end, and then see the result. More value of this height will result into somewhat narrow belt, but more rubbing with skirt plates. Contrarily lesser value of aforesaid height will result into somewhat wider belt but less rubbing with skirt plates.

The mentioned values have one important use that they help to create ready-to-use tables / data (in context of stated values) for quick selection of belt feeder size at a glance (first stage design). Thereafter analysing the result, the designer can review the chosen opening height as per his choice (say factor 0.6, 0.65, 0.9 and so on), to suit his need. The designer will have free choice for granular material, but for lumpy materials the designer has to see that the chosen value is compatible with lump size.

The mentioned values of (feed zone exit end opening height) ÷ (feed zone exit end width) as 1.0, 0.875 and 0.75, do not signify any technical preference for said values. They only provide handy information, at the disposal of designer.

Regarding use of ratio or factor (feed zone exit end opening height) ÷ (feed zone exit end width), it helps to eliminate one quantity in calculation, and thereby corresponding ease in calculation. Such approach is quite common in engineering design practice like choosing hub length equal to 1.25 times diameter, etc. and then find the solution.

Ishwar G. Mulani

Author of Book : Engineering Science And Application Design For Belt Conveyors (new print November, 2012)

Author of Book : Belt Feeder Design And Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: conveyor.ishwar.mulani@gmail.com

Website: www.conveyor.ishwarmulani.com ■