There is no need, for most installations, to adjust the rated capacity. There are a number of factors that control this condition:

1. material is compacted during transport by the wing rolls and belt resulting in a reduced cross-section at the head end and fines filling the intersitial voids increasing the shear strength of the cross-section.

2. it takes time and a disturbance to unsettle the ore configuration in the compacted state

3. the transition distance is very short taking about 1-3 seconds to traverse the transition zone

4. as the wing roll is relaxed, the material begins to break into three distinct regions, center and to wing elements which are still compacted - this holds to the beginning of the head pully curve

5. the transition is under high tension, thus little sag

6. on;y two or three transition idlers need to be tranversed so there is little disturbance or time

7. ore does begin to move laterally and occupy free belt edge that has been allowed for in the capacity calculation, however round large lump and mis-centered loading can cause spillage at the transition

8. all the above assumes the belt has not been unreasonably overloaded excepting point 7

9. I have taken a number of movies of this condition

10. We model the condition using the DEM model to study the event with large rock and with a head pulley raised to reduce edge tensions but cause more of an upset condition

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com ■

Ash,

If you abide by the 6% clear edge allowance per CEMA and DIN, there is no need to reduce the capacity to that of a lower trough angle within the transition.

If the transport time across the transition zone is more than 2 seconds, or you have a prill shaped rock that has a tendency to roll, a reduced cross-sectional capacity or retainer system may be advised.

These comments are for flat transitions.

Long overands, where a high cost penalty may occur, to increase the belt width or belt speed, we would model the ore flow in the transition, using discrete element (DEM), to define the sensitivity to this condition.

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com ■

You may be over analyzing. The material need only make it to the chute before it sluffs and spills. Experience has demonstrated that it does so hence this area is not considered in the capacity calculations. Even in the case of stopping under load there is typically not side spillage in the transition area as high surcharge angles (as high as the angle of repose, possibly higher where the material has some cohesion) are realized, since there is not the repeated agitation.

Joseph A. Dos Santos, PE ■

Sorry , I may be digressing somewhat , but I would like to know if there is a clearer basis for determining permissible belt width based on lumpsize considerations. The information in CEMA is not exhaustive enough - for eg. what could be the tentative width for coal of (-) 250 mm with lumps upto 400 mm sometimes recieved?

Regards.

Vishy. ■

Mr. Vishy

According to CEMA, for coal, max lump size shall be (Belt Width)/5 for material which is predominantly lumpy (more than 10% lumps) and may be up to (Belt Width)/3 for material which is occassionaly lumpy (less than 10%). Your belt must thus be 1200 mm minimum width. The effects of large lumps are felt at the transfers where typically the material stream is narrowed to (2/3)x(Belt Width) at the skirts. Jamming can be a problem as two lumps (in case of (BW)/3 size lumps) side by side can fill the width. Care must be taken in the material transfer to insure that the material flows while minimizing adverse impact.

Joe Dos Santos, PE ■

The area of the load in the trougth secction and in the area of the load at the tansition have the same numeric value with diferent sape , Q and V are constants.

The fact is that the surcharge angle change in the transition point, and the belt edge are deduced to minimum.

For big lumps is preferable to use moderate surcharge angles, and need to know the exact angle of internal friction.

joseamc@prodigy.net.mx ■

Dear Shri A K Bhatnagar,

The explanation / information given by various participants is more than adequate. However I would just like to state that:

1) Material / belt travels the transition distance in about 1 second (less or more as per actual data). The material tends to spread but will not reach to the belt edges during this time, and hence it is not necessary to reduce the capacity because of transition zone.

2) As the belt speed diminishes and comes to stop, the material agitation also diminishes, and surcharge angle value increases to repose angle. So no spillage in transition whether loaded belt is moving / stationery. This is the experience of the industry for usual situations.

Above answers are generally in line with the answers given by Mr. Nordell and Mr. Joseph A. Dos Santos.

Regards,

Ishwar G Mulani.

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

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 5882916 ■

Another Addition:

Transition zone spillage and rock size difficulties restated. This has little to do with the general or normalized belt cross-sectional loading and its behavior in the transition zone, and more to do with the occasional rock that is perched on the edge of the belt in the free unloaded zone. This rock is put into motion by the transition geometry change. The rock is given momentum towards the belt edge as the transition accelerates downward. When belts are allowed to reach the limits of CEMA/DIN/ISO based on edge clearance for belt tracking, large rock size exceeding twice the edge clearance is a candidate for spilling. Look around the conveyor. If you see rock about, be concerned.

L Nordell ■