This is the dynamic angle of repose.

The angle of repose is considered the static rilling angle of a stockpile. Agitate it, such as with idler bounce, launching over each idler trough set, adjusts the static rilling angle to another stable granular position which some call the surcharge angle.

Care should be taken with this angle on incline and decline slopes. Here the crossection should be reduced by the true maximum rilling angle which is a combination of the crossectional surcharge angle and incline angle.

THe angle should also be adjusted for acceleration and deceleration. ■

Dear Mr.Baker,

As stated by Mr. Nordel, it is dynamic angle of repose.While designing the conveyor ( like its cap.,loading etc..), one must take into consideration of the surcharge angle.

Regards.

A.Banerjee ■

George I applogize for the lack of clarity on my first response.

The products crossectional area is defined by the idler trough shape and the product pile atop the idler trough trapezoid shape. This section atop the trapezoid is often described as a sector of a circle ( refer to CEMA). THe angle, inclined above the horizon, the product makes at its visual edge with the belt is the surcharge angle.

I hope this is reasonable clear. ■

Then, how can we get info about the surcharge angles of the materials? I think, this angle also depends on conveyor dynamics (speed, distance between the idlers etc.). If that's not the case, how can i find the table that lists te surcharge angles of materials?

Thanks. ■

Please take care on how to apply the value to the surcharge angle. The value can vary for many reasons. Here are some:

1. Within the skirts the loading can induce a negative surcharge angle, usually from improper chute design

2. Products such as cement can aerate causing an apparent fluidization; however, with due diligence loading and eliminating hydraulic jump, the surcharge can be positive

3. Inclines will reduce the apparent cross-sectional surcharge

4. Travel along the conveyor can cause consolidation of the cross-section whereby the product fines percolate to the bottom and the apparent surcharge can increase as does its apparent bulk density. Narrowing of the flow stream can be observed

5. High speed and wide pitched idlers can have many effects that may widen the apparent product width on the belt, cause wavelets due to belt, and or structural undulations, et al. which manifest in transverse variations in width. ■

Please also take care of how the surchage angle is referanced.

The descriptions and values in DIN vary in method and description to corresponding values refernced in ISO and CEMA. ISO and CEMA being the industry norm in most documentation. ■

Dear sirs and friends;

I have been loking for the angle of repose of gypsum (just got from quarry) for a grain size of 0-100 mm. Could you please help me about the topic?

Regards. ■

Get a sample and measure it! ■

Gypsum, dust, non -aerated 20 deg

Gypsum, dust, aerated 30 deg

Gypsum, 1/2" screened 25 deg

Gyspsum, 3" lumps, 10 deg

not an exact science but near enough.

Regards

Jon Scarrott

Lodestone Electronics Ltd ■

Thank you very much Mr. Scarrott ■

??

Are you sure, in my experience aerated dusts can run like water.

(Had them over my boots a few times) ■

These figures are quoted from the Dunlop Conveyor Belt Guide hand book. I thought 30 degrees was a bit extreme. My guess is that it depends where you measure it.... i.e. in a lab or on a conveyor.

Jon ■

Hi everybody

I want to know the surcharge angle of cement (portland) . Could you please help me about it?

Regards. ■

Surcharge angle portland cement 25 degress

Surcharge angle portland cement, aerated, 5 degress

Jon ■

I thought 20 degrees . By the way Can you send me the dunlop manual.

Regards ■

My copy of the Dunlop Guide fell apart in about 1981. I rescued what I could and laminated the tatters that were left after a fire in our works. The damage was done by water from the fire hoses.

The problem is that there is a copyright issue here.

I suggest you go to the Duplop web site. There is loads of info on this site. I don't know if they still issue the booklet, but I suspect that what ever you need to know is on the web site.

Regards

Jon ■

Dear Sir,

Kindly suggest

How can we calculate the angle of surcharge. ■

Surely the angle of surcharge is a material property that is measured. ■

In following the above discussion I detect confusion in the various posts in defining the angle of repose and in defining the surcharge angle.

The angle of repose is well defined and I believe it is according to ASTM. It is the slope angle at a conical pile that is created by a bulk material poured on the ground from a specified height. I believe there are some stipulations about the surface onto which the pile is poured. I welcome comments on the specifics since I haven't researched these.

I am not aware of any such precise definition of surcharge angle. The latest CEMA Book (sixth edition) summarizes surcharge angle generally, (recommended I suppose) as related to angle of repose and materials characteristics, on table 3.3 page 47.

The comments made, from experience, in the previous posts are very good and helpful but they also highlight the ambiguities.

Unless I missed it I didn't see comments that relate the surcharge angle to the conveyor's length. Maybe relating it to the material dynamics (bouncing) is saying the same thing.

Just some snipits of my experience:

1.) Surcharge angles higher than 20 degrees are overly ambitious even with materials having high angle of repose, at short conveyors.

2.) On long conveyors, say over 300 meters, I tend to use a 10 degrees surcharge angle.

3.) Materials that fluidize (cement and others), by definition, have zero surcharge angle. Conveyor incline angle is also limited to nearly zero for such materials.

My two cents.

Joe Dos Santos ■

Mr. Phil Cummings sent me an Email with very valuable comments relating to my post. I believe these are worth sharing and, with his permission, I am happy to share these with the Forum readers.

"Joe,

It seems to me, a short to medium length (400 meters) conveyor designer for

~18 years, that some responders on the bulk forums are attempting to be too precise in their potential use of surcharge angle data. Using it to obtain an "exact" load carried per unit length of conveyor belt seems unrealistic and too precise for most bulk material handling applications. Where precision is required, such as in a blending application with strict requirements, some more positive means of controlling flow should be employed rather than relying on a standard flat belt conveyor material cross-section and belt speed for regulation.

Most bulk material handling operations do not require such precision, so we use an estimated surcharge angle thought to be very conservative (lower rather than higher) in design of belt speed for volumetric flow to assure being able to carry at least the desired bulk capacity for the application.

The same design philosophy is also used with regards to material density and volumetric flow. Then having sized the belt for width, cross-section, and speed conservatively, we then change philosophy with regards to power and use a higher end estimate of density and material cross-section to assure having enough power for belt overloading conditions. Using this methodology we have never had a conveyor of under capacity nor under power. This works well for short to medium non-precision belt conveyors that make up probably 90%+ of all conveyors. It is understandable that for extremely long conveyors, the high angle conveyors that you specialize in designing, or other special applications there are more exact methods that may be required. In these cases thorough material testing before design seems prudent and should not ignore the sometimes dramatic variations of particle size variation, moisture content, etc.

I am confident that you where aware of these and other various design philosophies used in the industry and that I have been preaching to the choir so to speak. I did want to comment to you though about the last statement contained in your recent post, "3.) Materials that fluidize (cement and others), by definition, have zero surcharge angle. Conveyor incline angle is also limited to nearly zero for such materials." Well, in my experience it depends...

I have done a few projects with cement and alumina, both designing successful air slides and belt conveyors, though I would not consider myself a master of all such applications. When either material is highly aerated I would agree that the surcharge angle is essentially zero, thus we have supplied air slides with a horizontal slope as low as 4-degrees. We successfully conveyed 1200 MTPH alumina at 4-degrees decline slope in an air slide. Ah, but when it is de-aerated it does not have zero surcharge angle!!! We successfully conveyed that same 1200 MTPH of alumina on a standard flat belt conveyor with maximum incline slope of 15-degrees for ~150 meters right after it came out of the previously stated air slide!!!

Of course we didn't try loading the material directly out of the airslide onto a 15-degree inclined section of belt! Thus conveyor incline angle is NOT limited to nearly zero for materials that CAN be aerated to zero surcharge angle as long as they are NOT. Somehow I think you knew this too!

Best Regards,

Phil Cummings

Enco Engineering" ■

Amen Mr. Cummins,

We see the same higher slope fine cement transport angles once the material is dearated. Some comments given earlier, but, not responded to are:

1. Material will consolidate on the belt when conveyed long distances. This results in a higher conveyed bulk density, and higher surcharge angle when the fines percolate to the bottom layer and intermediate rocks fill the interstitial voids of large rock agitated by idler disturbances.

2. Material can produce a negative surcharge angle when the falling flow stream causes a void from high pressure in the belt’s central zone between skirts. However, once the material leaves the skirt confinement it rather quickly tends to a positive surcharge angle within a 100m (5 m/s). I am sure there is a set of conditions which govern the angle based on belt sag, belt speed, granular dynamics, material properties (pellets, angular slabby, wet and fine, very fine), and idler trough configuration. Thus, your estimate has a lot to do with what problem you are trying to solve. Some of these finer points are not in any text and are in the domain of experience. Until somebody produces a scientific document, this is it.

3. Surcharge angle science requires detail on the individual particles and their interface asperities which creates the sliding or rilling surfaces. This precludes Einstein, Van der Waal and other chemical, molecular, or atomic forces. Such shape insight can be gleaned from such texts as McCrone’s Particle Atlas. ■

Maybe more correctly -

"Materials that are fluidised ...."

A good chunk of problems in materials handling are that while a powder may at one time be fluidised and have a zero angle of repose, at other times may arch and rathole and so be anything but fluidised. ■

- Larry,

All your points are very interesting but especially "1.". Have the affects of density increase, surcharge angle increase been quantified?

- Mr. Designer,

Indeed, materials that are fluidized.

Joe Dos Santos ■

Mr. Jack Sonc, how reliable are the results of these calculations. According to Mr. J. Willis the surcharge angle is not predictable from one end of a conveyor to the other.

For these reasons I have always taken a very conservative approach to surcharge angle, especially if the conveyor is very long with the repetition of disturbance that can cause what Mr. Willis describes.

Joe Dos Santos ■

Dear Willis & Dos Santos,

During the commissioning of Channar, I observed the ore left a trail of fine powder associated with individual rocks, starting closer to the belt edge = leaving the skirts. As the rocks were transported toward the head, the trail of fine powder moved in a curved path. The curve configuration of the powder path, tended slowly inward toward the belt center and backward toward the tail. As the transport length increase to about 5 km, the trail moved backward about 50mm and inward about 20 mm. The dimensions of this path, over 10 km (no hood covers), were from memory (did photo the event), moved toward the center about 40mm and toward the tail about 100mm.

I could observe the shrinking of the ore crossection as it was agitated by the idlers. I could also notice the surcharge angle on the edge increased, but in a different configuration than as it left the skirts. My take is a consolidation of the ore crossection due to idler agitation. The finer particles percolate to the bottom and fill the interstitial voids between the larger particles , increase the ore's density and its shear strength. Moisture was not evident.

I did not notice any reverse shaping of the ore surcharge on either belt. This does baffle me. I only see such events when a discharge stream applies pressure like a fluid action. This quickly reverts (<100m) to a positive surcharge angle after it leaves the skirts. Within 200m it is stable.

I will hopefully go to Channar, in the future, to witness this event.

For those wishing to dig deeper (pun), the belt is 1050mm wide, ran at 4.1 m/s, 35 degree trough, equal rolls, 1.5 and 1.75 m spacing, -75mm lump, 2200 t/h, at 2200 kg/cm. ■

I should have drawn a picture. The rate was initially slower toward the center, maybe about 10mm over the first 5 km. The points is consolidation increased toward the head end while a slight backward movement was observed that did not change in rate. ■

Hi everyone.

I have found in practice that the surcharge angle is about 70% dependent on the flowability of the material conveyed, 28% dependent on how the stuff is loaded (and at what angle the loading point is where it is loaded), and only changes about 2% en route, even with a rolling vertical profile.

The last point does not apply to high moisture content fines. These go from either +5 degrees to zero or, - 5 degrees to zero en route.

Regards

LSL Tekpro ■

David

To quote you...

"I have measured surcharge angles on many conveyors, both directly after the loading point and after inclines, and I have never found evidence to suggest that the allowance recommended in DIN 22 101 for the theoretical change in surcharge angles resulting from the slope of the conveyor is necessary. However, if the conveyor is inclined at the loading point, the longer material acceleration time (compared to a horizontal loading area) will affect the surcharge angle"...

At last someone agrees with me.

You have made my weekend with your wisdom!

Cheers

LSL Tekpro ■

As I can recall the DIN 22 101 adjustment for surcharge angle accounts for the true surcharge angle which is not oriented perpendicular to the belt line (unless the conveyor has no incline) but is the resolved angle as affected by the incline angle contribution. This is no different then the calculation of the valley angle in a chute with two sloped walls.

On the other hand, loading on a slope prolongs the time and distance to achieve material settling. The unfavorable surcharge angle is likely due to incomplete settling and possibly continued back movement. Material back travel on a sloped conveyor can be seen on some materials such as iron ore pellets, grains etc.

The DIN 22 101 may actually be a somewhat accurate reflection of the affect of slope at an open troughed conveyor but the calculation and the cause don't have anything to do with each other.

So, applying precise calculations to such imprecise phenomenon seems a bit silly. That is why I tend to be very conservative when it comes to surcharge angles. The behavior of materials handled are governed by the theory of soils mechanics. Carl Terzaghi, the father of soils mechanics recognized the approximate nature of soils behavior and therefore kept his equations simple and his safety factors large.

Joe Dos Santos ■

Joe

Are you agreeing with me and David then?

Cheers

LSL Tekpro ■

All

Just to add regarding surcharge, I believe this is also actually the maximum guaranteeable conveying angle and I do agree it is not possible to be precise due to all the factors mentioned and it is better to be a little conservative.

A good case, we handles completely dry fly in a pipe conveyor, as you will know it is like water.

Although in Mechanical Handling Engineers Association Recmmended practice it says for dry fine ash angle of repose 38/45 max conveying 20/22 for fly ash angle of repose 42 max conveying 20/22, so we also looked at portland cement bulk angle of repose 39 max conveying 20 and aerated max conveying 6 deg.

We did some tests of our own and decided to put the conveyor in at max 6 deg and this was proved to be just OK any more and run back would have occured containment by the pipe belt clearly helping to work at all.

The moral is to be cautious with fluid like materials.

Paul Holt ■

Graham,

I do agree with you and David but even more generally, as evidenced by the many posts on this thread, the causes are wide and varying. The predictive model hopefully reflects the cause but even if it does not, as in the DIN 22 101 adjustment, that is also OK, as long as it has the affect of correctly lowering the expected surcharge angle.

Mr. Paul Holt,

My experience with cement has been posted here a few times. Cement will fluidize and, in that state, it will not convey at any angle. I hope to learn differently in the future.

Joe Dos Santos ■

Hi Joe..

My experience with surcharge angles is 99% empirical, i.e. from measurement and actual behaviour.

I find this to be more accurate than deriving them from the theories of those wearing the smart pants.

People still reckon I'm wrong though, which is why I find David's comments refreshing.

Cheers

LSL Tekpro ■

David,

I am surprised by your pat answer on not observing the change in the surcharge or repose angle when the ore is conveyed on an inclined. We have discussed this point before. We use this condition to control spillage of larger rock on sloped belts. They would not spill on the horizontal.

Try a simple experiment. Make a pile of broken stones centered on a wood, glass or metal plate. Be sure the plate can carry a pile of twice the size of the small pile you will construct. Do it with small stones like crushed sand. Pour the pile to cover half the plate width and length. Observe you have a nice conical pile.

Let us look down on the pile and note four quadrants as 12 o’clock, 3 o’clock, 6 o’clock and 9 o’clock. Now elevate the 3 o’clock edge about half the repose angle degree you have built.

A portion of the pile will rill backward toward 9 o’clock and the rest will stay put. Take a theoretical cut across the old center from the original 12 to 6 o’clock, across the pile at right angle to the elevated plate . The new crossection will not posses the repose angle you started with before elevating.

I am sure you do not need a pile of sand to see the point and its migration. The frame of reference to the observer, looking from 9 o’clock to 3 o’clock, up the elevated surface will view a very reduced repose angle. ■

David,

Yes, I agree to a point. I believe DIN is correcting for the capacity and selection of edge to control spillage. The edge clearance will only change whe the slope becomes greater than 25 degrees. At that point, a blanket or hugger belt will be required to stop rock rotation down the material's surcharge slope and spillage. ■