Dear Mr. Wibo,

Please describe the type of rock. Also, whether it is coming as a overburden or mineral rock, the lump size gradation of the rock etc.

Regards,

Ishwar G Mulani.

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

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Careful:

The internal friction angle is not the same as the free flowing friction angle. By your example, a feeder under a stockpile may have a hydrostatic/ hydrodynamic force acting on the material to be sheared by the feeder action below the slot and within the skirts.

A simple internal friction angle is the observed angle of repose of the stockpile. Usually, with bigger rock pieces this angle is in the rangle of 35-45 degrees, depending on how the pile is formed.

This is not a friction angle. It is the dilation angle of particles moving on its surface rilling over one another.

Moisture, rock shape, rock size fractions have a major influence on the apparent friction as does the reclaim opening and slot geometry.

The key word is internal friction angle. Dr. Andrew Jenike spent much of his professional career trying to get a grip on it. We know today that there are many more factors and difficulties in resolving frictions: a) particle to particle, b) particle to wall or container, c) particle to belt, d) other influences noted above.

The identification of internal friction is not single valued, can be time dependent, velocity dependent, and conditional on the startup of the feeder such as might be characterized as static to dynamic transition.

If you are attempting to use this to calculate the power and belt tension, refer to some accepted references that offer a reasonables and conservative approximation.

Lots a luck

Lawrence Nordell ■

A second thought:

Researches and well informed engineers, not versed in J&J measurements or completely trusting in their application, do use observations such as:

1. make the pile and observe the rilling angle mentioned in my prior posting - 35-45 degrees

2. look at the reclaim surface angle made with the horizon on the internal slot geometry, as material is withdrawn, which can vary with the prior noted conditions but often yields 70-85 degrees

3. the later approaches the internal arching angle which is close to to number you seek.

4. If the feeder power and belt tension are on you mind, then you need to evaluate the side pressure on the skirts within the slot under the stockpile and after it leaves the feeder opening.

I think you need to be more informative to get a better idea of the best approach.

Lawrence Nordell

www.conveyor-dynamics.com ■

WOW!

I just read the replies from IG MULANI and Nordell....way outa my league, but fascinating just the same.

I am more field practical type of guy vs scientific and if I could ask for some basic definitions please:

RILLING over each other? I have never heard the word rilling used in this industry.

DILATION ANGLE?

STATIC TO DYNAMIC Position: re feeder, meaning from stopped to running position?

Internal friction angle vs free flowing friction angle.

........................................

FRICTION in regards to screening of materials on different types of media (perf plate, woven wirecloth, rubber or urethane, wet and or dry)

What general rules of thumbs could be applied here as guidelines to troubleshooting different situations where blinding or plugging is occuring in screening applications.

I hear various people attempting to explain the effects of water say, travelling down urethane and the words friction and flowability and tension? characteristics of certain materials (lets say sand and gravel) have on efficiency or lack of while screening.

Does friction , internal friction or free flowing friction apply to the above scenario and if so how?

Looking to understand............George ■

Dear George,

I will comment on a few points you have raised:

1. free flowing friction angle:

free flowing refers to granular flow that is not confined by a hydrostatic head ( no confining stress field) - the discharge off a stacking conveyor where the material flows down the outer surface of a stock pile. The ore can rotate freely or slide with little restraint from rock pushing, with little vertical gravity force, on the granular motion.

2. internal friction angle:

is the measure flow angle, such as the result from a Jenike & Johanson shear cell test, with differing normal forces (confined stress field) that create a hydrostatic or normal force component which the granular product has to shear against. The scenario is similar to a Columb force and resulting coeficient of friction measurement. One difference is the coeficient of friction can increase at a decreasing rate with the increase in confining force such as seen in the J&J shear cell test.

3. As I have stated in a prior comment, the difference between the free flowing angle can be observed between the outer stockpile angle wrt horizontal and the internal angle observed reclaim angle of the product flowing into the feeder, after the stockpile has been formed.

4. rilling:

is a term used in soil mechanics refering to pathways made in the soil with unstable and higher flow rates such as in erosion channels - path of least resistance - which in soil mechanics usually indicates a combination of soil and water channels. The key word in unstable.

5. dialation angle: is a term I use to differentiate between factors that allow the product to rotate within the flow field or not and how it changes the apparent stockpile angle when product size distribution and shape are significant.

An example - pile up marbles. The marbles can have highly polished surfaces with low surface friction like ball bearings. How do you measure the friction if the mable/ball can rotate verses having rotation constraint. Constrain the perimiter of horizontal flow. Study the mix of marble sizes. Marbles, of the same size, will stockpile at 60 degree or 2:1 slope (look at 3 nested marble pile). Marbles of disimilar sizes can have varying pile slopes exclusive of surface friction.

6. granular flow characteristics:

is a vast subject for which a few features have been or can be discussed here. I have tried to identify many attributes that should be determined in order to characterize granular behavior. Some critical elements are:

a. Size distribution - signif. volume of particles > 1 mm vs < 1 mm

b. Shape - slabby, cublic, rounded, ..... enhance or inhibit flow

c. Moisture -see the definition of cohesion and adhesion, capillary attraction

d. Particles < .05 mm increasingly resist flow with smaller sizes such as clay like behavior with critical moisture levels

e. Surface shape or asperity or lack of smoothness changes the apparent friction.

Rule-of-Thumb:

The point to all this discussion is to advance and understanding on the sensitivities to flow and the danger to elementary labeling of friction. Measure a value at one condition does not mean it will behave within this measurement without knowing what is contributing to the apparent friction. There is no rule-of-thumb.

Lawrence Nordell

www.conveyor-dynamics.com ■

Lawrence:

I have read your response with great interest and thankyou for knowledgeable, detailed reply.

Man, you guys are smart.

Sincerely,

George Baker ■

Dear Wibo,

Please consult the following books:

1. Structural bin design by.....Gaylord & Gaylord

2. Bins & bunkers by Rothe & Jenike

Regards.

A . Banerjee ■

Wibo,

If you are still searching for aid with the design of your feeder, can I suggest that you contact either of Professor Peter Arnold at parnold@uow.edu.au or Professor Allan Roberts at alan.roberts@newcastle.edu.au. Either of these gentlemen will be able to help with your problem.

Good luck, Stuart Whitton ■