Friction coefficient

Posted in: , on 1. Jul. 2003 - 18:34


I went through your other articles and hence I am posting my question.

I am Enginneer working in India and I am in the process of design of Apron feeder with a belt.

I want to know

1. what value to be given for internal friction coefficient for various materials. (what does it depend on?)

2. what value should be given for friction coefficient between skirt plates and material. (what does it depend on?)

3. what value of coefficient to be given between steel and steel under full lubrication(e.g chain roller on steel)

4. does these values changes with respect to liner materials?

I hope u could help me in this regard.

Thanking you,


Friction Co-Efficient

Posted on 8. May. 2008 - 07:01

Dear Sir,

I have not seen any apron feeder with rubber belt. For friction, please refer any handbook. However the following may please be consider as referance.

1.Internal frictional angle for most of the material will vary between 40 t0 60 deg.

2.Skirt friction can be consider as 8 Kg/M.( Data collected from skirt manufacturer)



Re: Friction Coefficient

Posted on 8. May. 2008 - 10:41

Different bulk materials are not like dirfferent grades of steel where you just look up a specification book/chart and read off the answer. Internal friction needs to be MEASURED using a sample of bulk material to be handled.

Some figures for skirt friction are contained in publications like the CEMA belt conveyor guide. (Does anyone know how to measure these figures or are they just empirical??)

Re: Friction Coefficient

Posted on 8. May. 2008 - 11:01

I am very surprised at the comment that internal friction is 55 in most cases. Internal friction is considered to be one of the most difficult parameter to measure in powder handling and it also the most important in determining various silo design parameters.


Friction Coefficient

Posted on 8. May. 2008 - 11:34

Dear Mr. Mantoo,

Please open page no. 131 of book ( Bins & Bunker by W. Reisner& M Rothe) wherein it is written that angle of internal friction would be 40 to 60 deg.

Also I like to refer another book ( Weighing & proportioning by H. Colijn ) at page no. 257 it is written DELTA would be 30 to 60 deg. May be it is difficult to measure but form the book I indicated the figure.



Re: Friction Coefficient

Posted on 8. May. 2008 - 01:02

Indeed both books have given the normal range nothing wrong with that.

What I cannot understand is why give a range when 55 degrees will work for everything? it will make life of lot of people very easy.



Friction Coefficient

Posted on 8. May. 2008 - 01:16

Dear Mr. Mantoo,

You are very much correct that 55 deg. is may not applicable to every materials specially powder materials.



Re: Friction Coefficient

Posted on 8. May. 2008 - 01:17

I tend to be sceptical of data written in books without a full description of the material and methods used.

If I measure something myself I know what I'm talking about.

I get the impression these days that too many people are treating materials handling as "here's a formula, here's some numbers, there's the result". Fourty years of materials handling experience has illustrated that is the road to ruin, and frequently seems to lead to forums like this and the question "I designed a machine/system/bunker but it doesn't work, please help me fix it".

Friction Coefficient

Posted on 8. May. 2008 - 01:44

Barring few,almost all designer take data either from handbook or from reputated book & there is no much faliure.



Re: Friction Coefficient

Posted on 8. May. 2008 - 02:24

Originally posted by A.Banerjee

there is no much faliure.

Alas, my experience is to the contrary.

I visit site, problems are outlined to me and equipment shown to me, and like the Irishman I am known to reply "To be sure, if I wanted to get to there I wouldn't be starting from here".

Such is life!

Friction Coefficient

Posted on 12. May. 2008 - 11:21


Your queries has given rise to some interesting responses so it is probably worth while to go through some fundamental background to the points made in dealing with your specific questions.

1.The ‘Internal friction’ of a bulk solid is something of a misnomer because ‘friction’ generally implies a sliding relationship. The value of internal friction is actually a combination of contact sliding between particles and mechanical interference due to the irregularity of the particle overlap during bulk shear. As it is not practical or useful to differentiate these components, a global resistance is taken from measurements from shear cell tests, from which a yield loci is derived. There are many publications that describe this technique, starting with Jenike’s Bull. 123 of the University of Utah.

This value is material dependent because there are a host of factors that influence inter-particle behaviour. Using a set value that is on the high side may produce a design that works, if you're lucky, but it will almost certainly be either over-designed or inadequate, with expensive consequences either way. As an example of using specific values for design, two studies covering wide ranges of common bulk materials assessed that the probability of securing mass flow with a conical hopper that has a 70 degree wall angle is about 35 to 50 %, or 55 to 70 % prospect with a 75 degree wall angle of cone. These are hardly odds that a professional engineer would find acceptable and the chance of either being an optimum value is of the order of 5%, so the result will be either un-commercial at best or disastrous at worst. I cannot emphasis strongly enough that there is no substitute for measured values when dealing with bulk solids.

2.The fiction co-efficient between a bulk material and a skirt plate is essentially similar to wall friction. The measuring technique is simple, preferentially made with a long-stroke testing device as at A sliding plate test can be conducted for crude results, remembering to measure with a number of different normal loads and graph the results to secure an angle of friction and indicate any wall cohesion. This would be adequate for an apron feeder as skirt resistance should be a minor element of the total force on a well-designed unit. Wall friction on a smooth surface basically depends on the molecular relationship between the particles and surface of the contact material, so while it is unique to the specific materials there is usually only the choice of changing the contact surface. There is no such thing as a universal low friction liner because what may give easier slip with one bulk solid may offer higher resistance with another so the answer is measure every time to secure accurate design values and optimum results. More awkward to establish is the normal force acting on the skirt as this is a function of the outlet over-pressure, which is a key multiple of the internal friction coefficient.

3.Lubricated sliding generally produces low friction. Roller contact is not friction, but rolling resistance, which is even lower. Neither is likely to be ideal in bulk solids applications because of contamination, so the assessment calls for a close look at the specific circumstances and some experience.

4.Liners, surface finish, wear, corrosion, surface irregularities, all make a difference to friction. May best suggestion is to invest in a wall friction tester. This factor arises in all circumstances where bulk material has to move on a contact surface. The tests are easy to conduct and the results invaluable for design.

Outline designs for belt feeder interfaces for easy flow and difficult flow bulk materials were given in a paper that I presented at the MHEA Annual Seminar in 2006. Two key features of effective extraction are the securement of reliable flow and the creation of a progressively increasing dynamic arch to facilitate a change of flow direction with minimum shear resistance. Whilst the dimensions and geometry have to be chosen to suit the specific application, this shows the basic principles of exploiting Sigma Two relief and providing expansion to relax shear stresses. Contact me at if you require a copy.

Finally, I would recommend that Mr Banerjee read Ed Merrow’s report for the Rand Corporation on the performance of plants handling bulk solids compared with those handling liquids and gasses regarding his assertion that, sic ‘there is no much failure’.

Re: Friction Coefficient

Posted on 12. May. 2008 - 06:04

Yet again wise words of advice from Lyn!

Friction Coefficient

Posted on 28. May. 2008 - 06:18

I am late in commenting.

There have been comments regarding actual results from large scale testing of actual various materials. I performed such testing in the early 1980's associated with my development of sandwich belt high angle conveyors. Of course, knowing the friction coefficient is crucial to the success of our sandwich belt systems. Additionally, at that time we had a troubled rotary plow coal reclaimer so we did some coal on concrete testing as well as materials on steel testing.

At the time I did not trust the typical, traditional small scale testing to produce reliable results for our bulk application. I designed a system large enough to not require any conditioning of the bulk material before testing. The test samples were about one cubic yard. The test set-up consisted of a lower steel box, struck-filled with bulk and an upper bottomless and topless box also struck filled with bulk. The upper box was supported on very low friction wheels so that the box weight did not contribute to the friction.

We determined the friction in two manners. In the first case we pivoted the lower box (with the upper box above) to the angle at which the upper box began to move downward with its material sliding over the material of the lower box. In the second case we used a cable, dynamometer and winch to pull the upper box past the flat lower box. In the second case we also added increasing weights onto the material in the upper box.

The two manners of testing (tilt test and pull test) were just two different ways of calculating the coefficient of friction. Adding weights, in the pull test, was to determine if the crowding would affect the coefficient.

The results were consistent and repeatable. Bulk on bulk coefficient values varied from just less than 0.7 to just over 1.0, typically (though not always) the respective tangent of the angle of repose for the various bulk materials, from soybeans, iron ore pellets, to 10 inch minus crushed ores. The additional load increments onto the bulk did not typically affect the friction coefficient. Various levels of moisture in the material also did not greatly affect the coefficient of friction.

These are the purest measures of internal friction, performed pursuant to the definition. I suspect that the typical values specified for design of bins and hoppers reflect effective internal friction not true friction in order to account for the various effects that are not due to friction, such as combinations of cohesion, compaction, arching etc.

Joe Dos santos

Dos Santos International 531 Roselane St NW Suite 810 Marietta, GA 30060 USA Tel: 1 770 423 9895 Fax 1 866 473 2252 Email: jds@ Web Site: [url][/url]

Re: Friction Coefficient

Posted on 26. Dec. 2008 - 05:45

Hello all,

Sorry to somewhat hijack the thread but my question runs along the same line as the one first asked.

I am currently investigating hopper angles using

Is this a reasonable resource to use to be able to accurately predict a hopper angle from? Are they any/many others which are avalible to view?

I have been supplied a material testing sheet for the coal I am designing the hopper for. The information given :

Effective angle of internal friction = 55 to 61 degrees

Static angle of internal friction = 48 degrees

Instantaneous Flow Function = 5-22 kPa

3-day Instantaneous Flow Function = 5-24 kPa

Bulk Density = 0.9 t/m^3

Particle size = 50x50mm

moisture = 2-25%

I am a little confused as to where to go from here. Can i obtain a wall friction angle from this data? From there i take it i need to find the CAS (obtained from the Material flow function which i am loosly given and the flow factor which i can obtain if i find out the wall friction angle?).

Im sorry if that sounds alittle confusing Ultimately i am trying to work out a hopper angle and outlet size and am just wondering if it is possible to do so with the info i have been provided with.



Hopper Design

Posted on 27. Dec. 2008 - 10:30

You also require the wall friction angle to assess the wall slope for hopper design. This is a relatively easy measurement to take but must be made with a sample of the product that exibits the worst slip conditions that will be experienced on a surface as that of hopper construction. Another feature to be taken into consideration with lumpy material is that the outlet must be large enough to avoid the formation of structural blackages. This is a stochastic process subject to a large range of variables but, as a safe crude rule of thumb for a mass flow hopper, should be at least five times the size of uniform dimensioned particles for the width of a slot outlet and more than eight times for the diameter of a circular outlet. A more detailed review of this aspect is given in a publication available from