Friction Coefficient Calculation

Posted in: , on 15. Mar. 2007 - 04:14

Dear All

I was wondering if it is possible to obtain the friction coefficient (or equivalent friction coefficient) merely from knowing the wall friction angle.

By definition

Tan (Friction Angle) = Shear Stress / Normal Stress

how could I work with this and establish a formulae to give me the friction coefficient?

I have found a formulae which states

Equivalent Friction Coefficient = K * sin (Effective Angle of Internal Friction)

where K is an empiracle constant of 0.8

thanks in advance!

Re: Friction Coefficient Calculation

Posted on 15. Mar. 2007 - 09:42

I don’t think so it is possible to link Wall friction with internal friction angle they are measured by separate tests. But if you find a solution it will save everybody loads of time especially if one uses Jenike shear cell to measure internal friction.

Mantoo

Re: Friction Coefficient Calculation

Posted on 15. Mar. 2007 - 05:20

Dear Mr. Alex,

The friction coefficient between material and wall is a separate characteristic from material internal friction coefficient. For example if material is limestone then its internal friction coefficient could be around 0.6. Now, the friction coefficient between this material and steel wall or steel skirt plate could be say 0.42 for fairly even surface of steel without protruding edges (say flush and welded reasonably good welded steel surface would be of this type). However, if the steel surface is flush welded but of poor quality with reference to surface finish or it has protruding edges, bolt heads, rivets, etc. then the friction coefficient between material and the steel wall of the skirt plate could be 0.5. The above values are mentioned by making reference to DIN.

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.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916

praveen - Larsen & Toubro, Cochin, Kerala, India
(not verified)

Coefficient Of Friction

Posted on 13. Aug. 2007 - 07:42

Sir,

Will it depend on the moisture content also????

can u tell me from where will i get the frictional coefficient values of limestone on steel plates in relation with moisture content..

It will be very useful in determining the discharge chute angles.

Re: Friction Coefficient Calculation

Posted on 13. Aug. 2007 - 09:39

Yes moisture content will have an influence.

It is not advisable to use generic values for designing purposes. Wall friction angle varies from 15- 30 degrees for carbon – stainless steel. Best option is to test your materials on a wall friction tester.

Mantoo

Re: Friction Coefficient Calculation

Posted on 13. Aug. 2007 - 09:47

Originally posted by Mantoo

Best option is to test your materials on a wall friction tester.

Exactly!

Lyle Brown
(not verified)

Re: Friction Coefficient Calculation

Posted on 13. Aug. 2007 - 09:50

Addressing the original post, as I understand, Alex was seeking a relationship between friction angle and coefficient of friction.

We could consider tan (theta) = u, hence u = atan (theta) (consider f=uN, rearranging for u =f/F or shear stress / normal stress as Alex suggested earlier), where theta is the angle of the graph of f (shear stress) vrs N (normal stress).

Suggest the K is to account for head, or lack there of, (note that the relationship is not really hydrostatic is often assumed, and the K accounts for this).

I am not aware of a relationship between internal friction and wall friction.

Material moisture may affect the properties. There is some guidance in various sources (AS 3774 for one) or you could get someone like TUNRA to do a test of your material / wall liner.

Regards,

Lyle

Lyle Brown
(not verified)

Re: Friction Coefficient Calculation

Posted on 14. Aug. 2007 - 09:20

Stumbled across my TUNRA notes, they use sin of theta for their Apron Feeder calculations, I guess this has something to do with the failure plane (others may know). The K factor appears to be a shape factor.

The understerding I presented earlier was based on my understanding of TUNRAs chute notes.

Regards,

Lyle

Re: Friction Coefficient Calculation

Posted on 15. Aug. 2007 - 04:41

I don't know if this is what you want, but I have some notes copied from a publication (whose reference I've lost) which states :

Flank angles for mass flow -

Pyramid Bunker 50 degrees + 1.2*WFA

Conical Bunker 45 degrees + 1.2*WFA

V/Wedge Bunker 35 degrees + 1.2*WFA

(WFA = Wall Friction Angle in degrees)

NOTE, I make no comment on their accuracy!!

Re: Friction Coefficient Calculation

Posted on 15. Aug. 2007 - 05:15

Eurocode ENV 1991 and AS3774 also have graphs relating hopper half angle and wall friction for mass flow and core flow.

Mantoo