# Friction between wall-ball

Hello
I have a contact between wall and ball with different frictions for wall and ball. In addition, I am using rrlinear model with rr_fric for both ball-facet and ball-ball contacts. I would like to know how is the coefficient of friction between ball-facet being calculated.
Thank you

The rolling friction coefficient (`rr_fric`) is inherited using the minimum value of the values set for the pieces. See this page - scroll to the section Surface Property Inheritance.

– Derrick

@mehrdad could you also categorize this under PFC. This is a good question for other PFC users on the forum to view. Thanks!

Hello Derrick,
Thank you for your help. This is what I have for my parameters.

contact cmat default type ball-facet model rrlinear …
property kn 1e8 ks 1e8 dp_nratio 0.1 dp_sratio 0.01 rr_fric 0.3 dp_mode 1 lin_mode 1
ball property ‘kn’ 1e7 ‘ks’ 1e7 ‘fric’ 0.5
wall property ‘kn’ 1e8 ‘ks’ 1e8 ‘fric’ 0.15

You mean the minimum between ball-facet rr_fric, ball fric and wall fric is being considered as the friction coefficient between ball and wall?
Thanks

That would be the case, except you have specified an entry in the cmat that will give properties to ball-facet contacts. In your case any new ball-facet contacts will always have rr_fric = 0.3, surface property inheritance will be ignored. If a ball-ball contact is created, then the rr_fric will be set to 0.5 at the contact.

This is all the parameters I have for my model:

contact cmat default type ball-ball model rrlinear …
property kn 1e7 ks 1e7 dp_nratio 0.1 dp_sratio 0.01 rr_fric 0.3 dp_mode 1 lin_mode 1
contact cmat default type ball-facet model rrlinear …
property kn 1e8 ks 1e8 dp_nratio 0.1 dp_sratio 0.01 rr_fric 0.3 dp_mode 1 lin_mode 1

ball property ‘kn’ 1e7 ‘ks’ 1e7 ‘fric’ 0.5
wall property ‘kn’ 1e8 ‘ks’ 1e8 ‘fric’ 0.15
ball attribute density 2650 damp 0.7

Thank you very much.

Let me rephrase my question: when both kinetic and rolling resistance friction are specified, how is the overall frictional force calculated?
Thanks

That was my mistake @mehrdad , I did not read the post carefully. The kinetic friction (friction according the liner contact model), with the property name `fric`, is not related nor influenced by the rolling resistance friction coefficient (`rr_fric` see here).

The friction coefficient [`fric`] defines the shear strength on the contact plane. The rolling resistance friction [`rr_fric`] defines a limiting torque in the moment direction of the contact plane.

In your case, a new ball-ball contact will have a linear friction of 0.5 (inherited from ball property) and a rolling resistance friction of 0.3 (from the cmat).
A new ball-facet contact will have a liner friction of 0.15 (inherited as the lesser value of the ball and wall `fric` property) and the rolling resistance friction will be 0.3 (from the cmat).

I hope this clarifies the issue. Please let me know if something does not make sense.

– Derrick

Hello Derrick
Thank you for the information.
Regards