[MBDyn-users] Implementation of friction models

Marco Morandini marco.morandini at polimi.it
Tue Jan 24 12:38:19 CET 2017

> I understood the basics but I
> don't understand the need of a shape function or the average radius model
> definition.

The friction coefficient gives you a relation between the normal and 
tangent components of the joint reaction force at the point of contact.
To compute the moment from the tangential reaction component you need 
the joint radius.
The joint unknowns is the total reaction force, not the normal 
component. Thus, the need of the shape function that for a slightly 
loaded hinge is equal to f/sqrt(1.+f*f). See

ALI FARAZ and SHAHRAM PAYANDEH, Towards approximate models of Coulomb 
frictional moments in: (I) revolute pin joints and (II) spherical-socket 
ball joints, Journal of Engineering Mathematics 40: 283–296, 2001.

Fig. 2 and Eq. 2. You'll find there expressions for heavily loaded 
joints too, but right now they are not implemented in MBDyn.

Hope this helps,


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