Contact friction (friction that occurs between bodies in contact) is different than joint friction. The default coefficients of friction used are based on the materials you define for each part. When defining your contact (either intermittent curve-curve contact or 3D contact), you can choose from three friction options: full, dynamic, or none. Once you decide how much friction to include in your contact, you must evaluate the static and/or dynamic velocity and friction constants.
To edit the velocity and coefficient parameters,
you must clear the selection of Use Materials.
The friction velocity and coefficient constants are preset based on the materials of the parts. However, they may not be the most appropriate parameters based on the dynamics of the model. The Coulomb friction method that COSMOSMotion uses in contact models is explained in the graph below. For this explanation, the default friction parameters for dry steel in contact with dry steel is used.
Static Friction Transition Velocity: 0.1 mm/s
Dynamic Friction Transition Velocity: 10 mm/s
Static Friction Coefficient: 0.30
Dynamic Friction Coefficient: 0.25
Coulomb friction forces are calculated based on two friction coefficients:
Static: The constant used to calculate the force necessary to overcome friction when a body is at rest.
Dynamic: The constant used to calculate friction forces once the body is no longer at rest.
In reality, the static friction transition velocity is zero, but in COSMOSMotion, you must specify a nonzero value to avoid a singularity at the origin. More specifically, when a part is in transition from a negative to positive velocity, and when the velocity is zero, the force magnitude cannot instantaneously transition from a positive to negative value. Therefore, you must specify a static and dynamic transition velocity where the friction coefficients are used. COSMOSMotion fits a smooth curve to solve for the friction force (see image above).
While this may seem like an ideal solution, be careful when defining your velocity transition values. For example, if your contact body is moving relatively slow, select velocity transition values accordingly.