I had mentioned in a previous post that I was working on a motion study using a model of a pair of friction grabs. Here I have a short video of that motion study, demonstrating the use of friction when defining 3D contacts for motion analysis.  

When defining 3D contacts for motion analysis in SolidWorks Motion, we have the option of acknowledging there’s friction between the components or not. Sometimes, no friction works fine for the kind of model we are working with, but there are other times when our simulation won’t work at all unless we add friction to the 3D contacts between the components.  My friction grabs, as you will see in the video, are one of those models that need the friction in order to work.

Dry friction, also known as Coulomb friction force is calculated based on a couple of coefficients:  the static coefficient and the kinematic coefficient.

The static coefficient is a constant related to the force necessary to overcome friction when the body is at rest, as expressed in this formula.

Fm = µs N

Where Fm is the maximum value of the static friction force, µs is the static coefficient and N is the normal component of the reaction force acting on the body.

The kinematic coefficient is a constant related to the kinetic friction force, which is the one acting on the body once it’s already in movement and its magnitude is usually much smaller than that of the static friction force. The magnitude of this force remains pretty much constant as the body’s velocity increases.

Fk = µk N

Where Fk is the kinematic friction force and µk is the kinematic coefficient.

Both coefficients usually depend on the nature of the surfaces that are in contact, but not really on their shape or size. Some values have been obtained experimentally and organized in tables. To use these coefficients all we’d need to do would be to determine what kind of materials would be in contact and find the coefficients for that particular combination from the tables. SolidWorks simulation has a small collection of materials for contacts available. When we choose materials from this list for our 3D contacts and mark the option to use friction in the 3D contact property manager, the coefficients for that particular combination are already determined for us. However, we can always clear out the option of using materials from the list and enter our own values for the coefficients, if we know them from some table or from experimentation.

In this little video of the simulation that I put together, I run a simulation first without friction and then with friction added to the 3D contacts between the grabs and the block.  Notice that, in my model, the tongs and block are made out of steel; however, since I choose rubber (dry) for the material in my contacts, it is the coefficients for this kind of material that will be used in calculating friction forces and not the coefficients that would apply for a combination of steel on steel.

Enjoy the video! And in case you have trouble seeing it, it will also be availabe at Viddler.com, just follow the link: http://www.viddler.com/explore/lainge96/videos/20/