Computation of Friction Force and Torque

The forces and moments that are used to evaluate the frictional effects are taken at the beginning of the time step. The resisting force and torques have a sign that is opposed to that of the velocity which is also evaluated at the beginning of the time step. In other words, the friction force and torque, during a given time step, are evaluated with the forces and torques of the previous time step and have the opposite sign of the velocity evaluated at the previous time step.

Postprocessing Joint Friction

For translational, revolute, cylindrical, point on curve, spherical, slot, universal and general joints, the Joint Probe allows you to verify the friction force and the friction moment. Friction force is available for translational, cylindrical, point on curve, slot, general, and all imperfect joints. Friction moment is available for revolute, cylindrical, point on curve with free rotations, spherical, slot, universal, general, and all imperfect joints.

Setting Joint Friction With a Command

The command SetFrictionVariable(var) replaces the constant value already given to the friction coefficient with the expression given by var.

For example, to set

Joint = CS_Joint.Find(_jid)
Var = CS_Variable()
u0 = 0.1
u1 = 0.2
alpha = 0.5
Var.SetFunc('u0+u1*exp(-alpha*time)',0)
Var.AddInputMeasure(Joint.GetVelocityMeasure())
Joint.SetFrictionVariable(Var)

The command has no effect if no value for the friction coefficient has been provided in the UI.

Setting the Friction Tolerance

The command SetFrictionTolerance(tol) sets the friction tolerance.

Joint = CS_Joint.Find(_jid)
Joint = Joint.SetFrictionTolerance(1e-4)

Setting the Static Friction Coefficient with a Command

The static friction coefficient is by default set to the dynamic friction coefficient. The command SetStaticFrictionCoefficient(muS) changes its value to muS.

Joint = CS_Joint.Find(_jid)
Joint.SetStaticFrictionCoefficient(0.2)

Bending Moment for Translational Joints

When using a non-zero length when defining the friction in a joint, the bending effect is proportional to the constraint moment existing in the joint. This moment is always defined as the moment expressed at the reference coordinate system of the joint. For joints that have translational degrees of freedom, such as the translational or cylindrical joint, the reference coordinate system and mobile coordinate system are not always at the same location, and the constraint force generates a moment at the reference coordinate system:

In this example, the blue mass slides on a fixed red bar, under vertical gravity.

As the mass moves to the right the reported moment at the reference coordinate system, which stays at the end of the blue bar, increases linearly.

If a length has been specified in the translational joint, the friction bending forces will increase with the position. In this example, this is not the expected effect.

However, if the blue mass is grounded and the red bar is sliding in it, the moment reported in the reference coordinate system is also increasing and so should the friction bending forces.

You must choose the reference and mobile coordinate systems and effective length properly to represent the physics, as the "joint" abstraction can be sometimes misleading.