Mechanical APDL Solver

Using the Mechanical APDL solver, you can specify friction for a spherical joint.

Rigid Dynamics Solver

For revolute, cylindrical, translational, point on curve, spherical, slot, universal, general, in-plane radial gap, spherical gap, and radial gap joints, Mechanical enables you to introduce frictional behavior in the joint when using the Rigid Dynamics solver.

Friction Model

Joint friction is based on the sliding mode of Coulomb's friction model:

(9–1)

where:

= the resisting tangential force

= the friction coefficient

= the normal force

Depending on the joint type, the friction effect leads to a resisting force or torque. Additional geometric information is also required to compute the effect of joint friction in revolute and cylindrical joints. Computation of the normal force depends on the joint type and is described below.

The friction coefficient is set as a constant in the user interface. However, a command exists that allows you to introduce a friction coefficient with an expression.

When the Joint Friction Type property is set to Program Controlled, Friction with Sliding/Sticking Transitions, or Forced Friction Sliding the frictional joints are handled as kinematic joints, meaning that they are constraint equation based. Two events exist that correspond to the sliding-to-sticking and sticking-to-sliding friction transitions.

When the velocity is lower than a velocity tolerance (), the sliding-to-sticking event is active and the following system is solved:

(9–2)

where:

M is the inertia matrix and F is the forces vector

and are respectively the normal and tangent constraint equations

and are the Jacobians of the normal and tangent constraints, respectively

During the sliding phase, the traditional sliding Coulomb friction is written as

(9–3)

When the Forced Frictional Sliding option is not selected, and under the condition , with and the normal and tangent forces, the sticking-to-sliding event is active.

When the sliding velocity is low, the friction coefficient is ramped between 0 and its nominal value using the following equation:

(9–4)

where:

= the friction coefficient used in the force or torque calculation

= the value of the friction coefficient you have specified in Mechanical

= the tangential velocity

= the velocity tolerance

The velocity tolerance is computed internally, and is based on system velocities and a numerical tolerance. Since the numerical tolerance is problem-dependent, it is possible to change its value using a command.