Friction is a force that occurs in joints and parts in contact. When parts are in contact, friction is calculated based on the static and dynamic coefficients of friction and the normal force acting on the part. Joint friction is more complex, because the size of the joint can affect the magnitude of friction.
In 1699, Amontons rediscovered Leonardo da Vinci's two laws of friction: the frictional force is directly proportional to the normal load, and the size of the bodies does not affect the friction [Bowden and Tabor, 1950, 1974]. Engineers have relied on Amontons' laws, extensively and routinely for three centuries. Contrary to popular belief, the size of the bodies does affect the friction forces in the case of joint friction.
Joint friction is a resistive, sliding, surface force between parts that must be overcome for the parts to move with respect to one another. The force develops due to contact between the surfaces and the loads acting on the connection. For a pin in a hole, joint friction is experienced as an additional torque restricting the pin from rotating with respect to the hole. Joint friction is not anything more than standard friction between bodies, however it takes into account aspects of joint geometry in determining the net frictional forces acting.
For example, think of a pin in a hole, but with a little slope. In the first image shown below, the pin is resting in the hole under a centrally located force. This is the equivalent of a pure bearing load. The force needed to slide the pin back and forth is only dependent on the vertical load. The torque needed to rotate the pin is dependent on this force, but also on the radius of the pin (see second image below). In this example, the radius of the pin has no effect on the magnitude of the friction force, but does have an effect on the moment required to overcome friction to rotate the pin (mu.r.F).
Now, consider the case where there is an additional moment on the pin. The moment forces the pin to rotate, becoming supported at the outer edges of the hole (w). The moment is reacted as a force couple (M/w). Dividing the bearing load (F) between the ends results in local force of F/2 + M/w. Frictional forces are cumulative so you can sum these force couples to get the total force upon which friction is based (F+2M/w).
This is a simple extension to derive the torque necessary to rotate the pin as mu*r(F+2M/w).
This influence of the bending moment of a joint is an important factor in joint friction. You can see that if the hole supporting the pin is not thick (in terms of w), the moment component tends to be very high. If the hole supporting the pin is very thick, the moment component tends towards zero.
The COSMOSMotion joint friction model uses a combination of dimensional information assigned to a joint and a coefficient of friction that may be entered directly (or obtained automatically from the materials database).
Joint
friction is modeled as Coulomb friction and only represents the dynamic
friction in the joint.
Revolute, cylindrical, translational, spherical, universal, and planar joints all support the application of friction. When friction effects are enabled for these joint types, a force is induced that opposes the motion of the joint and is a function of the reaction forces acting on the joint.
To define joint friction:
Open the Joints dialog box.
Select the Friction tab.
Select Use
Friction.
The Joint Friction dialog
box appears.
Enter the parameters as needed.