Friction Phenomena

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Friction Phenomena

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There are different phenomena of friction. The following friction phenomena are described in the friction models in 20-sim:

static friction: the torque or force necessary to initiate motion from rest (the so called break-away force); it is often larger than the Coulomb friction,
Coulomb friction (kinetic friction, dynamic friction): the friction component that is only dependent of the direction of velocity, not of the magnitude of the velocity,
viscous friction: the friction component that is proportional to velocity and goes to zero at zero velocity,
Stribeck friction (Stribeck effect): the friction phenomenon that arises from the use of fluid lubrication and gives rise to decreasing friction with increasing velocity at low velocity,
pre-sliding displacement (Dahl effect): the spring-like behavior of friction that causes a displacement linear dependent on the applied force if this applied force is less than the break-away force,
varying break-away force (rising static friction): the dependence of the break-away force on the rate of increase of the applied force,
frictional lag: the delay in the change of the friction force as a function of a change in the velocity.


Beside the friction phenomena mentioned above, some other effects with respect to friction are reported in the literature. These effects are not incorporated into the 20-sim library of friction models:

Time-dependent friction: From experiments it is known that friction changes with time. These changes of friction with time are due to such things as loss of lubricant, deformation of the surface material, change in temperature due to generated heat and/or accumulation of wear debris.
Position-dependent friction: A dependence of the friction on the position of a system is another effect that is experimentally observed and is well known by a lot of researchers. This position-dependency is caused by spatial inhomogeneities in the transmission of the system due to contact geometry and/or loading which varies as a function of position. As the load varies, the normal force between the sliding surfaces varies, causing a varying friction (friction is linear dependent on the normal force). By preloading the transmission elements and roller bearings this dependency of friction on the load can be decreased.
Direction-dependent friction: A lot of researchers have found the friction to be dependent on the direction of the motion of a system. Different Coulomb and viscous friction levels in the left and right directions of a single, linear motion have been observed experimentally on many occasions. Theoretically, this may be due to anisotropies in material or geometry.