Translational Friction
Friction when moving bodies are in contact.
Description
Block Translational Friction represents friction at contact between moving bodies. The friction force is modelled as a function of relative velocity and is calculated as the sum of the components of fluid, viscous and mixed friction as shown in the following figure.
Mixed friction, or Stribeck friction, , represents the characteristic with a negative slope occurring at low velocities. Dry, or Coulomb friction results in a constant force at any speed. Viscous friction resists motion with a force directly proportional to relative velocity. The sum of mixed and dry friction near zero velocity is often called breakaway friction . Friction is approximated by the following equations:
Where:
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- friction force.
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- dry (Coulomb) friction.
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- rest friction.
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- critical velocity.
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- velocity threshold for Stribeck friction.
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- Coulomb velocity threshold.
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and are absolute velocities of ports R and C, respectively.
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- relative velocity.
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- viscous friction coefficient.
The exponential function used in the Stribeck part of the force equation is continuous and decays at velocity values greater than the critical velocity.
The hyperbolic tangent function used in the Coulomb part of the force equation ensures that the equation is smooth and continuous when passing through , but quickly reaches its full value at non-zero velocities.
The positive direction of the block is from port R to port C. This means that if the speed of port R is greater than the speed of port C, the block transmits force from R to C.
Ports
R - rod
translational mechanics
A mechanical progressive port corresponding to a rod, that is, a moving body.
C is the body
translational mechanics
A mechanical progressive port corresponding to a hull, that is, a stationary body.
Parameters
Breakaway friction force, N - resting friction force
`25.0 N (by default)
`25.0 N (by default)
Rest friction force. It must be greater than or equal to the Coulomb friction force parameters.
Breakaway friction velocity, m/s - critical velocity for Stribeck friction
0.1 m/s (By default).
The speed at which Strebek friction reaches its maximum. At this point, the sum of Stribeck friction and Coulomb friction is equal to the value of the Breakaway friction force parameters. This parameter sets a velocity threshold that affects the trade-off between simulation accuracy and velocity.
Coulomb friction force, N - dry friction force
`20.0 N (by default).
Coulomb friction force, i.e. friction opposing motion with constant force at any velocity.
Viscous friction coefficient, N*s/m - viscous friction coefficient
`100.0 N*s/m (by default).
Coefficient of proportionality between friction force and relative velocity. Parameters value must be greater than or equal to zero.
Initial value of velocity, m/s - initial value of velocity
0.0 (by default)
Initial value of velocity.
Initial value of force, N - initial value of force
`0.0 (by default).
Initial value of force.