Documentation

Loaded-Contact Rotational Friction

Loaded-contact friction between two rotating bodies

Library

Brakes & Detents/Rotational

Description

The Loaded-Contact Rotational Friction block simulates friction between two rotating bodies loaded with a normal force.

The block is implemented as a structural component based on the Fundamental Friction Clutch block. From the locked state, the clutch unlocks if the friction force exceeds the static friction, as defined by the static coefficient of friction and current normal force. For details on how the locking and unlocking are modeled, see the Fundamental Friction Clutch block reference page.

Friction torque is transmitted for normal forces larger than the Threshold force parameter.

Ports

B and F are rotational conserving ports associated with the driving and driven shafts, respectively. N is the physical signal terminal through which you import the normal force.

Dialog Box and Parameters

Dimensions

Force action region

Select how to parameterize the loaded-contact friction. The default is Define effective radius.

  • Define effective radius — Provide a value for the friction effective radius.

    Effective torque radius

    Effective radius reff. Must be greater than zero. The default is 130.

    From the drop-down list, choose units. The default is millimeters (mm).

  • Define annular region — Define the friction effective radius in terms of the inside and outside diameters of the friction disk. If you select this option, the panel changes from its default.

     Define annular region

Friction

Friction model

Select how to specify the kinetic friction coefficient. The default is Fixed kinetic friction coefficient.

  • Fixed kinetic friction coefficient — Provide a fixed value for the kinetic friction coefficient.

    Kinetic friction coefficient

    The kinetic, or Coulomb, friction coefficient. Must be greater than zero. The default is 0.3.

  • Table lookup kinetic friction coefficient — Define the kinetic friction coefficient by one-dimensional table lookup based on the relative angular velocity between disks. If you select this option, the panel changes from its default.

     Table lookup kinetic friction coefficient

Static friction coefficient

The static, or peak, value of the friction coefficient. Must be greater than the kinetic friction coefficient. The default is 0.35.

Velocity tolerance

Relative velocity below which the two surfaces can lock. For the surfaces to lock, the torque across the B and F rotational ports must be less than the product of the effective radius, the static friction coefficient, and the applied normal force. The default is 0.001.

From the drop-down list, choose units. The default is radians/second (rad/s).

Threshold force

The normal force applied to the physical signal port N must exceed the Threshold force parameter value to be applied to the contact. Forces below the Threshold force are not applied, and there is consequently no transmitted frictional torque. The default is 1.

From the drop-down list, choose units. The default is newton (N).

Viscous Drag

Viscous drag torque coefficient

Viscous drag coefficient μvisc for computing the drag torque. The coefficient depends on the type of operating fluid, fluid temperature, and the maximum distance between the disks. The default is 0.

From the drop-down list, choose units. The default is newton-meters/(radians/second) (N*m/(rad/s)).

Initial Conditions

Initial state

Select the initial condition applied to the Fundamental Friction Clutch state inside the block:

  • Locked — Rotational ports B and F are initially locked together. This is the default.

  • Unlocked — Rotational ports B and F are initially sliding relative to each other.

Loaded-Contact Rotational Friction Model

The block simulates friction between two rotating bodies loaded with a normal force. When the two rotating bodies are not locked, the friction torque is determined with the following equations:

τfr = N· μ· reff · sign(ω) + τvisc ,

reff=23ro3ri3ro2ri2 ,

τvisc = μvisc· ω ,

where:

τfrFriction torque
NNormal force
μFriction coefficient
reffEffective radius
roDisk outside radius
riDisk inside radius
ωRelative angular velocity
τviscViscous drag torque
μviscViscous drag torque coefficient

Limitations

  • The model does not account for inertia. Add inertia terms externally to the B and F ports as required.

  • The model computes the torque assuming a uniform distribution of the normal force.

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