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.

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

Modeling Thermal Effects

You can model the effects of heat flow and temperature change through an optional thermal conserving port. By default, the thermal port is hidden. To expose the thermal port, right-click the block in your model and, from the context menu, select Simscape > Block choices. Choose a variant that includes a thermal port. Specify the associated thermal parameters for the component.

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.

Ports

B

Rotational conserving ports associated with the driving shaft

F

Rotational conserving ports associated with the driven shaft

N

Physical signal terminal through which you import the normal force

H

Thermal conserving port. The thermal port is optional and is hidden by default. To expose the port, select a variant that includes a thermal port.

Parameters

Dimensions

Force action region

Select a parameterization method to model the loaded-contact friction. The default method 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 value is 130 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 a parameterization method to model the kinetic friction coefficient. The options and default values for this parameter depend on the variant that you select for the block. The options are:

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

    • Is only visible if you use the default variant of the block

    • Is the default method for parameterizing the default variant of the block

    • Affects the visibility of other parameters

     Fixed kinetic friction coefficient

  • Table lookup kinetic friction coefficient — Define the kinetic friction coefficient by one-dimensional table lookup based on the relative angular velocity between disks. This option:

    • Is only visible if you use the default variant of the block

    • Affects the visibility of other parameters

     Table lookup kinetic friction coefficient

  • Temperature-dependent kinetic friction coefficient — Define the kinetic friction coefficient by table lookup based on the temperature. This option:

    • Is only visible if you use a thermal variant of the block

    • Is the default method for parameterizing the thermal variant of the block

    • Affects the visibility of other parameters

     Temperature-dependent kinetic friction coefficient

  • Temperature and speed-dependent kinetic friction coefficient — Define the kinetic friction coefficient by table lookup based on the temperature and the relative angular velocity between disks. This option:

    • Is only visible if you use the thermal variant of the block

    • Affects the visibility of other parameters

     Temperature and speed-dependent kinetic friction coefficient

Velocity tolerance

Relative velocity below which the two surfaces can lock. The surfaces lock if the torque across the B and F rotational ports is less than the product of the effective radius, the static friction coefficient, and the applied normal force. The default value is 0.001 rad/s.

Threshold force

The normal force applied to the physical signal port N is applied to the contact only if the amount of force exceeds the value of the Threshold force parameter. Forces below the Threshold force are not applied, and there is consequently no transmitted frictional torque. The default value is 1 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 value is 0 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 option is the default.

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

Thermal Port

These thermal parameters are visible only when you select a block variant that includes a thermal port.

Thermal mass

Thermal energy required to change the component temperature by a single degree. The greater the thermal mass, the more resistant the component is to temperature change. The default value is 50 kJ/K.

Initial temperature

Component temperature at the start of simulation. The initial temperature alters the component efficiency according to an efficiency vector that you specify, affecting the starting meshing or friction losses. The default value is 300 K.

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