Spring-Damper

) for 3D, 2D, and 1D components to model — between two points — an elastic spring, a viscous damper, or both.

The spring can either act as an axial spring between the two points, or be defined be a general spring matrix, connecting all the degrees of freedom in the two points. The two points are referred to as the source and destination point, respectively. The points can be geometrical points, but there are also other methods for describing the attachment point of the spring as a virtual point in space.

A spring-damper can connect two points belonging to two different physics interfaces. If at least one of the physics interfaces possesses rotational degrees of freedom, the should be placed in such an interface.

The node is only available with some COMSOL products (see https://www.comsol.com/products/specifications/).

Coordinate System Selection

This section is only of importance when a full matrix representation of the spring-damper is used. In that case, the matrix is interpreted as acting on the degrees of freedom in the local directions provided by the local coordinate system.

This section is only available if is selected as . In that case, a conceptual representation of the spring-damper assembly is shown.

Attachment Selection

In this section, you select the two points between which the spring or damper is connected.

Select a . The default is . You will then do the actual selection in the subnode. If you select more than a single point, the spring will be connected to a virtual point, the location of which is the average of the selected points. If you make another choice than , the corresponding subnode will be removed.

The other option that is always available as is . When is selected, the source side of the spring is fixed in space, and not connected to any modeled parts. In this case, you need to specify the location of the source point. For , select and enter the coordinate Xs of the source point.

If a is defined in a Multibody Dynamics interface, it can be selected as . Selecting a base motion gives a behavior similar to using , with the exception that the base now can have a nonzero prescribed displacement, velocity, or acceleration.

Any node in the model can be selected as , irrespective of in which physics interface it is defined. The use of attachments required the Multibody Dynamics Module or the Rotordynamics Module.

From the list, you can also select any object that is of a rigid body type, irrespective on in which physics interface it is defined. Such objects are:

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(in the Multibody Dynamics interface)

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(in the Multibody Dynamics interface)

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(in the Multibody Dynamics interface)

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(in the Multibody Dynamics interface)

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(in the Multibody Dynamics interface)

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(in the Multibody Dynamics interface)

Destination Point

Most settings are the same as for the . The differences are the following:

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The options Fixed and Base Motion are not available for the destination point.

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There is one other option: .

The available inputs for depend on the selected in the section.

If the spring type is , enter an expression for the , ud, and for the , Xd. The entered values determine how the spring extension is computed.

If the spring type is , enter expressions for the , ud, and the , Θd.


	Using Prescribed displacement, it is possible to connect the spring to another physics interface. In order to do that, you can enter expressions using Nonlocal Couplings defined in the Definitions node for a component.


	When the Spring type is set to Matrix, the rotation of the source and the destination (Θs and Θd) are determined by availability of rotational degrees of freedom. If the physics interface itself has rotational degrees of freedom, these will be used in a point selection. For all physics interfaces, irrespective whether they have rotational degrees of freedom or not, a connection to an attachment or rigid body type object will provide a rotation. Some settings in the GUI are only available if rotational degrees of freedom are present.


	When the Spring-Damper node is used in the Multibody Dynamics interface, there is one more option: Use selection filter. In large models, the list of available attachments can become very long. You can then create geometric filters to narrow down the search. When the Use selection filter check box is selected, two subnodes named Source Filter and Destination Filter are added to the Spring-Damper node. In these subnodes you can make graphic selections of the objects whose attachments should be shown in the Source and a Destination lists.

Select a — or . The latter option is not available for 1D and 1D axisymmetry.

Select a — or .

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For enter a value for k.

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For enter an expression for Fs. The expression must be a function of the extension of the spring. The built-in variable for the spring extension has the form <physicsName>.<SpringNodeTag>.dl, for example solid.spd1.dl.

To add viscous damping in a dynamic analysis, enter a value or expression for the c.

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Under , enter the stiffness matrices defining the elastic connection between the source and destination. Input fields for the matrices ku and kΘ are always shown. Select to show input fields for the coupling matrices kuΘ and kΘu. In 2D, most elements of these matrices are by definition zero. Only elements that can be nonzero are shown; these are elements 13 and 23 for kuΘ and elements 31 and 32 for kΘu.

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Under , enter the damping matrices defining the viscous connection between the source and destination. Input fields for the matrices cu and cΘ are always shown. Select to show input fields for the coupling matrices cuΘ and cΘu. In 2D, most elements of these matrices are by definition zero. Only elements that can be nonzero are shown; these are elements 13 and 23 for cuΘ and elements 31 and 32 for cΘu.

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By default, the relative displacement between source and destination is computed as Δu = ud − us. In some cases, however, it can be more reasonable to also include displacements resulting from a rigid body type rotation. If is selected, an additional term is added to the expression for the relative displacement. It describes the additional displacement due to the rotation of the destination if the source and destination were connected by a rigid bar element.

This section is only available if is selected as .

The free length is the distance between the connection points when there is no force in the spring. Select an option from the list: or .

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For enter a value for Δl0. The free length is computed as lf = l0 − Δl0, where l0 is the initial distance between the connection points.

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For enter a value for lf.

Activation Conditions

If is selected as , you can select a — , , or .

If you want to activate or deactivate the entire spring-damper, select the check box. Then, enter a idac. The expression is treated as a Boolean expression, so that when it evaluated to a nonzero value, the spring or damper is deactivated.

Select the check box if the spring is supposed to be removed permanently from the simulation when the deactivation condition is fulfilled for the first time.

As an example, if the spring should break at a certain extension, you can write an expression like solid.spd2.dl>0.12[m], and select the check box. If the check box is not selected, the spring would become active again when its connection points come close enough to each other.


	Spring-Damper Theory

Location in User Interface

Physics tab with selected:

Physics tab with selected:

Physics tab with selected:

Physics tab with selected:

Physics tab with selected:

Physics tab with selected:

Physics tab with selected:

Physics tab with selected:

Physics tab with selected: