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# Spring Elements

A spring element has two basic forms. The  classic spring connects two nodes on the model. It can be axial or rotational. For an axial spring, a spring stiffness, k, is applied to the spring element. The spring deflects a distance x. This distance depends on the axial force in the spring element, F, and the spring stiffness. The distance the spring deflects can be calculated by the equation x=F/k. For a rotational spring, a torsional stiffness, kt, is applied to the spring element. The spring rotates through an angle J. This angle depends on the moment applied to the spring, M, and the torsional stiffness. The angle through which the spring rotates can be calculated by the equation M/kt.

The DOF spring connects a single DOF from each node to which it is connected. This spring has a stiffness value as described for the classic spring.

## When to Use Spring Elements

• Two parts are connected by a spring with a known spring stiffness value.
• Two parts are connected by a part that only transmits an axial force. The spring stiffness for this part can be calculated as AE/L where A is the cross-sectional area, E is the modulus of elasticity, and L is the length.

## Spring Element Parameters:

When using spring elements, first select the type of spring for the part in the Spring Type section in the General tab of the Element Definition dialog box. For most applications, the Spring type is appropriate. To transfer the loads from different degrees-of-freedom at each node, select the DOF Spring type. Next, specify the spring stiffness in the Spring Stiffness field.

If you selected the Spring radio button, specify if you want the spring to resist translation or rotation in the Element Type section. If you selected DOF Spring , specify the degree-of-freedom to which you want the stiffness applied to at the I-node and J-node in the I Node and J Node sections.

When the units of the spring stiffness are shown, the input is converted based on the Display Units. When a DOF Spring uses different degrees-of-freedom at each node, the physical meaning of connecting a translation to a rotation are difficult to envision. Therefore, no units are shown for the spring stiffness, and no conversion is performed. (Mathematically, mixing degrees-of-freedom on each end of the spring gives Resulti = Resultj = stiffness*(Ui-Uj), where Ui and Uj are the translation and rotation at the respective end of the spring, and Result is the force and moment at the respective end of the spring.

TipThe orientation of the spring elements can be displayed using the View Visibility Object Visibility Element Axis 1 command. Axis 1 points in the direction from the I Node to the J Node. If axis 1 must be reversed for some elements, select the elements (Selection Select Lines), right-click, and choose Invert I and J Nodes.

## Visualize Spring Elements in the Results Environment

Spring elements can appear in the Results environment either as a line or as an actual spring. To render the spring, activate the Visualize as spring check box in the Visualization tab of the Element Definition dialog box. You can then specify the dimensions of the spring. If the sum of the values in the Beginning Length field, the End Length field, and the product of the Number of Coils and the Wire Diameter fields is greater than the length of the spring element, the spring is not drawn in the Results environment. The value in the Coil diameter field refers to the diameter of the spring along the centerline of the wire. The Beginning Length field and the Begin attachment type drop-down menu refer to the I-node end of the spring element. The End Length field and the End attachment type drop-down menu refer to the J-Node end of the spring element.

If you specify attachments at either end of the spring, define an orientation point. The coordinate entered in the X, Y and Z fields is used to assign the plane in which the attachment is located. A vector is created perpendicular to the spring element passing through this point. The attachment lies in the plane that is perpendicular to this vector. If this coordinate is along the line of the element, the spring is not drawn in the Results environment.

## To Use Spring Elements

1. Be sure that a unit system is defined.
2. Be sure that the model is using a structural analysis type.
3. Right-click the Element Type heading for the part that you want to be spring elements.
4. Select the Spring command.
5. Right-click the Element Definition heading..
6. Select the Edit Element Definition command.
7. Select the type of spring in the Spring Type section.
8. If the Spring radio button is selected in the Spring Type section, specify if this spring will be axial or rotational in the Element Type section.
9. If the DOF Spring radio button is selected in the Spring Type section, select the DOF for the i node and j node in the I Node and J Node sections.
10. Specify the stiffness of the spring in the Spring stiffness field.
11. Go to the Visualization tab.
12. Activate the Visualize as spring check box.
13. Define the dimensions of the spring.
14. Press the OK button.