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Application of Joints and Springs in ANSYS


Introduction

This tutorial was created using ANSYS 5.7.1. This tutorial will introduce:
  • the use of multiple elements in ANSYS
  • elements COMBIN7 (Joints) and COMBIN14 (Springs)
  • obtaining/storing scalar information and store them as parameters.
A 1000N vertical load will be applied to a catapult as shown in the figure below. The catapult is built from steel tubing with an outer diameter of 40 mm, a wall thickness of 10, and a modulus of elasticity of 200GPa. The springs have a stiffness of 5 N/mm.

Preprocessing: Defining the Problem


  1. Open preprocessor menu/PREP7
  2. Give example a TitleUtility Menu > File > Change Title ...
    /title,Catapult
  3. Define Element Types
  4. For this problem, 3 types of elements are used: PIPE16, COMBIN7 (Revolute Joint), COMBIN14 (Spring-Damper) . It is therefore required that the types of elements are defined prior to creating the elements. This element has 6 degrees of freedom (translation along the X, Y and Z axis, and rotation about the X,Y and Z axis).

    1. Define PIPE16
      With 6 degrees of freedom, the PIPE16 element can be used to create the 3D structure.
      • Preprocessor > Element Type > Add/Edit/Delete... > click 'Add'
      • Select 'Pipe', 'Elast straight 16'
      • Click on 'Apply' You should see 'Type 1 PIPE16' in the 'Element Types' window.
    2. Define COMBIN7
      COMBIN7 (Revolute Joint) will allow the catapult to rotate about nodes 1 and 2.
      • Select 'Combination', 'Revolute Joint 7'
      • Click 'Apply'.
    3. Define COMBIN14
      Now we will define the spring elements.
      • Select 'Combination', 'Spring damper 14'
      • Click on 'OK'
    In the 'Element Types' window, there should now be three types of elements defined.

  5. Define Real Constants
  6. Real Constants must be defined for each of the 3 element types.

    1. PIPE16
      • Preprocessor > Real Constants > Add/Edit/Delete... > click 'Add'
      • Select Type 1 PIPE16 and click 'OK'
      • Enter the following properties, then click 'OK'OD = 40
        TKWALL = 10
      'Set 1' will now appear in the dialog box
    2. COMBIN7 (Joint)Five of the degrees of freedom (UX, UY, UZ, ROTX, and ROTY) can be constrained with different levels of flexibility. These can be defined by the 3 real constants: K1 (UX, UY), K2 (UZ) and K3 (ROTX, ROTY). For this example, we will use high values for K1 through K3 since we only expect the model to rotate about the Z axis.
      • Click 'Add'
      • Select 'Type 2 COMBIN7'. Click 'OK'.
      • In the 'Real Constants for COMBIN7' window, enter the following geometric properties (then click 'OK'):X-Y transnational stiffness K1: 1e9
        Z directional stiffness K2: 1e9
        Rotational stiffness K3: 1e9
      • 'Set 2' will now appear in the dialog box.Note: The constants that we define in this problem refer to the relationship between the coincident nodes. By having high values for the stiffness in the X-Y plane and along the Z axis, we are essentially constraining the two coincident nodes to each other.
    3. COMBIN14 (Spring)
      • Click 'Add'
      • Select 'Type 3 COMBIN14'. Click 'OK'.
      • Enter the following geometric properties:Spring constant K: 5
      In the 'Element Types' window, there should now be three types of elements defined.

  7. Define Element Material Properties
    1. Preprocessor > Material Props > Material Models
    2. In the 'Define Material Model Behavior' Window, ensure that Material Model Number 1 is selected
    3. Select Structural > Linear > Elastic > Isotropic
    4. In the window that appears, enter the give the properties of Steel then click 'OK'.Young's modulus EX: 200000
      Poisson's Ratio PRXY: 0.33
  8. Define NodesPreprocessor > (-Modeling-) Create > Nodes > In Active CS...
    N,#,x,y,z
    We are going to define 13 Nodes for this structure as given in the following table (as depicted by the circled numbers in the figure above):
    NodeCoordinates (x,y,z)
    1(0,0,0)
    2(0,0,1000)
    3(1000,0,1000)
    4(1000,0,0)
    5(0,1000,1000)
    6(0,1000,0)
    7(700,700,500)
    8(400,400,500)
    9(0,0,0)
    10(0,0,1000)
    11(0,0,500)
    12(0,0,1500)
    13(0,0,-500)
  9. Create PIPE16 elements

    1. Define element typePreprocessor > (-Modeling-) Create > Elements > Elem Attributes ...The following window will appear. Ensure that the 'Element type number' is set to 1 PIPE16, 'Material number' is set to 1, and 'Real constant set number' is set to 1. Then click 'OK'.

    2. Create elementsPreprocessor > (-Modeling-) Create > Elements > (-Auto Numbered-) Thru Nodes
      E, node a, node b
      Create the following elements joining Nodes 'a' and Nodes 'b'.
      Note: because it is difficult to graphically select the nodes you may wish to use the command line (for example, the first entry would be: E,1,6).
      Node aNode b
      16
      25
      14
      23
      34
      108
      98
      78
      125
      136
      1213
      53
      64
      You should obtain the following geometry (Oblique view)
  10. Create COMBIN7 (Joint) elements

    1. Define element typePreprocessor > (-Modeling-) Create > Elements > Elem Attributes
      Ensure that the 'Element type number' is set to 2 COMBIN7 and that 'Real constant set number' is set to 2. Then click 'OK'
    2. Create elements
      When defining a joint, three nodes are required. Two nodes are coincident at the point of rotation. The elements that connect to the joint must reference each of the coincident points. The other node for the joint defines the axis of rotation. The axis would be the line from the coincident nodes to the other node.Preprocessor > (-Modeling-) Create > Elements > (-Auto Numbered-) Thru Nodes
      E,node a, node b, node c
      Create the following lines joining Node 'a' and Node 'b'
      Node aNode bNode c
      1911
      21011
  11. Create COMBIN14 (Spring) elements

    1. Define element typePreprocessor > (-Modeling-) Create > Elements > Elem Attributes
      Ensure that the 'Element type number' is set to 3 COMBIN7 and that 'Real constant set number' is set to 3. Then click 'OK'
    2. Create elementsPreprocessor > (-Modeling-) Create > Elements > (-Auto Numbered-) Thru Nodes
      E,node a, node b
      Create the following lines joining Node 'a' and Node 'b'
      Node aNode b
      58
      86
    NOTE: To ensure that the correct nodes were used to make the correct element in the above table, you can list all the elements defined in the model. To do this, select Utilities Menu > List > Elements > Nodes + Attributes.
  12. MeshingBecause we have defined our model using nodes and elements, we do not need to mesh our model. If we initially defined our model using keypoints and lines, we would have had to create elements in our model by meshing the lines. It is the elements that ANSYS uses to solve the model.
  13. Plot ElementsUtility Menu > Plot > ElementsYou may also wish to turn on element numbering and turn off keypoint numbering
    Utility Menu > PlotCtrls > Numbering ...

Solution Phase: Assigning Loads and Solving


  1. Define Analysis Type
  2. Solution > New Analysis > Static
    ANTYPE,0

  3. Allow Large Deflection
  4. Solution > Sol'n Controls > basic
    NLGEOM, ON
    Because the model is expected to deform considerably, we need to include the effects of large deformation.

  5. Apply Constraints
  6. Solution > (-Loads-) Apply > (-Structural-) > Displacement > On Nodes

    • Fix Nodes 3, 4, 12, and 13. (ie - all degrees of freedom are constrained).

  7. Apply Loads
  8. Solution > (-Loads-) Apply > (-Structural-) > Force/Moment > On Nodes

    • Apply a vertical point load of 1000N at node #7.
    The applied loads and constraints should now appear as shown in the figure below.
    Note: To have the constraints and loads appear each time you select 'Replot' in ANSYS, you must change some settings under Utility Menu > Plot Ctrls > Symbols.... In the window that appears check the box beside 'All Applied BC's' in the 'Boundary Condition Symbol' section.


  9. Solve the System
  10. Solution > (-Solve-) Current LS
    SOLVE
    Note: During the solution, you will see a yellow warning window which states that the "Coefficient ratio exceeds 1.0e8". This warning indicates that the solution has relatively large displacements. This is due to the rotation about the joints.

Postprocessing: Viewing the Results


  1. Plot Deformed ShapeGeneral Postproc > Plot Results > Deformed Shape
    PLDISP.2

  2. Extracting Information as ParametersIn this problem, we would like to find the vertical displacement of node #7. We will do this using the GET command.

    1. Select Utility Menu > Parameters > Get Scalar Data...
    2. The following window will appear. Select 'Results data' and 'Nodal results' as shown then click 'OK'
    3. Fill in the 'Get Nodal Results Data' window as shown below:
    4. To view the defined parameter select Utility Menu > Parameters > Scalar Parameters...
      Therefore the vertical displacement of Node 7 is 323.78 mm. This can be repeated for any of the other nodes you are interested in.

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