How to define a local coordinate system

When setting up a model for analysis, it can be easier and more efficient to add loads and constraints in a coordinate system other than the default (particularly for off-axis loads or constraints or for curved geometry). The FEA Editor environment of Autodesk Simulation Mechanical provides the ability to define local coordinate systems (rectangular, cylindrical, or spherical with the origin located anywhere in space) and then add loads and constraints relative to them.

For example, consider a model with a cylindrical surface. To define the vector for a tangential edge force you can no longer use the global coordinate system. You will need to define a local coordinate system and use it to define your load vector.

Define a cylindrical local coordinate about the centerline of the cylinder as follows:

• In the FEA Editor environment tree view, right click on "Coordinate Systems" to access a pop-up menu of options and choose "New..."  The "Creating Coordinate System Definition" dialog will appear (see Figure 1).  This screen provides options for defining a new coordinate system.

Figure 1: The "Creating Coordinate System Definition" dialog is used to define local coordinate systems.

• Click on the "Coordinate System Type" field and choose from the pull-down menu of options.  For this example, "Cylindrical" was specified.
• Enter three sets of coordinates in the "Point A", "Point B" and "Point C" fields to define the local coordinate system per the diagram shown on the screen. 
• Alternatively, you can define coordinates by clicking on points in the model as follows:
• Click on the “Select A,” “Select B,” or “Select C” button.  Then click on a vertex in the model to define the coordinate.
• Click on the "Interactive" button. Then click on three vertices to define all three points sequentially. A graphical display of an axis will be drawn on the model to guide you as you click on the points.
• Click on the "OK" button to accept the coordinate system definition. It will then be listed in the model tree under "Coordinate Systems".

Now, you can conveniently add a force load to the model relative to the local coordinate system. 

For this example, select the edges and right click anywhere in the working area to access a pop-up menu of options. Choose "Coordinate Systems" and then the local cylindrical coordinate system to apply it to the selected edges (see Figure 2). Icons indicating the orientation of the local cylindrical coordinate system will appear on the vertices.

Figure 2: Apply the local cylindrical coordinate system to the selected edges

Right click in the working area and choose "Add" and "Edge Forces..." to access the "Creating Nodal Force Objects" dialog. Notice that the active local cylindrical coordinate system is shown in the "Coordinate System" field (see Figure 3). The axis icons on the selected vertices will indicate the direction that you should define for the force load. In this case, the Y direction was specified to designate the tangential direction.

Figure 3:  Add a force load to the selected Edges. The active local cylindrical coordinate system is shown in the "Coordinate System" field

After adding the force load, the display will show the force arrows pointing in relation to the local cylindrical coordinate system (see Figure 4).

Figure 4: The force load is applied in relation to the local cylindrical coordinate system

Thus, defining a local coordinate system in the FEA Editor can make it easier to apply loads and constraints. Local coordinate systems can either be reused in the Results environment or defined there to display results in an arbitrary orientation relative to them