Autodesk Inventor: Moving Right Along
By Bill Fane
In a previous Autodesk Inventor? tutorial, you learned how to use two of the software’s motion constraints—Rotation-Rotation (gears or pulleys) and Rotation-Translation (rack and pinion). With these constraints, as with the Transitional constraint you’ll explore in this tutorial, you can analyze a mechanical assembly to ensure that the built part will function correctly and to demonstrate the function to other people.
I can best describe the motion of the Transitional constraint as a “cam and follower.” You’ll use this constraint to model and simulate a range of motions significantly more complex than those available with Rotation-Rotation and Rotation-Translation. A quick demonstration will show you the some of the power of this constraint. What Goes Around... 1. Start by downloading the ZIP file containing the files you will need to complete the tutorial. They works with Inventor 5.0 and later. Download (zip - 2013 Kb)
2. Unzip the file content into a suitable folder.
3. Start Inventor, go to the folder where you placed the content, and open the assembly file BF-03-01.iam, which looks like Figure 1.
Figure 1: Assembly file BF-03-01.iam.
4. Click the Place Constraint function in the panel bar, in R5.0 and 5.3 or in R6.0 and 7.0. The Place Constraint dialog box opens.
5. Click the Transitional tab (see Figure 2).
Figure 2: The Transitional tab of the Place Constraint dialog box.
View Larger Image Figure 3: The follower and the cam, with their faces selected for the Transitional constraint.
6. To apply a Transitional constraint, click the curved face at the bottom of the red follower (see Figure 1), and then click the upper, curved face of the green cam. Your screen, including the dialog box, should now look like Figure 3.
Note: You must click the bottom, curved face of the red follower before you click the upper, curved face of the green cam. Do not click edges or any other faces.
7. In the Place Constraint dialog box, click Apply > Cancel, which adds a Transitional constraint as the last item in the Browser and closes this dialog box. 8. In the Browser, right-click the DRIVE ME angle constraint, which is just above the Transitional constraint. 9. When the context menu opens, click Drive Constraint, which opens the Drive Constraint dialog box. 10. Click the Forward button, and then watch in shock and awe as the green cam rotates three revolutions while the red follower faithfully follows its contour. 11. When the motion has finished, click Cancel.
12. Move the cursor to a point within the region of the green cam’s smaller arc.
13. Press and hold down the left mouse button.
14. Drag the cursor around in a circular motion, centered approximately on the center of the larger arc of the cam. As with other constraints in Inventor, you can drag the components around and the constraints update dynamically. The follower stays in contact with the cam and oscillates properly. You have now applied and used the Transitional constraint in a fairly rudimentary way. (Keep in mind that I said earlier that this constraint offered a great range of motion.) So why is this constraint called Transitional? Take a close look at the green cam. Now, compare it to the one shown in Figure 4, which is a screen shot of assembly file BF-03-02.iam.
Figure 4: A simple circular cam.
The cam in Figure 4 consists of a simple, circular shape, extruded eccentrically to the shaft. The contact face of the cam consists of a single, continuous, constant-radius surface constrained with a Tangent constraint. In this instance, the follower has a simple reciprocating motion when you rotate the cam.
Now look at the cam in Figures 1 and 3. Its contact face consists of two arc portions that transition (hence, Transitional constraint) between two straight portions. The contact face actually consists of four surfaces. The unique Shape Manager technology within Inventor makes it possible for this transitional surface to work as a cam.
But Wait, There’s More! The transitions do not even have to be smooth tangencies.
1. Open the assembly file BF-03-03.iam, which looks like Figure 5.