Autodesk Inventor: Dynamic Assembly Constraints
By Bill Fane
I assume you have a working knowledge of the basic assembly constraints in Autodesk Inventor? software. In this tutorial, you'll learn how to use this product's dynamic assembly constraint analyzer to:
Determine if your assemblies are working properly. Evaluate whether your assemblies are properly constrained. Create an AVI file that demonstrates how the assembly works.
In the process, you're going to improve your efficiency, your productivity, and the quality of your designs. For starters, the analyzer has no update function for handling changes to an assembly. (This is a good thing.) Instead, this tool analyzes changes continuously, and any necessary updating takes place in real time. For example, you can use your mouse to grab onto any partially constrained component and drag it to a new location—and any other components constrained to it follow along. You don't have to stop working to update other components. But let's work on a cylinder assembly file and you'll see what I mean.
1. Download the drawing file, which works with Autodesk Inventor 5 and later. Download (zip - 572 Kb)
View Larger Image Figure 1: The cylinder assembly.
2. Unzip it into a suitable folder.
3. Start Inventor, and open the assembly file BF-01-Cylinder.iam. It should look like Figure 1. 4. Position the cursor within the circular end of the gold crank pin.
5. Hold down the left mouse button and move the cursor in a circular motion. 6. Keep the cursor more or less within the circular end of the crank pin, and move it around the main bearing portion of the crankshaft.
Observe how the black connecting rod and the aluminum piston obediently follow along with proper rotary-to-oscillating and oscillating motions respectively. The assembly is functioning as it should and having that information sooner rather than later saves you the costs of redesign?time, money, and less than satisfied clients.
You can also use the analyzer to determine if a model is properly constrained. For example, in an engine the crankshaft, connecting rods, and pistons should all move properly. On the other hand, you probably do not want the cylinder head to move relative to the block. Use the analyzer to see if the head moves, and if it does, add more restraints. Let's look in more detail at using the analyzer to test assembly design.
First, an observation. When you were moving around the cursor just now, did you notice that the piston pulls completely clear of the green frame as it approaches the bottom of the stroke, but reenters the bore properly on the way back up? I applied a mate constraint between the centerlines of the piston and the bore, so that they remain aligned, regardless of whether the cylindrical portions remain engaged. Obviously, the real world does not work this way. We can deduce a fundamental rule from this: Just because an assembly model works properly does not necessarily mean that the real mechanism will work too. Let's Go for a Drive... Having said that, I still believe that creating an assembly model is a very valuable tool both for analysis and demonstration purposes. Let's start with demonstration mode. Once you have designed an assembly, you usually have to demonstrate how it functions to other people. Since a word is worth .001 of a picture, an animated demonstration is usually better than a verbal description. Try this:
1. In the browser window, the entries for the frame and the crank should already be expanded to show the constraints. If they aren't, go ahead and expand them. 2. (Optional) Select the Angle (325.00 deg) constraint, which is under the Crank entry (see Figure 2a). The relevant faces are highlighted in a cyan color (see Figure 2b). Notice that this constraint is shaded out in the browser window indicating that it is currently suppressed. If it were active, you would not have been able to rotate the crankshaft with your mouse earlier. This optional step doesn't affect the process we're exploring, but it highlights the constraint that will be driven.
Figure 2a: The browser window with the Angle constraint selected.