Analyzing Assembly Motion with Mates and Relations

1. Visualizing Motion: Animating Mates and Setting Limits

The first step in analyzing an assembly's motion is to visualize it. Onshape has a built-in tool to animate any mate that has degrees of freedom (like Revolute, Slider, or Cylindrical).

When you create a mate, you can right-click on it in the Mate Features list and select "Animate." This opens a dialog where you can define the motion parameters.

However, a part in a real-world mechanism rarely has unlimited movement. A lever arm has hard stops, and a linear slide has a finite travel distance. You can define these physical boundaries by setting limits within the mate itself.

To see how this works, let's watch a short segment from the Onshape overview video you saw previously.

Your Turn: Add Limits to Your Hinge

Let's apply this to the simple hinge assembly you created in the last lesson.

1. Open your "Project 1" Onshape document and go to the Assembly tab.
2. Find the Revolute mate in the Mate Features list on the left.
3. Right-click the mate and select Edit.
4. In the dialog box, check the box for Limits.
5. Enter a minimum value of 0 deg and a maximum value of 90 deg. Click the green check.
6. Now, try dragging the lever. You'll notice it stops at 0 and 90 degrees.
7. Right-click the mate again and select Animate. Set the playback type to "Reciprocate" and click Play. You will see the lever swing back and forth precisely within your defined limits.

This simple validation is a powerful tool to quickly check the intended kinematics of your design.

2. Creating Complex Motion: Mate Relations

Mates define motion between two parts. Relations take this a step further by coupling the motion of different mates together. This allows you to simulate more complex mechanical systems where the movement of one component drives another.

Your mechanical engineering background gives you a head start here; concepts like gear ratios and screw pitches translate directly into Onshape's Mate Relations. There are four primary types:

• Gear Relation: Links the rotational motion of two mates.
• Rack and Pinion Relation: Links the rotational motion of one mate to the linear motion of another.
• Screw Relation: Links the rotational and linear motion within a single Cylindrical mate.
• Linear Relation: Links the linear motion of two mates.

To get a clear picture of these relations, we'll consult an article from Onshape.

3. Practical Application: A Screw Mechanism

Let's build the mechanism from the exercise above. This will be fantastic practice for Project 1, which focuses on a linear motion system likely involving a lead screw or ball screw.

The workflow is:

1. Create the parts.
2. Assemble with a mate that allows the base motion (Cylindrical).
3. Add a relation to constrain the motion (Screw).

Your Turn: Model and Animate a Screw

1. In your Onshape document, create a new Part Studio.
2. Model two simple parts:
   • Screw: Create a cylinder (e.g., 10 mm diameter, 100 mm long). For visual effect, you can use the Helix tool with the Sweep (Remove) feature to cut a shallow thread, but this is not required for the mechanism to work.
   • Nut: Create a block (e.g., 30x30x20 mm) with a 10 mm diameter through-hole.
3. Create a new Assembly tab.
4. Insert the Screw and the Nut.
5. Right-click the Nut and select Fix to lock it in place.
6. From the toolbar, select the Cylindrical Mate. Mate the cylindrical face of the Screw to the cylindrical face of the hole in the Nut. Click the green check.
   • At this point, you can drag the screw to see it both rotate and slide freely.
7. Now, select the Screw Relation from the toolbar.
8. In the 'Mates' selection box, select the Cylindrical Mate you just created from the Mate Features list.
9. Set the 'Distance per revolution'. A value of 2 mm is a good starting point (representing a 2mm pitch). Click the green check.
10. Now, click and drag the screw. Try dragging it rotationally—you will see it translate. Try dragging it linearly—you will see it rotate.
11. Animate the Cylindrical mate to see the smooth, coupled motion.

You have now successfully modeled a kinematic system that directly translates rotational motion into linear motion, a cornerstone of mechanical design.

Conclusion

In this lesson, you've bridged the gap between static assemblies and dynamic mechanisms. You can now verify, analyze, and simulate the motion of your designs directly within Onshape.

Here are the key takeaways:

• Animate Mates for Visual Feedback: Use the Animate function to quickly check if a component's motion is correct.
• Use Limits for Realistic Motion: Mates like Revolute and Slider can be constrained with Limits to define a realistic range of motion.
• Relations Create Coupled Motion: Use Gear, Screw, Rack and Pinion, and Linear relations to link the movements of different mates, simulating complex mechanical systems.
• The Workflow: The process of defining motion is systematic: establish degrees of freedom with a mate, then constrain them further with limits and relations.

You are now equipped with the fundamental Onshape skills for both part modeling and assembly kinematics. In our next lesson, "Create a drawing in OnShape, place standard orthographic and isometric views," we will move on to another critical part of the engineering process: documenting your designs for manufacturing and communication by creating 2D engineering drawings.