Assembling Mechanical Components

1. The Onshape Philosophy: Mates and Mate Connectors

Before we begin, it's important to understand that Onshape approaches assemblies differently from some traditional CAD systems. Instead of constraining parts by removing degrees of freedom (DOF) one by one (e.g., one constraint for alignment, another for contact), Onshape uses a more efficient system built on Mate Connectors.

Think of a Mate Connector as a dedicated coordinate system attached to your part's geometry. When you create a mate, you are simply defining a single, intelligent relationship between two of these connectors.

Let's watch a video that explains this core concept.

To summarize the key mates we will be using today:

• Fastened Mate: Removes all 6 degrees of freedom. Use this for parts that are rigidly attached and do not move relative to each other.
• Revolute Mate: Allows only one rotational degree of freedom around the Z-axis of the mate connector. Perfect for hinges, wheels on an axle, and our shafts rotating in their bores.
• Cylindrical Mate: Allows rotation around the Z-axis and translation along the Z-axis. This is ideal for a shaft sliding and rotating within a bushing.

2. Building the Top-Level Assembly

Now, let's put this theory into practice. We will build our rotary mechanism assembly step-by-step.

Step 1: Create the Assembly and Fix the Base Component
Every stable assembly needs a fixed reference point. All other parts will be mated relative to this anchor.

1. In your Onshape document, click the '+' icon in the bottom-left corner and select Create Assembly.
2. Click the Insert button in the toolbar. Navigate to your current document's Part Studios.
3. Select the Support Plate you designed previously and click to place it in the assembly space. Click the green checkmark to finalize.
4. In the instance list on the left, right-click on the support plate and select Fix. The part is now locked to the origin and cannot be moved. This is the foundation of our assembly.

Step 2: Insert Components and Apply Mates
Now we will insert the remaining components and define their motion.

Now, apply this process to our mechanism:

1. Insert Bearings: Most standard bearings are available in Onshape's public documents.
   • Click Insert > Other documents > Public.
   • Search for a standard bearing, for example, "608 bearing". Select a simple, reliable model.
   • Insert four bearings into your assembly.
2. Mate the Bearings: Use a Fastened Mate to fix each bearing into its corresponding bore on the support plate.
   • Pro-tip: To select a Mate Connector hidden inside a hole, hover over the circular edge until the connector appears in the center, then hold Shift to lock the selection pane. This allows you to move your mouse freely to select the connector without it disappearing.
3. Insert Shafts and Pulleys: Insert the input shaft, output shaft, and the two pulleys you have modeled or sourced.
4. Mate the Shafts: Use a Revolute Mate to place each shaft into the inner race of its corresponding bearings. This allows the shafts to rotate freely but prevents any other movement. Select the Mate Connector on the end of the shaft and the one on the center of the bearing's inner race.
5. Mate the Pulleys: Use a Fastened Mate to attach each pulley to its respective shaft. Ensure they are positioned correctly along the shaft's length using the mate offset options if needed.
6. Add the Belt: In the assembly toolbar, find and select the Belt relation. Select the cylindrical faces of the two pulleys to create the belt connection.

3. Integrating Fasteners from Standard Content

An assembly isn't complete without the nuts and bolts that hold it together. Onshape has a built-in library of standard hardware that makes this process incredibly efficient.

Let's learn how to access and use it.

Step 3: Add Fasteners to the Housing
Now, apply this to your project.

1. Insert the Housing: Insert the two halves of the housing you designed in the previous lesson.
2. Mate the Housing: Use Fastened Mates to secure the bottom half of the housing to the support plate. Then, mate the top cover to the bottom half.
3. Insert Screws:
   • Open the Insert dialog and select the Standard Content tab.
   • Choose a standard (e.g., ISO), category (Bolts & screws), and class (Socket head screws).
   • Select the appropriate size (e.g., M4) and specify a length that is suitable for joining your housing halves.
   • Instead of placing the screw in empty space, click directly into one of the clearance holes on your housing cover. Onshape will automatically create a mate and place the screw.
   • Repeat for all the screw locations.

For repeating patterns of fasteners, the Replicate tool is a huge time-saver. After mating one fastener, you can use Replicate to automatically populate all other identical holes in the part.

Conclusion

Excellent work! You have successfully completed a major milestone in this project. You've taken a collection of individual parts and integrated them into a fully defined mechanical assembly. This process of mating components and adding standard hardware is fundamental to a design engineer's daily workflow. Your Onshape assembly now serves as a digital prototype that can be used for motion analysis, interference checking, and generating documentation.

Key Takeaways:

• Onshape assemblies are built on the concept of Mate Connectors, which define relationships between parts efficiently.
• A robust assembly starts with one fixed component that acts as the anchor for all other parts.
• Specific mates like Revolute and Fastened are chosen to permit or restrict degrees of freedom, defining the mechanism's intended motion.
• Onshape's Standard Content library is a powerful tool for quickly adding common hardware like screws and bolts to your design.

Preview of the Next Lesson:
Our assembly now looks complete, but will it withstand the forces it experiences during operation? In the next lesson, we will move to validation. The learning outcome is to perform a basic FEA on the housing to verify its structural integrity. We will apply your existing knowledge of FEA within the Onshape environment to analyze stresses and deflections in the housing, ensuring our design is robust enough for its purpose.