Designing Mounting and Support Features

1. Mounting Strategy: Locating vs. Non-Locating Bearings

The first step in designing a mounting strategy is to decide how the bearings will support the shaft. A shaft is typically supported by two bearings, but their roles are not identical. One bearing must locate the shaft axially (preventing it from sliding back and forth), while the other must accommodate small changes in length due to thermal expansion or manufacturing tolerances.

Your mechanical engineering background will make these concepts familiar, but we'll review them in the context of a practical design choice.

For our rotary mechanism, we will use a locating/non-locating arrangement for both the input and output shafts. This is a robust and common approach for general-purpose machinery.

• Locating Bearing: One bearing on each shaft will be fixed axially in the housing. It will react to any axial loads on the shaft.
• Non-locating Bearing: The other bearing on each shaft will be allowed to "float" axially within its housing bore. This prevents the bearings from fighting each other if the shaft expands or contracts.

2. Support Structure: A Custom Bearing Plate

Now that we have a strategy, how do we physically implement it? We need a structure to hold the bearings. There are two main approaches:

1. Using standard components: Mount off-the-shelf bearing housings (like pillow blocks or flange-mount units) onto a simple base plate.
2. Designing a custom structure: Machine a custom block or plate with integrated bores to hold the bearings directly.

To explore the trade-offs, let's watch a short video that analyzes this exact decision.

As the video explains, a custom-machined block or plate offers superior control over the alignment between rotating components. By machining the bearing bores for both of our shafts in a single setup, we can ensure the center-to-center distance is highly accurate. For our portfolio project, this is the approach we'll take.

Our goal is to design a single, thick support plate that will hold the two bearings for the input shaft and the two bearings for the output shaft.

3. In-Context Design: The Professional Workflow in Onshape

To ensure our support plate fits perfectly with the shafts and pulleys, we will model it in context of the assembly. This powerful technique allows you to create a new part while referencing the geometry of other parts around it.

First, let's select our bearings so we know the required bore dimensions.

• Input Shaft (12 mm): We'll use a standard 6201 deep groove ball bearing. (ID: 12 mm, OD: 32 mm, Width: 10 mm).
• Output Shaft (16 mm): We'll use a 16002 deep groove ball bearing. (ID: 16 mm, OD: 32 mm, Width: 8 mm).

Notice that both bearings have the same 32 mm outside diameter. This is a great design simplification, as we can use the same tool to machine all four bores in our plate!

Now, let's learn the in-context workflow.

4. Designing for Assembly: Self-Locating Features

A well-designed part shouldn't just be strong; it should be easy to assemble correctly. We can add features to our support plate that help locate other components, a principle known as "design for assembly" (DFA).

We will apply two of these concepts to our support plate:

1. Bearing Shoulders: For our "locating" bearings, we will machine the bore with a step or shoulder. This gives the bearing's outer race a surface to press against, providing positive axial location.
2. Dowel Pins: We will add holes for dowel pins. These will be used later to precisely align our support plate with a main base plate, ensuring the entire mechanism is square.

Your Task for Today's Session

Your assignment is to create the main support plate for our mechanism. This will involve creating a temporary assembly to define the layout, then using in-context design tools to model the plate.

1. Create the Layout Assembly:

• Create a new Assembly in your Onshape document.
• Insert the Input Shaft and Output Shaft you designed previously.
• Insert the 24T and 72T pulleys from the previous lesson's Part Studio.
• Mate the pulleys to their respective shafts.
• Use a Parallel Mate and an Offset to position the two shafts at the correct center distance of 120 mm. Fix one of the shafts in place so the assembly is stable.

2. Design the Support Plate In-Context:

• With the layout assembly open, click "Create Part Studio in Context" on the toolbar. Select the assembly origin when prompted.
• A new Part Studio will open with the shaft assembly visible as a ghosted reference.
• Create a sketch on the Front Plane. Draw a rectangle that encompasses the area where the four bearings will go. A size of roughly 200 mm x 80 mm should be a good starting point.
• Extrude the sketch to create the plate. A thickness of 15 mm will provide good rigidity.

3. Add Bearing Bores and Features:

• Create a new sketch on the face of your plate.
• Use the "Use" (Project) tool to project the circular edges of the four bearing locations from your ghosted shafts onto your sketch. This ensures the bores will be perfectly aligned.
• Use the Extrude-Remove tool to create the four 32 mm bearing bores through the plate.
  • Fits: For a stationary outer ring with point load, a loose fit like H7 is appropriate. For modeling purposes, we will model the bores at the nominal 32 mm diameter. The tolerance is specified on the drawing later.
• Add Shoulders for Locating Bearings: For one bearing on the input shaft and one on the output shaft, add a feature to locate it axially.
  • The simplest way is to use the Extrude tool again. Select the circular edge of the bore on one side of the plate and extrude a ring-shaped boss a few millimeters thick. This creates a counterbore where the bearing sits against a solid shoulder. (Alternatively, you can model the bore with two different diameters using Revolve-Remove).
• Add Mounting and Alignment Holes:
  • Add four M6 clearance holes (e.g., 6.5 mm diameter) near the corners of the plate for mounting it to a base.
  • Add two dowel pin holes (e.g., 6 mm diameter) on the centerline but at asymmetric locations. This is a poka-yoke (mistake-proofing) technique to ensure the plate can only be installed in one orientation.

Conclusion

Excellent work today. You've moved from individual parts to system-level thinking, which is a critical step in any design project. You've defined a robust mounting strategy, considered the manufacturing trade-offs, and used a professional-grade CAD workflow to create a key structural component.

Key Takeaways:

• A locating/non-locating bearing arrangement is a standard, robust method for supporting shafts.
• Designing a custom support plate gives you precise control over the alignment of critical components like shafts.
• In-context design in Onshape is an essential workflow for creating parts that must fit perfectly within an assembly.
• Simple Design for Assembly (DFA) features like shoulders and asymmetric dowel pins can dramatically improve the ease and reliability of assembly.

Preview of the Next Lesson:
Our support plate forms the heart of the mechanism's structure. In the next lesson, "Model the mechanism housing, applying DFM principles for stiffness and manufacturability," we will build upon this plate. We'll add features to enclose the belt and pulleys, consider how to make the housing stiff enough to resist operating forces, and apply principles of Design for Manufacturing (DFM) to ensure our design is practical to produce.