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Lesson 21

Mounting Bracket Design for Actuators and Motors

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Hello! Welcome back to our project.

In our last session, we brought our linear motion system's core components into Onshape, modeling the ball screw and its associated parts. We now have the linear guides to carry the load and the ball screw to drive the motion. This is excellent progress.

Today, we move from integrating standard parts to creating our own. The learning outcome for this lesson is to design and model mounting brackets for the actuator and motor. These custom parts are the "glue" that will hold our system together, interfacing our standard components with the machine's main structure. This is a fundamental task for any design engineer, and it's where your creativity and engineering judgment really come into play.

We'll approach this in two parts:

  1. The Motor Mount: We'll focus on accurately interfacing with a standard component based on its specification sheet.
  2. The Actuator Mounts: We'll focus on designing for structural loads, applying principles to ensure the longevity and performance of our ball screw assembly.

Let's begin.

1. Designing and Modeling the Motor Mount

The stepper motor drives the ball screw. Its mount must hold it rigidly in place, ensuring alignment with the screw, and react to the motor's torque. The design process starts not in CAD, but with data.

In a professional setting, the first step is always to get the datasheet for the component you need to mount.

Designing a Motor Mounting Bracket

This blog post from a student's portfolio shows the exact workflow we will follow. It demonstrates finding a dimensioned drawing of a motor and using it as a reference to design a bracket in Onshape.

Read the first two sections, 'Selecting and Importing a Reference Image' and 'Scaling the Image and Designing a Part'. Notice how the process starts with finding a dimensioned drawing and using it to drive the CAD model. We will use a similar drawing but create our geometry directly from the dimensions.

Now, let's look at the component we'll be using. For our project, we'll use a standard NEMA 17 stepper motor. Below is its mounting face specification.

Stepper Motor Engineering Drawing with Dimensions
This is the engineering drawing for our motor's mounting interface. The critical dimensions for our bracket are the mounting hole pattern (the four M3 holes) and the overall face size (42.3mm square).

Your task is to design a bracket that picks up these mounting holes and allows the motor to be attached to a flat plate (which will be part of our main structure later). A simple "L" bracket is a common and effective solution.

Motor Mounting Bracket Technical Drawing and Isometric View
This is a drawing of a typical off-the-shelf NEMA 17 mounting bracket. We will design our own custom version tailored to our project.

The following video, while focused on a combat robot, demonstrates an excellent, practical workflow for modeling a part that interfaces with an existing component.

Designing Combat Robot Parts in Onshape

Pay close attention to the workflow shown in this video by Maker's Muse. The creator designs a mount around a component, considering tolerances and using efficient modeling techniques. This process is directly applicable to our motor mount.

Watch from 03:43 to 16:38. Focus on these key steps in the process: Creating a Reference Body (04:15): He models a simple block representing the component he needs to hold. Sketching with Offset/Tolerance (06:00): He designs around the reference body, leaving a small gap. Efficient Sketching (09:40): He uses a construction line and the Mirror tool to create symmetric geometry, saving time and ensuring consistency. Adding Mounting Holes (12:45): He carefully places and sizes holes, discussing the need to ensure enough material ('meat') is left around them for strength.

Your Task: Model the Motor Mount

In a new Part Studio in your Onshape project, follow this professional workflow:

  1. Model the Reference: Create a simple block representing the NEMA 17 motor's body (42.3 x 42.3 mm, extrude it to ~40 mm). Also, model the raised circular boss on the front face (22 mm diameter, ~2 mm tall). This reference part is crucial for ensuring your bracket fits.
  2. Design the Bracket: Create a new part in the same Part Studio. Design an "L" bracket that fits around the motor's face.
    • Use the dimensions from the NEMA 17 drawing to create the large circular cutout for the motor boss and the four M3 mounting holes (31 mm x 31 mm square pattern).
    • Give the bracket a reasonable thickness (e.g., 3mm).
    • Add a flange to the bracket that will be used to mount it to the machine frame. Add some mounting holes to this flange as well.
  3. Refine: Use the Fillet tool to round sharp corners. This is good practice for reducing stress concentrations and improving safety.

2. Designing and Modeling the Actuator Mounts

Mounting the ball screw assembly is even more critical, as improper mounting is a primary cause of failure in linear motion systems. The forces involved are purely axial (push/pull), and the mounting must respect that.

The Engineer's Guide to Linear Actuators

This guide from Firgelli, a linear actuator manufacturer, contains essential principles for mounting. Your mechanical engineering background will make the concepts here, particularly regarding buckling, very familiar.

Read section 7, 'Mounting Geometry & Failure Modes'. Pay special attention to: 7.1 The Golden Rule: Actuators are not guide rails. They must be isolated from side loads. 7.2 Pivot Mounts are Mandatory: This explains why one end must be allowed to pivot slightly. In our case, the 'fixed' bearing block (like a BK-type) handles thrust loads rigidly, while the 'simple' or 'floating' support (like a BF-type) on the other end accommodates thermal expansion and minor misalignment. 7.3 Buckling: A great refresher on Euler's critical load, emphasizing why compressive load capacity decreases with length.

Based on these principles, we need to design brackets to rigidly hold our ball screw end supports. These brackets must be stiff enough to resist deflection under load. A simple L-bracket might not be sufficient; we can improve its performance by adding structural features like ribs.

This next video provides a clean, concise tutorial on how to model a robust angle bracket, including the use of the Rib tool.

Learn Onshape - Project 11 - Angle Bracket

This tutorial by CAD Video Tutorials will show you how to create a more robust bracket and introduce a very useful feature for adding stiffness: the Rib tool.

Watch these three short clips: Initial Shape (00:24 - 01:35): See how the basic 'L' profile is created and extruded symmetrically. Holes and Fillets (01:35 - 03:17): Observe how to add holes and round edges on the 3D model. Adding a Rib (07:01 - 08:38): This is the key part. See how a simple 2D line sketch on a mid-plane is used with the Rib tool to add significant structural support. This is a far more efficient way to increase stiffness than simply making the entire part thicker.

Your Task: Model the Actuator Mounts

  1. Measure Your Components: In your Onshape assembly from the last lesson, use the Measure tool to get the mounting hole pattern and overall dimensions of the fixed (BK-type) and simple (BF-type) ball screw end supports you downloaded.
  2. Design the Brackets: In a new Part Studio, design the two brackets needed to hold these supports. They will likely be sturdy block- or L-shaped parts.
  3. Add Structural Ribs: Following the method in the video, add ribs to your brackets to increase their stiffness. This is a critical step in designing for performance. Think about the direction of the force from the ball screw and place the ribs where they will be most effective in resisting bending.
Test your understanding!

You are designing a bracket to hold a ball screw's fixed end support. The bracket is a simple L-shape, 100mm tall and 50mm wide, made of 5mm thick aluminum plate. You find that it deflects too much under the actuator's thrust load.

Based on today's lesson, what is a more effective way to increase its stiffness than simply changing the thickness from 5mm to 10mm? Why is it more effective?

Show answer

Adding a triangular rib between the two faces of the L-bracket is a more effective solution.

Increasing the thickness to 10mm would double the weight and material cost. A well-placed rib can provide a similar or greater increase in stiffness with a fraction of the added material. This is because the rib dramatically increases the second moment of area (area moment of inertia) of the bracket's cross-section in the direction of bending, which is the primary factor governing stiffness. Your FEA experience has shown you this principle in action.

Conclusion

Fantastic work today. You've moved beyond using off-the-shelf parts and have now designed and modeled your first custom components for this project. This is the essence of mechanical design engineering.

Key Takeaways:

  • Design Starts with Data: The first step to designing a mounting bracket is to obtain and understand the dimensions and specifications of the component you are mounting.
  • Design for Function: The motor mount's primary job is positioning and torque reaction. The actuator mounts must handle high axial loads without deflecting, while respecting the need to avoid side-loading the screw.
  • Use Features Intelligently: Simple Onshape features like Mirror can make your workflow more efficient. Structural features like Rib are a lightweight and effective way to increase part stiffness, a key principle in performance-oriented design.

Preview of the Next Lesson:
We now have our core motion system components and the brackets to hold them. The next logical step is to create the foundation that everything mounts to. In the next lesson, you will design and model the structural supports for the guide rails and the overall assembly, effectively building the chassis of our vertical lift system.