Mastering Gear Creation in Onshape: A Comprehensive Guide

Onshape, a cloud-based CAD software, offers robust tools for creating complex parts, including gears. This comprehensive guide will walk you through the process of designing a gear in Onshape, from understanding gear parameters to generating the final model. Whether you’re a student, hobbyist, or professional engineer, this tutorial will provide you with the knowledge and steps necessary to create accurate and functional gears for your projects.

Understanding Gear Terminology and Parameters

Before diving into the Onshape interface, it’s crucial to understand the fundamental terminology and parameters associated with gears. These parameters dictate the gear’s size, shape, and performance.

Module (m): A fundamental parameter that defines the size of the gear teeth. It’s the ratio of the pitch diameter to the number of teeth.
Number of Teeth (N): The total number of teeth on the gear.
Pitch Diameter (d): The diameter of the imaginary circle where the gear teeth mesh perfectly with another gear. It’s calculated as d = m * N.
Pressure Angle (φ): The angle between the line of action and the line tangent to the pitch circle. Common pressure angles are 14.5° and 20°.
Addendum (a): The radial distance from the pitch circle to the top of the tooth. Typically, a = m.
Dedendum (b): The radial distance from the pitch circle to the bottom of the tooth. Typically, b = 1.25 * m.
Whole Depth (ht): The total depth of the tooth, which is the sum of the addendum and dedendum. ht = a + b.
Root Diameter (dr): The diameter of the circle at the base of the teeth. dr = d – 2 * b.
Outside Diameter (da): The diameter of the circle at the top of the teeth. da = d + 2 * a.
Backlash: The amount of clearance between mating gear teeth.
Face Width (F): The width of the gear tooth along the axis of rotation.

Step-by-Step Guide to Creating a Gear in Onshape

Now, let’s proceed with the step-by-step instructions for creating a gear in Onshape. We’ll use the spur gear as an example, but the principles can be adapted to other gear types.

Step 1: Create a New Document

Open Onshape in your web browser.
Click on the Create button in the top left corner.
Select Document and give your document a meaningful name (e.g., “Spur Gear Design”).
Click OK.

Step 2: Define Gear Parameters

Before starting the sketch, define the gear parameters using variables. This makes it easy to modify the gear design later.

Click on the Variable icon (it looks like an ‘x=’) in the toolbar.
Define the following variables:
- module (m): Enter the desired module value (e.g., 2 mm).
- num_teeth (N): Enter the desired number of teeth (e.g., 20).
- pressure_angle (phi): Enter the pressure angle in degrees (e.g., 20 deg). Ensure you add ‘deg’ to indicate degrees.
- face_width (F): Enter the desired face width (e.g., 10 mm).
You can also define calculated variables based on the initial variables:
- pitch_diameter (d): Set the value to module * num_teeth.
- addendum (a): Set the value to module.
- dedendum (b): Set the value to 1.25 * module.
- outside_diameter (da): Set the value to pitch_diameter + 2 * addendum.
- root_diameter (dr): Set the value to pitch_diameter - 2 * dedendum.

Step 3: Create the Base Circle Sketch

Select the Front plane and click on Sketch.
Use the Center Point Circle tool to create a circle at the origin.
Dimension the circle’s diameter to be equal to the pitch_diameter variable. Type #pitch_diameter into the dimension box and press Enter.
Rename this sketch to “Pitch Circle”.
Create another sketch on the same Front plane.
Use the Center Point Circle tool to create a circle at the origin.
Dimension the circle’s diameter to be equal to the root_diameter variable. Type #root_diameter into the dimension box and press Enter.
Rename this sketch to “Root Circle”.
Create a final sketch on the same Front plane.
Use the Center Point Circle tool to create a circle at the origin.
Dimension the circle’s diameter to be equal to the outside_diameter variable. Type #outside_diameter into the dimension box and press Enter.
Rename this sketch to “Outside Circle”.

Step 4: Constructing a Single Tooth Profile

This is the most complex step. We’ll create the involute curve that defines the tooth profile.

Sketch on the Front Plane: Start a new sketch on the Front plane.
Centerline: Draw a vertical centerline from the origin upwards. This will be a reference for our tooth profile.
Involute Construction: This requires some geometric understanding. We will create a point on the pitch circle and then generate a line tangent to the base circle. We will then rotate this line around the origin and generate a series of points. We will connect these points with a spline to create the involute curve.
- Base Circle Diameter Create another variable named base_diameter with a value of pitch_diameter * cos(pressure_angle).
- Base Circle Sketch Create a sketch named “Base Circle” on the Front Plane and add a circle centered at the origin with a diameter equal to the base_diameter variable.
- Initial Point Create a point on the Pitch Circle at the intersection with the centerline.
- Tangent Line Create a line tangent to the base circle starting from the point where the base circle intersects the X axis. This can be done by using the “Line” tool and clicking on the base circle and then defining the tangent constraint.
- Pattern Use the Circular Pattern tool to pattern the tangent line around the origin. Choose the origin as the center of the pattern. Enter the number of instances (e.g., 10 or more for a smoother curve). Spread the instances over a small angle (e.g., 5-10 degrees). This will create a series of tangent lines.
- Points on Tangent Lines Create points on each of the patterned tangent lines. Ensure these points are spaced evenly along the lines. You can achieve this by creating a variable representing the distance from the base circle intersection and then using this variable to position the points on each line.
- Involute Spline Use the Spline tool to connect the points created in the previous step, starting from the intersection of the base circle and the X axis and progressing outwards. This will form one side of the tooth profile.
Mirror the Involute: Mirror the involute curve across the centerline. This creates the other side of the tooth profile.
Tooth Tip and Root: Connect the top ends of the involute curves with an arc that follows the outside diameter circle. Similarly, connect the bottom ends of the involute curves with an arc that lies near the root circle. Note: For a more accurate root fillet, you can research and apply proper fillet equations.
Complete the Profile: Close the tooth profile by connecting the root arc to the root circle.
Rename this sketch to “Tooth Profile”.

Step 5: Extrude the Tooth Profile

Select the Extrude tool.
Select the “Tooth Profile” sketch.
Set the extrusion depth to the face_width variable.
Click OK.

Step 6: Circular Pattern the Tooth

Select the Circular Pattern tool.
In the pattern dialog, select Feature Pattern as the pattern type.
Select the extruded tooth as the feature to pattern.
Select the cylindrical face of the gear as the axis of rotation.
Set the number of instances to the num_teeth variable.
Ensure that the Equal spacing option is checked.
Click OK.

Step 7: Add a Bore (Optional)

Select the face of the gear.
Click on Sketch.
Use the Center Point Circle tool to create a circle at the origin.
Dimension the circle to the desired bore diameter.
Select the Extrude tool.
Select the circle you just created.
Choose Remove as the operation type.
Set the extrusion depth to Through all.
Click OK.

Step 8: Fine-Tuning and Refinement

After creating the basic gear, you can refine the design further:

Filleting: Add fillets to the sharp edges of the teeth to reduce stress concentration and improve durability. Use the Fillet tool.
Chamfering: Chamfer the edges of the gear to ease assembly. Use the Chamfer tool.
Gear Modifications: Adjust the gear parameters (module, number of teeth, pressure angle) to optimize the gear for your specific application. Since you used variables, this is very easy.

Advanced Techniques and Considerations

Creating Involute Curves with Equations: Instead of approximating the involute curve with splines, you can use mathematical equations to define it precisely. This results in a more accurate gear profile. Onshape allows the input of parametric equations for sketch curves.
Generating Gear Pairs: To create a functional gear system, you need to design a pair of gears that mesh correctly. Ensure that the gears have the same module and pressure angle. The number of teeth can be different to achieve the desired gear ratio.
Helical Gears: For helical gears, you’ll need to incorporate the helix angle into your design. This involves creating a helix curve and using it to sweep the tooth profile.
Gear Racks: A gear rack is a linear gear. You can create it by extruding a series of tooth profiles along a straight line.
Importing Gears: Onshape allows you to import gear models from other CAD software or online libraries.
Using FeatureScript: Onshape’s FeatureScript allows you to create custom features, including gear generators. This can automate the gear design process. This is beyond the scope of a beginner tutorial but is a very powerful tool.

Tips for Successful Gear Design

Accuracy is Key: Pay close attention to the accuracy of your dimensions and geometric constructions. Small errors can lead to significant problems with gear meshing.
Use Variables: Using variables to define gear parameters makes it easy to modify the design and experiment with different configurations.
Test Your Design: Before manufacturing your gear, test the design virtually using simulations or physical prototypes.
Consider Manufacturing Constraints: Design your gear with the manufacturing process in mind. Avoid overly complex geometries that are difficult to machine.
Understand Gear Standards: Familiarize yourself with relevant gear standards (e.g., AGMA) to ensure that your design meets industry requirements.

Troubleshooting Common Issues

Gear Teeth Not Meshing Properly: This can be caused by incorrect gear parameters, inaccurate tooth profiles, or improper alignment. Double-check your dimensions and constraints.
Interference: Interference occurs when the teeth of one gear collide with the teeth of the other gear. This can be resolved by adjusting the pressure angle, addendum, or dedendum.
Weak Teeth: Weak teeth can be caused by stress concentrations or insufficient tooth thickness. Consider adding fillets or increasing the face width.
Circular Pattern Errors: Ensure that the axis of rotation is correctly defined and that the number of instances is accurate.

Conclusion

Designing gears in Onshape requires a solid understanding of gear terminology, geometric construction techniques, and the Onshape interface. By following the steps outlined in this guide, you can create accurate and functional gears for a wide range of applications. Remember to experiment with different parameters, refine your designs, and always consider the manufacturing process. With practice and dedication, you’ll become proficient in gear design and be able to create complex mechanical systems with ease.

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