Moving assemblies
Give a joint the right movement, set its limits, and check the mechanism between its end positions.
A hinge should turn about its pin without sliding sideways or pulling apart. A slide should move along its rail without turning. Each independent movement a joint allows is called a . Choose that allow the movements you need and hold everything else in place.
Complete the rigid plate assembly first. Then use a separate document named Hinge practice for the moving example below.
Make a simple link
On the Top , a 60 × 16 mm center-point rectangle at the . Add two 4.5 mm diameter circles, centered at (−20, 0) and (20, 0) mm. the plate region 3 mm, leaving both holes open. Rename the part Link.
In an , insert two instances. Rename them Base link and Moving link. Fix Base link only. The second link should be free to drag before you add its mate.
Both instances share one source part. Changing Link changes both instances. Their position and movement belong to the Assembly, not to independent copies of the part's sketch.
Add a revolute mate
A Revolute mate allows one rotation about its primary axis. Onshape displays that primary axis as the connector's Z direction; it need not be the document's global Z axis.
- Start a .
- Select the circular rim of Moving link's left hole on its bottom face, then Base link's left hole on its top face.
- Check that the inferred connectors are at the hole centers and their primary axes follow the intended pin axis.
- Flip the primary alignment if necessary so the links occupy separate parallel layers.
- Use an offset to leave 1 mm between their facing surfaces. Check the resulting gap; the sign depends on the selected connector directions.
- Accept the mate. Drag Moving link: it should rotate around the shared hole axis without sliding away from it.
The 1 mm gap helps you see the two parts in this exercise. A physical hinge also needs a or pin, or bushings, spacing, and . Use Shaft supports to design that joint.
If the link will not rotate, check whether it is fixed or has another . If it both rotates and slides, you may have chosen a Cylindrical mate. Remove the unwanted freedom rather than adding unrelated constraints until the model stops moving.
Set zero and movement limits
The mate measures its angle between the two connectors, so zero may not put the links where you expect. Rotate the connectors so the links point in the same direction at zero, then move the link and watch the angle reading.
Enable the mate's Limits and set a minimum of 15° and maximum of 120°. Drag the link to both ends. It should stop at the limits in the intended direction. If it travels through the opposite sector, correct the connector orientation or limits.
These limits describe the model's allowed travel. They do not create a physical stop or configure the 's . Record the actual stop positions, references, and safe commanded range when the is built.
Inspect the whole movement
Right-click the mate and use Animate to inspect its range. Move slowly through intermediate positions, not just the two endpoints.
Look for collisions with the frame, screw heads, connectors, and the game piece. A claw can fit when fully open and fully closed but still scrape a rail as it moves between them. Include the opening motion of the claw's fingers as well as the arm's main rotation.
Use section views and analysis to find overlaps you cannot see from outside. Some overlaps may be intentional, such as a press fit, so inspect what the tool highlights. Keep checking as the mechanism moves; clearing the frame in one position does not mean it clears it everywhere.
Choose a different mate
| Mate | Movement | Typical use |
|---|---|---|
| Fastened | None | body attached to a mounting plate |
| Revolute | One rotation | Arm, roller, or wheel around an axis |
| Slider | One | Carriage along a straight guide |
| Cylindrical | Rotation and translation along one axis | A joint that really permits both motions |
For a , align the connector's primary axis with the travel direction. Choose a zero position you can identify on the real robot, then set the limits from the rail’s usable travel. Do not assume the rail's total physical length equals its travel.
Onshape normally respects a 's internal mates in the top-level assembly. If its movement stops there, inspect fixed instances, rigid groups, and the new parent-level mates. Dissolving the subassembly reorganizes its parts and mates; it is not the first repair for a joint constrained twice.
Relationships between joints
A mate relation connects existing movements. A gear relation can make two Revolute mates turn at a chosen ratio. A rack-and-pinion relation connects rotation to translation.
Create the mates first, then the relation. Check direction and ratio with an easy input: one full turn of a 20-tooth driver should produce half a turn of a 40-tooth driven wheel in a 2:1 . External meshing gears turn opposite ways; an ordinary open belt pair turns the same way.
The relation is a motion model. It does not calculate motor , belt stretch, tooth , or slip. Combine it with the load estimates in Forces and motion and a physical .
Check the result
Your practice assembly should have two link instances, one fixed base, and one Revolute mate. Moving link should rotate only between the chosen limits and stay in its layer with a 1 mm face gap.
Ask another student to drag it, identify the axis, and explain what limits its real movement. If only the mate is holding the joint together, work out which bearing, spacer, or will do that job on the real robot before going further.