Overview#
The Motion Study tool performs kinematic motion analysis based on the joints in an assembly. In Autodesk's words, you select the joints to participate, then specify points and values for the motion. It is the tool to reach for when you want to see how several joints move together, rather than testing one joint at a time.
Important distinctions:
- A motion study is kinematic. It moves joints; it does not compute forces, torque, or stress.
- It is different from a Motion Link, which relates the degrees of freedom of two joints (a position-to-position relationship) and stays in effect as you drag.
- It is different from the Animation workspace, which animates component transformations (for example exploded views) using storyboards.
- It is different from Simulation studies (static stress, modal, and so on), which are covered in the simulation section.
Note: Autodesk's help for Motion Study consists of an overview and a short reference page. It does not document keyframe tracks, durations, frame rates, easing, rendering a study, or how contact sets and motion links behave during playback. Earlier versions of this page described those in detail; they are not in the documentation and have been removed. Anything below marked as general guidance is practical advice, not Autodesk data.
Where to find it#
- Design workspace > Solid tab > Assemble > Motion Study.
- Command Search (S): type "motion study".
- Prerequisite: an assembly with joints that can move (a Rigid joint has nothing to study).
What the Motion Study dialog shows#
Autodesk's Motion Study reference lists these elements:
| Element | What it does |
|---|---|
| Joint | Displays the name of the joint. |
| Degree of Freedom (DOF) | Displays the type of degree of freedom (for example rotate or slide). |
| Value | Displays the value of the DOF at the position the slider is currently at. |
| Play buttons | Play backwards and forwards in a continuous motion, or step by step. |
| Mode | Play the motion in one direction, forwards then backwards, or loop. |
| Speed | A slider that controls the speed of playback. |
So a study is driven by a position slider per degree of freedom, with playback controls and a loop mode, rather than a hand-authored timeline.
Step by step#
- Build and test your joints (see Joints and Joint Types). Add Motion Limits so the ranges are meaningful, and Rest positions where useful.
- Check that the joints you want are not locked. Autodesk's Edit Joints page says a joint with Lock Motion on cannot be moved until you unlock it, so a locked joint will not move in a study (inference from that description).
- Choose Assemble > Motion Study.
- Select the joints that should participate, and set the points and values for their motion.
- Use the Mode setting to pick one-way, forward-and-back, or loop playback, and the Speed slider to set the pace.
- Press the play buttons, or step through the motion, and watch the assembly.
- Adjust limits or joints and replay until the travel looks right.
Related tools compared#
| Tool | Where | What it does |
|---|---|---|
| Drive Joints | Assemble > Drive Joints | Enter exact values for a joint's degrees of freedom (Rotate, Slide, Axis, Normal, Pitch, Yaw) or drag the manipulator handles. |
| Animate Joint | Right-click a joint | Animates that joint only; ignores other joints and relationships. |
| Animate Joint Relationships | Right-click a joint | Animates the joint in context with other joints, grounded components and contact sets. |
| Motion Link > Animate | Motion Link dialog | Animates the relationship between linked joints. |
| Motion Study | Assemble > Motion Study | Kinematic analysis across the selected joints, with play, mode and speed controls. |
| Animation workspace | Workspace switcher | Storyboards and transformations such as Auto Explode; component motion rather than joint motion. |
Interaction with other assembly tools#
Joint limits#
Joints use Motion Limits (Minimum, Maximum, Rest). Set them first so the study covers a realistic range. Drive Joints and Animate Joint use the same limits, so test there first (general guidance).
Motion links#
A motion link relates the motion of two joints. Autodesk does not describe study behavior with links, so drive the input joint only and verify the follower behaves as expected on your design (general guidance).
Contact sets#
Contact sets stop components that would pass through each other when motion is analyzed. Autodesk notes they need more computation than a motion joint, so playback may slow down. Whether a contact halts a study is not documented; test on your design.
Rigid groups#
A rigid group moves as a single body, so joints attached to the group move the group.
Position afterward#
Leaving the assembly in a mid-motion pose is easy to do. Use Go to Home Position on a joint (right-click it) to return to where it was created, or use Capture Position to record the pose you want (see that page).
Tips and pitfalls#
Tip: Test each joint individually with Drive Joints or Animate Joint before including it in a study. Most study problems are joint problems.
Tip: Use Rest positions so you have a known state to return to.
Warning: Heavy assemblies with contact enabled make playback slower. Disable contact (Assemble > Disable All Contact) or simplify the design (see Large Assemblies) for review.
Tip: For exploded views and assembly instructions use the Animation workspace; a motion study acts on joints, not arbitrary component paths.
Warning: A motion study does not calculate real loads. If you need forces, use a Simulation study; check which studies your license includes, since feature availability depends on your subscription or extensions.
Tip: Capture the pose you like with Capture Position before you leave, so the arrangement is stored.
Worked example: checking a lid and latch#
A box has a latch on a Slider joint (0 to 8 mm limits) and a lid on a Revolute joint (0 to 100 degrees limits). You want to confirm that the latch clears before the lid opens, and that the lid does not hit the latch at full travel.
- Make sure both joints have Minimum, Maximum and Rest values that match the real stops.
- Add a contact set between the lid and the latch only (not the whole assembly) if you want collisions to be considered.
- Open Motion Study and select both joints.
- Set the latch to its maximum (8 mm) first and play the lid through its range. Watch the lid edge near the latch.
- Set the latch back to 0 and play the lid again. If the lid passes through the latch with contact off, you have found a real interference, which a contact set or an Inspect > Interference check will confirm.
Planning arithmetic is general guidance: if you want a lid to open in about 2 seconds over 100 degrees, that is 50 degrees per second, a comfortable review speed. Because the study has a Speed slider rather than a typed duration, use that slider to land near your target pace.
Planning before you play#
Write down, for each joint, its start value, end value and limits. Check three things: each end value is inside the joint limits; no two joints are asked to conflict (for example a motion-linked pair driven independently); and the pace is sensible, since a fast rotation is hard to read on screen. These checks cost minutes and save debugging.
Failure modes and fixes#
| Symptom | Cause | Fix |
|---|---|---|
| The joint stops short of the target | The target lies outside the joint limits, or a contact stopped it | Edit the limits or clearance first. |
| A joint does not move | Lock Motion is on, or the joint is Rigid | Right-click the joint and unlock it, or change the joint type. |
| Playback is slow or choppy | Contact sets or a heavy assembly | Turn contact off for the review, hide fasteners, simplify. |
| Linked joints behave oddly | Both the input and the follower are being driven | Drive only the input joint. |
| Cannot find the command | You are in a different workspace | Switch to the Design workspace and look in the Assemble panel, or use Command Search. |
| The model reopens in an open pose | The mechanism was saved mid-motion | Use Go to Home Position, or capture the position you want, before saving. |
After the study#
Decide what pose the model should end in. If the file should reopen closed, drive back to Rest or Home before saving. A drawing view made while the mechanism is open will show it open, so check the pose before you document the design (general guidance).
Practice exercise#
- Build a hinged lid on a box with a Revolute joint (0 to 100 degrees) and a small latch slider (0 to 5 mm) that releases the lid.
- Make sure neither joint has Lock Motion on.
- Open Motion Study, select both joints, and set Mode to forwards then backwards.
- Play with the Speed slider at a slow setting, then a fast one. Note which is easier to review.
- Move the latch to 0 and try the lid; then move the latch to 5 mm and try again. Confirm the latch clears only when retracted.
- Return the joints to their Home positions when finished.
Expected result: you can watch both joints move together, loop the motion, and see whether the latch interferes with the lid.
Stretch challenge#
Add an indicator lever on a Revolute joint (limits 0 to 30 degrees) that should swing only while the lid is open. Because the Motion Study tool in Autodesk's documentation does not provide a hand-authored timeline, achieve the coupling with a Motion Link between the lid joint and the lever joint (for example 100 degrees of lid travel to 30 degrees of lever travel), then play the lid and verify the lever follows. Expected result: the lever reaches 30 degrees as the lid reaches 100 degrees; check the ratio by driving the lid to 50 degrees and reading 15 degrees on the lever.
Related#
- Joint Limits and Motion Links
- Contact Sets
- Capture Position
- Joints and Joint Types