How to Test Objects in Onshape Assembly Gravity
Introduction
Testing objects under gravity within an Onshape assembly is a powerful capability that allows engineers, designers, and students to validate how their 3D models behave in real-world physical conditions — without ever leaving the cloud-based CAD environment. Consider this: whether you are designing a mechanism that relies on gravity to operate — such as a trapdoor, a sorting system, or a simple drop test — or you simply want to verify that your assembly holds together under its own weight, understanding how to test objects with gravity in Onshape is an essential skill. Gravity simulation in Onshape enables you to apply gravitational forces to components within an assembly, observe how they fall, settle, rotate, or interact with one another, and identify potential design flaws before committing to physical prototyping. This article provides a comprehensive, step-by-step guide to setting up, running, and interpreting gravity simulations in Onshape assemblies, along with practical examples, best practices, and common pitfalls to avoid Took long enough..
Understanding Gravity Simulation in Onshape Assemblies
Gravity simulation in Onshape falls under the broader category of motion analysis and basic physics simulation available within the platform. When you build an assembly in Onshape, you define how parts relate to one another through mates — constraints that control position, orientation, and degrees of freedom. But by default, most parts in an assembly are fully constrained and do not move. But onshape is a fully parametric, cloud-based CAD system that integrates modeling, simulation, and collaboration into a single environment. Still, when you introduce a motion study and apply gravity, Onshape calculates the effect of gravitational acceleration on every part that has at least one degree of freedom, allowing you to watch the simulation unfold in real time Not complicated — just consistent..
The underlying principle is straightforward: Newton's second law of motion, where force equals mass times acceleration (F = ma). Onshape uses the mass properties of each part — derived from their geometry and assigned material — to compute gravitational forces. When gravity is activated in a motion study, each unconstrained or partially constrained part experiences a downward force proportional to its mass. The software then solves the resulting equations of motion over discrete time steps, updating the position and orientation of each part accordingly. Contact detection between parts ensures that objects do not pass through one another, adding realism to the simulation That's the part that actually makes a difference..
Counterintuitive, but true Small thing, real impact..
Good to know here that Onshape's gravity simulation is not a full finite element analysis (FEA) tool. Instead, it is a kinematic and dynamic simulation that focuses on motion, collision, and gravitational interaction. It does not account for structural deformation, stress distribution, or material failure. This makes it ideal for verifying assembly logic, testing gravity-driven mechanisms, and performing basic drop tests, but it should not be used as a substitute for structural simulation when stress or deformation analysis is required.
Some disagree here. Fair enough And that's really what it comes down to..
Setting Up Your Assembly for Gravity Testing
Before you can test objects under gravity, you need to confirm that your assembly is properly prepared. But an assembly is a collection of parts that are positioned relative to one another using mates. The first step is to create or open an assembly document in Onshape. Each part in the assembly should have material properties assigned, as these determine the mass and density that gravity will act upon Worth keeping that in mind. Turns out it matters..
To assign material to a part, right-click on the part in the Part Studio or Assembly tree, select Properties, and then choose a material from the library. Which means onshape includes a comprehensive database of materials with predefined densities. The density value directly affects the mass of the part, which in turn affects how gravity influences it. Take this: a part made of steel will behave differently under gravity than an identical part made of aluminum, because steel is denser and therefore heavier.
Once materials are assigned, you need to define the mates between parts carefully. That's why mates restrict degrees of freedom, and the number of remaining degrees of freedom determines how a part will respond to gravity. Still, a part that is fully constrained — with all six degrees of freedom (three translational and three rotational) locked — will not move at all when gravity is applied. A part with one or more degrees of freedom, however, will move, rotate, or fall according to the forces acting on it. Understanding this relationship between mates and degrees of freedom is critical to setting up a meaningful gravity test Small thing, real impact. No workaround needed..
Step-by-Step Guide to Testing Objects with Gravity in Onshape
Step 1: Create a Motion Study
handle to the Assembly tab and locate the Motion Study panel at the bottom of the screen. Onshape supports different types of motion studies, but for gravity testing, you will typically use a Basic Motion study. And click the + button to add a new motion study. This type of study allows you to apply forces, gravity, and contact relationships without requiring a full simulation license.
Step 2: Enable Gravity
With the motion study active, look for the gravity settings. In Onshape's motion study environment, you can access gravity through the Gravity option in the forces or loads menu. Because of that, by default, gravity is set to act in the negative Y direction (downward) with a magnitude of 9. Which means 81 m/s², which corresponds to Earth's standard gravitational acceleration. You can modify the direction and magnitude if your simulation requires a different gravitational environment — for example, testing a mechanism designed for the Moon (1.62 m/s²) or for a rotating platform where centrifugal effects substitute for gravity That alone is useful..
Step 3: Define Contact Relationships
For gravity testing to produce realistic results, you must define how parts interact when they collide. Contact detection prevents parts from passing through each other and allows them to bounce, slide, or rest against one another based on friction and restitution coefficients. In the motion study settings, enable Contact and specify which parts or bodies should interact. Without contact definitions, parts may fall through the ground plane or through each other, producing unrealistic results.
Step 4: Set Initial Conditions
If you want to test a specific scenario — such as a part being dropped from a height or tilted at an angle — you can set initial conditions for the motion study. Think about it: this includes initial positions, velocities, and angular velocities for individual parts. Which means for a simple drop test, you might position a part above the ground plane and set its initial velocity to zero. For a more complex scenario, such as testing a pendulum or a rolling object, you would set appropriate initial angular or linear velocities.
Step 5: Run the Simulation
Once all settings are configured, click Play to run the motion study. Onshape will calculate the motion of each part over the specified time range, displaying the animation in the graphics area. You can scrub through the timeline to examine the state of the assembly at any point during the simulation Simple, but easy to overlook..
Pay close attention to how parts fall, collide, settle, and eventually come to rest. Observe whether components interpenetrate unexpectedly, bounce with unrealistic energy, or slide with incorrect friction behavior—these are immediate indicators that contact parameters or mass properties need adjustment And that's really what it comes down to..
Step 6: Analyze Results with Measurement Tools
Animation alone is often insufficient for engineering validation. make use of the Measure and Plot tools within the motion study environment to quantify the simulation output. You can plot the linear or angular displacement, velocity, and acceleration of specific mates or part centroids over time. Worth adding: for gravity testing, key metrics often include:
- Settling time: How long it takes for a mechanism to reach static equilibrium. * Impact forces: Peak forces generated during collisions (available if contact force output is enabled).
- Final resting position: Verification that a latch engages, a door closes fully, or a part seats correctly in a pocket.
- Energy dissipation: Tracking kinetic energy decay to verify damper or friction effectiveness.
Export this data to CSV for further analysis in external tools or to include in design verification reports.
Step 7: Refine and Iterate
Rarely does the first simulation run yield perfect correlation with physical expectations. In practice, treat the motion study as an iterative design tool. In real terms, return to the assembly to adjust:
- Mass properties: Verify assigned materials and densities; override mass/inertia for purchased parts lacking metadata. * Contact parameters: Tune static/dynamic friction coefficients and restitution (bounciness) based on manufacturer datasheets or physical testing.
- Mate limits: Ensure mechanical stops and limits are defined correctly to prevent over-travel.
- Mesh density/Time step: If using the advanced solver options, reduce the maximum time step or increase contact stiffness for stiff impacts to improve numerical stability.
Re-run the study after each change to converge on a realistic behavioral model.
Step 8: Document and Communicate Findings
Onshape’s cloud-native architecture simplifies sharing. On top of that, use the Share dialog to grant stakeholders view-only or comment access to the specific motion study tab. For formal documentation, capture key frames using the Snapshot feature or record a short video of the animation playback. Annotate these visuals with the plotted data curves to create a comprehensive Gravity Test Report that lives alongside the CAD data—ensuring traceability from requirement to validation And that's really what it comes down to..
Conclusion
Simulating gravity in Onshape transforms a static assembly into a dynamic proving ground. Consider this: by systematically enabling gravity, defining contact physics, setting precise initial conditions, and rigorously analyzing the resulting motion data, engineers can validate mechanisms—from simple latches to complex robotic deployments—entirely within the browser. This workflow eliminates the friction of file translation and license management associated with traditional FEA or multi-body dynamics packages, democratizing motion verification for the entire design team. Mastering the Basic Motion study ensures that when your design finally meets the real world, gravity behaves exactly as you engineered it to It's one of those things that adds up..
