Simulation

Simulations are located at the top of the Assembly list on the left side of the screen, just above the Instances list. Any simulation loads are automatically added into a Loads dropdown in the Instance tree, between the Assembly instances and the Mate features. Both are outlined in the image below:

• Simulation - Title of the Simulation area in the Assembly list.

• Simulation - The editable label for the current simulation. Enter the name for the current simulation here.

• Show Results - When checked, this begins showing the results of the simulation. As the results are calculated, a blue progress bar indicates the time remaining before the final calculation is displayed. Early iterations are displayed in the stress scale. A green checkmark next to the Status indicates the calculation (convergence) is complete.

• Status - Indicates the current status of the simulation. If the simulation is in an error state, a yellow warning icon is visible to the right of the Status label. Hovering over the icon provides a tooltip to indicate the error and offers a suggestion on how to fix the error. If the simulation is ready to proceed, a gray information icon is visible indicating the show results can be checked to start the simulation:

  

  Status with warning indicator explaining that a fixed instance is required before a simulation takes place.

  

  Status with an information indicator explaining the Show results are required before the simulation is run.

A simulation cannot be performed until at least one Load is assigned to the Assembly. They are assigned on an instance-by-instance basis, with no limit to the number or type that may be assigned to any instance, with the exception of a fixed part/instance, which, in reference to the simulation, is immutable. A Load cannot affect any change to or through a fixed part/instance.

Simulation load tools are found at the top of the screen at the far right in the Assembly toolbar. There are five Load tools: Force, Moment, Bearing, Pressure, and Acceleration. Each are outlined below:

Force

Simulates the effect of an evenly distributed, linear force on an instance. By default, the force is distributed throughout the instance’s volume. If instead a Load region is enabled, and specific faces of the instance selected, the force is distributed evenly over the faces.

Moment

Simulates the effect of an evenly distributed Moment (torque) on an instance. By default, this moment is distributed throughout the instance’s volume. If instead a Load region is enabled, and specific faces of the instance selected, the moment is evenly distributed over the faces.

Bearing

Simulates the effect of a bearing load on the faces of an instance. This Bearing load produces the total equivalent load as a force, but it is only applied to an instance's faces, and its distribution is not uniform (unless all faces are coplanar). Instead of an even distribution, Bearing distributes its load proportionate to the degree to which it is aligned normal to, and facing toward, the loaded faces. If the bearing load has no positive projection onto the loaded faces, it contributes no load to the simulation.

Pressure

Simulates the effect of an evenly distributed Pressure on the faces of an instance. Similar to a Bearing load, Pressure is only applied to an instance's faces. Pressure magnitude is only positive, but by toggling the Load direction arrow, it either pushes into a face (default) or pulls away from a face (suction).

There are two ways to apply a pressure load:

1. In the Instance field, select an Assembly instance. Pressure is loaded on any instance the selected instance touches, not the selected instance.

2. In the Load region field, select all faces to which you want the pressure applied. This, instead, applies the pressure to the selected faces of the instance.

Acceleration

Simulates the effect of a static, uniform, linear acceleration acting on an instance. Acceleration is only applied to the whole instance; individual faces cannot be loaded. The result of an Acceleration load on an instance is precisely equivalent to that of a proportional force applied to the same instance, in the same direction, if the mass of the instance is taken into account.

Load tools dialog

Once a simulation load tool is selected, the load dialog opens. This is a common dialog shared by all 5 tools:

• Force (load) 1 - Editable load label, which is used under the Loads dropdown in the Assembly Instance list and the Simulation panel.

• Force (load) dropdown - The load dialog is common to all 5 tools. Use this dropdown to switch between tools for the current load; for example, from Force to Moment:

  

• Instance - The instance in the assembly on which the load is applied. Only one instance can be selected in this field. Multiple instances are not supported.

• Faces to load (Bearing and Pressure loads only) - With Bearing and Pressure loads, the loads must be applied directly to the instance faces. Any number of faces may be selected, but they must all belong to the Instance identified in the Instance field.

• Load direction - With this field highlighted, select an existing mate connector or click on a location to create a new Mate connector that drives the load direction. By default, Loads are aligned with the Mate connector’s Z-Axis; click the reverse arrow icon to reverse the direction of the load. A Pressure load does not require a Mate connector to define its direction. Use the reverse arrow icon to switch between pushing into and pulling away from the face. Load direction is configurable on a simulation-by-simulation basis.

A Mate connector created here cannot be referenced anywhere else in the Assembly.

• X/Y/Z Magnitude - The numerical value of the load being borne by the instance. Pressure requires only a single Magnitude entry. When applying a Load direction in the Load dialog, the mate connector's X, Y, and Z axes are used as the coordinate system to drive the corresponding load direction's X, Y, and Z axes and related directional loads. Magnitudes are configurable on a simulation-by-simulation basis.

• Load region - By default, the load is borne by the instance. If instead you wish to localize the applied load to only part of the instance, check the Load region box. Then, select one or more faces of the instance bearing the load. Bearing and Pressure do not have a Load region checkbox, but do require you to select Faces to load. Acceleration does not have a Load region option.

Onshape automatically suppresses new loads for non-active simulations.

Load direction arrow and context menu

When an Instance and Load direction is selected in the Load dialog, a solid line extends from the instance outward, and a load direction arrow is placed at the end of the line. Each load has a unique icon associated with it. If selected from the graphics area, the icon and loaded instance region turns orange.

Right-clicking on the load direction arrow opens the following context menu:

The commands related to simulation are:

• Edit Force (load) 1 - Opens the selected load dialog.

• Delete Force (load) 1 - Deletes the selected load from the Assembly.

For information on the other items found in this context menu, see Context menus.

When results are loaded for a simulation, the color scale and results appears at the top of the graphics area. These are outlined below:

• Results - Select from 5 different result options:

  

  • von Mises Stress - A scalar value of stress that can be computed and is used to predict yielding of materials under complex loading from the results of uniaxial tensile tests. The von Mises stress satisfies the property where two stress states with equal distortion energy have an equal von Mises stress. It is a measure of energy density, as opposed to principal stress, which is a real stress. It is often used to determine whether a material yields when subjected to a complex load condition. By default, the stress is set in MPa, unless your units are set to a different measurement (under Account > Preferences > Units).

    See https://en.wikipedia.org/wiki/Von_Mises_yield_criterion for more information.

  • Signed von Mises Stress - Similar to the von Mises Stress, the sign result calculates the sum of the X, Y, and Z axes normal stresses, and if the quantity is negative, a minus sign is added before the von Mises stress. By default, the stress is set in MPa, unless your units are set to a different measurement (under Account > Preferences > Units).

  • Safety factor - Yield strength divided by the von Mises stress. This allows you to view how the stress levels for any given instance relate to the Tensile yield strength of that instance. It helps to point out problematic and non-problematic areas of stress in the simulation.

  • Max principal stress - Maximum principal stress is the maximum normal stress acting inside the component and its position. By default, the stress is set in MPa, unless your units are set to a different measurement (under Account > Preferences > Units).

  • Displacement - Displays the simulation result as a measurement of displacement. By default, the displacement is set in mm, unless your units are set to a different measurement (under Account > Preferences > Units).

• Show deformation - Enable this checkbox to display the deformation (stress/displacement) of the load. Setting the scale lower shows less deformation. Setting the scale higher shows more deformation.

It is not required to check the Show deformation checkbox to view deformations during simulation animation.

• Animation icon - This icon animates the simulation. The animation loops as long as the animation is enabled. As it is animated, the starting position is shown as an outline of the instances, and the Assembly moves based on the applied load. The icon is a toggle. Once run, the icon switches to a stop button icon . Pressing this icon stops the animation.

• Deformation scale - Allows you to set the scale lower or higher, to visually display more or less deformation. When Show deformation is enabled (checked) and/or the simulation is animated, this scale is applied to the deformation. The Deformation scale is based on the maximum deformation across the assembly; the default is set such that the deformation is 15% of the overall bounding box diagonal — the overall movement is always the same across any scale assembly.

• Color Scale - The color scale represents the load measurements from lowest to highest. Click the lower or upper scale link to numerically enter the minimum and maximum scale values, respectively. Click the Reset icon to the right of these numbers to reset the minimum or maximum to the default value. Drag the minimum and maximum vertical scale lines to increase the minimum or decrease the maximum scale limits.

The Simulation fly-out panel is accessed at the far right of the window, and contains the Simulation table at the top, and Simulation settings at the bottom.

The Mate Simulation connection option is found at the bottom of all Mate dialogs (except Pin slot, Parallel, and Tangent Mate dialogs). This option provides a greater degree of precision and control over which regions are bonded by limiting the Mate’s effect over a specific region, as opposed to having the Mate bond anywhere two mated instances touch.

Check the Simulation connection and enter the Faces to connect:

Simulation connection has no impact on your typical Assembly modeling behavior; it is only seen during simulation. Use the Connectivity method and visualization to inspect the difference.

Using Bond all touching faces as the connectivity method overrides all Mate Simulation connections.

This workflow assumes you have a completed CAD model in an Assembly tab, with the instances mated and the assembly sufficiently constrained. For this simulation example, a Caliper bracket assembly is used. It contains 4 instances and 4 bolts. The 4 instances are: the Caliper bracket, Axle, Connection bracket, and Forcing bracket:

Before creating a simulation, the following two criteria must be met:

1. At least one instance must be fixed in the Assembly. This is the instance on which the load is bearing. Right-click on the Axle and select Fix from the context menu. Do the same for the Connection bracket instance:

   

2. All instances (including standard content) used in the simulation (except those that are fixed) require a material assigned to them. Do one of the following:

   1. Open the Assembly BOM table. Double-click on the Material field for the associated instance row (in this example, the Caliper bracket), and select a material from the Material dialog.

      

   2. From the Part Studio, right-click on the part in the Parts list, and select Assign material. Then select a material from the Material dialog:

      

To create and load a simulation:

1. Visualize the Mate connections in the assembly. Open the Simulation panel and check Show connections. If desired, select an alternate Method to determine how instances in the assembly are connected (bonded together). Connectivity method and visualization can be changed at any point before, during, and after the simulation:

   

2. From the Assembly tab, select one of the five load tools along the top right of the Assembly toolbar. In this example, Force is selected:

   

   The Force dialog opens:

   

3. Select an Instance. Only one instance can be selected. Multiple instances are not currently supported; however, you can create multiple simulation loads in the same assembly. For this example, the Forcing bracket instance is selected.

4. Select a Load direction. Select an existing Mate connector, or click on a location to create a new Mate connector that drives the load direction. Click the reverse arrow icon to reverse the direction of the load. For this example, a Mate connector is added to the edge of the Forcing bracket, with the Z axis pointing left:

   

5. Enter the Magnitude for the load along any of the three axes (X, Y, and/or Z). These are the forces borne on the fixed instance. For this example, a load of 50 N is applied to the Z axis:

   

6. By default, the load is applied to the whole body. If desired, optionally check Load region, and select Faces to load, to narrow the load to a specific region.

7. Click the green checkmark to close the Force dialog.

8. Check Show results. As the results are calculated, a blue progress bar indicates the time remaining before the final calculation displays. Early iterations are displayed in the stress scale. When finished, the blue progress bar reads complete, and then a green checkmark is placed next to the Status signifying the calculation (convergence) is complete:

   

   Alternatively, you can select Simulation results from the View tools menu:
   

   The simulation converts the Assembly geometry to a simulation mesh, and begins the first of five adaptive passes before converging on a result. Intermediate updates are streamed through the browser, giving you early simulation results. When results are finalized, the Scale is visible at the top of the screen, and the stress plot overlays the loaded instances with a color gradient in the graphics area. Simulation results are also temporarily cached.

   

9. Select from 5 different result metrics. For more information, see Simulation results and color scale:

   

10. Hover your mouse over the instance in the graphics area to display a numerical readout next to the cursor. This indicates the stress or displacement amount of the instance located under the cursor:

    

11. To animate the simulation, click the Play toggle above the color scale. To stop playback press the Stop toggle:

    

    

    The simulation load result with Show deformation checked. The deformation is overlaid with a view of the original assembly as a wire frame.

The following displays an alternate simulation load using Moment, selecting the Caliper bracket as the instance, and then selecting the Mate connector at the center point of the axle as the Load direction. A load of 1.000e+6 N*mm is applied along the Z axis, and Show deformation is checked in the legend. This allows you to view both the original assembly and resultant deformation at the same time:

Once a simulation is created, create alternate simulations and compare between them.

1. Open the Simulation panel at the far right of the screen (outlined in red below):

   

2. Click the Add simulation button at the bottom of the screen. You can add multiple simulations, which show up in columns in the Simulation table along the top, and edit their loads directly in the table:

   

3. To set a Simulation to active and show its results on-screen, right-click on the Simulation title in the table and select Set active from the context menu (shown in the first image below). Alternatively, select the simulation from the Simulation dropdown in the Assembly list at the left side of the window to make it active (shown in the second image below):

   

The following example outlines how to use an alternate method to solve a simulation. For this example, the same Caliper bracket assembly is used from the Basic workflow section above.

1. In the example below, Mates are used as the connectivity Method, and the Fastened Mates that connect the bolts between the Caliper bracket and Forcing bracket are suppressed in the Mate Features list. There is nothing to constrain the Forcing bracket to the Caliper bracket:

   

2. Check Show results in the Simulation area at the top left. The Simulation solver does not converge due to unconstrained rigid body motions:

   

3. Change the connectivity Method to Mates and touching faces. The touching faces between the bolts and Caliper bracket are treated as Simulation bonded; the simulation is now fully constrained and can be solved:

   

4. Check Show results in the Simulation area at the top left to view the simulation results:

   

• Onshape Simulation is based on a proprietary, adaptive-refinement implementation of the well understood finite element method. This approach is optimized to produce highly performant and accurate results for Parts of arbitrary shape and detail. However, the drawback to this universally robust solution is that special geometry classes, which are better served by special element types, may presently require too many computing resources to reasonably solve. This is most commonly observed among thin-walled structures. Onshape Simulation aims to enhance this performance in the future. At this time we do not recommend using simulation for geometries with aspect ratios above 50:1.

• When assigning materials to any non-fixed instances/parts used in simulations, their Density, Poisson's ratio, Young's modulus, and Tensile yield strength properties must be provided. Create your own library with your own data defined or use standard materials (for example, 6061, Steel, ABS, etc.). Some materials in the standard library (for example, Balsa wood, Graphite, Silicone rubber) do not work with simulations.

• The following Mates are not translated, and therefore, not supported by simulation: Tangent Mate, Pin Slot Mate, and Parallel Mate. Relations (Gear relation, Rack and pinion relation, Screw relation, and Linear relation) are also not supported.

• Interactions between instances are inferred wherever two instances touch. If two instances are not touching (or within very close proximity), they do not interact with one another when simulated.

• By default, any two instances which touch and are not otherwise Mated, are assumed to be bonded together. If Mated, then the bonded behavior is overruled by the kinematic relation prescribed by the Mate.

• Any range prescribed in a Mate has no effect on the simulation. Each simulation cares only about the initial starting conditions prescribed by the Mates; translations and rotations which occur thereafter follow as a direct consequence of the linear relationships assumed in the linear static analysis.

• By default, interactions between instances extend between small gaps designed into the model. Onshape simulation performs well whether instance interfaces are designed flush, or whether machining gaps/tolerances are included.

• If there are regions between parts that are near enough one another to infer a connection and you do not want them connected, use the Mate Simulation connection to explicitly define the interaction. This is particularly useful if your design has any wedge-shaped gaps, where, at the widest area of the wedge, interaction is unlikely to occur. The nearer the wedge’s point, the more likely an interaction will bridge the gap.

• Since at least one instance must be fixed in a simulation, it is a common practice to create a "base" assembly instance which acts as the ground. Fix the ground instance, and then mate any neighboring instances to this ground.

• Always check to see if your assembly is fully constrained kinematically. Though tolerant of under-constrained assemblies, particularly if the loaded system is in static equilibrium, the simulation may not converge unless the model is fully constrained. Check to see if any individual instance can translate or rotate. If not, then your Assembly is truly “static” and ready for simulation.