ANSYS Discovery SpaceClaim

Assigning assembly conditions

Assembly Closed tools allow you to create proximity/arrangement conditions between components. These conditions constrain the components so they can't be moved in ways that violate the condition. In the animated example below, two cylindrical components (one large and one small) are constrained with a tangent condition. The large cylinder can be moved vertically without moving the small cylinder, but both cylinders move if either is angled because tangency between them must be maintained.

Click to replay

See the printable Assembly constraints reference chart for descriptions of the constraints created between different combinations of geometry.

It is very important to note that, because conditions constrain movement, it is possible to create conditions where a desired movement is not possible. A constraint solver is used to calculate the degrees of freedom available for movement, and then minimize the (small) errors resulting from conflicts, if present.

Assembly conditions are displayed in the Structure tree Closed with the same icon as the condition tool chosen from the ribbon - one for each component in a matched pair of conditions.

An assembly condition that cannot be satisfied to a certain tolerance is shown with a small yellow triangle in the Structure Tree.
You can click on the assembly condition to highlight the faces that share the relationship.
You can toggle conditions on and off, or delete them, in the Structure tree.
A condition that is toggled Off allows that component to move unencumbered by that constraint.
Turning the condition back On restricts that degree of freedom accordingly.
Deletion of a pair of conditions deletes the matched condition, if present in another component.

Objects must belong to different components for assembly conditions to be assigned.

You can create multiple assembly conditions for your components.

When you use the Move Closed tool on a component that is constrained to have only one degree of freedom, the move handle adapts to this and only activates the available movement/rotation axes.

The assembly constraint solver behaves differently than those in a history-based modeler. Read the important notes about assembly conditions in the next section.

Important notes about assembly conditions

Once an assembly condition is applied to a component, the component is constrained as to translation or rotation, or both. For example, a block with one face aligned to another planar face, can move within that plane, and rotate on that plane, but cannot rotate out of the plane (so that the aligned faces are no longer parallel).

Since there is no notion of History, the order of adding assembly conditions is unimportant; all active constraints are evaluated at the same time. The constraint solver calculates the available degrees of freedom for components, and then solves for the least possible error. The manner in which it does this leads to some important considerations when assembling components.

All assembly conditions are calculated with NO sense of order or priority.
- An exception is the Anchor condition, as all Anchored components are not allowed to be moved by the solver.
All assembly components are positioned (i.e. moved) so as to satisfy all of the non-anchor conditions all at once
Positioning is considered complete when an internal error threshold is reached.
- Angular and positional errors are compared to the same value (1e-4).
- This value was chosen to provide the best results for angles and positions in real systems.
- Angular error is measured in radians.
- Positional error uses the ACIS base unit of meters. Converting to other unit systems will convert the error value accordingly.
Solving speed is proportional to the number of active constraints.

Even though there is no notion of history, there may be slight differences in the resultant error depending on the initial conditions and degrees of freedom. For example, applying constraints to two components that are initially close together will produce an error that may be different than if they are initially farther apart. Both error values, however, will be less than the error threshold of the solver.

If all components are created and assembled in a way that there are no conflicting constraints, there will be zero error and all components appear perfectly aligned (see below for an illustration).

A perfectly square block is to be assembled to a 90° L-shaped block so that it fits in the corner.
The bottom face is aligned to the horizontal face.
The left face is aligned to the vertical face.
Zoom in on the bottom-right corner and see perfect alignment.
Zoom in on the top-left corner and see perfect alignment.
HOWEVER; If the vertical face has significant draft, the bottom can be aligned but the vertical face cannot (or vice versa.) Doing so, would cause the bottom face to rotate out of alignment with the horizontal face. Because it is a large angular error, the internal threshold cannot be reached, and the Align condition is shown in the Structure Tree with a warning triangle.
FURTHER; The square block looks the same in this example but the left vertical face has a small (0.01°) incline. This incline is smaller than the solver tolerance so the condition can be created. What actually happens is that the positions of both blocks are adjusted to minimize the alignment error. This is potentially non-intuitive, especially as many users expect that a previously-assigned Alignment would never be “violated” by a conflicting condition.
Zoom in to the bottom-right corner and you can see that a very small interference has been created, "doing the best it can" given the conflicting instructions.
The same is true at the top-left corner.

EVEN FURTHER, in an unconstrained, unordered, error-minimizing system, the order of selecting components has no bearing on which one will move and/or rotate to satisfy the condition as it is added. The system attempts to calculate these so that the smallest overall change gets made in order to satisfy the newly added condition.

If a notion of order is desired, then it is recommended that you apply an Anchor condition to one of the components. The anchored component will remain stationary and the positional/rotational change will only affect the other component (and of course all components that are assembled to it.)

Best practices

The following are useful techniques when working with assembly conditions.

Uncheck the Create conditions option if you just want to orient/position a component (one time) and do not need to constrain its movement through later move-tool-requested transformations
For larger assemblies, delete conditions on components that you subsume into sub-assemblies, once you get them in their correct positions (because subcomponents are considered rigid within themselves, any conditions within a sub-assembly are redundant). You can use conditions on the sub-assemblies to move all the parts as a unit. This minimizes the solver calculations and improves performance, since the algorithm does not have to go through the categorization of useless conditions within subassemblies, as well as keeping the structure tree understandable.
Use the Anchor condition but do not overuse it. If you use more than one Anchor, you run the risk of locking up the components so they will not move.
Delete assembly constraints after positioning components.
- Many users have found that there is no need for assembly conditions to persist unless they are intended for mechanism studies.
- Performance is improved at later load times, since assembly constraints do not have to be verified.
- Restructuring and removal of unneeded constraints makes an easier-to-understand structure.

To toggle an assembly condition

Uncheck the assembly condition checkbox in the Structure tree to disable the assembly condition. Check the box to enable the assembly condition.

To select all assembly conditions

Click Select all conditions in the Options panel Closed when an assembly tool is enabled.

You can also select a single assembly condition in the Structure tree and press Ctrl+A.

To delete an assembly condition

Select the assembly condition in the Structure tree.

You can click Select all conditions in the Options panel when an assembly tool is enabled if you want to delete all conditions.
Press the Delete key or right-click and select Delete Assembly Condition.

To highlight faces that share an assembly condition

Select the assembly condition in the Structure tree.

You can click Select all conditions in the Assembly Options panel to highlight all conditions.

You can use the following tools from the Assembly group to create assembly conditions:

	Tangent	Aligns two faces so they are tangent or aligns a face tangent with a line, point, or plane. Possible face types include planes, cylinders, spheres, and cones.
	Align	Aligns two points, lines, planes, or combination of these elements. If you select a cylindrical or conical face, then the axis is used. If you select a spherical face, then the center point is used.
	Orient	Rotates components so the selected elements are oriented in the same direction.
	Rigid	Locks the orientation and position of two components to each other.
	Gear	Constrains two objects so one of the objects rotates in response to the rotation of the other object. Gear conditions can be created between two cylinders, two cones, a cylinder and a plane, or a cone and a plane.
	Anchor	Locks the position of a single component in 3D space.