A position
constraint defines a relationship between two part or model instances—one that
will move (the movable instance) and one that will remain stationary (the fixed
instance). When you apply a position constraint,
Abaqus/CAE
computes a position for the movable instance that satisfies this relationship;
you do not specify the position directly. You can apply the following position
constraints to instances in the
Assembly module:
Parallel face (three-dimensional instances only)
Face to face (three-dimensional instances only)
Parallel edge
Edge to edge
Coaxial (three-dimensional instances only)
Coincident point
Parallel coordinate systems
In general, applying a single position constraint is not sufficient to
define the precise location of a movable instance. You must apply several
position constraints—usually three for a three-dimensional assembly and two for
a two-dimensional assembly—to position an instance in the desired location.
Part and model instances can overlap as a result of applying position
constraints;
Abaqus/CAE
does not prevent overclosure between edges, faces, or cells. Similarly,
Abaqus/CAE
does not prevent you from overconstraining instances or duplicating a
constraint.
The definition of a constraint feature includes all the faces and edges that you originally
selected. If you subsequently modify a part or move a part or model instance, Abaqus/CAE automatically recalculates the constraint based on your original selection of faces and
edges. As a result, one or more instances might move after the assembly is regenerated. For
example, different edges might become parallel. For more information on features, see Manipulating features in the Assembly module and The Feature Manipulation toolset.
The following position constraints are provided by the
Assembly module:
Parallel Face
A parallel face position constraint causes a selected face of the movable
instance to become parallel with a selected face of the fixed instance.
However, the position constraint does not specify the precise location of the
movable instance, and the distance between the parallel faces is arbitrary. To
apply a parallel face position constraint between two instances, you do the
following:
Select the faces to be constrained to be parallel from the movable
instance and the fixed instance, as shown in
Figure 1.
Figure 1. Select the faces to become parallel.
Abaqus/CAE
displays arrows normal to the selected faces. You prescribe the orientation of
the movable instance by selecting the direction of the arrow normal to its
selected face.
Figure 2
illustrates the result of applying the position constraint and the effect on
the movable instance of reversing the direction of the arrow.
Figure 2. The result of applying a parallel face position constraint and the
effect of changing the direction of the arrow normal to the selected face of
the movable instance.
Abaqus/CAE
rotates the movable instance until the two selected faces are parallel and the
arrows are pointing in the same direction.
The faces you select from the movable and fixed instances must be planar.
The parallel face position constraint can be applied only to three-dimensional
instances.
Face to
Face
A face-to-face position constraint is similar to a parallel face position
constraint except that you define the clearance between the parallel faces. The
clearance is measured between the two selected faces, positive along the normal
to the fixed instance. Other than this clearance, the precise location of the
movable instance is not constrained. Assuming that you selected the same two
faces shown in
Figure 1,
the effect of applying a face-to-face constraint is shown in
Figure 3.
Figure 3
also illustrates the effect on the movable instance of reversing the direction
of the arrow normal to its selected face.
Figure 3. The result of applying a face-to-face constraint and the effect of
changing the direction of the arrow normal to the selected face of the movable
instance.
Abaqus/CAE
rotates the movable instance until the two selected faces are parallel and the
arrows point in the same direction. In addition, the movable instance is
translated to satisfy the clearance specified. The faces you select from the
movable and fixed instances must be planar. The face-to-face position
constraint can be applied only to three-dimensional instances.
Parallel
Edge
A parallel edge position constraint causes a selected edge of the movable
instance to become parallel with a selected edge of the fixed instance.
However, the position constraint does not specify the precise location of the
movable instance, and the distance between the parallel edges is arbitrary. To
apply a parallel edge position constraint between two instances, you do the
following:
Select the edges to be constrained to be parallel from the movable and
fixed instance, as shown in
Figure 4.
Figure 4. Select the edges to become parallel.
Abaqus/CAE
displays arrows along the selected edges. You prescribe the orientation of the
movable instance by selecting the direction of the arrow along its selected
edge.
Figure 5
illustrates the result of applying the position constraint and the effect on
the movable instance of reversing the direction of the arrow.
Figure 5. The result of applying a parallel edge constraint and the effect
of changing the direction of the arrow along the selected edge of the movable
instance.
Abaqus/CAE
rotates the movable instance until the two selected edges are parallel and the
arrows point in the same direction.
The edges you select from the movable and fixed instances must be straight.
You can select an edge from an instance, or you can select a datum axis or one
of the axes of a datum coordinate system. The parallel edge position constraint
can be applied only to two-dimensional and three-dimensional instances. It has
no effect on axisymmetric instances.
Edge to
Edge
An edge-to-edge position constraint is similar to a parallel edge position
constraint except that the clearance between the parallel edges is defined by
the constraint. Assuming that you selected the same two edges shown in
Figure 4,
the effect of applying an edge-to edge position constraint to a two-dimensional
assembly is shown in
Figure 6.
Figure 6
also illustrates the effect on the movable instance of reversing the direction
of the arrow along its selected edge.
Figure 6. The result of applying an edge-to-edge constraint and the effect of
changing the direction of the arrow along the selected edge of the movable
instance.
The modeling space of the assembly determines the behavior of
Abaqus/CAE
after you apply an edge-to-edge position constraint.
If the assembly is three-dimensional,
Abaqus/CAE
positions the movable instance so that the edges are coincident.
If the assembly is two-dimensional, you can specify the clearance
between the selected edges. The clearance is measured between the two selected
edges, positive along the normal to the fixed instance.
Other than this behavior, the precise location of the movable instance is not constrained. The
edge-to-edge position constraint can be applied to two-dimensional, three-dimensional,
and axisymmetric instances.
Coaxial
A coaxial position constraint causes a selected cylindrical or conical face
of the movable instance to become coaxial with a selected cylindrical or
conical face of the fixed instance. However, the coaxial position constraint
does not constrain the precise location of the movable instance. To apply a
coaxial position constraint between two instances, you do the following:
Select the cylindrical or conical faces to be constrained to be coaxial
from the movable and fixed instance, as shown in
Figure 7.
Figure 7. Select the faces to become coaxial.
Abaqus/CAE
displays arrows along the axis of revolution of the selected instances. You
prescribe the orientation of the movable instance by selecting the direction of
the arrow along its axis of revolution.
Figure 8
illustrates the result of applying the coaxial position constraint.
Figure 8. The effect of applying a coaxial constraint.
Abaqus/CAE
rotates and translates the movable instance until the two selected faces are
coaxial and the arrows are pointing in the same direction. The coaxial position
constraint can be applied only to three-dimensional instances.
Coincident
Point
A coincident point constraint causes a selected point on the movable
instance to coincide with a selected point on the fixed instance. However, the
coincident point constraint does not constrain the orientation of the movable
instance. The orientation of the movable instance does not change after the
constraint is applied, as shown in
Figure 9.
For detailed instructions, see
Constraining two instances with coincident points.
Figure 9. The effect of applying a coincident point constraint.
Parallel
CSYS
A parallel coordinate systems constraint causes the axes of a datum coordinate system on the
movable instance to become parallel with the axes of a datum coordinate system on the
fixed instance. However, the parallel coordinate systems constraint does not specify the
precise location of the movable instance. Figure 10 illustrates the effect of applying a parallel coordinate systems constraint and a
coincident point constraint to two instances.
Figure 10. The effect of applying parallel coordinate systems and coincident
point constraints.
You can use datums to position part and model instances. When you are
prompted to select a face, you can also select a datum plane. When you are
prompted to select an edge, you can also select a datum axis or one of the axes
of a datum coordinate system. You can select a datum that you created in the
Part module
because the datum is associated with an instance of the part and moves with the
part instance. However, if the position constraint uses a datum that you
created in the
Assembly module
by selecting from a part instance (such as a face of a part instance),
Abaqus/CAE
changes its regeneration behavior and regenerates features in the order that
you created them. For more information, see
How are position constraints regenerated?.
You cannot select a datum as the movable part instance if you created the datum
in the
Assembly module
and it depends on more than one part instance; for example, a datum axis that
runs through vertices of two part instances.
