About Engineering Connections

In a mechanical design, the number of degrees of freedom (DOF) is the number of independent parameters required to determine the position of every component constituting an assembly.

The relative motion between two components is driven by six degrees of freedom:

• three translational directions
• three rotational directions

As you assign constraints to components, you restrict the number of degrees of freedom available. For example, if you assign a revolute-type engineering connection to a body, you restrict its movement to a rotation about a single axis. The degrees of freedom are reduced from six to one.

Before you create engineering connections for your mechanism, you must know what movement you want to restrict or allow.

For more information, see Engineering Connections and Degrees of Freedom.

This example shows how to create a simple mechanism by using constraints and engineering connections between two parts.

The following figure shows the creation of the mechanism. The block is positioned in the slot on the bottom plate and is free to slide back and forth:

To define this mechanism, a single prismatic engineering connection consisting of three constraints is used:

• A driving coincidence constraint between an edge on the bottom of the block and an edge on the bottom of the slot (marked "A" in the figure). This constraint moves the block into contact with the plate and defines the sliding direction, reducing the degrees of freedom from six to two.
• A driving contact constraint between the bottom face of the block and the face at the bottom of the slot (marked "B" in the figure). This constraint aligns the block with the plate and prevents rotation about the previous coincidence edge; the degrees of freedom are reduced from two to one.
• A controlled offset constraint between a face on the end of the plate and a parallel face on the side of the block (marked "C" in the figure). The controlled constraint accounts for the final degree of freedom. In the mechanism representation, the offset distance between the faces can be adjusted to determine the position of the block in the slot.