Assembly models Contact relationships

Parts are placed in the assembly in relation to other parts by contact, coincidence, and other relationships. Relationships are associativities between parts and act as constraints. They define the assembly and any change of them produces a changed or erroneous assembly. In Figure 5-13, three pairs of flat surfaces are in contact between part 1 and part 2. The result is complete positioning of part 2 by its placing on part 1. A complete set of relationship definitions carries placement information for a part in the assembly model. A placed part is in connection with one or more referred parts through relationship definitions. Automatic positioning and relationship definition by automatic orientation and translations are offered by advanced procedures. 

The parts in Figure 5-16 are placed by, among others, contact assembly relationships Rj, R2, and R3. Decrease of dimension H2 breaks relationship R2. Fortunately, dimension Hj is not constrained. The solution is to increase dimension Hj to such an extent so as to compensate for the decrease of dimension H2. Analysis and recommended solutions are available in advanced assembly modeling. 

Elements and their relationships in an FE model are shown in Figure 6-1. The part model and the assembly model together with their modifications for FEM and material data constitute the shape and contact information for the FE model. The second structural unit of FEM consists of the mesh and information about finite elements. The environment of the analyzed shape is modeled in the load model where loads and boundary conditions are described. Boundary conditions are restraints and supports at connections between parts as they are described in the assembly model. Less advanced systems cannot get contact information from the assembly model. In this case, single part models should be applied and the boundary conditions are defined by the engineer. Material data, finite element definitions, meshes, and load model element definitions are stored in databases as background information and knowledge for the creation of FE models. Complete FE models can be stored, retrieved, and applied with the same or modified geometry and load model. 

The model of the kinematics is the last subset of information required to complete the model of a mechanism. The first kinematics related decision is place and type of joints. The assembly model includes information for the places where the parts are in contact relationships. Joints are proposed to be placed at these connections automatically. This quick and easy construction of mechanisms has replaced the earlier separate definition of structures and joints. Part geometry and inertia properties are accessed from the assembly model. 

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