Frames and machines are built-up structural and mechanical systems consisting of multiple parts assembled together in a number of possible ways. For example, the parts could be pin-connected or welded together. The primary difference between frames and machines is that there are moving parts in machines and not in frames. The method of analysis is the same. Although machines involve moving parts, we analyze the system at one instant in time when the positions of individual parts and the applied loads are clearly defined. The solution to these systems usually requires the drawing of the free-body diagrams of individual parts, and the application of the equilibrium equations. Therefore, one must be careful in recognizing the existance of two-force members that would identify the direction of the unknown force.
When a frame or machine is broken up into multiple parts, the forces and/or moments present at the points of separation must be shown on the free-body diagrams of the separated parts in a manner consistent with the third law of Newton. That is the forces and moments are shown as equal and opposite on the two parts that have been separated.