Joints and linkage Part 1

Pagina 1

1. JOINTS Two mechanical components (e.g. two rigid bodies) can be connected each other by means of a suitable joint (or link) in order to remove/allow one or more relative displacements (degrees of freedom, dofs) between the couple of mechanical components. If no relative motion is allowed, the joint is a rigid joint. In general, in mechanics, one is interested in designing joint s that allow from one to five free degrees of freedom, in the 3 dimensional space, and from one to two degrees of freedom, in the 2 dimensional space. Obviously, the maximum number of relative dofs between two mechanical components is six, in the 3 dimensional space (3-D), and three in the 2 dimensional space (2-D). Many types of joints can be designed and, more in particular, it is possible to mak e different configurations for joints that are characterized by the same degrees of freedom. The mechanical joints can be classified according to different criteria. A first classification considers the type of dimensional space in which the motion of the couple of mechanical components can occur, in accordance with the free dofs of the joint. 2 -D joints allow only planar relative motions between the two mechanical components mutually interconnected. Conversely, the relative motions allowed by a 3-D joint can occur in a three dimensional space. Each joint is characterised by an index, often called class , ci, that is the number of the free independent dofs of the joint, regardless of the fact that the free dofs are translations or rotations. Besides, a specific name is associated with each type of joint. In technical literature there are many tables that summarize the main joints, their class, name and type of the free degrees of freedom , configuration, etc.. Here below some examples of such t…