Linking

Usually, a technical object is made up of several parts. These parts need to be held together to perform their function. They can be connected through simple contact, but generally an intermediate part, called a linking component, is required to do so. These components (screws, nails, rivets, glue, etc.) ensure that the two parts are connected to each other.

There are many ways to link parts together in a technical object. However, despite the variety of possible links, a link is still described using four characteristics. These characteristics are based on the four pairs of characteristics existing. Thus, a link can be direct or indirect, removable or permanent, rigid or flexible, complete or partial.

• Direct link: There is no linking component between the parts to be assembled. These parts have complementary shapes that provide a direct connection. The parts hold together on their own.

• Indirect link: The parts need a linking component to hold together. One or more linking components (screw, nail, glue, pin, etc.) are required in an indirect link.

• Rigid link: This type of link does not allow any deformation of assembled parts. The linking component is rigid and it does not allow any change in the position of the parts that it connects.

• Flexible link: A link is flexible when there is a flexible linking component or an elastic material which ensures a return motion (return to the initial position) of the components in the operation of the object. Springs are often present in this type of link.

• Removable link: This type of link allows the parts to be separated several times without damaging the surfaces of the parts or the linking component. This type of link is mainly used for maintenance or replacement of parts. Screws and nuts are often found in this type of link.

• Permanent link: The parts cannot be separated without damaging the linking component or the surfaces of the parts. This type of link is irreversible.

• Complete (or total) link: When no motion is possible between connected parts, the link is complete. In this connection, there is no degree of freedom since no motion is possible.

• Partial link: A link is partial when motion between the parts is possible. These links are classified according to the number and nature of the relative motions. The parts of a partial link can move relative to each other.

There are several types of links, depending on the motions that the linked parts might undergo. In all these types of links, excluding the rigid link, one of the parts is driven by the other. The type of link is thus determined by the possible motions of the driven part.

Link type	​Example	​Advantage	Disadvantage
Embedded link	​ Source	No degree of freedom between the parts (complete link). Can withstand heavy loads.	May cause deformation of parts.
Pivot link	Source	​Simple to design. Provides rotational guidance (only one degree of freedom).	High friction (may require lubrication). Precise guidance to achieve. Limited rotation speed.
​Sliding link	​ Source	​Provides guidance in rectilinear translation (only one degree of freedom).	High friction (may require lubrication). May limit range of motion.
Sliding pivot link	Source	Provides guidance in rectilinear translation and rotation on the same axis (two degrees of freedom).	​May limit range of motion.
Helical link	Source	​Ensures a horizontal displacement of a part while turning another part (two degrees of freedom on the same axis).	​Limits motion on a single axis.
Ball-and-socket link	Source	Provides rotational guidance in the three axes (three degrees of freedom).	Causes high friction (may require lubrication).
Planar link	​source	Ensure motions parallel to the plane (three degrees of freedom).	Limit motions to one dimension.

In a technical object, the link function can allow or prevent certain motions between the parts. If an object is completely free, in other words, it is not linked to any other component, it is considered as being able to move in space in translation and in rotation along the three axes of the Cartesian plane ($x$, $y$, and $z$). This freedom of motion corresponds to 6 degrees of freedom that enable 12 possible motions.