Among the countless operations do-it-yourself which often affect domestic environments lived daily, play a fundamental role Expansion dowelsoften simply called dowel and many other times identified with the name of Fishererroneously. Although different types exist, those who interest us in everyday life are substantially cable objects made with plastic materials That help to improve the local conditions of adherence of the metal screw with the basic material on which you want to make fixing. Their operating mechanism is based on birth of permanent deformations that, practically, transform The starting object making it effective to avoid any framing phenomena due to the external loads applied.
What are the pieces
The expansion dowel in plastic, invented in the 1950s/60s by Arthur Fisher (founder of the homonymous company from which the piece inherits the Municipality nickname), mates with a metal screw and is inserted inside a hole, previously to the vine itself. The shape of a piece may vary significantly according to the company that produces them. In essence, however, they are the extent of the loads at stake and the type of support on which we want to install it to identify them form, length And size from the screw coupled to it. However, They all keep everyone a-like form and quarrynecessary to create the encounter space of the screw inside them. The important detail to remember is that, generally, The piece has an incision along its development, which practically divides this cylindrical form idealized into two independent parts for a certain length. This cut is fundamental as directs and facilitates the deformation operations of the plastic material Once the screw has been inserted.
The operating mechanism: adherence
There genesis of the operating mechanism of a piece is as follows: this plastic element is inserted inside a holeperformed ad hoc. Once inside the hole, the piece lies in a rest condition that we could define passiveas it does not join Well to the lateral surface of the hole. However, when the screw is inserted, then The piece begins to expand and, thanks to the presence of the carving that we have first described, It significantly expands the space in the initially occupiedresulting in one strong adhesion with the wall of the hole. In this condition, we must imagine the birth of many small ones contact forces Between the piece and the hole that oppose its possible parade.
These forces, which were in part previously present, are now significantly greater in that the piece is pushed forcefully On the wall of the Forum and its deformation it makes the area of contact with the wall increase very much, therefore resulting in more forces and more entitled than the previous ones.
However, let’s make a clarification: over time, the dowel technology has evolved and has led to various types of shapes and ways to ensure correct fixing of the elements. In some cases, therefore, the mechanism of deformation of the piece is not properly what we have described, but rather something that approaches more to a punctual expansion close to the hole, to increase the contact area right there. This type of system is used when dealing with cable elements with reduced wall thickness. Conversely, classic pieces are used on full or even perforated elements, but in this case with lengths of the piece greater than ordinary ones.
From pieces to anchors
When the fixing system is called into question to bear high loads that go far beyond the furnishing objects of an apartment, then The use of the pieces of materials and shapes that we have described before can no longer be enough. In these cases, real anchoring systems must be used, of which two large families can be distinguished:
- That of mechanical anchors, also with expansion;
- That of chemical anchors.
The mechanical anchors expansion have a way of operating completely similar to those previously seen with simple pieceswith the only difference that the anchoring system is much more performing and usually made in a single block, that is, as if Tassello and Vite were already mounted together in a single element.
The chemical anchorson the other hand, they have no mechanical system that improves lateral adherence with the hole, but uses appropriate resins inserted in advance to create A binding matrix between the anchoring and the perforated surface. These mechanisms represent those currently most performing for the same size of the anchor and depth of inflammation.
In all this speech, however, the solid element where the anchoring goes to connect it seems to have gone into the background. The resistance of a given anchoring is also strongly conditioned by the type of support and its mechanical characteristics. Therefore, when we go to more complex systems, a real structural calculation is necessary to understand clearly What are the forces involved and what the resistances that oppose itby designing the fixing system that makes the specific case.
