Today, wood cork in concrete or brick wall for screwing in a screw looks more than archaic. Dowels – special factory products for making so-called blind fasteners (as opposed to sloppy ones, where it is possible to screw a nut onto the end of a connecting rod, weld an anchor washer or a transverse rod, rivet it or cotter it, etc.) have entered into construction use. Moreover, there are a great many options for dowels. Let’s get to know some of them.
Dowels are produced both for solid (massive) and for layered, hollow and hollow walls and are used when the free end of the rod (screw, anchor) is not available for the installation tool.
Dowels for massive (that is, solid, without voids) walls have an expanding principle of operation, for hollow ones – an anchoring.
Expanding dowels consist of either a nylon deformable sleeve (cartridge), or metal, with parts of the cartridge that open when the rod passes through. Anchor dowels are, as a rule, a rather tricky device from a technical point of view. For example, in some cases, when the fastening rod is screwed in, the end of the nylon cartridge of such a dowel can expand, turning into a “bulb”, in others, thanks to the transverse notches, the end of the cartridge turns into a kind of locking rhombic figure, in the third, it is screwed into a wide knot on the thread of the rod, turning into a thrust nut on the hollow side of the wall….
There are anchor dowels in the form of true little mechanisms, in which the tail section is opened by a spring or pivots around an eccentric hinge. There is also a model where the shank is installed in the anchoring (locking) position with the help of a “stem”, the end of which, after installing the dowel, remains outside.
The latest hit is the universal nylon dowel, which is used to attach all kinds of wooden bars and metal rails to walls and ceilings – partition frames, battens for facing slabs, suspended ceilings, door and window frames, etc. This dowel for hollow or laminated walls works as a spacer in areas of contact with a dense part of the walls, and as an anchor at the boundaries with voids due to the open annular parts in the coupling body. The ends of these open rings, resting on the side walls of the socket, provide additional friction when the screw rod is turned.
The outer end of the fastening rod is made in the form of a head of a nail or a screw, a head of a bolt, a threaded cylinder, a hook, a ring, etc..
The dowels differ not only in the material from which they are made, in the method of fastening in the thickness of the structure, but also in the calculated tensile and shear loads on the fastening rod. In other words, some of them are designed for fastening light household items, others are able to withstand the structures of large buildings and structures..
There are steel dowels that connect massive concrete foundation shoes to the supporting parts of steel columns for heavy bridge cranes with a lifting capacity of tens of tons. It is curious that if usually (without the use of dowels) akers with mounting plates are placed in the concreted shoe, to which the supporting parts of the columns are welded or attached, then the metal consumption for such embedded parts in concrete is comparable to the metal consumption of its main reinforcement. When using dowels in the upper part of the concrete shoe, according to the exact marking with the help of a drill – perforator, holes are made where all-metal bolt dowels are installed. With this method, the total weight of the dowels is literally a fraction of a percent of the metal consumption of the traditional version..
As for the repair work in the apartment, especially in modern houses, the floors and even the walls of which are made of reinforced concrete, here the dowel is simply irreplaceable, since all fastening operations to the walls, floors and ceilings have to be performed using dowels.
What type of dowel to choose in each specific case, it will become clear to you if you get acquainted with the figures on this page. The most important thing is to choose the right type and thickness of the dowel and the fixing rod (screw) for the load that it will carry..
With a horizontal hole, i.e. when the screw works “for a cut”, most often to bring bookshelves, kitchen cabinets, etc., use dowels with an outer diameter of 8 to 10 mm, which are buried in a solid concrete or brick wall by 30-50 mm. For more “critical” fastenings (for example, “wall bars” and other similar “wall” simulators), you can use dowels, buried in the wall thickness up to 80 – 100 mm.
In the case of a vertical arrangement, when the load acts downward (for example, fastening to ceilings when installing suspended ceilings), it is recommended to choose dowels with transverse notches and spacer “wings”. In this case, it is especially important that the hole corresponds to the diameter of the dowel (it must be hammered with some effort with a hammer, and not freely enter), and the drilling depth (dowel length) is at least 40-50 mm. Often, for greater reliability, the dowel installed in the ceiling and the hole for it are lubricated with PVA glue, and after hammering the dowel, before screwing in the screw, they give it a day to set. However, with the correct selection of the dowel and the correct calculation of the load, such “precautions” are seen as unnecessary.
Text: Alexander Naumov (remont2000.ru)