A dowel is a component used in connection technology . It is used for materials in which screwing in a screw can not cut a load-bearing screw thread in the counterpart. Originally, a dowel was only understood to mean the wooden dowel still used in the carpentry and joinery trade . Sometimes the term screw anchor is used to distinguish it from the latter .
When driving a screw into sufficiently strong and at the same time flexible materials such as wood, sheet metal and many plastics, no dowels are necessary.
Dowels are inserted into a cylindrical hole in bricked , concreted or paved walls , ceilings or floors as an intermediate part. When screwing in the wood or chipboard screw, the dowel expands. The screw is secured against being pulled out by a force fit between the dowel and the surrounding material and sometimes with a form fit . The dowel hole is set in hard material with carbide-tipped drills ( concrete drills ).
Standard dowels are made of plastic and are used together with commercially available wood or chipboard screws. There are various variants for special applications. In many cases, dowels with matching screws are sold as a set.
Only components made of metal, which are also often referred to as dowels, are used to introduce heavy loads primarily into concrete. The better term is steel anchor . Such an anchor is outside with metal thread for receiving a fastening nut provided. It can be combined with metal parts that are spread in the borehole, or it can be glued into the borehole as a shear connector.
Dowels are available in different diameters and lengths. The selection of the suitable anchor depends on various criteria.
- Condition of the subsoil
- Building material of the subsoil: concrete, masonry and panel building materials
- Homogeneity of the subsurface (an unplastered cellar ceiling) or multi-layered surfaces such as the top coat / base coat on sand-lime brick
- Cavities in the subsurface, as in plasterboard walls with insulating material behind. For the latter, special hinged dowels are available in stores.
- Subsurface depth: a non-load-bearing partition made of sand-lime brick can be 12.5 cm thick and the dowel chosen should be shorter so that it does not penetrate the wall
- Forces that the dowel must absorb and transfer from the screw or hook to the substrate
The term anchor was of wooden dowels (also: Dolle ) taken; however, both functions are different. The screw dowel enables a connection with a screw, while the wooden dowel - similar to a nail - creates the connection itself.
Before using screw dowels, holes were made or chiseled . A piece of wood was plastered or mortared into it . The screw could be screwed into this wood. In another method, a piece of wood was hammered into a drill hole and spread with the screwed-in screw and thus additionally pressed.
The first industrially manufactured expansion dowel was the type invented in 1910 by the British John Joseph Rawlings and registered with the patent office in London in 1911 (patent 22680/11 granted on January 14, 1913). The dowel was made from hemp string and an adhesive made from animal blood. The first expansion dowel industrially manufactured in Germany, a piece of hemp cord in a sheet metal sleeve, was supplied by Upat in Hamburg in 1926.
In 1928 the patent for the "sleeve expansion plug" was granted at the Reich Patent Office under DRP 555384 in the name of engineer Fritz Axthelm - founder and co-owner of the Niedax company (founded in 1920) - and it was officially tested for resilience at the Berlin State Materials Testing Office. The Niedax dowel was initially made of metal and after the Second World War made of plastic. Axthelm is thus the sure inventor of the plastic anchor.
On March 2, 1953, the Bern patent office granted protection of the invention to Richard Heckhausen (Tox dowel technology).
The first plastic expansion anchors from Thorsman (patent 1957) were made from nylon rods. They are put into a borehole. The most successful anchor in the world for decades came onto the market in 1958 by Artur Fischer . Fischer applied for his patent on November 7, 1958, and the patent specification 1097117 was issued on July 13, 1961 by the German Patent Office. Oswald Thorsman's son Mats Thorsman thinks the Fischer plug is a plagiarism of the Thorsman plug. According to a court decision, his father's invention was the original. Fischer's patent specification does not, however, claim the invention of the plastic expansion anchor, but refers to a shape that allows both expansion of the anchor in soft material ( form fit ) and deformation of the anchor without expansion in hard material ( frictional connection by clamping) so that screws with the same dimensions as with soft material can be screwed into hard material without hindrance.
Since shortly after 1945, dowels have been made of plastic, metal or both together, and are available in many designs for different walls, ceilings and floors.
The screw forms a mating thread in the inner part of the dowel, whereby it plastically deforms the dowel material and additionally displaces it radially outwards, it expands the dowel. Free spaces in the case of uneven or porous perforated walls are filled by the dowel material, creating a positive fit against pulling out. Mainly, however, this connection is non-positive or frictional. The radial forces generated in the dowel material mainly lead to its elastic radial deformation. The elastic forces act on the unyielding hole wall as normal force, which results in a proportional static friction force perpendicular to it. The elastic hollow cylinder between the screw and the masonry, for example, is thick enough that the radial forces are evenly distributed over a possibly uneven hole. The radial forces decrease steadily at the outer edge of the hole, so that nothing flakes off in the case of brittle masonry.
For building materials with a porous structure and low compressive strength, such as plasterboard, dowels with a particularly long expansion zone are used. This must be longer than the thickness of the plate to be pierced. These expansion dowels only diverge in the cavity behind the plate and thus provide support.
General purpose dowels
The most commonly used dowel is made of polyamide . It is used together with standardized, mostly wood screws. Its inside diameter is smaller than the diameter of the screw. The result is that the screw screwed into the dowel deforms the dowel radially. In order to facilitate the deformation, the dowel is partially slotted lengthways. The screw diameter and length influence the function.
A more recent development are nail anchors (also knock-in anchors ), in which a specially shaped screw supplied is hammered into the plastic sleeve . The length of the nail is matched to the thickness of the component to be fastened. The dowel is inserted with the nail through the component into the wall hole ( push- through installation ) , and then the nail is hammered in completely. This is why assembly is much faster, but nail anchors are not as stable as normal anchors (screw anchors ) . The thread on the nail is cut in a sawtooth shape. It's easy to turn in, but not easy to pull out. Its head has a connection shape for a screwdriver to facilitate disassembly.
Plasterboard has a low compressive strength and the panels are usually thin, which is why expansion dowels cannot be used. Special plasterboard dowels , also called aerated concrete , lightweight or plasterboard dowels , are screw-in sleeves : With their deep external thread, they cut a form fit into the plasterboard, which can be used to introduce smaller loads. They are screwed directly into commercially available plasterboard with an assembly aid that drills the required hole at the same time. They require a small cavity behind the plate for the commercially available tool and for fastening screws that have been inserted into them for too long. Cavity dowels are used for connections subject to higher loads, whereby sufficient space is required behind the plate in order to radially “extend” the dowel parts and to create an axial form fit with them over a larger contact area.
Insulation materials are even less strong than plaster. Insulation boards are usually thicker than plasterboard, so that the lower strength can be compensated for with a longer screw-in length. Insulation dowels are longer than plasterboard dowels, which they are otherwise very similar. Sometimes they have a plate-shaped outer closure, but should not be confused with the plate anchors with which an insulation board is attached to a solid surface. Such nail-like dowels have a long handle that is driven into a hole in the solid ground and is held there by clamping, thus securing the insulation board.
There are also insulation dowels made of metal: a helical spring, the lower tip of which is grinded to be screwed into Styrofoam, and the dowel head has an eye for screwing in the screw.
Tilting dowels, also known as hinged dowels or spring hinged dowels, are suitable for fastening in thin false ceilings. A spring spreads two stable folding wings when the folded toggle plug has been pushed through a hole through the ceiling from below. The hook can then be screwed up with a thread, and an intermediate washer then covers the relatively large hole.
Cavity dowels are made of metal; like a blind rivet, they deform in the cavity behind the plate and spread apart (see adjacent picture). With many such dowels, a part behind the plate is rotated around the protruding part (anchor, hook) towards the rear.
Heavy duty anchor
Metal expansion anchor
These dowels are made entirely of metal. Because there is no elastic material, their function is primarily based on a form fit. When screwing in, large forces are generated on the rear part of the wall hole via force-amplifying conical surfaces and these are plastically deformed. Because of the higher material strength and the usual use of larger diameters, metal anchors in solid materials (concrete) are more stable and safer than plastic anchors. For fixings on concrete ceilings, only metal expansion anchors are currently permitted in Germany, as there is a risk of plastic “flowing” (slow plastic deformation due to pull-out forces). Metal anchors are also required for certain applications because of their better heat resistance.
The shear connector is inserted into the borehole together with two-component reaction resin, in which it is not expanded, but rather bonds with the wall material . The metal threaded rod ( anchor rod or rod ) used for fastening is usually mounted together with the resins, which are located in a glass ampoule or a coaxial foil bag. The anchor often has a cutting edge on the tip so that the ampoule is safely destroyed and the resins can escape. The anchor is then rotated, thereby mixing and distributing the components of the resin. In addition to the cartridge system, there is an injection system in which the resin mixture is injected into the borehole using an extrusion device. The resin mixture fills the cavity between the drill hole and anchor rod, partially penetrates the pores of the surrounding masonry or concrete and hardens. The main advantage in contrast to expansion anchors is that no radial forces occur in the concrete, thus avoiding high stresses. Chemical anchors can be used close to the end of the wall.
Certain chemical anchors with an expansion reserve can be used in the cracked tension zone of a concrete component. Such anchors consist of many cones placed one behind the other , which have force components in the direction of the wall of the borehole in the event of cracks (e.g. due to earthquakes) and press the resin mixture against the wall. In practice, this dowel slips down a little if there is a crack and then stabilizes again.
The use of compound anchors is regulated by the building authorities.
In addition to the compound anchors, there are various undercut anchors approved by the building authorities for concrete. In these, a form fit is achieved by widening the bottom of the borehole using a special drill. Through a wedge construction or similar. it is achieved that the undercut anchor completely fills this widened borehole. Undercut anchors are often used for heavy assembly. They carry much higher loads than expansion dowels, which is partly related to the form fit mentioned, but also to introduce the loads further away from the surface in a defined manner. There are also undercut anchors that can be used in the cracked tension zone of a concrete component.
Screw anchor (concrete screw)
Screw dowels (better known as concrete screws) are a relatively new fastening system. The anchor is screwed into a pre-drilled cylindrical hole. The special thread of the anchor cuts an internal thread into the anchoring base when it is screwed in. The anchoring takes place through the form fit of the special thread. Whether the object can be fastened with screw dowels therefore mainly depends on the anchoring base. Depending on the approval, concrete screws are approved for heavy-duty fastening in cracked or non-cracked concrete indoors or outdoors (in this case only in A4 stainless steel) and are partially processed in the system with mortar. Typical areas of application are railing and noise protection fastenings on roads and bridges or the anchoring of high shelves indoors.
|DIN EN 14592|
|title||Timber structures - Pin-shaped fasteners - Requirements|
Shaped like wooden dowels, dowels are pin -shaped fasteners made of metal and standardized according to EN 14592. They are used to create load-bearing connections between wood and sheet steel and between wood and wood.
Dowels are arranged perpendicular to the shear surface and are mainly subjected to bending . They are cylindrical and chamfered at the ends . The quality feature is the smooth surface and low thickness tolerance. Their diameter is limited to between 6 and 30 mm. Preferably, dowels are made of steel S235JR, S275JR or S355J2 according to EN 10025. In wood, drill holes 0.2 mm-0.5 mm smaller than the dowel diameter are made, whereas the drill hole in steel is up to one millimeter larger.
Building authority approval
Especially if the failure of a component connection can lead to a risk to life or health of people or significant economic consequences, dowels should always be dimensioned and fastened in accordance with the approval.
Many of the plastic anchors on the market are not approved by the building authorities and may therefore not be used for security-relevant fastenings. Each pack of dowels approved by the building authorities is either accompanied by an instruction leaflet with the assembly instructions, or the instructions are shown on the packaging. The building inspectorate approval can be obtained from the manufacturer and is usually available as a PDF file on the manufacturer's homepage. Admission is to be obeyed. It regulates everything, from the dimensioning of the anchoring to the creation of the drill hole to the correct installation of the dowels. This document is legally relevant and must be added to the building files by the client.
In the case of dowels or anchors, the technical information can usually be found in the building authority approval; in the case of products which are not approved by the building authorities, the manufacturer's instructions for use. Typical data are the effective anchoring depth, the minimum spacing between the dowels, the minimum distance between the dowels from the edge, the minimum component thickness and the permissible load.
- For safety-relevant anchoring in concrete, where cracks in the anchoring area are to be expected, which affect the load-bearing capacity of the dowels, dowels suitable for cracking should always be used. Otherwise, evidence would have to be provided that the concrete in the installation area will not crack during the service life.
- In the case of fastenings on plaster, the thickness of the plaster is included in the thickness of the attachment.
- Corrosion (caused by stress cracks and on component connections) must always be avoided. To avoid contact corrosion , stainless steels, for example, must not be fastened with galvanized screws. While ordinary galvanized steel dowels may only be used in dry indoor areas, dowels made of stainless steel (A4) are usually used for fastenings outdoors (damp rooms, industrial atmosphere). In particularly aggressive environments (chlorine-containing atmosphere, road tunnels or direct contact with seawater), dowels made from special alloys should be used.
- It is recommended that the borehole be cleaned by blowing or vacuuming after drilling. Drilling dust reduces the adhesion of the anchor by frictional connection in the drill hole. Incorrect bores are to be mortared.
- ↑ Main Patent No. 286.793
- ↑ http://www.fischer.de
- ↑ http://www.dpma.de/ponline/erfindergalerie/patente/de1097117.pdf Patent 1097117
- ↑ Kaianders Sempler: Originalpluggen var Oswald! In: Ny Teknik. Ny Teknik, Stockholm, April 15, 2008, accessed November 9, 2018 .
- ↑ http://www.duebel-shop.at Dämmstoffdübel made of metal
- ↑ Peter Nause: Fire behavior of fastening systems in installation technology. (PDF; 229 kB) On: wuerth.de
- ↑ holzbau-kompetenzzentren.de: dowels, fitting bolts, ... , PDF
- Anchor manual (accessed June 5, 2020)
- Dowel guide (accessed June 5, 2020)
- Overview of anchor types (accessed on June 5, 2020)
- DOWELS AND FASTENING SOLUTIONS (accessed June 5, 2020)
- Technical manual of fastening technology for building construction and civil engineering (accessed on June 5, 2020)