The invention concerns a means for connecting two or more solid construction components, particularly bolts and dowels of metal, as stated in the introductory of claim 1.

Background

Bar dowels and bolts of metal are two of the most commonly used connection means used for connecting wooden structures. The surface of these connection means are usually relatively smooth, and there are standard dimensions for connections of this type. In Norway the standard NS 3470 is used - "Prosjektering av trekonstruksjoner, Beregnings- og konstruksjonsregler", 4th ed. (1989).

When calculating a connection of this type, a simplification is used assuming that the force is transmitted as a uniformly distributed stress across a width corresponding to the dowel diameter. However, this force is not uniformly distributed, and the stresses underneath the bar dowel or bolt will have a varying direction and magnitude. If a fracture should occur in two sections at both sides of the dowel or bolt, each of the fracture surfaces will be stressed by a horizontal force and a vertical force. The distance between the fracture sections is dependent on the mechanical properties of the wood. Rodd, P.D. "The Analysis of Timer Joints Made with Circular Dowel Connections", Sussex, The University of Sussex, Thesis Phd (1973) describes results from the testing of mounted "ring"-dowels having three different surfaces and therefore different friction coefficients. The ring dowel with a large friction coefficient had a rugged surface, and the results showed that the connection became stronger and more ductile than with a smooth surface.

The disadvantage of a bar dowel having a rugged surface for insertion into a long hole is that the bar dowel will act as a rasp during insertion into the hole in the wood material. When inserting long rod dowels, the rasping effect will cause the rugged surface to be filled with sawdust, and the resulting dowel surface will become relatively smooth and act more or less like a smooth dowel. At the same time the hole will have become enlarged and thus produce a weakened connection.

Connection means having a groove shaped surface are also known, especially in furniture manufacture. However, these connection means are formed of wood, and are not configured to stand stresses exerted peφendicular to the longitudinal axis of the connection means. The groove shape in this type of connection means is provided to facilitate distribution of glue along the surface of the means and displace excess glue from the hole or aperture of the furniture component.

Object

The main object of the invention is to provide an improved bolt, rod dowel or similar, offering a more secure connection of components, such as wood- wood or wood-metal than prior art method. Another object of the invention is to provide a connection means of the type above, which requires fewer overall connection units and therefore provide a less expensive construction.

The invention

These objects are achieved by a connection means according to the characterizing section of claim 1. Other beneficial features are described from the corresponding dependent claims.

The invention is constructed of metal or another material having similar mechanical properties, for connecting two or more solid construction components of wood or similar, or alternatively, wood combined with metal. The means comprises an elongate element having a substantially cylindrical external cross section with a rough external surface and is arranged for supporting forces acting radially to the longitudinal extent of the means.

According to the invention the external surface of the elongate element is provided with protrusions or similar extending substantially parallel with the longitudinal axis of the element and at least along that region of the element which is to transfer the forces between the different connected components.

Preferably, the protrusions and recesses (hereinafter referred to as "grooves") are formed with an identical distance therebetween and are evenly distributed along the circular cross section of the connection means. With this shape, the connection means can be arranged with an arbitrary rotational orientation within a correspondingly shaped circular hole or aperture in the construction component. The connection means can, preferably, be shaped

' with a beveled end section known per se, to facilitate insertion of the connection means into the component to be connected. The grooves can be V-shaped as well as U-shaped, or can be provided with further surface profiles in the groove walls to establish increased friction. The connection means according to the invention will minimize or practically eliminate the horizontal component of the force acting on the dowel or the wood material. Accordingly, there will only be small forces left for acting transversely on the fiber of the wood material to cause cracks or fissures, compared with the corresponding stress on a connection means having a smooth surface. The protrusions on the surface of the connection means will prevent the fibers in the aperture edge from slipping along the surface of the connections means and being squeezed to the side. Moreover, the longitudinal protrusions and recesses in the surface of the connection means result in minimum damage to the material at the aperture edge of the wood at the region being stressed. The best effect is achieved when the force direction is coincident with the grain of the wood material in the adjacent construction component. In other words: tree fibers which are stuck or shortened will give a larger contribution to the load capacity than fibers which are displaced horizontally. Moreover, a grooved dowel will transfer a pressure component in the vertical direction between grooves and wood fiber up to a location along the dowel surface which is higher than the dowel center. Grooved dowel connections, for that reason, probably have a higher strength than smooth dowel connections for this force direction. The depth, number and shape of the grooves is not critical, but preferably the groove is formed with a relatively narrow edge and shallow recesses so as to form the largest possible surface area to contact the aperture edge of the wood material to be connected. However, the depth between the elevations and recesses should not become too large, to maintain a sufficiently solid connection and faciliate insertion of the means into the aperture edge. A dowel with deep and narrow or shaφ elevations will have to be inserted a longer distance into the wood material before the dowel in a mounted condition is able to transfer forces completely. Flat-topped grooves will locally have the same effect as a smooth dowel without grooves because the wood fibers may slip along the inclined top surface in the same manner as they may slip along the surface of a smooth dowel. Wood fibers passing an inclined surface, however, are supported by the adjacent substantially radially extending surface of the adjacent groove and will be prevented from slipping and hence establish transversal support. The fibers will therefore not slip as a result of a corresponding stress

load as if all fiber ends had met a smooth surface. Narrow groove tops are therefore preferable to flat groove tops.

When inserting a grooved dowel into a pre-drilled hole having a smaller diameter than the external diameter of the dowel, the grooves will become squeezed into the aperture edge or surface. The smaller the hole, the larger the portion of groove bottoms become filled with fiber. With more fiber present between the grooves, less deformations will occur when the dowel is loaded, thereby resulting in a stiffer connection. For improved stiffness and local strength it is advantageous that the hole is narrow or tight. However, if a hole is too narrow or tight the insertion of a dowel will establish tensile stress in the wood along the hole edge. These tensile stresses alone, or in combination with tensile stress caused by a load, can become too large. If these tensile stresses do become too large, the wood may crack. When the wood cracks and these cracks extend beyond a certain distance, the load capacity of the connection will be weakened. Weaknesses may directly cause a rupture or become a source for rupture development. Because of this, there is a balance point between the hole dimension and the dowel diameter, which will have to be assessed for the actual type of construction to be supported and the properties of the material itself. In general, the hole in the wood material is preferably formed according to the common standard, such as -0.1 mm for a dowel, and maximum +1.0 mm for a bolt.

The dowel core itself can be solid or hollow. Hollow dowel cores can be desirable in situations where a more ductile connection is desirable.

Moreover, the dowel in accordance with the invention is also suitable for gluing because the grooves will cause glue filled into the hole before insertion of the dowel to be displaced upwards along the grooves when inserting the dowel. The grooves will keep the dowel surface at a distance from the aperture edge, and in this way the complete dowel surface will become coated by glue. This principle has been used with wood pins for furniture, as decribed above.

The invention is described in further detail below with reference to the accompanying drawings, where Figures la-c illustrate a section through a smooth rod dowel supporting a wood piece, including forces and stresses acting with the same,

Figures 2a-b illustrate a section corresponding to Fig. 1 above, but with the connection means in accordance with the invention,

Figure 3 illustrates a partial section of the connection means according to the invention in further detail, and Figure 4 illustrates the sample piece after testing according to the example below.

Figure la illustrates a principal section of a wood piece 102 and a smooth dowel, bolt or similar 101. In order to simplify the inteφretation of the forces acting with a dowel in a hole, the simplified model illustrates a dowel supporting two plane surfaces forming a V- shaped configuration. The force F acts on the dowel 101 and establishes two counteracting forces Ml and M2, respectively, on the underside of the dowel. Figure lb shows the force components Fl and F2 acting on the surface of the wood piece. As is apparent from the figures, the force can only be transferred to the surface of the wood piece in a direction peφendicular to the aperture surface. In Fig. lc the force Fl from the dowel is separated into a horizontal force component FH1 and a vertical force component FV1. (F2 is not illustrated in Figure lc). This force distribution results in a stress on the wood 102, in which a horizontal force component MV1 acts transversely to the grain (indicated at 105) in the wood material, and a vertical force component (MV1 ) acts with the grain. The substantial horizontal force component MH1 (1 refers to the contact location at one side of the dowel) will, at a sufficiently large stress, cause the wood fibers to tear apart and form a crack directly below the dowel.

Figure 2a and 2b illustrate a cross-section similar to Figure la and lc, but with a dowel in accordance with the invention. As is apparent from the sectioned drawing of Figure 3, which illustrates a part of the surface of a dowel according to the invention, the dowel has a solid core or external shell 201 and exhibits a surface having protrusions 204 and recesses 203 extending along with the longitudinal direction of the connection means (dowel). In Figure 2a the dowel 201 according to the invention is affected by a downwards directed vertical force F counteracted by counter- forces Ml and M2 from the aperture edge of the wood material 102. In Figure 2b the force has been separated into a first and second component Fl and F2 on either side of the dowel axis. Here there are no horizontal forces acting transversely on the grain as in Figure lc, and the counteracting force distribution MV is in this case relatively homogenously distributed along the projected dowel section.

Example

This example is provided in order to illustrate the effect of the connection means according to the invention compared with a connection means having a smooth surface, both in the form of dowels. A total of 10 dowels were provided, formed of stainless steel and having a smooth surface. The dowels were anti-corrosion treated electrolytically, and had a diameter of 12.0 mm. Five of the dowels were subjected to a surface treatement in a lathe by means of profiling tools to form elongate grooves in the dowel surface. The grooves were evenly distributed across the dowel surface having a center-to-center distance to neighbouring grooves of about 0.8 mm. The groove tops were apparently narrow to the bare eye, but with a 50x magnification the tops were flat. The elevation between groove top and groove bottom was within the range 0.25-0.30 mm. The dowel ends were, in accordance with common practice, beveled to facilitate the insertion of the dowel into a wood piece.

The dowels were then inserted in holes having a diameter of about 11.9 mm drilled in sample pieces of spruce plank. Each sample piece of the comparative test was taken from the same plank (totalling 5 planks) in order to obtain as equal conditions as possible, and had a thickness (hole length) of about 40 mm.

Samples were subjected to tensile tests according to the ISO 6891 and NS-EN 383 Standards to determine the aperture edge pressure or load capacity of the dowels. The higher the aperture edge pressure, the better the load capacity. The sole deviation from the Standard used with these tests was that the end distance was larger than prescribed, in order to establish a comparison basis for a commercial construction.

The visual result of the tensile test is illustrated in Figure 4. The left-hand sample piece supported by three smooth dowels cracked with one single fissure. The right-hand sample piece was supported by dowels according to the invention. As is apparent from the figure, the left-hand wood piece has cracked at two locations along each side edge of the hole, and the hole itself has become substantially enlarged in the length direction. From this visual result it is evident that the dowel in accordance with the invention establishes a far better distribution of the vertical forces than the smooth dowel, in which the vertical force is concentrated at the bottom of the dowel. Moreover, the smooth dowel establishes a horizontal force component forcing the wood fibers away from each other and forming a central crack in the sample piece. To the contrary, the dowel according to the present

' invention forms a minimum horizontal force component transversely to the grain of the wood piece. The force exerted becomes more evenly distributed on the lower semi-circle surface of the dowel and acts towards the fiber ends of the sample piece. The results from the tensile tests show that mean aperture edge pressure for a smooth dowel is 28 N/mm ² , whereas the aperture edge pressure of a dowel according to the invention is at least 40 N/mm ² . Accordingly, the load capacity increase by using grooved connection means, in this case in the form of a dowel, compared to a smooth surfaced means, is 45%. Moreover, the coefficient of variation of the smooth dowels was higher than for the dowels formed in accordance with the invention.