A fixture for anchoring a machine screw in a hole in brickwork, concrete or stone comprises a socket which can be inserted into the hole, the diameter of which is not more than 1.1 times larger than the diameter of the screw. The thickness of the socket is between 3 and 7 % of the diameter of the screw and the material of the socket is selected from those being highly ductile when stressed beyond its yield point. The material of the socket will then after undergoing plastic deformation when the screw is tightened, fill at least one-half of the volume between the screw thread and the walls of the hole. The material of the sockets may consist of a metal or plastic material of deepdrawing quality, such as steel, brass, a tombac alloy or Delrin (polyoxymethylene).

The present invention concerns a fixture for anchoring a screw in a hole in 
brickwork, concrete or stone, comprising a socket which can be inserted 
into the hole, and a machine screw. 
The invention is based upon the fact that with a type of socket that is 
simpler and requires less material than hitherto it is possible to obtain 
a fixture of the above type which requires a much stronger force to 
extract than the force previously achievable, and that in various 
practical cases the desired extraction force can be achieved by optimizing 
certain characteristics of components in the fixture. 
A fixture according to the present invention is characterized essentially 
in that the diameter of the hole is not more than 1.1 times larger than 
the diameter of the screw, the thickness of the socket is between 3 and 7% 
-- preferably about 5% -- of the diameter of the screw and the material of 
the socket is highly ductile when stressed beyond its yield point. After 
undergoing plastic deformation when the screw is tightened, the material 
of the socket fills at least one-half of the volume between the screw 
thread and the walls of the hole. 
Since the material of the socket undergoes plastic deformation when the 
screw is being screwed in and is compressed to its yield point after the 
screw is tightened, the extraction force depends upon the frictional force 
between the socket and the hole along the entire inside surface of the 
hole, the friction of which in theory equals the coefficient of friction x 
the yield stress of the socket x the wall-area of the hole. Owing to the 
plastic deformation of the material of the socket, the space between the 
thread and the wall of the hole is well filled to the above-mentioned 
extent and this in turn is a required condition for the high extraction 
force desired. 
It is preferable that the material of the socket and diameter of the screw 
are chosen so that the desired extraction force obtained lies within the 
limits of torque achievable in practice for tightening the screw. 
This tightening torque is determined, among other factors, by the yield 
point of the material, the depth of the screw thread and thread angle, and 
also to some extent by the friction between the screw and the socket. 
In practice it is preferable that the material of the socket should consist 
of a metal or plastic material of deep-drawing quality, such as steel, 
brass, a tombac alloy or Delrin (polyoxymethylene). A lower tightening 
torque for the screw in relation to the extraction force can then be 
obtained by using a comparatively hard material for the socket with a 
small diameter in preference to a soft material with a large diameter. 
If a metallic material without excessive strain hardening is used for the 
socket -- such as pure aluminium, deep-drawing steel or a plastic material 
such as Delrin -- high extraction forces can be obtained even with very 
moderate tightening torque. 
In practice, the socket should be designed so that it can easily be 
inserted in the hole, i.e. by reducing the outside diameter during 
insertion, followed by elastic spring-back, and also so that the screw 
thread grips relatively soon after insertion of the screw, without the 
socket rotating in the hole. For the simplest version, the socket can 
therefore consist of a cylinder with an oblique slot. On insertion, the 
socket is compressed to some extent, and owing to the oblique slot the 
socket is deformed asymmetrically. After insertion into the hole, the 
shape of the socket should be that of a substantially true geometrical 
cylinder -- and should preferably also have some residual elasticity so 
that it exerts some pressure against the walls of the hole. At one end, 
the cylinder should be tapered to some extent on the inside, enabling the 
screw thread to grip fairly easily.

The screw (1) shown in FIG. 1 may consist of, for example, a 
machine-threaded screw with the ISO type designation M6. A hole (2) with a 
diameter of 6.1 mm -- which in practice can be obtained with a 6 mm-drill 
-- is drilled in the wall (4). The socket (3) for the fixture may consist 
of, for example, brass and has a wall thickness of 0.3 mm. When the screw 
is tightened (FIG. 2), the socket fills approximately 70-80% of the space 
between the screw thread and the wall of the hole. 
Examples are given in the tables below of the relative extraction force 
that can be achieved with different fixtures designed in accordance with 
the present invention, for varying screw diameters and socket-material 
characteristics. The numeral 1 in the respective tables indicates the 
relative extraction force, which thus constitutes a reference value and 
which in a test carried out with a 1/4"-screw and a socket material with a 
.sigma.-value of 25 kgf/mm.sup.2 (carbon steel for deep drawing) was found 
to be approx. 1250 kgf. 
It will be seen from Table 1 that the extraction force decreases with 
increasing softness of the material of the socket used for the fixture. 
Table 1. 
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Extraction force; relative values for different 
socket materials and screw sizes 
______________________________________ 
Material 1 Material 2 Material 3 
Material 4 
= 25kgf/ = 15kgf/ = 10kgf/ 
= 7kgf/ 
Screw mm.sup.2 mm.sup.2 mm.sup.2 mm.sup.2 
______________________________________ 
1/4" 1 0.6 0.4 0.3 
M10 2.5 1.5 1 0.7 
M12 3.5 2 1.5 1 
M16 6 3.5 2.5 1.7 
M20 10 6 4 2.9 
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Table 2. 
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Tightening torque; relative values for different 
socket materials and screw sizes 
______________________________________ 
Mater- Mater- Mater- Mater- 
ial 1 ial 2 ial 3 ial 4 
=25kgf/ =15kgf/ =10kgf/ 
=7kgf/ 
Screw mm.sup.2 mm.sup.2 mm.sup.2 
mm.sup.2 
______________________________________ 
1/4" 1 0.6 0.4 0.3 ref. value: 
M10 4 2.4 1.6 1.1 1/4", material 
M12 7 4.2 2.8 2 1: Torque 
M16 17 10 7 4.7 (lubricated 
M20 33 20 13 9 screw 0.75 
kgf-m) 
______________________________________ 
Examples of materials (see Tables 1 and 2): 
Material 1, carbon steel for deep drawing, .sigma. = 25 kgf/mm.sup.2 
Material 3, deep-drawing brass (70-72% Cu, .sigma. = 10 kgf/mm.sup.2 
Material 4, pure aluminium, .sigma. = 7 kgf/mm.sup.2 
The above Table 2 shows the tightening torques. The reference value 
obtained with a 1/4" screw and material 1 with a .sigma.-value of 25 
kgf/mm.sup.2 is 1.5 kgf-m. If the screw is lubricated, the reference value 
is 0.75 kgf-m. 
The tables reveal a clear correlation between the socket material 
characteristics and the extraction force or tightening torque for screws 
of different sizes. Numerical values calculated theoretically were found 
to agree very closely with values obtained in practice. 
When the screw is screwed in, the socket is compressed with such force that 
the stress produced exceeds the yield point of the material of the socket, 
causing plastic deformation. Most of the space between the screw thread 
and the wall of the hole is filled with socket material when the screw is 
right home. The extraction force then depends upon the residual elastic 
stress in the socket -- i.e. the yield point of the material of the socket 
-- and the coefficient of friction between the socket and the walls of the 
hole. The tightening torque is determined in the same way -- i.e. by the 
surface area of the screw thread, the coefficient of friction between the 
socket and the screw, the socket material yield point and the mean screw 
radius. The above tables show that high extraction forces can be achieved 
by optimizing these material characteristics, without excessive tightening 
torque. 
It is preferable to use a socket material which exhibits purely elastic 
behaviour when stressed up to a specific yield stress beyond which plastic 
deformation occurs -- e.g. a metal such as brass, or a tombac alloy. Not 
quite so suitable are materials which show visco-elastic behaviour -- 
plastics, for example -- because the state of elasticity produced by 
tightening the screw decreases fairly rapidly with time, with the result 
that the contact with the walls of the hole is greatly reduced, and hence 
the extraction force as well. Such materials also undergo creep during 
long-time application of stress with a load considerably lower than the 
maximum extraction force recorded with short-time loading. For 
visco-elastic materials (plastics) both of these phenomena -- relaxation 
and creep -- are highly temperature-dependent, and with temperature 
increases even up to, e.g., 50.degree. C., these processes take place at a 
rate more than twice that at room temperature. Because of the behaviour of 
visco-elastic materials, large safety margins must be applied here -- 
which is not the case for sockets of materials with a well-defined yield 
point, such as metals. However, good results have been obtained with a 
socket of Delrin (polyoxymethylene).