Fastening member having threads, particularly a screw and screwing tool

A threaded fastening member includes an actuating end having a head. The head has a diameter projecting beyond a diameter of the thread of the fastening member and is coaxial with a thread axis. A centrally located recess is provided within the head for the application of interior forces. The recess includes radial outwardly projecting slots, each having two flanks lying in respective planes. Each flank plane is essentially tangential to an imaginary cone that is co-axial with the thread axis. The recess further includes a pair of segmented circumferential faces separating each slot from an adjacent slot, and a transitional linear inner edge formed between each pair of segmented circumferential faces and each flank of a respective slot. Each inner edge converges towards a common point on the thread axis at a position within the fastening member, and each face of each pair is inclined relative to the other face of the pair at an angle, whereby the inner edges define the radially innermost limits of the recess and are for the reception of the interior forces.

BACKGROUND OF THE INVENTION 
The invention relates primarily to a threaded fastening member including an 
actuating end having a head, the head having a diameter projecting beyond 
a diameter of the thread of the fastening member and being coaxial with a 
thread axis; and a centrally located recess within the head for the 
application of interior forces. 
The recesses of these types of prior art screws which are intended for the 
application of force from the interior, are configured symmetrically. 
Their axis of symmetry is the axis of the screw thread. The recesses of 
known screws of this type have different shapes. They may be cruciform, 
hexagonal, square, rectangular or circular with notched toothing, 
wedge-shaped toothing or arcuate toothing. 
It is known that significant improvements in securing screws against 
loosening and with respect to their fatigue strength are obtained if they 
are pre-tensioned to the highest possible level of their pre-tensioning 
force, i.e., approaching the force at which their shank would break. This 
high pre-tensioning force cannot be realized with many of the known types 
of force application to a screw, because the tightening torque is limited. 
This torque limitation, for example, has the following mechanical causes: 
reaming of the interior-force application zone, severing of the head due 
to insufficient strength of the material between the head and the 
transition to the shank and the base of the interior-force application 
zone, and breakage of the screwing tool. 
One important cause for the limit of the pre-tensioning force of 
conventional screws of this type is also that the screwing tool, due to 
axial forces during the assembly process, is pushed axially out of its 
force application zone. This cam-out effect is the result of a succession 
of reactive forces between the screwing tool and the force application 
zone, i.e., in the last analysis, it is the result of the screw torque 
that is applied. 
A further drawback of most configurations of interior-force application 
zones for a screw is that different tools are required for screwing in 
different nominal diameters. For example, for mounting many different 
types of conventional Phillips screws having a nominal diameter ranging 
from 1.6 to 10 mm, five different sizes of screwdrivers are required. 
Frequently, each size of screw requires a screwdriver whose size 
specifically matches the screw. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide fastening members, 
particularly, screws of the above-mentioned type, with a novel geometry of 
the screw's recess for the application of force from the interior, such 
that the axial reactive forces are minimized, even if very high torque is 
applied during the screwing process. This makes it possible for the 
screwdriver to generate a pre-tensioning force that comes close to the 
force under which the screw shank would break. This problem is solved by 
the provision of a recess comprising radial outwardly projecting slots 
each having two flanks lying in respective planes, each flank plane being 
essentially tangential to an imaginary cone being co-axial with the thread 
axis; a pair of segmented circumferential faces separating each slot from 
an adjacent slot; and a transitional linear inner edge formed between each 
pair of segmented circumferential faces and each flank of a respective 
slot, wherein each inner edge converges towards a common point on the 
thread axis at a position within the fastening member, and each face of 
each pair is inclined relative to the other face of the pair at an angle, 
whereby the inner edges define the radially innermost limits of the recess 
and are for the reception of the interior forces. 
Axial reactive forces are prevented on account of this solution. A further 
advantage is that this geometry of the interior-force application zone may 
be used in the same manner for screws of any size, so that all screws may 
be tightened with the same screwing tool. 
To facilitate the realization of the above object with respect to 
manufacturing technology, the interior edges forming the transition from 
the slot flanks to the segment-like peripheral surfaces of the central 
recess are in their desired position independent of the angle they form 
together with the axis of the thread. 
An essential feature of the above is that each of the afore-mentioned 
interior edges lies in an imaginary plane which contains the thread axis 
and thus is spaced radially from this projecting imaginary plane. This 
must be the case with respect to the entire planar slot flanks. However, 
manufacturing technology does not allow for the production of such an 
ideal form at acceptable cost. The recesses of screws intended for 
interior-force application are conventionally manufactured by means of 
non-cutting deformation, namely, pressing. The principal direction of 
pressing, in this case, extends in the direction of the thread axis. In 
order to be able to remove the pressing tool from the formed recess in 
this conventional manufacturing method, the slot flanks must not be 
aligned precisely parallel to the thread axis. The slope angles of the 
individual slot flanks employed here, which are in the range of 
1.5.degree. to 2.degree. and, with respect to the thread axis, are 
generally essentially negligible with respect to the desired prevention of 
reactive axial forces. 
The peripheral surfaces of the central opening lying between the slots are 
not of great significance for the transfer of torque. However, they must 
be configured in such a way that they do not prevent the engagement of the 
tool for transmitting torque and its interaction with the slot flanks. 
The angle between the interior edges forming the transition from the slot 
flanks to the segment-like circumferential faces of the central opening 
and the axis of the thread may be matched in such a way with the axial 
length of the screw head that point P lies within the screw head. This 
ensures increased mechanical strength of the screw. 
Point P on the thread axis must not be configured concretely in the 
appearance of the recess. It may also be an imaginary point of 
convergence. 
A further part of the invention relates to the configuration of the tool 
head for introducing torque and for transferring the torque to the screw 
configured according to the invention. The configuration of the 
circumference of the tool head is generally complementary to the spatial 
contour of the screw recess. Thus the screwing tool basically has 
approximately the form of a central body that corresponds to the central 
opening of the screw with wings projecting radially from the screw axis 
which, during the screwing process, sink into the slots in the recess of 
the screw. The central body of the screwdriver, in this case, has 
approximately the shape of a nose. The characteristics of the screwing 
tool ensure that the interior edges forming the transition from the slot 
flanks to the segment-like circumferential faces of the central recess in 
the circumferential direction of the screw recess are charged by the 
application of force originating from the screwdriver. Advisably, it is 
only those interior edges that are subjected to the application of force 
if the occurrence of axial reactive forces must be significantly 
suppressed. This advantageous limitation of the force applied by the wings 
on the screwdriver head is ensured by the configuration of the flanks of 
the wings on the screwdriver. The flanks of the screwdriver wings must not 
necessarily be planar in this case. They may even be given a curved shape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The screw shown in FIG. 1 comprises essentially the screw shank 1 
containing the screw thread (not shown) and the screw head 2 forming the 
activating end of the screw. The thread axis 3 is the axis of symmetry of 
shank 1 and head 2. A recess, generally designated 4, is made starting 
from the end face 5 of the screw head. The recess 4 comprises a central 
opening and radially projecting outwardly slots 6 whose flanks 7 form the 
surfaces for applying the force to introduce the torque. 
The interior edges 9 which, in the circumferential direction 22, form the 
transition from the slot flanks 7 to the segment-like circumferential 
faces 8 of the central opening have a linear configuration. Typically, the 
segmented circumferential faces 8 are divided into pairs. Each face of 
each pair is inclined at an angle relative to the other face, so that the 
interior edges form the radially innermost limits of the recess. Referring 
also to FIG. 2, the interior edges 9 each extend in an imaginary plane 10 
containing the thread axis 3 and thus project radially from the same. 
In the direction II away from the end face 5, the interior edges 9 converge 
in a common point P of thread axis 3. 
As shown in FIGS. 6 and 10, the planar slot flanks 7 in their entirety lie 
within the imaginary planes 10 containing thread axis 3 and projecting 
radially from the same. 
Alternatively, as illustrated in FIG. 2, the slot flanks 7, which form a 
common slot 6 between them, converge slightly in the direction II facing 
away from end face 5 in order to form a lift-out slope on both sides with 
respect to a shaping tool, so that the screw can be provided with recess 4 
without cutting. Measured in the direction of penetration II, the slope 
angle of the slot flanks 7 relative the central plane of each of the slots 
6 containing the thread axis 3, is approximately 1.5.degree. to 2.degree., 
and is thus negligibly small with respect to the desired prevention of 
axial reactive forces. Stated alternatively, each flank 7 lies in a 
respective plane that is essentially tangential to an imaginary cone 23 
that is co-axial to the thread axis and that has a vertex corresponding to 
common point P. The imaginary cone thus tapers outward away from common 
point P at an angle of about 1.5 to 2 degrees relative to thread axis 3. 
The imaginary planes of all slot flanks 7 also intersect at point P lying 
on screw axis 3. The point of intersection between the interior edges 9 
and the surface 5 of the screw head 2 is designated Q. 
The circumferential faces 8 of the central opening of the recess lying 
between slots 6 in circumferential direction 22 are surfaces of an 
imaginary regular pyramid whose axis coincides with thread axis 3 and 
whose tip is point P lying on thread axis 3. However, it is also possible 
without jeopardizing the purpose of the invention, that the 
circumferential faces 8 lie outside of these surfaces of a pyramid. As 
illustrated in FIG. 9, the interior edges 9, together with thread axis 3, 
enclose an angle 12 of approximately 30.degree., 28.degree. in the 
exemplary embodiment. 
The surfaces of the slots 6 lie on an imaginary cone or imaginary regular 
pyramid surface, whose axis coincides with thread axis 3, and whose cone 
tip coincides with point P on axis 3. The angle 13 of the cone tip is 
shown in FIG. 9. 
As illustrated in FIG. 5, point P on thread axis 3 may also lie as an 
imaginary point on the axis of and in a blind hole 14. Blind hole 14 forms 
the end at the bottom of the recess 4. 
The screwdriver head shown in FIG. 7 essentially shows a surface contour 
which is complementary to the circumferential contour of recess 4. In the 
engaged state, as shown in FIG. 10, its wings 16 lie within the slots 6 of 
recess 4. When inserted into the recess 4 of the screw, wing flanks 17 
together with slot flanks 7 of screw recess 4 form an acute angle 18 which 
closes toward thread axis 3. Angle 18 may be relatively small. It must 
merely be ensured that a contact between the wing flanks 17 and slot 
flanks 7 occurs in the region of the interior edge 9 and that it is 
limited as much as possible to this interior edge region 9. To this end 
and referring to FIGS. 6 and 8, the wing flanks 17 of each wing 16 
advisably enclose an acute angle 19 which closes outwardly substantially 
in the radial direction whereas the flanks 7 on the screw head 2 forming a 
common slot 6 form an angle 20 which closes in the direction toward thread 
axis 3. 
The illustration according to FIG. 8 shows in the left half that wing 
flanks 17 there are planar. However, they may also be configured 
spherically convex in the same manner. This is illustrated in FIG. 8 on 
the right side. 
While the tool is in use, the wing flanks 17 make contact practically only 
with the interior edges 9 of the recess 4 in the region of the dashed 
contact line 21 (FIG. 7) between the end points designated q and p there. 
The end p of line 21 lies on the screwdriver axis 3 to form the tip of the 
screwdriver.