Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02452521 2007-01-15
SELF-TAPPING SCREW FOR COMPOSITE MATERIALS
The present invention concerns a self-tapping screw, especially for wood-
plastic
composite materials or the like, consisting of a screw head and a screw shaft
adjacent to it and
a screw tip, where two threaded sections have been formed on the screw shaft,
which are
separated by a thread-free section, and where each of the threads of the two
threaded sections
has a different pitch.
From the German Gebrauchsmuster 200 07 321.4 a screw of the kind described
above
is known. This design of a screw leads to the result, that when two adjacent
fasteners are to be
joined by means of screws, they can be screwed together without pre-drilling
in such a manner
that the absence of a gap between the two fasteners can be ensured and an
initial tensioning
force can be generated. For this purpose, the two threaded sections contain
threads with
different pitches, where the threaded section closest to the screw tip has a
greater pitch than the
other threaded section,
Such a screw is also known from GB-PS 2 169 051. The WO 99/15796 and the
EP 0 460 264 A1 also show similar screws.
When the known screws are used for threading them into so-called wood-plastic-
composite materials, it has been found that an increased production of chips
occurs, while the
chips are transported away from the upper threaded portion, and that the chips
are thrown up
below the screw head.
The present invention is based on the objective of improving the screw
mentioned
above in such a manner that the chips are largely prevented from exiting in
the region of the
upper thread section.
In accordance with the invention this is achieved in that the threads of the
two thread
sections have an opposite pitch.
-1-
CA 02452521 2007-01-15
Accordingly, the present invention provides a self-tapping screw comprising: a
screw
head; a screw shaft adjacent to the screw head; a screw tip; and wherein a
first and second
thread sections have been formed on the screw shaft and are separated by a
thread-free section,
and the threads of the first and second thread sections having an opposite
pitch, and wherein
an outer thread diameter of the first thread section near the screw head is
smaller than an outer
thread diameter of the second thread section near the screw tip and where the
first and second
thread sections have the same root diameter.
By means of the formation of counter-rotational thread sections chips are not
transported away from the upper thread region, which excludes the possibility
of generating
chips below the screw head.
The tip of the screw near the thread section may be configured as a multiple
screw-
thread, in particular a double screw-thread, where the pitch of the threads of
the multiple screw-
thread is greater than the pitch of the single screw-thread of the thread
section near the head of
the screw.
The formation of a relatively large pitch for the threads of the multiple, in
particular the
double screw-thread, makes it possible that the pitch of the single screw-
thread can be enlarged
as well. Here, a pitch ratio of 2:1 is especially advantageous. In this
manner, the overall
resistance to starting the threaded engagement is reduced as compared to known
screws without
a reduction of the achievable holding forces, because a reduction of the
holding forces resulting
from the greater pitch of the thread is essentially compensated for as a
result of the multiple
thread of the threaded section which is adjacent the tip of the screw.
Furthermore, the use of the single screw-thread with the reduced pitch as
compared to
the multiple screw-thread results in the advantage that the chips being
generated are retained
or pushed back into the screw hole.
By virtue of the overall increase of the pitch in both threaded sections
achieved in
accordance with the invention as compared to known screws, the production of
chips as such
-2-
CA 02452521 2007-01-15
is also reduced.
Furthermore, because of the design in accordance with the invention, when the
screw
threads are first engaged, a different advance occurs as a result of the
different pitch of the two
threaded sections, as soon as both thread sections are engaged, so that the
upper, i.e. in the
direction of thread insertion the front fastener part, is pulled against the
lower, i.e. the following
part when viewed in the direction of thread insertion. In this manner the
generation of a gap
between the two fastener parts is prevented and at the same time an initial
tension force is
generated. At the same time, it is contemplated that the threaded section and
the thread-free
sections are dimensioned in such a way, that when the thread is fully engaged,
the section,
which is free of threads, bridges the transition region between the two
fastener parts.
Advantageous embodiments of the invention are contained in the subordinate
claims.
The invention will be explained in more detail on the basis of the examples of
the
embodiments shown in the enclosed drawings.
The following is shown:
Fig. 1 is a view of a self-tapping screw in accordance with the invention,
Fig. 2 is a section through a connection of two fasteners by means of a screw
in
accordance with the invention according to Fig. 1,
Fig. 3 is a view of a second embodiment of a self-tapping screw in accordance
with the
invention; and
Fig. 4 is a plan view of a screw head of the screw in accordance with the
invention
according to Fig. 3.
A screw in accordance with the invention, which is designed as a self-tapping
screw or
a self-drilling screw, consists of a screw head I and an adjacent screw shaft
2 with a screw tip
3. The screw head 1 is shown in the example of the embodiment as a flat head.
The screw
shaft 2 has a constant root diameter d, which becomes continuously smaller in
the region of the
screw tip 3, and which runs out at the end of the screw tip 3. On the screw
shaft 2 there is a
first threaded section 5 which is close to the screw head, and which in turn
is followed by
-3-
CA 02452521 2007-01-15
thread-free section 6, which is again followed by a second screw tip near the
threaded section
7, the thread of which runs out at the screw tip 3. The threaded section 5 has
a thread 9, which
is designed as a single thread. A thread 10 on the threaded section 7 is
designed as a multiple
thread, especially as a double thread, with two parallel threads. The pitch of
the threads of the
double thread is greater than the pitch of the thread of the single-thread of
the threaded section
5. The pitch ratio is preferably 2:1. Thus, the advance in the threaded
section 7 with its greater
pitch is greater than the advance in the threaded section 5 with its smaller
pitch. In the example
of the embodiment shown the threads 9, 10 of the two threaded sections 5, 7
have the same
outer thread diameter D. However, it would also be within the scope of the
invention, if these
two threaded sections would have a different outer thread diameter, while it
would be
advantageous, in such a case, if the outer thread diameter of the thread 9
would be greater than
the outer thread diameter of the thread 10. In this case, the root diameter d
of the two threaded
sections 5, 7 is, therefore, also dimensioned in the appropriate different
magnitudes. In the
example of an embodiment shown, the thread 10 continues on the tip of the
screw up to its end
point. But it is also within the scope of the invention, if the screw point 3,
for example, is
designed as a drill tip or a friction tip. By means of having a thread on the
threaded section 5,
which has a smaller pitch, it is further achieved that the chips, which are
being generated, are
held back or pushed back into the drilled hole. By virtue of the fact that the
thread of the
threaded section 5 has a relatively large pitch, an overall reduced resistance
to an initial thread
engagement is achieved, while the configuration as a multiple thread or a
double thread still
ensures, as before, a strong holding force. The screw head I has on its
underside a
circumferential edge 14, which protrudes with respect to its lower head
surface 15. By means
of this edge 14, any protruding material chips are cut through while the screw
is screwed in. By
virtue of fact that the lower head surface 15 is curved slightly toward the
inside, an additional
space for receiving the chips is generated.
As can be seen from Fig. 2, where a connection in accordance with the
invention by
means of a screw in accordance with the invention, as described above, is
shown, the thread-
free section 6 bridges the contact area of the two fasteners, which are joined
by means of the
two fasteners 12 and 13 in accordance with the invention. It may advantageous
if these two
fasteners 12, 13 are composite parts made of a wood-plastic mixture. As can be
gathered from
-4-
CA 02452521 2007-01-15
Fig. 2, it is thus ensured that the screw in accordance with the invention,
when it is screwed in,
has individual screw sections which proceed in such a manner that the thread
section 5 runs
preferably exclusively in the front fastener 12, when viewed in the direction
in which it is
screwed in, and that the thread section 7 runs preferably exclusively in the
fastener 13, which
is located behind it when viewed in the direction in which it is screwed in.
By virtue of this
design, it is then ensured that, when the screw according to the invention is
screwed in, the two
fasteners 12, 13 are in contact with each other free of gaps and under an
initial tension force.
The magnitude of the initial tension force is determined by the pitch
difference and the advance
difference associated with it, when the screw in accordance with the invention
is screwed into
the two fasteners 12, 13. For the screw in accordance with the invention a pre-
drilling of the
fasteners is not required.
A further embodiment of the screw according to the invention, see fig. 3,
which has been
designated as a self-tapping or self-drilling screw, consists of a screw head
101 and screw shaft
102 adjacent to it with a screw tip 103. The screw head 101 in the embodiment
shown
according to Fig. 3 is fashioned as a flat-head. The screw shaft 102 has a
constant root diameter
d, which is progressively reduced in the region of the screw tip 103 and runs
out at the end of
the screw tip 103. On the screw shaft 102 a first thread section 105 is formed
near the screw
head, which is followed by a thread-free section 106, which in turn is
followed by a second
thread section 107 near the screw tip, the thread of which thread section 107
runs out at the
screw tip 103. The thread section 105 has a thread 109, which is formed as a
single-thread
screw. A thread 110 on the thread section 107 has a pitch opposite to that of
thread 109. The
thread 110 is, in its preferred embodiment, fashioned as a single thread, but
it can also be
designed as a multiple thread, especially a double thread. The pitch angles of
the two threads
109, 110 are of equal magnitude, but they can also be of different sizes.
By means of the embodiment of the invention with the two oppositely threaded
thread
sections 105, 107 it is avoided that the tips are transported away from the
upper thread region
when the screw is threaded into parts to be joined. Rather, the chips remain
in the region of the
thread section 105 near the screw head and are transported advantageously in
the direction of
the thread section 107 near the screw tips. By this means an accumulation of
chips below the
-5-
CA 02452521 2007-10-10
screw head is excluded.
In the example of the embodiment of the invention according to Fig. 3, the
threads 109, 110 of the two thread sections 105, 107 have different outer
thread
diameters D2, D3. The outer thread diameter D2 of the thread section 105 near
the
screw head is smaller than the outer thread diameter D3 of the thread section
107 near
the screw tip. Preferably the outer thread diameter D2 is 5.3 mm, while the
outer thread
diameter D3 preferably amounts to 5.5 mm.
The thread-free section 106 has, in the examples of embodiments described, a
diameter D4, which is larger than the root diameter d of the respective thread
sections
105, 107.
The height of the thread between two threads - i.e. the pitch - of the thread
section 105 near the screw head is smaller than the pitch of the thread
section 107 near
the screw tips. The pitch of the thread near the screw head is preferably 2.5
mm, while
the pitch of the thread section 107 near the screw tip is preferably 2.8 mm.
In the example of the preferred embodiment according to Fig. 3, the screw head
101 has on its underside a circumferential edge 114, which protrudes with
respect to the
surface 115 beneath its head. By means of this edge 114, any protruding
material chips
are severed while the screw is screwed in. By virtue of the fact that the
surface 115
underneath the head is curved slightly toward the inside, an additional space
for
receiving the chips is generated.
In the example of the preferred embodiment shown in Fig. 3, the length of the
thread section 107 near the screw tips is about 2 to 3 times greater than that
of the
thread section 105 near the screw head. The length of the thread section 107
near the
screw tips is preferably 2.6 to 2.7 times greater than that of the thread
section 105 near
the screw head.
-6-
CA 02452521 2007-10-10
The screw head 101 is up to 2 to 3 mm high. It is, in particular, 2.6 mm high.
The screw head diameter at the end near the threaded shaft is larger than the
diameter at
the opposite end, while the screw head diameter at the end near the threaded
shaft is, in
particular, 8.1 mm,
- 6a -
CA 02452521 2007-01-15
and the diameter at the opposite end has a size of about 7.7 mm.
The screw head 101 includes a recess 116 (Fig. 4) for a turning tool for
turning the
screw into the parts to be joined. The recess 116 is preferably formed as an
interior square. But
it is also within the scope of the invention that the screw head be equipped
with a recess, which
can serve for accepting a common turning tool, where the turning tool may be
fashioned as a
screw driver for slotted-head screws or even as a hex-head screw driver. The
screw head in
accordance with Fig. 1 can, of course, have this configuration.
When the screw is screwed in, the configuration of the individual threaded
sections is
such that the thread section 105 is located preferably exclusively in the
forward part to be joined
as viewed in the direction in which it is screwed in, and the screw section
107 is located
preferably exclusively in the rearward part to be joined when viewed in the
direction in which
it is screwed in. Pre-drilling of the parts to be joined is not required in
the case of the screw
according to the invention.
-7-
