Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVEMENTS RELATING TO
SCREWS AND THREADFORMS
BACKGROUND
This invention concerns the construction of threads for screw fasteners,
particularly
those for screwing into timber, and has particular application to fasteners
for attaching
railway track to timber sleepers.
Many forms of screws have been proposed and used for holding railway track to
timber sleepers. They have been used with new sleepers and for insertion into
holes
previously created by spike fasteners, where the holes had enlarged to the
point where
the spikes were loose, or for so-called spike killed sleepers where no further
sound
timber was available on a sleeper to drive in a further spike.
It has now been found that a screwed fastening with the novel thread
characteristics
described hereunder has significant advantages during installation and in
track
operation performance compared with existing alternatives. Although developed
particularly for rail track application, the thread has much wider
applications.
SUMMARY OF THE INVENTION
In one aspect the present invention provides a screw thread having a
threadform
comprising a ridge rising from root to crest with the crest having two peaks
separated
by a trough the depth of which is less than the height of the ridge from root
to crest.
Preferably the trough depth is between 10% and 40% of the ridge height, more
preferably between 15% and 35%.
In another aspect the invention provides a screw thread having a twin start
helical
thread configuration consisting of two ridges helically winding around a
shank, each
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of the ridges having flanks rising from a root to a crest, and at least one of
the crests
having a pair of peaks. The crests of both of the helically wound ridges may
have a
pair of peaks. One of the ridges may be higher than the other ridge.
Preferably the
lower ridge is between 30% and 70% of the height of the higher ridge.
In another aspect the invention provides a twin start screw thread for a
fastener, the
thread having a repeated threadform profile comprising:
- a first crest, having a first pair of peaks, rising from a first root and
falling to a
second root, and
- a second crest, having a second pair of peaks, rising from the second root
and
falling to the first root.
The first crest may be higher than the second crest. Preferably the second
crest is
between 30% and 70% of the height of the first crest.
In another aspect the invention provides a screw fastener for securing a
railway track
rail to a timber sleeper, the fastener having a thread as described above.
In a further aspect the invention provides a method of rolling a helical screw
thread
onto a cylindrical shank of a metal workpiece comprising:
- rolling into the shank initial helical grooves by plastically deforming the
metal
into a hump immediately to either side of the grooves,
- subsequently deepening and widening the initial grooves by further rolling
which increases the size of the humps and displaces the humps increasingly
further from the initial grooves,
- further deepening and/or widening the grooves to press the two humps
between adjacent grooves into each other until they produce a single ridge
with a crest carrying a pair of peaks, and
- finish rolling the crest to more uniformly define the two peaks along the
crest
of the ridge.
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Preferably the height of each pair of peaks, when measured from a trough
between
said pair, is between 10% and 40% of the height of their respective ridge.
More
preferably it is between 15% and 30% of the height of the ridge.
Preferably:
- the screw thread has a twin start with said two initial helical grooves
axially
offset from each other by less than 45% (160 ) of their lead,
- the finished thread has the helical crests of the ridges offset by
substantially
50% (180 ) of their lead, and
- as one helical groove is displaced axially relative to the other helical
groove
during the rolling process, one helical ridge is produced which is taller than
the other.
More preferably the axial offset is between 20% (70 ) and 35% (125 ) of the
thread
lead.
In a further aspect the invention provides a screw fastener for securing a
railway track
rail to a timber sleeper, the fastener having a thread as described above.
Preferably
the fastener has a head for applying torque to the fastener and a collar
integrally
formed with the head, the collar having a sloping face facing the thread, said
slope
matching that of the upper surface of the foot of the rail.
The invention will now be described with reference to the attached drawings
which
illustrate particular embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an illustration showing a rail fastening screw carrying a thread
according
to one embodiment of the present invention.
3o Figure 2 is a diagram showing in detail the threadform on the screw shown
in Figure
1.
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Figures 3 to 7 show diagrammatically steps in a thread rolling process which
produces
the threadform shown in Figure 2.
Figure 8 is an illustration showing a rail fastening screw carrying a thread
according
to another embodiment of the invention.
Figure 9 is a diagram showing in detail a threadform similar to that formed on
the
screw shown in Figure 8.
Figures 10 to 13 show diagrammatically steps in a thread rolling process which
produces the threadform shown in Figure 8.
1o DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS
Referring to Figure 1, the rail fastening screw 2 has a head 4, flange 6,
plain shank 11,
tapered shoulder 12 and tip 13. Between the shoulder 12 and tip 13 the screw
has a
portion into which a thread 15 is rolled.
For the embodiment shown, the screw has the following approximate dimensions:
total length = 125 mm
diameter of shank 11 = 16 mm
pre-roll diameter for thread 15 = 14.5 mm
diameter of flange 6 = 40 nun
head = 18 mm diameter 6-lobe head
The flange 6 is tapered, with its top face 8 perpendicular to the major axis
17 of the
screw and its bottom face 9 angled at about 11.5 to the top face. This taper
is to
conform with the corresponding taper on the foot of railway rails which the
bottom
face 9 bears against in use. The screws may be used to affix a rail with or
without the
use of a tie plate between the rail and sleeper.
The thread 15 has a 5mm pitch and 10mm lead. Accordingly it is a twin start
thread
with two ridges 21 and 31 of equal height helically winding around a core 19.
The
thread is continuous for its length on the screw. The crest of each ridge 21
and 31
carries a pair of peaks along its length and these will now be described.
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With refcrcncc to Figure 2, the threadform is indicated as the solid line in
the
illustration. It should be noted that the cross section tkvough the tb,read so
illustrated
is not parallel to the axis 17 of the sCrew, but is instcad at the helix angle
to the axis
5 17 in ordcr to be at right angles to the line of the ridges 21 and 3l . The
illustratian
shows the twin start thread consisting of ideritia of ridges 21 and 31
respectively
separated by roots 23 where the thread rolling process has pressed most deeply
into
the metal of the shank 14. The distance of the roots 23 from the axis ] 7
defines the
radius of ihe core 19 of the threadcd shank 14.
Working from the left side of Figure 2, the th.readforzn profile rises from a
root 23 to
the ridge 21 by way of a flank 24 which riscs to a crest 26. This crest
carries two
peaks 27 and 28 with a trough 29 between tbem. From peak 28 the ridge falls
down a
flank 25 to the root 23 which is of the same depth as the root on the other
side of the
ridgc 21. The threadform thcn rcpcats its scqucncc for ridgc 31. Ridges 21 and
31
are the two ridges which form the twin start thread.
Figure 3 illustrates diagrammatically the positioning of initial tips 71 to 73
of contact
upon a cylindrical workpiece by a die in a thread rolling operation which is
to produce
a twin start thrcad with evenly spaced, evenly sized ridgcs in the thrcadform.
The tips
71 to 73 are evenly spaced along the workpiece. Another way of expressing this
is to
say the helical grooves the tips would produce are offset from each other by
180 or
50% of their lead.
Referring to Figure 4, when the tips 71 to 73 are rolled into the surface of
the
workpieee, two helical grooves 177 and 178 are pxoduced . Thc thrcad rolling
dic is
configured so that the groove 177 made by tip 71 is contiguous with the groove
made
by tip 73. Grooves 177 and 178 are evenly spaced along the workpiece.
To either side of grooves 177 and 178 is a liump of metal 122 to 125 which has
been
plastically dcformcd from the groove area. Figures 5, 6 and 7 illustrate
successive
stages in the thread rolling operation as the grooves 177 and 178 are deepened
and
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widened through their stages 277 and 278, to 377 and 378 and to 477 and 478.
The
skilled person will be able to follow the operations from the illustrations
without
further detailed description. As the grooves are deepened and widened, the
humps
show an increased size and increased displacement of position in successive
stages,
and in Figures 3 to 7 their identifying numerals have been changed only by the
hundreds digit in order to identify them more easily.
Of particular relevance is that hump 122/222/322 and hump 123/223/323 converge
to
form a ridge, with its two peaks separated by the shallow trough 328, and that
lo similarly hump 124/224/324 and hump 125/225/325 converge to form the other
ridge
with its two peaks separated by the shallow trough 329.
The final stage of the rolling operation is for the rolling dies to finish
roll the crests,
including down into the troughs 428 and 429, to more uniformly define the
peaks 422
to 425 and the troughs 428 and 429 between the peaks.
Referring now to the embodiment in Figure 8, this shows a rail fastening screw
43
similar to the screw in Figure 1, the only significant difference being the
thread
configuration. The thread 45 of this screw 43, like the thread of the screw in
Figure 1,
has a 5mm pitch and 10mm lead. Accordingly it is a twin start thread with two
ridges
51 and 61 helically winding around a core 49. Ridge 51 is significantly higher
than
ridge 61 and both maintain their heights for the length of the thread, so the
thread is
continuous for its length on the screw. The dimensions of the screw are the
same as
those given above for the embodiment shown in Figure 1, except that for the
Figure 8
embodiment, the pre-roll diameter for the thread 45 is somewhat smaller being
approximately 12mm. The crest of each ridge 51 and 61 carries a pair of peaks
along
its length and these will now be described with reference to Figure 9.
The threadform shown in Figure 9 is slightly different to that in Figure 8. In
particular, the root 53 in Figure 9 is more squared off than the corresponding
part of
the threadform in Figure 8. However this difference may be allowed for by the
skilled
reader.
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With reference to Figure 9, the threadform is indicated as the heavy line
highest in the
illustration. It should be noted that the cross section through the thread so
illustrated
is not parallel to the axis 47 of the screw, but is instead at the helix angle
to the axis
47 in order to be at right angles to the line of the ridges 51 and 61. The
threadform
can be seen to consist of alternating high and low ridges 51 and 61
respectively
separated by roots 53 where the thread rolling process has pressed most deeply
into
the metal of the shank 44. The distance of the roots 53 from the axis 47
defines the
radius of the core 49 of the threaded shank 44.
l0
Working from the left side of Figure 9, the threadform profile rises from a
root 53 to
the ridge 51 by way of a flank 54 which rises to a crest 56. This crest
carries two
peaks 57 and 58 with a trough 59 between them. From peak 58 the ridge falls
down a
flank 55 to the root 53 which is of the same depth as the root on the other
side of the
ridge 51. The threadform profile then rises to the ridge 61 by way of a flank
64 which
rises to a crest 66. The ridge 61 is significantly lower than the ridge 51.
The crest 66
carries two peaks 67 and 68 with a trough 69 between them. From the peak 68
the
ridge falls down a flank 65 to the root 53 from where the threadform repeats
its
sequence. The trough 69 has a shallower form than trough 59.
The scales on the axes of Figure 9 indicate the dimensions in mm for the
threadform.
The higher ridge 51 rises 2.5mm from the root while the lower ridge 61 rises
1.25mm.
The higher trough 59 is 0.7mm deep while the lower trough 69 is 0.25mm deep.
The
lower ridge is thus 50% of the height of the higher ridge, and thus within the
preferred
range of 30% to 70%. Also, the higher trough is 28% of the height of the
higher
ridge, while the lower trough is 20% of the height of the lower ridge, thus
within the
more preferred range of 15% to 35%.
The threadform profile may be defined such that it resembles a compound of
sinusoidal wave forms. Two such curves are shown in the lower portion of
Figure 9.
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Curve 40 illustrates the relationship:
f(t) = aisinwt for 39 <_ t<_ 219
Curve 42 illustrates the relationship:
f(t) = a2sin3wt for 39 < t< 219
Accordingly the threadform can be approximated by the compound relationship:
f(t) = A(alsinc)t + a2sin3cot + C) for 39 _ t<_ 219 and
f(t)=0 for219 5t<_219 +cp
where al ? a2
This relationship defines a single cycle of the threadform which is repeated
along the
threaded shank 44.
Alternatively the threadform can be approximated by defining as linear
dimensions
the height, width and separation of the ridges and defining their shape by a
series of
straight lines at set angles connected by radiused curves.
Figure 10 illustrates diagrammatically the positioning for initial tips 81 to
84 to make
contact upon a cylindrical workpiece by a die in a thread rolling operation
where the
intent is to produce a twin start thread with evenly spaced, unevenly sized
ridges in
the threadform in accordance with one embodiment of the present invention.
When
the tips 81 to 84 are rolled into the surface of the workpiece, two helical
grooves 187
and 188 are produced . The thread rolling die is configured so that the groove
187
made by tip 81 is contiguous with the groove made by tip 83. In the same way
the
tips 82 and 84 together make groove 188. Grooves 187 and 188 are not evenly
spaced
along the workpiece. They are axially offset from each other by 90 or 25% of
their
lead.
To either side of grooves 187 and 188 are ridged humps of metal 102 to 105
which
have been plastically deformed from the groove area. Figures 10 to 13
illustrate
successive stages in the thread rolling operation as the grooves 187 and 188
are
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deepened and widened through their stages 287 and 288, to 387 and 388, and to
487
and 488. From the study of Figures 10 to 13 the skilled person will be able to
follow
the operations without furher detailed verbal description. For the purposes of
explaining the process, as the humps develop an increased size and increased
displacement of position in successive stages, their respective identifying
numerals in
Figures 10 to 13 have been changed only by the hundreds digit in order to
identify
them more easily.
Of particular relevance is that hump 102/202/302/402 and hump 103/203/303/403
1 o converge to form the low ridge, with its two peaks separated by the
shallow trough
411, and that similarly hump 104/204/304/404 and hump 105/205/305/405 converge
to form the high ridge with its two peaks separated by the shallow trough 412.
The final stage of the rolling operation is for the rolling dies to finish
roll the crests,
including down into the troughs 411 and 412, to more uniformly define the
peaks 402
to 405 and the troughs between them.
The embodiment of the invention described with reference to Figures 10 to 13
involves the initial helical grooves 187 and 188 being axially offset from
each other
2o by 90 or 25% of their lead. The extent of offset chosen for other
embodiments is
greatly influenced by the intended difference in size wanted between the sizes
of the
final ridges. The size difference becomes significantly beneficial at an
offset less than
45%, and even more so at less than 35% offset. But at less than 20% the amount
of
metal relocation required in the thread rolling process tends to become too
high. A
20% offset corresponds to about 70 of lead, 35% to about 125 and 45% to
about
160 .
Although the above descriptions with reference to Figures 3 to 7 and Figures
10 to 13
refer to stages of a thread rolling operation, such an operation need not be a
series of
separate steps. In fact the stages preferably form a continuous procedure by
careful
design of the thread rolling dies and this will be understood by the skilled
worker.
*rB
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RAMIFICATIONS AND CONCLUSION
While testing has confirmed screw fasteners incorporating the present
threadform
5 perform better than conventionally threaded fasteners of the same size, the
full
reasons are not completely understood. However a possible explanation for at
least
part of the advantage has been developed. While it is now offered in order to
explain
the physical effects at work in a screwed fastener's operation, it is not
intended to
limit the scope of the invention thereby.
Conventional threads for wood screws have crests which have a single sharp
edge. It
has previously been thought that this is advantageous because it cuts through
the
wood fibres and so achieves maximum penetration. Failure of the fastening
system
usually occurs by tearing of the timber as the screw pulls out, bringing with
it a torn
plug of timber the diameter of which is the same as the outside diameter of
the thread.
In contrast the present invention attempts to reduce the degree to which the
wood
fibres are cut. Instead, the penetration of the threadform into the adjacent
timber is
more by way of compressing the timber rather that cutting it. This leaves the
wood
fibres longer and more intact, thereby causing less weakening of the timber's
structure. Also, because the timber is significantly compressed, the timber
exhibits a
higher strength. The process of compressing the timber instead of cutting into
it is
particularly prevalent at the crest 66 of the lower ridge 61 of the threadform
shown in
Figure 9.
It will be appreciated that fasteners incorporating the threadform described
above are
not restricted to rail track applications. The characteristics which make them
desirable for that purpose make them similarly useful for fastening to any
timber item
or to other similar types of dense fibrous or fibre reinforced materials.
Particular
applications are envisaged in landscaping and in fastening planks on piers and
jetties
for example. Fasteners incorporating the threadform may also be screwed into
non
fibred materials, such as plastic plugs inserted into spike killed timber
sleepers, and
............~~.~._... _ __.._~._.,.._..,o...~._._..a_._....~.~..s.~.. ._
..._.~__
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may be installed in other applications where conventional screws would have
otherwise been used.
Throughout this specification, unless the context requires otherwise, the word
"comprise", and variations such as "comprises" and "comprising", will be
understood
to imply the inclusion of a stated integer or group of integers but not the
exclusion of
any other integer or group of integers.
Finally, it is to be understood that various alterations, modifications and/or
additions
1 o may be introduced into the constructions and arrangements of parts
previously
described without departing from the spirit or ambit of the invention.
