Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 9513a8~s 2 ~ ~ ~ 2 2 8 i . I /Dr , ~ "
PROFILED FENCE POST MADE OF POLYMER MATERIAL
The invention relates to a profiled fence post, made of
polymer material, which can be used for electric fences and
5 other applications.
Such posts are implanted in the ground, and when a transverse
force is exerted on the post, specifically the location where
the post emerges from the ground i s subject to a 1 arge
bending moment. On order to better resist hereto, the posts
are usually tubular, or, in general: profiled. This means
that the surface area of the useful cross-section (cross-
section of the material~ is maximally 30% of the surface area
of the smallest possible circumscribed convex figure, as is
lS the case with most tubular shapes, L-profiles, T-profiles and
I-prof i 1 es, ori ented i n accordance wi th the ant i c i pated
direction of the transverse forces And to be useful as a
post, such a convex figure must have an equivalent diameter -
i.e. the diameter of the circle having the same surface area
-ranging between 2 and 25 cm, and usually between 3 and 12
cm. Although from this point on, the invention will be
further explained speclfically in relation to tubular fence
posts, it should be clear that the invention extends to
lnclude any profiled post of any specially adapted dimensions
whatsoever.
Such tubular fence posts are already known, for example, from
Dutch patent application no. 8600065. In this case the post
is further reinforced with a number of continuous steel
reinro-, t wires running in the longitudinal direction
from one end to the other, these wires having a diameter of
around 3 mm and may be embedded in adherent relationship in
said polymer material. In general, the invention will be used
for posts in which the reinforcement wires have a thickness
ranging between 1.5 and 5 mm, and preferably between 2 and 4
mm .
WO9513081S 218~228 r~llD~
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Furthermore, the polymer material for such a post will in
general be a hard PVC or a copolymer of PVC with another t
monomer, such as vinyl acetate, or a polypropylene or HD-
polyethylene. Such tubes are produced by means of an
extrusion process in which the reil~ro~, L wires are fed in
the direction of the extrusion towards the entrance of the
extrusion machine. In the case where, due to chemical and/or
mechanical adhesion, the reil,ro,, L wires of themselves
already adhere firmly enough to the polymer material, and/or
the wires exhibit sufficient surface irregularities or
roughness for the adhesion, then, upon being fed into the
extrusion machine, the wires do not need to be coated with a
special adhesive layer, a so-called "primer". In other cases,
such as with hard PVC on smooth reinforcement wire; it will
be necessary first to coat the latter with a primer, a well-
known manufacturing technique for plasticized wires.
As already mentioned, such posts are implanted in the ground,
often cast in a concrete block, and when the post is
subjected to a transverse force, the place on the post where
it emerges from the ground is subjected to a particularly
great bending moment. On increasing this transverse force, a
conventional post will give way relatively quickly under the
increasing bending moment and exhibit a permanent crease,
thus having to be replaced in the fence. It is an object of
the invention to provide a post of the type described above,
i.e. made of polymer material and reinforced with steel
wires, by means of which a higher resistance to such a
bending moment is obtained, without having to complicate the
rei~ru~, L and/or to jeopardize the extrudability, of the
polymer material together with the reinforcement.
According to the invention, reil~ro~ ~ L wires are used with
an elastic limit of at least 1200 N/mmZ, and preferably more
than 14û0 N/mm2. As will become apparent below, wires with an
oil-hardened martensitic structure shall here preferably be
WO95/3081S ~l 8q2~/~ r~,l/Dr,~
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used .
The idea underlying the invention relates to the fact that,
up till now, efforts to improYe the bending strength have
always focused on making the reinforcement wires adhere
better in the polymer material, so that these wires would be
better able to take over the tensile forces from the polymer
material. But herein the fact was overlooked, that, when
bending such a post, the conventional force distribution
pattern does not apply, in which over the cross-section of
the post there would only be a tensile zone, a pressure zone
and a neutral line between the two. Upon bending indeed, the
polymer material between the wires is subject to great
shearing stresses in the longitudinal direction. And since,
in order to keep the extrusion process simple and easy, there
is no transverse reinforcement prov~ded, the polymer material
between the wires begins to flow relatively easily in the
direction of longitudinal shearing. And because of this, the
force distribution pattern begins to deviate from the
conventional pattern and begins to tend towards a
superposition thereof, on a configuration in which each
separate reinru~ ~ -t wire with the polymer material around
it exhibits its own pressure zone, tensile zone and neutral
line. This entails that the reinforcement wires, and more
especially those located far from the conventional neutral
line, in fact are much less than normally subject to tensile
and pressure forces, but are greatly subject to bending
forces. For these wires, therefore, it is not a question of
better taking up the tensile forces, but rather of being well
able to resist permanent bending. Therefore, stiff wires are
required: thick and made of steel with a high elastic limit.
Up till now, no attention was paid to this fact and it was
proposed, among other things, that the reil~rc~- rt wires
could be replaced by bundles of wires.
There is yet another important side effect when such a
WO95130815 2I g9228 r~ 7~
reinro~, t with stiff wires is utilized: these posts can
be bent surprisingly far without leaving a permanent crease. r
They can in fact be bent far beyond the elastic limit of the
relatively soft polymer material, so that the latter indeed
S shows a plastic deformation. But at this point, the
reinforcement wires have not yet reached their elastic limit:
neither in flexure, nor, for the wires that are located far
from the conventional neutral line, in tension or pressure.
When the post is released, all the wires seek to recover both
their original straightness and their original length. And
thanks to their great stiffness, combined with the good
adhesion, they will draw the polymer material along in this
movement and hence deform it in the opposite sense back into
the original straight condition of the wires and post.
When a post is reinforced with a number of longitudinal steel
reinroI~ ~ elements, one must ensure with respect to each
individual reinforcing element, that each element, in
addition to having a normal resi$tance to tension or
pressure, would also be able to offer a large resistance
against bending in the expected direction. Thus, in the first
place, it is better for such an element to be in the form of
one thick wire instead of a bundle of several wires having
the same total steel cross-section. The conventional measure
taken with reinforcement wires, wherein thick wires (which
normally speaking have not undergone a large cross-sectional
reduction during drawing and thus have no appreciable tensile
strength) are replaced by a bundle of finer wires, which then
have undergone a higher cross-sectional reduction and thus
possess a higher tensile strength, is therefore not
reL ~ here. It can however be useful to use profiled
wires which have a larger bending resistance in the expected
direction of bending. For a round post, flattened wires with
the largest cross-sectional dimension running towards the
central axis of the post, can so advantageously be used.
WO 9~/3081~ 8 9 2 ~ 8
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However, even when it is advantayeous for each individual
reinforcement element to consist of one single stiff wire,
this still does not mean that the entire rei"ru~, t for
the entire useful cross-section of the post must be
concentrated in one single very thick reinforcement element,
or in a very small number of such elements. A certain even
distributiûn ûf the reinforcement steel is necessary, i.e.
over different elements spread over the cross-section. In
bending indeed, as has already been mentioned, each element
develops its own pressure and tensile zone (including a
neutral line) around itself in the polymer material. And the
lesser the number of elements over which this reinforcement
steel is distributed, the further these zûnes will extend.
Upon bending, then, the parts that 1 ie too far from such a
neutral line can start to flow too soon and exhibit an
excessive and irreversible plastic deformatiûn. It can thus
be put as a norm that the distribution is not, or
insufficiently uniform, when, when exerting a transverse
force on the top of the post to prûduce a 5 deviation from
vertical (straight line from the top to the foot of the post)
and then removing this transverse force, the post no longer
returns tû its original vertical position.
The distribution tû be utilized will depend on the geometry.
Thus, it will be aimed at achieving a greater density in the
parts of the cross-section of the material which are located
further from the cûnventional neutral line than those parts
in the center. These parts lying further out will participate
to a greater extent in the development of the bending
strength due to the fact that their tensile or pressure
res~stance is better utilized. Thus a tubular form in which
the reillror, ,l wires are uniformly distributed over the
perimeter of the tube wall is a preferred embodiment of the
i nvent i on .
In general, it will thus be appropriate, for ghe conventional
WO951308~5 2~ 89228 ~ D~
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reillru~ perc~ntages of 4 to 20%, to maintain the usual
order of magnitude or diameters, in the range between 1.5 and
5 mm, with a limited number of single reinforcement wires
being fed into the e~trusion machine (by ,uleréI~nce from 2 to
4 mm for a per~ntage of 6 to 14%). "Reinforcement
percentage" here me1ns: the percentage of steel surface area
compared to the total surface area of the transected material
in the cross-sectiJn of the post.
lû Where, as to geometry, it seems to be appropriate to maintain
the usual dimensi~ls and geometries, then, for steel
material, this is nqi~: the case. The second point that should
be given attention to is the stiffness of the rei"ruic t
wire, and the fact that this is not defined by the tensile
strength of the steel, but by its elastic limit. Under a
gradually increasing tensile force on the wire, as is known,
this is the tension IJer unit of cross-sectional surface area
(and therefore in N/mm2) under which the material begins to
flow, i.e. under which it begins to show plastic deformation
in a permanent manner. ln this respect, then, the
reinforcement wires ran be improved, in accordance with the
invention, by ensuri~g that the elastic limit of the steel is
at least 1200 N/mm2, and preferably more than 1400 N/mm2. By
this elastic limit, t.le so-calle~ "0.2 limit" is meant. Since
the "beginning" of the flowing is difficult to observe, in
most standard tests this beginning is defined as the
situation in which an elongation of 0.2% is reached. In order
to obtain such a high elastic limit, it is necessary to
harden the wire.
The necessary hardness can be obtained by cold deformation
during the drawing of the wire from an initial diameter to
the final diameter. ln this case, as is known, the wire
possesses a cold-drawn perlitic structure. For relatively
thick wires in the range already mentioned of 1.5 to 5 mm
diameter, however, this value is not obtainable without
~1 8q2~
~0 95/30815 ~ q
taking the special measure of starting the process with a
relatively large initial diameter in order to be able to
carry out a reduction to a final diameter that will produce
the required amount of cold deformation and the resulting
5 increase in the elastic limit. Therefore it is preferable to
achieve the required hardness by means of oil hardening.
Here, as is known, wire with the final diameter is
continuously led through a continuous furnace where it is
heated to austenitization temperature, and then upon exit
from the continuous furnace it runs through an oil bath where
it is quenched and thus obtains a hard martensitic structure,
and thereupon is fed through a heating element where said
structure is softened to a certain extent because the pure
quenched structure i s too bri ttl e . When these thermal
treatments are carried out on straight wire, then a good
straight wire is thus obtained for use in the extrusion of
the post.
The invention is further explained here on the basis of a
number of figures representing a cross-section of the post.
Figure l is a round tubular post w~th an even wall
thickness and uniform distribution of the
reinforcement wires;
Z5
Figure 2 is an essentially round, tubular post with a
substantially uniform distribution of the
reinforcement wires;
Figure 3 is a round tubular post with a substantially
even wall thickness and with thickenings around
the reinforcement wires;
Figure 4 is a round tubular post with a less uniform
distribution of the reinforiement wires;
WO9513081S 2 ' ~9228 ~ " ~,
Figure 5 is a post with an I-shaped cross-sectional
prof i l e .
Figure I shows a post in the shape of a round tube 1 with an
even wall thickness. Here 3 is a relnru., t wire. The
broken line 2 represents a fence wire mesh which is attached
to the post. The shape of the tube, however, may however only
be essentially round, which means that at certain locations,
1û it deviates from the round shape, as for example the
locations 4 where a fence wire mesh will be attached with a
snap connection (Figure 2). The wall thickness does not
necessarily need to be the same everywhere. Thus it can vary,
and the wall can display local thickenings for the purpose of
providing space for the reinforcement wires when they are
relatively thick in relation to the average wall thickness
(Figure 3). The tube can also be square or rectangular. The
average wall thickness over the cirl rel~e,,ce will generally
l ie in the range between 3 and 15% of the diameter or
equivalent diameter of the post, and preferably between 5 and
lû% .
When it is not known from which direction the possible forces
will be applied to the tubular post, or when one does not
wish to take this into account, then the selected number of
wires will preferably be distributed as uniformly as possible
around the ci\, re~_.,ce of the tube wall. However, at
locations where the shape of the post deviates from
roundness, for example for attaching the fence mesh, it is
also possible to deviate from the strictly uniform
distribution (Figure 2, the reinror, L wires 5 and 6), in
which case the distribution is only substantially uniform.
The wires shall however not be distributed over too small a
number of reinforcement wires, for reasons which have already
been presented above, nor over too large a number of
reinforcement wires, because then the wires will be thinner
_, . .. . . _ ....... .
2~ ~9228
~WO 95/30815
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and less stiff. A distribution over a minimum of 4 and a
maximum of 30 wires is therefore most suitable.
Deviations from a strictly uniform distribution are also
5 possible when one wants the post to offer more resistance
against transverse forces in the plane perpendicular to the
fence. it is then taken into account that part of the force
distribution pattern is still the standard pattern over the
entire cross-section, with a pressure zone, a tensile zone
and a neutral line running parallel with the fence. A
distribution is therefore used which involves a greater
concentration of wires far from the neutral line. Reference
is made here to Figure 4, in which the broken line 6
represents the neutral line and the broken line 2 once again
represents a fence mesh. In this case, a different post
profile can also be chosen, for example an I-profile, with
the reinforcement wires more concentrated far from the
neutr~l line (Figure i).
.
