Note: Descriptions are shown in the official language in which they were submitted.
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Floor covering and locking systems and an equipment for production of
floorboards.
Field of the Invention
The invention relates generally to the technical field of locking systems
for floorboards. The invention concerns on the one hand a locking system for
floorboards which can be joined mechanically and, on the other hand,
floorboards
and floor systems provided with such a locking system and a production method
to
produce such floorboards. More specifically, the invention relates above all
to locking
systems, which enable laying of mainly, floating floors in large continuous
surfaces
and laying with floorboards that exhibit considerable changes in shape after
installation.
Field of Application
The present invention is particularly suited for use in floating wooden
floors and laminate floors, such as massive wooden floors, parquet floors,
floors with
a surface of veneer, laminate floors with a surface layer of high pressure
laminate or
direct laminate and the like.
The following description of prior-art technique, problems of known
systems as well as objects and features of the invention will therefore as non-
limiting
examples be aimed mainly at this field of application. However, it should be
emphasised that the invention can be used in any floorboards, which are
intended to
be joined in different patterns by means of a mechanical locking system. The
invention may thus also be applicable to floors which are glued or nailed to
the sub
floor or floors with a core and with a surface of plastic, linoleum, cork,
varnished
fibreboard surface and the like.
Definition of Some Terms
In the following text, the visible surface of the installed floorboard is
called "front side", while the opposite side of the floorboard facing the
subfloor is
called "rear side". By "floor surface" is meant the major
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outer flat part of the floorboard, which is opposite to
the rear side and which is located in one single plane.
Bevels, grooves and similar decorative features are parts
of the front side but they are not parts of the floor
surface. By "laminate floor" is meant a floor having a
surface, which consists of melamine impregnated paper,
which has been compressed under pressure and heat. "Hori-
zontal plane" relates to a plane, which is extended
parallel to the outer part of the floor surface. "Verti-
cal plane" relates to a plane perpendicular to the hori-
zontal plane.
The outer parts of the floorboard at the edge of the
floorboard between the front side and the rear side are
called "joint edge". By "joint edge portion" is meant a
part of the joint edge of the floorboard. By "joint" or
"locking system" are meant cooperating connecting means,
which interconnect the floorboards vertically and/or
horizontally. By "mechanical locking system" is meant
that joining can take place without glue. Mechanical
locking systems can in many cases also be joined by glue.
By "vertical locking" is meant locking parallel to the
vertical plane. As a rule, vertical locking consists of
a tongue, which cooperates with a tongue groove. By
"horizontal locking" is meant locking parallel to the
horizontal plane. By "joint opening" is meant a groove
which is defined by two joint edges of two joined floor-
boards and which is open to the front side. By "joint
gap" is meant the minimum distance between two joint edge
portions of two joined floorboards within an area, which
is defined by the front side and the upper part of the
tongue next to the front side. By "open joint gap" is
meant a joint gap, which is open towards the front side.
By "visible joint gap" is meant a joint gap, which is
visible to the naked eye from the front side for a person
walking on the floor, or a joint gap, which is larger
than the general requirements on joint gaps established
by the industry for various floor types. With "continuous
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floating floor surface" is meant a floor surface, which
is installed in one piece without expansion joints.
Background of the Invention
Traditional laminate and parquet floors are usually
installed floating on an existing subfloor. The joint
edges of the floorboards are joined to form a floor sur-
face, and the entire floor surface can move relative to
the subfloor. As the floorboards shrink or swell in con-
nection with the relative humidity RH varying during the
year, the entire floor surface will change in shape.
Floating floors of this kind are usually joined by
means of glued tongue and groove joints. In laying, the
boards are brought together horizontally, a projecting
tongue along the joint edge of one board being inserted
into a tongue groove along the joint edge of an adjoining
board. The tongue and groove joint positions and locks
the floorboards vertically and the glue locks the boards
horizontally. The same method is used on both long side
and short side, and the boards are usually laid in paral-
lel rows long sid against long side and short side
against short side.
In addition to such traditional floating floors,
which are joined by means of glued tongue and groove
joints, floorboards have been developed in recent years,
which do not require the use of glue but which are
instead joined mechanically by means of so-called mecha-
nical locking systems. These systems comprise locking
means, which lock the boards mechanically horizontally
and vertically without glue. The vertical locking means
are generally formed as a tongue, which cooperates with a
tongue grove. The horizontal locking means consist of a
locking element, which cooperates with a locking groove.
The locking element could be formed on a strip extending
from the lower part of the tongue groove or it could be
formed on the tongue. The mechanical locking systems can
be formed by machining the core of the board. Alterna-
tively, parts of the locking system such as the tongue
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and/or the strip can be made of a separate material,
which is integrated with the floorboard, i.e. already
joined with the floorboard in connection with the manu-
facture thereof at the factory.
The floorboards can be joined mechanically by
various combinations of angling, snapping-in, vertical
change of position such as the so-called vertical folding
and insertion along the joint edge. All of these instal-
lation methods, except vertical folding, require that one
boards, which are possible to displace, and which at the
same time are connected to each other with a fit, which
is as tight as possible. A very small displacement play
of for instance 0,01-0,05 mm is often sufficient to
reduce the friction between wood fibres considerably.
According to The European Standard EN 13329 for laminate
floorings joint openings between floorboards should be on
an average 0,15 mm and the maximum level in a floor
should be 0,20 mm. The aim of all producers of floating
floors is to reduce the joint openings as much as pos-
cult to install.
Wooden and laminate floors are also joined by gluing
or nailing to the subfloor. Such gluing/nailing counter-
acts movements due to moisture and keeps the floorboards
joined. The movement of the floorboards occurs about a
centre in each floorboard. Swelling and shrinking can
occur by merely the respective floorboards, and thus not
the entire floor surface, changing in shape.
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Floorboards that are joined by gluing/nailing to the
subfloor do not require any locking systems at all. How-
ever, they can have traditional tongue and groove joints,
which facilitate vertical positioning. They can also have
5 mechanical locking systems, which lock and position the
floorboards vertically and/or horizontally in connection
with laying.
Prior-Art Technique and Problems thereof
The advantage of floating flooring is that a change
in shape due to different degrees of relative humidity RH
can occur concealed under baseboards and the floorboards
can, although they swell and shrink, be joined without
visible joint gaps. Installation can, especially by using
mechanical locking systems, take place quickly and easily
and the floor can be taken up and be laid once more in a
different place. The drawback is that the continuous
floor surface must as a rule be limited even in the cases
where the floor consists of relatively dimensionally
stable floorboards, such as laminate floor with a fibre-
board core or wooden floors composed of several layers
with different fibre directions. The reason is that such
dimensionally stable floors as a rule have a change in
dimension, which is about 0.1% corresponding to about
1 mm per meter when the RH varies between 25% in winter
and 85% in summer. Such a floor will, for example, over a
distance of ten meters shrink and swell about 10 mm. A
large floor surface must be divided into smaller sur-
faces with expansion strips, for example, every tenth or
fifteenth meter. Without such a division, it is a risk
that the floor when shrinking will change in shape so
that it will no longer be covered by baseboards. Also
the load on the locking system will be great since great
loads must be transferred when a large continuous surface
is moving. The load will be particularly great in pas-
sages between different rooms.
According to the code of practice established by the
European Producers of Laminate Flooring (EPLF), expansion
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joint profiles should be installed on surfaces greater
than 12 m in the direction of the length of the indivi-
dual flooring planks and on surfaces greater than 8 m in
the width direction. Such profiles should also be in-
stalled in doorways between rooms. Similar installation
guidelines are used by producers of floating floors with
a surface of wood. Expansion joint profiles are generally
aluminium or plastic section fixed on the floor surface
between two separate floor units. They collect dirt, give
an unwanted appearance and are rather expensive. Due to
these limitations on maximum floor surfaces, laminate
floorings have only reached a small market share in com-
mercial applications such as hotels, airports, and large
=
shopping areas.
Unstable floors, such as homogenous wooden floors,
may exhibit still greater changes in shape. The factors
that above all affect the change in shape of homogenous
wooden floors are fibre direction and kind of wood. A
homogenous oak floor is very stable along the fibre
direction, i.e. in the longitudinal direction of the
floorboard. In the transverse direction, the movement can
be 3% corresponding to 30 mm per meter or more as the RH
varies during the year. Other kinds of wood exhibit still
greater changes in shape. Floorboards exhibiting great
changes in shape can as a rule not be installed floating.
Even if such an installation would be possible, the con-
tinuous floor surface must be restricted significantly.
The advantage of gluing/nailing to the subfloor is
that large continuous floor surfaces can be provided
without expansion joint profiles and the floor can take
up great loads. A further advantage is that the floor-
boards do not require any vertical and horizontal locking
systems, and they can be installed in advanced patterns
with, for example, long sides joined to short sides. This
method of installation involving attachment to the sub-
floor has, however, a number of considerable drawbacks.
The main drawback is that as the floorboards shrink, a
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visible joint gap arises between the boards. The joint gap can be relatively
large,
especially when the floorboards are made of moisture sensitive wood materials.
Homogenous wooden floors that are nailed to a subfloor can have joint gaps of
3-5 mm. The distance between the boards can be irregularly distributed with
several
small and some large gaps, and these gaps are not always parallel. Thus, the
joint
gap can vary over the length of the floorboard. The large joint gaps contain a
great
deal of dirt, which penetrates down to the tongue and prevents the floorboards
from
taking their original position in swelling. The installation methods are time-
consuming, and in many cases the subfloor must be adjusted to allow
gluing/nailing
to the subfloor.
It would therefore be a great advantage if it were possible to provide a
floating floor without the above drawbacks, in particular a floating floor
which
a) May consist of a large continuous surface without expansion joint
profiles,
b) May consist of moisture sensitive floorboards, which exhibit great
dimensional changes as the RH varies during the year.
Summary of the Invention
The present invention relates to locking systems, floorboards and floors
which make it possible to install floating floors in large continuous surfaces
and with
floorboards that exhibit great dimensional changes as the relative humidity
(RH)
changes. The invention also relates to production methods and production
equipment to produce such floors.
A first object of some embodiments of the present invention is to
provide a floating floor of rectangular floorboards with mechanical locking
systems, in
which floor the size, pattern of laying and locking system of the floorboards
cooperate
and allow movements between the floorboards. According to the invention, the
individual floorboards can change in shape after installation, i.e. shrink and
swell due
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to changes in the relative humidity. This can occur in such a manner that the
change
in shape of the entire floor surface can be reduced or preferably be
eliminated while
at the same time the floorboards remain locked to each other without large
visible
joint gaps.
A second object of some embodiments is to provide locking systems,
which allow a considerable movement between floorboards without large and deep
dirt-collecting joint gaps and/or where open joint gaps could be excluded.
Such
locking systems are particularly suited for moisture sensitive materials, such
as wood,
but also when large floating floors are installed using wide and/or long
floorboards.
The terms long side and short side are used in the description to
facilitate understanding. The boards can according to the invention also be
square or
alternately square and rectangular, and optionally also exhibit different
patterns and
angles between opposite sides.
It should be particularly emphasised that the combinations of
floorboards, locking systems and laying patterns that appear in this
description are
only examples of suitable embodiments. A large number of alternatives are
conceivable. All the embodiments that are suitable for the first object of the
invention
can be combined with the embodiments that describe the second object of the
invention. All locking systems can be used separately in long sides and/or
short
sides and also in various combinations on long sides and short sides. The
locking
systems having horizontal and vertical locking means can be joined by angling
and/or
snapping-in. The geometries of the locking systems and the active horizontal
and
vertical locking means can be formed by machining the edges of the floorboard
or by
separate materials being formed or alternatively machined before or after
joining to
the joint edge portion of the floorboard.
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According to a first aspect, the present invention
comprises a floating floor, which consists of rectangular
floorboards, which are joined by a mechanical locking
system. The joined floorboards have a horizontal plane,
which is parallel to the floor surface, and a vertical
plane, which is perpendicular to the horizontal plane.
The locking system has mechanically cooperating locking
means for vertical joining parallel to the vertical plane
and for horizontal joining parallel to the horizontal
plane of a first and a second joint edge. The vertical
locking means consist of a tongue, which cooperates with
a groove, and the horizontal consist of a locking element
with a locking surface cooperating with a locking groove.
The floor is characterized in that the format, installa-
tion pattern and locking system of the floorboards are
designed in such a manner that a floor surface of
1 * 1 meter can change in shape in at least one direction
at least 1 mm when the floorboards are pressed together
or pulled apart. This change in shape can occur without
visible joint gaps.
According to a second aspect, the present invention
comprises a locking system for mechanical joining of
floorboards, in which locking system the joined floor-
boards have a horizontal plane which is parallel to the
floor surface and a vertical plane which is perpendicular
to the horizontal plane. The locking system has mechani-
cally cooperating locking means for vertical joining
parallel to the vertical plane and for horizontal joining
parallel to the horizontal plane of a first and a second
joint edge. The vertical locking means consist of a
tongue, which cooperates with a groove and the horizontal
of a locking element with a locking surface, which coope-
rates with a locking groove. The first and the second
joint edge have upper and lower joint edge portions
located between the tongue and the floor surface. The
upper joint edge portions are closer to the floor surface
than the lower. The locking system is characterised in
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that, when the floorboards are joined and pressed against
each other, the two upper joint edge portions are spaced
from each other and one of the upper joint edge portions
in the first joint edge overlaps a lower joint edge por-
5 tion in the second joint edge.
According to several preferred embodiments of this
invention, it is an advantage if the floor consists of
rather small floorboards and many joints, which could
compensate swelling and shrinking. The production tole-
10 rances should be rather small since well-defined plays
and joint openings are generally required to produce a
high quality floor according to the invention.
Small floorboards are however difficult to produce
with the required tolerance since they have a tendency to
turn in an uncontrolled manner during machining. The main
reason why small floorboards are more difficult to pro-
duce than large floorboards is that large floorboard has
a much large area, which is in contact with a chain and a
belt during the machining of the edges of the floor-
boards. This large contact area keeps the floorboards
fixed by the belt to the chain in such a way that they
cannot move or turn in relation to the feeding direction,
which may be the case when the contact area is small.
Production of floorboards is essentially carried out
in such manner that a set of tools and a floorboard blank
are displaced relative to each other. A set of tools
consists preferably of one or more milling tools which
are arranged and dimensioned to machine a locking system
in a manner known to those skilled in the art.
The most used equipment is an end tenor, double or
single, where a chain and a belt are used to move the
floorboard with great accuracy along a well defined
feeding direction. Pressure shoes and support unites are
used in many applications together with the chain and the
belt mainly to prevent vertical deviations. Horizontal
deviation of the floorboard is only prevented by the
chain and the belt.
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The problem is that in many applications this is not sufficient, especially
when panels are small.
A third object of some embodiments of the present invention is to
provide equipment and production methods which make it possible to produce
floorboards and mechanical locking systems with an end tenor but with better
precision than what is possible to accomplish with known technology.
The present invention therefore also comprises equipment for
production of building panels especially floorboards. The equipment consists
of a
chain, a belt, a pressure shoe and a tool set. The chain and the belt are
arranged to
displace the floorboard relative the tool set and the pressure shoe, in a
feeding
direction. The pressure shoe is arranged to press towards the rear side of the
floorboard. The tool set is arranged to form an edge portion of the floorboard
when
the floorboard is displaced relative the tool set. One of the tools of the
tool set forms
a guiding surface in the floorboard. The pressure shoe has a guiding device,
which
cooperates with the guiding surface and prevents deviations in a direction
perpendicular to the feeding direction and parallel to the rear side of the
floorboard.
It is known that a grove could be formed on the rear side of a floorboard
and that a ruler could be inserted into the groove to guide the floorboards
when they
are displaced by a belt that moves the boards on a table. It is not known that
special
guiding surfaces and guiding devices could be used in an end tenor where a
pressure shoe cooperates with a chain.
A fourth object of some embodiments of the present invention is to
provide a large semi-floating floor of rectangular floorboards with mechanical
locking
systems, in which floor the format, installation pattern and locking system of
the
floorboards are designed in such a manner that a large semi-floating
continuous
surface, with length or width exceeding 12 m, could be installed without
expansion
joints.
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According to an other aspect, the present invention provides a semi-
floating floor which consists of rectangular floorboards joined with a
mechanical
locking system and in which locking system the joined floorboards have a
horizontal
plane which is parallel to the floor surface and a vertical plane which is
perpendicular
to the horizontal plane, said locking system having mechanically cooperating
locking
means for vertical joining parallel to the vertical plane and for horizontal
joining
parallel to the horizontal plane of a first and a second joint edge and in
which locking
system the vertical locking means consist of a tongue which cooperates with a
tongue groove and the horizontal locking means consist of a locking element
with a
locking surface which cooperates with a locking groove, wherein the locking
element
and the locking groove have cooperating locking surfaces, and that upper joint
edge
portions are formed in an area of the joint edge which is defined by upper
parts of the
tongue and the groove and the floor surface, whereby a play is formed between
said
locking surfaces when the floorboards are joined and pressed against each
other in a
horizontal direction, and whereby a joint gap arises between the upper joint
edge
portions when the floorboards are pulled apart in the opposite horizontal
direction,
said play being larger or equal to said joint gap when these edges are pressed
together and pulled apart, the format, installation pattern and locking system
of the
floorboards are designed in such a manner that a floor surface of 1 * 1 meter
can
change in length in at least one direction at least 1 mm when the floorboards
are
subjected to a compressive or a tensile load in the horizontal plane, wherein
said
change in length is calculated as the average play multiplied with the number
of joints
per meter floor length, wherein this change in length can occur without
visible joint
gaps, and wherein a surface layer is laminate or wood veneer, the core of the
floorboard is a wood based board such as MDF or HDF, the change in floor
length is
at least 1,0 mm when a force of 100 kg/m of the joint edge is used, the change
in
floor length is at least 1,5 mm when a force F of 200 kg/m of the joint edge
is used,
the average joint gaps do not exceed 0,15 mm when the force is 100 kg/m of
joint
edge and they do not exceed 0,20 mm when the force is 200 kg/m of joint edge.
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Brief Description of the Drawings
Figs la-b show floorboards with locking system.
Figs 2a-2f show locking systems and laying patterns.
Figs 3a-3e show locking systems.
Figs 4a-4c show locking systems.
Figs 5a-5d show joined floorboards and testing
methods.
Figs 6a-6e show locking systems.
Figs 7a-7e show locking systems.
Figs 8a-8f show locking systems.
Figs 9a-9d show locking systems.
Figs 10a-10d show production equipment
Figs ha-lid show production equipment
Figs 12a-12c show locking system.
Figs la-b illustrate floorboards which are of a
first type A and a second type B according to the inven-
tion and whose long sides 4a and 4b in this embodiment
have a length which is 3 times the length of the short
sides 5a, 5b. The long sides 4a, 4b of the floorboards
have vertical and horizontal connecting means, and the
short sides 5a, 5b of the floorboards have horizontal
connecting means. In this embodiment, the two types are
identical except that the location of the locking means
is mirror-inverted. The locking means allow joining of
long side 4a to long side 4b by at least inward angling
and long side 4a to short side 5a by inward angling, and
also short side 5b to long side 4b by a vertical motion.
Joining of both long sides 4a, 4b and short sides 5a, 5b
in a herringbone pattern or in parallel rows can in this
embodiment take place merely by an angular motion along
the long sides 4a, 4b. The long sides 4a, 4b of the
floorboards have connecting means, which in this embodi-
ment consist of a strip 6, a tongue groove 9 and a tongue
10. The short sides 5a also have a strip 6 and a tongue
groove 9 whereas the short sides 5b have no tongue 10.
There may be a plurality of variants. The two types of
floorboards need not be of the same format and the
. .
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locking means can also have different shapes, provided
that as stated above they can be joined long side against
short side. The connecting means can be made of the same
material, or of different materials, or be made of the
same material but with different material properties. For
instance, the connecting means can be made of plastic or
metal. They can also be made of the same material as the
floorboard, but be subjected to a treatment modifying
their properties, such as impregnation or the like. The
short sides 5b can have a tongue and the floorboards can
then be joined in prior-art manner in a diamond pattern
by different combinations of angular motion and snap
motions. Short sides could also have a separate flexible
tongue, which during locking could be displaced horizon-
tally.
Fig. 2a shows the connecting means of two floor-
boards 1, l' that are joined to each other. In this
embodiment, the floorboards have a surface layer 31
of laminate, a core 30 of, for instance, HDF, which is
softer and more compressible than the surface layer 31,
and a balancing layer 32. The vertical locking D1 con-
sists of a tongue groove 9, which cooperates with a
tongue 10. The horizontal locking D2 consists of a strip
6 with a locking element 8, which cooperates with a
locking groove 12. This locking system can be joined by
inward angling along upper joint edges. It could also be
modified in such a way that it could be locked by hori-
zontal snapping. The locking element 8 and the locking
groove 12 have cooperating locking surfaces 15, 14. The
floorboards can, when joined and pressed against each
other in the horizontal direction D2, assume a position
where there is a play 20 between the locking surfaces 14,
15. Figure 2 b show that when the floorboards are pulled
apart in the opposite direction, and when the locking
surfaces 14, 15 are in complete contact and pressed
against each other, a joint gap 21 arises in the front
side between the upper joint edges. The play between the
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locking surfaces 14, 15 are according to the invention
defined as equal to the displacement of the upper joint
edges when these edges are pressed together and pulled
apart as described above. This play in the locking system
is the maximum floor movement that takes place when the
floorboards are pressed together and pulled apart with a
pressure and pulling force adapted to the strength of the
edge portions and the locking system. Floorboards with
hard surface layers or edges, which when pressed together
are only compressed marginally, will according to this
definition have a play, which is essentially equal or
slightly larger than the join gap. Floorboards with
softer edges will have a play which is considerable
larger than the joint gap. According to this definition,
the play is always larger or equal to the joint gap. The
play and joint gap*can be, for example, 0.05-0.10 mm.
Joint gaps, which are about 0.1 mm, are considered
acceptable. They are difficult to see and normal dirt
particles are too big to penetrate into the locking
system through such small joint gaps. In some applica-
tions joint gaps up to 0,20 mm, with a play of for
example 0,25 mm could be accepted, especially if play and
joint gaps are measured when a considerable pressure and
pulling force is used. This maximum joint gap will occur
in extreme conditions only when the humidity is very low,
for example below 20% and when the load on the floor is
very high. In normal condition and applications the joint
gap in such a floor could be 0,10 mm or less.
Fig. 2b shows an ordinary laminate floor with floor-
boards in the size of 1.2 * 0.2 m, which are installed
in parallel rows. Such a laminate floor shrinks and
swells about 1 mm per meter. If the locking system has
a play of about 0.1 mm, the five joints in the transverse
direction D2 B will allow swelling and shrinking of
5 * 0.1 = 0.5 mm per meter. This compensates for only
half the maximum swelling or shrinking of 1 mm. In the
longitudinal direction D2 A, there is only one joint per
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1.2 m, which allows a movement of 0.1 mm. The play 20 and
the joint gap 21 in the locking system thus contribute
only marginally to reduce shrinking and swelling of the
floor in the direction D2 parallel to the long sides. To
5 reduce the movement of the floor to half of the movement
that usually occurs in a floor without play 20 and joint
gap 21, it is necessary to increase the play 20 to
0.6 mm, and this results in too big a joint gap 21 on the
short side.
10 Fig. 2c shows floorboards with, for instance, a core
30 of fibreboard, such as HDF, and a surface layer of
laminate or veneer, which has a maximum dimensional
change of about 0.1%, i.e. 1 mm per meter. The floor-
boards are installed in parallel rows. In this embodi-
15 ment, they are narrow and short with a size of, for exam-
ple, 0.5 * 0.08 m. If the play is 0.1 mm, 12 floorboards
with their 12 joints over a floor length of one meter
will allow a movement in the transverse direction D2 B of
1.2 mm, which is more than the maximum dimensional change
of the floor. Thus the entire movement may occur by the
floorboards moving relative to each other, and the outer
dimensions of the floor can be unchanged. In the longitu-
dinal direction D2 A, the two short side joints can only
compensate for a movement of 0.2 mm per meter. In a room
which is, for example, 10 m wide and 40 m long, installa-
tion can suitably occur, contrary to the present recom-
mended installation principles, with the long sides of
the floorboards parallel to the width direction of the
room and perpendicular to the length direction thereof.
According to this preferred embodiment, a large contin-
uous floating floor surface without large visible joint
gaps can thus be provided with narrow floorboards which
have a locking system with play and which are joined in
parallel rows perpendicular to the length direction of
the floor surface. The locking system, the floorboards
and the installation pattern according to the invention
should thus be adjusted so that a floor surface of
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1 * 1 m can expand and be pressed together about 1 mm or
more in at least one direction without damaging the
locking system or the floorboards. A mechanical locking
system in a floating floor which is installed in home
settings should have a mechanical locking system that
withstands tensile load and compression corresponding to
at least 200 kg per meter of floor length. More speci-
fically, it should preferably be possible to achieve the
above change in shape without visible joint gaps when the
floor surface above is subjected to a compressive or ten-
sile load of 200 kg in any direction and when the floor-
boards are conditioned in normal relative humidity of
about 45%.
The strength of a mechanical locking system is of
great importance in large continuous floating floor sur-
faces. Such large continuous surfaces are defined as a
floor surface with length and/or width exceeding 12 m.
Very large continuous surfaces are defined as floor sur-
faces with length and/or width exceeding 20 m. There is a
risk that unacceptable joint gaps will occur or that the
floorboards will slide apart, if the mechanical locking
system is not sufficiently strong in a large floating
floor. Dimensionally stable floorboards, such as laminate
floors, which show average joint gaps exceeding 0,2 mm,
when a tensile load of 200 kg/m is applied, are generally
not suitable to use in a large high quality floating
floor. The invention could be used to install continuous
floating floors with a length and/or width exceeding 20 m
or even 40 m. In principle there are no limitations.
Continuous floating floors with a surface of 10.000 m2 or
more could be installed according to invention.
Such new types of floating floors where the major
part of the floating movement, in at least one direction,
takes place between the floorboards and in the mechanical
locking system are hereafter referred to as Semi-floating
Floors.
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Fig. 5d illustrates a suitable testing method in
order to ensure that the floorboards are sufficiently
mobile in the joined state and that the locking system is
strong enough to be used in a large continuous floating
floor surface where the floor is a Semi Floating Floor.
In this example, 9 samples with 10 joints and with a
length L of 100 mm (10% of 1 meter) have been joined
along their respective long sides so as to correspond to
a floor length TL of about 1 meter. The amount of joints,
in this example 10 joints, is referred to as Nj. The
boards are subjected to compressive and tensile load
using a force F corresponding to 20 kg (200 N), which is
10% of 200 kg. The change in length of the floor length
TL, hereafter referred to as A TL, should be measured.
The average play, hereafter referred to as AP or floor
movement per joint is defined as AP = A TL/Nj. If for
example A TL = 1,5 mm, than the average play AP = 1,5/10
= 0,15 mm. This testing method will also measure dimen-
sional changes of the floorboard. Such dimensional
changes are in most floorboards extremely small compared
to the play. As mentioned before, due to compression of
top edges and eventually some very small dimensional
changes of the floor board itself, the average joint gap
will always be smaller than the average play AP. This
means that in order to make sure that the floor movement
is sufficient (A TL) and that the average joint gaps 21
do not exceed the stipulated maximum levels, only A TL
has to be measured and controlled, since A TL/Nj is
always larger or equal to the average joint gap 21. The
size of the actual average joint gap 21 in the floor,
when the tensile force F is applied, could however be
measured directly for example with a set of thickness
gauges or a microscope and the actual average joint gap =
AAJG could be calculated. The difference between AP and
AAJG is defined as floorboard flexibility = FF (FF=AP-
AAJG). In a laminate floor ATL should preferably exceed
1 mm. Lower or higher force F could be used to design
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18
floorboards, installation patterns and locking systems
which could be used as Semi Floating Floors. In some
applications for example in home environment with normal
moisture conditions a force F of 100 kg (1000 N) per
meter could be sufficient. In very large floating floors
a force F of 250 - 300 kg or more could be used. Mecha-
nical locking systems could be designed with a locking
force of 1000 kg or more. The joint gap in such locking
systems could be limited to 0, 2 mm even when a force F
of 400 - 500 kg is applied. The pushback effect caused by
the locking element 8, the locking surfaces 15, 14 and
the locking strip 6 could be measured by increasing and
decreasing the force F in steps of for example 100 kg.
The pushback effect is high If A TL is essentially the
same when F is increased from 0 to 100 kg (=A TL1) as
when F is increased from 0 to 200kg and than decreased
back to 100 kg (=A TL2). A mechanical locking system with
a high pushback effect is an advantage in a semi-floating
floor. Preferably A TL1 should be at least 75% of A TL2.
In some applications even 50% could be sufficient.
Fig. 2d shows floorboards according to Fig. 2c
which are installed in a diamond pattern. This method
of installation results in 7 joints per running meter
in both directions D2 A and D2 B of the floor. A play
of 0.14 mm can then completely eliminate a swelling and
shrinking of 0.1% since 7 joints result in a total mobi-
lity of 7 * 0.14 = 1.0 mm.
Fig. 2e shows a m2 floor surface which consists of
the above-described floorboards installed in a herring-
bone pattern long side against short side and shows the
position of the floorboards when, for instance, in summer
they have swelled to their maximum dimension. Fig. 2f
shows the position of the floorboards when, for instance,
in winter, they have shrunk. The locking system with the
inherent play then results in a joint gap 21 between all
joint edges of the floorboards. Since the floorboards are
installed in a herringbone pattern, the play of the long
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19
sides will help to reduce the dimensional changes of the
floor in all directions. Fig. 2f also shows that the cri-
tical direction is the diagonal directions D2 C and D2 D
of the floor where 7 joint gaps must be adjusted so as to
withstand a shrinkage over a distance of 1.4 m. This can
be used to determine the optimal direction of laying in a
large floor. In this example, a joint gap of 0.2 mm will
completely eliminate the movement of the floor in all
directions. This allows the outer portions of a floating
floor to be attached to the subfloor, for example, by
gluing, which prevents the floor, when shrinking, to be
moved outside the baseboards. The invention also allows
partition walls to be attached to an installed floating
floor, which can reduce the installation time.
Practical experiments demonstrate that a floor with
a surface of veneer or laminate and with a core of a
fibreboard-based panel, for instance a dimensionally
stable high quality HDF, can be manufactured so as to
be highly dimensionally stable and have a maximum dimen-
sional change in home settings of about 0.5 - 1.0 mm per
meter. Such semi-floating floors can be installed in
spaces of unlimited size, and the maximum play can be
limited to about 0.1 mm also in the cases where the
floorboards have a width of preferably about 120 mm. It
goes without saying that still smaller floorboards, for
instance 0.4 * 0.06 m, are still more favourable and can
manage large surfaces also when they are made of mate-
rials that are less stable in shape. According to a first
embodiment, the invention thus suggests a new type of
semi-floating floor where the individual floorboards are
capable of moving and where the outer dimensions of the
floor need not be changed. This can be achieved by opti-
mal utilisation of the size of the boards, the mobility
of the locking system using a small play and a small
joint gap, and the installation pattern of the floor-
boards. According to the invention, a suitable combina-
tion of play, joint gap, size of the floorboard, instal-
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lation pattern and direction of laying of the floorboards
can thus be used in order to wholly or partly eliminate
movements in a floating floor. Much larger continuous
floating floors can be installed than is possible today,
5 and the maximum movement of the floor can be reduced to
the about 10 mm that apply to current technology, or be
completely eliminated. All this can occur with a joint
gap which in practice is not visible and which is not
different, regarding moisture and dirt penetration, from
10 traditional 0,2 m wide floating floorboards which are
joined in parallel rows by pretension or with a very
small displacement play which does not give sufficient
mobility. As a non-limiting example, it can be mentioned
that the play 20 and the joint gap 21 in dimensionally
15 stable floors should preferably be about 0.1 - 0.2 mm.
An especially preferred embodiment according to the
invention is a semi-floating floor with the following
characteristics: The surface layer is laminate or wood
veneer, the core of the floorboard is a wood based board
20 such as MDF or HDF, the change in floor length A TL is at
least 1,0 mm when a force F of 100 kg/m is used, the
change in floor length A TL is at least 1,5 mm when a
force F of 200 kg/m is used, average joint gaps do not
exceed 0,15 mm when the force F is 100 kg/m and they do
not exceed 0,20 mm when the force F is 200 kg/m.
The function and joint quality of such semi-floating
floorboards will be similar to traditional floating
floorboards when humidity conditions are normal and the
size of the floor surface is within the generally recom-
mended limits. In extreme climate conditions or when
installed in a much larger continuous floor surface, such
semi-floating floorboard will be superior to the tradi-
tional floorboards. Other combinations of force F, change
in floor length A TL and joint gap 21 could be used in
order to design a semi-floating floor for various appli-
cation
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21
Fig. 3a shows a second embodiment, which can be used
to counteract the problems caused by movements due to
moisture in floating floors. In this embodiment, the
floorboard has a surface 31 of direct laminate and a core
of HDF. Under the laminate surface, there is a layer 33,
which consists of melamine impregnated wood fibres. This
layer forms, when the surface layer is laminated to HDF
and when melamine penetrates into the core and joins the
surface layer to the HDF core. The HDF core 30 is softer
and more compressible than the laminate surface 31 and
the melamine layer 33. According to the invention, the
surface layer 31 of laminate and, where appropriate, also
parts of, or the entire, melamine layer 33 under the sur-
face layer can be removed so that a decorative groove 133
forms in the shape of a shallow joint opening JO 1. This
joint opening resembles a large joint gap in homogeneous
wooden floors. The groove 133 can be made on one joint
edge only, and it can be coloured, coated or impregnated
in such a manner that the joint gap becomes less visible.
Such decorative grooves or joint openings can have, for
example, a width JO 1 of, for example, 1 - 3 mm and a
depth of 0.2 - 0.5 mm. In some application the width of
JO 1 could preferably be rather small about 0,5 - 1,0 mm
When the floorboards 1, 1' are pressed towards each
other, the upper joint edges 16, 17 can be compressed.
Such compression can be 0.1 mm in HDF. Such a possibility
of compression can replace the above-mentioned play and
can allow a movement without a joint gap. Chemical pro-
cessing as mentioned above can also change the properties
of the joint edge portion and help to improve the possi-
bilities of compression. Of course, the first and second
embodiment can be combined. With a play of 0.1 mm and a
possibility of compression of 0.1 mm, a total movement of
0.2 mm can be provided with a visible joint gap of 0.1 mm
only. Compression can also be used between the active
locking surfaces 15, 14 in the locking element 8 and in
the locking groove 12. In normal climatic conditions the
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separation of the floorboards is prevented when the
locking surfaces 14, 15 are in contact with each other
and no substantial compression occurs. When subjected to
additional tensile load in extreme climatic conditions,
for instance when the RH falls below 25%, the locking
surfaces will be compressed. This compression is facili-
tated if the contact surface CS of the locking surfaces
14, 15 are small. It is advantageous if this contact
surface CS in normal floor thickness 8 - 15 mm is about
1 mm or less. With this technique, floorboards can be
manufactured with a play and joint gap of about 0.1 mm.
In extreme climatic conditions, when the RH falls below
25% and exceeds 80%, compression of upper joint edges and
locking surfaces can allow a movement of for instance
0.3 mm. The above technique can be applied to many diffe-
rent types of floors, for instance floors with a surface
of high pressure laminate, wood, veneer and plastic and
like materials. The technique is particularly suitable in
floorboards where it is possible to increase the compres-
sion of the upper joint edges by removing part of the
upper joint edge portion 16 and/or 17.
Fig. 3b illustrates a third embodiment. Figure 3c
and 3d are enlargements of the joint edges in figure 3b.
The floorboard l' has, in an area in the joint edge which
is defined by the upper parts of the tongue 10 and the
groove 9 and the floor surface 31, an upper joint edge
portion 18 and a lower joint edge portion 17, and the
floorboard 1 has in a corresponding area an upper joint
edge portion 19 and a lower joint edge portion 16. When
the floorboards 1, 1' are pressed together, the lower
joint edge portions 16, 17 will come into contact with
each other. This is shown in figure 3d. The upper joint
edge portions 18, 19 are spaced from each other, and one
upper joint edge portion 18 of one floorboard l' overlaps
the lower joint edge portion 16 of the other floorboard
1. In this pressed-together position, the locking system
has a play 20 of for instance 0.2 mm between the locking
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23
surfaces 14, 15. If the overlap in this pressed-together
position is 0.2 mm, the boards can, when being pulled
apart, separate from each other 0.2 mm without a visible
joint gap being seen from the surface. This embodiment
will not have an open joint gap because the joint gap
will be covered by the overlapping joint edge portion 18.
This is shown in figure 3c. It is an advantage if the
locking element 8 and the locking grove 12 are such that
the possible separation i.e. the play is slightly smaller
then the overlapping. Preferably a small overlapping, for
example 0,05 mm should exist in the joint even when the
floorboards are pulled apart and a pulling force F is
applied to the joint. This overlapping will prevent
moisture to penetrate into the joint. The joint edges ,
will be stronger since the lower edge portion 16 will
support the upper edge portion 18. The decorative groove
133 can be made very shallow and all dirt collecting in
the groove can easily be removed by a vacuum cleaner in
connection with normal cleaning. No dirt or moisture can
penetrate into the locking system and down to the tongue
12. This technique involving overlapping joint edge
portions can, of course, be combined with the two other
embodiments on the same side or on long and short sides.
The long side could for instance have a locking system
according to the first embodiment and the short side
according to the second. For example, the visible and
open joint gap can be 0.1 mm, the compression 0.1 mm and
the overlap 0.1 mm. The floorboards' possibility of
moving will then be 0.3 mm all together and this consi-
derable movement can be combined with a small visible
open joint gap and a limited horizontal extent of the
overlapping joint edge portion 18 that does not have to
constitute a weakening of the joint edge. This is due to
the fact that the overlapping joint edge portion 18 is
very small and also made in the strongest part of the
floorboard, which consists of the laminate surface, and
melamine impregnated wood fibres. Such a locking system,
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24
which thus can provide a considerable possibility of
movement without visible joint gaps, can be used in all
the applications described above. Furthermore the locking
system is especially suitable for use in broad floor-
boards, on the short sides, when the floorboards are
installed in parallel rows and the like, i.e. in all the
applications that require great mobility in the locking
system to counteract the dimensional change of the floor.
It can also be used in the short sides of floorboards,
which constitute a frame FR, or frieze round a floor
installed in a herringbone pattern according to Fig. 5c.
In this embodiment, shown in figures 3b-3d, the vertical
extent of the overlapping joint edge portion, i.e. the
depth GD of the joint opening, is less than 0.1 times the
floor thickness T. An especially preferred embodiment
according to the invention is a semi-floating floor with
the following characteristics: The surface layer is
laminate or wood veneer, the core of the floorboard is a
wood based board such as MDF or HDF, the floor thickness
T is 6 - 9 mm and the overlapping OL is smaller than the
average play AP when a force F of 100 kg/m is used. As an
example it could be mentioned that the depth GD of the
joint opening could be 0,2-0,5 mm (= 0,02*T - 0,08 T).
The overlapping OL could be 0,1-0,3 mm (= 0,01*T -
0,05*T) on long sides. The overlapping OL on the short
sides could be equal or larger than the overlapping on
the long sides.
Figure 3e show an embodiment where the joint opening
JO 1 is very small or nonexistent when the floorboards
are pressed together. When the floorboards are pulled
apart, a joint opening JO 1 will occur. This joint
opening will be substantially of the same size as the
average play AP. The decorative groove could for example
be coloured in some suitable design matching the floor
surface and a play will not cause an open joint gap. A
very small overlapping OL of some 0,1 mm (0,01*T-0,02*T)
only and slightly smaller average play AP could give
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sufficient floor movement and this could be combined with
a moisture resistant high quality joint. The play will
also facilitate locking, unlocking and displacement in
locked position. Such overlapping edge portions could be
5 used in all known mechanical locking systems in order to
improve the function of the mechanical locking system.
Figs 4a and 4b show how a locking system can be
designed so as to allow a floating installation of
floor-boards, which consist of a moisture sensitive
10 material. In this embodiment, the floorboard is made of
homogeneous wood.
Fig. 4a shows the locking system in a state subject-
ed to tensile load, and Fig. 4b shows the locking system
in the compressed state. For the floor to have an attrac-
15 tive appearance, the relative size of the joint openings
should not differ much from each other. To ensure that
the visible joint openings do not differ much while the
floor moves, the smallest joint opening JO 2 should be
greater than half the greatest joint opening JO 1. More-
20 over, the depth GD should preferably be less than
0.5 * TT, TT being the distance between the floor sur-
face and the upper parts of the tongue/groove. In the
case where there is no tongue, GD should be less than
0.2 times the floor thickness T. This facilitates clean-
25 ing of the joint opening. It is also advantageous if JO 1
is about 1 - 5 mm, which corresponds to normal gaps in
homogeneous wooden floors. According to the invention,
the overlapping joint edge portion should preferably lie
close to the floor surface. This allows a shallow joint
opening while at the same time vertical locking can occur
using a tongue 10 and a groove 9 which are placed essen-
tially in the central parts of the floorboard between the
front side and the rear side where the core 30 has good
stability. An alternative way of providing a shallow
joint opening, which allows movement, is illustrated in
Fig. 4c. The upper part of the tongue 10 has been moved
up towards the floor surface. The drawback of this solu-
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26
tion is that the upper joint edge portion 18 above the
tongue 10 will be far too weak. The joint edge portion 18
can easily crack or be deformed.
Figs 5a and 5b illustrate the long side joint of
three floorboards 1, l' and 1" with the width W. Fig. 5a
shows the floorboards where the RH is low, and Fig. 5b
shows them when the RH is high. To resemble homogeneous
floors, broad floorboards should preferably have wider
joint gaps than narrow ones. JO 2 should suitably be at
least about 1% of the floor width W. 100 mm wide floor-
boards will then have a smallest joint opening of at
least 1 mm. Corresponding joint openings in, for example,
200 mm wide planks should be at least 2 mm. Other combi-
nations can, of course, also be used especially in wooden
floors where special requirements are made by different
kinds of wood and different climatic conditions.
Fig. 6a shows a wooden floor, which consists of
several layers of wood. The floorboard may consist of,
for example, an upper layer of high-grade wood, such as
oak, which constitutes the decorative surface layer 31.
The core 30 may consist of, for example, plywood, which
is made up of other kinds of wood or by corresponding
kinds of wood but of a different quality. Alternatively
the core may consist of or wood lamellae. The upper layer
31 has as a rule a different fibre direction than a lower
layer. In this embodiment, the overlapping joint edges 18
and 19 are made in the upper layer. The advantage is that
the visible joint opening JO 1 will consist of the same
kind of wood and fibre direction as the surface layer 31
and the appearance will be identical with that of a
homogeneous wooden floor.
Figs 6b and 6c illustrate an embodiment where there
is a small play 22 between the overlapping joint edge
portions 16, 18, which facilitate horizontal movement in
the locking system. Fig. 6c shows joining by an angular
motion and with the upper joint edge portions 18, 19 in
contact with each other. The play 20 between the lock-
.
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27
ing surface 15 of the locking element 8 and the locking groove 12
significantly
facilitates joining by inward angling, especially in wooden floors that are
not always
straight.
In the above-preferred embodiments, the overlapping joint portion 18 is
made in the tongue side, i.e. in the joint edge having a tongue 10. This
overlapping
joint portion 18 can also be made in the groove side, i.e. in the joint edge
having a
groove 9. Figs 6d and 6e illustrate such an embodiment. In Fig. 6d, the boards
are
pressed together in their inner position, and in Fig. 6e they are pulled out
to their
outer position.
Figs 7a-7b illustrate that it is advantageous if the upper joint edge 18,
which overlaps the lower 16, is located on the tongue side 4a. The groove side
4b
can then be joined by a vertical motion to a side 4a, which has no tongue,
according
to Fig. 7b. Such a locking system is especially suitable on the short side.
Fig. 7c
shows such a locking system in the joined and pressed-together state. Figs 7d
and
7e illustrate how the horizontal locking means, for instance in the form of a
strip 6 and
a locking element 8 and also an upper and lower joint portion 19, 16, can be
made by
merely one tool TO which has a horizontally operating tool shaft HT and which
thus
can form the entire joint edge. Such a tool can be mounted, for example, on a
circular saw, and a high quality joint system can be made by means of a guide
bar.
The tool can also saw off the floorboard 1. In the preferred embodiment, only
a
partial dividing of the floorboard 1 is made at the outer portion 24' of the
strip 6. The
final dividing is made by the floorboard being broken off. This reduces the
risk of the
tool TO being damaged by contacting a subfloor of, for instance, concrete.
This
technique can be used to produce a frame or freize FR in a floor, which, for
instance,
is installed in a herringbone pattern according to Fig. 5c. The tool can also
be used
to manufacture a locking system of
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a traditional type without overlapping joint edge por-
tions.
Figs 8a-8f illustrate different embodiments.
Figs 8a-8c illustrate how the invention can be used in
locking systems where the horizontal locking consists
of a tongue 10 with a locking element 8 which cooperates
with a locking groove 12 made in a groove 9 which is
defined by an upper lip 23 and where the locking groove
12 is positioned in the upper lip 23. The groove also has
a lower lip 24 which can be removed to allow joining by
a vertical motion. Fig. 8d shows a locking system with a
separate strip 6, which is made, for instance, of
aluminium sheet. Fig. 8e illustrates a locking system
that has a separate strip 6 which can be made of a
fibreboard-based material or of plastic, metal and like
materials.
Fig. 8f shows a locking system, which can be joined
by horizontal snap action. The tongue 10 has a groove 9'
which allows its upper and lower part with the locking
elements 8, 8' to bend towards each other in connection
with horizontally displacement of the joint edges 4a and
4b towards each other. In this embodiment, the upper and
lower lip 23, 24 in the groove 9 need not be resilient.
Of course, the invention can also be used in conventional
snap systems where the lips 23, 24 can be resilient.
Figs 9a-9d illustrate alternative embodiments of the
invention. When the boards are pulled apart, separation
of the cooperating locking surfaces 14 and 15 is prevent-
ed. When boards are pressed together, several alternative
parts in the locking system can be used to define the
inner position. In Fig. 9a, the inner position of the
outer part of the locking element 8 and the locking
groove 10 is determined. According to Fig. 9b, the outer
part of the tongue 10 and the groove 9 cooperate. Accor-
ding to Fig. 9c the front and lower part of the tongue 10
cooperates with the groove 9. According to Fig. 9d, a
locking element 10' on the lower part of the tongue 10
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cooperates with a locking element 9' on the strip 6. It
is obvious that several other parts in the locking system
can be used according to these principles in order to
define the inner position of the floorboards.
Figure 10a shows production equipments and produc-
tion methods according to the invention. The end tenor ET
has a chain 40 and a belt 41 which displace the floor-
board 1 in a feeding direction FD relative a tool set,
which in this embodiment has five tools 51, 52, 53, 54
and 55 and pressure shoes 42. The end tenor could also
have two chins and two belts. Figure 10 b is an enlarge-
ment of the first tooling station. The first tool 51 in
the tool set makes a guiding surface 12 which in this
embodiment is a groove and which is mainly formed as the
locking groove 12 of the locking system. Of course other
groves could be formed preferably in that part of the
floorboard where the mechanical locking system will be
formed. The pressure shoe 42' has a guiding device 43'
which cooperates with the groove 12 and prevents devia-
tions from the feeding direction FD and in a plane
parallel to the horizontal plane. Figure 10 c shows the
end tenor seen from the feeding direction when the
floorboard has passed the first tool 51. In this embodi-
ment the locking groove 12 is used as a guiding surface
for the guiding device 43, which is attached to the
pressing shoe 42. The figure 10 d shows that the same
groove 12 could be used as a guiding surface in all tool
stations. Figure 10 d shows how the tongue could be
formed with a tool 54. The machining of a particular part
of the floorboard 1 can take place when this part, at the
same time, is guided by the guiding device 43. Figures 11
a shows another embodiment where the guiding device is
attached inside the pressure shoe. The disadvantage is
that the board will have a grove in the rear side. Figure
11 b shows another embodiment where one or both outer
edges of the floorboard are used as a guiding surface for
the guiding device 43, 43'. The end tenor has in this
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embodiment support units 44, 44' which cooperate with the
pressure shoes 42, 42'. The guiding device could alterna-
tively be attached to this support unites 44, 44'.
Figures 11c and lid show how a floorboard could be
5 produced in two steps. The tongue side 10 is formed in
step one. The same guiding groove 12 is used in step 2
(fig. 11d) when the groove side 9 is formed. Such an end
tenor will be very flexible. The advantage is that
floorboards of different widths, smaller or larger than
10 the chain width, could be produced.
Figures 12a-12c show a preferred embodiment which
guarantees that a semi-floating floor will be installed
in the normal position which preferably is a position
where the actual joint gap is about 50% of the maximum
15 joint gap. If for instance all floorboards are installed
with edges 16, 17 in contact, problems may occur around
the walls when the floorboards swell to their maximum
size. According to the invention, the locking element and
the locking groove could be formed in such a way that the
20 floorboards are automatically guided in the optimal posi-
tion during installation. Figure 12c shows that the
locking element 8 in this embodiment has a locking
surface with a high locking angle LA close to 90 degree
to the horizontal plane. This locking angle LA is higher
25 than the angle of the tangent line TL to the circle C,
which has a centre at the upper joint edges. Figure 12b
shows that such a joint geometry will during angling push
the floorboard 4a towards the floorboard 4b and bring it
into the above-mentioned preferred position with a play
30 between the locking element 8 and the locking groove 12
and a joint gap between the top edges 16, 17.
