Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A TRAMPOLINE
FIELD OF INVENTION
The invention relates to improvements to a trampoline for sporting and/or
recreational
use which is soft-edged relative to conventional trampolines which support the
mat of
the trampoline via a solid peripheral frame and exposed springs between the
frame and
the mat.
BACKGROUND
US patent 6,319,174 discloses a form of soft-edged trampoline in which the mat
of the
trainpoline is supported by a plurality'of resiliently flexible rods received
in a frame of
the trampoline at the lower ends of the rods and coupled to the periphery of
the
bouncing mat of the trampoline at their upper ends, and which avoids the need
for a
solid frame about the exterior of the bouncing mat and exposed springs between
the
frame and periphery of the mat.
SUMMARY OF THE INVENTION
The invention provides an improved or at least alternative form of such a soft-
edged
trampoline.
In broad terms in one aspect the invention comprises a trampoline including a
flexible
mat, a plurality of resiliently flexible rods the lower ends of which are
retained in a
frame of the trampoline and the upper ends of which are coupled to the mat
about a
periphery of the mat to support the mat, in which the rod stiffiiess against
bending is
greater than about 1200 and up to about 3000 N/m.
In broad terms in another aspect the invention comprises a trampoline
including a
flexible mat, a plurality of resiliently flexible rods the lower ends of which
are retained
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in a frame of the trampoline. and the upper ends of which are coupled to the
mat about a
periphery of the mat to support the mat, in which the rods are deflected at
their upper
ends, from a natural state of rest when the rods are in position in the frame
at their lower
ends but before connection with the mat edge at their upper ends, to
connection with the
mat, by greater than about 300 and up to about 450 mm.
In accordance with the invention the rods which support the mat are deflected
so as to
apply an optimum load radially or laterally outwards to tension the mat
appropriately for
normal fiuictioning of the trampoline, while at the same time the rods are of
a length and
diameter which gives the mat edge a vertical stiffness that is sufficiently
resilient,
impact-absorbing and safe for a jumper who might land on the edge out of
control.
In this specification (including claims) the term "trampoline" is intended to
extend to
smaller trampolines commonly referred to as rebounders also, as well as larger
trampolines of all sizes. Trainpolines of the invention may be circular,
square,
rectangular, or of other shapes such as octagonally shaped in plan view for
example.
The term 'comprising' as used in this specification and claims means
'consisting at least
in part of, that is to say when interrupting independent claims including that
term, the
features prefaced by that term in each claim will need to be present but
otlier features
can also be present.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further described with reference to the accompanying drawings
by way
of example and without intending to be limiting, wherein:
Figure 1 is a perspective view of a preferred form trampoline,
Figure 2 is a side view of the trampoline of Figure 1,
Figure 3 is similar to Figure 1 but of one side of the trampoline only and
showing a portion of the edge of the mat of the trampoline cut away, and
Figure 3 a is an enlarged view of the cut away edge portion of the trampoline,
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Figure 4. shows the rod deflection measurement at the trampoline edge,
Figure 5 shows the range of rod stiffnesses and rod deflections that achieve
the
edge and mat properties required in accordance with the invention,
Figure 6 illustrates the cantilever rod notation for the equations given
subsequently, and
Figure 7 shows rod length vs diameter for GRP rods to achieve the shown
stiffnesses.
DETAILED DESCRIPTION OF PREFERRED FORMS
Referring to Figures 1 to 3, a preferred form trampoline comprises a flexible
mat 1 on
which users may bounce, a plurality of resiliently flexible rods 2, and a base
frame 3.
The preferred form trampoline shown is circular in shape but the trampoline
could be of
any other desired shape such as oval, square, rectangular or similar.
The base frame of the preferred foirn trampoline comprises a circular beam 4
typically
formed of steel or aluminium for example, which may be supported from the
ground by
legs 5.
The rods 2 are typically fibreglass rods and may be pultruded glass reinforced
plastic
rods, but may alternatively be formed of spring steel for example. The lower
ends of the
rods are retained by the base frame 3 and the upper ends of the rods connect
to fittings 6
as will be further described, which are coupled to the mat 1 about the
periphery of the
mat. In the preferred form the lower ends of the rods 2 enter into tubular
holders 7 fixed
to the circular beam as shown, but the lower ends of the rods may be coupled
to the
circular beam, or a base frame of the trampoline of any other form, in any
suitable way.
In the preferred form the mat, which is typically heavy canvas or a woven
synthetic
material, is doubled back upon itself and fixed by stitching for example about
the
periphery of the mat to form a continuous pocket 8 extending about the
periphery of the
mat. A number of the fittings 6 are positioned within this pocket in the
peripheral edge
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of the ,mat as shown in Figures 3 and 3a in particular. The fittings may be
loosely
captured within the pocket or alternatively may be stitched to the mat within
the edge
pocket, or mechanically fastened to the mat via rivets for example.
The rods in the preferred form each have a ball-shaped upper end 12 which
connects to
a socket cavity in the underside of one of the fittings 6. Typically the
fittings 6 will be
formed from a plastics material, by injection moulding for example. The
fittings may
optionally include a slight dome on the body of the fitting over the socket
cavity, on the
underside of the fitting. Also in the preferred fonn the fittings have an
outer edge which
in use is closest to the outer peripheral edge of the mat, which edge is wider
in the
planar than in the inner edge of fittings, so that the fittings have an
approximate
truncated triangular shape and plan view, with concave sides, but this is non-
limiting
and in other forms the fitting could be alternatively shaped.
In set up of the trampoline from its component parts, after assembly of the
base frame 3
as required, the lower ends of the rods 2 are inserted into the holders 7. At
this point the
upper ends of the rods are free so that each rod will be in an undeflected
position, as
indicated at 2a in Figure 4. The mat 1 is then draped over the frame and rods
and the
upper ends of each of the rods are coupled to the fittings 6 one by one, about
the
periphery of the trampoline. After some initial rods have been connected, then
subsequently as the upper end of each rod is coupled to the trampoline mat,
the rods will
be deflected inwardly towards the centre of the trampoline, to the deflected
position
indicated at 2b in Figure 4. When the upper ends of all of the rods have been
connected
to the matter about the periphery of the mat, all of the rods will be in this
deflected
position 2b. The distance between the upper end of any rod in its deflected
position as
at 2b, and its undeflected position when not coupled to the mat (with the mat
held by the
other rods) as indicated at 2a is the rod deflection x indicated as in Figure
4. In
accordance with the invention, this deflection is up to about 450 mm,
preferably
between about 300-450 mm. At the same time the rods are chosen so that they
have a
stiffness against cantilever bending in the range about 1200-3000 N/m.
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Typically the rods will be between about 0.1 and 1.5 metres in length, and,
more
typically between about 0.5 and 1 metre in length.
We have found that the trampoline will then have a good balance of properties
of, first,
sufficient vertical stiffness at the mat edge so that that the edge will not
"collapse" when
a jumper lands on the edge, and second tension of the mat laterally to provide
good
jumping performance when a jumper is juinping centrally on the mat. That is,
within
these parameters, the rods are deflected so as to apply optimum load radially
outwards
to tension the mat for good jumping performance of the trampoline. At the same
time,
the rods are of a length and diameter which gives the mat edge a vertical
stiffness that is
sufficiently resilient, impact-absorbing and safe for a jumper who might land
on the
edge out of control. Figure 5 plots these two measures, defining the ranges of
the
invention.
The following further analysis makes use of the rod stiffness given above:
First the simple cantilever deflection formula is used. The notation is shown
in Figure 6
and is defined below. The deflection x is given by:
x = SL3 (equation 1)
3EI
where the second moment of area I is given as:
4
I = 64 .C, (equation 2)
The stiffness k can be derived from equation 1 as:
S 3EI
k = - _ (equation 3)
x L3
and this can be expanded with equation 2 to give:
3E;cd 4
k = 64L3 . CI (equation 4)
Rearranging, the rod length can be found as:
5
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L CI (equation 5)
This final equation 5 allows calculation of the rod length L for any given rod
diameter d
and Young's Modulus E, assuming the stiffness, k, is known, whicll has been
defined
above as nominally 700N/m; and a value for CI, which is given below
Where:
I = Actual measured 2"d moment of area (corrected for GRP if appropriate)
(ln)
S = Rod tip load (N)
L = Rod free length from socket exit to ball center (m)
d = Rod diameter (m)
Ci = Coefficient correcting to actual measured I value (C1= 0.83 for pultruded
GRP of glass fraction 70%+; but 1 for homogeneous materials such as
steel)
x = Rod tip deflection, perpendicular to undeflected centreline (m)
E = Young's Modulous (for GRP rods=41 Gpa)
k = "Spring stiffness" for cantilever typically 700N/m
As an example equation 5 is plotted in Figure 7 for the range of rod
stiffnesses, being
greater than 1200 N/m to up to 3000 N/m, but nominally 2000N/m. This is for
glass
reinforced pultruded rods. Other materials need another plot to define the
length to
diameter. Figure 7 shows the combinations of rod length and rod diameter that
will give
performance for a trampoline edge in accordance with the invention. With the
specified
initial deflection x (nominally 220mm) these coinbinations will at the same
time also
give optimum performance to the trampoline mat as well.
Thus the figure shows the rod combinations required to achieve the two key
functions of
mat tension and edge stiffness. These two functions can be achieved with
different
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materials in the same way as shown in the example, but using the appropriate
Young's
Modulous, E, and coefficient, C1.
The foregoing describes the invention including preferred forms thereof.
Alterations
and modifications as will be obvious to those skilled in the art are intended
to be
incorporated within the scope hereof as defined in the accompanying claims.
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