Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02246104 1998-08-27
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The invention r_ela.tes to a device for damping the vibrations
of a ball racket and, in particular, a tennis racket, squash
racket or racketba=_1 racket.
Conventional devices for damping the vibrations of a tennis
racket are based on the absorption of energy by friction or
internal friction of materials arranged between the grip shell
of a racket handle and the frame members of the racket frame.
In order to be able to absorb an appreciable amount of energy,
relatively heavy ~~tructural parts are usually required. This
holds all the more if the weight of the racket is relatively
low such that the damping elements to be provided for the
absorption of ener~~y will attain a considerable portion of the
overall weight of the racket. Sinr_e such damping structural
parts, moreover, 'usually are arranged in the region of the
handle grip, '.hi~~ will result ir~ a grip-heaviness of the
racket. In order t:o produce head-heavy rackets, compensation
weights, therefore, have to be arranged in the head region,
which again leads to an increase in the overall weight of the
racket.
Other conventional. damping elements are arranged between the
strings of a stringing. In this connection, a damper has
become known f=rorr. US-A 5,651,545; in which a viscoelastic
carrier for a movable element is mounted between strings of a
stringing. As the racket gets into contact with a ball, what
constitutes a transient procedure, the racket vibrates at its
eigenfrequency, wherein the racket, if exhibiting a relatively
low mass, is excited to vibrate at its eigenfrequency, being
damped to a ~~_esser degree. The eigenfrequency of modern
rackets having relatively low weights and, in particular,
weights ranging between 200 and 250 g varies between
frequencies of about 180 to 280 Hz depending on the racket
frame material chosen, that eigenfrequency corresponding to
the first mode of bending under free-free constraint. Typical
of such a free-free vibration is the fact that a maximum
vibration amplitude is each present on the head-side and grip-
CA 02246104 1998-08-27
side ends as well as in a central region. The area of the
strung surface, like the end region of the grip facing the
strung surface, is located in a node of vibration. The
arrangement of darnping elements in the stringing area always
will bring about relatively little effect and hence relatively
slight damping if such an element is arranged near a node of
vibration. Moreover, additional superpositions will be caused
in the stringing area. Besides, thc= effect of a damping means
provided in the stringing area also is a function of the
hardness of tr,e stringing and of the strings chosen for the
stringing. The effects of damping, therefore, cannot be
precisely predicted in such cases. Due to the mass vibrating
between the strings in a viscoelastic element, a two-mass
system having an. equal number of degrees of freedom is
involved, ratios changing to a high degree as a function of
the weight of t_he strings and the material of the strings.
The invention aim: at providing effective damping for rackets
having low dead weights, by as low an additional weight as
possible and appropriate tuning of damping irrespective of the
strings chosen for the stringing and of the weight of the
stringing selecaed..
To solve this ox>ject, the configuration according to the
invention essentially consists in that a damper weight is
mounted on or in t:he grip-side end of the ball racket so as to
be able to elastically excurse from the axis of the handle
grip, its weight being selected between 0.6 to 3.5 % of the
weight of the st_=ung racket and the elastic mounting being
designed or dimensioned with a view to an at least monoaxial
excursion at ~~n eigenfrequency of between 100 and 300 Hz. By
providing but an extremely low weight, the dimensioning of
which is to be se=~ected between O.E> and 3.5 ~ of the weight of
the strung racket, the characteristics of the racket will
hardly change and it will be ensured, in particular, that a
head-heavy racket can be produced without any additional
weights in the head region at an extremely low overall weight.
CA 02246104 1998-08-27
By mounting that r<~lat.ively low damper weight elastically in a
manner so as to obtain an at least monoaxial excursion at an
eigenfrequency of between 100 and 300 Hz and by arranging the
relatively low damper weight on the grip-side end of the ball
racket, it is fea~;ible to almost cancel vibrations by inter-
ference without having to markedly increase the overall weight
of the ball r~~cket for that purpose. The relatively light
racket, after excitation by the ball contact, reacts nearly
undamped with its eigenfrequency, the damper reacting to the
same impact with its eigenfrequenc5~. The extent of damping of
the vibration of tape damper weight in the elastic mounting has
hardly any influence on the effect of damping, since the two
vibrations, i.e., the vibration of the racket and the
vibration of the damper weight are linked with each other. To
the player only the vibrations of the racket will be decisive,
and, if the eigenfrequency of the damper is tuned to the
eigenfrequency of the racket with a view to safeguarding an
appropriate am~~litude of the dampen weight, effective damping
with the slighte;~t masses on the smallest space will be
feasible. The arrangement in the grip-side end of the ball
racket ensures that the damper weight will always be located
in the antinode of the eigenfrequency of the racket.
Advantageously, the configuration according to the invention
is devised such that the grip-side damper weight is mounted so
as to be capable of excursing in a biaxially elastic manner
and elastically pivotable about the axis of the handle grip,
such a biaxia~_ elastic excursion, as a rule, also enabling
damping of vibrations about the axis of the handle grip on
account of the inertia of the mass of the damper weight. V~lith
an eccentric excitation of the racket frame, the damper weight
at first remains in its original position on grounds of the
inertia of the mars and subsequently is excited to rotational
vibration by r_he elastic deformation of the mounting, said
rotational vibration acting against the vibration of the
racket frame about the longitudinal axis of the handle grip.
CA 02246104 1998-08-27
Advantageously, the damper weight amounts to about 3 and 10 g
and, preferably, about 5 g. Taking into account the initially
mentioned racket vaeights of between 180 and 250 g of modern
tennis rackets and the slightly lower weights of squash
rackets, an ext_=emely small damper weight mass will,
therefore, suffice.
In a preferred man:zer, the arrangement of the damper weight is
realized in a hollow space on the end of the grip, the damper
weight preferably being arranged in an elastic carrier which
is supported on the wall of the hollow space in at least two
points located diametrically oppos_~te each other with respect
to the axis of the handle grip. Such an elastic carrier simply
may be made of an elastomer material such as, in particular,
silicone rubber, wherein the carrier, in the manner of a
carrier of uniform strength, has ~ cross section increasing
from the damper weight towards the walls of the hollow space,
viewed in the axial direction of the handle grip. Thus, an
extremely smal=L structural component is required also for the
elastic carrier, which structural component, in turn,
contributes to the overall mass of the racket to as slight an
extent as possible. In order to ensure that any vibration will
actually be compensated for in the region of the antinode of
the free-free vibration of the racket, the configuration
advantageously is devised such that the carrier and the damper
weight extend in the axial direction of the handle grip over
less than 1/10 of the length of the handle grip, whereby
further substantial weight reduction is feasible as compared
to conventional constructions.
Appropriate tuning of the amplitudes of the damper relative to
the amplitude of vhe racket is advantageous with a view to a
most effectivt~ absorption of vibrations and the pertinent
reduction of the :>ame. The eigenfrequency of the vibration of
the damper weight is a function of its stiffness and mass. The
respective physic~~l formula for that eigenfrequency f - ~c/m
is applicable universally, c being a measure for the
CA 02246104 1998-08-27
stiffness, and m denoting the mass, of the damper weight. The
stiffness of the elastic carrier may be influenced by
compression as well as the mode of constraint of the carrier
and its shape. As a rule, the stiffness of the damper is
decisively influenced by the flexural strength of the
elastomer. Measure::nent of the eigenfrequency of the damper is
relatively difficult from a measuring engineering point of
view, since measurements must be carried out by means of
extremely sma~_1 acceleration probes. Good approximation,
however, results from the determination of the eigenfrequency
by calculation knowing the material for the elastomer and the
shape of the carrier as well as 'she mass of the vibration
damper.
In an advantageous manner, the invention thus contemplates
that the eigenfrequency of the vibration of the damper weight,
by selecting the stiffness and/or compression of the carrier
and by selecting the mass of the weight, is adjusted to a
frequency corresponding to the frequency of the free-free
vibration of the first mode of the ball racket. Dimensioning
of the eigenfrequency of the vibration of the damper weight in
such a manner results in a nearly immediate extinction of the
vibrations of the racket, slight differences in the
frequencies still causing extremely rapid damping. In order to
provide for optimum damping at a low weight, the arrangement
according to the invention in that case is devised such that
the damper weight .and, in particular, the center of gravity of
the damper weight is arranged on the grip end in the region of
the antipode and, in particular, of the plane which extends
normal to the axi;~ of the handle grip and through which the
antipode of the free-free vibration of the first mode of the
ball racket passes.
In order to cancel vibrations effecvtively, appropriate tuning
and, in particular, enlargement of the amplitude is
particularly advantageous in addition to the measure of tuning
the eigenfrequency of the damper tc~ the eigenfrequency of the
CA 02246104 1998-08-27
racket. The damper amplitude that is required for damping is
proportional to the ratio of the ;stiffness of the racket to
the stiffness of t:~e damper. The lighter the racket the larger
the amplitude; that must be admitted for the damper, the
design advantageously being devised such that the vibration
amplitude of the ~3amper weight proportional to the ratio of
the stiffness of the racket to thn~ stiffness of the carrier
with a lower racket weight is chosen to be larger than with a
higher racket weight.
A further measare characteristic of the effect of the damper
is the socal=-ed enlargement function. This enlargement
function is delwined as the amplitude of th.e racket divided by
the excitation amplitude. If that ratio between the eigen-
frequency of tie damper and the eigenfrequency of the racket
assumes values smaller than l, particularly intensive damping
of the overall system is reached. Consequently, the
configuration advantageously is devised such that the ratio
between the amplitude of the ball racket and the excitation
amplitude in the range between the eigenfrequency of the
damper weight and the eigenfrequency of the ball racket is
chosen to be small.=_r than 1.
Particularly effective damping at _i.ow weights may be achieved
in that the weight of the mounting of the damping element
amounts to a maximum of 30 o and, preferably, 25 ~ of the
damper weight. In this manner, the influence of the mounting
on the vibration. behavior of the damper weight may be
minimized and particularly weight-saving and effective damping
by interferencE, ma:y be realized.
In the follow.:Lng, the invention ,gill be explained in more
detail by way of exemplary embodiments schematically
illustrated in the drawing. Therein, Fig. :L is a top view on a
racket; Fig. 2 is a side view in tine direction of arrow II of
Fig. 1 with ~.he schematic illustration of the free-free
vibration in the first. mode; Fig. 3 is a view in the direction
CA 02246104 1998-08-27
10
of arrow III of Fig. 1, on the bottom grip end; and Fig. 4 is
a view analogous to Fig. 3, illustrating the schematic
arrangement o3- a damper weight suspended in a biaxially
elastic manner.
Fig. 1 depicts a tennis racket 1 whose stringing oval is
denoted by 2. The tennis racket 1 has a handle grip 3 whose
axial height is dE>_noted by a. The axis of the handle grip or
tennis racket is d<~noted by 4.
On the lower e:zd of the grip 3 a damper 5 is arranged, whose
axial height b extend;> over less than 1/10 of the axial length
a of the grip ~ort:ion 3.
In the illustration according to Fia. 2, the free-free
vibration of the first mode is indicated by 6. The nodes of
vibration 7 of that free-free vibration of the first mode pass
throught the st:rir..ging oval 2 and the grip portion 3, whereas
the maximum amplitudes c in the zegion of the antinode are
each present on the head-side end and on the grip-side end of
the ball racket 1 as well as closely below the stringing oval
2 in the chosen illustration.
From Fig. 3, the damping device 5 is apparent in a view
according to arrcw III of Fig. 1. The damping device 5 is
comprised of a c,~rrier and a damper weight 8, the damper
weight 8 being .retained between carriers 9 for elastic
excursion. Excita'ion of the racket frame by a ball contact
causes the damper weight 8 to make an elastic excursion, the
way of clamping represented in Fig. 3 allowing for a
multiaxial exc-arsion. An excursion may, thus, take place both
in the direction of double arrow 10 and also transverse
thereto in the direction of double arrow 11, the respective
elastic torsional vibration of the mass of the damper weight 8
being likewise ac~mit:ted whenever eccentric impacts on the
stringing oval stimulate the rac:vket frame to torsionally
vibrate.
CA 02246104 1998-08-27
_. g _
The type of elasti~~ suspension is schematically illustrated in
Fig. 4, wherein a biaxial suspE:nsion via springs 12 is
provided. By selecting suitable m<~terials for the carrier 9
and the springs 12, respectively, and selecting suitable
biassing, the eigenfrequency of tree damping device 5 may be
tuned to the free-free eigenfrequency of the vibration of the
racket frame after a ball contact, thereby obtaining effective
damping.
