[0001] This invention relates to a frame of a racket for tennis, badminton etc., and particularly
to those made of metals.
[0002] Generally, rackets with metal tubular frames are constructed in two forms, oval-shaped
tubular structure and I-beam with two tubular ends. As racket frames are principally
designed for strength and stiffness, such metal frames usually cannot possess a minimum
desired weight due to the characteristic of the materials. Additionally, when striking
the ball, shock and vibration will be transmitted to the hand of the player, especially
when the stroke point is not at the centre of the racket. Therefore the player will
easily feel fatigued and sometimes even get hurt. The preferred form of metal racket
frame to be employed in this invention is oval-shaped tubular structure. It is proposed
by the inventor after he made a test for rigidity of the above two structures.
[0003] Figure 1 is a diagrammatic illustration with data of tests for rigidity in respect
of the above two forms of frame, both of which have similar dimensions A, B, C and
D as shown. During the tests, the handle portions thereof are supported fixedly and
a load of 15 pounds is hung at the other free end of the frame. It is seen that the
oval shaped tubular structure is bent such that the free end thereof deflects to a
position having a distance of 20.2 mm from the original position, the cross-sections
being 152.4 mm and 304.8 mm distant from the free end deflect to a position 9.9 mm
and 5.6 mm distant from the original position respectively. Similarly, the I-beam
frame makes a deflection that the free end, the cross-sections being at 152.4 mm and
304.8 mm distant from the free end move to the positions 23.92 mm, 11.2 mm and 6.11
mm distant from the original positions.
[0004] From these data it is seen that the oval-shaped tubular frame has higher strength
and rigidity than the other one. The object of the invention is further to improve
the characteristics of the frame.
[0005] A racket frame according to this invention comprises an elongated metal tubular member
of hollow oval cross-section bent to form a head portion and a handle portion, characterised
in that the oval section metal tubular member includes an elongated rigid foamed core
filled therein. In this way the wall thickness of the metal may be reduced, thereby
reducing the weight of the frame. Furthermore,- the foamed core reduces the vibration
induced upon stroke. Advantageously, the oval section tubular member is made of aluminium
alloy and the foamed core is formed of a foamable polyurethane based composition.
[0006] This and other objects, features and advantages of the present invention will be
more apparent in the following description of a preferred embodiment with reference
to the accompanying drawings, in which:-
Figure 1 is a diagrammatic illustration of prior arts showing some experimental data
with respect to tests on rigidity for two kinds of frame;
Figure 2 is a perspective view of a frame constructed according to the present invention;
Figure 3 is a sectional view taken along the plane A-A of Figure '2; and
Figure 4 is a diagram comparing the vibrational condition of the frame according to
the present invention and the oval section frame of the prior art.
[0007] As shown in Figure 2, a racket frame 10 of the present invention comprises a thin,
elongated metal tube of oval cross-section bent to form a head portion 11 and a handle
portion 12 of the frame. This metal tube is made of aluminium alloy and the inside
thereof is provided with a rigid foamed core 15 as shown in Figure 3. This rigid foamed
core is formed by inserting a foamable polyurethane based composition into the tube,
heating to a certain temperature, and allowing it to expand and cure. The formed frame
10 has an elongated metal shell 13 and a rigid foamed core 15 to reduce the weight
of the frame. Thus, the wall thickness of an aluminium alloy frame may be reduced
to from 0.70 to 1.05 mm, compared to the 1.1 mm wall thickness shown in Figure 1 as
being commonly used in prior art frames. Due to the presence of the foamed core 15
the metal tubular frame 10 is also improved in that the vibration characteristic thereof
becomes less when comparing with the metal frame without foamed core. Figures 4(b)
and (a) are the diagrammatic representations of the vibration of the frame 10 constructed
according to the invention and the frame without foamed core.
[0008] With the invention thus explained, it is apparent that obvious modifications and
variations can be made without departing from the scope of the invention. It is therefore
intended that the invention be limited only as indicated in the appended claims.
1. A racket frame comprising, an elongated metal tubular member of hollow,oval cross-section
bent to form a head portion and a handle portion, characterised in that the oval section
tubular member includes an elongated rigid foamed core filled therein.
2. A racket frame as claimed in Claim 1, wherein said metal tubular member is made
of aluminium alloy.
3. A racket frame as claimed in claim 1 or claim 2, wherein the wall thickness of
the metal tubular member is from 0.70 to 1.05 mm.
4. A racket frame as claimed in any one of the preceding claims, wherein said rigid
foamed core is formed of a foamable polyurethane based composition.
