DETAILED DESCRIPTION OF THE INVENTION

[0018] FIG. 1 is a preferred embodiment of the sport bat 10, such as a softball or baseball bat 11, shown in longitudinal sectional view An alternative embodiment of baseball bat 11, also shown in longitudinal sectional view, is shown in FIG. 2.

[0019] Baseball bat 11 generally includes an elongate body 20 with a longitudinal axis 21. Body 20 includes a handle 45 for gripping, a batting portion 30, and taper portion 44 between handle 45 and batting portion 30. Batting portion 30 is cylindrical and is for striking a sports ball.

[0020] It is well known that striking, or hitting, a ball with a bat typically causes vibration in the bat. The vibration can be severe enough to sting the hands of the person batting It is also well known that some hits do not result in vibration being felt in the hands of the batter Such hits also tend to result in the ball traveling especially far and fast. There are various hypotheses of what causes this phenomenon; one especially credible explanation is the “acoustic” theory of Rod Cross as cited by Daniel Russell.

[0021] The acoustic explanation involves the fact that a typical ball bat has three main vibrational bending modes that are activated when as ball is struck Cross defines the zone between the [farthest from the handle] nodes of the first and second modes of vibration as the “sweet zone”, analogous to what is traditionally called the “sweet spot”, a point on the bat that yields strong hits without vibration of the hands. Herein this zone will be called sweet spot 12 Sweet spot 12 can be empirically located by suspending the handle of the bat from the fingers and tapping the length of batting portion 30 until the minimum of vibration is felt in the fingers. Russell's more analytical method is to suspend a bat with an attached accelerometer and strike the bat at many different points. Modal analysis is done on the accelerometer data to determine the shapes and frequencies of the various vibrational modes.

[0022] Batting portion 30 includes one or more internal chambers 31 that decrease the weight of bat 10, shift the center of mass of bat 10, and influence the location and width of sweet spot 12 The embodiment of FIG. 1 includes one internal chamber 31 and the embodiment of FIG. 2 includes three internal chambers 31. It has been found that for baseball bat 11, one or more internal chambers 31 with total volume of two to six cubic inches will reduce the weight of baseball bat 11 to within the desired range.

[0023] It is preferred that internal chambers 31 be located between taper portion 44 and sweet spot 12. Sweet spot 12 may be located empirically, as discussed above, but in general, it has been found that sweet spot 12 is five inches or less from distal end 40 of body 20.

[0024] FIG. 3 shows the laid-up billet 53 from which the bat of FIG. 1 will be cut FIG. 4 is a transverse cross-sectional view of billet 53 of FIG. 3, taken along line 4-4 Dashed line 10′ indicates the outline of bat 10 that will be lathe cut from billet 53.

[0025] Billet 53 is formed from sticks 52 of bamboo or other suitable wood. Sticks 52 are dipped into an adhesive 55, such as epoxy, and stacked into the size and shape of billet 53 required. Sticks 52 may be added to the assembly in a freeform manner or may be laid into a mold, or by other techniques as are well known to those skilled in the art

[0026] Bat 10 is preferably made from laminated wood 50, such as laminated bamboo strips 51. Laminated bamboo 51 is a strong, durable material, but denser than most woods used for traditional baseball bats. To decrease the weight of bat 10, internal chamber 31 must have a density less than that of laminated wood 50 or laminated bamboo 51

[0027] In the preferred embodiment of FIG. 1, internal chamber 31 is a simple void in laminated bamboo 51, having a wall 36 enclosing internal space 35, which is filled with air. Wall 36 includes an upper end 37 and a lower end 38. In the alternative embodiment of FIG. 3, internal chamber 31 includes sleeve insert 32 to reinforce internal chamber 31 against collapse Sleeve insert 32 may be of metal, graphite, or plastic Although metal is denser than laminated bamboo 51, the combination of metal sleeve insert 32 and the air of internal space 35 has a density less than that of laminated bamboo 51.

[0028] Sleeve insert 32 may be circular, square, rectangular, or other shape in cross section. A sleeve insert 32 that is square or rectangular in cross section fits most snugly into a chamber 31 that is formed by omitting certain sticks 52 from the billet. A sleeve insert 32 that is circular or other shape in cross section should have sticks 52 that are tangent to three or more segments on the perimeter of sleeve insert 32 such that sleeve insert 32 does not rattle or shift within internal chamber 31 Some small voids will likely be present between chamber wall 36 and some parts of a sleeve insert 32 that is non-rectangular in cross section, but this is not detrimental to the performance of bat 10.

[0029] Sleeve insert 32 is shown as a hollow pipe in the figures, but sleeve insert 32 could alternatively be a solid piece of graphite, plastic, or foam. The main requirement for sleeve insert 32 is that the density of the material used be low enough to decrease the weight of baseball bat 11 to the desired total weight and balance. It is also desirable that sleeve insert 32 absorb or dampen shock and vibration.

[0030] Internal chamber 31 may alternatively include a rubbery coating 34 on chamber wall 36, as shown in FIGS. 3 and 4 Rubbery coating 34 is easily introduced during lay-up of billet 53 by dipping one side of each of sticks 52 that enclose internal chamber 31.

[0031] In the preferred embodiment shown in FIG. 2, multiple internal chambers 31 are filled with a porous filler 33, such as an elastomeric foam, such as polyurethane foam, or a fibrous material, such as jute. Porous filler 33 must have a lower density than laminated wood 50 or laminated bamboo 51. It is desirable that porous filler 33 absorb or dampen shock and vibration.

[0032] Although particular embodiments of the invention have been illustrated and described, various changes may be made in the form, composition, construction, and arrangement of the parts herein without sacrificing, any of its advantages. Therefore, it is to be understood that all matter herein is to be interpreted as illustrative and not in any limiting sense, and it is intended to cover in the appended claims such modifications as come within the true spirit and scope of the invention.