CROSS REFERENCE TO RELATED APPLICATION [0001] The present application claims priority under 35 U.S. C. §119(e) to U.S. Provisional Patent Application No. 60/588,434, filed July 16, 2004.

BACKGROUND OF THE INVENTION Field of Invention [0002] This invention relates to portable tools, and more particularly, to a device for centering a flexible drive cable within a shaft boom of the tool.

Description of Related Art [0003] Hand-held string trimmers are well known in the art and are equipped with either a straight or curved shaft boom. Typically, the shaft boom supports a trimmer head at one end and a drive motor at the other end. It is known in the art to support the drive shaft connecting the power source with the trimmer head within the shaft boom using at least one centering liner. For this purpose, the centering liner conventionally included radial supporting ribs which extend in the longitudinal direction of the shaft boom. [0004] One problem with conventional centering liners is that tolerance differences with the interior diameter of the shaft boom or the outside diameter of the centering liner often lead to an unsatisfactory fit. If the centering liner is undersized, then the supporting ribs are not in radial contact engagement with the shaft boom, so that the centering liner defines a clearance space with the shaft boom, and the centering liner can be subjected to radial oscillations therein. On the other hand, if the centering liner is oversized, forces develop which are applied to the supporting ribs. These forces are directed radially inwardly when the centering liner is inserted into the shaft boom and can deform the centering liner and severely limit its journaling characteristic. [0005] Additionally, some disadvantages occur even when a centering liner is seated in the shaft boom with a precise fit. Oscillations and vibrations produced by the drive shaft are transmitted directly to the shaft boom because of the considerable radial stiffness of the supporting ribs of the centering liner. These oscillations and vibrations are disturbing to the operator. Additionally, an amplified radial force is applied to the centering liner because of the ovalization of the curved shaft boom in the region thereof which is bent, causing the centering liner to become deformed. [0006] Accordingly, it would be desirable to have an improved centering liner that enables a better fit within the shaft boom, absorbs the oscillations caused by the drive shaft and exhibits suitable life.

SUMMARY OF THE INVENTION [0007] The invention provides a flexible centering liner for a portable handheld tool such as a string trimmer which includes a work tool assembly such as a trimmer head and a power source connected by a drive shaft and wherein the drive shaft is held within a tubular shaft boom with the centering liner. The centering liner includes a central sleeve defining a center cavity for receiving the drive shaft and a plurality of curved support ribs that extend outwardly from the central sleeve having their distal ends against an inner wall surface of the shaft boom, wherein the cross section of the ribs tapers towards their distal ends, hi one embodiment, the centering liner has a plurality of ribs that extend substantially perpendicular from an outer surface of the central sleeve and then curve such that a body section of the rib is substantially parallel to a tangent of the central sleeve drawn through at a point on the outer surface of the central sleeve radially inward from said body section. [0008] In another embodiment, the invention is directed to a portable handheld tool having a working mechanism powered by a power source, a tubular shaft boom extending between the working mechanism and the power source, and a drive shaft positioned within the tubular shaft boom mechanically connecting the power source with the working mechanism. The drive shaft is held within the tubular shaft boom with a centering liner. The centering liner includes a central sleeve defining a center cavity for receiving the drive shaft and a plurality of curved support ribs that extend outwardly from the central sleeve having their distal ends against an inner wall surface of the shaft boom, wherein the cross section of the ribs tapers towards their distal ends. In one embodiment, the working mechanism of the tool is a trimmer head and the power source is one of an internal combustion engine and an electric motor. [0009] These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.

BRIEF DESCRIPTION OF THE DRAWINGS [0010] The above mentioned and other features of this invention will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: [0011] Figure 1 illustrates a perspective view of a string trimmer having a shaft boom; and [0012] Figure 2 illustrates a cutaway view of a centering liner within the shaft boom according to one embodiment of the invention. [0013] Corresponding reference characters indicate corresponding parts throughout the views of the drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS [0014] The invention will now be described in the following detailed description with reference to the drawings, wherein preferred embodiments are described in detail to enable practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. But to the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description. [0015] Referring now to the drawings, FIG. 1 shows a portable string trimmer 10 essentially comprising an elongated protective tubular shaft boom 12 having a forward end on which a rotating trimmer head 14, broadly a working mechanism, is mounted. The trimmer head 14 is driven by a power source 16 positioned at the opposite end of the shaft boom 12. The power source 16 of the trimmer 10 can be either a gasoline engine or an electric motor, both of which are well known in the art and need not be discussed in further detail. The trimmer head 14 permits the trimmer 10 to be used for trimming and cutting vegetation. A handle bar 18 is attached to the shaft boom 12 between the attachment of the power source 16 and the trimmer head 14. The operator guides the string trimmer 10 with the handle bar 18. [0016] The protective tubular shaft boom 12 houses a rotatable flexible drive shaft 20 (FIG. 2) that extends throughout the shaft boom 12 and operatively connects the power source 16 to the trimmer head 14 to provide the necessary rotating motion to the trimmer head 14. Flexible drive shafts in and of themselves are known in the art and need not be described in detail. However, means for securing the flexible drive shaft 20 within the shaft boom 12 will be described below. Although the illustrated embodiment is a handheld tool configured to be used as a string trimmer 10, the invention is not so limited, but may also include other tools using flexible drive shafts housed in a shaft boom. [0017] Turning now to FIG. 2, the flexible drive shaft 20 is arranged within the shaft boom 12 and is held in its central position within the shaft boom 12 by means of a centering liner 24. In one preferred embodiment, the centering liner 24 extends over substantially the entire length of the shaft boom 12. However, the centering liner can comprise several short centering liner segments spaced one from the other along the length of a shaft boom 12 without departing from the scope of the invention. The centering liner 24 preferably comprises a plastic extrusion made of Nylon 6/6, although similar materials can be used without departing from the scope of the invention. [0018] The centering liner 24 is inserted into the inside diameter of the shaft boom 12 and has a central sleeve 26 to provide a circular cavity 28 within which the drive shaft 20 is captured and rotated in. The centering liner 24 includes outwardly extending supporting ribs 30 which extend along the longitudinal length of the shaft boom 12 and lie with their distal ends 32 against an inner wall surface 33 of the shaft boom 12. The plurality of ribs 30 extend from the central sleeve 26 to the inside surface 33 of the boom tube, centering the circular shaft cavity 28 substantially in the center of the shaft boom 12. [0019] According to invention, the supporting ribs 30 extend outward from the central sleeve 26 in a curved manner. In one illustrated embodiment, the ribs 30 extend substantially perpendicular from the outer surface of the central sleeve 26 and then curve approximately 90 degrees or more. Desirably, a body section 34 of the rib 30 in at least one point is substantially parallel to a tangent of the central sleeve 26 drawn through at the point on the outer surface of the central sleeve 26 radially inward from the body section 34 when the centering liner 24 is inserted into the shaft boom 12. Preferably the central sleeve 26 and the ribs 30 are extruded as a single body. In one desirable embodiment, the liner 24 has four ribs 30. However, other number of ribs 30, such as three and five, may be used without departing from the scope of the invention. Providing a liner 24 with four ribs 30 allows for easy inspection of the distance across the ribs (i.e., the outside diameter of liner). [0020] In one embodiment, the cross section of the ribs 30 tapers or becomes thinner towards the outer tips 32 thereof. Desirably, the cross-sectional area of the ribs 30 at the outer end 32 thereof is between 40% and 90% the cross-sectional area of the rib 30 at the end adjacent the central sleeve 26, more preferably between about 50% and 80%, and even more preferably between about 60% and 80%. The rate at which the taper occurs controls the flexibility characteristics of the ribs 30 and maintains a desired level of rigidity towards the central sleeve 26 and flexibility towards the outer ends 32 of the ribs 30. [0021] Preferably, the curved shape of the rib 30 and the taper work together to increase structural strength toward the central sleeve 26 of the liner 24 while reducing stress points that may allow the liner 24 to fatigue over time. It also improves control for keeping the center drive shaft cavity 28 centered, especially in when the centering liner 24 encapsulated in a curved section of a shaft boom 12. [0022] Desirably, the mounting of the centering liner 24 within the shaft boom 12 is adapted with respect to tolerances in order to prevent damaging deformations of the centering liner 24 and to prevent the centering liner 24 from vibrating within the shaft boom 12. The supporting ribs 30 are configured so as to be resilient in the radial direction so that they provide a tolerance compensation as well as attenuate vibration. The spring force provided by the ribs 30 desirably provides a sufficient force on the inside wall 34 of the shaft boom 12 so as to hold the liner 24 in the desired location within the shaft boom 12 so that it is not necessary to require a separate retainer to prevent motion lengthwise in the shaft boom 12 unlike many conventional liners. [0023] While this invention has been described in conjunction with the specific embodiments described above, it is evident that many alternatives, combinations, modifications and variations are apparent to those skilled in the art. Accordingly, the preferred embodiments of this invention, as set forth above are intended to be illustrative only, and not in a limiting sense. Various changes can be made without departing from the spirit and scope of this invention. [0024] What is claimed is: