Abstract:
A novel abrasive cutting string for use in a string trimmer provides for a thermoplastic string with abrasives in the surface, but not in the central core of the string, the string being wound on a spool for rotation and incremental extension of at least one extremity in combination with the operation of the string trimmer.

Description:
[0001]    The present application is a continuation of U.S. Ser. No. 11/006,024, filed Dec. 7, 2004 which in turn claims priority of U.S. Provisional Application No. 60/527,348 filed Dec. 8, 2003, which is incorporated herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to an improved cutting string, and more particularly, to an improved cutting string for cutting grass, weeds and the like when mounted on a spool in combination with a rotary string trimmer. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known to use extruded nylon thermoplastic string such as of nylon to cut grass, weeds and the like in combination with a rotary string trimmer. Conventional thermoplastic strings, however, suffer from several disadvantages. For example, conventional strings have a substantially smooth exterior surface. More particularly, the exterior surface of conventional strings is devoid of any abrasive or aggregate materials which produce cutting surfaces. Instead, conventional strings rely on the rotational velocity and impacting force of the string to cut weeds, grass and the like. This, however, is an inefficient way to cut grass, weeds and the like. In addition, the smooth surface of conventional strings is devoid of any air voids which cool the string when it is rotated. As a result, conventional strings are known to melt and fuse in the head of a rotary string trimmer under normal or high temperature conditions. This results in costly and time-consuming repairs and requires frequent replacement of conventional strings. 
         [0004]    Further, the cross-sectional shape of conventional strings is circular. Consequently, conventional strings do not have any edges or raised profiles which improve cutting efficiency, and instead they rely on the rotational velocity and impacting force of the string to cut grass, weeds and the like. As noted above, this is an inefficient way to cut grass, weeds and the like. Still further, as a result of the high rotational velocity and composition of conventional strings, they are susceptible to damage and breaking, and thus they have a limited lifespan. Finally, because conventional strings are inefficient (e.g., they rely upon the rotational velocity and force of impact of the string rather than any cutting surfaces on the string), they impart undue stress upon the rotary string trimmer with which they are used. 
         [0005]    Some attempts have been made to improve upon conventional circular strings used to cut grass, weeds and the like in combination with a rotary string trimmer. For example, cutting strings have been extruded in different cross-sectional shapes. More particularly, the circular cross-sectional shape of conventional strings has been modified to a cross-sectional shape having one or more edges or raised profiles such as an octagon. This modification of conventional strings, however, also suffers from many of the same disadvantages as conventional strings of circular cross-section. Like conventional strings having a circular cross-sectional shape, conventional strings having a cross-sectional shape with raised profiles or edges have a generally smooth exterior surface. Consequently, conventional strings having a cross-sectional shape with raised profiles or edges still rely substantially on the rotational velocity and impacting force of the string to cut, and the result is an inefficient way to cut grass, weeds and the like. 
         [0006]    Further, like conventional smooth surface circular strings, conventional smooth surface strings having a cross-sectional shape with raised profiles or edges are susceptible to melting and fusion in the head of the rotary string trimmer under normal or high temperature conditions. Again, this results in costly and time-consuming repairs and frequent replacement of the string. Still further, like conventional strings having a smooth, circular cross-section, conventional strings having a smooth, cross-sectional shape with raised profiles or edges have a limited lifespan due to the composition of the string and the high rotational velocity of the string. Finally, because conventional strings having edges or raised profiles are inefficient for cutting purposes inasmuch as they rely upon the rotational velocity and force of impact of the string rather than cutting surfaces on the string, they impart undue stress on the rotary string trimmer with which they are used. 
         [0007]    In an effort to overcome these shortcomings, some attempts have been made to mix abrasive particles with thermoplastic resins to produce string. These attempts generally are very taxing on string extrusion equipment, tend to leave most of the abrasive particles buried in the string, and have abrasive particles even in the core of the string. The result is a relatively weak or breakable string without substantial external abrasive particle cutting surfaces. Alternatively, gluing abrasives to the string also results in difficulties in manufacturing and string performance. 
         [0008]    It would be desirable, therefore, if an improved cutting string could be provided for cutting grass, weeds and the like when mounted upon a spool in combination with a rotary string trimmer. It would also be desirable if such an improved cutting string could be provided that would resist melting and fusion under normal or high temperature conditions. It would be further desirable if such an improved cutting string could be provided that would be less time-consuming and less expensive to maintain, repair and replace. It would be still further desirable if such an improved cutting string could be provided that would have a longer lifespan. It would also be desirable if such an improved cutting string could be provided that would reduce the stress imparted upon the rotary string trimmer with which it is used. It would be further desirable if a simple and inexpensive method for making such an improved cutting string could be provided. 
         [0009]    Accordingly, it is an advantage of the invention described herein to provide an improved cutting string for cutting grass, weeds and the like when mounted upon a spool in combination with a rotary string trimmer. It is also an advantage of the invention to provide an improved cutting string that resists melting and fusion under normal or high temperature conditions. It is a further advantage of the invention to provide an improved cutting string that is less time-consuming and less expensive to maintain, repair and replace. It is a still further advantage of the invention to provide an improved cutting string with a longer lifespan. It is another advantage of the invention to provide an improved cutting string that reduces the stress imparted upon the rotary string trimmer with which it is used. 
         [0010]    It is a yet another advantage of the invention to provide a simple and inexpensive method for making such an improved cutting string. 
         [0011]    Additional advantages of the invention will become apparent from an examination of the drawings and the ensuing description. 
       SUMMARY OF THE INVENTION 
       [0012]    The invention comprises a cutting string for cutting grass, weeds and the like for mounting upon a spool for incremental unwinding in combination with a rotary string trimmer. The preferred cutting string is composed of a plastomeric material and comprises abrasives which are embedded on the string. The abrasives are preferably embedded on the cutting string by heating the abrasive and applying the abrasive with pressure onto the exterior surface of the string. Also in a preferred embodiment, the cutting string is exposed to an acidic substance that softens the string such that the abrasives may be embedded on the string. 
         [0013]    A method is provided for making the improved cutting string. More particularly, the preferred method comprises the steps of extruding a plastomeric cutting string, exposing the cutting string to a heated abrasive such as sand, and cooling the cutting string and abrasive combination. In a preferred method, the extruded cutting string is exposed to an acidic substance adapted to soften the cutting string before the cutting string is exposed to the abrasive. 
         [0014]    In another preferred embodiment, the extruded cutting string may be heated before it is exposed to the abrasive. 
         [0015]    In order to facilitate an understanding of the invention, the preferred embodiments of the invention are illustrated in the drawings, and a detailed description thereof follows. It is not intended, however, that the invention be limited to the particular embodiments described or to use in connection with the apparatus illustrated herein. Various modifications and alternative embodiments such as would ordinarily occur to one skilled in the art to which the invention relates are also contemplated and included within the scope of the invention described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The presently preferred embodiments of the invention are illustrated in the accompanying drawings, in which like reference numerals represent like parts throughout, and in which: 
           [0017]      FIG. 1  is a partial cross-sectional view of a preferred embodiment of the cutting string in accordance with the present invention. 
           [0018]      FIG. 1A  is an enlarged view of the circled portion of the preferred cutting string illustrated in  FIG. 1 . 
           [0019]      FIG. 2  is a schematic diagram of a preferred embodiment of the method for making the preferred cutting string in accordance with the invention. 
           [0020]      FIG. 3  is a schematic diagram of an alternative embodiment of the method for making the preferred cutting string in accordance with the invention. 
           [0021]      FIG. 4  is a sectional view of a prior art string trimmer spool mounted in a cutting head. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0022]    Referring now to the drawings, an exemplary apparatus and methods for making the apparatus of the invention are illustrated by  FIGS. 1 through 4 . As shown in  FIG. 1 , the cutting string is designated generally by reference numeral  10 . The cutting string  10  is composed of a plastomeric material such as nylon that is extruded to have a substantially circular cross-sectional shape. It is contemplated within the scope of the invention, however, that cutting string  10  may be composed of any suitable material that may be formed into a string-like configuration. Further, it is contemplated within the scope, of the invention that cutting string  10  may be formed to have any suitable cross-sectional shape such as oval, crescent, semi-circular, triangular, square, rectangular, pentagonal, hexagonal, octagonal or any other polygonal shape. It is further contemplated within the scope of the invention that cutting string  10  may be produced by a process other than extrusion such as molding or any other suitable process for producing string. 
         [0023]    Still referring to  FIG. 1 , cutting string  10  includes a plurality of abrasives  12 . Abrasives  12  are preferably composed of very fine foundry sand or some other similar material such as very fine aggregates and the like. Abrasives  12  are attached to cutting string  10 . Preferably, abrasives  12  are partially embedded on the exterior surface of cutting string  10 . Also in the illustrated embodiment, abrasives  12  cover the substantially entire exterior surface of cutting string  10 . Abrasives  12  are adapted to produce a non-smooth surface on the exterior of the cutting string. As shown in  FIG. 1A , the non-smooth surface produced by abrasives  12  produces a plurality of cutting surfaces  14 . The plurality of cutting surfaces improves the cutting efficiency of the cutting string as compared to conventional smooth surface strings. 
         [0024]    As a result of cutting surfaces  14 , cutting string  10  need not be rotated at the same high velocity as conventional smooth surface strings in order to cut the same grass, weeds and the like. Further, cutting string  10  having cutting surfaces  14  reduces the amount of stress imparted upon the rotary string trimmer with which it is used because of its improved cutting efficiency. Still further, the abrasives on cutting string  10  permit the string to cut through larger, thicker and denser objects than conventional smooth surface strings. In experiments conducted to date, it has been found that preferred cutting string  10  with abrasives  12  is capable of cutting objects up to ten times larger than conventional smooth surface strings and up to forty percent (40%) faster than conventional smooth surface strings. 
         [0025]    Abrasives  12  produce another advantage. As shown in  FIG. 1A , the abrasives produce a non-smooth surface which results in a plurality of air voids  16  in the trimmer head coil. Air voids  16  cool cutting string  10  as it rotates, thereby reducing the temperature of the cutting string and inhibiting melting and fusion of the cutting string. 
         [0026]    Referring now to  FIG. 2 , a method of making cutting string  10  is illustrated. More particularly, as shown in  FIG. 2 , the method of making cutting string  10  includes the steps of feeding nylon pellets into hopper  22 , which are melted and extruded in the form of cutting string  10  by extruder  24 . As discussed above, nylon or any other suitable plastomeric material may be used to produce the cutting string. Further, as discussed above, cutting string  10  may be extruded by extruder  24  or any other suitable method or device for producing a cutting string. 
         [0027]    Still referring to  FIG. 2 , the method also includes the step of exposing the cutting string to an abrasive material such as very fine foundry sand. According to this method, the cutting string is placed into super heated sand bath  26  where the sand is heated, and the heated sand is applied to the cutting string under pressure using sand covered rollers  28 . The heating of the string is sufficient to soften the external surface layer, but not so intense as to compromise the strength of the core of the string. As discussed above, the abrasive may be very fine foundry sand or any other suitable fine aggregate material that will produce a non-smooth surface, including a plurality of cutting surfaces and air voids, around the exterior surface of the cutting string. It is further contemplated within the scope of the invention that the abrasives may be applied to the cutting string by any suitable means for attaching the abrasives to the cutting string such as adhesives and the like. 
         [0028]    Still referring to  FIG. 2 , the method also includes the steps of cooling the cutting string in cooling bath  30  and winding the cutting string onto storage reel  32  for storage. Cooling bath  30  preferably contains water or any other suitable coolant, but it is also contemplated that the string may also be cooled by means other than a cooling bath such as cooled air, liquid coolant sprayed on the string, ice or some other solid form of coolant, and the like. It is also contemplated within the scope of the invention that the cutting string may be stored in any suitable manner using any suitable storage device. 
         [0029]    Referring now to  FIG. 3 , an alternative embodiment of the method of the invention is illustrated. More particularly, according to the alternative embodiment, after the extruding step described above, the cutting string is stretched by stretching rollers  34 . It is contemplated within the scope of the invention, however, that the string may be stretched by any suitable device for stretching string. The stretched cutting string is then exposed to water or some other suitable coolant in cooling bath  36  in order to cool the string. It is contemplated within the scope of the invention, however, that the string may cooled by any suitable means such as cooled air, a sprayed liquid coolant, ice or the like or by using any suitable cooling device. 
         [0030]    Still referring to  FIG. 3 , the cutting string is then exposed to an acidic substance such as hydrochloric acid (HCl), muriatic acid or the like in softening bath  38  to soften the external layer of the string. While as described the cutting string is placed into softening bath  38 , it is contemplated within the scope of the invention that the acidic substance may be applied to the cutting string in any suitable manner by any suitable device. The softening is focused on the external layer of the string, and the string core is not compromised. The softened cutting string is then exposed to an abrasive material as described above. Next, the cutting string is exposed to water or some other rinsing agent adapted to substantially remove the acidic substance from the cutting string. While the string is preferably placed into rinsing bath  40  for the purpose of substantially removing the acidic substance from the string, it is contemplated within the scope of the invention that the acidic substance may be substantially removed from the cutting string by any suitable rinsing agent and by using any suitable rinsing device. 
         [0031]    Finally, the cutting string is wound onto storage reel  32  for storage. It is contemplated within the scope of the invention that the cutting string may be stored in any suitable manner using any suitable storage device. 
         [0032]    As shown in  FIG. 4 , a typical prior art rotary cutting assembly with cutting string feed is normally mounted on and driven by a suitable drive unit indicated generally at  110 . The drive unit may be an electric or gasoline motor, the housing of which is coupled through a sleeve member  112  to a suitable support handle  115  for positioning the cutting assembly in the cutting of weeds. A suitable annular bumper  120  extends from the base of the housing and a drive shaft  122  extends therefrom to mount the rotary cutting assembly which is indicated generally at  125 . The details of the drive assembly and the support or handle structure may take varying forms. 
         [0033]    The rotary cutting assembly is comprised of a driven member having first and second plate members  130  and  132  respectively. Plate member  130  has a cylindrical hub  134  projecting from one surface thereof through which the drive shaft  122  extends. This surface of the plate member advantageously has a plurality of extending radial fins which, upon rotation, will provide cooling for the assembly. The plate member  130  has a second cylindrical hub portion  140  extending from the opposite surface with a cylindrical aperture therethrough concentric with the aperture of the hub portion  134  and receiving the extremity of the shaft  122 . In addition, plate member  130 , has a pair of elliptical apertures  42  therein and a plurality of posts  45  extending from the surface of the plate member opposite the radial fins. The plate member  130  is mounted on the drive unit  110  through the shaft  122  thereof. The shaft extends through the hub  134  and into hub  140  in a recessed portion  48  therein wherein a threaded extremity  49  of the drive shaft is located. A retainer flange  50  having an internal threaded surface or threaded recess is positioned in the recessed portion  48  of the hub  140  and threaded onto the threaded extremity of the shaft with a collar portion  52  bearing against the end of the hub  140  to retain the hub and the plate member  130  on the end of the shaft. 
         [0034]    The hub  140  has an outer peripheral surface with an annular flange portion adjacent the plate member  130  which portion, as will be later noted, serves as a guide surface. The lower extremity of the hub is recessed from this guide surface and a plurality of triangular shaped teeth members  60  and  62  are distributed about the periphery of the same. Teeth members  60  are distributed along a common plane adjacent the annular flange guide surface and have a height equal to the external dimension or surface of the guide surface of the hub. These teeth or ratchet means are angularly spaced about the peripheral surface of the hub and the number of teeth may be varied, for purposes of determining the amount of cutting string to be incrementally fed from the storage spool within the driven member. A second set of teeth  62  are disposed in a plane parallel to and spaced from the plane of the teeth  60  and angularly distributed so that they are positioned radially in between the teeth  60  and equidistantly spaced having the same angular spacing peripherally on the hub. 
         [0035]    The outer plate member  132 , has a pair of post members  70  projecting therefrom with notched surface  72  in the extremity of the same. Plate member  132  has a central aperture  75  therein with an external dished surface  76  leading to the same. The posts  45  of plate member  130  have wear sleeves positioned over the same which act as spacers to space the plate member  130 ,  132  apart with the posts  70  having the notched extremities  72  extending through the apertures  42  in the plate member  130  and locking the same to the plate member  130 . The plate member  132  around the post members  70  is slightly deformable such that the notched surfaces may be moved into and out of the apertures  42  for assembly and release of the plate members. These plate members of the driven member define a spaced area therebetween in which there is positioned the spool  80  which mounts the cutting string  85  of the rotary cutting assembly. Although not specifically shown, one or more strands of cutting string may be wound on the spool and secured to the hub of the same with the opposite extremity or extremities extending from the spool to provide the cutting surface conventional with a rotary cutting assembly of this type. Where two or more such lines are used, they will be wound in the same direction, but coming out through the posts  45  equidistantly spaced such that upon a predetermined relative rotation of the spool to the driven member, the lines will be unwound from the spool and extend out of the driven member. Except for the posts  45  and  70  the area between the plates  130  and  132  is open to expel dirt from the housing through centrifugal force. The plate  132  extends beyond the posts  45  and  70  to form a lip  79  which prevents the line from shearing off near the posts. 
         [0036]    As will be seen in  FIG. 4  the spool  80  is formed in a conventional manner with a pair of spaced sides  82  held together by a common hub portion and in the illustrated embodiment, has a hemispherically shaped hub member  90  formed integral therewith and projecting from one side of the spool. The interior of the hub portion of the spool has a plurality of projecting triangular teeth members  92  distributed about the inner periphery of the same. The interior of the hub has a recessed shoulder portion  95  which fits over the end of the hub  140  with the retaining flange  50  thereon to be guided thereon for longitudinal movement of the spool in an axial direction relative to the hub  140 . Positioned between the interior of the hub  90  and the retainer flange  50  is a spring member  100  which fits into a recessed surface  51  in the retaining flange  50  and into a recessed surface  101  in the top of the hub  90 . The spring is a compression spring which will bias the hub and hence, the spool  80  formed integral therewith, relative to the hub  140  such that the surface of the spool will bear against the plate  132 . However, when the hub  90  is engaged by a surface, such as by depressing the hub against a solid surface, the spring  100  will be compressed allowing the hub  90  with the spool  80  integral therewith to slide on the hub  140  through the guiding surfaces on the respective hubs to permit the teeth  92  to disengage from the row of teeth  62  on the hub  140 . With the driven member rotating, a relative rotation will take place between the spool  80  and the hub  140  or the parts of the driven member allowing the spool to rotate until the flat surfaces of the teeth  92  next to adjacent teeth  60  are engaged. Once the pressure on the spring is released by withdrawal of the hub  90  from the solid surface, the compression of the spring will move the spool  80  back towards the plate member  132  disengaging the teeth  92  from the teeth  60  and allowing them to engage with the teeth  62  on the hub  140 . This will allow another step of relative rotation between the spool and the driven member, the purpose of which is to unwind an increment of cutting string from the spool and allow the free extremity of the cutting string to extend out of the confines of the driven member between the posts  45  or  70  to provide a longer cutting surface for the rotary cutting assembly. This will replace any portion of the cutting string which has been broken off due to wear, fatigue, or impact with solid objects in the cutting operation. 
         [0037]    Whenever it is desired to disassemble the driven member for replacement of the spool, the posts  70  will be deflected, releasing the notched surfaces  72  from the sides of the apertures  42  in the plate member  130  and permitting separation and removal of the plate member  132  therefrom. Thereafter, the spool  80  and the hub  90  integral therewith will be lifted off of the hub  140  and new cutting line may be mounted thereon or a replacement unit with cutting string thereon may be inserted in its place. Thus, no separate or special tools will be required for assembly or disassembly of the driven member for replacement of the spool or the addition of cutting string thereon. The posts  45  and  70  also serve as a supporting surface for the free end of the line from the spool which is extended by centrifugal force upon rotation of the head directing the cutting string out from the driven member between the posts in a conventional manner. Where the extended portion of a cutting string is broken upon impact or fatigue, the user of the rotary cutting assembly while rotating need only press the entire assembly against a surface causing the hub through its contact with the surface to overcome the compression of the spring  100  and move the spool assembly out of engagement with one set of ratchet teeth and into engagement with the other. Upon lifting or releasing of the rotary cutting assembly away from the surface, the spring will return the spool assembly within the driven member and against the plate member  132  allowing another increment of rotary movement between the spool assembly and driven member to complete the advance or automatic feed of an increment of cutting string from the spool through the action of centrifugal force on the line rotating the spool relative to the housing 
         [0038]    In operation, several advantages of the apparatus and method of the invention are achieved. The abrasives of the preferred cutting string produce a plurality of cutting surfaces and air voids about the exterior surface of the string. The non-smooth exterior surface of the preferred cutting string results in several advantages. For example, the plurality of cutting surfaces produced by the abrasives of the preferred cutting string improves the cutting efficiency of the string. More particularly, the cutting surfaces of the preferred cutting string contribute to the ability of the string to cut grass, weeds and the like. Unlike conventional smooth surface cutting strings which rely almost solely upon the rotational velocity and force of impact of the string to cut grass, weeds and the like, the preferred cutting string of the invention relies on the cutting surfaces produced by the abrasives as well as the rotational velocity and impacting force of the cutting string to cut grass, weeds and the like. The result of the plurality of cutting surfaces on the preferred cutting string is that the cutting efficiency of the preferred cutting string is increased, the rotational velocity of the preferred cutting string need not be as high as the rotational velocity of a conventional smooth surface cutting string in order to cut the same grass, weeds or the like, the amount of stress imparted upon the rotary string trimmer with which the cutting string is used is decreased, the preferred cutting string cuts grass, weeds and the like up to 40% faster than conventional smooth surface strings, and the preferred cutting string is capable of cutting grass, weeds and the like that are up to ten times larger than the grass, weeds and the like that conventional smooth surface cutting strings are capable of cutting. 
         [0039]    The plurality of air voids produced by the abrasives of the preferred cutting string also produce several advantages. For example, the air voids of the preferred cutting string have a cooling effect upon the string as it is rotated such as by a rotary string trimmer. The cooling effect of the air voids reduces the likelihood that the preferred cutting string will melt or fuse under high temperature conditions. Consequently, repairs and replacements of the preferred cutting string are reduced as compared to conventional smooth surface cutting strings. As a result, the preferred cutting string is less expensive and less time-consuming to repair and requires less frequent replacement. 
         [0040]    The preferred cutting string of the invention will also have a longer lifespan than the conventional cutting string. As discussed above, the preferred cutting string may be exposed to an acidic substance during the manufacturing process. While the acidic substance is used to soften the preferred cutting string for application of the abrasive material, it is believed that the acidic substance will also produce a stronger, more resilient cutting string which will withstand greater forces without damage as compared to conventional cutting strings. Consequently, the preferred cutting string will resist breaking better than conventional cutting strings, and it will have a longer lifespan than conventional strings. As a result, the preferred cutting string is less expensive and less time-consuming to maintain, repair and replace than conventional strings. 
         [0041]    The methods of making the preferred cutting string also achieve several advantages. For example, the preferred methods provide a simple and inexpensive way to manufacture the preferred cutting string. 
         [0042]    Although this description contains many specifics, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments thereof, as well as the best mode contemplated by the inventors of carrying out the invention. The invention, as described herein, is susceptible to various modifications and adaptations, as would be understood and appreciated by a person having ordinary skill in the art to which the invention relates.