Patent Abstract:
A cutting head for a string trimmer is provided which accommodates any string gauge thickness and may be easily and quickly re-threaded upon failure of a string previously disposed therein. The cutting head has spring-biased clamping members which are each mounted within a rotatable clamp housing within the head, such that, as a string encounters a solid object, the clamping member and its associated housing are rotated about a generally vertical axis so as to obviate or minimize the development of a stress concentration in the string in the vicinity at which it is engaged by the clamp housing.

Full Description:
This application claims priority from U.S. Provisional Application No. 60/562,844, filed Apr. 16, 2004, which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   This invention is directed to garden string trimmers and, more particularly, to a cutting head for string trimmers which can be easily re-strung, and which is designed to minimize breakage of the strings when encountering solid objects. 
   String trimmers are most often used to cut vegetation along a border of a flower bed or plot of grass which is adjacent to a sidewalk, driveway or other solid structure. Since the cutting head rotates at a speed that makes the strings nearly imperceptible, an operator of the string trimmer is not always able to see that the strings may come into contact with a hard surface and react accordingly. As a result, strings on a string trimmer are commonly broken. Prior art designs of string trimmer cutting heads have attempted to ease the re-stringing process which may be quite time consuming. 
   One type of prior art cutter head design is generally known as a weave-type head. The latter usually includes an array of grooves and cut-outs in the hub of the cutting head which act to anchor a string therein. An example of this type of cutter head design is disclosed in U.S. Pat. No. 4,190,954 to Walto entitled “Cutting Head” which issued Mar. 4, 1980. This type of cutting head has several drawbacks. First, the grooves and cut-outs include a number of sharp corners which, in time, fray, weaken and break the string disposed therein. Second, a relatively thin string can only be used due to the numerous twists and turns the string must pass through to anchor the string to the cutter head. Third, disassembly of the weave-type cutting head is required to re-string the cutter head with subsequent bending and manipulation of the strings to conform with the grooves and cut-outs formed therein. 
   A second type of string trimmer cutting head design is generally known as a tap-and-go head. The latter includes a spool of string enclosed within the cutting head, where the string is paid out through peripheral apertures formed in the circumference of the cutting head as required, when the head is tapped against the ground. An example of this type of string trimmer cutting head design is disclosed in U.S. Pat. No. 3,708,967 to Geist, et al. entitled “Rotary Cutting Assembly” which issued on Jan. 9, 1973. This cutting head design also has several drawbacks. A failure near an aperture may cause the end of the string to retract within the cutting head, thus requiring disassembly of the cutter head and re-threading of the string through the aperture. Also, the operator of the string trimmer must carry the weight of an entire spool, including about twenty feet of string, during the course of operation, which for a commercial gardener may be a substantial time and even an entire work day, resulting in a strenuous effort. 
   The shortcomings of the prior art string trimmer cutting head designs have been overcome by the inventor of the subject invention in a new and improved head for string trimmer which is disclosed in the subject inventor&#39;s prior U.S. Pat. No. 5,758,424 which issued on Aug. 23, 1996 and U.S. Pat. No. 5,896,666 which issued on Apr. 27, 1999, both of which patents are entitled “Head for String Trimmer”. 
   In applicant&#39;s above-mentioned U.S. patents, a cutting head for a string trimmer is provided which accommodates discrete lengths of string of any gauge thickness and which may be easily and quickly re-threaded upon failure of a string previously disposed therein. Spring-biased clamping members are provided to clamp the strings within the cutter head, with the clamping force being provided by the springs and possibly supplemented by centrifugally generated force moments. The teachings of applicant&#39;s U.S. Pat. Nos. 5,758,424 and 5,896,666 are incorporated in their entirety herein by reference. 
   As illustrated in FIGS. 1-3 of U.S. Pat. No. 5,896,666, applicant&#39;s cutting head design includes a substantially cylindrical body having a disc-shaped base plate formed to define a central drive shaft aperture and a surrounding side wall. A plurality of slots are formed in the side wall with a radially inward extending pressing wall forming one edge of each of the slots. A spring-biased clamping member is pivotally mounted adjacent each aperture, opposite the corresponding pressing wall so that its center of gravity is disposed between the pivotal mounting and the corresponding pressing wall. 
   The spring-biased clamping members of U.S. Pat. No. 5,896,666 are adapted and formed to generate two degrees of clamping force in cooperation with the corresponding pressing walls. Each spring is provided to generate one degree of clamping force, wherein the clamping force is sufficient to grippingly engage and maintain strings within the cutting head. A second supplemental clamping force is generated with the clamping head being in use in that the rotation of the cutting head creates centrifugal force that acts on the center of gravity of the clamping members and enhances the gripping force thereof. 
   The spring-biased clamping force of the cutter head of U.S. Pat. No. 5,896,666 is overcome by a string being forcibly introduced from a location outside the cutting head, through the aperture and between the clamping member and the pressing wall, and into the central volume of the head. During operation, the clamping members are rigidly locked into a clamping position from which the supplemental clamping force is generated. Since the discrete strings are threaded through the apertures from a location outside the cutting head, no time-consuming disassembly of the cutting head is required to re-string the cutting head upon failure of a string. An operator of the cutting device, as disclosed in U.S. Pat. Nos. 5,758,424 and 5,896,666, may carry a bundle of strings pre-cut to a pre-determined length which can be easily threaded into the cutting head as needed. 
   Notwithstanding the outstanding results and efficiencies obtained using the cutting head of U.S. Pat. Nos. 5,758,424 and 5,896,666, depending on the physical characteristics of the string used with the cutting head, stress concentrations may develop at the point where the string is captured within the cutting head (i.e., where the spring-biased clamping member engages the string) as the string contacts a solid object. Repeated stress concentrations could result in the premature breaking of the string. 
   It is an object of the subject invention to provide a new and improved cutter head which includes means for minimizing or obviating the development of stress concentrations in the vicinity at which the clamping member engages the string when the string contacts a solid object. 
   It is also an object of this invention to provide a string trimmer cutter head which can be easily and quickly strung with any gauge string of discrete length. 
   It is yet another object of this invention to provide a lightweight string trimmer cutter head which contains an amount of string needed only for operation. 
   It is a further object of this invention to provide a string trimmer cutter head which can be re-strung without disassembly or bending and twisting of the string within the cutting head. 
   SUMMARY OF THE INVENTION 
   The above-stated objects are met by a new and improved string trimmer cutting head which can be easily and quickly re-strung and is capable of using thin, as well as thick, gauge string. 
   The subject invention achieves the above objectives by providing, as in the case of applicant&#39;s prior U.S. Pat. Nos. 5,758,424 and 5,896,666, a cutting head with spring-biased clamping members which are each mounted within a rotatable housing within the head, such that, as the string encounters a solid object, the clamping member and its associated rotatable housing are rotated about a generally vertical axis so as to obviate or minimize the development of a stress concentration in the string in the vicinity at which it is engaged by the clamping member. The spring-biased clamping members may be pivotally or slidably mounted within the associated rotatable housings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of the cutting head of the subject invention with four cutting lines inserted into the cutting head; 
       FIG. 2  is a top plan view of the cutting head of  FIG. 1 ; 
       FIG. 3  is a side elevational view of the cutting head and strings of  FIG. 1 ; 
       FIG. 4  is a cross-sectional view taken along lines  4 - 4  in  FIG. 3 ; 
       FIG. 5  is a perspective view of a rotatable housing which forms a part of the cutting head of the subject invention; 
       FIG. 6  is a perspective view of the rotatable housing of  FIG. 5  taken from the opposite side thereof; 
       FIG. 7  is a top plan view of the rotatable housing of  FIG. 5 ; 
       FIG. 8  is a side elevational view of the rotatable housing of  FIG. 5 ; 
       FIG. 9  is a front elevational view of the rotatable housing of  FIG. 5 ; 
       FIG. 10  is a cross-sectional view of the rotatable housing taken along lines  10 - 10  in  FIG. 9 ; 
       FIG. 11  is a front elevational view of a second embodiment of the rotatable housing of the subject invention; and 
       FIG. 12  is a cross-sectional view of the second embodiment of the rotatable housing of the subject invention taken along lines  12 - 12  in  FIG. 11 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1-4 , the cutter head of the subject invention is generally designated by the numeral  10 , and is shown with cutting lines or strings  12  extending from the cutter head. Cutting string  12  usually is made of a plastic material, while the cutter head  10  may be made of metal or plastic. 
   As shown in  FIGS. 1 and 2 , each string  12  is of a discrete length, on the order of eight inches, and is generally diamond-shaped in cross-section and includes ridges  14  extending along the length thereof. Each diamond-shaped string  12  is mounted within the cutter head  10  so that a ridged edge  14  of the string engages the vegetation to be trimmed and effectively saws the vegetation. Alternatively, the string  12  may be of circular cross-section and may be various diameters, depending upon the type of vegetation to be cut. 
   As shown in  FIGS. 1-3 , the cutter head  10  includes upper cover  16 , lower cover  18 , and a plurality of posts  20  which are fixedly connected to and maintain the spacing between the upper and lower covers  16 ,  18 . 
   Each discrete length of string  12  is selectively fixedly connected to the cutter head  10  by a rotatable clamp housing  22 . As illustrated in  FIG. 1 , each rotatable clamping structure  22  is uniformly spaced about the periphery of the cutting head  10 , intermediate the posts  20 . 
   Each rotatable clamp housing  22  is rotatable about an axis, designated “a”, extending perpendicular to or generally vertical to the upper and lower covers  16 ,  18 . By virtue of the rotation of the clamp housings  22 , should a string  12  contact a rigid structure during the operation of the cutting head, the respective clamp housing  22  will rotate about axis “a”, thereby obviating or minimizing the development of a stress concentration in the cutting string adjacent the connection of the cutting string  12  to the respective clamp housing  22 . When the cutting head  10  is rotating at operational speed, the centrifugal force action on each string  12  will maintain the string in the positions as shown in  FIGS. 1 and 2 , until such time as the string encounters a rigid object, at which time the clamp housing  22  will rotate. 
   As more fully described hereinafter, disposed within each rotatable clamp housing  22  is a spring-biased clamping member of the type disclosed in applicant&#39;s U.S. Pat. Nos. 5,758,424 and 5,896,666. Each spring-biased clamping member disposed within a clamp housing  22  is adapted and formed to generate two degrees of clamping force. First, the cam of the clamping member is spring-biased to generate a clamping force which is sufficient to grippingly engage and maintain the string  12  within the rotatable clamp housing  22 . Depending on the design of the cam of the clamping member, a supplemental clamping force may be generated when the cutting head is rotated at operational speed. More particularly, rotation of the cutting head  10  may create a centrifugal force that acts on the center of gravity of the respective cam of the clamping member and thus enhances the gripping force of the clamping member on the string. The cam structure within the clamping member may be pivotally mounted or slidably mounted within the clamp housing  22 , as more fully described hereinafter. 
     FIGS. 5-10  illustrate various views of a rotatable clamp housing  22 , with  FIG. 10  being a cross-sectional view of the rotatable clamp housing  22 . The latter includes a generally cylindrically shaped housing  24 , the lower end of which includes a circular support portion  26  of smaller diameter than the cylindrically shaped housing  24 . Extending from the upper end of the cylindrically shaped housing  24  is a curved tang  28  which provides a stop for limiting the amount of angular rotation of the rotatable clamp housing  22  about the axis “a” within the cutter head  10 . 
   At the front portion of each rotatable clamp housing  22  is a protrusion  30  including a diamond-shaped opening  32  leading to a passageway  34  which extends completely through the housing  24  for receiving a string  12 . 
   As illustrated in  FIG. 10 , the passageway  34  extends through the housing  24  to the rear opening  36  in order to allow the through passage of the string  12 . 
   Referring to  FIGS. 7 and 10 , disposed within each rotatable clamp housing  22  is a clamping member  40  of the type disclosed in applicant&#39;s U.S. Pat. Nos. 5,758,424 and 5,896,666, with the cam  42  of said clamping member  40  being spring-biased by torsion spring  44  and being disposed within the rotatable clamp housing  22 . As shown in  FIG. 10 , the pivot axis  46  of the cam  42  is orthogonal to the rotatable axis “a” of the rotatable clamp housing  22 . As shown in  FIGS. 7 and 10 , the spring  44  is of the torsion type, although various other types of springs may be utilized in order to provide the spring-biased force of the cam  42  against the string  12 . 
   During operation of the cutter head, as the cutter head  10  is rotating at high speed, the spring-biased cam  42  clamps the cutting line  12  to the cutter head  10 . 
   As illustrated in  FIG. 4 , each rotatable clamp housing  22  is mounted within the cutting head  10  with the circular support portion  26  being rotatably supported within circular opening  50  provided in the lower cover  18 . The curved tang  28  of each rotatable clamp housing  22  is accommodated in a curved recess  52  within the upper cover  16 , with the curved recess  52  extending about 180 degrees, thus enabling the rotatable clamp housing  22  to be rotated 90 degrees about axis “a” in either direction from the neutral position as shown in  FIG. 1 . 
   As in the case of applicant&#39;s trimmer head as disclosed in U.S. Pat. Nos. 5,758,424 and 5,896,666, the passageway  34  extends completely through the cylindrically shaped housing  24 . Hence, should a string  12  be broken during operation of the cutter head  10 , it can be readily pushed radially inwardly and through rear opening  36  to the central volume  60  within the cutter head  10  in order to extract the string  12  from the cutter head, and enable a new string  12  to be loaded into the trimmer head via opening  32 . 
   The volume  60  which is radially inward of the rotatable clamp housings  22  enable the latter to be rotated through a range of 180 degrees, without the string  12  contacting the inner surface of the cutting head  10 . 
   As illustrated in  FIG. 4 , the upper cover  16  includes drive shaft aperture  70  for receiving the drive shaft of a motor (not shown) for powering the cutter head. 
   During operation of the cutter head  10 , a string  12  is inserted through opening  32  and through the passageway  34  and out of the rear opening  36 . At such time, one end of the string  12  is engaged by the spring-biased clamping member  40  and is maintained in that position during rotation of the cutting head. The distal end of the string  12  which extends out of the rear opening  36  of the rotatable clamp housing  22  is disposed in the central volume  60  of the rotatable clamp housing  22 . Hence, if a string  12  breaks during operation of the trimmer head, the operator turns off the machine and pulls on the radially inner distal end of the string  12  through volume  60  in order to remove the broken string preparatory to insertion of a new string  12  through the opening  32  in the protrusion  30 . 
   As illustrated in  FIGS. 4 and 10 , spring-biased cam  42  is pivotally mounted about axis  46  that extends generally parallel to the planes of the upper and lower covers  16 ,  18 . Cam  42  includes teeth  48  for gripping the cutting line  12 , and for preventing the cutting line or string  12  from being pulled out of the rotatable clamping structure  22  through the diamond-shaped opening  32 . The cam  42  is spring-biased and cooperates with the opposing pressing wall  34 A (see  FIG. 10 ) within the passageway  34  of the rotatable clamp housing  22  to clamp the string  12  within the clamp housing  22 . Preferably, each cam  42  is pivotally mounted about generally horizontal axis  46  such that the center of gravity of the cam  42  is disposed between the pivot axis  46  and the pressing wall  34 A. The construction and operation of cams  42  generally correspond to the operation of the cam structure in applicant&#39;s prior U.S. Pat. Nos. 5,758,424 and 5,896,666. 
   In operation, if a cutting string  12  should contact a rigid object, the rotatable clamp housing  22  would react by rotating about the circular support portion  26  within the circular opening  50 , and the tang  28  would slide within the curved recessed opening  52  in the upper cover  16 , with the rotation of the rotatable clamp housing  22  thereby minimizing the development of a stress concentration in or breaking of the string  12  in the region where the string engages the rotatable clamp housing  22 . 
   Should a cutting line  12  be broken, it is merely necessary for the user to turn off the trimmer and rotate the rotatable clamp housing  22  in order to pull the broken string through the rear opening  36  and out through the volume  60 . 
     FIGS. 11 and 12  illustrate an alternate embodiment of a clamping member disposed within the rotatable clamp housing  22 . Instead of being pivotally mounted, the cam  80  is slidably mounted in a guide structure  82  and is biased by a compression spring  84  into engagement with the string  12 . 
   Although the cutting head illustrated in  FIGS. 1-10  includes four cutting strings  12 , the subject cutting head may also be operated with only two diametrically opposed cutting strings. 
   As is readily apparent, numerous modifications and changes may readily occur to those skilled in the art and, hence, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modification equivalents may be resorted to falling within the scope of the invention as claimed.

Technology Classification (CPC): 0