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'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'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'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'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.