Generally, in a filament trimmer, to accomplish rotation of the filament, whereby it may cut grass or weeds, one end of the filament is attached to a rotary means while the other end, together with a selection portion of the filament, projects and rotates freely beyond the assembly. An output shaft, usually a hollow-tubular member, attaches to the rotary means and provides a conduit for the filament through the assembly. Generally, the output shaft outer portion is configurated to support or reinforce the gyration or rotation of the filament. For example, in U.S. Pat. No. 3,928,911 a hollow bell-shaped head is attached to the shaft at the trimming end. A groove within the bell mouth is designed to capture the rotating filament and cause its rotation. However, in practical demonstrations this has proved to be impractical, if not impossible. Typically, the bell mouth head begins its rotation independently of the rotation of the filament, and there is no opportunity for the groove in the bell mouth to capture the filament. Indeed, to ensure proper operation, it is necessary to hand enclose the filament in the bell mouth groove before the device is put into operation. Furthermore, because the filament rotates or twists, the filament will tend to rotate out of the groove. Once out of the groove, the filament tends to orient itself axially of the rotating shaft.
This axial alignment is the result of the high speed rotation of the rotary means wherein the upper edge projection of the bell mouth groove drives the filament to the center line of rotation. Since there is no radial force to cause the filament to initially lie in the groove, there is no means for it to catch in the groove for diverting the end into a planar sweep.
Other trimmer devices include a rotary head wherein the filament is a part of that head and rotates therewith. See, for instance, U.S. Pat. Nos. 3,664,102; 3,693,255; 3,708,967; 3,826,068; and 3,831,278. Such devices are known to have a tendency to split the ends of the filament since the exposed sweeping end is oriented always in the same plane; and it is recommended that the split end be trimmed to increase the efficiency of cutting.
In the present application the filament is twisting and untwisting as it sweeps in a planar fashion and presents an ever changing surface to the vegetation to be cut. This results in an even wear surface and a tendency to become pointed which increases the efficiency of cutting.