A filament type trimmer for cutting vegetation such as grass and weeds and particularly for trimming along walks, fences and flower beds and around trees comprises a motor driven rotary head comprising a housing carrying one or more spools of synthetic monofilament an end portion of which extends out through a guide provided on the head. As the head rotates at high speed, the projecting end portion extends out from the head by centrifugal force and serves as a cutting "blade".
The spool of filament is rotatably mounted on the housing so as to supply additional filament as the projecting end portion of the filament is worn or broken off. However, as the pull on the filament resulting from centrifugal force acting on the projecting end portion of the filament tends to rotate the spool relative to the housing to unwind the filament, means must be provided for holding the spool against rotation relative to the housing except when it is desired to feed more filament. Thus it has been proposed to provide means for locking the filament spool against rotation. When it is desired to supply more filament, the trimmer is stopped and held in inverted position so that the spool can be manually released and rotated to unwind the desired amount of filament from the spool.
To avoid the need of stopping the trimmer to adjust the filament length, it has been proposed to supply filament from a stationary coil which is concentric with the rotary head. The coil is fed through clamping jaws which are releasable by means of a manually operable release rod while the trimmer is running so that additional filament is pulled out from the coil by centrifugal force acting on the projecting end poriton of the filament. However, with this arrangement it is not possible to tell how much filament is being fed out.
To overcome this disadvantage it has been proposed to release the filament by increments in the manner that only a predetermined amount of filament is released each time the trigger is pulled. However, the operation still depends on the judgement of the operator as to when additional filament is required.