GB2255264A - Vegetation cutter using a flexible filament - Google Patents

Abstract

In a string trimmer having a so-called "bump feed" mechanism, the filament supply spool 13 and feed mechanism 14 are mounted separately from the cutting head 18 so that the cutting head 18 rotates independently thereof. <IMAGE>

Description

Vegetation Cutter Using a Flexible Filament This invention relates to vegetation cutters and has particular reference to such cutters that employ a length of flexible filament as the cutting element and are intended to be hand-held by a user. The filament is rotated rapidly about an axis that is usually vertical but, for edging, can be horizontal.

Many forms of such cutters incorporate a reel on which the flexible filament is wound and from which the actual cutting length extends. During use, the cutting length becomes worn and ultimately breaks at some point along its length and it is then necessary to withdraw a further length of filament from the reel to restore the cutting length to its original value. The withdrawal of filament from the reel can be effected manually but, in many cases, a mechanism controlled by the user is provided. The mechanism, when actuated by the user, allows a controlled length of filament to be withdrawn from the reel by the action of centrifugal force or by the feeding action of a rotatable reel.

The mechanism may be mounted upon a shaft carrying, at one end, a rotatable head which, in use, rapidly rotates the cutting length, and at the other end, a carrying handle by which the user supports and guides the cutter.

In one form, the cutting length of filament is rotated by means of a spool having a radial passageway through which the filament extends. The spool is rotated at high speed independently of the supply reel, which is rotatable from time to time solely for the purpose of supplying an extra length of filament to the spool. In U.S. Patent No. 4211004, for example, the supply reel is mounted above an electric drive motor for the spool, and has an incremental rotational advance mechanism controlled remotely by way of a cable. Thefilament is fed through the armature shaft of the motor.

This arrangement is cumbersome and the supply reel is difficult to replace.

U.S. Patent No. 4233735 discloses a similar arrangement, but one in which the filament is fed manually by turning pinch rollers gripping the filament between the supply reel and the spool. The motor drive is taken indirectly through a flexible drive shaft, improving the compactness of the head, but at the expense of introducing extra complexity.

European Patent Application No. 145284 discloses an arrangement in which the spool is rotated by an electric motor whose output spindle is offset transversely from the spool axis, and in which the manually-controlled supply reel is mounted close to the carrying handle, the filament being fed down through the shaft to the head. Although this form of command feed" for the filament operates satisfactorily, and the head of the cutter is compact, the supply reel housing and manual feeding mechanism adjacent the cutter handle are relatively expensive to produce, and fitting a fresh filament supply reel involves the extra task of feeding the filament through the cutter shaft.

A different type of cutter uses what has come to be known as a "bump feed" mechanism, as disclosed for example in U.S. Patent No. 4584771. The supply reel itself is rotated at high speed, and there is no separate spool. The supply reel is mounted with its axis vertical and with an outer, convex surface projecting downwardly for "bumping" contact with the ground. Depression of the cutter against the ground causes the supply reel to shift axially upwards, which allows incremental angular advancement of the reel, to feed extra filament. This has the advantage of facilitating supply reel replacement, and allows the head to be compact. However, the rotation of the entire store of filament imposes a significant load on the motor, especially when frequent stopping and starting is necessary.Accordingly, such a mechanism is considered to be unsatisfactory for cordless, battery-powered string trimmers, for example.

In order to satisfy the need for a vegetation cutter capable of being powered by batteries, yet which is relatively compact and economical to manufacture, the invention provides a vegetation cutter having a cutting element in the form of a flexible filament, the cutter comprising a frame supporting a filament supply reel, an independently rotatable spool having a passageway through which filament fed from the supply reel may pass freely and twist about its own axis, and means for rotating the spool such that in use a free end portion of the filament projecting from the cutting head rotates in a cutting plane and tends to pull more filament from the supply reel, and in which the supply reel is shiftable axially relative to the supporting frame, by bumping the cutter on the ground, from an outer position at which supply reel rotation is prevented to an inner position at which supply reel rotation and consequential filament feed through the spool are allowed.

This invention combines some of the advantages of the two types of known cutter described above. The spool can be made very lightweight and well-balanced rotationally, minimising the loads on the drive system and reducing power consumption, and the head can still be relatively compact.

There is no real limit to the storage capacity of the supply reel. Moreover, by separating the elements for spinning and for storing the filament, the axial thrust load borne by the "bump feed" mechanism need not be transmitted to the spool driving means, which may for example be an electric motor whose bearings are subject to wear.

The same inventive concept can be used, however, in a vegetation cutter using a different type of filament feed mechanism, and in another aspect the invention provides a vegetation cutter having a cutting element in the form of a flexible filament, the cutter comprising a frame supporting a filament supply reel, an independently rotatable spool having a passageway through which filament fed from the supply reel may pass freely and twist about its own axis, and means for rotating the spool such that in use a free end portion of the filament projecting from the spool rotates in a cutting plane and tends to pull more filament from the supply reel, in which the supply reel is positioned below the spool at the operative orientation of the cutter with the cutting plane generally horizontal.

This relative positioning of the supply reel and the spool allows the spool rotation means, for example an electric motor, to be housed immediately above the spool, and places no restrictions on either the size of the supply reel or the filament feed control mechanism, which may be of the bump feed type or, for example, of the type responsive to the speed of rotation of the spool to allow a rotation of the supply reel through a predetermined angle whenever the spool decelerates below a threshold speed and accelerates once again to above the threshold speed.

One advantage of having a spool distinct from the supply reel is that the part intended for rotation at high speed, i.e. the spool, may have a low moment of inertia, substantially lower than that of the supply reel. This minimises power consumption, an especially useful feature in cordless string trimmers.

One way in which the invention may be used will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a side elevation, partly in axial section, of the lower half of a cordless bump-feed electric string trimmer, and Figure 2 is a transverse section on the line II-II of Figure 1.

The vegetation cutter 1 illustrated is of the type known as a string trimmer, and uses as the cutting filament a supply of circular-section Nylon (regd. trade mark) thermoplastics flexible string 20. The trimmer has an elongate housing with a handle portion at the top containing a rechargeable battery pack and an on-off switch (these features not being illustrated). Electric cable from the switch is fed down to a permanent magnet D.C. electric motor 21 housed in the enlarged base of the shaft, with its axis generally vertical as shown. An upper guard 11, generally dish-shaped with a down-turned edge flange, surrounds the base of the shaft 10, and is connected to a lower guard and skid 12 for guiding the trimmer along the ground where necessary, and for guarding the rotating string from beneath.

The upper and lower guards 11, 12 together form a housing at the base of the shaft.

The string 20 is wound at 16 on a supply reel 13 which is journalled within a recess in the lower guard 12 for rotation on a boss 22 about a vertical axis off-set transversely from the motor 21 axis. The supply reel 13 is removably retained in its recess by a snap-fitting cap 121, and is biassed downwardly with its lower flange 15 engaging an inner surface of the cap 121 by means of a compression spring 24. The rotation of the spool 13 relative to the lower guard 12 is controlled by an escapement mechanism 14 so that string is fed only in discrete increments corresponding to a quarter turn of the spool.The escapement mechanism 14 comprises a set of four equi-angularly spaced teeth, or axial splines, on the inner surface of the hub 141 of the spool 13, which engage either of two axially-spaced sets 172, 173 of four similar teeth or splines projecting outwardly from a cylindrical boss formed on the underside 17 of the upper guard coaxially with the boss 22. As shown in Figure 1, the spool is locked by the engagement of the spool teeth with the lower set 172 of teeth on the guard 12, 17. Axial displacement of the spool upwardly frees the spool to turn through one-eight of a turn until the upper set 173 of teeth are engaged; the spool turns under the torque derived from tension in the string 20 whilst a spool 18 rotates, as described below.When the spool is then returned under the force of the spring 24 to its initial position, a further rotation of one-eight of a turn is permitted, until the lower set 172 of teeth are re-engaged. The supply reel 13 has a knob 131 which projects axially through the cap 121 to allow it contact the ground beneath the trimmer. Thus bumping the trimmer on the ground causes the supply reel 13 to shift axially and to feed extra string.

The string is fed through an aperture 23 on the axis of the motor 21 and remote from the supply reel 13, into a passageway 19 through a cutting head 18. In this example, the aperture 23 and the mouth of the passageway are in close proximity, and are coaxial. The cutting head 18 is substantially lighter in weight than the supply reel 13, and is preferably winged to minimise its moment of inertia. The cutting head 18 is connected directly and coaxially to the output spindle of the electric motor 21, and is balanced for smooth rotation at high speed.

The passageway 19 is a smooth, circular-section bore with an L-shape, having an axial leg and a radial leg, as shown. The relative diameters of string 20 and passageway 19 are such as to allow the string 20 to pass through freely in order to feed extra string or initially to reload the trimmer with string, and to allow rapid twisting of the string about its own axis as the spool rotates.

The end portion of the string 20 leaves the passageway 19 at the periphery of the cutting head 18 and terminates at a predetermined length, so that when the spool is rotated at high speed a predetermined length is caused to rotate in a horizontal cutting plane. Of this predetermined length, only a portion projects radially beyond the front sector of the upper guard 11, so that it is only the fastest moving portion of the string which is exposed at the front for cutting duty.

The string is limited automatically in length by means of a blade on the edge of the upper guard, to one side of the front sector; the blade is positioned so as to trim the string to its operative length as the string passes at high speed.

In use, the string wears and may fracture and shorten, necessitating an extra length to be fed. This is drawn from the supply reel while the string is rotated, under the centrifugal pull on the end of the string, by bumping the trimmer on the ground to release momentarily the supply reel 13 for rotation.

As an alternative, the rotation of the supply reel 13 may be controlled by an automatic system responsive to the length of filament or to the speed of rotation of the spool; or a partially automatic system which allows incremental rotation each time the trimmer is switched off and on again with a consequential reduction and increase in string tension or of spool speed below and then above a predetermined threshold level.

In this case, the operative orientation of the cutter may be changed, for example to move the cutting plane to a vertical direction to cut edges of lawns. For this purpose, there may be a joint in the shaft to allow the lower portion of the cutter to be swivelled, e.g. through 1800, relative to the handle such that the cutting plane is appropriately tilted.

Claims (10)

Claims

1. A vegetation cutter having a cutting element in the form of a flexible filament, the cutter comprising a frame supporting a filament supply reel, an independently rotatable spool having a passageway through which filament fed from the supply reel may pass freely and twist about its own axis, and means for rotating the spool such that in use a free end portion of the filament projecting from the spool rotates in a cutting plane and tends to pull more filament from the supply reel, and in which the supply reel is shiftable axially relative to the supporting frame, by bumping the cutter on the ground, from an outer position at which supply reel rotation is prevented to an inner position at which supply reel rotation and consequential filament feed through the spool are allowed.

2. A vegetation cutter having a cutting element in the form of a flexible filament, the cutter comprising a frame supporting a filament supply reel, an independently rotatable spool having a passageway through which filament fed from the supply reel may pass freely and twist about its own axis, and means for rotating the spool such that in use a free end portion of the filament projecting from the spool rotates in a cutting plane and tends to pull more filament from the supply reel, in which the supply reel is positioned below the spool at the operative orientation of the cutter with the cutting plane generally horizontal.

3. A vegetation cutter according to Claim 2, in which the supply reel is shiftable axially relative to the supporting frame, by bumping the cutter on the ground, from an outer position at which supply reel rotation is prevented to an inner position at which supply reel rotation and consequential filament feed through the spool are allowed.

4. A vegetation cutter according to Claim 2, comprising filament feed control means responsive to the speed of rotation of the spool or length of line projecting to allow a rotation of the supply reel through a predetermined angle whenever the spool decelerates below a threshold speed and accelerates once again to above the threshold speed.

5. A vegetation cutter according to any preceding claim, in which the rotating means is an electric motor coupled directly to the spool.

6. A vegetation cutter according to any preceding claim, in which the spool has a moment of inertia substantially lower than that of the supply reel.

7. A vegetation cutter according to any preceding claim, in which the axis of rotation of the spool and the supply reel are spaced apart and generally parallel.

8. A vegetation cutter according to any preceding claim, in which the frame comprises means for guiding the filament fed from the supply reel into a supply path substantially on the axis of the spool, on which axis it enters the passageway through the spool.

9. A vegetation cutter according to any preceding claim, in which the passageway through the spool has a portion extending generally radially of the spool ending at a peripheral opening.

10. A vegetation cutter substantially as described herein with reference to the accompanying drawing.