GB2255265A

GB2255265A - Vegetation cutting apparatus

Info

Publication number: GB2255265A
Application number: GB9109303A
Authority: GB
Inventors: Craig Duncan Webster
Original assignee: Black and Decker Inc
Current assignee: Black and Decker Inc
Priority date: 1991-04-30
Filing date: 1991-04-30
Publication date: 1992-11-04
Anticipated expiration: 2011-04-30
Also published as: GB9109303D0; GB2255265B

Abstract

A vegetation cutting apparatus comprises a support frame 3, a rotary body 11 mounted on the support frame for rotation about a principal axis, means 9 for driving the rotary body into rotation, one or more elongate, substantially rigid, plastics cutting blades 12 adapted for pivotal connection to the rotary body for cutting vegetation in a cutting plane GP normal to the principal axis, and a swathe-defining blade guard 4 connected to the support frame, the periphery of the guard extending below the cutting plane wholly within the annular swathe traversed in use by the or each cutting blade from the or each anchoring formation to the blade tip. The guard 4 minimises the chances of impact of the blades 12 with anything over its inner region and thereby minimises power consumption and wear on the blades. <IMAGE>

Description

Vegetation Cutting Apparatus This invention relates to apparatus for cutting vegetation, especially light growths of vegetation such as weeds or grass in portions of gardens inaccessible to conventional lawn mowers or along paths or edges, and is particularly useful in the form of a hand-held, elongate grass trimming device.

String trimmers are well-known examples of such apparatus; a flexible plastics line or flail is caused to rotate at speeds of around 6,000 - 10,000 r.p.m. in a cutting plane, which may be horizontal or vertical (for edges), and the line is replaced during use as it wears by feeding increments from a storage spool in the same housing as the motor. A disadvantage, however, of string trimmers is their high power consumption, and this has placed practical limitations on the design of any cordless electric string trimmer. Even with the rechargeable battery packs presently available, the period of use before recharging is necessary is relatively low, and may not be sufficient to allow the user to complete all the trimming required in one go.

British Patent Specification No. 1570252 describes a vegetation cutting device in which an electric motor drives a rotary disc about an upright axis, the disc supporting a rigid, elongate, flat, plastics safety blade at an eccentric pivot point. The blade is rotated at high speed in its own plane about the axis, to cut vegetation in an annular horizontal swathe. Because the blade is free to pivot about its inner end, it is protected to some extent from severe impact, thus limiting the risks of damage to the blade and of injury to the user.

Similar safety blades, which are readily replaceable when worn, have also been used in a rotary lawn mower in place of the more usual metal blade or blades, for reasons of safety, but at the expense of more frequent replacement operations.

We have found that the use of such safety blades in a device such as that of GB-1570252 can be rendered much more economical in power consumption than a corresponding string trimmer, by the provision of an appropriate guard to limit the effective cutting portion of the blade or blades to the outer end region.Accordingly, the invention in its broadest aspect provides vegetation cutting apparatus comprising: a support frame; a rotary body mounted on the support frame for rotation about a principal axis; means for driving the rotary body into rotation; one or more blade anchoring formations on the rotary body at a common radius relative to the principal axis; a corresponding number of elongate, substantially rigid, plastics cutting blades each having at one end an attachment formation adapted for pivotal connection to a corresponding one of the blade anchoring formations such that it may be driven into rotation about the principal axis whereby to extend it radially thereof for cutting vegetation in a cutting plane normal to the principal axis, and such that the or each blade is free to pivot in the cutting plane; and a swathe-defining blade guard connected to the support frame, the periphery of the guard extending below the cutting plane wholly within the annular swathe traversed in use by the or each cutting blade from the or each anchoring formation to the blade tip.

The saving in power consumption is believed to be due primarily to the prevention of drag caused by inner portions of the blade impacting, but failing to cut, the grass or other vegetation, because their speed is of course lower than that of the blade tip. A further contributory factor is probably that the blades may be made flat in the cutting plane with a sharp cutting edge, which minimises drag in the air; the shape may be optimised aerodynamically.

A cordless, rechargeable electric vegetation cutting device with all its advantages of freedom of use in remote areas, is now capable of production at a reasonable cost and with a satisfactory performance, using the present invention.

Preferably, the swathe-defining guard has a major portion whose periphery is arcuate with its centre of curvature on the principal axis. We have found that the performance is best when the radius of the periphery of the major portion of the guard is such in relation to the length of the or each cutting blade that the tip of the or each of the blades projects in use beyond that major portion over only a limited range of angular positions of the blade about its pivot, the range being no greater than 900 . In an advantageous example of this, the periphery of the major portion is at a radius approximately three quarters of the blade length from the blade anchoring formations, so that the cutting swathe is effectively an annulus whose width is about one quarter of a blade length.

Preferably, the swathe-defining blade guard extends over an angle of no more than 1800 about the principal axis.

Guards for protecting the user from rotating parts are of course well-known in the context of vegetation cutters, and there is preferably a further guard which extends across the cutting plane and around the path traversed by the blade tips in use, over an angular range about the principal axis separate from that of the swathe-defining blade guard.

Conveniently, the angular ranges of the guards are complementary.

Preferably, as with conventional string trimmers, the support frame comprises an elongate handle substantially in the upright direction in use to enable it to be guided parallel to the cutting plane. In its preferred form, the driving means comprises an electric motor and a selfcontained, re-chargeable electric power source for the motor.

A light vegetation cutter embodying the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic perspective view of the whole cutter, but omitting, for the sake of clarity, the rotary portions; Figure 2 is an underneath plan view, to an enlarged scale, of the base portion of the cutter of Figure 1; and Figure 3 is a section through the principal axis of rotation, taken on the line III - III of Figure 2, of part of the base portion of the cutter, with the electric motor shown in elevation.

The cutter 1 is a hand-held, cordless, re-chargeable electric device, with a housing 2, 6, 7 formed as two unitary plastics mouldings mating along their peripheries extending the full length of the device, rather like a clamshell. A cylindrical widening at the base 2 houses an electric motor 9; this is connected at an acute angle of about 300 to an upright stem 6, whose upper end is formed as an elbow connected to a handle 7. A rechargeable battery pack, providing 9 volts D.C., is housed in the handle 7, and has an electric socket 8 for connection during recharging to a power source, not shown; and an on-off switch linking the battery output to the motor 9 by way of a cable within the stem 6.

A plastics shield 3 is connected to the bottom edge of the housing 2, and comprises a horizontal plate perpendicular to the principal axis A of rotation of the motor shaft, with a peripheral, downwardly-depending flange 5. The motor output shaft 10 drives directly a blade-support disc 11 mounted coaxially beneath the top plate of the shield 3. A stud 13 projects from the underside of the disc 11 at its edge, and a rigid, flat, elongate, plastics safety blade 12 is anchored pivotally on to the stud 13 at its inner end.

The blade has a key-hole shaped aperture 14 in its own plane at the inner end, whose major, circular portion (shown in Figure 2) is just large enough to fit over the eye of the stud 13, and whose narrower portion defines the surface which bears pivotally against the stud. In this example, the narrower portion is a part-circular aperture of smaller diameter (not shown). In this way the blade 12 is locked on to the stud in normal use but is readily removeable by sliding it radially inwards to place the circular-opening 14 in register with the eye of the stud 13.

In this example, there is one blade 12, and the disc 11 is preferably counterbalanced to place the centre of inertia on the axis A. However, it would be feasible to provide two identical blades at diametrically opposed positions, or more than two identical blades placed preferably at equi-angularly disposed positions around the axis A, each blade-anchoring stud 13 being at a common radius so that the tip of each blade follows the same path.

The blade 12 is symmetrical about its medial horizontal plane and its medial vertical plane, and has two sharp cutting edges 15 along its length. Otherwise it is generally flat; the inner portion around the key-hole formation 14 is broadened in the horizontal plane, for strengthening.

The upper guard plate 3 is disc-shaped but with a part-annular cut-out over about 1800 at its front. The circular path P of the blade tip is at a radius of about nine-tenths that of the rear part of the upper guard plate 3; the front edge of the upper guard plate 3 has a radius about three-fifths of the rear part, so that the blade path P is exposed at the front. About half the length of the blade 12 extends in use beyond the front edge 51. The rear part of the guard 3 has a deep flange 5 which depends from its edge across the cutting plane CP (Figure 3) in which the blade rotates, to protect the user from the blade at the rear. The flange is continued at 51 around the front edge, but at about one third the depth, so that it lies wholly above the cutting plane CP.

A swathe-defining cylindrical metal wire guard 4, of stainless steel, is mounted by its ends in the underside of the upper guard 3. Its diameter is similar to the width of the blade 12. It comprises a major, active portion extending in a plane just below and parallel to the cutting plane CP, in the shape of an arc of a circle centred on the axis A, over nearly 1800, so that its angular range is complementary to that of the rear edge 5 of the upper guard 3. The outer edge of the wire guard 4, projected normally onto the cutting plane CP, coincides with a point on the blade about three-quarters of its length from the pivot point 13, so that about one quarter of the blade projects beyond the swathe-defining guard 4. This has the effect of defining an annular cutting swathe of width one quarter of a blade length, over nearly 1800 of arc.Of course in use the cutter is moved parallel to the cutting plane CP to form the actual swathe of cut vegetation. The function of the wire guard 4 is to minimise the chances of impact of the blade 12 with anything over its inner region. This in turn minimises power consumption and wear on the blades. It ensures that the most efficient speed of the cutter is maintained, by placing a limit on the load it bears.

The wire guard is symmetrical in a plane through the axis A. The arcuate, front portion 41 is connected by 1000 bends to two diametral lengths 42 in the same plane, then by 900 bends to two axial lengths 43, and then by further 900 bends to arcuate end portions 44 which are a push fit by sliding along the upper inner edge of the rear flange 5, between gripping formations 54, 55 formed integrally with the upper guard 3.

In use, the inertia of the blade causes it to adopt a near radial position, affected slightly by air resistance.

As shown in Figure 2, the blade 12 is free to pivot over a wide arc in the cutting plane CP. At the position 12' shown in phantom lines, at about 450 to its normal radial orientation, the blade tip becomes protected by the wire guard 4 (as the cutter is advanced forwardly or to the side or both). There is therefore an angular range of about 900 of pivotal movement over which the tip is exposed for cutting duty, to a greater or lesser extent. Impact with vegetation, such as a blade of grass, will cause the grass to shear, and the resulting reaction on the blade 12 will cause it to pivot about the stud 13. Where a piece of vegetation too thick to cut, or indeed a wall or path, is encountered, the retraction of the blade in this manner reduces the drag on the motor and thus the power consumption, and minimises the risk of fracture or severe wear to the blade.

The cutter may be provided with a swivel joint (not shown) intermediate the stem 6 and the motor housing 2, to allow a 1800 turn to cause the cutting assembly to be inclined with the cutting plane vertical instead of horizontal, for lawn-edging. Such a joint is well-known in string trimmers, and need not be described here.

Although the cutter described is electric and cordless, the invention is applicable to any vegetation cutting apparatus, domestic or agricultural, whether powered by an internal power source or an external drive, and whether powered electrically or otherwise, e.g. by an internal combustion engine.

In comparative tests carried out on a cutter as shown in the drawings and a string trimmer, both with a 180mm diameter swathe and a rotation speed of 11,750 r.p.m. using an electric motor, the cutter embodying the invention used 14% less power, for the same speed of traverse along a grass lawn.

Claims

1. Vegetation cutting apparatus comprising: a support frame; a rotary body mounted on the support frame for rotation about a principal axis; means for driving the rotary body into rotation; one or more blade anchoring formations on the rotary body at a common radius relative to the principal axis; a corresponding number of elongate, substantially rigid, plastics cutting blades each having at one end an attachment formation adapted for pivotal connection to a corresponding one of the blade anchoring formations such that it may be driven into rotation about the principal axis whereby to extend it radially thereof for cutting vegetation in a cutting plane normal to the principal axis, and such that the or each blade is free to pivot in the cutting plane; and a swathe-defining blade guard connected to the support frame, the periphery of the guard extending below the cutting plane wholly within the annular swathe traversed in use by the or each cutting blade from the or each anchoring formation to the blade tip.

2. Apparatus according to Claim 1, in which the swathe-defining guard has a major portion whose periphery is arcuate with its centre of curvature on the principal axis.

3. Apparatus according to Claim 2, in which the radius of the periphery of the major portion of the guard is such in relation to the length of the or each cutting blade that the tip of the or each of the blades projects in use beyond that major portion over only a limited range of angular positions of the blade about its pivot, the range being no greater than 900.

4. Apparatus according to Claim 2 or 3, in which the periphery of the major portion is at a radius approximately three quarters of the blade length from the blade anchoring formations, so that the cutting swathe is effectively an annulus whose width is about one quarter of a blade length.

5. Apparatus according to any preceding claim, in which the swathe-defining blade guard extends over an angle of no more than 1800 about the principal axis.

6. Apparatus according to any preceding claim, comprising a further guard which extends across the cutting plane and around the path traversed by the blade tips in use, over an angular range about the principal axis separate from that of the swathe-defining blade guard.

7. Apparatus according to Claim 6, in which the angular ranges of the guards are complementary.

8. Apparatus according to any preceding claim, in which the support frame comprises an elongate handle substantially in the upright direction in use to enable it to be guided parallel to the cutting plane.

9. Apparatus according to any preceding claim, in which the driving means comprises an electric motor and a self-contained, re-chargeable electric power source for the motor.

10. Vegetation cutting apparatus substantially as described herein with reference to the accompanying drawings.