CA1126965A - Cord-whip vegetation cutting appliance - Google Patents

Abstract

ABSTRACT
A grass-cutting appliance has a handle and a driven rotary cutting head comprising a flexible cord-like cutting element the end-section of which projects from the periphery of the housing within which the cord is wound on a spool. The cord is lengthened by a specific amount, when it becomes worn during use, by means of a clutch associated with the spool and the housing, the spool rotating through a specific angle in relation to the housing. The clutch is moved from the locked position to the released position automatically when the cord cutting element is too short and an incremental length of cord is paid off from the spool.

Description

l~Z6965 The invention relates to a vegetation-cutting appliance of the type commonly used for trimming grass around the edges of lawns, trees etc.
At the necessarily high r.p.m. of an appliance of this kind ~German Disclosure Text 2 734 734), the cutting element wears away relatively quickly and must frequently be adjusted in length by winding off the spool. To this end, the appliance while in operation must be pressed in an approximately perpendicular position against the ground. This displaces the spool axially and releases the clutch. As a result of the rotational impulse acting upon it, the spool simultaneously rotates into a second stop position, which causes the required length of cutting element to unwind from the spool. The di.sadvantage ol this urrangement :is tl-at adjustmellt -i.s not n.utomati.c, but has lo be :i.ni.t.iuto(l by the ol)erator who must therofore checl~ the length of the cutting element at frequent intervals. As a rule, the operator is unable to estimate the length of cutting element required to allow the appliance to operate at high efficiency. I:urthermore, the force requ:ired to release the clutch is dependent upon the condition of the ground, which means that var:ious levels of :force are re(luired to release the clutch. When the operator encleavours to cleterm-ine the force required, or if he underest:i.mates the actual length of the CUttillg elenlellt, he m;ly p:ross tho ut)~ i.Llllce Ug.l:i.nSt the ground too ofton un~l thus UllW:i lld too mucll CUttillg e:lemellt. An excessive lengtll of cutt.ing elonlellt muy eas:i.ly over:Loacl the motor, thus ~lamagi.ng or even destroying .it. 'rhe excess length must therefore be cut o:Ef, to which encl it :is known to Eit ucld.it:i.on;l.l cutting devices to some of these appliances.
Ilowever, those uclclit:ional cutt:ing devices :i.ncrease the cost o:E the appliance ancl also lea~ to excessive consumpt:ion of the relat:ively experlsive cutting C? lement.
It is the purpose of the invention to design an appliance of the type mont:ionecl at tllo beginning hereof, so that when the cutting element d~i ~69~S

wears it may be lengthened by an accurately defined amount automatically i.e.
without the operator's intervention.
The invention provides a vegetation-cutting appliance having a handle and a driven rotary cutting head comprising: a rotatable housing, at least one flexible cord-like cutting element having an end-section projecting from the periphery of the housing, the cutting element being wound on a spool arranged in the housing, said end-section being adapted to be lengthened by a specific amount when it becomes worn during use, such lengthening being effected by means of a clutch associated with said spool and said housing and operative to permit the spool to rotate through a given angle in relation to the housing, the clutch being movable from a locked condition to a released condition against the force of a ~prlng, and including a clutch member comprising a first and a second locking element which define respectively a first and a second locking po6ition of the clutch member through engagement with corresponding locking detents, angularly spaced around the spool, said clutch member comprising a clip having two parallel legs on each of which is provided a said locking element pro~ecting outwardly from the leg.
The centrifugal force may be utilized for the automatic extension of the cutting element ln different ways. When the cutting element end section has been shortened by wear, its reHlHtance to alr and to cutting will drop.
This will cause the drive motor and thus the cutter head to reach a higher r.p.m.
which causes increasingly higher centrifugal forces to act upon the clutch member. This effect is used to move the clutch member by the increased centrifugal force from a first into a second locking position. This causes the spool to rotate relative to the housing sufficiently to restore the cutting element to its original length.

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; .
~ ' ~Z6965 In a second embodiment the continuous switching operation of the spool is achieved independent from a change in the r.p.m. In both cases it will not be necessary to control the length of the cutting member while the implement is in use, so that the work can be carried out without interruption and thus in a shorter time. The automatic readjustment of the cutting member offers the further advantage that the cutting element never exceeds the desired length, and the motor cannot therefore be damaged by overloading. There is also no need for an additional knife for shortening the cutting element after too much has been unwound. This makes the appliance simpler and less expensive to produce, and eliminates waste of the cutting-element material.
As a rosult of tho uutolllatic roadjustlllent of the cutting element, the appliance according to the invention is particularly easy to handle and ensures the high efficiency.
Further characteristics of the invention may be gathered from the specification, claims and drawings.
'I'he invent:ion is described in greater detail hereinafter, in con-junction with the exemplary embodiment illustrated in the drawings attached hereto, wherein:
[:igure I SIlOWS u plallt-cuttillg appliullce uccordillg to tho invention, hold by an operator in its operuting position;
l~igure 2 is an exploded view of the cutting head of the appl.iance of l~igure l;
l~iguro 3 is all underneatll plan v;ew of the CUttillg head, with the housing cover romovedJ in a first locked position of the clutch, in which the cutting element is still suff:iciently long;
Figure ~ is a view similar to that in Figure 3, but with the clutch in a second locked position, immediately after the cutting element has been ~mwound to its original length;

l~Z6~65 Figure 5 shows a cutting head of another design, with its spool and clutch, in plan view towards the bottom of the housing;
Pigure 6 is a section along the line VI-VI in Figure 5 passing through the plane of symmetry of the housi.ng;
Figure 7 is a section along the line VII-VII in Figure 5.
As shown in Figure 1, the plant-cutting applia.nce consists of a cutting head 1 having a cutting element 3 extending radially outwards when in opcration, a carrying tube 5 to one end of which the cutting head is attached by a connecting part 4, a motor 6, which drives the cutting head, being secured to the othcr end of thc tube. The appl.i.ance has handles 9 and a sllou].dor strap 8, by moans o:f whlch :it may easily be carried and guided by the operator 7.
The motor 6 is connected to the cutting head 1 by means of a drive-shaft (not shown) in the carryin~ tube 5, and gearing (not shown) housed in the connecting part 4. The part 4 projects through a central aperture 33 (I:igure 2) in the top 20 of a cup-shaped cutting-element housing 2 the bottom of thc housing 2 being closed by a cover 13. The top 20 carries a guide-ring 19, coaxial with the aporture 33, to wh:icll a spool ].0 is ~ccut~c(l. WoUlld onto tho spool i.s a flox.i.ble cord mLIdO 0~ nylo1l, the froe erld 3 of wll:ich ptojocts outwardly througll a passago 12 :in the wall of housing 2 by a specific amount.
When the cutting head :is cause~d to rotate, this end 3 const:itutes the cutting olement, by moans of which ptants, such as grass, weeds and even small twigs may be satisfuctorily cut, the length of the cutting element being governed by the power of the motor and the cutting resistance.
The spool 10 comprises two annular flanges 14, 14', of different widths, extending radially outwardly and in spaced relationship with each other. The outside diameter of the wider flange 14' corresponds approximately to the :insido diameter of housing 2 and, when the unit is assembled, flange ~Z6g~5 14' faces the cover 13. The edge 17 of the narrower flange 14 has a plurality of cam-like detents 18 spaced equally apart thereon and projecting outwardly. In plan view, these extensions are approximately trapezoidal in shape. The inside 15 of the cylindrical portion of spool 10 has cam-shaped detents 16 projecting radially inwardly and similar to the detents 18 and, at corresponding angular spacings. The detents 16 are all at the same height and have the form of steps with triangular outlines. As seen in the diroction of rotation 38 of cutting head 1, they are each preferably located immediately behind a corresponding one of the outer detents 18. Associated with the inner detents 16 is a first locking element 28, and a second locking olement 31 is ussociatod with the outer detents 18, so that the detents constituto matching locking elcmerlts. Tllese elements consist of thc ends of an approximately U-shaped clip 23 acting as the cutting-appliance clutch.
These ends are bent at right angles to the plane of the U, both in the same direction. 'I'he clip 23 is preferably made of spring wire and is mounted betweon the narrow flango 14 of the spool 10 and the top 20 of the housi.ng 2, in radial relationship with spool 10.
The locking elements 28, 31 may bo formed by a clip of a d:iEEo-ront shape, or may be in two purts, Eor irl.~t;ltlco a l~ivotubly moullto~ singlo-or doublo-ondod lovor. 'I'hcy noed only bc adupted to IIIOVQ, undcr the centrifugal forco whicll increases as cutting eloment 3 wears, in such a mallner that first tho locking member 28 is disengaged from inner detent 16 to allow the spool to rotute irl tho direct:ioll of arrow 35 under the action of centrifugal force, more particularly of cutting elemont 3 (Inigures 3 and 4). The spool can then be rotated until the second locking olement 31 is in register with an outer detent 18 i.e. until an outer extension in front, as seen in the direction of rotation, comes to rest against it.
Tlle clip 23 is asymmetrical, having one short leg 23' and one ~ 6965 longer leg 23". These legs are joined by an approximately V-shaped cross-piece 23a enclosing an obtuse angle with the apex 23b thereof pointing out-wardly. Because of the a~ymmetrical design of the clip, the centre of mass thereof does not lie upon the axis of rotation of cutting head 1, even if, as in the embodiment shown, it is arranged in relation to the housing in such a manner that its longitudinal centre plane passes through the axis of rotation of cutting head 1. This ensures that the clip 23 is acted upon by a centrifugal force which will shift the clip in relation to the housing 2 and the spool 10.

For the purpose of positioning the clip 23 and ensuring accurate displacement thoreo~, to control adjustmcnt of the lengt}l of the CUttillg elomont 3, thc top 2() of the ht)using 2 (which is preferably an injection moulding), has integral cams 21, 22. Three outer cams 21, arranged one behind the other and spaced radially apart, bear against the longer leg 23" ofthe clip 23, while an inner cam 22 is provided on a level with the central cam, whereas preferably two inner cams 22, and an outer cam 21 (facing the inner cam at the free end 28 of leg 23') bear against the short leg. Two of the outer cams 21 associated with longer leg 23" aro located at free end 31 thereof and in tlle area adjacent c-rosspiece 23a rosl~octively.

Whon tho loc~ing clement 2~ is ln its first locked posltion, the apex 23b of crosspioce 23a of clip 23 bears against an approximately semi-circularly rounded cam 27 attached to the top 20 of the hous:ing 2, while the opposite surfacG of the apex bears against a rod-shaped spring 24 located approximatoly in thc outer third of tho said housing. 'I'his spring rod extends substantially over the en~ire width of the llousing, the ends 29, 32 thereof being clamped between radially adjacent retaining cams 25, 26 respectively on the top. Before the cutting element 3 becomes worn, the force of the spring rod 2~ prcsses the first locking element 28 on short leg ~126965 23' of clip 23 against the front surface 39, as seen in the direction of rotation of cutting head 1, of one of inner detent 16, applying a preload thereto. As the cutting element wears, the air- and cutting-resistance thereof decrease and the r.p.m. of the appliance increases. An ;ncreasing centrifugal force the~n acts upon the spring rod 24 and, as the cutting element continues to wear, this centrifugal force overcomes the opposing force of spring rod 24. The centrifugal force IIOW causes the apex 23b of the clip 23 to shift radially outwards into a second stop position, the spring rod bowing to such an extent that it comes up against a radially outer second stop 30 facing the cam 27. When the clip shifts radially outwards, the locking eloment 28, lying at riuht angles to the plane of the legs 23", 23' (Figuros 3 and 4), disongages from relevant inner detent 16. The centrifugal force acting upon cutting elemont 3 then causes spool 10 to rotate in the direction of the arrow 35 until front of detent 18, as seen in direction 35, comes up against second locking element 31 of clip 23 which is simultaneously moving in the direction of arrow 36 (Figure 4). The spacing a between stops 27 and 30 which lim:it MoVement in the direction of arrow 36, is such that the second locking element 31, in the second locking posit:ion of clip 23, then lies in the path of movement o~ tho outor dototlts 18. 'I'ho d;stullco b bctween tho detents 18, lG detorlllillos tho reudjustmollt longtll of cutting elelllellt 3.
It is preferable for the thickness of the clip legs 23', 23", of the locking elements 28, 31, and the width of detents 16, 18, measured in the direction of rotation 38 of cutting head 1, espocially that of the outer detents, to be small in rolation to distance b.
When the clip 23 is :in the second locking position, the~ air- and cutting-resistance of cutting element 3 increase, because the latter is now back to its original length. This slows down the motor and reduces the centrifugal force acting upon the clip and the spring rod. This force is now less than that of the said spring rod which therefore shifts clip 23, in a direction opposite to that of arrow 36, back to its first locking position, placing the locking element 28 in the path of the inner detent 16.
This procedure is repeated whenever the cutting element 3 becomes so worn that the centrifugal force acting upon the clip 23 and spring rod 24 is greater than the force exerted by the said spring rod on the clip.
Clip 23 may be positioned in the housing by retaining and guiding means other than cams 21, 22 described above.
The cutting head 101 illustrated in Figures 5 to 7 has a housing lO 102 in the form of a cassette in whicll a spool 110 ;s rotatably mounted and wouncl w:ith a cord l]l, preferably ~ perlon corcl. The housing 102 comprises a lowor part 104 and an upper part 105, with skirts 106, 107 engaging one another, the internal surface 108 of the upper part of the housing being seated upon a shoulder 109 of a hub-like guide ring 119 formed in the lower part 104 of the housing.
Sk:irts 106, 107 of the housing parts 104, 105 define a passage 112 for the cord 111. The end 103 of this cord projecting from housing 102 constitues, when the cutting head rotates, tho CUttillg olemont ot cho applianco, the carrying tubo ol wl~icll is connoctecl to tllo cutting lloa(l 101 by 20 means of a connecting piece 113.
'Io recluce wear of the cord 111 and the housing 102 the cord is arranged botween two wear-sleevos 115, 117 arranged in the vicinity of the pussago 112 on opposite sidos of a radial plane of symmetry passing through the axis of cutting head ]01. Reliable guidance of inserted wear-rings 115, 117 is assured by edges 106, 107 having thickened end sections 121, 121 ' with concave encl-faces 122, 122' (Figure 5~. The passage 112 is defined on its top and bottom sides by rounded edge zones 125, 125' in the housing parts 104, 105 (I:igure 6) extending to the area between adjacent wear-sleeves ., :

1~69~5 115, 117. This means that the actual passage 112 between the wear-sleeves llS, 117, and the edge zones 125, 125', is only slightly larger than the thickness of cord 111, thus reducing chatter and vibration of the cord.
To inhibit movement of the cord, and the wear of the cord and the housing parts associated therewith, a roller 126 is mounted rotatably upon a stud 127 adjacently in front of the wear-sleeve 117, as seen in direction of rotation 138 of the cutting head 101, one turn of the cord being wrapped tautly around the roller. Roller 126 is located in a partly rounded inner-edge-section 129 of lower part 104 of the housing. On the side associated with wear-sleeve 115, the housing part 104 has an inner-edge-section 129' whicll, as i.n the case of odge-soct:ion 129, is assoc:iated with a plug-:in aperture 132 located in a sloping wall-section 130 which merges into top 120 of lower housing part 104. The roller 126 and stud 127 can be moved to this aperture 132 when the cutting head is to be driven in the reverse direction, i.e. in a direction opposite to that of arrow 138. I:rom the inner-edge-sections 129, 129', skirts :L06, 107 merge into cylindrical edge-areas 133, 133' coaxial with the axis of housing 102, these edge-areas being displaced radially inwardly to define project:ing r:ims 136, 139 oxtend:i.llg rad.i.ally outwards (I'iguro 7).
'I'o accon~llodate a clutch pa:rt 123 :tor CUttillg head 10l tlle edge-areas 133, 133' in the lower part 104 are stepped down and merge into an intermediate section 140, 140' (I~:igure 5) which curves radially outwards, in a part circle, around tho axis of housing 102, the intermediate section then merging into a semi-c:ircul.ar, sharply curved bearing section 141. Section 141 extends into the outer surface of the housing 102 which is defined by supports 136, 139 and by the thickened end-sections 121, 121'. Except for the thickened end-sections, the skirts 106, 107 are of constant thickness.
In the vi.cinity of bearing section 141 of the skirts 106, 107, an additional _ 9 _ aperture is provided in the top 120 of lower part 104 of the housing, a bearing stud 142 for the clutch part 123 projecting from this additional aperture. The bearing section 141 is coaxial with the bearing stud 142, the axis being displaced radially outwards in relation to the ilmer wall of intermediate sections 140, 140'.
The skirt 106 of lower part 104 of the housing extends ~except in the area of the edge-zone 125) over rather more than three-quarters of the height of the housing, whereas tlle height in the vicinity of this cdge zone 125 is approximately equal to half the height of the housing, so that the cord 111 passes to the outside at about the mid point in the height of the housing. ~ntormcdi,lto scctions 140, 140', and bcaring section 141, have approximatcly an cqual circumferential length to the area between the inner-end-sections 129, 129'; they extend over an arc of about 45.
The clutch part 123 is in the form of an approximately rectangular plate and carries an extension 144 on its radially outer longitud:inal edge 143, tlle extension defining a passuge for the bearing stud 142, the edge of the extension being part circular to correspond to bearing section 141.
~urthermore, edge 143 slopes away from the extension 144, so th(lt the back of clutch part 123 is upproxilrlatoly V-sll;lpcd.

2~ ~t its oppos;te longitudinal ed~e 145, clutch part 123 has two outer detents 128, 131 in the form of prongs of d;fferent widths and lengths.
The centre of gravity S of tho clutcll part 123 is eccentric in relation to its pivot uxis (i..e. tho bearillg stud 142) to which end a gap 146 may be provided in the part of tho plate whicll is to the front as seen in direction of rotation 138.
The rear prong 128 is shorter and thicker than the front prong 131, so that it can be relied upon to absorb the forces arising during operation.
The prong 128 co-oporates with detents 116, 116' provided as projections on a lower flange 114' (Figure 7~ of the spool 110. The flange 114', and an upper flange 114 of approximately the same radial width, deiine the height of the spool. Detent 116' is followed, as seen in direction of rotation 138 and at equal angular intervals, by additional detents, of which only detents 118 and 147, located in front of detent 116 - in the position of the spool shown - are illustrated. In plan view, all of the detents are approximately trapezoidal in shape, increasing in width radially outwards.
The distance between prongs 128, 131 is greater than the angular distance between adjacent detents and smaller than the distance between alternate detents. Front prong 131 is narrower and is ot a length such that in one locked position of clutch part 123 it projects as far as rear prong 128 between adjacent detents. Bccause of its eccentric mounting, the clutch part 123 forms a doublcd-ended lever movement of, which is limited by, abutment of its rear longitudinal edge 143 upon the relevant intermediate sections 140, 140' of the skirt 106 of lower part 104 of the housing, the slope of the edge 143 being selected accordingly. Moreover, the prongs 128, 131 diverge in relation to the pivot axis of clutch part 123 and have rounded external surfaces, whereby pivoting of the clutch plate 123, in particular from the first locking position into the releasin~ positioll is facilitat~d, In uddltion to this, tho internal surfaces 148, 1~8' of prongs 128, 131 also diverge. Particularly in tho second locked position of clutch purt 123, therefore, the front prong 131 is in linear contact only with relevant detent 118, as a result of which the clutch part 123 may be easily pivoted back to its startin~ position (the first locked position).
To retain the clutch part 123 in its iirst locked position -when there is a desired and predetermined cutting length of cord-end 103 -until wear has shortened cutting element 3 to a specitic length, a retaining member, more particularly a helical spring 124 is provided, the spring l~Z696S

surrounding the bearing stud 142, and having a lower leg 149 engaged in a corresponding aperture 150 in the clutch part 123; according to Figure 5, this aperture is located, as seen in direction of rotation 138 and in the radial direction, between the centre of gravity S and the pivot axis of the clutch part. The other leg 149' of the spring 124 projects into open lower end 151 of an adjusting ring 152 screwed to the bearing stud 142. The upper end 153 of the adjusting ring 152 is located in an aperture 15~ in upper part 105 of the housing. To compress spring 124 to a greater or lesser extentJ
depending upon the required preload on the clutch part 123, the adjusting ring 152 may be adjusto~ by rotating the bearing stud which is provided, to thut end, with u slot 155. It is possible to use, instead of the helical spring, a tension or compression spring located at right angles to the direction in which lever 23 pivots. The desired working diameter of the cord and power output may be adjusted simply and quickly by means of the adjusting ring, even by the operator of the appliance.
A change of this kind may also be achieved by displacing the centre of gravity S of the part 123, to which end a pin may be arranged in an elongated hole in the clutch part, for example.
Deponding upon the lellgtll ol cord-oll(l 1()3 projocting from llousing 102, i.e. tho cuttillg elelllent, grass, twigs or the like may be satisfactorily cut, the thread-ond being adjusted to a suitable length when the cutting appliance is placod in servico. At this time, clutch part 123 assumes its first locked position (I:igure 5), ;n which the prong 128 is located betweell detents 116 and 118 and the clutch part is preloaded by the force of spring 124 in such a manner that rear longitudinal edge 143 thereof bears against front intermediate section 140', as seen in direction of rotation 138. As soon as the cutting head starts to rotate, spool 110 can rotate, in relation to the housing 102 and in direction of rotation 138, until the detent 116 ;9~

engages the prong 128. The spool 110 remains initially in this position, because the centrifugal force acting upon the cutting element 103 loads the spool in direction of rotation 138. This applies to the clutch part 123 a resultant force FR which is proportional to torque M arising at the clutch part and is inversely proportional to radius r of cutting element 103, i.e. M, radius r being measured from the axis of rotation of housing 102.
The clutch part is held to the detent 116 by friction. As soon as the cutting element has worn down to a specific length, centrifugal force Fsp, acting upon spool 110, becomes smaller than the resulting force FR acting upon the clutch part, multiplied by coofficient of friction /~. When the following condition is fulfilled:
FR . /lu>Fsp a shifting force comes into effect and pivots the clutch part 123 (counter-clockwise as seen in Figure 5) until the longitudinal edge 143 thereof comes to rost against the rear intermediate section 140 of the skirt 106 ~the position shown in ~otted lines :in Figure 5). At this time the prong 128 is disengaged from the detent 116 (the release position) and prong 131 moves between front detents 118 an~ l47, as seon in diroction oi rotation l38.
Spool 110 then rotatoC;~ in relation to housing 102, In the direction of rotation 138, until detent 118 comes up against tho prong 131 (the second locked position - cam shown in dotted lines in Figure 5). The lengtll of the CUttillg elonlont 3 increases in accordance with the rotational movement of the spool 110. As a result of the detent 118 conling to rest against the prong 131, a torque acts upon this prong, causing clutch part 123 to be pivoted back to its starting position (bearing against front intermediate section 140'). At this time, the spool 110 continues to rotate in the direction of rotation 138 until the detent 116' comes to rest against the prong 128 which has now moved in between these detents (the first locked position). As a ~i93Ei~

result of the rotary motion of spool 110 in relation to the housing 102, the cutting element 3 is now readjusted to its initial length. The shifting operation described hereinbefore may also be introduced by moving the appliance out of the cutting position, so that there is no load on the cutting element and the tensile force acting upon the spool is reduced. If the rotational velocity of the motor is dependent upon the load, this velocity will now be increased. All that is essential, however, is that the ratio of the forces acting upon clutch part 123 be altered in such a manner as to initiate the shifting operation.
If the spool is wound in the opposite direction, and the cutting llo.l~ i.s to operuto in the rovorso direction (the direction opposite to d.irection of rotation 138), stud 127 is shifted to aperture 132 and c1utch part 123 is inverted so that the top side of the plate in Figure 5 faces the bottom 120 of the lower part 104 of the housing and the prong 131 is in front, in the direction of rotation.
'I'he dimensiolls of the Ullit may be such that the shifting force ar:ising at a given r.p.m. of the motor lies within the given operating r.p.m.
range, since the shift is govorned by the opposin~ forco~ uct:ing upon tho clutch part und cunnot occur with u cuttin~ olomon~ 1()3 o-f suf:~'iciellt lCIlgth.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A vegetation cutting appliance having a handle and a driven rotary cutting head comprising: a rotatable housing, at least one flexible cord-like cutting element having an end-section projecting from the periphery of the housing, the cutting element being wound on a spool arranged in the housing, said end-section being adapted to be lengthened by a specific amount when it becomes worn during use, such lengthening being effected by means of a clutch associated with said spool and said housing and operative to permit the spool to rotate through a given angle in relation to the housing, the clutch being movable from a locked condition to a released condition against the force of a spring, and including a clutch member comprising a first and a second locking element which define respectively a first and a second locking position of the clutch member through engagement with corresponding locking detents, angularly spaced around the spool, said clutch member comprising a clip having two parallel legs on each of which is provided a said locking element projecting outwardly from the leg.

2. A vegetation-cutting appliance according to claim 1, wherein the clip is U-shaped and said legs are of different lengths, a first of said locking elements being formed by the end of the shorter leg bent approximately at right angles to its length, said first locking element being adapted to engage a selected one of a series of first detents projecting radially inwards from an annular flange on the spool.

3. A vegetation-cutting appliance according to claim 2 wherein the second locking element is formed by the end of the longer leg bent approximately at right angles to its length, and adapted to engage a selected one of a series of second detents projecting radially outwards from an annular flange on the spool.

4. An appliance according to claim 1 wherein said clip engaged by said spring is supported in said housing to be movable longitudinally of said legs in a plane substantially normal to the axis of rotation of said housing, said clip being movable between said first locking position, wherein the first lock-ing element blocks the path of movement of one detent on said spool, and said second locking position wherein the second locking element blocks the path of movement of a second detent on said spool, movement of said clip between said first and second locking positions permitting incremental rotation of said spool to lengthen said end-section of the cutting element, said clip having a centre of gravity which is at all times eccentric with respect to said axis, the centrifugal force acting on said clip upon rotation of said housing urging said clip away from said first and towards said second locking position against the force of said spring.

5. An appliance according to claim 4, wherein said clip is held in said first locking position by means of said spring when the end-section of the cutting element is of desired length, radial displacement of the said clip into its second locking position, in which the second locking element is in active connection with the spool, being effected when the centrifugal force acting upon the said clip and said spring overcomes the force of said spring, as a result of an increased speed of rotation of said cutting head when said end-section of the cutting element is too short.

6. An appliance according to claim 5, wherein the spring lies transversely of the locking elements and is in the form of a rod, the ends of which are held in appropriate mountings attached to the top of the housing.

7. A plant-cutting appliance according to claim 5, wherein the spring is arranged between two stops spaced in the radial direction of the spool on the top of the housing such that when the spring is against the radially outermost stop the second locking element is located in the path of movement of one of a plurality of second detents projecting radially outwards of the spool.

8. An appliance according to claim 7, wherein the U-shaped clip has an apex of approximately V-shapes extending radially outwardly of the clutch part, which apex bears against the radially innermost stop of housing part and against the spring.

9. An appliance according to claim 6, 7 or 8, wherein the legs of the clip bear against a plurality of guides on the top of the housing, the shorter leg bearing against two such guides and the longer leg against three such guides, the said guides being spaced apart one behind the other, with at least two facing each other at the same level.

10. An appliance according to claim 7, wherein the width, measured in the direction of rotation of the cutting head of the outwardly projecting second detents, and the thickness of the locking elements are kept small as compared with the distance between adjacent ones of said second detents.

11. An appliance according to claim 7, wherein a plurality of first detents projecting inwardly from said spool are located, in the direction of rotation of the cutting head, immediately behind the outwardly projecting second detents and at the level of an annular flange of the spool.