Note: Descriptions are shown in the official language in which they were submitted.
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;
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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
-- 4 --
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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
-- 5 --
~ 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
-- 6 --
~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
-- 7 --
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
- 10 -
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
- 12 -
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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
- 13 -
~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.
