WO2010055532A1

WO2010055532A1 - A garden tool

Info

Publication number: WO2010055532A1
Application number: PCT/IT2008/000709
Authority: WO
Inventors: Nicola Cerreto
Original assignee: Emak S.P.A.
Priority date: 2008-11-13
Filing date: 2008-11-13
Publication date: 2010-05-20
Also published as: EP2346315A1

Abstract

A portable garden tool comprising a rotating tool (2) and a transmission system (7) which connects the rotating tool (2) to an activating motor (6), characterised in that the transmission system (7) comprises a rotating shaft formed by at least two tracts (70, 71 ) having a different torsion coefficient.

Description

A GARDEN TOOL

Technical Field

The invention relates in general to portable tools for gardening, which are provided with a rotating tool and a motor for powering the tool.

In more detail, the present invention relates to bush trimmers which are commonly used for cutting shrubs, bushes and the like.

Background Art

As is known, a bush trimmer generally comprises a rotating cutting tool, typically a disc provided with rigid blades or flexible nylon wires, which is protected by a surrounding safety casing and borne at an end of a rigid support rod provided with a grip.

The support rod is internally hollow and houses a transmission system for connecting the rotating cutting tool to an activating motor, typically a small internal combustion motor, which is generally fixed to the opposite end of the support rod.

A typical transmission system comprises a single rigid transmission shaft, which is rotatably housed in a coaxially position internally of the support rod, and is connected both to the cutting tool and to the activating motor.

A drawback of this transmission system however consists in the fact that during passage through medium-to-low rotation speeds, the transmission shaft is subjected to torsional resonance phenomena, generated by the irregularities of functioning of the activating motor, which resonance is not only transmitted to the user in the form of unpleasant vibrations, but is also responsible for rapid wear of the transmission shaft, which can be subject to early breakage.

A further known transmission system comprises a single flexible transmission shaft, which is generally used as it is more economical than the rigid shaft.

This transmission system is not, however, free of torsional resonance, which is manifested at different frequencies than with the rigid shaft, due to the different torsion coefficient. Beyond this, the flexible-shaft transmissions exhibit further drawbacks, among which, for example, the drawback of transmitting only low-entity powers, lack of sufficiently strong resistance to impacts and, in general, to turn in a lower performance than transmissions with a rigid shaft.

Disclosure of Invention

The aim of the present invention is to obviate the above-mentioned drawbacks, connected to torsional resonance phenomena, with a solution that is simple, rational and relatively inexpensive. These aims are attained by the characteristics of the invention as reported in the independent claim 1. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention.

In particular, the invention makes available a portable gardening tool comprising a rotating tool and a transmission system which connect the rotating tool to an activating motor, where the transmission system comprises a rotating shaft formed by at least two tracts having different torsion coefficients.

By torsion coefficient, reference is made to the ratio between the applied torque momentum and the relative rotation of two sections of the shaft located at a unitary distance.

In analytical terms, where the relative rotation of two sections located at-a unitary distance on a torsion shaft is expressable with the known relation dθ _ Mt ~fa^{~ ~ ~}GΪp where Mt is the torque moment applied, G is the modulus of tangential elasticity which depends on the material and Ip is the moment of polar inertia which depends on the transversal section of the shaft, the elastic torsion coefficient K is defined by K = GIp which depends only on the material and the geometric characteristics of the

shaft, and is expressable for example by the unit of measurement [ ]. rad In this way, the most elastic tract, i.e. the tract with the lowest elastic torsion coefficient, is able to absorb the torsional vibrations transmitted by the most rigid tract during the transitory speeds, thus eliminating excessive vibrations and reducing the wear to which the whole transmission system is subjected. The most elastic tracts are preferably interposed between the most rigid tracts and the rotating tool.

In a preferred aspect of the invention, the most elastic tract is realised by a flexible shaft, which is located coaxially and is connected in series to a more rigid shaft. The flexible shaft is preferably constituted by steel wires which form a series of helical spirals wound one upon another, alternatively in opposite directions, welded at ends thereof to connection sleeves, and possibly closed inside a flexible sheath which serves to keep the spirals in well-lubricated condition. Flexible shafts of this type are, for example, Bowden cables.

In a further preferred aspect of the invention, the transmission shaft formed by the most elastic tract and the most rigid tract is all inserted in the hollow and rigid support rod, with a rotating tool being fixed to an end thereof. Brief description of the Drawings Further characteristics and advantages of the invention will better emerge from a reading of the following description, which is provided by way of non- limiting example, with the aid of the figures of the accompanying drawings. Figure 1 is a schematic illustration of a bush trimmer. Figure 2 is a section view of the transmission system connected the bush trimmer motor of figure 1 to the relative rotating cutting tool.

Figure 3 shows the flexible shaft used in the transmission system of figure 2.

Figure 4 is the shaft of figure 3, seen from IV.

Figure 5 is the constructional structure of the flexible shaft 3.

Best Mode for Carrying Out the Invention The bush trimmer 1 comprises a cutting tool 2, which is typically constituted by a rotating disc provided with rigid blades or belts or flexible nylon wires. The cutting tool 2 is protected by a surrounding safety casing 3, and is borne at an end of a rigid support rod 4, which is provided with at least a grip 5 which can be gripped by the user during use.

The bush trimmer 1 further comprises an activating motor 6, in the example a small internal combustion engine, which is fixed to an end of the support rod

4, which end is opposite to a cutting tool 2.

The activating motor 6 is connected to the cutting tool 2 via the transmission system denoted globally by 7 and illustrated in figure 2.

The transmission system 7 schematically comprises a rigid rotating shaft 70 and a flexible rotating shaft 71 , which are reciprocally coaxial and are connected in series by means of a connecting joint 72.

The rigid shaft 70 and the flexible shaft 71 are both inserted coaxially internally of the hollow support rod 4 of the bush trimmer 1.

The rigid rotating shaft 70 exhibits a fluted end 73, which projects externally of the support rod 4 for mechanically coupling with the activating motor 6, and an opposite threaded end 74, which is screwed into a first axial hole of the connecting joint 72.

The rigid shaft 70 is rotatably coupled and supported internally of the support rod 4 by one or more guide bushings 75, and is axially blocked by means of known type and not illustrated in the figures.

The flexible rotating shaft 71 is substantially conformed as a Bowden cable.

As illustrated in figure 5, the flexible shaft 71 comprises a central core 76 made of metal or plastic on which steel wires 77-80 are wound, which wires

77-80 form a series of helical spirals wound one on another, alternatively in opposite directions.

The steel wires 77-80 can be en masse closed within a flexible sheath which serves to maintain the spirals in well-lubricated conditions.

As can be understood from figures 3 and 4, the ends of the helical spirals are welded to two connecting sleeves 81 and 82, which both have a polygonal transversal section which in the illustrated example is square. Thanks to the particular conformation, the flexible shaft 71 has a torsion coefficient K which is decidedly lower than that of the rigid shaft 70, which is formed by a single steel bar with a preferably solid transversal section. The flexible shaft 71 must have an overall length Q which is preferably above 50 mm, for example about 100 mm.

The connecting sleeve 81 is coupled to the connecting joint 72, while the connecting sleeve 82 projects externally of the support rod 4 where it is coupled to a conical cogged pinion 83, which is rotatable and axially blocked with respect to the support rod 4. In more detail, the connecting sleeve 81 is inserted in a second axial hole of the connecting joint 72, which exhibits a corresponding polygonal transversal section, such as to realise a sliding coupling which blocks them reciprocally in a rotational direction. Similarly, the connecting sleeve 82 is inserted coaxially in a central through- hole of the cogged pinion 83, which exhibits a corresponding polygonal transversal section, such as to realise a sliding coupling which blocks them reciprocally in a rotational direction.

The connecting sleeve 82 is further axially blocked by a block 84 of larger diameter, which is fixed to an end of the sleeve 82 and is received in an annular recess of the central hole of the cogged pinion 83.

In this way, the flexible shaft 71 is substantially stretched between the connecting joint 72 and the pinion 83, such as always to be coaxial to the rigid shaft 70. The conical cogged pinion 83 enmeshes with a conical cogged crown 85, which is keyed on a tool-bearing chuck 86 having a rotation axis which is inclined with respect to the flexible shaft 71 and the rigid shaft 70. The cogged pinion 83 and the tool-bearing chuck 86 are received internally of a single hollow rigid body 87, which comprises two cylindrical portions 88 and 89, reciprocally communicating, which are destined respectively to receive the cogged pinion 83 and the tool-bearing chuck 86.

The cylindrical portion 88 is inserted and fixed to an end of the support rod 4. The cogged pinion 83 is rotatably coupled to the cylindrical portion 88 by means of interposing a pair of radial ball bearings 90, which are axially blocked between the conical head of the cogged pinion 83 and the front edge of the support rod 4. The tool-bearing chuck 86 is in turn rotatably coupled to the cylindrical portion 89 by interposing a pair of radial ball-bearings 91 , which are axially blocked by a ferrule 92 which closes the open end of the cylindrical portion 89. The tool-bearing chuck 96 exhibits a tract which projects from the closing sleeve 92 towards an outside of the cylindrical portion 89, on which the cutting tool 2 is keyed.

In use, the motor 6 sets the rigid shaft 70 and the flexible shaft 71 in rotation about the axis thereof, such that the enmeshing between the cogged pinion 83 and the cogged crown 85 causes the tool-bearing chuck 86 to rotate, which chuck 86 in turn draws the cutting tool 2 in rotation.

The torsional vibrations transmitted by the motor 6 are in part absorbed by the flexible shaft 71 , the coils of which tend to twist, squeezing against one another and thus demonstrating a certain torsional elasticity. In this way, it is possible to limit the vibrations which are produced and transmitted during the passages through average-to-low rotation speeds, consequently reducing the stresses and therefore the wear to which the transmission system 7 is subjected.

Obviously a technical expert in the sector might make numerous modifications of a technical-applicational nature to the above-described bush- trimmer 1 , without forsaking the ambit of the invention as claimed herein below.

Claims

1 ). A portable garden tool comprising a rotating tool (2) and a transmission system (7) which connects the rotating tool (2) to an activating motor (6), characterised in that the transmission system (7) comprises a rotating shaft formed by at least two tracts (70, 71 ) having a different torsion coefficient. 2). The tool of claim 1 , characterised in that the tract having a lower torsion coefficient is a flexible shaft (71 ).

3). The tool of claim 2, characterised in that the flexible shaft (71 ) comprises metal wires (77-80) spiral-wound about one another.

4). The tool of claim 2, characterised in that the flexible shaft (71 ) is coaxial and is connected in series to a rigid shaft (70), which is the tract with a higher torsion coefficient.

5). The tool of claim 4, characterised in that the rigid shaft (70) and the flexible shaft (71 ) are connected by a connecting joint (72).

6). The tool of claim 1 , characterised in that the rotating transmission shaft is inserted internally of a hollow and rigid rod (4).

7). The tool of claim 6, characterised in that the hollow and rigid rod (4) bears the rotating tool (2).

8). The tool of claim 1 , characterised in that the tract (71 ) having a lower torsion coefficient is interposed between the rotating tool (2) and the tract (70) having a higher torsion coefficient.

9). The tool of claim 8, characterised in that the tract (71 ) having a lower torsion coefficient is connected to the rotating tool (2) by means of a gearing

(83, 85).

10). The tool of claim 1 , characterised in that the rotating tool (2) is a cutting tool.