IMPROVEMENTS IN OR RELATING TO SHREDDER CUTTERS

The present invention relates to improvements in or relating to cutters for shredders, in particular to cutters for shredders, in particular for vegetation shredders, suitably of the type used for shredding tree branches and garden waste. The present invention relates to a novel cutter and method for manufacturing the cutter and to a shredder including the cutter.

Figure 1 shows a typical prior art shredder assembly for a vegetation shredder. The assembly comprises a cylindrical shredder element 10 carried by a shaft 11 of a reduction gear box connected to an electric motor 12 and thereby caused to rotate. Element 10 includes teeth 13 which, in use, bear against a reaction plate 14. The assembly is surrounded by a housing 15 which acts to contain any vegetation impacting upon the assembly and direct it to be crushed and cut between the shredder element 10 and reaction plate 14. In a typical shredder, shredder element 10 has eight blades and rotates at around 40 revolutions per minute.

A disadvantage of such arrangements is that crushing, whilst useful for breaking fibres, is comparatively inefficient in its use of the power available compared with cutting. Accordingly, alternative arrangements have been proposed. One such arrangement is described in US 5 836 528. The shredder cutter is generally of frusto- conical appearance comprising a cast aluminium hollow rotatable drum formed with a plurality of axially elongate tapering apertures angularly spaced around the body of the drum. A steel blade is secured adjacent to each aperture at an inclination of between 30° and 45° to the base of the frusto-conical drum. This arrangement is effective but complex to manufacture and to maintain in good working order ensuring correct orientation of the blades is maintained. Accordingly, there is a need to provide an alternative shredding assembly for a vegetation shredder.

In one aspect, the present invention provides a cutting element for a vegetation shredder, the cutting element being a hollow unitary element comprising a plurality of

elongate blades arranged in a generally cylindrical configuration between a proximal and distal support elements.

Suitably, the proximal support element is in the form of a disc.

Suitably, the distal support element is in the form of a ring.

Preferably, the cylindrical configuration has a generally frusto-conical form, the distal support element forming a base of said frusto-conical form.

Preferably, each blade is aligned to taper inwardly from the base towards the proximal support element.

Preferably, the cutting element is formed with elongate apertures between adjacent blades.

Preferably, the distal end includes a circumferential ring joining the end faces of the blades.

Preferably, each blade has a body having an axis at an angle of 20° to 40° to the radius of the cutting element, more preferably 23° to 37°.

The present invention also provides a method of manufacturing a vegetation shredder cutting element, the method comprising the technique of investment or lost-wax casting, wherein the cutting element comprises a plurality of elongate blades arranged in a generally cylindrical configuration between proximal and distal support elements and wherein the cutting element is formed as a unitary element.

Preferably, the cutting element is cast from steel. More preferably, the cutting element is cast from a low carbon steel. Even more preferably, the cutting element is cast from low carbon alloy steel.

Preferably, the method further comprises a step of hardening, more preferably gas carbonitriding or induction hardening, most preferably, gas carbonitriding.

Suitably, the cutting element is formed having a surface hardness of 55HRC.

The present invention also provides a vegetation shredder comprising a shredder body, housing a shredder cutter assembly within a shredder cutter chamber wherein the shredder body includes a feed inlet for receipt, in use, of vegetation to be shredded, wherein the feed inlet is in operative communication with the shredder cutter chamber by means of a generally elongate feed chute; characterised in that the shredder cutter assembly comprises a cutting element as described above.

Preferably the distal end of the cutting element is supported by a bearing having a low coefficient of friction. More preferably, the coefficient of friction is 0.1 or less, even more preferably 0.05 or less. Most preferably, the bearing is a PTFE loaded polymer- faced metal-backed plain bearing of low friction.

Preferably, the vegetation cutter further comprises a reaction plate formed with a front face having two distinct surfaces such that their joint intersection with the cutter diameter generally forms a V-shape.

Preferably, the lowest point of the V-shaped form of the intersection is provided from 15 to 30mm from the proximal end of the cutting element.

Preferably, a first surface of the reaction plate has a profile edge which extends over a degree of cutter rotation of from 20° to 35°, preferably about 30°.

Preferably, a second surface of the reaction plate has a profile edge which extends over a degree of cutter rotation of from 0° to 10°.

The above and other aspects of the present invention will now be illustrated, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is an exploded view showing a typical prior art shredder assembly; Figure 2 is a perspective view of an embodiment of a vegetation shredder in accordance with the present invention;

Figure 3 is a perspective view of an embodiment of a shredder cutter element in accordance with the present invention;

Figure 4 is a sectional end view along line IV-IV of Figure 7; Figure 5 is an exploded perspective view of a shredder assembly incorporating the cutter element of Figure 3;

Figure 6 is an exploded view of the shredder cutter of Figure 3 with a reaction plate;

Figure 7 is a plan view of the cutter and reaction plate of Figure 6;

Figure 8 is a detail front and side view of the reaction plate of Figures 6 and 7; and Figure 9 is a sectional view through a assembled shredder assembly.

Figure 2 shows a vegetation shredder including a shredder body 20 housing a shredder cutter assembly mounted on a frame 21 above a collection box 22 in operative communication with an outlet 23 of the shredder body 20. The shredder body 20 has a feed inlet or hopper 24 for receipt of vegetation to be shredded, such as branches or twigs of a tree. Feed inlet 24 is spaced from the cutter assembly by means of a feed chute 25, to prevent user access to the cutter assembly during use.

Referring to Figure 5, there is shown the cutter assembly of the shredder, comprising an electric motor and gearbox unit 30 having a rotatable shaft 31 carrying a torque transmission means 32 for transferring torque to the cutter. A cutting element 33 in accordance with the present invention is mounted upon transmission means 32. The cutter assembly includes a housing 34 to contain the cutting element 33 and define a shredding cavity. The housing includes an operatively upper opening or inlet 35 for receipt of vegetation to be shredded and an operatively lower opening or outlet 36 for ejection of shredder vegetation. The housing 34 also includes a cover 40 which acts

to hold and support cutting element 33 in position. The cutter is supported by a thrust and radial plain bearing at its proximal end. The distal end bearing supporting the outside diameter of the distal ring should be a low friction bearing due to this large diameter. Rolling contact bearings are not preferred due to cost, space and assembly reasons. A polymer faced low friction bearing 41c has been used satisfactorily. This is illustrated more clearly in Figure 9. Typically constructed of a backing sheet faced with a bronze fibre matrix impregnated with polytetrafluoroethylene (PTFE), such bearings exhibit a remarkably low coefficient of friction, being of the order of 0.05 under test conditions. The bearing is protected by seal elements 41b and scraper ring 41a.

Mounted within housing 34 is a reaction plate 42 against which the cutting surfaces of cutting element 33 bear in operation.

Cutting element 33 is shown in more detail in Figure 3. In the preferred embodiment, the cutting element 33 has a generally frusto-conical configuration having a proximal disc 43 for receipt of transmission means 32 and a distal ring 44 which is supported by and, in use, bears against bearing 41c in housing cover 40.

Between disc 43 and ring 44 are a plurality of blades 45 spaced around the periphery of the cutting element each blade having a cutting edge 50 formed between an outer blade surface 50a making an angle B to the radius of the cutter and an inner blade surface 50b making an angle A to the radius of the cutter. Suitably angle B is about 45° and angle A is about 15°. The individual blades 45 are substantially elongate and uniform in cross section and are arranged with the main axis C (Figure 4) of this section at an angle of 20° to 40°, preferably 23° - 37°, to the radius of the cutter. At the distal end the blades are joined to the distal ring with the inclusion of a full circumferential lip 44a to ensure good load spreading. This lip nevertheless does not extend to the full blade outer diameter so as to allow access for grinding or milling both front and rear faces of the blade after heat treatment.

At the proximal end, the blades merge into the proximal disc (again with clearance to enable grinding or milling of the cutting edges) which has a diameter of 70 - 90mm preferably 80mm giving a cone angle of around 15 - 20° per side. The speed of rotation is preferably about 40 rpm and the number of blades 8.

As the cutting element 33 rotates, in use, vegetation such as branches or twigs, entering the shredding cavity is urged in the direction of the reaction plate 42 by virtue of the rotation of the cutting element 33, whereupon blades 45 bear against the reaction plate 42 to cut and tear the fibres of the vegetation. The shredded material is ejected in two ways, from the outlet 36 at the bottom of the shredding cavity and from within the cutting element itself. Shredded vegetation falls between the blades 45 through elongate apertures 51 into the open body of the cutting element 33. Due to the frusto-conical shape of the cutting element, the vegetation is caused to tumble towards the distal end of the cutter where it is ejected through the distal end ring 44 and thence through a central aperture 52 provided in housing end cover 40. In preferred embodiments, blades 45 are provided with projections on a rear edge or surface thereof, which projections 53 project into the open body of the cutting element 33, providing a base to transfer stresses into the rest of the structure of cutting element 33.

For optimal outflow of shredded material it has been found that an inner diameter of the distal ring in the range of 100 - 130mm is particularly advantageous, preferably about 115mm.

The cutting element 33 and a preferred embodiment of a reaction plate 42 are illustrated further in Figures 6, 7 and 8. Reaction plate 42 is pivotally mounted towards an upper edge thereof by means of a pin 61 about an axis suitably parallel to the axis of rotation of the motor/gearbox output shaft 31. The reaction plate 42 is adjustable into contact with the cutting element by means of an adjusting screw, or other suitable means. The reaction plate 42 has a profiled edge 60 against which

blades 45 bear to produce a scissor-like motion, thereby cutting vegetation passing into the shredding cavity.

Ordinarily, vegetation is free to slide to either end of the cutting element where wood fibres can wrap around the motor/gearbox output shaft or the chippings produced can have restricted freedom to move and be ejected from the shredding cavity. In these circumstances, blocking of the cutter assembly can occur or a reduced cutting width is produced and thus lower efficacy. Accordingly, in preferred embodiments, the front face of reaction plate 42 is formed with two distinct surfaces, a distal surface A and a proximal surface B, such that their joint intersection with the cutter diameter (line 60) forms a generally "V" shape (see Figure 8). Therefore as the cutter rotates an individual cutter blade first contacts line 60 at the distal end (point C). As the cutter rotates further this point of contact travels along the cutter blade in a scissor like action. At or shortly before the moment when this point of contact reaches point D the other end of the blade also contacts line 60 at the proximal end (point E) producing a second point of contact also travelling simultaneously towards point D as the blade rotates. This action naturally guides vegetation to the optimal position suitably distant from the obstruction of the proximal disc 43 while providing a lower energy scissor cutting action for larger diameters of material. Preferably point D is 15 -30mm ideally 20 mm from the proximal end. Preferably the point of intersection to surface A extends over a large degree of cutter rotation (20-35°), preferably about 30°) while for the intersection to surface B the cutter rotation is from 0 to 10°. In an alternative embodiment, not shown, portions A and B are formed as an arcuate surface rather than as two discontinuous portions.

A further advantage of the cutting element of the present invention lies in its method of manufacture and this forms a further aspect of the invention. As the cutting element 33 is formed as a unitary component, we have developed a process for the manufacture of the cutting element by means of investment casting, also known as lost-wax casting. Casting allows the cutting element to be formed as a unitary component with complex shaping, including undercutting as necessary, and requiring

little post-casting machining. Following casting, the cutting-element 33 is hardened. Gas carbonitriding is preferred to induction hardening. The casting is formed from steel, preferably a low carbon steel, suitably including alloying elements and having a surface hardness of around 55HRC on the Rockwell C hardness scale, at which level edge chipping is resisted. A molybdenum/chromium/nickel steel suitable for surface hardening is particularly suitable. Vanadium at around 0.15wt% can be included to provide a refined grain structure if required.