The invention relates to cutters for a rotary- shredding machine, and to rotary shredding machines of the kind (hereinafter referred to as a "machine of the kind hereinbefore specified") having, a comminuting chamber, a pair of parallel cutter shafts arranged for simultaneous contra-rota ion in the comminuting chamber, and a plurality of said cutters carried by the shafts, at least one of the shafts having more than one said cutter secured thereon and the cutters of one shaft being interleaved with the cutter or cutters of the other, so as to co-operate in comminuting material fed into the chamber. The cutters to which the invention relates are of the kind comprising a generally disc-like body having at least one radially-projecting peripheral tooth provided with a cutting edge, along a leading edge thereof, and the body defining a coaxial shaft aperture therethrough to accommodate a said shaft and defining a cutter axis. Such a cutter will be called a "cutter of the kind hereinbefore specified".

Although machines of the above kind are normally referred to as shredding machines or shredders, their comminuting action takes a form or forms which depend largely on the nature of the material being comminuted, and on the design of the cutters. The latter may in practice perform very little cutting as such; for example, glass will tend to be crushed into small pieces, whilst other common materials, such as thin metal, will

tend to be torn and/or deformed by crushing. The materia to be comminuted ismost usually scrap or waste material, though shredders can be used to break up solid materials as part of, or in preparation for, industrial processes of various kinds.

Various types of shredding machine of the kind hereinbefore specified are in commercial use or have been proposed. For example, such patent specifications as those of British patents Nos.13 53-7, 1310057. and 1491611 variously teach the use of cutter shafts of cylindrical o hexagonal cross-section, with various means for securing the cutters to the shafts. Hexagonal shafts, by virtue o their shape, have an advantage in that if the cutter has a hexagonal hole fitting the shaft, the former cannot slip around the latter. On the other hand, a circular shaft is very much easier to make and probably cheaper, but may require additional expedients such as splining, keying or the use of suitable adhesives, to ensure that t cutter will not rotate with respect to the shaft.

According to the present invention, in a first aspect, in a machine of the kind hereinbefore specified, each cutter shaft is of substantially square cross-sectio a shape which is both simple to machine and enables the shaft aperture of each cutter to be made with a minimum number of stress raisers without the disadvantages attendant on a cylindrical shaft.

In a second aspect, the invention provides a

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-3- cutter for mounting on such a cylindrical cutter shaft in a machine according to the invention, wherein the shaft aperture of the cutter is itself substantially square, the cutting edge of the or each tooth being intersected by a radial plane inclined at an angle in the range 0° to 60° with respect to a diametral plane bisecting one side of the square shaft aperture, said angle being defined forward of the cutting.edge in the direction of intended rotation of the cutter. The value of this angle is preferably chosen so that the optimum strain pattern is set up in the cutter body members when the cutting edge of the tooth is subjected to a tangentially applied force, i.e. the most even stress distribution throughout the cutter consistent with the greatest local strain at any one point in the cutter being at an acceptable level. Tests for one design of cutter according to the invention have established on this basis an optimum value of 50° for this angle. A cutter having a square shaft aperture, according to the invention, is preferably of split form having two or more cutter body members held together by releasable fastening means, with a tooth carried, integrally or as a separate member, by at least one of the body members and each body member having two first surface portions extending chordally from the body periphery, with each first surface portion juxtaposed w th a parallel, corresponding portion of the next

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-4- adjacent body member. Such a cutter is preferably further constructed according to the specification of co-pending British patent application No.28953/78 _*

Furthermore, the machine is preferably of the kind generally described in our co-pending British patent application No.34262/76.

Embodiments of the invention will now be described, by way of example only, with reference to t drawings hereof, in which:

Figure 1 is a simplified side elevation, as from the bottom end of Figure 2, of a rotary shredding machine;

Figure 2 is a plan view taken on the line II in Figure ;

Figure 3 is a plan view of part of a cutter shaft carrying cutters, according to the invention;

Figure 4 is a sectional view taken on the li IV-IV in Figure 3 ,

Figure 5 is a scrap sectional view similar t Figure 4 but showing an effect of an impact force on a cutter; and

Figure 6 is an axial elevation of a cutter i one possible modified form.

The shredding machine (shredder) shown in Figures 1 and 2 has a base frame 10 on which are mount a cutter box 11 and a gearbox 14. The cutter box 11 encloses a rectangular comminuting chamber 12 which is

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-5- open at top and bottom. A loading hopper 13 is fixed on top of the cutter box 11. Extending through the chamber 12 and gearbox 14 are a pair of parallel cutter shafts 16-.17- A motor 15, mounted on the gearbox 14, has a shaft 19 driving- a clutch 20, whose driven shaft 26 carries, a worm 21 which drives a worm wheel 22 carried on, but rotatable independently of, the cutter shaft 17. The wheel 22 drives the cutter shaft 16 through a pinion 23 on the latter, whilst the cutter shaft 17 is driven by a pinion 24 on the shaft 16 through a gear 25 on the shaft 17 so that the latter is rotated in the opposite direction to the shaft 16, as indicated by the arrows in Figure 1, and at a slower speed.

The shredder is preferably constructed according to the principles described in our co-pending British patent application No.34-262/76 aforementioned.

Each of the cutter shafts 16,17 is mounted in end bearings in the opposite end walls 27,28, and also a bearing in a centre plate (not shown), of the gearbox and cutter box respectively, and that part of each cutter shaft that extends through the cutter box is of square cross- section as indicated in Figure 1. Each shaft 16,17 carries six cutters 18 which are secured on the shafts, each cutter being spaced by an equal amount from the next such that the cutters of the shaft 16 are interleaved with those of the contra-rotating shaft 17, so as to co-operate with them in comminuting material fed from the hopper 13 into the

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-6- chamber 12.

Each of the cutters 18 comprises a generally disc-like body having at least one radially-projecting peripheral tooth provided with a cutting edge. Each cutte body, furthermore, comprises two body members each having two first surface portions extending chordally from the body periphery and joined by a second surface portion which engages the cutter shaft, so that these second surface portions together constitute the sides of a square coaxial through aperture in which the respective cutter shaft is accommodated. This aperture defines the cutter axis which is coincident with the axis of the correspondin shaft 16 or 17. Each of the chordal first surface portions of one of the body members is juxtaposed with, but spaced from, a parallel, corresponding one of the chordal first surfaces of the other body member, and the two body members are held together and clamped on the shaf by releasable fastening means. An embodiment of such a cutter which may advantageously be incorporated in the shredder of Figures 1 and 2 will now be described.

Referring therefore to Figures 3 and 4, four identical cutters 30,31,32,33 are in this example mounted on part of the square-section cutter shaft 16. The body of each cutter 30 to 33 comprises a first and larger body member 34 and a second and smaller segmental body member 3 The member 35 has a chordal plane surface whose first or outer portions 43,44, extending from the cylindrical

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-7- peripheral surface 53 of the cutter body, are joined by the shaft engaging surface portion or face 48. The outer surface portions 43 and 44 are juxtaposed with plane surface 45 and 46 respectively of the member 3 , with which they are parallel but from which they are spaced by a narrow gap 47. The sides of the square shaft aperture, the centre of which is the cutter and shaft axis 54, consist of the face 48 and three chordal faces 49,50,5 joining the surfaces 45 and 46 of the larger member 34.

The releasable fastening means comprises a pair of elongate fasteners in the form of a stud 38 and a stud 39, both fixed in the body member 34 and extending through, respectively, the pair of surfaces 43,45 and the pair of surfaces 44,46. The head of each stud lies in a respective recess 42 in the outer peripheral surface of the segmental member 35, and bears on the bottom of the recess through a Belleville washer 40,41. The larger body member 34 is thus mounted, through the studs and the Belleville washers, resiliently upon the segmental member 35; the two members _\k- and 35 together constitute a disc¬ like body having opposed, parallel, flat side faces .

The cutter can be removed from the shaft 16 by removing the studs 38 and 39 and drawing the two body members _>k- and 35 radially outwards.

Each of the larger body members 36 has a single, integral, radially-projecting tooth 36 whose cutting edge 37, at the leading end of the tooth in the direction of normal

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rotation of the cutter (indicated by the arrow 13 in Figure 4) is parallel with the axis 5 , and lies in a radial plane 55 which is displaced, rearwardly with respect to the direction B_, by an angle A from the diametral plane 6 which bisects the shaft 16 and the face 48 of the segmental member. The angle A is in the range 0° to 60°, but in this example it is 50°.

In operation, the cutters are rotated as indicated in Figure 1 and matter to be comminuted is fed down on to them from the hopper 13, to be broken up by th cutters in known manner and discharged through the open bottom of the chamber 12. If an object of tramp material (e.g. an iron bar or other object which the cutters cannot break up) is introduced, the drive mechanism is reversed several times and, if the object is still there, the machine is then stopped. This is achieved automatically by a suitable control system not shown.

Impact of the cutting edge 37 of a cutter upon an object produces a force on the edge 37 having a tangential component F _, (Figure 4). During normal operation such a force exists as the cutting edge comes into contact with material to be comminuted, but the cutt continues to rotate with the cutter body members clamped together in the relative disposition shown in Figure 4. If however, due for example to impact of the cutting edge upon an object of tramp material, the force _ is greater than a value which can be predetermined by providing a

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-9- suitable stiffness of the Belleville washers 40,41, this force exerts a rearward turning movement upon the body member 3 which overcomes the stiffness of the washer 41 and causes the member 3 to undergo a' limited tilting movement with respect to the segmental member 35. This tilting, which takes place in a matter of a few micro¬ seconds, is shown (somewhat exaggerated) in Figure 5. The washer 40 is such that it continues to exert a force between the head of the stud 38 and the bottom of the corresponding recess 42.

It will be realised that the shaft aperture 57 (defined by the faces 48 to 5 ) in the cutter is a close, sliding fit on the shaft 16, though not an interference fit. The .tilting action of the cutter body member 34 is thus accompanied by some simultaneous elastic deformation of the latter in the vicinity of the faces 49 to 51, so that much of the energy imparted by the force F_ under crash-stop conditions is dissipated as strain energy due to this momentary deformation. In Figure 5 the faces 49 to 51 are shown diagra matically, their deformation not being illustrated. As soon as the rotation of the shaft 16 is reversed and/or the force F is otherwise removed, e.g. by removal of the tramp material, the body member 34 is restored automatically to its normal position relative to the member 35 as shown in Figure 4.

Referring now to Figure 6, this shows one of a number of variations which are possible in the construction

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of a cutter according to the invention. The cutter in Figure 6 is a double-toothed cutter having one tooth 36 formed in each of its two identical body members 60. The members 60 are again arranged to be clamped, by studs 38.39 resiliently mounted by Belleville washers in recesses 42 in the body members, around the square shaft 16, and for this purpose each body member in this particular embodiment has two shaft-engaging faces 61 at right angles to each other and at 45° to the pairs of chordal surfaces, 62, which in this case define opposed diametral gaps 63 between them to allow for tilting of either one of the members 60 relative to the other under crash stop conditions. The shank of the stud 38 is in this embodiment secured in one of the members 60 and that of the stud 39 is secured in the other.

Figure 6 shows each fastening stud 38,39 mounted by a pair of Belleville washers 64 instead of a single washer as in Figure ^' s 4 and 5« It will be realised that in either embodiment, or indeed in any other embodiment of cutter according to the invention having resilient mounting means -in the form of Belleville washers, the latter may be provided singly or in groups of two or more. Furthermore, in the latter case they may be arranged back-to-back as in Figure 6, i.e. in series, or in nesting relationship, i.e. in parallel.

Although Figure 6 shows the shaft 16 orientated with a diagonal plane coincident with the diametral plane,

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-11- 65, defined by the gap 63 between the two cutter body members 60, each of the latter may be formed with a rectangular recess such that the two rectangular recesses together form a square shaft aperture in which the diametral plane 65 bisecting the shaft is parallel with two sides of the shaft.

Furthermore, it is not essential that the two chordal surfaces of each cutter body member associated with the fastening means (e.g. the surfaces 43;44; 45,46; or 62) lie in a common plane. Thus, for example in Figure 4, the member 35 could be made with a second shaft- engaging face perpendicular to the face 48 and engaging the side of the shaft which in Figure 4 is engaged by the face 51 of the member 34. The face 44 would then be continuous with, this second shaft-engaging face, with the member 34 modified accordingly. Such an arrangement may be convenient irrespective of the number of teeth 36 per cutter, but may be especially useful if it is desired to provide an odd number of teeth, for example three.

The fastening means of the cutter need not consist of studs, though threaded studs as shown, or bolts with separate nuts, are a convenient form of fastening. Preferably the fastenings will be provided with a suitable locking device, in any known form, for resisting rotation of the stud, bolt or nut during operation of the machine due to vibration or other similar causes.

It will also be appreciated that the Belleville

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washers instead of being interposed under the heads of the studs 38,39, could be mounted in recesses in the faces 43,44, to bear directly on the faces 45,46 respectively. Furthermore coil springs may be employed instead of Belleville washers.

It will be understood that in cutters according to the invention, the cutting teeth may be separate members attached by suitable means to the body members.

The ^' machine itself may or may not have a clutch. The cutter shafts may or may not be arranged for rotation at different speeds; the cutter shafts may have their axes in a common horizontal plane; there may be any desired number of cutters on each shaft; and any suitable arrangements for delivering material to the cutters for-comminution, and for collecting it after comminution, may be provided.

In Figures 3 and 4 each cutter is shown displace y 90° with respect to the next one on the same shaft, so that the cutting edges 37 define a helix. It will be understood however that any relative orientation, i.e. angular displacement, of the cutters, may be chosen according to the particular application of the machine, subject to the condition being met, that, the cutting edge of each tooth is intersected by a radial plane inclined, at an angle of 0° to 60°, to a diametral plane bisecting one side of the square shaft aperture, the angle being

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-13-. defined forward of the cutting edge in the direction of intended rotation of the cutter. It will be observed, by way of illustration, that this condition is in fact met in Figure 4, in which the angle A of 0° is defined forward of the edge 37• Again, provided that this condition is met, the cutter may be provided with any number of teeth consistent with there being enough space around the circumference to accommodate them.

Each .cutter may comprise more than two body members, particularly for use in very large machines where a large cutter diameter may be called for. The construction of such a cutter may for example be a straightforward adaptation of that shown in Figure 6, but with one tooth on each body member and with the two surfaces 62 of each body member lying in radial planes subtending an angle which depends on the number of body members. Here again the abovementioned condition must be satisfied.

It will also be understood that, whilst the provision of resilient mounting means (such as the Belleville washers) between adjacent cutter body members is highly advantageous, if they are omitted tilting and elastic deformation of one body element relative to another, under crash-stop or other exceptional impact conditions, can still take place with consequent rapid absorption of part of the energy released by the impact.

It will also be appreciated that any suitable

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-14- type of cutter may be used in a machine according to th invention having substantially square cutter shafts, provided that the abovementioned condition as to the relative orientation of the tooth leading edge and the square shaft is satisfied. Thus the cutters may be of kind comprising a single body member in which the shaft aperture is formed as a through hole. Even if the cutters are of a "split" kind, having more than one bod member, they need not have the facility for tilting und abnormal impact conditions as described herein, but may be of a kind that is clamped rigidly on to the cutter shaft.

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