GB2387341A

GB2387341A - Shredding machine with radially-tipped cutting teeth

Info

Publication number: GB2387341A
Application number: GB0208445A
Authority: GB
Inventors: Paul Arthur Aries; David Mead
Original assignee: Acco UK Ltd
Current assignee: Acco UK Ltd
Priority date: 2002-04-12
Filing date: 2002-04-12
Publication date: 2003-10-15
Anticipated expiration: 2022-04-12
Also published as: GB2387341B; GB0208445D0

Abstract

A shedding machine comprises two hexagonal shafts mounted for rotation about respective parallel axes, and a plurality of cutting discs 40 mounted on the shafts, with spaces between the cutting discs, with the cutting discs on one shaft inter-mashing with the discs on the other shaft, and being rotated in opposite directions so as to provide a nip into which a sheet of paper may be fed, between rollers 60a, 60b of shafts 62a, 62b. The circumference of each cutting disc is provided with an array of shallow circular recesses 52, between which is located a series of teeth 50 which are equally spaced around the circumferential surface of the disc 40. The tip of each tooth 50 projects radially outwardly from the centre of the disc 40, and the tip of each tooth is connected to its neighbour along a cutting edge by a circular arc 52. In the preferred embodiment the radius of the circular arc is between 1.5mm and 3mm, and subtends an angle of less than 180{, preferably approximately 100{. This results in a path length along the circumference of the disc which is less than 4mm, and thus shredding to a particle size of less than 4mm may easily be achieved.

Description

PATENTS ACT 1977

A10532GB - WL/AAL Title: Shredding Machines

Description of Invention

This invention relates to shredding machines of the kind (hereinafter referred to as being of the kind specified) comprising a cutting mechanism which comprises two shafts mounted for rotation about respective parallel axes, and cutting discs arranged at spaced intervals on each shaft, the cutting discs intermeshing and the shafts being rotated in opposite directions so as to provide a nip into which documentary material (such as paper) may be fed. The engagement between the circumferential edges of adjacent discs subjects the documentary material to a plurality of longitudinal cuts and the discs are provided with transverse cutting edges by which the material is subjected to a transverse severing.

Conventionally such shredding machines have been manufactured with two types of transverse cutting profile. The first type of cutting profile comprises a series of teeth which are angled in the direction of rotation of the discs. This maintains tension on the paper whilst it is being cut. The second type of cutting profile comprises a series of slots disposed around the circumference of the discs, edges of the slots parallel to the longitudinal axis of the discs providing transverse cutting edges of the discs.

The advantage of using slots rather than teeth is that the slots, in theory, cut the paper both on a leading edge and a trailing edge of the slot, although in practice the cut on the trailing edge is not wholly reliable. This makes the slots more efficient, particularly when shredding material into small particles, since twice the number of teeth would need to be provided compared to slots for the same particle size. Originally radial slots were used, but radially angled slots, such as those shown in Figure 1, have been developed since these have a greatly improved cutting action on the leading edge of each slot, the cut on the trailing edge having been sacrificed for an improved cut on the leading edge.

It is, of course, generally desirable to reduce the particle size produced by the shredding machine, since this leads to higher security. Indeed, one way of classifying shredding machines is by the size of particles that they produce. Shredders are designated dependent on this particle size, and certain government departments require their contractors etc. to have a certain security level before they will deal with them.

Currently shredders can produce a particle of, at best, 5mm by lmm. Changes in print technology, however, such as scaleable fonts and landscape style formats, have led to a requirement for a shorter particle to maintain current levels of security. A new standard of security has therefore been proposed, having a maximum particle size of 1mm x 4mm.

It is difficult to cut particles into such small sizes using traditional angled slot cutting profiles since, for a good quality of cut it is necessary to precisely intermesh the cutting discs on the two shafts. To do this, the leading edge of each slot of the cutting discs on one shaft must be aligned with a flat on the cutting discs on the adjacent shaft. With only a 4mm particle size the path length between two leading edges must be 4mm or less. The size of the flat between the slots will therefore be less than 2mm, perhaps significantly less than 2mm depending on the angle of the slots. This means that the two shafts must be very precisely aligned and that this alignment must be maintained throughout their operation. With typical conventional cutters such precision cannot be reliably achieved.

In addition, at such small particle sizes the power required to drive the cutting blades increases disproportionately, which means that such shredding machines tend to have a low sheet capacity.

There is, therefore, a need for a shredding machine which can reliably and economically achieve small particle sizes.

According to this invention there is provided a shredding machine of the kind specified in which the discs are provided with a series of teeth characterised in that the teeth have tips which are directed substantially radially outwardly.

Since teeth rather than slots are used the tips of the teeth do not need to be aligned with the cutting edge on the adjacent disc. There is, therefore, no problem with alignment. In addition, since the teeth are not angled radially, any increase in the distance along the cutting edge between the tips of the teeth may be equated with an increase in the depth of the trough between two teeth, which potentially increases the sheet capacity of the shredding machine.

Preferably the distance between the tips of the teeth along the circumference of the disc is less than 5mm, in a preferred embodiment substantially 4mm. This enables smaller particles sizes to be achieved.

The teeth may be joined at their base by an arc of a circle of any convenient radius. This prevents paper snagging in the gap between the teeth and further reduces the energy required for paper to be moved through the shredder.

Preferably the teeth have a cross-section which comprises a plurality of joined circular arcs. The arcs may each subtend an angle of between 600 and 1400, and preferably subtend an angle of between 900 and 1100.

Such teeth are convenient to manufacture and achieve a good balance between the height of the tooth and the amount of material necessary to provide it.

Preferably such arcs have a radius of between around 1.5mm and around 3mm, in a preferred embodiment the radius is substantially 2.25mm. Again, such a radius has been found to achieve a good balance between the strength of the tooth and height of the tooth.

According to this invention there is also provided a shredding machine of the kind specified, wherein the angle of attack, viz the angle between the cutting edge and the paper, is less than 900, preferably between 600 and 900.

It will be appreciated that this is converse to current thinking, where the tendency has been for the angle between the cutting edge and the paper to be increased, as the cutting edge is rotated to attack the paper at a more acute angle.

Additionally, it has been found, not only in shredding machines of the kind set out in the last preceding paragraph, but also shredding machines of conventional design, that paper fed into the nip is subject to lateral inward force, tending to cause the paper to be drawn inwardly in a "concertina" manner, as the paper tends to become drawn around the cutting discs. This tendency is particularly great, where the shredding machine is of the kind specified, particularly of the kind comprising slots disposed around the circumference of the discs, with edges of the slots extending parallel to the longitudinal axis of the discs slot. This tendency can under certain circumstances increase the size of the shredded pieces, which can result in a failure of a shredding machine to meets its design criteria regarding security.

According to this invention there is also provided a shredding machine of the kind specified, comprising restraining means adjacent to the entry to the nip to restrain a sheet of paper fed into the nip against lateral inward movement.

Such restraining means may be operative to subject the sheet to a widthwise tension, but is conveniently provided by a means to restrain movement of the sheet into the nip, such restraint simultaneously effecting a restraint against laterally inward movement (ie. concertina movement).

Thus, preferably the machine comprises restraining rollers through which a sheet of paper is fed, located adjacent to the entry to the nip, such rollers conveniently comprising a high friction surface such as a coating of rubber or plastics. The rollers may be caused to rotate, eg. against a restraint, by movement of the paper into the nip, but preferably are driven to convey the sheet into the nip, although preferably being driven at a rotational speed operationally less than the rotational speed of the cutting discs.

In this matter, the tension which is applied to the sheet between the restraining rollers and the cutting discs holds the sheet in width-wise tension, preventing inward movement of the sheet.

Whilst the invention set out in the last preceding paragraph but three is particularly advantageous in a shredding machine of the kind set out in the last preceding paragraph but thirteen, it is of course to be appreciated that the advantages may be utilised in other shredding machines of the kind specified.

According to this invention there is also provided a shredding machine of the kind specified comprising a pair of in-feed rollers mounted for rotation about axes generally parallel to the axes about which the cutting discs rotate, and located a short distance upstream of the nip, wherein the in-feed rollers produce a restraint to movement of a sheet through the nip of the machine.

Preferably the nip of the in-feed rollers is located a distance no greater than 10cm from the entry to the nip of the cutting mechanism, preferably a distance between 3cm and 8cm.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIGURE 1 is a perspective view showing the machine which is the preferred embodiment of this invention; FIGURE 2 is an end elevation of the machine; FIGURE 3 shows schematically a cross section through a prior art cutting disc; FIGURE 4 shows schematically a cross section through a cutting disc according to the invention; and FIGURE 5 shows schematically an enlarged front view of a nip roller.

The machine which is the preferred embodiment of this invention comprises two hexagonal shafts (not shown) mounted for rotation about respective parallel axes, and a plurality of cutting discs 10 mounted on the shafts, with spaces between the cutting discs, with the cutting discs on one shaft intermeshing with the shafts on the other disc, being rotated in opposite directions so as to provide a nip N into which a sheet of paper may be fed. As with the conventional machines, engagement between circumferential edges of adjacent discs subjects the documentary material to a plurality of longitudinal cuts, whilst, in accordance with the invention, the tip of each tooth subjects the strips to a transverse severing.

Each cutting disc 10 comprises a main face 12, a hexagonal bore 14, for mounting the cutting disc 10 on one of the shafts, a location hole 16 and a cutting edge 18.

The cutting edge 18 of a conventional cutting disc, shown in Figure 3, comprises a series of radially angled slots 20 equally spaced around the circumferential edge of the disc 10. Each slot has a leading edge 22, a trough 24, a trailing edge 26 and a flat 28. As the shaft rotates the paper moves around with the cutting edge 18 of the disc 10. It is pierced by each leading edge 22 and, at least in theory, by each trailing edge 26. The obtuse angle of the trailing edge means that this cut is not particularly reliable. The depth of each trough 24 limits the paper capacity of the shredding machine.

The cutting disc 40 in accordance with this invention, shown in Figure 4, has a superior cutting action. It comprises a main face 42, a hexagonal bore 44, a location hole 46 and a circumference 48. The circumference is provided with an array of shallow circular recesses 52, between which is located a series of teeth 50, which are equally spaced around the circumferential surface of the disc 40.

The tip of each tooth 50 points radially outwardly from the centre of the disc 40. In cross section, as shown in Figure 2, the tip of each tooth is joined to its neighbour along the cutting edge by a circular arc 52, which has a radius of 2.25mm and subtends an of less than 1800, specifically less than 1400. Preferably, the arc subtends an angle of approximately 1000. This results in a path length along the circumference of the disc of less that 4mm between the tips of adjacent teeth. Thus particle sizes of less than 4mm can easily be achieved.

The teeth are manufactured by laser cutting the circumferential profile into the disc. Alternatively the shape of the teeth could be formed by a milling operation. In order to improve the cutting properties further, in either case, the disc can then be sharpened by a grinding operation.

In accordance with the second aspect of this invention, the shredding machine comprises a pair of restraining rollers 68a, 60b mounted a short distance above the nip N, the rollers 60f, 60b providing an exterior surface of rubber, and being mounted on parallel shafts 62a, 62b.

The shafts 62a and 62b are driven by the drive mechanism of the machine which rotates the drive shafts of the cutting mechanism, at a rotational speed such that the surface speed of the rollers 60a and 60b is slightly less than the speed at which a sheet of documentary material is drawn through the nip N into the shredding mechanism.

In this manner, the sheet S is subject to a restraint, providing a longitudinal tension in the sheet over the area between the rollers 60a, 60b and the shredding mechanism, reducing tendency for the sheet to "concertina" in the width-wise direction, caused-by tendency of the sheet to be drawn into the circular recesses 52.

Preferably the rollers 60a, 60b are located close to the nip N, preferably the distance therebetween being less than 10cm, conveniently being between 3cm and 8cm.

To further improve the tensioning the rollers 60a may be inscribed with knurled lines inclined, for example, at 450 to the longitudinal axis of the rollers, as shown in Figure 5. These exert width-wise tension on the sheet S as it is fed into the nip N of the machine. If the rollers are inclined at 45ù to the longitudinal axes, tension is applied to the sheet in both the width-wise and the fed direction, stretching the paper in these directions and preventing "concertina-ing".

In the present specification "comprises" means "includes or consists of' and "comprising" means "including or consisting of'.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A shredding machine of the kind specified in which the discs are provided with a series of teeth characterised in that the teeth have tips which are directed substantially radially outwardly.

2. A shredding machine according to Claim 1 in which the distance between the tips of the teeth along the circumference of the disc is less than 5mm.

3. A shredding machine according to Claim 2 in which the distance between the tips of the teeth along the circumference of the disc is substantially 4mm.

4. A shredding machine according to any preceding claim in which the teeth are joined at their base by an arc of a circle of any convenient radius.

5. A shredding machine according to any preceding claim in which the teeth have a cross-section which comprises a plurality ofjoined circular arcs.

6. A shredding machine according to Claim 5 in which the arcs each subtend an angle of between 60ù and 140ù.

7. A shredding machine according to Claim 6 in which the arcs each subtend an angle of between 90ù and 1100.

8. A shredding machine according to any of Claims 5 to 7 in which the arcs have a radius of between around 1.5mm and around 3mm.

9. A shredding machine according to Claim 8 in which the radius of the arcs is substantially 2.25mm.

10. A cutting mechanism for a shredding machine comprising a plurality of shafts having cutting discs according to any preceding claim.

11. A shredding machine wherein the angle of attack, viz the angle between the cutting edge and the paper, is less than 900, preferably between 600 and 900.

12. A shredding machine comprising restraining means adjacent to the entry to the nip to restrain a sheet of paper fed into the nip against lateral inward movement.

13. A shredding machine according to Claim 12 wherein said restraining means is operative to subject a sheet to a width-wise tension.

14. A shredding machine according to one of Claims 12 and 13 wherein the restraining means is operative to restrain movement of the sheet into the nip, such restraint simultaneously effecting restraint against lateral inward movement of the sheet.

15. A machine according to any one of Claims 12 to 14 comprising restraining rollers through which a sheet is fed.

16. A machine according to Claim 15 wherein said restraining rollers are located adjacent to the entry to the nip of the machine. I1

17. A machine according to one of Claims 15 and 16 wherein said rollers are provided with a high friction surface.

18. A machine according to any one of Claims 15 to 17 wherein the rollers are caused to rotate at a rotational speed operationally less than the rotational speed of the cutting discs.

19. A shredding machine according to any one of Claims 1 to 11, being in accordance also with any one of Claims 12 to 18.

20. A shredding machine of the kind specified comprising a pair of infeed rollers mounted for rotation about axes generally parallel to the axes about which the cutting discs rotate, and located a short distance upstream of the nip, wherein the in-feed rollers produce a restraint to movement of a sheet through the nip of the machine.

21. A shredding machine substantially as hereinbefore described with reference to the accompanying drawings.

22. A cutting mechanism for a shredding machine substantially as hereinbefore described with reference to the accompanying drawings.

23. Any novel feature or novel combination of features described herein and/or shown in the accompanying drawings.