GB1589214A - Rotary shredding apparatus - Google Patents

Abstract

The comminution machine serves for comminuting waste products such as household refuse, industrial waste and the like. It has two shafts (14, 15) which bear the comminution tools (16). The shafts (14, 15) are driven in opposite directions via a downstep gear (25, 26). A motor (9) drives the drive shaft (22) of the gear via a clutch (21), the clutch (21) being constructed such that it is automatically triggered when the motor (9) exceeds a predetermined maximum torque. As a result, components of the machine are prevented from being damaged. <IMAGE>

Description

(54) IMPROVEMENTS RELATING TO ROTARY SHREDDING APPARATUS (71) We, METAL Box LIMITED, a British company of Queens House, Forbury Road, Reading RGl 3JH, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to rotary shredding apparatus of the kind comprising a plurality of parallel cutter shafts carrying rotary cutter elements in a comminuting chamber of the apparatus; drive means for rotating one of the cutter shafts and including a rotor normally operating at a predetermined high rotational speed whereby the rotor exhibits substantial inertia; reduction gearing coupling the drive means with said one cutter shaft; and further gearing coupling said shafts together for contra-rotation of each shaft with respect to the or each next adjacent shaft at velocities substantially less than that of the rotor, whereby the cutter elements of respective contra-rotating cutter shafts co-operate to comminute matter introduced into the comminuting chamber. Such apparatus will be referred to hereinafter as "shredding apparatus of the kind hereinbefore specified".

Many forms of such apparatus have been proposed and are well documented in published literture; in addition a considerable number of shredding machines embodying such apparatus have been marketed and used in various countries for at least the past seven or eight years. Their principal application is of course in the destruction of waste products, for example general household refuse, refuse from catering establishments, certain kinds of industrial waste or refuse on board ship. Larger machines of the same general kind are in use for shred ding discarded rubber tyres.

Whether the apparatus is used for the comminution of general or heterogeneous matter, or whether it is used for more specific single purposes such as the shredding of tyres, there is always a substantial danger of some object being introduced into the apparatus such that the apparatus cannot readily deal with it. Such an object is dne which offers such a high resistance to the cutting elements that, in apparatus of the kind hereinbefore specified which is designed for general household or similar refuse, it usually jams the apparatus and so brings the latter suddenly to a standstill. Other examples of intractable objects, such as iron bars can be readily imagined, as can the ease with which such objectionable objects can be introduced, inadvertently or deliberately, into the apparatus.

The immediate effect of such an intractable object being introduced into the apparatus consists of course in a sudden deceleration of the moving parts. Because it is usually upon engagement of the object between the working surfaces of the coperating cutting elements that it causes the machine to stall in this way, the applied force (which, by virtue of the high deceleration rate, tends to be very large) may be transmitted as a reaction force through the whole apparatus from the cutter element working surfaces back to the drive means (which usually consists of an electric motor).

It is common to provide in apparatus of the kind hereinbefore specified an automatic reversing facility which reverses the drive motor under stall conditions, so as to free the object causing the condition. The motor then drives forwards again and is likely to undergo successive reversals, until either the motor overheats or the cutters succeed in demolishing the obstructing object. Such a reversing facility may comprise suitable means responsive to the torque on one or more cutter shafts, and connected with the control system of the drive motor, so as to reverse the direction of rotation of the latter immediately upon the cutter shaft torque increasing to a predetermined value.

It has however been found that, even where an efficient automatic reversing facility is provided in apparatus of the kind hereinbefore specified, failures of various components are not uncommon under stall (or' "crash stop") conditions Components vul nerable to such failure include bearings, gears, shafts and cutters. This failure is due to dynamic overloading of the components concerned arising from the effect of the inertia of moving parts of the apparatus itself. These inertia effects arise from the rotating cutter/shaft assemblies, from the gearing and from the rotor of the drive motor, but mainly from the latter, which, because it rotates substantially faster than the remaining components, exhibits very high inertia.

Whilst it is theoretically possible to design apparatus 1n. which every component has a factor of safety such as to enable it to survive without failure even under the most several crash stop conditions, apparatus so design would be uneconomically expensive and the size of the components in relation to the power of the drive means would be such that the cutter box (i.e. the housing in which the cutters are housed and which defines the comminuting chamber) has to be unduly large. Thus a machine with such a factor of safety in all components will be unacceptably cumbersome and, for normal operation, underpowered and uneconomically inefficient.

As will be seen from the foregoing, apparatus of the kind hereinbefore specified has a capability of self-destruction or selfdamage due to the aforementioned inertia effects. It is an object of the present invention to provide apparatus of the said kind which does not have this capability, but which can at the same time be made such that in normal operation, i.e. not under crash stop conditions, it can be provided with a power unit of sufficient power to make the apparatus economically viable.

According to the invention, shredding apparatus of the kind hereinbefore specified has, interposed between the rotor and the said reduction gearing, a clutch adapted to isolate the rotor with its inertia from the cutter shafts in direct response to the torque on the shafts reaching a predetermined limiting value, the reduction ratio of the reduction gearing being such that the value of the lowest rotational cutter shaft speed obtaining for any value of the output torque of the drive means is always greater than a predetermined critical value, which is defined by the ratio of said output torque to the corresponding value of the said lowest cutter shaft speed when the force applied to a working surface of a cutter element by an object introduced into the comminuting chamber reaches a maximum permitted value corresponding to crash-stop conditions, whereby the said limiting value of torque on the shafts is encountered only under crash stop conditions.

This arrangement ensures that the motor is isolated from the rotating components connected to the driven side of the clutch when the shaft torque reaches its limiting value under crash-stop conditions. The effects of the inertia of the motor rotor are thereby controlled.

-An equally important advantage of the invention is that it permits those working components directly or indirectly controlled by the clutch to be designed so that they will not fail under crash-stop conditions. This is so because any individual component can be designed to withstand, without damage, a fixed ultimate loading condition, and the arrangement according to the invention ensures that this ultimate loading condition is the greatest that any component will be called upon to endure, even under the indeterminate and excessive loads applied under crash-stop conditions.

The shafts may be arranged to rotate at a common speed. However, it is common practice to arrange for there to be two cutter shafts rotating at different speeds. In such a case the lowest rotational cutter shaft speed mentioned above is the speed of the slower of the two shafts. The cutter elements are preferably of the kind each comprising a disc-like member fixed on the corresponding cutter shaft and having at least one radially projecting portion defining a cutter tip which pushes the matter to be comminuted down between the co-operating cutters. Preferably the cutter tip also has a piercing effect, to initiate the cutting action on the said matter.

For given values of maximum motor output torque, gear ratios, cutter size and masses of rotating components, the torque applied to the clutch is a function of the force applied at the cutter tips. Thus the clutch will disengage if this cutter tip force reaches the particular value that represents a crashstop situation. Therefore the reduction ratio of the gearing can be more specifically defined, where the cutter elements have the said cutter tips, as being such that the said critical value of lowest cutter shaft speed is defined by the ratio of the said output torque to the corresponding value of the said cutter shaft speed upon application of a predetermined applied force on a said cutter tip, the value of this force being chosen so that it will only be. encountered under crash-stop conditions. The clutch is such that this value of the applied force is the greatest permissible by the clutch without disengagement of the clutch.

It will be appreciated that the mere provision of a clutch by itself would merely isolate the motor and does not by itself prevent there being a possibility of selfdestruction elsewhere in the apparatus; what it does do is to set limits upon the effects of inertia of the rotating components upon the bearings,. the machine housing and those rotating components themselves, so that they may be designed to withstand such effects under crash-stop conditions. The potentiality for self-destruction is eliminated by suitable choice of reduction gear ratio as above defined according to the invention.

The stresses imposed by the effects of reversal are in general less than those imposed by crash-stop conditions, so that if provision is made for clutch disengagement at a cutter tip load substantially higher than that which will induce a shaft torque such as to initiate temporary reversal, the latter will be initiated without, in most cases, recourse to clutch disengagement. It has been found possible to make this provision in apparatus according to the invention without having to provide unduly bulky or expensive components.

A further important advantage of apparatus according to the invention is that, it is a simple matter to provide a range of interchangeable gear elements, and indeed of other components, within a standard cutter box. Interchangeability of reduction gear elements and/or of different types of cutter, for example, enables the machine to operate effectively over a wide range of shredding conditions. Apparatus according to the invention is thus potentially capable of substantially increased versatility, combined with a high degree of standardisation leading to reduced cost, as compared with existing shredding apparatus in which each machine tends to be suitable only for a relatively restricted range of shredding conditions, and thus has a relatively higher first cost owing to the number of components peculiar to that machine.

One preferred form of shredding apparatus, of the kind hereinbefore specified and embodying the invention, will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a simplified sectional elevation of part of the apparatus, taken on the line I-I in Figure 2; Figure 2 is a simplified sectional plan taken on the line II--II in Figure 1; and Figure 3 is a diagram in which the horsepower of the driving motor for the apparatus is plotted against cutter shaft speed to illustrate the criterion whereby parameters of the apparatus are chosen according to the invention.

Referring to Figures 1 and 2, a rotary shredder has a frame, parts of which are not shown, but which includes a cutter box 10.

The cutter box 10 is open at its top and bottom and its interior defines a comminuting chamber ll. One end wall 12 of the comminuting chamber 11 is formed by one side of a gearbox 13, the interior of which is totally enclosed to. constitute an oil bath.

Two parallel, hexagonal cutter shafts 14 and 15, each carrying rotary cutter elements 16, extend across the chamber 11. A cylindrical extension, 18, 19 respectively, of each shaft 14, 15 extends through the gearbox 13.

An electric, reversible drive motor 9 has its output shaft 20 connected directly to the driving side of a clutch shown diagrammatically at 21. The driven side of the clutch 21 drives a worm shaft 22 which in turn drives the cutter shafts 14, 15 in contrarotation as indicated by the arrows in Figure 1, through reduction gearing 25 to 28 in the gearbox 13, whereby the drive means (comprising the motor and the clutch 21) is coupled with the cutter shafts, and whereby also the cutter shafts are coupled together.

Each- cutter element 16 comprises essentially a disc coaxial with the hexagonal shaft 14 or 15 to which it is secured, and these cutter discs are interleaved as shown in Figure 2. Each cutter elements 16 has peripheral edges 23 co-operating with those of adjacent cutter elements, and piercing teeth 24 extending radially from the circumference of each cutter disc and being integral therewith. Each tooth 24 terminates in a cutter tip 33. Refuse or other matter to be comminuted is introduced via a hopper (not shown) through the top of the cutter box 10, to be pierced and cut into small pieces in know manner by the rotating cutter elements 16, and discharged through the bottom of the cutter box 10.

The reduction gearing comprises a worm 25 on the worm shaft 22, and a worm gear 26 driven by the worm 25. The gear 26 is mounted on the cutter shaft 14, which carries a spur gear 27 driving a second spur gear 28 on the cutter shaft 15. The gear ratio between the gears 27 and 28 is 2:1 so that the shaft 15 is the slower of the two.

The cutter shafts 14 and 15 rotate in roller bearings 29, whilst the lower end of the worm shaft 22 is supported by a suitable thrust bearing 30, shown in this example as a pair of opposed tapered roller bearings.

Seals 31 are provided around the cutter shafts 14, 15 in the wall 12, and labyrinth seals 32 at the outer ends of the cutter shafts.

The control system (not shown) of the motor 9 incorporates a reversing device responsive to a transducer (also not shown) which actuates the reversing device when a reverse torque on one of the cutter shafts 14, 15 reaches a predetermined level; this happens when a body engaged by the cutter elements 16 offers excessive resistance and thereby tends to slow the apparatus down or stop it. It should be noted that, insofar as it has been thus far described, though with the exception of the clutch 21, the apparatus is a conventional twin-shaft waste shredder. The various bearings can be of any suitable kind, preferably of a rolling type; the cutters may also be of any suitable con figuration, not necessarily double-sided as shown, and in particular can have any suitable circumferential shape.

The clutch 21 is of any suitable type capable of disengaging automatically upon being subjected at its driven side to a reverse torque of a predetermined limiting value.

This is the torque corresponding to a maximum permitted force at the cutter tips 33 high enough to be encountered only under crash-stop conditions. The clutch 21 is preferably adjustable so that it can be set to disengage at the required limiting or maximum permitted value of the cutter tip force.

This limiting value is chosen during design of the machine, and may for example be five times the nominal mean cutter tip force operative during normal operation of the apparatus. In this connection it should be noted that the value of the cutter tip force corresponding to the predetermined torque level at which the above-mentioned reversing device is actuated will be lower than the said limiting value. Thus the clutch 21 will normally remain engaged during reversal and reverse running, and will disengage only if, under crash-stop conditions (due typically to the presence of an undesired solid, intractable object among the cutters, for example an iron bar), the cutter tip force rises to its limiting value.

It will be clear from Figures 1 and 2 that at any given instant during the operation of the apparatus, the force applied between the cutter tips 33 and the matter to be comminuted will be the sum of the forces actually being applied at that instant by those of the tips 33 which happen to be in contact with the said matter. This force, for a given cutter diameter, is proportional to a torque on the cutter shaft, transmitted from the motor via the gearing 25 to 28.

The power deriving from any constant nominal value of this torque can be represented as a straight line on a graph of motor horsepower plotted as a function of the speed of the cutter shaft.

Referring now to Figure 3, which is an example of such a graph and which relates to a typical example of apparatus such as that described above with reference to Figures 1 and 2, it is assumed that the speed of the slow cutter shaft 15 is one-half that of the fast shaft 14. The abscissa represents the speed of the slow shaft 15 and the line A represents the torque on the slow shaft (i.e. twice that on the fast shaft) at the value of cutter tip force selected as limiting for this example. Also plotted in Figure 3 are seven discrete points P representing four different motor horsepower values corresponding to foui values of the reduction ratio between the worm 25 and the worm gear 26. The four values of horsepower plotted are 4, 5i, 7t and 10.

It will be seen that all seven of these points P lie below the line A. In general terms, the overall reduction ratio of the gearing 25 to 28 between the motor and the slow cutter shaft 15 (e.g. 80:1 for a 40:1 worm/gear ratio, and 30:1 for a 15:1 worm / gear ratio) is such that the rotational speed of the slowest cutter shaft obtaining at any time is always above a value of the latter defined by the ratio of the motor horsepower (and therefore its output torque) to the particular corresponding value of the speed of the slow shaft at the predetermined limiting or maximum permitted value of cutter tip force corresponding to the crashstop conditions When the speed of the slow shaft 15 is decreased, under crash-stop conditions, by application of an excessive force to the cutter tips 33; the clutch 21 will disengage when the torque on the slow shaft reaches a value which is a predetermined multiple of the nominal torque represented by the line A.

It will of course be understood that the limiting cutter tip force is selected so as to provide a reasonable factor of safety for all components of the apparatus; this factor need not, however, be nearly as great as if the clutch 21 were not present.

For any apparatus of the kind described, and for which the characteristic point P lies below the line A in Figure 3 or in an equivalent diagram, it is possible by conventional design techniques to determine the maximum static and dynamic stresses to which the other components, such as shafts, cutters, bearings, gears and clutch will be subjected. These components are individually selected accordingly.

It should be noted that Figure 3 illustrates how, in a particular apparatus of the kind described, its capacity for self-destruction or self-damage under abnormal loading conditions remains absent even if the worm/ gear ratio and/or the motor size is changed.

It is thus possible to provide, for a standard apparatus, interchangeable worm and worm gear sets, 25, 26 and interchangeable motor.

Furthermore, the spur gears 27, 28 can be replaced by gears giving a different ratio between the normal speeds of the two cutter shafts 14 and 15, for example 1:1 or 11: 1, provided the normal speed of each shaft remains below the line A in Figure 3 or in the appropriate equivalent diagram of the apparatus concerned corresponding to Figure 3. The cutter elements 16 can also be replaced by other such elements of different configuration (but of the same diameter as the elements 16).

The motor need not be an electric motor, but may for example be a hydraulic motor.

With all these possible variations, such a standard machine can readily be adapted to operate under a wide variety of shredding conditions, i.e. with wide variations in the nature of the material being shredded, without losing the inherent integrity afforded it by application of the invention.

WHAT WE CLAIM IS:- 1. Shredding apparatus of the kind hereinbefore specified, having, interposed between the rotor and the said reduction gearing, a clutch adapted to isolate the rotor with its inertia from the cutter shafts in direct response to the torque on the shafts reaching a predetermined limiting value, the reduction ratio of the reduction gearing being such that the value of the lowest rotational cutter shaft speed obtaining for any value of the output torque of the drive means is always greater than a predetermined critical value, which is defined by the ratio of said output torque to the corresponding value of the said lowest cutter shaft speed when the force applied to a working surface of a cutter element by an object introduced into the comminuting chamber reaches a maximum permitted value corresponding to crash-stop conditions, whereby the said limiting value of torque on the shafts is encountered only under crash-stop conditions.

2. Apparatus according to Claim 1, wherein said drive means comprises a motor and said clutch, the driving side of the clutch being coupled directly with the motor.

3. Apparatus according to Claim 1 or Claim 2, wherein the said further gearing couples the cutter shafts together for rotation at different speeds, the said lowest cutter shaft speed being that of the slower shaft.

4. Apparatus according to Claim 3, wherein each cutter element has at least one radially projecting portion defining a cutter tip constituting the said working surface.

5. Shredding apparatus of the kind hereinbefore specified, constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. conditions, i.e. with wide variations in the nature of the material being shredded, without losing the inherent integrity afforded it by application of the invention. WHAT WE CLAIM IS:-

1. Shredding apparatus of the kind hereinbefore specified, having, interposed between the rotor and the said reduction gearing, a clutch adapted to isolate the rotor with its inertia from the cutter shafts in direct response to the torque on the shafts reaching a predetermined limiting value, the reduction ratio of the reduction gearing being such that the value of the lowest rotational cutter shaft speed obtaining for any value of the output torque of the drive means is always greater than a predetermined critical value, which is defined by the ratio of said output torque to the corresponding value of the said lowest cutter shaft speed when the force applied to a working surface of a cutter element by an object introduced into the comminuting chamber reaches a maximum permitted value corresponding to crash-stop conditions, whereby the said limiting value of torque on the shafts is encountered only under crash-stop conditions.

2. Apparatus according to Claim 1, wherein said drive means comprises a motor and said clutch, the driving side of the clutch being coupled directly with the motor.

3. Apparatus according to Claim 1 or Claim 2, wherein the said further gearing couples the cutter shafts together for rotation at different speeds, the said lowest cutter shaft speed being that of the slower shaft.

4. Apparatus according to Claim 3, wherein each cutter element has at least one radially projecting portion defining a cutter tip constituting the said working surface.

5. Shredding apparatus of the kind hereinbefore specified, constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.