Patent Publication Number: US-8974710-B2

Title: Pellet mill having improved construction

Description:
TECHNICAL FIELD 
     Pellet mills are generally known for pelletizing raw materials, foodstuffs, feedstuffs and more recently biofuels. The pelletizing process results in the transformation of a solid powdery or pasty material into hard pellets or granules which are easier to handle for a consumer than the unpelletized materials. A popular type of pellet mill comprises a rotating annular die, into the interior of which the raw material to be pelletized is fed, and one or more cylindrical rollers rolling along the interior surface of the annular die to compress the raw material and force the raw material through bores in the annular die, the bores extending from the interior surface of the annular die to the exterior surface of the annular die. The raw material which is compressed enters these bores and emerges as rods of raw material, which rods are cut into pellets by a knife positioned adjacent to the outer surface of the annular die, the knife being held stationary. An enclosure is typically provided around the annular die for catching the pellets and funneling the pellets towards a bottom outlet provided beneath the annular die, from where the pellets are collected for subsequent processing. 
     Depending on the type of raw material being pelletized, the raw material may be pre-treated such as by soaking, heating, or steam treatment, and the pressing action of the rollers may also heat the material. 
     In the popular type of pellet mill described above, the annular die is mounted at one end of a rotating main axle mounted in a frame and driven by a motor unit. The roller holder is in turn mounted to one end of a roller holder axle which is coaxial with the main axle and extends throughout the main axle. The other end of the roller holder axle is connected to the frame by a torque limiter. This allows the roller holder axle to remain stationary, i.e. non-rotating, relative to the annular die and the frame during normal operation of the pellet mill, and also allows the roller holder axle to rotate with the rotating annular die in the event of an overload, whereby the pellet mill is protected from damage. An overload event may occur if the amount of raw material fed to the pellet mill is beyond the capacity of the pellet mill, whereby the raw material in the annular die causes the rollers to seize against the annular die causing a high strain on the motor unit and large loads on the annular die, the rollers and the other components of the pellet mill. 
     However, also during normal operation, heavy loads act on the components of the pellet mill. Particularly the roller holder axle bearings which suspend the roller holder axle within the main axle are subjected to high loads, as the raw material to be compressed is pressed between the rollers (which are mounted on the roller holder and hence the roller holder axle) and the annular die (which is mounted on the main axle). Ideally, where the rollers are evenly spaced along the inner surface of the annular die and where each roller would receive the same amount of material, the forces between the rollers and the annular die would cancel out. In this ideal situation, the load on the roller axle bearings would be reduced to simply the weight of the roller holder with rollers. In practice, however, this ideal situation is never achieved and therefore the rollers are loaded differently requiring that the roller holder axle bearings are always sufficiently oversized to ensure a sufficient service life before replacement of the roller holder axle bearings. 
     The oversizing of the roller holder axle bearings, while providing a useful service life between overhauls of the pellet mill, however, results in a disadvantage in the construction of the pellet mill. The oversized roller holder axle bearings, which are provided within the main axle, necessitate very large main axle bearings for rotatably suspending the main axle in the frame. Using very large main axle bearings are, however, uneconomical as the main axle bearings are subjected to significantly lower loads, which could have been carried by much smaller main axle bearings than the roller holder axle bearings. 
     A further disadvantage of the construction of the popular type of pellet mill is that the safety feature provided by the possible co-rotation of the roller holder with the annular die inconveniences the provision of an adjustment mechanism for adjusting the distance between the rollers and the inner surface of the annular die as is often desired. Such adjustment mechanisms, as exemplified in U.S. Pat. No. 4,770,621, EP396200, GB989603, U.S. Pat. No. 4,838,779, U.S. Pat. No. 5,248,469, EP2105292, U.S. Pat. No. 6,244,850, WO2004073963A1 and DE2917453 are often complex as they must be carried by and controlled through the roller holder and/or may require special precautions for preventing damage to the adjustment mechanism if the roller holder axle begins to rotate with the main axle due to an overload event. 
     The above mentioned safety feature further precludes the placing of a feeding mechanism, for feeding raw material into the annular die, in any position which may be occupied by the rollers or the roller holder if the roller holder axle begins to rotate with the main axle due to an overload event. This inconveniences the feeding of raw material evenly to the rollers with the result that the rollers are differently loaded as described above. 
     In FR 2 548 957 is disclosed a pellet mill having an annular die and rollers rolling along the inner side of the annular die. Each roller revolves around a corresponding eccentric bushing. Each eccentric bushing is mounted around a corresponding roller axle. One end of each roller axle is mounted in a roller holder. A gear is providing for turning the eccentric bushing to adjust the distance between the roller and the annular die. A shear pin is positioned between a gear, for adjusting the position of the rollers, and the eccentric bushing. The shear pin is configured to shear in the event of an accidental blocking of the system. 
     In EP 0 694 380 is disclosed a pellet mill in which the rollers with their roller axles and motors are connected at a fixed angle to a basic frame on the side of the die facing away from the carrier shaft around which the annular die and the holder for the annular die rotates. The annular die and the rollers are driven by respective motors to provide a predeterminable slip between the rollers and the annular die. 
     In DE 77 14 334 is disclosed a pellet mill in which roller axles carrying rollers are mounted in a frame. The rollers protrude from the frame, and by moving one of the roller axles inwards to a first position, an annular die can be slipped over the roller so that it covers the rollers. The roller axle in the first position is then moved out to a second position corresponding to the position of normal use in which all rollers contact the inside of the annular die. There is thus no need for a main axle or die holder, instead the annular die is held in position during use by the rollers. 
     Yet a further disadvantage inherent from the construction of the popular type of pellet mill is that the coaxial arrangement of the roller holder axle within the main axle is possible only if the components of the pellet mill have small tolerances on their dimensions, thus requiring the pellet mill to be assembled from carefully selected components in order to ensure proper function. The coaxial arrangement further requires that the rotating main axle, which surrounds the normally non-rotating roller holder axle, is driven by the motor unit via a belt, chain or a gear provided around the main axle, thus inconveniencing service of the pellet mill. The belt, chain or gear further, unless properly covered, poses a risk for the personnel operating the pellet mill. In addition, the use of a belt, chain or gear may cause the main axle bearings to be non-uniformly loaded due to the tension needed in the belt or chain and the thrust generated by the gear. 
     It is thus an object of the present invention to provide a simpler and more economical construction of a pellet mill. 
     A further object of the present invention is to facilitate the provision of an adjustment mechanism for adjusting the distance between the rollers and the inner surface of the annular die in a pellet mill. 
     Yet a further object of the present invention is to facilitate the provision of a feeding mechanism for feeding raw material to the annular die in a pellet mill. 
     These and further objects are achieved by a first aspect of the present invention pertaining to a pellet mill, for preparing pellets from a pelletizable material supplied to the pellet mill, comprising
         a frame,   a main axle rotatably mounted in the frame and having a first end and an opposite second end,   an annular die being supported by the second end and having an inner surface for receiving a portion of the pelletizable material supplied to the pellet mill, an outer surface, and a plurality of radial channels extending from the inner surface to the outer surface for preparing pellets from the portion of the pelletizable material,   a roller holder supported by the frame,   a number of roller axles, each of the number of roller axles being moveably mounted in the roller holder, the roller axles may move in the roller holder in the event of an overload to prevent damage to the pellet mill,   a number of rollers, each of the number of rollers being rotatable around a corresponding one of the number of roller axles, each of the number of rollers being adapted to press the portion of pelletizable material against the inner surface for rolling, in response to rotation of the annular die, along the inner surface of the annular die for forcing the portion of the pelletizable material from the inner surface to the outer surface through at least one of the plurality of radial channels for preparing the pellets, and,   a number of actuators, each of the number of actuators being operatively connected to a corresponding one of the number of roller axles and being adapted to prevent movement of the corresponding one of the number of roller axles relative to the die holder for maintaining a distance between a corresponding one of the number of rollers and the inner surface of the annular die.       

     The roller axles may move in the roller holder such that the distance between each of the number of rollers and the inner surface of the annular die increases in the event of an overload to prevent damage to the pellet mill 
     As the roller holder is supported by the frame instead of being supported by a roller holder axle extending through the main axle, the construction of the pellet mill is simpler. A further advantage derived from the construction of the pellet mill is that the number of bearings may be reduced and each bearing may be dimensioned solely from the loads applied to the specific bearing. This renders the pellet mill more economical. 
     As the roller holder is supported by the frame it becomes easier to provide an adjustment mechanism for adjusting the distance between the rollers and the inner surface of the annular die in a pellet mill as the control lines do not have to extend through a roller holder axle. 
     Further, the construction of the pellet mill, particularly the feature of the roller holder being supported by the frame, facilitates the provision of a feeding mechanism for feeding raw material to the annular die in a pellet mill. 
     Further, as the roller axles are moveably mounted in the roller holder in the pellet mill according to the first aspect of the present invention, the safety feature provided by the roller holder axle in the previously known pellet mill is also present in the pellet mill, namely that the roller axles may move in the event of an overload to prevent damage to the pellet mill according to the first aspect of the present invention. 
     In the context of the present invention, the term preparing pellets is to be understood as also comprising the preparing of intermediates of pellets, including the preparing of rods or strands of material which are subsequently cut, divided, broken off or otherwise treated to form the final pellet. 
     In the context of the present invention, the term pelletizable material is to be understood as any material from which pellets may be prepared by forcing the material through a channel in a die. Pelletizable materials include foodstuffs, feedstuff, waste, wood particles etc. The pelletizable material may include additives and/or be treated by heat, soaking or grinding to increase the coherence of the prepared pellets. Preferably the pelletizable material is a biofuel comprising wood particles. 
     The frame may comprise a main frame part and an auxiliary frame part. The frame may further be supported by a fundament. 
     The main axle is preferably solid, as there is no roller holder axle extending within the main axle. It is, however, contemplated within the context of the present invention that the main axle may be hollow as this may increase the rigidity of the main axle while lowering the amount of material needed for the main axle. The main axle is preferably made of steel and mounted rotatably in the frame by main axle bearings. The main axle bearings may comprise rolling element bearings. The main axle is preferably substantially horizontal. 
     The annular die is preferably made of steel. The annular die preferably has a cylindrical shape with open ends. The annular die is preferably positioned so that the rotational axis of the annular die is substantially horizontal. 
     The pelletizable material may be supplied directly to the inner surface for, or may alternatively be supplied to a point within the annular die from which the pelletizable material may accumulate in the bottom of the annular die due to the force of gravity. 
     In the context of the present invention, the term a portion of the pelletizable material is to be understood as also comprising a separate amount of the pelletizable material for running the pellet mill in batch mode, as well as the continuous supplying of pelletizable material for running the pellet mill in continuous mode. 
     The radial channels may have a diameter from 2-20 mm, and may have a uniform diameter or a tapering diameter or inlet, or may have different diameters on the inner and outer side of the annular die. 
     The radial channels may have a first diameter from the inner surface to a certain position between the inner surface and the outer surface of the annular die, and a second diameter from the certain position to the outer surface of the annular die. 
     The roller holder preferably comprises a first roller holder plate, a second roller holder plate, and a connecting member. Each of the roller axles is preferably supported with one end of the roller axle supported by the first roller holder plate and the other end of the roller axle being supported by the second roller holder plate. 
     In the context of the present invention, the term supported by the frame means that the element referred to is stationary relative to the frame. The roller holder may be supported by the frame by being directly attached to the frame, or by being attached to the frame via one or more further elements. Preferably the roller holder is attached to an end plate or door mounted to the frame. The roller axles are preferably made of steel. 
     In the context of the present invention, the term moveably is to be understood as also comprising rotatably, displaceably, tiltably. 
     The rollers are preferably made of steel. The rollers may roll, in response to rotation of the annular die, either due to direct contact between the roller and the inner surface of the annular die, or alternatively by contact with the annular die via a layer of pelletizable material present between the roller and the inner surface of the annular die. 
     The actuators may be linear actuators or rotary actuators. 
     In the context of the present invention, the term prevent movement is not to be restricted to the meaning that movement is impossible, rather the term is to be understood as also comprising the meaning that movement is prevented until a sufficiently large force is applied to the element referred to, whereby movement can no longer be prevented. 
     Further, in the context of the present invention, the term maintaining a distance is not to be restricted to the meaning that the distance cannot be changed, rather the term is to be understood as also comprising the meaning that the distance is maintained until a sufficient large force is applied to the element referred to, whereby the distance can no longer be maintained. 
     In a preferred embodiment of the pellet mill according to the first aspect of the present invention movement of each of the number of roller axles is prevented until a sufficiently large force is applied to the roller axle, whereby movement can no longer be prevented. This is advantageous as it allows the roller axles to move in the event of an overload, corresponding to a sufficiently large force applied to the roller axle, to prevent damage to the pellet mill. 
     In a preferred embodiment of the pellet mill according to the first aspect of the present invention, each of the number of actuators is further adapted to move the corresponding one of the number of roller axles with respect to the roller holder for changing the distance between the corresponding one of the number of rollers and the inner surface of the annular die. This is advantageous as it allows the pressure exerted by the roller on the pelletizable material to be varied, for example depending on the type of pelletizable material. 
     The roller axles may be moved by displacing, tilting and/or rotating the roller axles. 
     In some embodiments of the pellet mill according to the first aspect of the present invention, each of the number of roller axles is moveable by being eccentrically and rotatably mounted in the roller holder, and,
         each of the number of actuators is operatively connected to a corresponding one of the number of roller axles for rotating the corresponding one of the number of roller axles relative to the die holder. This is advantageous as it provides a strong and simple mechanism for changing the distance between the rollers and the inner surface of the annular die.       

     In the present context, the term eccentrically and rotatably mounted is to be understood as comprising the situation where the axis of rotation, as rotatably mounted, is offset from the geometrical centre axis. 
     In further embodiments of the pellet mill according to the first aspect of the present invention, each of the number of actuators is operatively interposed between the frame and a corresponding one of the number of roller axles. This is advantageous as it provides a simple way of positioning the actuator. The actuator may be operatively interposed between the frame and a corresponding one of the number of roller axles by being attached to the frame directly, or via a further element, and by being attached to the roller axle directly, or via a further element. 
     In a preferred embodiment of the pellet mill according to the first aspect of the present invention, the roller holder is attached to an end plate or door mounted to the frame, and each of the actuators ( 114 ) are fastened to the end plate ( 62 ) or door. This is advantageous as it provides easy access to the actuators, the rollers and the annular die, for maintenance and adjustments 
     In some embodiments of the pellet mill according to the first aspect of the present invention, the pellet mill further comprises
         a number of circular end pieces,   a number of collars,   a number of shear pins,   each of the number of end pieces being connected to an exterior end of a corresponding one of the number of roller axles and having a seat comprising a first connector connected to a portion of a corresponding one of the number of shear pins,   each of the number of collars being operatively connected to a corresponding one of the number of actuators and encircling a corresponding one of the number of end pieces, each of the number of collars having a second connector connected to a remainder of the corresponding one of the number of shear pins, and   each of the number of shear pins transferring rotational motion from a corresponding one of the number of collars to a corresponding one of the number of end pieces. This is advantageous as it provides a secure and convenient way of connecting the actuators to the roller axles.       

     The circular end pieces are preferably made of steel. 
     The collars are preferably made of steel. 
     The shear pins are preferably made of steel, but may be made of other metals and alloys for attaining a specific force for shearing the shear pins. The shear pins preferably comprise hollow cylinders. 
     In the context of the present invention, the term exterior is to be understood as also comprising the situation of the element being referred to being accessible from the exterior. 
     The seat is preferably positioned on the circumference of the circular end piece and preferably extends in the axial direction of the circular end piece from the surface of the circular end piece. 
     The first connector is preferably a through-going hole provided in the seat, the length of the hole being no less than the length of the portion of the shear pin. 
     The second connector is preferably a through-going hole provided in the collar, the length of the hole being no less than the length of the remainder of the shear pin. 
     In preferred embodiments of the pellet mill according to the first aspect of the present invention, wherein the pellet mill comprises a number of circular end pieces, a number of collars and a number of shear pins, the first connector is a first through-going hole provided in the seat, the length of the first through-going hole is no less that the length of the portion of the shear pin, and the second connector is a second through-going hole provided in the collar, the length of the second through-going hole is no less than the length of the remainder of the shear pin. This is advantageous as it provides easy mounting of the shear pin and easy removal of the remains of a sheared shear pin. 
     The first and the second through-going hole, respectively, is preferably oriented laterally in relation to the corresponding one of the number of roller axles. More preferably the first and the second through-going hole, respectively, is oriented radially in relation to the corresponding one of the number of roller axles. 
     In embodiments of the pellet mill according to the first aspect of the present invention, in which embodiments the pellet mill further comprises a number of circular end pieces, a number of collars, and a number of shear pins, each of the number of collars comprises an inner side and an outer side and a stopper provided on the inner side, and,
         the stopper is engageable by the seat on a corresponding one of the number of end pieces for limiting rotation of the corresponding one of the number of end pieces in the event of shearing of the corresponding one of the number of shear pins. This is advantageous as it prevents excessive rotation of the end piece and thereby excessive rotation of the roller axle.       

     In embodiments of the pellet mill according to the first aspect of the present invention, in which embodiments the pellet mill further comprises a number of circular end pieces, a number of collars, and a number of shear pins, each of the number of collars further comprising an inner side and an outer side and a stopper, the stopper engages the seat thereby limiting the rotation of the roller axle for preventing the roller axle from rotating past the position of maximum distance between the roller and the inner surface of the annular die for preventing the roller from striking the inner surface ( 44 ) of the annular die and damaging the annular die and/or the roller. This is advantageous as it prevents damage to the annular die and/or the roller in the event of shearing of the corresponding one of the number of shear pins, for example due to severe overloading. 
     The stopper is preferably positioned at a position on the collar 90° offset from the position of the second connector on the collar. 
     In further embodiments of the pellet mill according to the first aspect of the present invention, each of the number of end pieces further comprises a tapering sleeve, a compression ring, and a tightening mechanism,
         the exterior end of the corresponding one of the number of roller axles being inserted into the tapering sleeve,   the compression ring encircling the tapering sleeve, and   the tightening mechanism being adapted to displace the compression ring along the tapering sleeve for engaging the tapering sleeve for clamping the exterior end of the corresponding one of the number of roller axles. This is advantageous as it provides a simple way of attaching the end piece to the roller axle.       

     The tapering sleeve is preferably attached to one side of the circular end piece and preferably has its larger diameter at its connection to the circular end piece. The wall of the tapering sleeve may comprise slits for simplifying the clamping of the roller axle. 
     The tightening mechanism may comprise holes in the circular end piece, threaded holes in the compression ring, and bolts connecting the end piece to the compression ring for forcing the compression ring towards the circular end piece for clamping the roller axle. 
     In embodiments of the pellet mill according to the first aspect of the present invention, each of the number of actuators comprises a hydraulic cylinder. This is advantageous as it provides an actuator having great strength and precise movement. Further, a hydraulic actuator may easily be loaded with fluid at a set pressure for preventing movement of a roller axle, whereby the actuator may allow movement if the forces on the roller axle become large enough to counteract the force exerted by the fluid at the set pressure. The hydraulic cylinder may be single acting or preferably double acting. 
     In embodiments of the pellet mill according to the first aspect of the present invention, wherein each of the number of actuators comprises a hydraulic cylinder, each of the hydraulic cylinders is loaded with fluid at a set pressure for preventing movement of the corresponding one of the number of roller axles, whereby the actuator allows movement if the forces on the corresponding one of the number of roller axles become large enough to counteract the force exerted by the fluid at the set pressure. This is advantageous as it prevents damage to the pellet mill by allowing the roller axles to move when the force on the roller axle becomes large. 
     The set pressure may be effected by providing a relief valve connected to the hydraulic cylinders. 
     In embodiments of the pellet mill according to the first aspect of the present invention, wherein each of the number of actuators comprises a hydraulic cylinder, the hydraulic pressure within the hydraulic cylinder is preset for providing a specific pressure between a corresponding one of the number of rollers and the inner surface of the annular die such that the distance between the corresponding one of the number of rollers and the inner surface of the annular die can increase when an increased amount of the pelletizable material present between the corresponding one of the number of rollers and the inner surface forces the corresponding one of the number of rollers away from the inner surface of the annular die against the hydraulic pressure within the hydraulic cylinder. This is advantageous as it prevents damage to the pellet mill by allowing the roller axles to move when the force on the roller axle becomes large. 
     Further the preset or set pressure results in a fixed maximum force with which the hydraulic cylinder can hold the rollers against the inner surface of the annular die. This limits the possible maximum imbalance, i.e. the resultant force on the rollers from the annular die and the pelletizable material between each roller and the annular die, by allowing each roller to move away from the inner surface of the annular die when the force on that roller overcomes the force from the pressure within the hydraulic cylinder. 
     The preset pressure may be effected by providing a relief valve connected to the hydraulic cylinders. 
     In embodiments of the pellet mill according to the first aspect of the present invention, the first end of the main axle extends outside the frame, and the pellet mill further comprises
         a motor unit and a coupling, the motor unit supplying rotational movement to the coupling, and,   the coupling being adapted to transfer the rotational movement from the motor unit to the first end of the main axle for rotating the main axle.       

     The coupling may be a rigid coupling such as a sleeve, muff or flange coupling, or may alternatively be a flexible coupling such as a Cardan joint or an Oldham coupler. This is advantageous as it simplifies service of the pellet mill-motor unit assembly. Further, the transferring of rotational movement via a coupling limits risk of the main axle being non-uniformly loaded. 
     In embodiments of the pellet mill according to the first aspect of the present invention, the pellet mill further comprises:
         a die holder operatively interposed between the annular die and the second end of the main axle. This is advantageous as it allows the main axle and the die holder to be manufactured separately which permits the main axle and die holder to be manufactured using simpler methods and further renders the die holder and the main axle less cumbersome to handle during manufacturing.       

     In embodiments of the pellet mill according to the first aspect of the present invention, the pellet mill further comprises
         a number of feed conduits extending from outside the pellet mill into the annular die for supplying the pelletizable material to the inner surface of the annular die. This is advantageous as it provides an effective way of supplying the pelletizable material to the inner surface of the die.       

     The feed conduits preferably supply the pelletizable material to the inner surface of the annular die between the rollers. The feed conduits are further preferably supported directly, or via a further element, by the frame and held stationary relative to the frame. This is advantageous as it facilitates the provision of a feeding mechanism for feeding pelletizable material to the annular die in a pellet mill. 
     In preferred embodiments of the pellet mill according to the first aspect of the present invention, wherein the pellet mill comprises a number of feed conduits, each of the number of feed conduits is associated with a corresponding one of the number of rollers and each of the number of feed conduits comprises a side opening for feeding the pelletizable material into the substantially wedge-shaped space between the corresponding one of the number of rollers and the inner surface of the annular die, the side opening extending axially along the feed conduit and having an extension that is approximately equal to the length of the corresponding one of the number of rollers, wherein at least one of the feed conduits is configured for feeding the pelletizable material directly to a substantially wedge-shaped space which is at a higher location in the annular die than the at least one of the feed conduits. 
     This is advantageous because in the pellet mill according to the first aspect of the present invention, the roller holder is supported by the frame and is stationary, even in the event of an overload. Further, the roller axles may move in the roller holder such that the distance between each of the number of rollers and the inner surface of the annular die increases in the event of an overload to prevent damage to the pellet mill. This allows the feed conduits in the preferred embodiments of the pellet mill according to the first aspect of the present invention, wherein the pellet mill comprises a number of feed conduits, to be placed within the annular die between the rollers such that the side opening of each of the number of feed conduits is parallel and aligned axially with the corresponding one of the number of rollers associated with the feed conduit. 
     By providing a side opening in the feed conduit extending axially along it, and having an extension that is approximately equal to the length of the corresponding one of the number of rollers, an even distribution of the pelletizable material over the outer surface of the rollers is achieved. This causes a more efficient use of the pellet mill, because virtually the full lengths of the rollers are used in the process. Furthermore, an even distribution of the pelletizable material over the full lengths of the rollers gives an even distribution of the load on the individual roller. 
     In a pellet mill having a number of rollers, wherein the number is at least two, one of the wedge-shaped spaces will be placed at a higher location than the feed conduit feeding pelletizable material to it. Hence, by feeding the pelletizable material directly to a substantially wedge-shaped space, which is at a higher location in the die chamber than said conduit from which it is fed, it is achieved that the pelletizable material is fed directly to the place where it is needed, and the unbalances caused by the pelletizable material having a tendency to accumulate in the bottom of the die chamber are therefore completely alleviated or at least greatly reduced. 
     In embodiments of the pellet mill according to the first aspect of the present invention, wherein the pellet mill comprises a number of feed conduits, the pellet mill may further comprise means for angular adjustment of each of the feed conduits about its longitudinal axis, thereby adjusting the direction, in which the pelletizable material is expelled through the side opening. Hereby is achieved that the feed of pelletizable material into the die chamber may be accurately adjusted, for example in dependence of the other structural parts of the pellet mill. 
     Preferably, the conduits have a circular cross section, and each comprises a screw conveyor having a shaft tapering in the direction of feed. 
     In a preferred embodiment, the pellet mill may further comprise drive means for driving the screw conveyors, which extend into the feed conduits, independently of each other. Hereby is achieved that the quantity of pelletizable material that is fed into the wedge-shaped spaces may be individually adjusted, for example by adjusting the speed of the screw conveyor, or the feeding of pelletizable material to the feed conduits. 
     Some embodiments of the pellet mill according to the first aspect of the present invention further comprises
         a knife operatively positioned relative to the outer surface of the annular die for severing said pellets from said outer surface of said annular die. This is advantageous as it allows further treatment of the pellets in the pellet mill and/or ensures uniform dimensions for the pellets.       

     Preferably the pellet mill according to the first aspect of the present invention comprises a plurality of knives operatively positioned relative to the outer surface of the annular die for severing the pellets from the outer surface of the annular die. One knife may be provided for each roller. 
     In embodiments of the pellet mill according to the first aspect of the present invention, the number is three. This is advantageous as it allows the forces between the rollers and the annular die to even out. 
     The above mentioned and further objects are moreover achieved by a second aspect of the present invention pertaining to a method of preparing pellets from a pelletizable material comprising the steps of: 
     (a) providing a pellet mill, for preparing pellets from a pelletizable material supplied to the pellet mill, comprising 
     
         
         
           
             a frame, 
             a main axle rotatably mounted in the frame and having a first end and an opposite second end, 
             an annular die being supported by the second end and having an inner surface for receiving a portion of the pelletizable material supplied to the pellet mill, an outer surface, and a plurality of radial channels extending from the inner surface to the outer surface for preparing pellets from the portion of the pelletizable material, 
             a roller holder supported by the frame, 
             a number of roller axles, each of the number of roller axles being moveably mounted in the roller holder,
 
said roller axles may move in said roller holder ( 54 ) in the event of an overload to prevent damage to the pellet mill
 
             a number of rollers, each of the number of rollers being rotatable around a corresponding one of the number of roller axles, each of the number of rollers being adapted to press the portion of pelletizable material against the inner surface for rolling, in response to rotation of the annular die, along the inner surface of the annular die for forcing the portion of the pelletizable material from the inner surface to the outer surface through the plurality of radial channels for preparing the pellets, and 
             a number of actuators, each of the number of actuators being operatively connected to a corresponding one of the number of roller axles and being adapted to prevent movement of the corresponding one of the number of roller axles relative to the die holder for maintaining a distance between a corresponding one of the number of rollers and the inner surface of the annular die,
 
(b) rotating the main axle, and
 
(c) supplying the pelletizable material to the pellet mill, is provided.
 
           
         
       
    
    
    
     
       The roller axles may move in the roller holder such that the distance between each of the number of rollers and the inner surface of the annular die increases in the event of an overload to prevent damage to the pellet mill. 
       The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments, and in which 
         FIG. 1  shows, in section view, a first embodiment of a pellet mill according to the first aspect of the present invention, 
         FIG. 2  shows, in cross sectional side view, the first embodiment of the pellet mill according to the first aspect of the present invention without ( FIG. 2A ) and with ( FIG. 2B ) the end plate, 
         FIG. 3  shows, in partial cutaway side view, the first embodiment of the pellet mill with the end plate, 
         FIG. 4  shows, in side view, the adjustment of a roller relative to an inner surface of the annular die of the pellet mill according to the first aspect of the present invention, and, 
         FIG. 5  shows a feed conduit suitable for use in the pellet mill according to the first aspect of the present invention. 
     
    
    
     When further embodiments of the invention are shown in the figures, the elements which are new, in relation to earlier shown embodiments, have new reference numbers, while elements previously shown are referenced as stated above. Elements which are identical in the different embodiments have been given the same reference numerals and no further explanations of these elements will be given. 
       FIG. 1  shows, in section view, a first embodiment of a pellet mill  20  according to the first aspect of the present invention. The pellet mill is driven by a motor unit  10 , which will be described in further detail below. 
     The pellet mill comprises a steel frame  22  being composed of a main frame part  24  and an auxiliary frame part  26 . A main axle  28 , made from steel, has a first end  30  driven by the motor unit  10 , and an opposite second end  32 . The main axle  28  is rotatably supported in the main frame part  24  by first and second main axle bearings  34  and  36  respectively, which rotatably support the main axle  28 , and a die holder  38  which is mounted to the second end  32  by means of bolts, one of which is designated the reference numeral  40 . The die holder carries an annular die  42 , having inner and outer surfaces, designated by the reference numerals  44  and  46 , respectively, which is made from steel and comprises a plurality of radial channels  48 , through which the pelletizable material is pressed as described in more detail below. The annular die  42  is positioned inside a die compartment  50  having a pellet outlet  52  through which pellets produced by the mill are collected. The die compartment  50  is defined by the volume enclosed by the auxiliary frame part  26  and a roller holder, in its entirety designated the reference numeral  54 . The roller holder  54  comprises a first roller holder plate  56  connected to a second roller holder plate  58  via a connecting member  60 . The connecting member  60  may be integral with the first and/or the second roller holder plates  56  and  58 . The roller holder  54  is further attached to the frame  22  of the pellet mill  20  by being attached to the auxiliary frame part  26  by being mounted to an end plate  62 , not shown in  FIG. 1 , as will be described in more detail below. The mounting of the roller holder  54  to the end plate  62  includes bolts or suitable releaseable fasteners to allow the roller holder  54  to be removed from the pellet mill  20  as needed for repairing or exchanging the annular die  42 . The roller holder  54  carries at least two roller axles, one of which is shown in  FIG. 1  and which is designated the reference numeral  64 , which has a first end section  66 , and a second end section, representing an exterior end and designating the reference numeral  68 . Between the end sections  66  and  68 , a middle section  70  is provided. The end sections  66  and  68  have a first centre axis  72  and the middle section has a second centre axis  74 , which is offset from the first centre axis  72 . The first and second end sections  66  and  68  of the roller axle  64  are rotatably supported in the roller holder  54  by corresponding first and second journal bearings  76  and  78 , respectively. 
     The main frame part  24  and the auxiliary frame part  26  may be displaceable in relation to each other, for example by providing a traverse (not shown) mounted to a rail or beam (not shown) attached to and extending from the main frame part  24 , for supporting the auxiliary frame part  26  as it is disengaged from the main frame part ( 24 ) and distanced from the main frame part  24  for enabling access to the die compartment  50 . 
     The pellet mill  20  further includes a plurality of rollers, one roller for each roller axle  64 . One of the rollers is shown in  FIG. 1  and is designated the reference numeral  80 , the roller  80  being rotatably supported around the middle section  70  by first and second roller bearings  82  and  84  provided on the middle section  70 . The roller axle  64  is eccentrically mounted in the roller holder  54  as the second centre axis  74 , around which the roller  80  rotates, is offset from the first centre axis  72 , around which the roller axle  64  is rotatably mounted in the roller holder  54 . 
     A collar  86  is connected, as will be further shown below, to an end piece  88  which is attached to the second end section  68  of the roller axle  64  via a compression ring  90  and bolts, one of which is designated the reference numeral  92 . The end piece  88  further comprises a tapering sleeve, not shown, which is positioned between the compression ring  90  and the second end section  68  in such a way that tightening of the bolts  92  causes the compression ring  90  to move towards the end piece  88 , thereby compressing the tapering sleeve for causing the tapering sleeve to engage the second end section  68  for attaching the end piece  88  to the second end section  68 . The collar  86  further comprises a pin  94  provided in a handle part  96  for allowing the rotation of the collar  86  by means of an actuator, as discussed further below. 
     The motor unit  10  shown in  FIG. 1  comprises an electric motor  12  having an output shaft  14  which is connected to the first end  30  of the main axle  28  of the pellet mill  20  via a gearbox  16  and a coupling (not shown). 
     By attaching the roller holder  54  to the frame  22 , the roller holder axle with roller holder axle bearings used in conventional pellet mills may be omitted. Thus, the first and second main axle bearings  34  and  36  can now be dimensioned solely based on the actual loads applied to the first and second main axle bearings  34  and  36 , without being unnecessarily overdimensioned due to the now omitted roller holder axle with roller holder axle bearings. The pellet mill  20  is thus simpler and more economical as compared to a conventional pellet mill having a roller holder axle supported within the main axle. The construction of the pellet mill  20  further allows the main axle  28  to be coupled directly to the motor unit  10  as there is no roller holder axle supported within the main axle  28 . As the main axle  28  is directly driven by the motor unit  10 , there is significantly less non-uniform loading of the first and second main axle bearings  34 ,  36  as there are no lateral forces, due to the tension needed in a belt or chain for driving a conventional pellet mill. This further allows for providing the motor unit  10  in line with the pellet mill  20 , thus simplifying service of a pellet mill assembly comprising the pellet mill  20  and the motor unit  10 . 
     As will be described in further detail below with reference to  FIGS. 2-4 , the operational safety of the pellet mill  20  matches or surpasses that of conventional pellet mills. 
       FIG. 2A  shows, in cross sectional side view, the pellet mill  20  with annular die  42  and three rollers including the roller  80 . The roller holder  54  is not shown in  FIG. 2A . The annular die  42  rotates clockwise as indicated by the arrow  98  around a main centre axis  100  corresponding to the centre axis of the main axle  28 . During the operation of the pellet mill  20 , the roller  80  is caused to rotate clockwise around the second centre axis  74  as indicated by the arrow  102 . Three feed conduits, one of which is designated the reference numeral  104  are positioned between the rollers to supply portions of pelletizable material  106  to the rollers. During the operation of the pellet mill  20 , the portion of pelletizable material  106  is pressed between the roller  80  and the inner surface  44  of the annular die  42  and pressed through the radial channels  48  to form pellets (not shown). A plurality of knifes (not shown) provided within the die compartment  50  are positioned close to the outer surface  46  of the annular die  42  to sever the pellets from the outer surface  46  of the annular die  42  for allowing the pellets to exit the die compartment  50  through the pellet outlet  52 . 
     As seen in  FIG. 2A , the construction of the pellet mill  20  further facilitates the provision of the feed conduits  104 . As the roller holder  54  supported by the frame  22  it is stationary, even in the event of an overload, so that the feed conduits  104  may be simply positioned between the rollers, including the roller  80 , without risk of being damaged in the event of an overload. 
       FIG. 2B  shows the pellet mill  20  with the end plate  62 . The end plate  62  is attached to the auxiliary frame part  26  and carries the roller holder  54  (not shown in  FIG. 2B ). The collar  86  with handle part  96 , as well as the end piece  88 , extends through the end plate  62 . The end piece  88  further comprises two seats, one of which is designated the reference numeral  108 . Rotation of the collar  86  is transferred to the seats  108  via shear pins, one of which is designated the reference numeral  110 , extending through corresponding bores (not shown) in the collar  86  and the seats  108 . Two oppositely positioned stoppers, one of which is designated the reference numeral  112 , are provided within the collar  86  for engaging the seats  108  to prevent excessive rotation of the end piece  88  in the event that the shear pins  110  break, as will be described in further detail below. 
     Collar  86  may be acted upon by an actuator, in its entirety designated the reference numeral  114 , being fastened to the end plate  62 . The actuator  114  comprises a hydraulic cylinder  116  in which one end of a piston  118  is positioned such that the supply or withdrawal of hydraulic fluid to the hydraulic cylinder  116  causes the piston  118  to extend or retract relative to the hydraulic cylinder  116 . The piston  118  is connected to the handle part  96  of the collar  86  via the pin  94  and a link part  120 , which is threadedly connected to the piston  118  via a rod  122 , which rod  122  is also threadedly connected to the piston  118 . A nut  124  is fastened to the rod  122  for allowing the rod  122  to be rotated for fine tuning/zero point adjustment of the rotational position of the collar  86 . The hydraulic cylinder  116  is further connected to the end plate  62  via a pin  126  and a bracket  128 , which bracket  128  is fastened to the end plate  62  by bolts  130  and  132 . 
     As seen in  FIG. 2B , the construction of the pellet mill  20  facilitates the provision of the actuators  114 . As the roller holder  54  is supported by the frame  22  it is stationary, even in the event of an overload. Thus the actuators  114  are not confined to the roller holder  54  but can instead be connected to the frame  22 , directly, or as shown in  FIG. 2B , via the end plate  62 . The function of the actuator  114  will now be described in further detail with reference to  FIGS. 3-4 . 
       FIG. 3  shows, in partial cutaway side view, the pellet mill  20  with the end plate  62  partly cut away. Referring to the roller  80 ,  FIG. 3  shows the offset of the second centre axis  74  from the first centre axis  72 . The second centre axis  74  is the axis around which the roller  80  rotates, while the first centre axis  72  is the axis around which the first and second end sections  66  and  68  of the roller axle  64  rotate. 
     The adjustment of the distance between the roller  80  and the inner surface  44  of the annular die  42  is shown in  FIGS. 4A-4B .  FIG. 4A  shows the roller  80  in contact with the inner surface  44  of the annular die  42 . In  FIG. 4B  the actuator  114  has been activated to retract the piston  118  into the hydraulic cylinder  116  thereby pulling on the handle part  96  for turning the collar  86  causing the rotation of the collar  86  as indicated by the arrow designated the reference numeral  136 . The turning of the collar  86  is transferred to the end piece  88  via the shear pins  110  causing the roller axle  64  to turn. As the roller axle  64  turns, the second centre axis  74  moves in a clockwise circular motion around the first centre axis  72 , whereby the distance between the roller  80  and the inner surface  44  of the annular die  42  increases as indicated by the dashed line indicating the previously occupied position of the roller  80 , designated the reference numeral  138 . The increase of the distance between the roller  80  and the inner surface  44  of the annular die  42  may, as described above in relation to  FIGS. 4A-4B , be deliberate such as upon the detection of an overload event, in which case the distance may be decreased again by extracting the piston  118  for turning the roller axle  64  counter-clockwise and decreasing the distance. The increase of the distance between the roller  80  and the inner surface  44  of the annular die  42  may also be spontaneous due to an increased amount of pelletizable material  106  being present between the roller  80  and the inner surface  44  of the annular die  42 , which increased amount of pelletizable material  106  forces the roller  80  away from the inner surface  44  of the annular die  42  against the hydraulic pressure present within the hydraulic cylinder  116  of the actuator  114 . The hydraulic pressure may be preset to provide a specific pressure between the roller  80  and the inner surface  44  of the annular die  42 . Thus, the pellet mill  20  possesses, by means of the actuator  114  and the roller axle  64 , safety features for preventing damage to the pellet mill  20  in the event of an overload event. Further, in the event of extraordinarily severe overloading events or seizures of the pellet mill  20 , such as for example being caused by the introduction of a foreign object, such as a forgotten hand tool etc. in the annular die  42 , the shear pin  110  prevents damage to the pellet mill  20 , particularly to the actuator  114  and the first and second roller bearings  82  and  84 , by shearing, thereby allowing the roller axle  64  to rotate to the position where the distance between the roller  80  and the inner surface  44  of the annular die  42  is at a maximum, i.e. when the first centre axis  72  is positioned between the second centre axis  74  and the inner surface  44  of the annular die  42 . Even if the shear pin  110  shears, the stopper  112  engages the seat  108  thereby limiting the rotation of the roller axle  64  for preventing the roller axle  64  from rotating past the position of maximum distance between the roller  80  and the inner surface  44  of the annular die  42  for preventing the roller  80  to strike the inner surface  44  of the annular die  42  and potentially damaging the annular die  42  and/or the roller  80  and/or the first and second roller bearings  82  and  84 . 
       FIG. 5  shows a preferred embodiment of the feed conduit  104 . The feed conduit  104  shown in  FIG. 5  has a circular cross section and comprises a screw conveyor  140  extending from a feed inlet  142  to a side opening  144 . The side opening  144  extends axially along a front portion  146  of the feed conduit  104  and the side opening  144  has a length which is approximately equal to the length of the roller  80 , so that the pelletizable material  106  ejected laterally through the side opening  144  forms an even distribution of the pelletizable material over the outer surface of the roller  80 , i.e. the full length of contact between the roller  80  and the inner surface  44  of the annular die  42 . 
     The screw conveyor  140  has a shaft tapering in the direction of feed and is driven by an electrical motor  148 . The electrical motor is preferably set up to run at a set speed whereby adjustment of the amount of pelletizable material  106  to be delivered to each roller  80  is carried out by controlling upstream feed screw units (not shown), one for each feed conduit  104 . The feed screw units are controllable for individually portioning out, i.e. feeding, pelletizable material  106  to the feed inlets  142  of the individual feed conduits  104 . The feed screw units may receive pelletizable material  106  from a common supply, such as a conditioner unit where the pelletizable material may be heated, steam treated, etc. 
     If the feed conduit  104  cannot keep up with the amount of pelletizable material  106  being fed to it from the feed screw units its speed must be adjusted, i.e. controlled. Alternatively, or in addition to controlling the feed screw units, the electric motor  148  may be controllable for adjustment of the amount of pelletizable material  106  to be delivered to each roller  80 . 
     The feed conduit  104  provides a very efficient feeding of pelletizable material  106  to the corresponding one of the roller  80  as the pelletizable material  106  is fed, by means of the side opening  144 , selectively to the roller  80 . The pelletizable material  106  may, depending on the angular adjustment of the feed conduit  104  about its longitudinal orientation, even be fed upwards, such as to a substantially wedge-shaped space formed between a roller  80 , positioned at a higher location in the pellet mill than the feed conduit  104 , and the inner surface  44  of the annular die  42 . Further, the amount of pelletizable material  106  which is fed to each roller  80  can be individually adjusted by individually controlling the feed screw units as described above. 
     Means (not shown) may be provided for angular adjustment of the feed conduit  104 , or only the front portion  146 , about the longitudinal axis of the feed conduit  104  for adjusting the direction of the pelletizable material  106  ejected from the side opening  144 . The means may include a hydraulic cylinder or servomotors and gears. 
     The feed conduit  104  may be attached to the end plate  62 , the auxiliary frame part  26 , or the roller holder  54 . 
     As the roller holder  54  is supported by the frame  22 , it is stationary, even in the event of an overload. This allows the feed conduit  104  to be positioned such that the front portion  146  of the feed conduit  104  is positioned within the annular die  42  between the rollers  80  such that the side opening  144  is parallel and aligned axially with the roller  80 . This in turn allows the pelletizable material  106  ejected laterally through the side opening  144  to be evenly distributed over the outer surface of the roller  80 . 
     If, as in prior art pellet mills, the roller holder  54  were mounted on a roller holder axle held stationary relative to the frame  22  by a shear pin, an overload event would cause the shear pin to break and the roller holder  54  to rotate relative to the frame, thus destroying the feed conduits  104  which are fastened to the end plate  62 . 
     
       
         
           
               
             
               
                   
               
               
                 List of parts with reference to the figures: 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 10. 
                 Motor unit 
               
               
                 12. 
                 Electric motor 
               
               
                 14. 
                 Output shaft 
               
               
                 16. 
                 Gearbox 
               
               
                 20. 
                 Pellet mill 
               
               
                 22. 
                 Frame 
               
               
                 24. 
                 Main frame part 
               
               
                 26. 
                 Auxiliary frame part 
               
               
                 28. 
                 Main axle 
               
               
                 30. 
                 First end 
               
               
                 32. 
                 Second end 
               
               
                 34. 
                 First main axle bearing 
               
               
                 36. 
                 Second main axle bearing 
               
               
                 38. 
                 Die holder 
               
               
                 40. 
                 Bolt 
               
               
                 42. 
                 Annular die 
               
               
                 44. 
                 Inner surface 
               
               
                 46. 
                 Outer surface 
               
               
                 48. 
                 Radial channel 
               
               
                 50. 
                 Die compartment 
               
               
                 52. 
                 Pellet outlet 
               
               
                 54. 
                 Roller holder 
               
               
                 56. 
                 First roller holder plate 
               
               
                 58. 
                 Second roller holder plate 
               
               
                 60. 
                 Connecting member 
               
               
                 62. 
                 End plate 
               
               
                 64. 
                 Roller axle 
               
               
                 66. 
                 First end section 
               
               
                 68. 
                 Second end section 
               
               
                 70. 
                 Middle section 
               
               
                 72. 
                 First centre axis 
               
               
                 74. 
                 Second centre axis 
               
               
                 76. 
                 First journal bearing 
               
               
                 78. 
                 Second journal bearing 
               
               
                 80. 
                 Roller 
               
               
                 82. 
                 First roller bearing 
               
               
                 84. 
                 Second roller bearing 
               
               
                 86. 
                 Collar 
               
               
                 88. 
                 End piece 
               
               
                 90. 
                 Compression ring 
               
               
                 92. 
                 Bolt 
               
               
                 94. 
                 Pin 
               
               
                 96. 
                 Handle part 
               
               
                 98. 
                 Arrow indication direction of rotation 
               
               
                 100. 
                 Main centre axis 
               
               
                 102. 
                 Arrow indication direction of rotation 
               
               
                 104. 
                 Feed conduit 
               
               
                 106. 
                 Pelletizable material 
               
               
                 108. 
                 Seat 
               
               
                 110. 
                 Shear pin 
               
               
                 112. 
                 Stopper 
               
               
                 114. 
                 Actuator 
               
               
                 116. 
                 Hydraulic cylinder 
               
               
                 118. 
                 Piston 
               
               
                 120. 
                 Link part 
               
               
                 122. 
                 Rod 
               
               
                 124. 
                 Nut 
               
               
                 126. 
                 Pin 
               
               
                 128. 
                 Bracket 
               
               
                 130. 
                 Bolt 
               
               
                 132. 
                 Bolt 
               
               
                 134. 
                 Arrow indicating direction of rotation 
               
               
                 136. 
                 Line indicating previously occupied position 
               
               
                 140. 
                 Screw conveyor 
               
               
                 142. 
                 Feed inlet 
               
               
                 144. 
                 Side opening 
               
               
                 146. 
                 Front portion 
               
               
                 148. 
                 Electric motor