Agricultural hammermill and method of fine grinding grain

Abstract

 An agricultural hammermill achieves fine grind at practical production rates. Hammer tip speeds of 7620m/min are provided at 1800 rpm by providing hammers (140) of sufficient length to provide a tip path diameter of approximately 1.37m. In order to avoid excessive centrifugal stresses on the hammer rotor and components, hammers are tapered to reduce tip mass. Because of the high tip speeds, fine grind of feed grains is obtained usually with only a single impact of the hammers. Thus, since it is not necessary to retain the grain in the grinding chamber (150) for multiple hammer impacts, a large apertured screen can be used.

Description

[0001] This invention relates generally to grain processing machinery and more particularly to agricultural hammermills for impact grinding of grains.

[0002] In the agricultural industry, cereal grains and feed grains are commonly dried, ground, mixed, conditioned and pelletised or flaked. Depending upon the formulation being prepared, conditioning and pelleting may require that the grain be ground to different degrees of coarseness. Thus, one application may require grain particles of 1000 microns or larger size while others may require sizes of 500 microns or smaller (1 micron = 1µm).

[0003] Grinding is most often accomplished in hammermills in which a rotor within a chamber with an apertured wall has a number of hammers pinned about its periphery. When the rotor spins, the hammers are centrifugally extended so that they pass very close to the chamber wall and strike any objects which pass through the space between the rotor and the chamber wall. The axis of the rotor is horizontal, and the chamber wall, viewed along the rotor axis, is approximately tear-drop shaped. The grain is fed into the narrow (top) part of the chamber and the ground particles of grain are discharged through the apertured chamber wall (screen). Comminution of the grain occurs by impacts with the hammers and the screen.

[0004] Depending upon the type of grain, its state of dryness and the operating conditions of the mill, a single impact from a hammer will produce a certain distribution of particle sizes. The fine fraction of the particles will pass through the screen while the coarse particles will strike the screen, shatter further, rebound into the path of the hammers, and be struck again. After a few impacts, all the particles will be fine enough to pass through the screen. Of course, the trajectories of the particles also affect the likelihood of passage of the particles through the screen.

[0005] The most common approach for producing fine grind grain particles is to use a screen having fine apertures. This retains the particles within the chamber for a longer time and, thus, subjects them to additional hammer impacts. However, this limits capacity due to reduced particle and air flow capacity and also due to increased screen plugging when high moisture is encountered in the grain or as ambient humidity. In addition, fine aperture screens are much more expensive than are coarse aperture screens.

[0006] Agricultural hammermills usually operate with constant speed motors at either 1800 rpm or 3600 rpm. The higher speed mills are usually restricted to smaller sizes; because the high centrifugal loads at high speed require either heavier rotor and frame structures which increases the cost of the mill or a decrease in rotor diameter which causes a decrease in mill capacity.

[0007] Heavy duty hammermills used in other industries such as mineral dressing and metals processing and recycling may provide higher capacity but at prohibitive cost. In order to economically achieve the grinding performance required for feed grain processing, it is necessary to achieve hammer tip speeds required for desired fineness of grind with a single impact. This permits use of large aperture screens and maintains mill capacity by maintaining high flow of material and air through the mill and by reducing plugging of the screen.

[0008] Generally, particle size of the ground grain is inversely proportional to tip speed of the hammers, so that particles of 800 micron or larger size are produced at about 12,500 feet per minute (3810m/min), 700 to 800 microns at about 17,500 feet per minute (5334m/min), 550 to 700 microns at about 20,000 feet per minute (6096m/min), and less than 500 microns at about 25,000 feet per minute (7620m/min). Therefore, achievement of a fine grind of 500 microns or less requires a high speed hammermill with all the strength and cost penalties already described. These penalties have effectively limited hammermill grinding of grains such that achievement of particle sizes smaller than the 550 to 700 micron range at practical production rates has been very costly if not impossible.

[0009] According to a first aspect of the present invention, there is provided an agricultural hammermill for fine grinding of feed grains, comprising:
   a housing having a grain inlet in an upper portion, a ground particle discharge in a lower portion, and a grinding chamber between said inlet and said discharge;
   an apertured screenwall, surrounding said grinding chamber, having an open top adjacent said inlet for receiving grain, having apertures of at least 2000 microns, being formed as a portion of a horizontal cylinder above said discharge, and having straight tangential portions converging to said inlet;
   a rotor on a rotatably driven shaft mounted coaxially within said horizontal cylinder portion of said apertured screenwall, said rotor comprising a plurality of discs mounted on said shaft, each said disc having at least two hammer support pivot pins symmetrically disposed near the periphery of said disc;
   means for rotating said shaft and rotor at a substantially constant speed of 1800 rpm; and
   a plurality of hammers, one said hammer being pivotally supported on each said pin;
   characterised in that said hammers each have a length such that, when said rotor is rotated at 1800 rpm, the outboard tip of said hammer swings at substantially 25,000 feet per minute (7620m/min) along a path in close proximity to the cylindrically formed portion of said screenwall.

[0010] According to a second aspect of the present invention, there is provided a method of fine grinding grain using an agricultural hammermill for fine grinding of feed grains having an apertured screenwall surrounding a grinding chamber, said screenwall having apertures of at least 2000 microns and being formed as a portion of a horizontal cylinder above a discharge, there being a rotor mounted coaxially within said horizontal cylinder portion of said screenwall, said rotor comprising a plurality of discs mounted on said shaft, each said disc having at least two hammer support pivot pins symmetrically disposed near the periphery of said disc and a plurality of hammers, one said hammer being pivotally supported on each said pin, said hammers each have a specified length; and the method including rotating said rotor at a substantially constant speed of 1800 rpm, characterised in that the outboard tip of said hammer swings at substantially 25,000 feet per minute (7620m/min) along a path in close proximity to the cylindrically formed portion of said screenwall.

[0011] For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 is a cross-sectional elevation view of an agricultural hammermill illustrating the overall structure of the mill;

Figure 2 is a schematic view illustrating the effect of the trajectory of a particle approaching a screen with small apertures (a) and a screen with large apertures (b); and

Figure 3 is a view, parallel to the rotor axis, illustrating a standard hammer (a), a single taper hammer (b), and a double taper hammer (c).

[0012] Referring to Figure 1, the present hammermill has a housing 100 enclosing the mill from a top-connected inlet 110 to a bottom discharge 200. From the bottom edges of the inlet 100, a screen wall 120 is suspended to separate the inlet 110 from the discharge 200 by virtue of the continuous disposition of the screenwall 120 from one edge to the other edge of the inlet 110.

[0013] Starting at one edge of the inlet 110, the screen 120 consists of a straight portion 115 extending downwardly and diverging from the vertical centreline of the mill to a point where the straight portion 115 is tangent to and smoothly blends into a cylindrical portion 113 which curves downward and then upward again in a smooth circular path until it reaches another point of tangency with another straight portion 115 which converges to the opposite edge of the inlet 110 from the starting point. The upper portions of straight portions 115 of the screen 120 may not be perforated since grinding does not take place adjacent the inlet above a grain deflector 90. The deflector 90 serves to prevent grains and particles from being driven upwardly into the inlet 110 by impacts with a hammer 140 travelling upwardly at the right side of the illustrated mill.

[0014] A plurality of hammers 140 are pivotally mounted on hammer support pivot pins 135 and swing freely about the pins 135 which are symmetrically disposed about the peripheries of rotor discs 130 which are mounted upon and rotate with a rotatably driven shaft 125. When the rotor is rotating, the outboard tips of the hammers 140 travel along a path in close proximity to the screen 120.

[0015] The space between the outer surfaces of the discs 130 and screenwall 120 defines the grinding chamber 150 in which comminution of the grain occurs. When the grain has been ground to the desired particle size, it passes through the screen 120 and out of the mill through the discharge 200.

[0016] In operation, grain enters through the inlet 110 and is deflected leftwardly by the deflector 90 into the left side of the grinding chamber 150 where it is struck by the hammers 140, which are swinging past the screen 120 at approximately 1800 rpm, carried on the pins 135 by the discs 130 on the shaft 125, which is driven by a drive motor unit (not illustrated).

[0017] By increasing the outward reach of the hammers 140 (lengthening them), it has become possible to achieve the required hammer tip speed of 25,000 feet per minute (7620m/min.) for grinding grains to particle sizes of less than 500 microns, in most cases with only a single hammer impact. This maintains practical production rates without the expense of heavy- duty hammermills as used in other industries while still achieving the desired fineness of grind.

[0018] Referring now to Figure 2, another feature is illustrated. Figure 2(a) shows the high angle trajectory 122 required for a particle 10 to pass through small holes 20 in the screen 120, while Figure 2(b) shows that large hole 25 will allow particles 10 through the screen 120 at a low angle trajectory. Since the trajectories indicated in Figures 2(a) and (b) are the minimum for each screen, it is clear that all particles at higher angles will pass through if they are of small enough size. To pass 500 micron particles at the low angle trajectory shown in Figure 2(b), an aperture size of about 2000 microns is required, as seen here. Thus, since the higher hammer tip speeds provided by the longer hammers 140 provide finer single-impact particle size, it follows that, even at the lower angle particle trajectories attending high speed impacts, the present hammermill will maintain practical production rates.

[0019] Figures 3(a), (b) and (c) show a standard (known) hammer 140, a single taper hammer 145 of the present invention, and a double taper hammer 146 of the present invention, respectively. These three configurations address the desired performance characteristics of hammers. Hammer 140 is reversible so that, when one side is worn, it can be turned over to place the unworn side forward. Pin hole 141 is symmetrical. Single taper hammer 145 has an edge taper portion 144 on one edge between a minimum width portion 142 and a maximum width portion 143. The straight edge of portion 142 maintains the direction of impact of the hammer tangential to the arc of the tip travel. Thus the objective of reducing the centrifugal forces acting on the rotor components at high hammer tip speeds is accomplished without changing the grinding characteristic of the hammers. The double taper of hammer 146 provides the desired reduction of hammer tip mass needed to reduce centrifugal forces.

[0020] It will be appreciated that the present hammermill provides for fine grinding of agricultural feed grains at practical production rates by providing hammers having increased length and, therefore, higher tip speeds at the same rotor speed (in revolutions per minute) to produce finer particle sizes in a single impact. This produces fine ground particle sizes (less than 500 microns) using large apertured screens which avoids screen plugging and promotes flow of materials and air through the screen. The provision of tapers on the hammer bodies also reduces centrifugal forces on the rotor components and permits operation of larger diameter grinding chambers.

Claims

1. An agricultural hammermill for fine grinding of feed grains, comprising:
   a housing (100) having a grain inlet (110) in an upper portion, a ground particle discharge (200) in a lower portion, and a grinding chamber (150) between said inlet and said discharge;
   an apertured screenwall (120), surrounding said grinding chamber, having an open top adjacent said inlet (110) for receiving grain, having apertures of at least 2000 microns, being formed as a portion of a horizontal cylinder above said discharge (200), and having straight tangential portions (115) converging to said inlet (110);
   a rotor on a rotatably driven shaft (125) mounted coaxially within said horizontal cylinder portion of said apertured screenwall, said rotor comprising a plurality of discs (130) mounted on said shaft, each said disc having at least two hammer support pivot pins (135) symmetrically disposed near the periphery of said disc;
   means for rotating said shaft and rotor at a substantially constant speed of 1800 rpm; and
   a plurality of hammers (145, 146), one said hammer (140) being pivotally supported on each said pin;
   characterised in that said hammers each have a length such that, when said rotor is rotated at 1800 rpm, the outboard tip of said hammer swings at substantially 25,000 feet per minute (7620m/min) along a path in close proximity to the cylindrically formed portion of said screenwall.

2. A hammermill according to claim 1, wherein each said hammer (140) has a middle portion (144) tapered from a portion (143) of maximum width near said pin (141) to a portion (142) of minimum width near said outboard tip.

3. A hammermill according to claim 1 or 2 and further comprising a grain deflector (90) extending downwardly across said inlet (110) from a lower edge thereof, said deflector serving to prevent reflow of grain particles into said inlet from said grinding chamber (150) by occluding a line of sight between upwardly moving hammer tips and said inlet.

4. A method of fine grinding grain using an agricultural hammermill for fine grinding of feed grains having an apertured screenwall (120) surrounding a grinding chamber (150), said screenwall having apertures of at least 2000 microns and being formed as a portion of a horizontal cylinder above a discharge (200), there being a rotor mounted coaxially within said horizontal cylinder portion of said screenwall, said rotor comprising a plurality of discs (130) mounted on said shaft, each said disc having at least two hammer support pivot pins (135) symmetrically disposed near the periphery of said disc and a plurality of hammers (145, 146), one said hammer (140) being pivotally supported on each said pin, said hammers each have a specified length; and the method including rotating said rotor at a substantially constant speed of 1800 rpm, characterised in that the outboard tip of said hammer swings at substantially 25,000 feet per minute (7620m/min) along a path in close proximity to the cylindrically formed portion of said screenwall.

Drawing