US3021953A - Sifter - Google Patents

Description

1952 w. A. BRASTAD ETAL 3,021,953

SIFTER Filed May 29. 1958 FIG. I

FIN

INVENTORS mmumn A. BEQSTQD REI- F. DIEKHOFF STANLEY c. BUSTED Bvykzz mww United States Patent 3,021,953 E TER William A. Brastad, Earl F. Diekhotf, and Stanley C. Rusted, Minneapolis, Minn, assignors to General Mills, Inc, a corporation of Delaware Filed May 29, 1958, Ser. No. 738,873 9 Claims. (Cl. 20339) The present invention relates generally to sifting apparatus and more specifically to an improved sifting device for classifying finely ground or powdered materials.

Sifting devices are known in which material to be sized is conveyed across a sifting screen-or sizing surface with the result that some of the particles pass through the screen and others pass over the screen. Thus the ma terial is classified into separate fractions of different particle size.

In such devices a sifter box is generally employed to enclose the sizing surface or screen and prevent the escape of fine particles of stock. The sifter box is reciprocated or vibrated to convey the material across the screen and to provide the desired sifting action.

While known sifter constructions are satisfactory for many purposes, certain problems require special consideration when finely ground or powdered materials, such as flour milling stocks, are processed. One major problem involves the deleterious effect of dynamic air pressures within the sifter box. It is this factor with which the present invention is primarily concerned and the effect of which is cancelled out to provide improved sifting as will be hereinafter disclosed.

The effect of air within the sifter box has been found to create undesirable pressure differentials on opposite sides or faces of the sizing screen. Such dynamic pressure changes may cause the stock to be held fast to the screen when it should be free to slide, and conversely to be blown upward away from the screen when it should be in close contact therewith. The pressure changes are particularly encountered at rapid frequencies of reciprocation, e.g., of the order of six strokes per second.

It is one object of our invention to provide an improved sifter having means to increase conveying and sifting efficiency by cancelling out the effect of dynamic air ressur s within the sifting chamber.

Another object of our invention is to provide an improvide sifting device having rigid partitions disposed at particular locations within the sifting chamber to gencrate equal dynamic pressures above and below the surfaces of the sizing screen.

A further object of our invention is to provide an improved reciprocating or vibratory sifter having a screen surface which is enclosed to prevent dusting-out of the material conveyed and having means for cancelling out the effect of dynamic air pressures which affect the sifting operation.

A still further object of our invention is to provide an improved sifting apparatus wherein rigid means is provided perpendicular to the direction of flow of the material to be sized for controlling fluctuations in pressure on opposite sides of its sizing surface.

Another object is to provide a sifter construction having increased symmetry in construction above and below the ends of the sifting surfaces to equalize the generation of dynamic pressures.

Other objects and advantages will become apparent from the following specification in which certain preferred embodiments of the invention are described.

In the drawings which form a part of this application:

PEG RE 1 is a schematic side elevation of-an improved sifter embodying features of the present invention; 1..

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1; and

FIG. 3 is a partial side elevation view of a further embodiment of the present invention.

The instant invention is exemplified in FIG. 1 in a reciprocating sifter 10 designed in a particular manner to prevent or cancel out the undesired effects of dynamic air pressures generated by the reciprocation of the sifter. The sifter 10 includes an enclosed body portion formed of suitable wall means such as a removable top wall 12, a bottom wall 14, side walls 16 and 18 and end walls 20 and 22. Within these walls are generally horizontal classifying surface 24 having a main sifting or screen area 25 extends longitudinally of the sifter Hi. This surface divides the body portion into substantially closed upper and lower chambers or compartments 26 and 23.

The sifter 10 also includes in the top plate 12 an inlet opening 30 through which material can be fed onto the imperforate receiving portion or plate 32 at the head or receiving end of the classifying surface 2 5. At the opposite or discharge ends of compartments Z6 and 23 the sifter 10 isprovided with an imperforate discharge portion or plate 34 adjacent independent outlets 36 and. 38 which receive the separate discharge of the overs and throughs from the classifying surface 24.

The sifter 10 is supported for reciprocation along a desired path by hangers 4t) and 42 which are pivotally mounted to a suitable supporting structure 44. The hangers 40 and 42 are pivotally connected to the sifter 10 at their lower ends and provide swinging movement thereto along an arcuate path generally perpendicular to the hangers, to provide conveying and sifting action in moving the material across the screen area 25.

The desired reciprocation is given to the sifter 10 in known manner by the connecting rod 46 which is pivoted at its one end to the sifter 1G and at its other end to the eccentric 48 carried by a rotating drive shaft 50, driven by a motor 52.

As described up to this point, the sifter is designed to receive stock through inlet 30 and to move the stock across the sifting surface 24 from the imperforate receiving end 32 toward the imperforate discharge end 34. Stock which can pass through the perforated screen 25 is carried along the lower chamber and discharged at 38,

while the overs of screen 25 are discharged at 36.

In actual operation, however, such a sifter often exhibits uneven and erratic movements of the stock, which we have termed surging. For example, the conveying movement of stock at the head or receiving end has been observed to slow up and even back up toward the inlet, while stock at the tail end moves even faster off the screen and leaves a gap or bare areas on the screen 25. A thick pile of stock then accumulates at the head end and gradually expands along the screen, but without apparent conveying action. As the stock finally expands toward the discharge end and covers the screen, the conveying movement of the layer may begin again. Stock near the tail end moves off more rapidly, until the layer again seems to break in the center screen area, with the portion nearest the inlet backing up toward the inlet and the other portion moving more rapidly to the outlet. Such surging is believed to impair the sifting action, since some stock is held and sifted too long at the head end, while other stock is discharged too soon with inadequate sifting.

It is our belief that the inertia of the fluctuating air columns within the upper and lower chambers of the sifter tends to affect the sifting and conveying operation by creating pressure differentials on opposite sides 11 and 13 of the sizing screen 14. These pressure differentials appear to work in opposition to the desired sifting and conveying'action. F Y

For example the construction of the sifter is such that the ends of the upper and lower chambers are not symmertical or balanced in their effects on the enclosed air columns. Thus the area 54 at the head end of the lowermost chamber or compartment 28 is essentially closed by the lower portion of head end wall 26. On the other hand, the inlet 36 through which the material enters the Sifter body provides a sizeable opening for the corresponding head end 56 of upper chamber or compartment 26 above the screen 25 and receiving surface 32. During sifting as noted above, the difierences in relative freedom of movement of the air in these compartments (excluding for the moment the partitions to be described below) will cause pressure differentials which can give rise to the undesirable results noted previously.

Thus air or fluid columns in the compartments 26 and 28 above and below the screen 25 have aninertiawhich. tends to hold the fluid column stationary as the sifter vibrates. If the sitter it} is moving to the right, however, as indicated by the arrow 58 in FIG. 1, the tendency of the air to remain in its original position, and the movement of end wall 2% to the right, will produce a region of higher density or higher pressure at 54 during such movement. Conversely, when the sitter moves to the left as indicated by the arrow 69, the end wall tends to move away from the enclosed column of air in compartment 23 to produce an area of lesser density or lower presure at 54. These pressure variations within the sifter 10 are for practical purposes termed air slugging or dynamic air pressures.

The effect of air-slugging is somewhat dilferent above the screen 25, however, since the inlet 30 will permit the escape of part of the air column from the head end 56 of upper compartment 26 during movement of the upper portion of end wall 20 and Sifter 10 to the right. Likewise the inlet 3% will admit further air as the end wall 20 and sifter 16 move to the left. Thus the inertia of the body of air above the screen 25 will not cause air-slugging or dynamic pressure of the same magnitude as that below the screen. Hence the higher pressure at 54 tends to lift the material being conveyed away from the screen 25 just when it should be in contact with the shifting surface in order to obtain the desired conveying and sifting movement to the right. Likewisethe lower pressure at 54 during the movement of the sifter 10 to the left tends to draw the material conveyed down against the screen 25 and move the material to the left, just when it should remain free of the screen in order to obtain relative movement toward the tail end of the sitter.

At the tall or discharge ends 62 and 64 of the respective upper and lower chambers 26 and 28, the effects of dynamic pressures appear to increase the conveying action. Here, when the upper portion of'tail end wall 22 moves to the right (arrow 53), it tends to create an area of low pressure above the tail end of screen 25.

While the lower portion of end wall 2 may have 'a similar effect in the region 64, an even more important factor in this area is the presence of the usual frame member 66 which supports the end of screen 25. This member acts as a wall portion immediately below the screen. As it moves to the right it further decreases the dynamic pressure at 64 beneath screen 25. The net result is a pressure dilferential in which the lower pressure below the tail end of the screen holds stock on the screen as the screen moves to the right. H g Conversely, the increased dynamic pressures duet o movement of wall 22 and frame 66 to the left (arrow 60) result in a pressure differential which pushes the stock upwardly away from the screen on this return stroke. The net elfectis to increase the conveying efiect and hurry the material off the discharge end of the screen.

According to the present invention means are introduced to increase the effective symmetry of dynamic pressure. generation in the upper and lower chambers 26 and 2a. For this purpose part tions of apprc riate construo L on are added at specified locations. One such partition 68 is located in the head end of the upper chamber between inlet 30 and the head end of screen area 25. Another partition 76 is preferably located at the tail end of the upper chamber 26 above the frame member 66. These partitions are constructed with sufficient rigidity to prevent them from flexing substantially in response to reciprocation of the sifter. Thus the partitions serve as walls which can close the major cross section of the chamber in which they are located and can generate dynamic pressure which neutralize or overcome the undesirable effects of dynamic pressures generated in the other chamber.

The rigid partitions 68 and 70 which are suspended beneath the top wall *12 within the upper chamber 26 may be identically constructed. Because they do not completely close the ends of the chamber but provide a limited pas-agew-ay for stock just above the surfaces 32 and 34, they may he referred to as semi-partitions.

Each semi-partition consists of an inverted L shaped plate 72 and a substantially flat plate 74 which extends across the width of the chamber 26 generally normal to the direction of flow of the stock 76. Plates 72 and 74 are secured together by a plurality of screws or bolts 78 positioned in vertically slotted openings 80 in plates 74 and bolt holes 82 in plates 72-. The bolts 78 are provided with wing nuts 84 which hold or retain the plates 72 and 74 together so that the lower edges 86 of the plates 74 can be held in a desired adjusted spaced relation above the top surface of the receiving and discharge plates 32 and 34. Accordingly one can readily loosen the wing nuts 34- and slide plate 74 vertically to adjust the spacing between the lower edges 86 of the plates 74 and the top surface of the receiving and discharge plates 32 and 34.

By means of these semi-partitions 68 and 70, the deleterious effects of dynamic air pressures within the charm hers 26 and 28 are overcome or cancelled out, and interference with conveying and sifting is prevented. This, apparently results from the increased effective symmetry of the upper and lower chambers.

For example, by adjustment of the wing nuts 84, the lower edges 86 of the plates 74 are raised or lowered as. required so that a minimum opening is provided whichwill allow the stock 76 to pass, but will allow little or no air to flow in or out. Thus in effect the semi-partitions- 68 and 70 retain the air above the screen 25 captive during reciprocation of the sifter 10 and exert forces on the air mass or column in upper chamber 26. These forces generate dynamic pressures in phase with reciprocation of thesifter, which overcome the undesired effects previously described. Thus the conveying effect at the head end is not slowed down or reversed, and the unduly rapid con-- veying at the tail end is avoided.

The undesired pressure differences generated by recipro-- cation of a conventional sitter are accordingly neutralized or overcome, and an equalization of pressures above and:

below the screen 25 within the chambers 26 and 28 provides improved sifting and conveying to the stock at both the head and tail of the sifting surface.

In establishing the desired efiective symmetry of the upper and lower chambers, one should consider the effects of the stock outlets'as well as inlet 30.

During operation of the sitter 10 stock admitted through inlet 30 'mayarrive at a rate which effectively closes ofl? all-or part of the inlet. One may recognize the degree of pressure relief -'afiorded by the'dischar'ge outlets 36 and 38 which receive the overs 88 and the throughs 90 of the stock 76, andlocat'e the semi-partitions 68 and 70 accordingly. The partitions are adjusted to provide a minimum opening adjacent the lower edges 86 of the plates 74 which will allow the stock 76 to be conveyed across the surface of the screen 25.

'As willbe noted in FIG. 1 the sitter 1t! issubstantially closed to outside atmosphere with the inlet 30 connected to stocking 94 and a hopper 96. Likewise the discharge outlets 36 and 38 may be connected to stockings 98 and 100 and to a hopper 102 and further sifter 104, respectively. Thus the present device provides not only an improved sifter having increased sifting and conveying efiiciency, but also one in which the dusting-out of particles is substantially prevented.

By adjustment of a speed changer 92 attached to the motor 52 the rate of reciprocation of the sifter may be controlled so that the stock 76 is conveyed at a given rate.

In FIG. 3 a further embodiment of the invention is shown. Under certain operating conditions the air within the chamber 26 may tend to blow underneath the lower edge 36 of the plate 74 and escape through the outlet 30 during reciprocation. To prevent such action, the preferred form of FIG. 3 replaces plate 74 by an L shaped member 106 having'an arm 108 extending generally parallel to the direction of flow of the stock 76. This arm provides a relatively long restricted passage between the partition and screen as indicated by the numeral 110. The stock effectively closes most or all of this passage and further retards any movement of the air past the incoming stock.

The above description presents a disclosure of the instant invention. The specific construction details of the sifting device 10 are used for illustration only and should accordingly not be considered as limiting the invention to the exact form disclosed. For example, depending on the shape and location of such frame members as 66 (FIG. 1) it may be desirable to increase the effective symmetry at the discharge ends of the upper and lower chambers by adding a further semi-partition 112 at the end of the lower chamber directly beneath upper partition 70, as shown in the dotted outline in FIG. 1.

We have set forth in this application the principles believed to be involved and some of the ways in which the invention may be practiced.

We claim:

1. In a reciprocating sifter comprising a conveying and sifting surface having an imperforate receiving end, a discharge end, and a perforated sifting area between said ends, a body portion having walls enclosing said conveying and sifting surface and providing separate substantially closed upper and lower chambers extending longitudinally above and below said surface, means for reciprocating said body chambers and sifting surface longitudinally as a unit and thereby conveying stock across said sifting area from the receiving end to the discharge end, an inlet for stock at the receiving end of the upper chamber and an outlet for stock at the other end of each chamber, the improvement comprising an anti-surging partition located in said upper chamber between said inlet and said sifting area at a point adapted to equalize dynamic air pressures in said upper and lower chambers, said partition being secured for reciprocating movement with said body and sifting surface and having an upper edge adjacent said top wall, side edges adjacent said side walls, and a bottom edge spaced closely above said surface and thereby providing a restricted passage adapted to be substantially closed by stock passing from the inlet to the sifting area. 7 a I 2. A sifter according to claim 1 in which said partition has a flange at its lower edge, said flange extending parallel to the sifting surface and thereby providing a relatively long restricted passage adapted to be closed by said stock.

3. A sifter according to claim 1 having a similar partition at the discharge end of the upper chamber.

4. A sifter according to claim 3 having a similar partition in the lower chamber extending downwardly from said surface at the discharge end of said perforated sifting area.

5. An improved sifting device comprising a sifting chamber enclosed by a plurality of wall members, inlet means in said chamber for feeding stock to be classified thereto, outlet means for discharging classified stocks, a sifting surface intermediate said inlet and outlet means for classifying said stock during said reciprocation, a substantial'y imperforate receiving plate at one end of said sifting surface below said inlet means, a relatively air-tight discharge plate at the opposite end of said sifting surface adjacent said discharge means, means for reciprocating said devices including said chamber, wall members and sifting surface as a unit along a given path, and rigid antisurging partition means disposed above each of said plates and having an area transversely of said direction of reciprocation, which partition reciprocates with said wall members and thereby moves a volume of air within said chamber above the sifting surface in desired phase and correlation with the reciprocation of said chamber to provide improved sifting and conveying.

6. The device of claim 5 wherein each of said rigid partition means comprises a pair of plate members extending across the width of said chamber and wherein at least one of said plate members on each partition is adjustably mounted with respect to the sifting surface of said chamber.

7. An improved sifting device comprising a sifting chamber enclosed by a plurality of wall members, means for reciprocating said chamber along a given path, an inlet and outlet means in said chamber, a sizing surface dividing the sifting chamber into an upper and a lower compartment and movable as a unit with said enclosed chamber, and rigid partition means secured for reciprocating movement with said chamber and disposed within said upper chamber adjacent said outlet means and having a substantial area transversely of said path and thereby moving the volume of air within the outlet end of said chamber in correlation with said reciprocation.

8. A sifting device comprising an enclosed sifting chamber having an inlet and outlet means for receiving and discharging material to be classified, classifying means within said chamber between said inlet and outlet means, means for moving said chamber and said classifying means as a unit relative to said material whereby it is classified, and dynamic air pressure generating means adjacent one of said inlet an outlet means in close proximity with said material and located and arranged to generate dynamic air pressures in response to relative movement of said chamber, thereby preventing air slugging and surging of the stock within said chamber during said relative movement.

9. An improved sifting device comprising a reciprocating sifting box, means for reciprocating said box along a given path, a sizing screen dividing said box into an upper and a lower chamber and movable therewith, an inlet located at a first end of said upper chamber, outlets located at the second end of each chamber, rigid antisurging partitions between said inlet and said screen and between said screen and at least one of said outlets, said partitions extending transversely of said path and having a shape and area substantially closing the passage through "the upper chamber except for a minimum opening required by the stock to be passed through the device for sifting, and said partitions being secured for reciprocating movement with said box.

References Cited in the file of this patent UNITED STATES PATENTS 398,692 Bittinger Feb. 26, 1889 794,936 Harris July 18, 1905 1,214,506 Brasack Feb. 6, 1917 1,686,107 Sewell Oct. 2, 1928 1,793,018 Runge Feb. 17, 1931 2,047,713 Simpson July 14, 1936 2,589,957 Pearman Mar. 18, 1952 2,903,135 Dryg Sept. 8, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,02l 953 February 20 1962 William A. Brastad et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 52 for "areas" read area column 3. line 42, for "shifting" read sifting line 56, for "2" read 22 column 4, line 11, for "pressure" read pressures --3 line 18, for "pasageway" read passageway column 6,, line ll after "material" insert along a desired Signed and sealed this 12th day of June 1962.

(SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Commissioner of Patents Attesting Officer

Images