US3809133A

US3809133A - Hopper with variable level hopper feed tube

Info

Publication number: US3809133A
Application number: US00381008A
Authority: US
Inventors: D Skinner
Original assignee: Beloit Corp
Current assignee: Beloit Corp; First National Bank of Chicago; First National Bank of Minneapolis
Priority date: 1973-07-20
Filing date: 1973-07-20
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07

Abstract

A feed tube extends downwardly into a hopper, preferably a vacuum hopper wherein vacuum pressure is created within the hopper to draw material through the feed tube thereinto, and it is preferably a split hopper having at least two sections filled with separate material at a predetermined ratio. The lower end of the feed tube includes an arrangement such as openings in the tube itself and a relatively rotatable surrounding sleeve for varying the upper limit of lateral through openings through the side of the feed tube. When the material in the hopper reaches the said upper limit and hence covers the through openings, the vacuum will no longer be communicated to the interior of the feed tube and hence the flow of material therethrough will terminate. Consequently, the relative positions of the inner tube and the surrounding sleeve determine the amount of material delivered to the section of the hopper associated with that feed tube.

Description

Primary Examiner-Stanley H, Tollberg Assistant Examiner-Norma'n L. Stack, Jr. 7 V Attorney, Agent, or Firm-Larson, Taylor & Hinds I United States Patent 11 1 1111 3, Skinner May 7, 1974 HOPPER WITH VARIABLE LEVEL HOPPER [57 ABSTRACT FEED TUBE A feed tube extends downwardly :into a hopper, pref- [75] Inventor: David Skinner, B910", erably a vacuum hopper wherein vacuum pressure is [73] Assign; a Corporation Belch, w created within the hopper to draw material through v the feed tube thereinto, and it is preferably a split hop- 2] Filedi J y 1 1973 per having at least two sections filled with separate [21] APPL NOJ 381,008 material at a predetermined ratio. The lower end of v p the feed tube lncludes an arrangement such as openings in the tube itself and a relatively rotatable sur- [52] US. Cl. 141/198 rounding sleeve for varying the upper i i f lateral [51] Int. Cl 1365b 1/30 through openings through the Side f the f d tuba of Search n when the material in the hopper reaches the said 141/7, 59, 198, 8 upper limit and hence covers the through openings,

1 the vacuum will no longer be communicated-to the in- [56] References Cited terior of the feed tube and hence the How of material UNITED STATES PATENTS therethrough will terminate. Consequently, the rela- 2,138,356 11 1938 Ryan et al 141/7 five Positions. of the inner tube and the Surrounding 2,708,542 5/1955 Gray et al 141/7 sleevedetermine h amount 0 material delivered to 1,163,734 12/1915 Birns 239/538 the section of the hopper associated with that feed 3,737,074 6/1973 Davies 302/59 X tuba 3,00l,465 9/1961 Donelson, Jr 302/59 X 3,423,130 1/1969 Milner 302/59 X 1. 3, 9 a 7 D awin Figures HOPPER WITH VARIABLE LEVEL HOPPER FEED TUBE BACKGROUND OF THE INVENTION This invention relates to vacuum hoppers, and in particular it relatesto a new and improved feed tube for such a hopper, which feed tube has the capability of varying the level of the material delivered through the feed tube to the hopper. A vacuum hopper of the type with which the present invention is concerned is illustrated and described in U.S. Pat. No. 3,309,146; This type of hopper is normally split into two sections each of which is filled from a separate source and both of which are emptied through a common bottom gate. A suitable means such as a motor-blower creates a vacuum within the entire hopper, and this vacuum is used to draw material through feed tubes from separate sources to the two separate sections of the hopper. One

particular application of this type of hopper is in the plastic industry where it is often desirable to supply at least two different materials such as virginand regrind material in a fixed volume ratio to a machine for proother side of the hopper. The volume delivered to each of the two sides is carefully controlled in accordance with a predetermined ratio therebetween. After the two sides have been filled to the appropriate levels, the common gate is opened and all of the material in both sides is discharged into a common receiving means for further processing.

Various means are known for supplying the predetermined amounts of material to the two sections of the hopper. As indicated above, in this type of arrangement,.the vacuum created within the hopper draws the material through the feed tubes. Consequently, when the material in a given section of the vacuum hopper fills up to a level so as to cover the opening from the feed tube to the hopper, the vacuum is no longer communicated back through the feed tube and hence the flow of material through the feed tube is terminated.

This may be sensed for immediately cutting off the motor-blower. In the said U.S. Pat. No. 3,309,146, the feed tube in one section of the vacuum hopper is at a fixed height while the lower end of the feed tube in the other section of the hopper is vertically adjustable. In

this manner, the height of the material in the second section can be varied, and the feed tube will be at a height so as to terminate material flow therethrough at a level which will provide the proper ratio to the fixed SUMMARY OF THE INVENTION Thus, it is a purpose of the present invention to pro-- vide a new and improved feed tube of the type for delivering material into a vacuum hopper and concurrently determining the level of the material delivered thereinto.

This purpose of the present invention is achieved by providing a feed tube which extends downwardly into upper limit of the openings, one can vary the level of material delivered into the hopper before the flow is terminated (by terminating communication of the vacuum pressure into the feed tube).

In accordance with a preferred embodiment of the invention, the feed tube itself forms at its lower end an inner tube which is surrounded by an outer sleeve.

Openings are made in the side of the inner tube and in the side of the outer sleeve. These openings are related.

to each other such that by turning one of the said sleeve and innter tube relative to the other, the vertical posi tion of the overlap of the two cutouts can be varied. In a preferred arrangement, the opening through the feed tube will initially be formed at the lower end of the side thereof and upon further relative rotation of the two members, the opening through the feed tube .will remain continuous, enlarging in size andjsimply. moving up the side of the feed tube. In other arrangements, the opening can be of a relatively fixed'size, this fixed size through opening moving up the side of the feed tube as the members are rotated relative to each other.

Normally, the inner tube will form a part of the continuous conduit back to the source and this will be fixed while the outer sleeve is mounted so as to rotate about the inner tube. Indicia means may be provided on the inner tube and outer sleeve to visually indicate the exact position of the opening, and hence also the level to which the material will fill in the hopper.

The openings in the inner tube and the outer sleeve can take various forms; In accordance with a first embodiment, one member may include downwardly outwardly diverging slots while the other member includes parallel sided slots, the width being equal'to the smallest width of the said diverging slots in the other member. With this arrangement, as the straight sided slots gradually start to overlap the diverging slots, the through opening gradually enlarges in size and moves up the side of the feed tube. A similar result can be obtained using a helical row of separate openings on one member and enlarged cutout on the other member which diverges on one side at an angle equal to the helical angle and is parallel to the axis of the feed tube on the other side. With this arrangement, the helically arranged ports overlap with the larger area on the other member one at a time, starting with the lowermost cutout and then moving upwardly; but as each next cutout is exposed, the cutouts beneath it remain exposed.

In other embodiments, the opening through the feed tube can remain of the same size and move gradually up the side of the tube as the inner tube and sleeve are relatively rotated. This may be accomplished, for example, by providing on the tube members helical slots or helical rows of openings extending up and about the tube members at the same pitch but in the opposite direction.

' ternatively, a further sleeve or the like may be provided so as to partially cover the uppermost open cutout.

Thus, it is an object of the invention to provide a new and improved feed tube for use in a vacuum hopper.

It is another object of this invention to provide, for use in a vacuum hopper, a new and improved feed tube which permits variation of the material delivered into its respective section of the hopper in a highly simplified manner.

It is still another object of this invention to provide, for use in a vacuum hopper, a feed tube having openings in the side thereof, the vertical position of which openings can be varied without changing the vertical position of the feed tube itself, to thereby control the level of the material delivered into that respective section of the vacuum hopper.

It is still another object'of this invention to provide, for a vacuum hopper, a feed tube having relatively rotatable inner tube and outer sleeve members, these two members having openings therethrough, and the vertical extent of the openings being varied upon relative rotation of the members.

Otherobjects and the advantages of the present invention will become apparent from the detailed description to follow, together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS There follows a detailed description of preferred embodiments of the invention which are to be read together with the accompanying drawings which are provided for illustrative purposes only.

FIG. I is a schematic side elevation view of a split vacuum hopper including feed tubes constructed inaccordance with the features of the present invention.

FIG. 2 is an enlarged view, partially in section and partially in side elevation of either of the feed tubes of FIG. 1.

FIG. 3 is a development of the portions of the feed tube of FIG. 2 with the inner tube show in dotted lines and the outer sleeve superimposed thereon (in the closed position of the feed tube) in dotted lines.

FIG. 4 is a plan view .of FIG. 2.

FIG. 5 is a side elevation view similar to FIG. 2 showing another embodiment of a feed tube of the present invention.

FIG. 6 is an enlarged development view of the inner tube and outer sleeve of FIG. 5, with the outer sleeve shown in solid lines and the inner tube superimposed thereon in dotted lines, the members being shown in the closed position of the feed tube.

FIG. 7 is a development view similar to FIGS. 3 and 6 showing another embodiment of the invention with the inner tube shown in solid-lines and the outer sleeve superimposed thereon in dotted lines in the closed position of the feed tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, like elements are represented by like numerals throughout the several views.

FIG. 1 illustrates schematically a split chamber vacuum hopper 10 having a pair of chambers 10a and 10b separated by a divider plate 1 1. As explained in the said U.S. Pat. No. 3,309,146, the two chambers 10a and 10b are filled-with different materials such as for example virgin and regrind materials, respectively, when utilizing the vacuum hopper in the plastic forming industry. The important factor is the volume ratio of the two different materials after the delivery of those materials has been completed and the bottom gate 12 is opened by pivoting the same about pivot axis 12a for discharging all of the material in chambers 10a and 10b into a common receptacle. a

Also shown in FIG. 1 is a feed tubeIS extending downwardly into each of the chambers 10a and 10b, each feed tube including an inner tube 16 which is generally fixed and which continues to the source of material being delivered to the chambers and an outer sleeve 17. This outer sleeve 17 is preferably rotatable relative to the fixed inner tube 16. Referring momentarily to FIGS. 2 and 4, the sleeve 17 includes a collar 18 fixed thereto which may rest loosely on the top 13 of the hopper 10 so that it can be turned relative to the tube 16. The tube 16 may include a collar 19 fixed thereto to rest on the collar 18. Also, has shown in FIG. 4, indicia means 30 and 31 may be provided on the rotatable and fixed

collars

18 and 19, respectively, so as to indicate the degree of rotational movement of the sleeve 17 relative to the fixed tube 16. A motor-blower 14 is shown schematically in FIG. 1 communicating with the chambers 10a and 10b. As explained in the said US. Pat. No. 3,309,146, the most important purpose of this type of apparatus is to establish the proper ratio between the two materials in the two different chambers (as opposed, for example, to the absolute amount of the materials). Consequently, as in the said patent, it is feasible to utilize in one chamber a feeding means which always feeds to a constant level and then to use the variable feeding means of the present invention in only the other chamber. However, there are times when it is preferable to control the volume in both levels so as not to always be restricted to a fixed level in one of the two chambers, and hence there is some advantage in having a variable level feed tube in both chambers. Consequently, in the illustrated embodiment, a feed tube is provided in each of the two chambers 10a and 10b.

In the operation of a vacuum hopper of the present type, the hopper 10 is generally enclosed and the interior thereof is evacuated by a motor-blower 14, thus creating a vacuum within the chambers 10a and 10b. This vacuum is communicated through the open bottom of After the material has been discharged, and the gate 12 closed, a new cycle may commence.

In accordance with a main feature of the present invention, lateral openings are provided in the side of the tube 16 and the outer sleeve 17, the patternof openings being such that as the sleeve 17 turns about tube 16, different portions of the two sets of openings overlap, thereby varying the position, and most importantly the vertical position, of the complete

openingthrough elements

16 and 17, this complete opening constituting a through opening for communication of the vacuum pressure to the interior of tube 16. 1

A preferred embodiment of the present invention is illustrated in detail in FIGS. 2-4. The bottom of the feed tube is of course open so that material canflow directly through the tube 16 and directly outthe end of feed tube 15 as it fills the major portion of the respective chamber beneath the feed tube.-In this particular embodiment, the outer sleeve '17 includes a pair of straight sided slots and 26 and the inner tube 16includes a pair of downwardly diverging

slots

27 and 28..

As illustrated in the figures, the side of the feed tube 15 is completely closed since the

slots

25 and 26 do not overlap the

slots

27 or 28. However, referring to FIG. 3which shows themember l6 and 17 in development view and superimposed on each other, as the member 17 is moved (to the right in FIG. 3), the edge25a of slot 25 will cross the inclined (actually helical in the curved tube) edge 27a of the slot 27 andthe greater the angle of turning, the greater the overlap between

slots

25 and 27. Also, of course the upper extent of the through opening formed bythe overlap rises upwardly. Thus, by turning the member 17 relative to the member 16, the upper extent of the through opening can be'raised, thus raising the level to which the material will be delivered before the delivery thereof is terminated.

Edges

26a and 28a of

slots

26 and 28 operate in the same manner as describedwith respect to edges 25a and 27a. Thus, only a single set of overlapping slots need be provided.

' ,However, an advantage of providing a secondset of slots diametrically opposed from the first set of slots is that itaids in equalizing the distribution of the material passing through the lateral openings afterthe material has covered the bottom of the feed tube 15. This might be especially important if one of the two through openings is adjacent a wall of the hopper. Also, .a single opening could possibly become plugged while this would be less likely with a plurality of through openings.

FIGS. 5 and 6 illustrateanother embodiment of the invention. For convenience, only the portionbelow the top 13 of the hopper is shown in thesefigures. In this embodiment the inner tube 116 includesa plurality of cutouts 50-54 arranged spirally along a helical pathon the inner tube 116. These cutouts arenotvisible in FIG. 5 because this figure shows the feed tubein the position with none of the openings of the two

members

116 and 117 overlapping. The opening in the outer sleeve 117 is generally of a downwardly diverging shape. One side of this opening, whichis visible in FIG. 5 and which is shown at the left-hand side of FIG. 6, is formed by a straight: vertical edge 39 from which project a plurality of fingers 40-44, the spaces between these fingers being horizontally aligned with respective ones of the cutouts -54. The opposite side of the opening in the sleeve 117, which side is partially visible in FIG. 5 and completely visible at the right-hand side of element 1 17 in FIG. 6 comprises a set'of steps 45-49 which are arranged to cover respective ones of the cutouts 50-54 in the completely closed position as illustrated.

In the operation of the embodimennsleeve 117 rotates in a direction corresponding to movement to the right in FIG. 6 while tube 116 remainsfixed. Upon initial rotation, the cutout 50 will cross the edge 39 and communicate with the space beneath-finger 44 thus providing a lower through opening. Upon further movement of sleeve 117, the cutout 51 will communicate with the space betweenfingers53 and 54. This will continue until the cutout 54 communicates with the space betweenfingers 40an d 41. With the present arrangement, it-willbe notedthat as each new higher cutout'communicates with the space to the left of edge 39, all of the cutouts beneath it remain exposed.

Inboth of the embodiments of FIGS. 1-4 and 5-6, as the through openingor openings move up the side of the feed tube, the lower openings or the lowerpart of a. single opening remains exposed. This is a preferred arrangement since it provides the advantage that since the material flowingthrough the tube will seek the lowermost opening, as the hopper is filled, after the lower end of the tube has been covered, the spiral 'dispostition of the lower openings or opening portions will aid in distributing material generally about the chamber.

However, the inventionis also applicable to arrangements wherein the lateral through opening. remains of an essentially constant size and moves up the side of the feed tube as the sleeve is rotated. FIG. 7 illustrates schematically two possible structures for operating in this manner; Inthiscase, the fixed feed tube 216 and the rotatable outer sleeve 217 may be provided with either opposite hand equal pitch helical slots and 61 which form through openings as the edges60a and 61a of the slots overlap or alternativelyrthese slots can be replaced by rows of spirally arranged "openings, the rows being of equal pitch and in an opposite direction, andof course the openings in eachrow corresponding vertically with an openingin the other row. Such openings are illustrated schematically M62 and 63for the

elements

216, and 217, respectively, in FIG. 7.

, ratherthan continuous slots. For example, referring to The present invention also includestheprovision of meansfor effecting a fine adjustmentofthe through openings. This would be particularly advantageous whenusing an embodiment having separate cutouts the embodiment of FIGS. S and 6, when a given cutout of the cutouts 50-54 constitutes the uppermost exposed cutout, it may be desirable to change the vertical location of the upper end of that through openingwithout moving to a differenvcutout: This can be accomplished in several ways. Firstly, oneof the

members

16 and 17 can be madeto move vertically. relative to the other. For example, the collar l9ycan engage alongitudinal groove in thetubes' 16 to move vertically away from the collar 18, thuspermitting movement of the collar 18 upwardly along;the tube 16. Similarly, the inner tube-16 can be moved vertically. relative to the outer sleeve .17. Alternatively, a further sleeve in addition to the elements 16and17 can be provided on the feed tube, which additional element could be spaced and arranged to cover the uppermost opening and move vertically relative to the

members

16 and 17. For example, in the embodiment of FIGS. and 6, a plate could be provided on the exterior of element 17 ex? tending circumferentially only for the length of the fingers 40-44. This would be sufficient for covering the upper portion of the uppermost exposed cutout. Alternatively, a further sleeve could be provided having openings identical to the openings of either one of the

members

16 or 17 and movable relative to both of them. It will be understood that these modifications are equally applicable to all of the described embodiment s.

Although the invention has been describedin considerable detail with respect to a preferred embodiment,

- it will be apparent that the invention is capable of numerous modifications and variations apparent to those skilled in the art without departing from the spirit and scope of the invention.

1 claim:

1. An enclosed vacuum hopper having pump means for creating a vacuum pressure therein, a feed tube extending downwardly into the hopper to be subjected to the vacuum pressure in the hopper to move material through the feed tube into the hopper, said feed tube including means defining lateral through openings in the side thereof near the lower end thereof for communicating the vacuum pressure to the interior of the feed tube, and means for varying the vertical position of the lateral through openings while maintaining the lower end of the feed tube at a given vertical position within the hopper.

2. A hopper according to claim 1, the lower end of the'tube comprising an inner tube member and an outer sleeve member, each member having lateral openings in the sides thereof, and said varying means including means for rotating one of the members relative to the other to vary the relative position of the sleeve member and the inner tube member openings.

3. A hopper according to claim 2, including indicia means on said sleeve and inner tube for indicating the position of one relative to the other.

4. A hopper according to claim 2, wherein the opening in one of said members comprises at least one slot diverging from an upper edge to a lower end at the lower end of that member, and the opening in the other member comprising a slot extending from an upper edge to the lower end of that member at a divergence angle less than that of the first said member, such that when the two members are turned relatively to overlap the slots, the upper extent of the through opening moves upwardly away from the lower end while the opening remains open beneath said upper extent.

5. A hopper according to claim 4, wherein the slot in the first said member has at least one helical edge and the slot in the said other member includes a pair of straight parallel sides parallel with the axis of the feed tube.

6. A hopper according to claim 5, the first said member being a inner tube and the second said member being the outer sleeve. .7

7. A hopper according to claim 2, said inner tube being fixed against rotation and said outer sleeve being rotatable about the inner tube.

8. A hopper according to claim 2, the-said openings in the two members being shaped such that upon relative rotation of the two members, the through opening travels up the feed tube, closing beneath it.

9. A hopper according to claim 8, said openings comprising a pair of equal pitch opposite hand helical slots located one in each of said members.

10. A hopper according to claim 8, said openings comprising a pair of equal pitch, opposite hand helical rows of openings, one row in each member, the holes in the two rows being at corresponding heights to each other.

11. A hopper according to claim 2, including a third member coaxial with the outer tube and sleeve members, said third member including openings registering with the openings in at least one of the said first or second members, said third member being movable to at least partially cover the through openings while the first two members are fixed relative to each other for a fine adjustment of the said through opening.

12. A hopper according to claim 2, at least one of said inner tube or sleeve members being movable axially relative to the other for fine adjustment of the said through opening.

13. A hopper according to claim 2, the openings in a first of said members comprising a plurality of openings arranged in a helical path about that member, the openings in the second member being a diverging slot, one edge of the slot diverging at an angle less than the angle of said helix so that upon relative rotation of the members, first the lower opening and then successive upper openings of the first member cross said one edge to form through openings, the slot diverging downwardly and outwardly a sufficient amount such that as each successive opening crosses said one edge and hence forms a through opening, those openings below it remain exposed through the slot to form lower through openings.

14. A hoppercomprising means defining an enclosed chamber, pump means for creating a vacuum pressure within the hopper chamber, a feed tube extending downwardly into the hopper chamber to a lower end portion open to the interior of the hopper chamber, such that the vacuum pressure is communicated into the feed tube to move material therethrough into the hopper chamber, whereby when the said material in the hopper chamber rises to a level to cover the open end portion, the material blocks the vacuum pressure from the feed tube to terminate movement of the material therethrough to the hopper chamber, said feed tube including, at said lower end portion, an inner tube and an outer sleeve coaxial with the inner tube, each of said tube and sleeve including lateral openings'in the sides thereof and one of said tube or sleeve being rotatable about the axis of the feed tube relative to the other, the lateral openings in the inner tube and sleeve being positioned such that at a first relative position of the inner tube and sleeve the respective openings overlie each other to form a lateral through opening through the side of the feed tube to communicate the vacuum pressure within the hopper chamber into thefeed tube, and

upon relative rotation of the inner tube and the sleeve from the said first position, theupper edge of the lateral vmember, and the opening in the other member comprising a slot extending from an upper edge to the lower end of that member at a divergence angle less than that of the first said member, such that when the two members are turned relatively to overlap the slots, the upper extent of the through'opening moves upwardly away from the lower end while the opening remains open 7 10 below said upper extent.

16. A hopper according to claim 14, said inner tube and outer sleeve comprising feed tube members, the opening in a first of said members comprising a plurality of openings arranged in a helical path about that member, and the opening in the second member being a diverging slot, one edge of the slot diverging at an angle less than the helical angle so that upon relative rotation of the members, first the lower opening and then successive upper openings of the first member cross said one edge to form through openings, the slot diverging downwardly and outwardly a sufficient amount such that as successive openings cross said one edge and hence form through openings, those openings below it remain exposed through the slot to form lower through openings. r

Claims

2. A hopper according to claim 1, the lower end of the tube comprising an inner tube member and an outer sleeve member, each member having lateral openings in the sides thereof, and said varying means including means for rotating one of the members relative to the other to vary the relative position of the sleeve member and the inner tube member openings.

6. A hopper according to claim 5, the first said member being a inner tube and the second said member being the outer sleeve.

8. A hopper according to claim 2, the said openings in the two members being shaped such that upon relative rotation of the two members, the through opening travels up the feed tube, closing beneath it.

14. A hopper comprising means defining an enclosed chamber, pump means for creating a vacuum pressure within the hopper chamber, a feed tube extending downwardly into the hopper chamber to a lower end portion open to the interior of the hopper chamber, such that the vacuum pressure is communicated into the feed tube to move material therethrough into the hopper chamber, whereby when the said material in the hopper chamber rises to a level to cover the open end portion, the material blocks the vacuum pressure from the feed tube to terminatE movement of the material therethrough to the hopper chamber, said feed tube including, at said lower end portion, an inner tube and an outer sleeve coaxial with the inner tube, each of said tube and sleeve including lateral openings in the sides thereof and one of said tube or sleeve being rotatable about the axis of the feed tube relative to the other, the lateral openings in the inner tube and sleeve being positioned such that at a first relative position of the inner tube and sleeve the respective openings overlie each other to form a lateral through opening through the side of the feed tube to communicate the vacuum pressure within the hopper chamber into the feed tube, and upon relative rotation of the inner tube and the sleeve from the said first position, the upper edge of the lateral through opening moves upwardly along the feed tube to permit a higher level of material to be delivered to the hopper chamber before the material covers the opening from the hopper chamber to the feed tube.

15. A hopper according to claim 14, said inner tube and sleeve comprising feed tube members, one of said members comprising at least one slot diverging from an upper edge to a lower end at the lower end of that member, and the opening in the other member comprising a slot extending from an upper edge to the lower end of that member at a divergence angle less than that of the first said member, such that when the two members are turned relatively to overlap the slots, the upper extent of the through opening moves upwardly away from the lower end while the opening remains open below said upper extent.

16. A hopper according to claim 14, said inner tube and outer sleeve comprising feed tube members, the opening in a first of said members comprising a plurality of openings arranged in a helical path about that member, and the opening in the second member being a diverging slot, one edge of the slot diverging at an angle less than the helical angle so that upon relative rotation of the members, first the lower opening and then successive upper openings of the first member cross said one edge to form through openings, the slot diverging downwardly and outwardly a sufficient amount such that as successive openings cross said one edge and hence form through openings, those openings below it remain exposed through the slot to form lower through openings.