US 3598439 A
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United States Patent  inventors Ernst A. Dahlquist;
Stanley J. Polakowski; Teunis Vaalburg, all of Grand Rapids, Mich.  Appl. No. 781,823  Filed Dec. 6,1968  Patented Aug. 10, 1971 [73) Assignee Raplstan Incorporated Grand Rapids, Mich.
[541 LIFTING HEADS FOR PALLETIZERS 10 Claims, 14 Drawing Figs.
 11.8. CL 294/65  Int. Cl. l B66c H02 (501 Field of Search 294/64, 64 A. 64 B, 65, 65.5; 214/1 BS, 6 FS, 651; 137/5253, 525.5; 251/75; 248/363  References Cited UNITED STATES PATENTS 2,208,605 7/1940 Schmidt 294/65 UX 2,599,499 6/1952 Thorstenson.... 137/5253 X 2,694,337 11/1954 Anander 248/363 X 2,715,038 8/1955 Billner et al. 294/65 2,841,433 7/1958 Pagdin et al. 294/65 2,880,030 3/1959 Horn 294/65 2,960,364 11/1960 Herzog 294/65 2,970,608 2/1961 Doeg 137/5255 X 3,057,373 10/1962 Bragg 251/75 X 3,307,819 3/1967 Cocito 137/5253 X FOREIGN PATENTS 541,721 12/1941 Great Britain Primary Examiner-Evon C. Blunk Assistant Examiner- Douglas D. Watts Attorney- Price, Heneveld, Huizenga & Cooper ABSTRACT: The invention is directed to a device of the type frequently used in palletizers for lifting, by air pressure differential, articles arranged in any of a number of different patterns. The device consists of a head having a primary chamber from which the air is exhausted. The bottom of the primary chamber is formed by a grate creating a number of small individual air passages. A number of these individual openings are equipped with valves, each individually operable. These valves open and close in response to a predetermined amount of air pressure differential across the valve. The valves close to shut off the flow of air into the primary chamber through those individual air passages not closed off by the articles to be lifted.
IPATENTEDAUBIOISII 3.598.439 sum 1 or 4 I FIG] INVENTORS ERNST A. DAHLQUIST STANLEY J POLAKOWSKI TEUNIS VAALBUR BY 63 M W ATTOR NEYS P TEmEnmmmen I 3,598,439
SHEET h 0? 4 FIG. 10
9o 35 l 42 90a riu i 3 1111111110 n m A INVENTORS ERNST A. DAHLQUIST STANLEY J. POLAKOWSKI TEUNIS VAALBURG lWZ M M W ATTORNEYS LIFIING HEADS FOR PALLETIZERS BACKGROUND OF THE INVENTION In the palletizer art, articles such as case goods are automatically arranged on a conveyor surface in groups and then, by a head having vacuum-creating equipment. the articles as a group are lifted from the conveyor surface and loaded onto a pallet. Such a group of articles is called a tier and a number of tiers are stacked, one upon another, to form a pallet load. This system works satisfactorily when the articles are such that the articles in every tier can be so arranged that the tier perimeter has the same dimensions and geometry and no internal chimneys or voids exist within the tier. However, for stability of the pallet load, alternate tiers should have different patterns f so that the articles of one tier overlap the articles of the tier on which it rests. This ties the load together. Frequently, internal chimneys are formed which are in different positions in each tier. The problem of nonidentical tier geometry occurs under some operating circumstances when the same palletizer is used, first for articles of one size and shape and then for articles of another size and shape.
conventionally, the vacuum lift heads of palletizers are built to have an overall air intake opening sized to fit the tier of greatest perimeter. The opening is normally subdivided by a grid or other means into a plurality of subopenings or individual air passages. Certain groupings of articles leave substantial areas at the perimeter of the tier which are not covered by the articles. Also the chimneys remain open. Often the volume of air which can enter through these unused openings is so great that the head must have excessive airpumping capacity to create sufficient vacuum to lift the articles. Such an arrangement is initially expensive and bulky. It is also both expensive and noisy to operate. Further, even with excessive air-pumping capacity, the equipment can only handle relatively minor variations in tier size.
To overcome these problems, various devices have been employed including flexible curtains which close in around the sides of the articles. This system works reasonably well if the shape of the perimeter is reasonably regular and does not involve any significant indentations or recesses. It does not solve the problem of internal chimneys.
Another solution has been the use of individual valves, shaped somewhat like cuff links, located in the individual air passages. These remain closed until opened by contact with the articles. These have not been entirely satisfactory. To prevent them from opening in response to the vacuum alone, they have to be heavy and their weight pressing down on the articles is added to that of the articles. In efiect, this reduces the efficiency of the equipment by reducing its payload capacity, collectively, the weight of these valves represents a substantial reduction in equipment capacity. Also, should the edge of an article happen to contact the valve and open it, even though the article is closing off only a relatively small portion of the entire air passage, large volumes of air are allowed to enter. When a number of valves are so affected, the capacity of the equipment is seriously impaired.
SUMMARY OF THE INVENTION' This invention overcomes these problems by providing a head having its air intake subdivided into a plurality of small individual air passages by a grate. In the preferred embodiment, the individual air passages around the perimeter of this grate are equipped with individual leaf valves, each formed from a thin resilient material such as spring steel. However, the nature of this invention is such that all air passages in the grate may be equipped with these valves. Each valve has at least a portion adjacent the opening to the primary chamber to form a normally open air passage. However, when the air pressure differential across the valve exceeds a predetermined value, the valve is caused to close. In the preferred embodiment, this portion is an arch biased outwardly away from this opening. In addition to the perimetrical areas of the grate, these valves are provided in those areas where chimneys'may be formed. By this arrangement, all of the air passages, not closed by the presence of an article, are automatically closed off by the valves. This permits the vacuum lift head automatically to adjust the overall shape of its air intake to that of the tier of articles and to limit its air intake only to those air passages covered by the presence of an article. The construction of the valve is such that it adds substantially no weight'to the head, applies no load to the articles acting against the effect of the vacuum and is positive in its closing action when the required air pressure differential is attained.
DESCRIPTION OF THE DRAWINGS FIG. I is a bottom view of a vacuum-operated lifting head equipped with this invention;
FIG. 2 is a side elevation view of the lifting head;
FIG. 3 is an enlarged bottom view of one of the valves;
FIG. 4 is a sectional elevation view taken along the plane IV-IV of Fig. 3 showing the valve open;
FIG. 5 is a sectional elevation view taken along the plan V-V of Fig. 4;
FIG. 6 is a view similar to Fig. 4 showing the valve closed;
FIG. 7 is a view similar to Fig. 1 showing a modified construction;
FIG. 8 is a plan view of a typical tier of articles;
FIG. 9 is a somewhat schematic view illustrating the operation of the valves;
FIG. 10, 11 and 12 are schematic views illustrating the functional principles of the invention;
FIG. 13 is a fragmentary perspective view of the backing plate used with the valves; and
FIG. 14 is a fragmentary sectional view taken along the plane XIV-XIV ofFig. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring specifically to Fig. 2, a side elevation view of the vacuum head 10 is shown. This consists of a rectangular frame 1 1, an air pump or blower chamber l2 and a motor 13 for the blower. Clamped to the lower edge of the frame .11 is a subframe 14. The attachment is made by the fasteners 15. It will be understood that the top of the frame 1 1 is closed except for the opening into the blower chamber 12 and a relief port 17. This forms a primary chamber 16 (Fig. 2) above the subframe 14. The relief port 17 communicates with the primary chamber. Air exhausted by the fan or blower exits via the blast port 18 which is equipped with a closure gate. All of the preceding structure is conventional and further description of it is unnecessary.
It will be understood that in the palletizer assembly, the entire head is mounted by suitable means by which it can be raised and lowered and, in addition, has means to guide it for both vertical and horizontal movement. This again is conventional, forms no part of the invention and, therefore, is not il lustrated.
The lower face of the head is illustrated in Fig. 1 which shows the subframe l4 surrounding a basically rectangular area, the center of which is occupied by a main grate 20, having on each side an auxiliary grate 21. The grates form a backstop for articles held against the head by air pressure differential. The main and auxiliary grates are formed of bars of rectangular cross section which intersect to form a rigid structure characterized by a plurality of individual air passages or openings 22 of generally rectangular shape. That portion of the lower face of the head not occupied by the main and auxiliary grates is closed off as by the closure plates 23 and 23a and the closure strips 24 and 24a.
It will be recognized that other construction could be utilized for the grate. For example, it could be a single sheet of material with the depending structure supplied by the bars formed by sharply defined depending ridges surrounding the recesses. The sheet material would have an air passage in each of the recesses. Irrespective of whether this member is constructed as the grate illustrated or made in some other manner, it is a perforate member which forms a stop for the articles and provides numerous individual air passages opening into the primary chamber.
Referring first to the auxiliary grates 21 (Fig. 1), it will be noted that all of the openings 22 in the central portion of these grates are occupied by valves 30, one in each of the openings. At the ends of the grate and on each side of the valves, the openings are closed by seals 31. The seals 31 are formed of a suitable material such as a foam rubber which is compressible and projects beyond the face of the grate for contact with articles. These seals prevent flow of air parallel to the face of the grate between the grate and the articles of the tier. The seals 31 collectively form gaskets which isolate the valved openings or air passages from those which are not valved. It will be noted that a similar arrangement is provided along each of two sides of the main grate 20.
Arranged symmetrically about the center of the grate 20 are four groups 32, 32a and 32b and 320 of valves 30. The purpose of these will be explained more fully hereinafter. Except in the areas where either the gaskets 31 or the valves 30 are provided, the openings 22 in the grate are free of obstruction, permitting air to pass through the grate into the primary chamber 16 under the inducement of the fan. When the fan is operating and the blast port is open, there is a differential in air pressure between the air below the grate and that within the primary chamber.
The main grate 20 and the auxiliary grates 21 are both seated within the subframe 14 and below the main frame 11 and, thus, below the primary chamber 16.
In those areas where valves 30 are provided, a backing plate 34 is mounted on the upper and inside face of the grate and separating the openings within the grate from the primary chamber. A fragmentary portion of a typical backing plate is illustrated in Fig. 13. Typically, these plates have a plurality of elongated slots 35 which form air passages communicating between the openings in the grate and the primary chamber. As is best seen in Fig. 3, the openings or air passages 35 are substantially smaller than the openings in the grate. This arrangement provides a valve seat against which the valve member 40 can effect a seal. The backing plates are secured to the grate so that even though a substantial air pressure differential exists between the outside of the head and the primary chamber, the plates cannot lift from the grate. It is also true that the grate is secured to the head so that it may not be raised by the force of the articles held against it as a result of the air pressure differential generated by the fan.
It will be recognized that the backing plate and the grate could be a single integral structure. Such would be possible if the entire perforate member were cast.
As is best seen in Figs. 3, 4, and 6, the valve 30 consists of thin closure member 40 which is archedat the center to form a closure portion and on each end has an inclined or ramplike terminal portion 41. Approximately, at the juncture between the central arched portion and the terminal portions 41, the closure member 40 has openings 42 to receive the ends of the clip hanger 43. The clip hanger 43 is generally U-shaped in cross section and has its ends inserted through the spaced openings 44 in the plate 34 and, by friction, the hanger is thus secured to the backing plate. This is all that is necessary inasmuch as the closure member 40 is a thin, light piece of material. Any other suitable means could be used to secure the valve member 40 such as drive screws, so long as the ends of the valve member are free to slide in response to the flexing of the valve.
The central portion of the hanger clip 43 is preferably arched toward the closure member to limit the closure members downward movement, although such an arrangement is not essential to the function of this invention.
The closure member 40 may be made of any lightweight, resilient material which also has sufficient strength to withstand the pressures exerted against it when it is in sealing or closure position. The material should also be resistant to fatigue failure resulting from frequent flexing as the valves open and close. A suitable material for this member is a thin spring steel. The closure member is free to collapse and flatten against the backing plate 34 since the openings 42 are oversize with respect to the legs of the clip hanger 43. The width of the closure member 40 must be such as to completely cover the air passage 35 and preferably seat over the adjacent portions of the backing plate on each side of the air passage to form an effective seal and also to support the closure member when it is in air-sealing position.
Since each valve 30 is a separate, self-contained assembly occupying only one of the openings 12, they can be arranged in various patterns within grates, and the number employed can be limited to those, necessary to satisfy the operating circumstances of the lifting head.
In the particular arrangement illustrated in Fig. 1, a pattern of the valves 30 is arranged about the perimeter of the opening in the head. The fact that along two sides, the rows of valves are arranged in side-by-side relationship and alongthe other two sides each row is spaced from the adjacent row by a row of seals 31 is illustrated only. Any other pattern arrangement suitable to the operating circumstances of the head could be utilized.
The valve groups 32, 32a 32b and 320 are located to coincide with the chimneys which will be formed in the particular tier patterns to be employed. The valves 30 in these groups are identical to those used at the perimeter of the head. To support these valves, backing plates 34a are provided. These are the same as the backing plates 34 except for their shape. The purpose of these valves becomes clear when the tier (Fig. 8) is studied in which the articles 81 are so arranged as to create two diagonally positioned voids or chimneys 82 in the tier. The two chimneys will align with two of the valve groups 32, 32a 32b and 320. Because overlapping of articles from tier to tier strengthens the load on the pallet, the next tier will normally be arranged with the chimneys 82 at the opposite corners. Thus, in handling one tier, the valve groups 32 and 32c may coincide with the chimneys in the tier and in the next tier it will be the valve group 32 a and 32b.
Fig. 7 illustrates a modification of the invention which would enable a lift head to accommodate much greater variation in tier size and shape. In this arrangement, a central grate 50 is utilized, surrounded by a plurality of auxiliary grates 51a and 51b. All of the openings 52 in the auxiliary grates, except those having seals, are equipped with valves 30 (not illustrated). These valves are arranged in groups in the auxiliary grates separated from each other by seals 31a. Thus, tiers of articles varying in size from that of the main grate 50 to that which would cover the entire head surface can be accommodated. Further, the shape of the tier could be rectangular or square. The operation of this modified head is the same as that of the head illustrated in Fig. 1 except for its greater flexibility of tier size and shape. Because the seals 31a occupy a substantially greater portion of the lower face of the head, there may be some reduction in the overall lifting capacity of the modified head.
lt will be recognized that this invention could be applied in a slightly different manner. The entire grate could be made up of sections similar to auxiliary grate 21 which are removably mounted in the head. For each section, two grates would be maintained, one with valves and one without. To adapt the head to tier sizes having a greater variation in overall size than, for example, the head illustrated in Fig. 1, additional grate sections equipped with valves could be substituted for grates without valves. To make this substitution, all that would be necessary would be to detach the subframe 14, exchange grates and reattach the subframe to the head. This type of arrangement would be quite satisfactory to adapt the lift head in situations where the use of the head for the different tier sizes was to continue for several hours.
OPERATION OF THE INVENTION The head is lowered until the grate and the seals 31 contact the articles of the tier. As is more clearly shown in Fig. 14, the seals 31 project slightly beyond the bottom face of the grate to have an effective sealing action against the articles. This gasket-forming action may be further increased by having the rows of seals projecting progressively greater distances from the grate, the more remote the seals are from the center of the grate. This is particularly important in effecting a seal adjacent the edge of the tier where the top of an article may slope downwardly away from the grate such as is illustrated in Fig. 9. In this type of situation, the greater projection of the peripheral seals will assure an effective airflow barrier. This will enable even those air passages at the periphery of the tier to exert full lifting capacity on the articles. When a number of air passages are so affected, the result is a marked increase in a the efficiency of the equipment. The stepped arrangement of the seals is illustrated in Fig. 14 where seal 31b projects further than the seal 31.
With the fan operating and the blast gate open, contact between the grate and the articles immediately closes off the airflow through those of the openings covered by the articles. Until this happens, the air pressure differential across the grate, even with the fan operating and the blast gate open, is not enough to cause the valves 30 to close because the total number of air passages or openings in the grate by which air can enter the primary chamber provides an airflow capacity of sufficient volume that the air pressure differential required to close the valves cannot be obtained. However, the closing of a number of the openings by the articles upsets this balance and the air pressure differential across the grate increases sharply.
As a result, those valves located in grate openings which remain unobstructed are caused to collapse and thus shut off the airflow. Such a valve d is schematically illustrated in Fig. 9. However, a valve seated in an opening which is entirely closed off by an article remains open because the air pressure differential remains too small to close it. This condition is illustrated by valve 30e in Fig. 9.
In situations in which the article only partially obstructs the opening, the reaction of the valve will depend upon the degree to which the obstruction is effective in reducing the airflow. As is illustrated by valve 30f in Fig. 9, the obstruction by the article 55 must be substantially complete before the valve will close. From tests run on this invention, it has been ascertained that a pressure difierential of l 1 inches of water is a satisfactory differential at which the valve can be designed to close. Of course, the particular air pressure differential at which the valve will close can be varied in several ways such as by selection of the stiffness of the material used in making the valve. This can be controlled, for example, by the particular alloy used or by selection of a thinner or thicker material. The maximum vacuum which can be generated in the primary chamber will of course be determined by the capacity of the fan. A fan which can generate a differential of 25 to 26 inches of water has been found satisfactory. It has also been ascertained that a valve designed to close when the differential reaches 1 1 inches of water will close, if the effective air intake area to the opening is reduced to 0.2 of a square inch of efiective air intake area when the vacuum in the primary chamber has a value of approximately 20 inches of water. it will be seen that valve 30f as illustrated in FIG. 9, is nearing the critical threshold of pressure differential which will result in closure.
It is desirable that the valve remain closed unless a substantial portion of the opening is obstructed by an article. However, once the obstruction is such that the threshold pressure differential is not attained, the value must remain open. Otherwise, insufiicient openings will remain operative to retain the peripheral articles against the grate. It is at this point that the gaskets formed by the seals 31 become important. They form an effective seal against the articles. Those of the openings closed by articles and within areas surrounded by the gaskets will generate maximum air pressure differential and thus maximum lifting capacity.
The principle of operation of the individual valves is illustrated in Figs. l012. The arched configuration of the valve creates a pair of air passages and 90a (Fig. 12), one on each side. The flow of air through these passages is indicated by the arrows A. As the pressure differential between the primary chamber and the air passage increases, the velocity of the air increases as it bypasses the valve through the passages 90 and 90a. As the pressure differential increases, the unbalance between the pressure acting against the upper face 91 of the valve and that acting against the lower face of the valve (arrows B) increases. When this pressure differential reaches approximately 1 1 inches of water, the resistance of the valve member 40 to collapse is overcome and the valve closes. When the vacuum falls so that there is less than about 11 inches of water differential, the valve members 40, because of their resilient bias, will automatically snap open.
To release the load, the gate on the blast port 18 is closed This immediately reduces the fan's air-pumping capacity and the pressure differential between the primary chamber and the air passages decreases. This effect can be accelerated by open ing the relief port 17 to bleed air into the primary chamber. When the air pressure differential drops below than necessary to support the tier of articles, they will be released.
It will be understood that the value of ll inches of water, while established by tests satisfactory, is not the only value which could be used. A greater or lesser differential could be selected for effecting closure of the valve-depending upon various design factors and the operating circumstances of the lift head. It is important that the valve not be so sensitive that it fails to stay open when the air passage is blocked by the presence of an article since this could seriously impair the lifting capacity of the head. It also must close when any significant portion of air passage remains unobstructed.
It will also be understood that while the preferred embodiment of this invention utilizes the valves 30 only at the peripheral areas of the head, it is entirely feasible to apply the valves to the entire head. This will slightly decrease the lifting capacity of the head but this loss of capacity is more than compensated for by the increase in adaptability of the equipment, particularly in situations in which the equipments use is expected to involve a wide variety of articles.
The invention provides an inexpensive, low-cost means which permits the lifting head to automatically adjust itself to accommodate a wide variety of load shapes and sizes. It also permits the use of a fan having only that capacity necessary to lift a tier of articles when substantially the entire bottom of the head is blocked off by articles because this is the condition which exists when the valves are closed even though only portion of the lower face of the head is covered by the articles. Reducing the capacity of the fan reduces the initial cost, operating cost and the noise incident to its use. Further, irrespective of fan capacity, it permits much greater variation in load size and shape than is possible without the valves.
While a preferred embodiment of the invention, together with certain modifications have been shown and described, it will be recognized that other modifications can be made within the principles of the invention.
The embodiments of the invention in which an exclusive property is claimed are defined as follows: i
The embodiments ofthe invention in which we claim an exclusive property or privilege are defined as follows:
1. A device for lifting by air pressure differential, a group of articles arranged in any one of several different predetermined patters, said device having a housing defining a chamber, a perforate member forming a flat bottom face of said chamber and means to evacuate from said chamber air entering through said perforate member, the improvement therein comprising: said perforate member being a grate having intersecting walls defining a plurality of closely spaced recesses each forming an air passage through said grate; a plurality of normally open, air pressure differential responsive valves, one in each of a plurality of predetermined ones of said recesses, said predetermined recesses forming a band of recesses adjacent the perimeter of said housing; the remainder of said recesses including a plurality of valveless recesses located inwardly and adjacent the said band of recesses; a first flexible sealing gasket surrounding said grate and band of recesses at the outer perimeter thereof; a second flexible gasket at the inner perimeter of said band of recesses between said band of valved recesses and those of the valveless recesses enclosed within said band; said first and second gaskets depending below said bottom face of said grate.
2. The device described in claim 1, wherein certain of the said remainder of the recesses enclosed within said band are also equipped with said valves; said certain recesses being arranged in groups spaced from each other and from said band in a pattern to coincide with one or more chimneys formed by the group of articles.
3. A device for lifting by air pressure differential a group of articles arranged in any of several predetermined patterns, said device having a housing-defining chamber, a perforate member forming one face and means to evacuate air from said chamber entering through said perforate member; said perforate member having depending elements forming a plurality of recesses each having an air passage therefrom communicating with said chamber the improvement in said device comprising: predetermined ones of said recesses being arranged in groups; a plurality of recesses in each of said groups; each group being adjacent a periphery of said perforate member; means forming a valve seat in each of said predetermined recesses; a valve member in each of said predetermined recesses; said valve member being of a thin resilient material and having a portion normally spaced from said valve seat means and supported in said spaced position by said resilience to permit the flow of air from said recess to said chamber; the resilient support of said portion being overcome and said portion shifting into sealing engagement with said valve seat means when the differential in air pressure between said chamber and the recess in which said valve is located exceeds a predetermined value; and the remainder of said recesses including a plurality of valveless recesses located inwardly and adjacent substantially the said groups of recesses.
4. The device described in claim 3, wherein each of said groups of recesses is surrounded by a resilient gasket, said gasket projecting downwardly beyond the lower face of said perforate member.
5. The device described in claim 4, wherein said gaskets comprise a plurality of individual sections, each section being mounted in one of the air passages in said perforate member.
6. The device described in claim 4, wherein the gaskets adjacent the periphery of the device project a greater distance beyond the lower face of said perforate member than those remote from said periphery.
7. A device for lifting by air pressure differential a group of articles arranged in any of several predetermined patterns, said device having a housing defining a chamber, a perforate member forming one face and means to evacuate air therefrom entering through said perforate member; said perforate member having a plurality of separate recesses each having an air passage communicating with said chamber, the improvement in said device comprising: said perforate member being divided into a plurality of separate and independent first sections and second sections each removably mounted to said housing and each having a plurality of said recesses; said first sections including a plurality of valveless recesses; said second sections each having a plurality of valves, one mounted in each of the recesses; sections of the type of said first sections being interchangeable with said second sections whereby the number and arrangement of recesses having valves may be changed by substituting one section for another.
8. The device described in claim 7, wherein each of said second sections has a peripheral gasket.
9. A device for lifting by air pressure differential a group of articles arranged in a predetermined pattern said pattern having one or more chimneys entirely surrounded by the articles, said device having a housing defining a chamber, a perforate member forming one face and means to evacuate air from said chamber entering through said perforate member; said perforate member having depending elements forming a plurality of recesses each having an air passage therefrom communicating with said chamber the improvement in said device comprising: individual valve means in predetermined ones of said recesses for controlling the flow of air therethrough, said predetermined recesses being arranged in a group or groups located on said perforate member to correspond to the location of the chimneys; means forming a valve seat in each of said predetermined recesses; a valve member in each of said predetermined recesses; said valve member being of a thin resilient material and having a portion normally spaced from said valve seat means and supported in said spaced position by said resilience to permit the flow of air from said recess to said chamber; the resilient support of said portion being overcome and said portion shifting into sealing engagement with said valve seat means when the differential in air pressure between said chamber and the recess in which said valve is located exceeds a predetermined value; and the remainder of said recesses including a plurality of valveless recesses surrounding the said group or groups of predetermined recesses.
10. The device described in claim 9, wherein said articles are arranged in several patterns, each pattern having a different location for the chimney and said certain recesses are arranged in groups, each group located on said perforate member to correspond to one of the chimney locations.