Sheet stacking apparatus
SHEET STACKING APPARATUS BACKGROUND OF THE INVENTION In the material handling field. sheet stackers which accumulate sheets one at a time from a conveyor and automatically lower the stack support to maintain the uppermost sheet at a constant vertical elevation are known. Sheet handling means which support sheets from above by fluid flow through noncontacting noz zles operating on Bernoulli&#39;s principle are also known. Examples may be found in the US. Pats. to Forrester No. 3.054.6l3. Walton et al No. 3.168.307. Stanley No. 3.I98.499. Hori et al No. 3.219.340. Robinson No. 3.220.723. Olsson et a] No. 3.438.668 and Albright et al No. 3.4] 1.770. This principle is useful wherever the sheet being handled would be adversely effected by contact with vacuum grippers or the like and one example of such material is freshly silvered glass.  
 BRIEF SUMMARY OF THE INVENTION In accordance with the present invention. a transfer means operating on Bernoullis principle transfers sheets one at a time from a conveyor to a stack. A plurality of nozzles are arranged to overlie the conveyor and the stack support and are arranged in a common plane which is inclined to the horizontal with the higher end over the conveyor and the lower end over the stack support. A fluid such as air under positive pressure is fed to all nozzles to elevate sheets from the conveyor into closely spaced but noncontacting relation to the nozzles. Due to the inclined plane of the nozzles. the sheets then slide by gravity in a plane parallel to the plane of the nozzles until they overlie the stack receiver. At this point in time. the flow of air to those nozzles which are supporting the sheet over the stack is momentarily cut off and the sheet permitted to fall by gravity onto the uppermost sheet of the stack and its fall is cushioned by the blanket of air trapped between the under surface of the sheet and the uppermost sheet of the stack.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic side elevational view of the overall apparatus of the present invention:  
  FIG. 2 is a top plan view of the apparatus shown in FIG. I;  
  FIG. 3 is a side elevation partially in section of one of the nozzles; and  
  FIG. 4 is a diagrammatic showing of one means for automatically maintaining a constant elevation of the uppermost sheet of the stack by lowering the stack support in step-wise fashion in response to the addition of each successive sheet to the stack.  
 DETAILED DESCRIPTION OF THE DRAWINGS Referring first to FIG. 1, a sheet stack supporting and level maintaining means is identified generally by the reference numeral 10. A belt conveyor 12. driven by means not shown, conveys the sheets 14 to be stacked one at a time in the direction indicated. A transfer means ofthe non-contacting type is indicated generally at 16 and is effective to transfer the sheets 14, without contacting them. from a conveyor 12 to the stack receiving means 10.  
  The sheet transfer means includes a plurality of nozzles 18 supported on the underside of a common housing 20. The housing 20 includes a pair of separate manifolds 20A and 208 so that the flow of air to the left hand group of nozzles 18 may be controlled separately from the others. Air under pressure is supplied to both manifolds from a blower 22 and a closed conduit 24. A valve 26 is positioned in the conduit 24 to permit interruption of fluid flow to the manifold 20A without disturbing flow to the manifold 208.  
  By reference to FIG. 2 of the drawings, it will be appreciated that the manifolds 20A and 20B are of substantial horizontal extent and in fact are nearly as large as the sheets which are being handled. FIG. 3 is a view partially in section of one of the nozzles 18. It will be seen that the under surface of the nozzle has an annular air passage 28 formed between a core piece 30 held in position on a spider 32 in spaced relation to the surrounding curved surface 34. Air admitted under pressure from the manifold is therefore directed outwardly along the plane of the lower surface of the nozzles as indicated by the arrows in this figure.  
  Referring again to FIG. I, for a description of the operation ofthe apparatus. the sheets 14 are individually advanced along the conveyor I2 from a remote point toward the stack receiving means It]. As each sheet on the conveyor passes beneath the nozzles IS. the decrease in pressure between the sheets and the nozzles due to the increase in fluid velocity thcrebetvveen re sults in the sheets being picked up and held in closely spaced non-contacting relation to the nozzles as shown by the sheet 14A in FIG. I. Due to the fact that the plane ofthe nozzles is lightly inclined to the horizontal (an inclination of as little as 2 is sufficient]. the thus elevated sheets have a strong tendancy to slide downhill toward the stack receiving means until they reach the position indicated by the sheet shown in NB in FIG. I. At this point. by means to be hereinafter described. the valve 26 is momentarily closed thus cutting off the supply of fluid to the manifold 20A and the sheet drops from the position of [48 onto the uppermost sheet of the stack. Its fall is vrtical and is cushioned by the blanket of air trapped between the undersurface of the sheet HB and the uppermost sheet on the stack.  
  One possible means for automatic operation of the valve 26 is illustrated in FIGS. 1 and 2. A four way valve 36 is connected to supply fluid pressure selectively to opposite ends of the cylinder 38 to operate the linkage 40 which interconnects the valve 26 with the piston in the cylinder 38. The valve 36 in turn is controlled by sensors 42. 44 and 46 shown most clearly in FIG. 2. These sensors are triggered by the arrival of a sheet at the position indicated at 148 in FIG. I and are effective to momentarily reverse the valve 36 to momentarily close the valve 26 and permit the sheet to drop onto the stack.  
  The vertical spacing between the sheets still on the conveyor belt and the under surface of the nozzles is not particularly critical which makes the system partie ularly attractive from a practical point of view. The partial vacuum created by the nozzles is sufficient to lift a sheet. for example. a glass sheet 6mm thick through a lcm spacing and attract it toward the under surface of the nozzles. In its supported position the air film between the upper surface of a sheet and the under surface of the nozzles is between about 0.5 and I5 mm in thickness. Nozzle size, of course. is dependent on the minimum size of the sheets being handled. In one embodiment. ttoY/les covering an area lfitlmm by l5llmm have been effective to handle sheets as small as 3th) by 300mm.  
  Referring lastly to Flti. 4. this merely diagrants otte means for maintaitting tlte vertical elevation of tlte uppermost sheet of the stack. It is of course. essential that this elevation be ratlter closely controlled so that tlte free fall distance of sheets from tlte no/lles to the top of the stack is substantially constant As shown in l&#34;l(i 4 tlte stack is supported by a piston 50 e\tending out of a fluid pressure cylinder 52 and the position of tlte piston within tlte cylinder is under the control of the valve 54. light source 50 is positiottcd to t&#39;ellect ligltt at a low attgle frotn the upper surface of the uppermost sheet on the stack to a pair of photoelectric cells 58 and fill. lf the stack is to high. the photocell 58 is activated to shift the \alve 54 to admit fluid to the upper portion of cylinder 52 to lower the stack. Any o\ersltoot&#34; of the support is then detected by the photocell 60 to re- \erse the process When the heigltt is correct. the rellccted light beam will be centered witlt respect to the two photocells aml the al\c 54 will remain iii a neutral position.  
  l rom the foregoing, it will be apparent to those skilled in this art that there is herein shown arid disclosed a new and useful sheet material handling stack ing apparatus attd while a preferred embodintent ltas been illustrated and described. applicant claittts the benefit of a full range of equivalents witltin the scope of the appended claims.  
 I claim I. sheet stacking apparatus contprising iii combinatiott.  
 a stack receiving means;  
 conveying means for conveying sheets one at a time front a remote point to a station adjacent said recei\ing means;  
 a manifold having a plurality of Ito/lies on the under surface thereof. positioned to overlie at least a sheet length of said com eying means as well as at least a portion of said stack recei\ing means. said ttol/les being arranged to direct fluid uttdcr pressure front said manifold along the upper sut&#39;tltce of sheets on said conveyor iit order to elevate sheets from said conveyor toward said nonles and maintain them in closely spaced but non contacting relation to said nonles. said nol/les lying in a contitton plane which is inclined to the hori/ontal with that end remote front said stack recci\ ittg means at till the highest elevation so that sheets suspended beneatlt said nol/lcs will slide downhill toward said recei\ ing means:  
 means for supplying fluid under pressure to said manifold; and  
 means for lttomentarily cutting off the flow of air to selected ones of said nozzles positioned over said receiving means to permit sheets to drop vertically on to said stack recei\ing means.  
  2. Apparatus as defined by claim 1 iii which said last mentioned means is a valve and in which said stack re cei\ing means includes means responsive to the prosintity of an approaching sheet for operating said valve.  
  3. Apparatus as defined by claim I including means responsive to the accumulation ofsltects on said receiv ing means to progressively lower the said receiving means whereby the upper surface of the stack remains at substantially the same elevation.  
  4. Apparatus as defined by claim 3 including hydrauhe means for supporting said stack and photoelectric means for sensing the height of the stack and con nected to control said hydraulic means for maintaining a constant vertical weight of the stack.  
  5. In a sheet stacking de\ ice of the type including a means for accumulating and maintaining a constant ele\ation of the uppermost sheet iii the stack. aitd a cottveyor for advancing sheets one at a time front a remote point to a point adjacent the said means, apparatus for transferring slteets front said conveyor to the stack comprising in combination:  
 a plurality of lluid directing nollles lying itt a contmon plane, o\erlying at least a sheet length of said conveyor and at least a portion of said stack said plane being inclined downwardly toward said stack:  
 manifold means communicating with said noules&#39;.  
 means for supplying fluid under pressure to said manifold. said nollles being configured to direct fluid downwardly and along the upper surface of sheets on said comeyor to elevate sheets into close but non-contacting relatiott to said noules aml permit the thus elevated sltcets to slide downltill toward said stack; and  
 means responsive to a sheet being positioned over the stack to momentarily stop fluid flow to those no. /les then holding the sheet to permit it to fall by gravity to the upper surface of the stack.