US 3727911 A
This is a method of providing an overlap between individual sheets in preparation for subsequent stacking by accelerating each sheet in comparison with the web feeding speed and then decelerating it down to stacking speed. It is characterized by applying to each sheet, during the time interval required to provide the desired overlap, either only accelerating or only decelerating forces.
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Description (OCR text may contain errors)
United States Patent 1 91 Vits [ Apr. 17, 1973 54 METHODS AND APP TUS FOR 2,769,495 11/1956 Pomper et a1. ..271 74 X Y BETWEEN 3,123,354 3/1964 Ungerer ..271/74 PROVIDING 3,218,897 1 1/1965 Geigenmiller et a1. ..271/68 X INDIVIDUAL 3,258,265 6/ 1966 Beauiieu et a1 ..271/68 PREPARATION F SUBSEQUENT 3,411,772 11/1968 Rovin ..271 74 STACKHNG 3,41 1,829 1 1/1968 Albright ..271/74 X [75} Inventor: Hilmar Vits, Huschelrath, Germany Primary Examiner Richard E. Aegerterv  Assignee: VITS Maschinenbau GmbH, Lan- Assistant ExaminerB'rue q genfeld (Rhin land), G m AttorneyMichae1 S. Striker 1 PP NOJ 1361348 This is a method of providing an overlap between in- I dividual sheets in preparation for subsequent stacking  Foreign Application Priority Data by accelerating each sheet in comparison with the web feeding speed and then decelerating it down to Apr. 30, 1970 Germany ..P 20 21 375.6 stacking peed It is characterized to each sheet, during the time interval required to provide the  1.1.8. C1 ..271/711, 271/74 desired overlap, either only accelerating or only [51 Int; CL ..B65h 29/24 decelerating forces. 1  Field of Search ..271/74, 68, 71, 80, 1 271/86, 77, 78
[ 56] References Cited 14 Claims, 3 Drawing Figures g UNITED STATES PATENTS 2,381,719 I 8/1945 Brintnall ..271/68 PATENTED APR 1 7 I975 SHEET 1 OF 2 NS Q METHODS AND APPARATUS FOR PROVIDING AN OVERLAP BETWEEN INDIVIDUAL SHEETS IN PREPARATION FOR SUBSQUENT STACKING Such sheets consist of paper, cardboard, foils of plastic, metal or the like, fibers, fleece, textile fabrics, and similar flexible materials, generally occurring in the form of webs which first have to be cut into the corresponding sheets. This is done by cross cutters, e.g. in the form of rotating cutting cylinders.
. To enable the sheets so obtained to be stacked, an overlap must be provided between the individual sheets in order to keep machine lengths within economically and technically reasonable limits. To accomplish the principal object, which consists in preventing the front edge of a succeeding sheet from butting against the rear edge of the precedingsheet, the most varied arrangements have been employed in the past. This object is accomplished by substantially accelerating each sheet as compared with the speed at which the undivided web is fed to the cross cutter. Once the necessary spacing of the sheets in relation to each other has thus been achieved, all that is required to achieve said overlap, and consequently, a scaly arrangement of the sheets in relation to each other is a deceleration of the speed preceding, and an acceleration of the speed of the sheet succeeding, each sheet. This deceleration of the sheet speed following upon its acceleration is indispensable because, with the stack immediately following upon the zone of acceleration, the sheet speed in an arrangement employing high-speed cross cutters would be too high to enable the delivery impact of the sheet to be taken up by the start, i.e. the front portion, of the sheet without damage to the latter.
This procedure, although widely employed, is dependable only if the individual sheets are sufficiently stable in proportion to their surface area and if their surfaces are conducive to good sliding. However, if the individual sheets are too stiff or too strong in proportion to their area, the difficulties arise which the present invention proposes to eliminate. The same difficultiesoccur if the sliding ability of the sheets is substantially reduced, e.g. because they carry electrostatic charges, so that transition from the scaly to the stacked arrangement is unsatisfactory. These difficulties are aggravated by the fact that the sizes of the individual sheets have increased, that materials are now employed with greater thicknesses and that frequently the sheets are provided with coatings affecting the mobility of the sheets to a degree where the conventional stacking methods and apparatus will fail.
Various attempts have been made to eliminate these difficulties, including, for example, collecting a major number of sheets on cylinders while forming them into packs. This method requires that the circumference of the respective cylinder be greater than the maximum length of sheet to be collected. The apparatus is controlled in such a manner that the sheets collected on the cylinder will form the said pack which is then ejected as a whole. However, even such arrangements cannot dispense with conveyors such as belts which are bound to generate electrostatic charges as a result of the unavoidable relative speeds. Consequently, recourse was taken to conveyors gripping a sheet at its top surface only. To that end, perforated conveyor cloths are caused to move over a suction box. In certain arrangements a plurality of conveyorvbelts has been used in conjunction with a suction box which is perforated between said conveyor belts. By arranging these suction boxes in series, cut sheets can be stacked at predetermined locations. However, with very large sheets and very high stacking speeds it has been found that the drawbacks resulting from the necessity of advancing a sheet at high speed along only part of its length cannot be eliminated.
A substantial improvement was achieved by the introduction of ducts provided with nozzles operable to produce a floating condition in accordance with US. Pat. No. 3,633,281. (Ser. No. 8,656 of Feb. 4, 1970) which have the advantage of combining modest space requirementwith the ability of simultaneously floating and conveying a sheet. Owing to these features they can also be used to advantage in providing an overlap between sheets to be stacked under particularly difficult conditions.
However, one feature which all these suggestions have in common is that the conveying elements must be operating continuously. This means that one and the same sheet is influenced both by the accelerating and by the decelerating elements during its transition from the higher speed imparted to it by acceleration to the lower speed achieved by deceleration. This is a basic drawback inasmuch as the apparatus employed to provide an overlap between successive sheets cannot handle more than one sheet size. If the sheet size changes or if, even worse, differently sized sheets are fed to the apparatus during one and the same operation, proper stacking of such sheets can no longer be ensured. Only by abandoning the idea that in view of the prior art methods of providing said overlap it is essential that during said transition the sheet be kept under the influence of both the high-speed and low-speed conveying elements to avoid a stepped and, consequently, jerky sequence of operations is it possible to realize that gradual and steady overlapping can also be achieved by cyclically exposing the sheet, during said transition, to the alternating exclusive influences of the high-speed and low-speed elements respectively, which, in fact, is the idea underlying the present invention. As already mentioned, the advance in the art which can be achieved thereby includes the advantage thatsince the said functions are separated the apparatus is no longer tied to one specific sheet size, which was an unavoidable drawback of the methods and apparatus of the prior part.
These and other requirements will be met by the present invention.
Setting out from known methods of providing an overlap between individual sheets in preparation for subsequent stacking by accelerating the sheet speed in comparison with the web feeding speed, followed by deceleration down to stacking speed, the solution found in accordance with the present invention is characterized in that during the cycle by which said overlap is provided the sheet is alternately exposed to only accelerating or only decelerating forces.
This enables a second advantage of great practical importance to be accomplished inasmuch as it has now become possible to expose only one of the two surfaces of the sheet to the accelerating forces. If this be, for example, that surface of the sheet which with the usual horizontal design of the machines employed to provide the overlap is facing downward, an important practical advantage is obtained in that prior to, and during, overlapping the sheets are accessible for unrestricted visual inspection so that joints, for example, can be readily detected and sheets which might cause trouble during subsequent processing can be removed during the operation. With continuously operating cross cutters such joints cannot be avoided, but they are definitely undesirable because they lead to discontinuities which invariably cause operational difficulties unless they are eliminated.
The method can be substantially improved if during overlapping the start of a sheet is floated while its end is still being decelerated. Another advantageous possibility of performing this step consists in that the forces causing the sheet to float can be momentarily rendered ineffective and that from that moment the same sheet can be subjected to forces which eject, i.e. push, the sheet toward the stack. In discussing the apparatus for performing the method it will be pointed out that the control operations involved in performing this step can be interlocked on the one hand and synchronized on the other hand so that on the one hand the various control phases will follow each other in the proper sequence without unnecessary intermissions and on the other hand will not interfere with each other in particular where such control operations are dependent on automatic tripping mechanisms such as light barriers whose interruption will cause an automatic sequence of operations to be initiated by the sheet tripping the control impulse.
In accordance with the present invention, an apparatus for performing the method is, therefore, characterized in that in the zone of transition from high-speed to low-speed conveyance only such conveying elements are arranged which operate alternately. If, in accordance with a further embodiment of the present invention, said conveying elements are controlled by a system by which the conveying elements decelerating the sheet are switched on at the very moment in which the elements accelerating the sheet are switched off, the control phases will follow eachother in direct succession without any jerks occurring during overlapping of the sheets so that the same effect is achieved which the inventors of prior art methods and apparatus believed to be obtainable only by keeping the sheet, during overlapping, under the influence of the high-speed conveying means even while the slowspeed conveying means were already acting on it.
One embodiment of the new control system provides for synchronism between preferably pneumatic elements influenced by said system which cause the sheet to assume a floating position and the conveying elements for accelerating the sheet feeding speed. If the sheets consist of metal foil, the floating position may also be achieved with the aid of magnetic means provided that such foil is made of ferromagnetic material.
Alternatively, the control system may be designed so that pneumatic elements influenced by said system which cause a sheet, once it has reached a position above the stack, to be blown onto the stack operate in synchronism with the conveying elements for decelerating the sheet from its previous speed.
Using suction rolls as pneumatic conveyors, a difference in height between the sheet positions which will favor the formation of the overlap can be readily achieved by suitably disposing the suction rolls provided in the zone in which the sheet feeding speed is accelerated with respect to the location of at least one suction roll disposed in the zone of deceleration. The previously mentioned ducts provided with nozzles operable to produce a floating condition in accordance with US. Pat. No. 3,633,281 (Ser. No. 8,656 of Feb. 4, 1970) can be used to advantage as means for causing the sheets to assume a floating position. In particular, the very compact construction of these ducts provided with nozzles operable to produce a floating condition permits them to be combined with other devices such as ducts provided with ejector nozzles, e.g. by disposing above the stack of sheets, in alternating succession, one duct provided with nozzles operable to produce a floating condition followed by one duct provided with ejector nozzles or one ductprovided with nozzles operable to produce a floating condition followed by a group of ducts provided with ejector nozzles or a group of ducts provided with nozzles operable to produce a floating condition followed by one duct provided with ejector nozzles or a group of ducts provided with ejector nozzles, unless other arrangements are more advantageous, depending on the specific conditions of each application. These ducts provided with nozzles operable to produce a floating condition can also be used between the individual suction rolls or ahead or aft of said rolls to provide air lubrication for the floating sheets, resulting in virtually inertialess operation since the mass of each sheet is negligible in comparison with the mass of moving machine parts.
As already mentioned, light barriers may be provided which may, for example, scan the position of the rear end of a sheet or the front end of a succeeding sheet, thus providing a means of automatic switch-over from elements conveying the sheets at high speed in order to first accelerate them to said higher speed to elements which convey the sheets at a lower speed and therefore first have to decelerate them to said lower speed.
Similarly, the start of the next sheet can be scanned, using its momentary position to switch off the initially decelerating conveying means and to switch on again the accelerating conveying means. The ducts provided with nozzles operable to produce a floating condition and/or the ducts provided with ejector nozzles can be switched on or off in an identical manner.
A particularly simple arrangement of the control system is one in which the essential components of said system are disposed so that they act directly on the delivery and/or suction branches of the blower. However, the invention is not limited to this particular arrangement and does not exclude other possibilities such as controlled dampers in the blower ducts opening and closing said ducts or controlling by-passes and/or short-circuits by means of which the suction and delivery branches of the blowers may, for instance, be interconnected so that a pressurized fluid delivered by the blower, which may consist of gases, vapors, gasvapor mixtures and, preferably, air, is momentarily prevented from acting on the sheets, unless it is precisely this action which is caused by actuating said blowers. If the delivery and suction branches of the blowers are controlled by solenoid-operated valves whose valve discs momentarily close or open the clearance sections of said delivery and suction branches, suction rolls connected to said suction branches will be rendered ineffective instantly or within a few milliseconds since no fluid can be conveyed even though the rotor of the blower takes a certain time to slow down to a stop. A similar effect is achieved by closing, for instance, the suction branches of the blowers by which the ducts provided with ejector nozzles are supplied with the working fluid.
A suitable apparatus for performing the method of the present invention will now be described in more detail by way of example with reference to the acompanyin g drawing, in which:
F IG. 1 is a greatly simplified longitudinal elevation of the complete apparatus in a schematic representation, while 1 FIG. 2 represents a cross-section through those parts of the apparatus which are located above the formed stack of sheets, and
FIG. 3 shows a simplified circuit diagram for the solenoid-operated valves employed and their electric interlock.
Through a pair of feed rolls 3, the material web 1 is fed in the direction indicated by the arrow 2 to a rotating cross cutter in accordance with US. Pat. application Ser. No. 136,347, filed Apr. 22, 1971, corresponding German Patent Application P 20 20 431.3 of Apr. 27, 1970, with the cutting cylinders 4. The cross cutter is followed by a zone of acceleration equipped with three suction conveyor rolls 5, 6 and 7. Between these rolls and below the plane of conveyance there are grate-type units 8 and 9 made up of ducts provided with nozzles operable to produce a floating condition in accordance with US. Pat. No. 3,633,281 (Ser. No. 8,656 of Feb. 4, 1970). Thus, the material web 1, after passing through the pair of rolls 3 and the cross cutter 4, enters the zone of acceleration between the rolls 5 and 7, in which the sheets travel at an increased speed as compared with the speed of the web, in the form of sheets cut to size.
Almost flush with a plane connecting the tops of the suction rolls 5 through 7, ducts 10 provided with nozzles operable to produce a floating condition are disposed which, in the direction of sheet travel, extend almost up to the end face 1 1 of the stack 19 formed by the starts of the deposited sheets.
Aft of the suction roll 7 of the zone of acceleration, which is the roll located in the most forward position in the direction of material travel, the suction roll 12, located somewhat lower, is running at stacking speed, i.e. at a speed which, unlike that of comparable rolls in prior art systems, is entirely independent of the speeds of those parts of the apparatus which, again viewed in the direction of sheet travel, are located ahead of the suction roll 12. While the suction rolls 5, 6 and 7, even at maximum machine speed, have a higher speed than the pair of feed rolls 3, the speed of the suction roll 12 is only a fraction of that speed. For the purposes of the present invention it is irrelevant whether the speed of conveyance in the zone of acceleration comprising the suction rolls 5, 6 and 7 is related to the speed of the feed rolls 3 by a fixed or variable ratio or whether it can be adjusted entirely independently of the feeding speed as long as the sheet speeds in the zone of accelerating exceed the maximum possible feeding speed, in order to make sure that the individual cut sheets can still be spaced apart.
It is also irrelevant whether the speed of conveyance of the suction roll 12 can be controlled or adjusted as a function of, or without any relation to, the feeding speed of the rolls 3 as long as that speed is substantially lower than the speed of the suction rolls 5, 6 and 7 to make sure that an overlap can be produced above the suction roll 12. The suction effect of the suction rolls 5, 6, 7 and 12 is limited to that part of their circumference which is located above V-shaped sectors disposed inside the suction rolls as shown in the accompanying drawing. Each suction roll has its own suction blower. The suction end of each blower is connected with the associated suction roll or, more precisely, with the sector-shaped suction chambers inside the suction roll. The blower 13 of the suction roll 7 and the blower 14 of the suction roll 12 are each provided with valve discs 15 in front of their delivery branches, which valve discs can be moved by one pneumatic control element 16 each, e.g. a simple diaphragm, so that they can completely shut the outlets of the blowers 13 and 14. Since the weight of each valve disc 15 is negligible and since the valve need be lifted only slightly, the valve disc can be moved with extreme rapidity by the pneumatic control element 16. The control elements 16 at the two blowers 13 and 14 are interlocked so that only one of the suction rolls 7 and 12 is active at any one time. According to an alternative arrangement, the pneumatic control elements and the valves which they control may be replaced by solenoid-operated valves controlled in a manner known per se.
When the end of a sheet passes through a light beam emitted by a light source 18, a photoelectric cell 17 receives an impulse and closes the delivery branch of the blower 13 while simultaneously opening that of the blower 14. Consequently, the end of the sheet which has been advanced to a position above the stack at a speed in excess of the machine speed is attracted by the negative pressure of the suction roll 12, braked and thus reduced to stacking speed. Since the top of the suction roll 12 is located somewhat below the top of the suction roll 7, the sheet so advanced will assume a position below the start or leading portion of the succeeding sheet so that the latter will be moved over the former. Before arriving above the stack, the start of the succeeding sheet passes through the light barrier 18, causing the valve discs 15 at the blowers 13 and 14 to reverse their positions so that the start of the succeeding sheet is advanced at high speed under the ducts 10 provided with nozzles operable to produce a floating condition without interfering with the process by which the preceding sheet is deposited on the stack at reduced speed, any possibility of such interference being, in fact, positively precluded.
The light barrier is made up of the photoelectric cell 17 and the light source 13. Both are disposed in spaced relation to the suction rolls 7' and 12 in the direction opposite to the direction of sheet travel, the distance from said rolls being selected so that at the moment in which the conveyance of the sheet is transferred from the faster roll 7 to the slower suction roll 12 the end of the sheet, owing to its inertia, will have passed the suction roll 7 but not yet the suction roll 12. This switchover causes a delay of approx. 20 milliseconds referred to the time at which the measuring impulse is received in the photoelectric cell 17. The distance of the light barrier 17, 18 from the conveyor roll 7 is equal to the distance traveled by the sheet at the increased speed in the zone of acceleration and also equal to the distance traveled by a point on the suction rolls 5, 6 and 7 per unit time, i.e. equal to the high circumferential speed of the suction rolls. As the speed of conveyance of the suction rolls 5, 6 and 7 can be maintained constant irrespective of variations in the feeding speed of the feed rolls 3, there is no need of varying the distance of the light barrier from the suction roll 7 in accordance with varying feeding speeds. Thus, the invention also enables sheets of any size or different sizes to be overlapped in the same manner as those produced by the constant-speed cross cutter 4.
The ducts provided with nozzles operable to produce a floating condition, which are disposed above the stack, can be used to assist in depositing sheets having a particularly delicate surface, such as coated aluminum foils for offset printing. The floating condition of the sheets permits such assistance.
A plurality of the ducts provided with nozzles operable to produce a floating condition extend in the longitudinal direction across substantially the whole stack 19 starting preferably at the suction roll 7, but not farther aft than the suction roll 12. An advantageous arrangement is one in which the ducts provided with nozzles operable to produce a floating condition are interconnected by a transverse duct 20 connected to the blower 21. At its suction end, the blower 21 has a valve disc and a, preferably pneumatic, control element 16 so that the combined floating and conveying actions of the ducts provided with nozzles operable to produce a floating condition can be switched off in pulses or cyclically. In accordance with the present invention, this switching control is synchronized with the switching control of the suction roll 7.
As shown in FIG. 2, ducts 22 provided with ejector or pusher-type nozzles alternate with the ducts 10 provided with nozzles operable to produce a floating condition. According to FIG. 1, the ducts 22 provided with said ejector nozzles are interconnected by the duct 23 mounting a blower 24. At its suction end, this blower 24 is again provided with a valve disc 15 capable of shutting the suction end of the blower 24 under the influence of a pneumatic control element 16. Thus, the blower 24 can be operated in synchronism with the blower 14 so that each sheet will first be advanced to a position above the stack 1? underneath the ducts provided with nozzles operable to produce a floating condition and then, at the moment when it is to be deposited, ejected by the nozzles of the ducts 22 and blown onto the stack with its full surface area.
FIG. 3 shows a simplified circuit diagram for electrically controlling and interlocking solenoid-operated valves which close and open the outlets of the blowers l3 and 14 and the inlet of the blower 24. A and B are the supply terminals of the electric circuit. As shown in the drawing, the light source 18 is directly connected to the electric supply system. Alternatively, this light source could also be supplied with electricity from a transformer or a rectifying and stabilizing device. To illustrate the control function of the photoelectric cell 17, this cell has been incorporated directly in the circuit of a relay 25. In practice, known amplifying control elements such as transistor switches will be inserted between the photoelectric cell 17 and the relay 25. The relay 25 may also take the form of a power contactor. When the relay 25 is deenergized, a contact bridge 26 closes the circuit 27 which includes the solenoidoperated valves 28 and 29 equipped with the valve discs 15. The outlet of the blower 14 is kept open as long as the valve disc 15 of the solenoid-operated valve 28 is not operated and closed when said valve disc is operated. The valve disc 15 of the solenoid-operated valve 29 serves to open and close the inlet of the blower 24. When the relay 25 is excited, a contact bridge 26 closes a circuit 30 which includes the solenoid of a solenoid-operated valve 31. The outlet of the blower 13 is kept open as long as the valve disc 15 of the solenoidoperated valve 31 is not operated and closed when said valve disc is operated.
When the light beam from the light source 18 to the photoelectric cell 17 is interrupted by a sheet, said cell is in a blocked condition so that the relay 25 is deenergized and its contact bridge 26 closes the circuit 27, thereby energizing the solenoid-operated valves 28 and 29 and causing the outlet of the blower 14 and the inlet of the blower 24 to be closed by their respective valve discs 15. The solenoid-operated valve 31 is'deenergized and the outlet of the blower 13 open. Consequently, a negative pressure takes effect at the suction roll 7 so that the suction effect produced at that roll will attract the sheets. The sheets are not influenced by the suction roll 12 and the ejector nozzles of the ducts 22. When the end of a sheet has passed the light barrier and the beam from the light source 18 can again reach the photoelectric cell 17, the latter will be made conductive, causing the relay 25 to be operated. The contact bridge 26 then closes the circuit 30 and opens the circuit 27, causing the solenoid-operated valve 31 to be energized and the valves 28 and 29 to be switched off so that the suction effect of the suction roll 7 is canceled, that of the suction roll 12 is initiated, air is blown through the ducts provided with the ejector nozzles and the sheet is thus braked and simultaneously deposited on the stack 19. As soon as the light beam from the light source to the photoelectric cell is again interrupted by the start of a newly arriving sheet, the positions of the solenoid-operated valves 28, 29 and 31 will again be reversed automatically, causing the action of the ejector nozzles in the ducts 22 and of the suction roll 12 on the sheet to be canceled and that of the suction roll 7 to be initiated.
A comparison of apparatus designed in accordance with the present invention with apparatus of the prior art reveals a superiority of the invention on the following counts:
1. Only metals come in contact with the sheets so that electrostatic charging is completely avoided and the electrostatic charges, if any, carried over from preceding operations and machines are discharged so that their drawbacks are eliminated;
2. the accelerating zone is exclusively equipped with conveying elements acting on the underside of the ac 'celerated sheet so that waste can be readily detected and removed during operation with the result that the stacking of sheets with defects such as joints in continuously fed web material can be avoided;
3. 'at the delivery end, the cut sheets are guided above the stack along their full length and deposited on their entire surface area;
4. any desired stacking speed can be selected so that the starts of the sheets can no longer be damaged by excessive impact;
5. working conditions at the point of overlap are the same at all speeds of the machine, eliminating a change in conditions as a sourceof possible trouble;
6. there is no contact between successive sheets while they overlap each other, which precludes any possible trouble due to excessive or insufficient friction, sticking due to surface properties or electrostatic charges subjecting the surfaces so affected to forces of attraction;
7. at the point of overlap only one of the two modes of conveyance is operative at any one time so that the sheets cannot be bent by being conveyed, at increased speed, into a zone of reduced speed.
For the rest, apparatus in accordance with the present invention for continuous stacking of cut sheets can be equipped with known means and devices. Thus it is, for instance, possible to dispose a bar grate below the zone of acceleration in an arrangement permitting said bar grate to be displaced in the direction of material travel and temporarily accepting the sheets delivered by the apparatus while the stack is being lowered and removed until another stack base, such as a pallet, has been moved into working position by a lift table.
The invention relates to each of its features disclosed, even if mentioned in conjunction with other features only, as well as to every possible partial combination of features and, finally, the total combination of all features to the extent .that such individual features, partial combinations and/or total combinations are technically significant, practicable and useful, even where the new technical effects that can be achieved thereby are not mentioned and described in detail.
l. A method of forming a stack of sheets from a continuous web comprising the steps of feeding the web in longitudinal direction with a first speed; cutting the web while being fed at said first speed at'a cutting station into successive sheets;vtransporting the sheets in floating condition substantially in a plane from said cutting station to a stacking station while accelerating each sheet to attain a second speed greater than saidfirst speed so as to space the sheets in the direction of their movement from each other; decelerating the speed-of each sheet as it reaches said stacking station to a speed smaller than said second speed so that successive sheets start to overlap each other as they approach said stacking station; and moving each sheet at the stacking station in a direction substantially normal to said plane so as to form a stack of sheets.
2. A method as defined in claim 1, wherein said plane is a substantially horizontal plane, and'including the step of moving each sheet during the deceleration of its speed downwardly out of said plane.
3. A method as defined in claim 2, wherein the step of decelerating the speed of each sheet comprises the step of applying a braking force only to the trailing end portion of the respective sheet while maintaining the remainder of said sheet in floating condition.
4. A method as defined in claim 3, and including the step of maintaining the whole sheet in floating position above the stack to be formed and accelerating the sheet toward the stack to be formed.
5. A method as defined in claim 4, wherein the step of accelerating the sheet towards the stack comprises the step of blowing the sheet onto the stack.
6. A method as defined in claim 1, wherein said plane is a substantially horizontal plane, and wherein the step of decelerating each sheet comprises the step of applying a braking force only to the trailing end of the respective sheet while simultaneously moving said trailing end in downward direction out of said plane and while maintaining the remainder of said sheet in floating condition.
7. Apparatus for forming a stack of sheets from a continuous web comprising means for feeding a web in longitudinal direction at a first speed along a given path; cutting means at a cutting station along said path for cutting the web into successive sheets; pneumatic transporting means for transporting the sheets in floating condition substantially in a plane from said cutting 'station to a stacking station while accelerating each sheet to attain a second speed greater than said first speed so as to space the sheets in the direction of the movement from each other; pneumatic decelerating means for decelerating the speed of each sheet as it reaches said stacking station to a speed smaller than said second speed so that successive sheets start to overlap each other as they approach said stacking station; pneumatic means to maintain each sheet above the stacking station in floating condition; and further pneumatic means to move each of said sheets in said floatingcondition in a direction substantially normal to said plane soas to form a stack of sheets.
8. An apparatus as defined in claim 7, wherein said pneumatic decelerating means is spaced from said plane in a direction substantiallynormal thereto.
9. An apparatus as defined in claim 8, wherein said plane is a substantially horizontal plane and wherein said pneumatic deceleration means is downwardly spaced from said plane.
10. An apparatus as defined in claim 7, wherein said means for accelerating and decelerating the speed of each sheet comprises rotating suction rolls rotating at different speeds.
11. An apparatus as defined in claim 10 and including control means for alternatingly actuating at least one suction roll for accelerating the sheets and at least another suction roll for decelerating the sheets.
12. An apparatus as defined in claim 1 1, wherein said one and said other suction roll are located adjacent and upstream of said stacking station with said one suction roll located upstream of the other.
13. An apparatus as defined in claim 12, wherein said control means are actuated by the trailing end edge of each sheet and the leading edge of the following sheet.
14. An apparatus as defined in claim 13, wherein said control means comprise an electric circuit and relay means in said circuit for actuating and deactivating said further pneumatic means simultaneously with the suction roll for decelerating each sheet.