Sheet feeding apparatus
United States Patent 1 Melehan 1 1 SHEET FEEDING APPARATUS [75] Inventor: George M. Melehan. Downers Grove. Ill.  
 [73] Assignee: Chippewa Paper Products Company.  
 Chicago Ill.  
 [22] Filed: July 23, 1973 [21] Appl. No.: 381.628  
 [52] US. Cl 271/11: 214/85 D; 271/30 R;  
  271/104 [51] int. Cl B65h 3/08 [58] Field of Search 271/14, 12. 5. ll, 15.  
 27l/l3. 30. 31. 104. 152-155. 171. 167; 214/85 D. 1 BT Apr. 22, 1975 2.960.805 11/1960 Herman ct al. 271/155 X 3.225.891 l2/l965 Hickin ct al. 2l4/l BT X 3.411.640 ll/l9h8 Wallis ..1 2l4/8.5 D  
 [57] ABSTRACT A carriage is horizontally movable above a stack of sheet goods between a position adjacent to the stack and an advanced position adjacent to a succeeding processing stage for the sheet goods. A pickup assembly is mounted on the carriage for vertically reciprocal movement with respect thereto between a raised position and a lowered position. A power unit releasably engages the pickup assembly and then moves it to its lowered position to grasp the top sheet on the stack. Thereafter, the pickup assembly is returned to its raised position. and the motor is disengaged. The carriage is transported to its advanced position to deliver the sheet to the succeeding processing stage.  
 23 Claims. 11 Drawing Figures 32 i I I 27 PMENTEBAPRZZWS 3,879,031 sumsurgs FIG.9  
 l I I 49 FIG. IO  
 PATENTEUAPRZZISYS m: ommwmmmzou SHEET FEEDING APPARATUS BACKGROUND OF THE INVENTION This invention is directed to a feeding mechanism which picks up sheets of material such as cardboard. paper, cloth, etc., from a stack of such sheets. one at a time and delivers them to a succeeding processing stage for the sheets. In one known apparatus of this type, the sheets of material are individually lifted from the stack by one mechanism and then transferred to a carriage which then transports the sheets to the succeeding stage. This is a relatively complicated apparatus requiring separate lifting and transporting mechanisms and the capability to effect transfer therebetween.  
  Another known sheet feeding apparatus has not only the lifting mechanism mounted on the horizontally reciprocal carriage, but also the power unit which reciprocates the lifting mechanism vertically. The carriage and associated transporting structure must be capable of accommodating the added weight of the power unit carried thereby.  
  A preferred construction of the lifting apparatus incorporates suction cups which are pressed against the top sheet of the stack. In certain types of feeding apparatus incorporating suction cup units. the suction cups traverse a somewhat arcuate path while moving downwardly. It has been found that such movement has an adverse effect on the life of the cups. In other words. the cups tend to wear a great deal faster when any horizontal component is imparted to them as they are lowered onto the stack of sheets.  
  Certain types of sheet goods have a tendency to warp, and it is therefore desirable that the feeding apparatus be capable of compensating for such warpage. Cardboard is particularly prone to become warped. Presently available feeding apparatus are not capable of satisfactorily handling warped cardboard.  
  It is desirable that provision be made to accommodate sheet goods ov various widths.  
  Certain presently known sheet feeding apparatus have elaborate structure to cause initial separation of the top sheets of the stack preparatory to their being picked up by the lifting mechanism.  
  It is known to provide a table which automatically elevates the stack of sheet goods as it becomes depleted. However, there is some tendency in presently available systems for the apparatus to break down because the elevating mechanism elevates the stack at an inopportune time.  
 SUMMARY OF THE INVENTION It is, therefore, an important object of the present invention to provide an improved sheet feeding appara tus for feeding sheets of material from a stack to a sue ceeding processing stage.  
  Another object is to improve the wearability of suction cups used in a sheet feeding apparatus.  
  Still another object is to provide a table which automatically elevates the stack of sheets thereon as the stack is depleted, but only elevates the stack when all the moving parts of the sheet feeding apparatus are at rest.  
  Yet another object is to simplify the construction of a carriage in a sheet feeding apparatus and the associated transporting structure used for the carriage, by reducing the amount of weight the carriage must support.  
  A further object is to provide an inexpensive means to separate the top sheets of the stack. preparatory to their being picked up by the sheet feeding apparatus.  
  A still further object is to provide a sheet feeding apparatus utilizing suction cups to pick up sheets which have a tendency to be warped.  
  In summary, there is provided a sheet feeding apparatus for delivering sheets of material. one at a time, to a given location from a stack of such sheets at another location, the feed mechanism comprising carriage means reciprocally mounted for substantially horizontal movement above the stack between a retracted position adjacent to the stack and an advanced position in front of the stack. carriage operating means operatively coupled to the carriage means for movement thereof between the retracted and advanced positions. pickup means reciprocally mounted on the carriage for substantially vertical movement between raised and lowered positions with respect to the stack. pickup operating means releasably engaging the pickup means for movement thereof between the raised and lowered positions, and control means for causing the pickup operating means to engage the pickup means for movement thereof to the lowered position to grasp the top sheet of material of the stack. thereafter causing the pickup operating means to move the pickup means to the raised position, thereafter causing the pickup operating means to be released from the pickup means. thereafter causing the carriage operating means to move the carriage means to the advanced position to deliver the top sheet to the given location. and thereafter causing the carriage operating means to retract the carriage until the pickup means is again in releasable engagement with the pickup operating means preparatory to a subsequent cycle.  
  The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details of the apparatus may be made without departing from the spirit. or sacrificing any of the advantages of the invention.  
 BRIEF DESCRIPTION OF THE DRAWINGS For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description. the invention, its construction, and operation. and many of its advantages should be readily understood and appreciated.  
  FIG. 1 is a perspective view of a sheet feeding apparatus incorporating the features of the present invention, delivering sheets from a stack to a cutting machine;  
  FIG. 2 is a perspective view of the sheet feeding apparatus from a different direction;  
  FIG. 3 is a perspective view of the sheet feeding apparatus from the rear;  
  FIG. 4 is a perspective side view of a portion of the sheet feeding apparatus;  
  FIG. 5 is a side elevational view of the sheet feeding apparatus;  
  FIG. 6 is an enlarged view in cross section taken along the line 66 of FIG. 2;  
  FIG. 7 is a view in horizontal cross section taken along the line 7-7 of FIG. 2&#39;.  
  FIG. 8 is a view like FIG. 7. but with the bottom of the feeding apparatus and the bottom of the cutting machine being cut away. illustrating the feeding mechanism in condition to pick up a sheet of material;  
  FIG. 9 is a view like FIG. 8. but with the sheet having been picked up by the pickup assembly;  
  FIG. 10 is a view like FIG. 9. but with the carriage having been advanced forwardly to deliver the top sheet to the cutting machine; and  
  FIG. 11 is a schematic view of the control system used to operate the sheet feeding apparatus.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the drawings. and more particularly to FIG. I thereof. there is illustrated a cutting machine which cuts out circular blanks from a sheet of card&#39; board. The cutting machine 20 illustrated includes a plurality of shafts 21. each carrying a pair of sprockets 22. The front lower shaft 21 and associated sprockets 22 are illustrated in FIG. I. All the sprockets 22 on one side are interconnected by an endless chain 23; and the sprockets on the other side are interconnected by another chain 23. A suitable drive mechanism (not shown) is provided for the chains 23. The chains 23 carry a set of plates 24, on each of which are two cutting die members 25 adapted to cut two circular blanks from a sheet of cardboard. There may be a sufficient number of plates 24 that the total area of the plates matches the area of the cardboard being fed to the machine 20. As is best seen in FIG. 2, the cutting machine 20 includes a pair of spaced-apart upper rollers 26 carried by a shaft 26a. and a pair of spaced-apart lower rollers 27 carried by a shaft 270. The upper rollers 26 are respectively vertically aligned with the lower rollers 27. The rollers 26 and 27 are continuously rotated by the drive mechanism. and are operative to pull a sheet of cardboard into the machine to be cut successively by pairs of the die members 25. It is to be understood that the cutting machine is merely one example of a processing stage which can be fed by the invention to be described.  
  The sheet feeding apparatus is designated generally by the numeral 30 and is adapted to pick up sheets of cardboard, one at a time. from a stack 31, and then deliver each such sheet to the cutting machine 20. While the sheet feeding apparatus 30 is described as feeding sheets of cardboard, it is to be understood that the invention is applicable to feeding of any sheet material which is furnished in stack form.  
  The sheet feeding apparatus 30 comprises a main frame having two side members 41, a pair of end members 42 welded respectively to opposite pairs of ends of the side members 41, and a center member 43 welded to the side members 41 between the end members 42. Referring specifically to FIG. 5, the main frame 40 includes a front leg 44 in the form of an angle iron. and a shorter rear leg 45. A cross brace is welded to the top of the rear leg 45 and to the front leg 44 be tween the ends thereof. A guide 47 is secured to a side member 41 near the front thereof, whereby the space between the guide 47 and the adjacent protruding end of the front end member 42 provides a channel through which the front leg 44 may slide. An extension member 49 depends from the side member 41 and is secured thereto by means of a bracket 48. A bracket 50, welded to the lower end of the extension 49, has a threaded hole for ajacking screw SI. The lower end of the jacking screw 51 contacts the top of the rear leg 45 to en- 5 able adjustment of the height of the rear of the main frame 40. Another jacking screw 52 passes through the protruding end of the front end member 42. The lower end of the jacking screw 52 rests on a horizontal shelf of the cutting machine 20. Thus, by rotating the jacking l0 screw 52, the height of the front of the main frame 40 may be adjusted. A lower front member 42a (FIG. 5) is secured to the legs 44 beneath the front end member 42. Structure substantially identical to the elements 4452 is provided on the side of the frame 40 visible in IS FIG. I. An equipment plate 53, secured to such structure, has mounted thereon electrical equipment such as a fuse box, switches, outlets. etc.  
  The stack 31 is arranged on a table located at the rear of the sheet feeding apparatus 30. The table 60 in- 20 cludes a base 61 carrying a scissor-type linkage 62 which supports a platform 63. The platform 63 is constructed of a pair of parallel, longitudinally extending main members 64 arranged relatively close together. An end member 65 is welded to the main members 64 25 at the rear thereof, which end member 65 is substantially longer than the spacing between the main members 64 and therefore protrudes laterally outwardly therefrom. A similar end member 65 is welded to the main members 64 at the fronts thereof. Two extensions 30 66 are directed laterally outwardly from the main members 64 and are disposed between the end members 65. A cylinder 67 mounted on the base 61, has a piston rod 68 operatively connected to the linkage 62. In the particular form illustrated, the cylinder 67 is hydraulically 35 operated. Means to be described presently automatically operate the cylinder 67, so that the platform 63 is automatically raised from time to time to maintain a constant level of the top sheet of the stack 31.  
  The sheet feeding apparatus 30 includes a pair of adjustable stack-retaining mechanisms 70 respectively located on the sides of the stack 31. Each stack-retaining mechanism 70, as is best seen in FIGS. 4 and 5, includes a plate 71 in abutting relationship with the associated side of the stack 31. The plate 71 carries a pair of spaced-apart, outwardly directed rods 72. A depending bar 73, secured to the adjacent side member 41 of the main frame 40, supports a pillow block 74. A pair of bushings 75 in the block 74 respectively slidably receive the rods 72. The plate 71 is held at a selected position relative to the stack 31 by means of thumb screws 76 which pass through the block 74 and engage the rods 72. A similar adjuatable stack-retaining mechanism 70 is furnished on the other side of the sheet feeding apparatus 30, whereby the distance between the plates 71 is adjustable to accommodate stacks of various widths.  
  As can be best seen in FIGS. 2 and 3, a brush 78 is disposed at the front of the stack 31 and is held thereat by means of a mounting arm 79 secured to the lower front member 42a. The bristles of the brush 78 are directed rearwardly and are in contact with the front edges of the stack 31. The brush 78 serves to loosen the frictional bond between adjacent sheets in the stack 31 to facilitate their being removed one at a time.  
  The sheet feeding apparatus 30 further comprises a carriage 80 having a U-shaped frame 81. The frame 81 has a long end member 82 and a pair of short side members 83. As is best seen in FIG. 6, a block 84 is secured by means of screws 85 to the lower front end of each side member 83 on the inner surface thereof. Each block 84 has a vertically extending bore 86 therethrough, for use in connecting the pickup assembly 90.  
  The pickup assembly 90, which comprises a further element of the sheet feeding apparatus 30, includes two end blocks 91 attached to the ends of a pair of spacedapart, substantially parallel rods 92. Associated with each block 91 is a bolt 93 extending through the associate bore 86, and having a threaded end 94 threadably engaging in the associated block 91, and having a head 95. A compression spring 96 is arranged on each bolt 93 between the block 84 and the head 95,10 urge the block 91 against the associated block 84. Thus, the springs 96 bias the pickup assembly 90 upwardly against the blocks 84. A set ofthree plate-like members 97 (see FIG. 2) is secured to the rods 92 at spacedapart points thereon. Each member 97 carries an upstanding bar 98. An upper pin 99 extends through each bar 98 near the upper end thereof, and a lower pin 100 extends through each bar 98 beneath the associated pin 99.  
  The pickup assembly 90 includes a plurality of gripping units 101. In the embodiment being described, there are four gripping units 101, two of which are arranged between the first and second members 97, and the other two of which are arranged between the second and third members 97. In the instant embodiment, each gripping unit 101 is provided with a depending suction cup 102. A hollow tube 103, mounted between the end members 82 of the carriage frame 81, is connected to a vacuum generator (not shown). The suction cups 102 are connected to the tube 103 by means of hollow tubing (not shown). The gripping units 101 respond to compressed air in the tube 103 to produce a continuous vacuum in the suction cups 102. Thus, when the cups 102 are pressed onto a sheet of cardboard, the cups will retain such sheet. If any suction cup is worn, this construction enables the other gripping units to operate satisfactorily.  
  The sheet feeding apparatus includes a carriage supporting structure 110 having two lugs 111 respectively depending from the frame members 42 and 43. The lugs 111 are longitudinally aligned and support a rod 112. Similar structure is provided adjacent to the other side of the frame 40. As is best seen in FIGS. 3 and 4, a block 113 is secured to each side member 83 of the carriage frame 81. Each block 113 has a bushing 114 which slidably receives the adjacent rod 112. Thus, the carriage 80 is suspended from the main frame 40 and is reciprocally movable forwardly and rearwardly with respect to the main frame 40.  
  To lower the pickup assembly 90, the sheet feeding apparatus 30 is provided with an operating mechanism 120. As is best seen in FIG. 2, such operating mechanism 120 includes a rod 121 journaled at 122 to the side members 41 of the main frame 40. There is provided a plurality of arms 123, here three, each having a clamp 124 at one end and a forked portion 125 at the other end. Each forked portion 125 is defined by a slot in the forward end of the associated arm 123. The arms 127 are fixedly anchored to the rod 121 at spaced-apart points thereon by means of the clamps 124. The arms 123 are so positioned that they are in longitudinal alignment with the upstanding bars 98 of the pickup assembly 90. Each arm is bent slightly to furnish one portion directed downwardly and forwardly and another portion disposed generally horizontally.  
  To rotate the rod 121 about its longitudinal axis, there is provided a pneumatic cylinder 126 supported by an arm 127, which is attached to the rear end member 42 of the main frame 40 by means of a bracket 128. A piston rod 129 associated with the cylinder 126 has a clevis 130 at its outer end, which is pivotally attached to one end of a lever 131, the other end of the lever being attached to the rod 121. By actuating the cylinder 126 to extend the piston rod 129, the lever 13] will be caused to move clockwise. as shown in FIG. 2, to rotate the rod 121 clockwise and thus swing the three arms 123 clockwise. Similarly, when the piston rod 129 is withdrawn into the cylinder 126, the rod 121 will be caused to rotate in a counterclockwise direction, and swing the arms 123 counterclockwise.  
  As was previously noted, the forked portions 125 of the arms 123 are respectively longitudinally aligned with the bars 98 of the pickup assembly 90. As illustrated in FIG. 2, the forked portion 125 of each arm 123 is adapted releasably to engage the associated bar 98, whereby such bar 98 and the pins 99 and thereon define a keeper structure for the pickup assembly 90. When the cylinder 126 is operated to extend the piston rod 129, the arms 123 swing clockwise. The forked portions of the arms 123 act like presser feet against the associated lower pins 100 to cause the suction cups 102 to be pressed against the top sheet 32 of the stack 31. When the piston rod 129 is withdrawn, to rotate the rod 121 counterclockwise and swing the arms 123 upwardly, the biasing furnished by the springs 96 causes the pickup assembly 90 to rise with the front of the top sheet 32 adhering to the suction cups 102.  
  The sheet feeding apparatus 30 further comprises an operating mechanism for the carriage 80. The operating mechanism 140 includes a pneumatic cylinder 141 suspended from a leg 142 which depends from the rear end member 42 of the frame 40. A bracket 143 secures the cylinder 141 to the center member 43. A piston rod 144 associated with the cylinder 141 is attached, by means of a connector 145, to the rear surface of the end member 82 of the carriage frame 81. When the piston rod 144 is extended, the carriage 80 is moved to its advanced position, that is, it is moved forwardly. When the piston rod 144 is drawn into the cylinder 141, the carriage 80 is reciprocated to the retracted position illustrated in FIG. 2.  
  Turning now to FIG. 11, the operation of the sheet feeding mechanism 30 will be described. FIG. 11 depicts the control system for operating the actuating mechanisms 120 and 140. The control system in FIG. 11 is electrically powered from a l lO-volt supply, which is coupled to the solenoid 156 of a timer 155 by means of a switch 150. Associated with the switch is an actuator 151. As is best seen in FIG. 6, the actuator 151 is generally J-shaped the shorter leg of which is attached to the end member 82 of the channel frame 80 and the longer leg of which is adapted to engage the switch 150 when the carriage 80 is in its retracted position. In that position, the switch 150 is closed, as illustrated in FIG. 11. The timer 155 has a pair of normally closed contacts 157 and a pair of normally opened contacts 158. The timer 155 includes adjustable delay, so that the contacts 158 close and the contacts 157 open a selected time interval after the solenoid 156 is energized. After a predetermined time, which is also adjustable, the contacts 158 automatically open and the contacts 157 automatically close.  
  The control system also includes a solenoidoperated valve unit 160. A first valve 161 in the unit 160 has an inlet port, an outlet port. and an exhaust port. The valve 161 has a valve member 162 which is movable to an &#34;on position to cause the inlet port to communicute with the outlet port, and is movable to an exhaust position to cause the outlet port to communicate with the exhaust port. When the solenoid 163 is energized. the valve 161 is placed in its on&#34; position. and when it is de-energized, the valve 161 automatically reverts to its exhaust condition. The valve unit 160 includes a second valve 164 constructed similarly to the valve 161, having a valve member 165 and an associated solenoid 166. When the solenoid 166 is energized, the valve 164 is placed in its on position, and when deenergized. it automatically reverts to its exhaust conditron.  
  The inlet ports of the valves 161 and 164 are connected to a supply of compressed air; the outlet port of the valve 161 is connected to the front of the cylinder 126 (air admitted to the front&#34; will urge the piston rod 129 rearwardly); and the outlet port of the valve 164 is connected to the rear of the cylinder (air admitted to the &#34;rear&#34; will urge the piston rod 129 forwardly).  
  When the solenoid 156 is energized to place the contacts 157 and 158 in the condition illustrated, the solenoid 163 is de-energized to cause the valve 161 to exhaust air in the front of the cylinder 126, and the solenoid 166 is energized to deliver compressed air to the rear of the cylinder 126, thereby extending the piston rod 129. When the solenoid 156 is de-energized, the contacts 157 close and the contacts 158 open, to cause the valve 161 to deliver compressed air into the front of the cylinder 126, and the valve 164 to exhaust air in the rear of the cylinder 126, thereby withdrawing the piston rod 129.  
  The control system includes a second valve unit 170 having a first valve 171 with a valve member 172 to control its condition (on or exhaust). A solenoid 173 in the valve unit 170 places the valve 171 in its on condition when energized, and in its exhaust condition when de-energized. The valve unit 170 includes a second valve 174 having an associated valve member 175 and a solenoid 176. When the solenoid 176 is energized, the valve 174 is placed in its on condition, and, when deenergized. the valve 174 is placed in its exhaust condition. The inlet ports of both valves 171 and 174 are coupled to the air supply; the outlet port of the valve 171 is coupled to the front of the cylinder 141; and the outlet port of the valve 174 is coupled to the rear of the cylinder 141.  
  A switch 177 has one stationary contact coupled to the solenoid 173 and another stationary contact coupled to the solenoid 176. The switch 177 is mounted on the cutting machine (see FIG. 2, for example) and has its movable contact operated by means of an actuator 178. As the plates 24 on the cutting machine 20 traverse their endless paths, they will periodically strike the actuator 178 to place the switch 177 in the dashedline condition of FIG. 11. The solenoid 176 is thereby energized to cause the valve 174 to transmit compressed air into the rear of the cylinder 141, and the deenergized solenoid 173 causes the valve 174 to exhaust compressed air from the front of the cylinder 141,  
 thereby extending the piston rod 144. When the actuator 178 is not being struck by the plates 24, the switch 177 is in the solid-line condition illustrated in FIG. 11 to energize the solenoid 173 and cause the valve 172 to transmit compressed air into the front of the cylinder 141; at this time, the valve 174 is in its exhaust condition to exhaust the compressed air from the rear of the cylinder 141. Thus, the piston rod 144 is withdrawn.  
  Turning now to FIGS. 7-10, the details of operation of the sheet feeding apparatus 30 will be described in the context of the operation of the portion of the con trol system just described. Prior to the time the main on-off switch (not shown) of the sheet feeding apparatus 30 is first turned on, the switch 150 is closed in accordance with the position of the actuator 151 in FIG. 6. The solenoid 156 is de-energized, so that the normally closed contacts 157 are closed and the normally opened contacts 158 are opened. When the power is first supplied, the solenoid 163 is energized, so that compressed air is delivered to the front of the cylinder 126, and the solenoid 166 is maintained tie-energized to exhaust air from the rear of the cylinder 126. Thus, the piston rod 129 is maintained in the withdrawn condition illustrated in FIG. 7. The switch 177 is in the position illustrated by the solid line, so that the solenoid 173 is energized to cause compressed air to be delivered to the front of the cylinder 141, and the solenoid 176 is maintained deenergized to cause air in the rear of the cylinder 141 to be exhausted. Thus, the piston rod 144 associated with the cylinder 141 is maintained in the withdrawn condition illustrated in FIG. 7.  
  A predetermined time after the solenoid 156 has been energized, the contacts 158 close, energizing the solenoid 166 to cause the valve 164 to deliver compressed air to the rear of the cylinder 126; and the contacts 157 open, de-energizing the solenoid 163 to cause the valve 161 to exhaust the air in the front of cylinder 126. The piston rod 129 is therefore extended, as shown in H0. 8, to pivot the lever 131 clockwise about the axis of the rod 121. The forked portions 125 of the arms 123 engage respectively the lower pins 100, thereby lowering the pickup assembly 90 against the action of the springs 96. The cups 102, producing a continuous vacuum, are pressed onto the top sheet 32 of the stack 31.  
  After a predetermined delay furnished by the timer 155, the contacts 157 close, and the contacts 158 open, respectively energizing the solenoid 163 and deenergizing the solenoid 166. Compressed air is deliv ered into the front of the cylinder 126, and the air in the rear of the cylinder 126 is exhausted by the valve 164. thereby withdrawing the piston rod 129. The biasing action furnished by the springs 96 automatically causes the pickup assembly 90 to return to its raised position, illustrated in FIG. 9. The suction cups 102 grasp the sheet 32, as illustrated, and lift it, preparatory to being delivered to the cutting machine 20. At about this time, the leading edge of the leading plate 24 strikes the actuator 178 to place the switch 177 in the position indicated by the dashed lines in FIG. 11. The solenoid 173 is deenergized, and the solenoid 176 is energized, respectively to cause the valve 171 to exhaust air in the front of the cylinder 141 and to cause the valve 174 to deliver compressed air into the rear of the cylinder 141. the piston rod 144 is extended, as illustrated in FIG. 10, to advance the carriage and the sheet 32 carried thereby to the continuously rotating rollers 26 and 27. These rollers take over and deliver the sheet 32 into the cutting machine 20. After the sheet 32 has completed its path in the cutting machine 20, the absence of the plates 24 causes the actuator I78 to return the switch I77 to the dashed condition illustrated in FIG. 11. The valve 174 becomes operative to exhaust air in the rear of the cylinder I41, and the valve 17] becomes operative to deliver compressed air to the front of the cylinder 141. The piston rod I44 is withdrawn into the cylinder 14I, as is illustrated in FIG. 7. The bars 98 are respectively received in the slots in the forked portions 125 of the arms I23. preparatory to the next cycle. The actuator II engages the switch I50 thereby closing the switch 150 to re-energize the solenoid I56. Because of the built-in delay furnished by the timer I55, the contacts 158 close a short time later to enable the preceding sheet to have sufficiently cleared the stack 31 before a new cycle commences.  
  Turning now to the right-hand portion of FIG. 11, the details of the automatic elevating mechanism 180 will be described. A hydraulic pump 181, driven by a motor I82 receives fluid such as oil from a reservoir I83. A solenoid-operated valve unit 190 includes a solenoid 193 which operates a valve I91 having a valve member 192. The inlet port of the valve 191 is coupled to the pump ISI. One outlet port of the valve 19] is coupled to the reservoir I83, and the second outlet port is coupled to the cylinder I67. Coupled in series with the solenoid I93 is a switch 194 for which the actuator 195 is a rigid wire resting on the top of the stack 31 (see FIG. 7 for example). When the switch 194 is closed, the solenoid I93 is energized to place the valve I91 in the condition shown in FIG. I], so that the pump I81 pumps oil from the reservoir 183 into the cylinder 167. The piston rod 68 is caused to be extended, thereby raising the platform 63. When the switch 194 opens by virtue of the stack 31 reaching a predetermined height, the solenoid 193 is de-energized, thereby placing the valve I91 in a position to divert the oil from the pump I81 away from the cylinder 67 and back into the oil reservoir I83. When the height of the stack 31 falls below a predetermined level, the switch I94 will close for an instant and thereby incrementally raise the level of the stack.  
  If the stack 31 becomes depleted, it is desirable that the platform 63 not elevate. To this end, there is provided a solenoid-operated valve unit 200 including a valve 201 and a solenoid 202. A switch 203 has one stationary contact coupled to the solenoid 202 and the other stationary contact coupled through the switch 196 to the motor 182. The movable contact of the switch 203 is controlled by an actuator 204 (which can be seen in FIGS. 4 and 5, for example). When the stack 31 is on the platform 63, so that the actuator 204 is not operated, the movable contact of the switch 203 will be in the position indicated by the solid line. However, if the stack becomes depleted, the platform 63 raises to strike the actuator 204 and cause the switch 203 to be placed in the position indicated by the dashed line, to operate the solenoid 202 and open the valve 20]. The oil in the rear of the cylinder 67 will be exhausted through the valve 201 into the oil reservoir 183, thus causing the piston rod 68 to be withdrawn. Moreover, the shifting in position of the switch 203 disrupts power to the motor 182, so that the pump 181 no longer pumps fluid into the cylinder 67.  
  The switch I96 is controlled by an actuator 197 (FIG. 2) mounted on the carriage 80. As long as the carriage is in its retracted position, the switch 196 is closed. and, therefore. power can be delivered to the motor 182 when the stack 3| drops below a predeter mined level and as long as the stack 31 is not depleted, so that the switch 203 is closed. However, as soon as the carriage 80 moves forwardly. the actuator I97 disengages the switch 196 to open same and therefore to prevent power from being delivered to the motor I82, irrespective of the height of the stack 3I. As soon as the carriage 80 returns to its initial position to close the switch I96, the motor I82 is operated for a very short period of time to raise the platform 63 incrementally.  
  The sheet feeding apparatus 30, as described. has a number of advantages. For example. it is noted that the pickup mechanism is reciprocally carried by the carriage 80. but that the carriage 80 need not support the mechanism which operates the pickup assembly 90. Specifically, the entire operating mechanism I20 is mounted on the main frame 40, and the arms I23 releasably engage the pickup assembly 90. Thus, when it is desired to pick up a sheet, the arms I23 releasably engage the pins 99 and cause the suction cups 102 to be pressed downwardly against the top sheet of the stack 31. The pickup assembly 90 is then raised. and thereafter the operating mechanism I20 is disengaged. The carriage 80, which does not carry the operating mechanism 120, is then advanced. Thus, the carriage can be of a simpler and less expensive construction.  
  In addition, the sheet feeding mechanism 30 is so constructed that the suction cups I02 move substantially vertically with little or no horizontal component. This is also achieved in part by virtue of the selective engagement of the operating mechanism I20 with the pickup assembly 90. It has been found that the life of the suction cups 102 is substantially increased by insuring only vertical movement as they contact the top sheet of the stack 31.  
  Another feature of the present apparatus is the adjustable retaining mechanism 70 for the stack 31. The retaining mechanisms 70 enable adjustment of the sheet feeding apparatus 30 to accommodate stacks of a variety of widths. The brush 78 provides inexpensive means to separate the sheets while they are being raised by the suction cups 102. Also, the elevating mechanism I80 for the table 60 operates only when the two operating mechanisms I20 and are at rest. This insures that the platform 63 will not be moving at a time when other parts are moving and thereby jam or damage the apparatus 30. The particular construction of the sheet feeding apparatus 30 enables it to accommodate material that has a tendency to warp, such as is the case with cardboard. The gripping units I01 of the pickup assembly 90 are somewhat independently movable by virtue of the rod 92 which support them. The three arms I23 resiliently engage the bars 98, so that a different force may be applied by different suction cups I02 depends upon the warpage present.  
  It is believed that the invention, its construction and operation, and its advantages should be readily understood from the foregoing without further description, and it should also be manifest that, while a preferred embodiment of the invention has been shown and described for illustrative purposes, the details as to the structure are, nevertheless, capable of wide variation within the purview of the invention, as defined in the appended claims.  
 What is claimed is:  
  1. A sheet feeding apparatus for delivering sheets of material. one at a time. to a given location from a stack of such sheets at another location. said sheet feeding apparatus comprising carriage means reciprocally mounted for substantially horizontal movement above the stack between a retracted position adjacent to the stack and an advanced position in front of the stack, carriage operating means operatively coupled to said carriage means for movement thereof between said re tracted and advanced positions. pickup means reciprocally mounted on said carriage means for substantially vertical movement between raised and lowered positions with respect to the stack, pickup operating means releasably engaging said pickup means for movement thereof between said raised and lowered positions, and control means for causing said pickup operating means to engage said pickup means for movement thereof to said lowered position to grasp the top sheet of material of the stack. thereafter causing said pickup operating means to move said pickup means to said raised position, thereafter causing said pickup operating means to be released from said pickup means, thereafter causing said carriage operating means to move said carriage means to said advanced position to deliver the top sheet to the given location. and thereafter causing said carriage operating means to retract said carriage until said pickup means is again in releasable engagement with said pickup operating means preparatory to a subsequent cycle.  
  2. The sheet feeding apparatus of claim 1, wherein said carriage operating means includes a pneumatic cylinder and a piston rod fixedly attached to said carriage means.  
  3. The sheet feeding apparatus of claim I, wherein said pickup means has substantially no horizontal movement as it is being vertically moved between the raised and lowered positions thereof.  
  4. The sheet feeding apparatus of claim 1, wherein said pickup means includes at least one suction cup adapted to be pressed onto the top sheet of the stack.  
  5. The sheet feeding apparatus of claim 1, wherein said pickup means includes at least one rod and two end pieces respectively secured to the ends of said rod, said end pieces being reciprocally mounted on said carriage for substantially vertical movement between said raised and lowered positions, and a plurality of gripping units carried by said rod and being axially adjustably located thereon.  
  6. The sheet feeding apparatus of claim 1, and further comprising an adjustable retaining mechanism including a pair of laterally spaced apart plates disposed substantially parallel to each other, means for mounting said plates for reciprocal movement toward and away from each other, and means for locking the position of the plates at a selected spacing therebetween, said plates being adjusted to be spaced slightly outwardly respectively from the sides of the stack.  
  7. The sheet feeding apparatus of claim 6, wherein said mounting means includes at least one outwardly directed rod at each side of the frame, and a pillow block secured each side of the frame and slidably receiving the associated rod, said locking means including a thumb screw to secure the rod at a selected position with respect to the pillow block.  
  8. The sheet feeding apparatus of claim 1, and further comprising a table for the stack, and means for automatically elevating the table as the stack becomes depleted 9. The sheet feeding apparatus ofclaim 8, and further comprising means for permitting said platform to be elevated only when both said carriage operating means and said pickup operating means are at rest.  
  10. The sheet feeding mechanism of claim 1, and fur ther comprising a brush having its bristles in contact with an edge of the stack to facilitate separation of the individual sheets of the stack while said pickup means is raising the top sheet.  
  11. A sheet feeding apparatus for delivering sheets of material, one at a time, to a given location from a stack of such sheets at another location, said sheet feeding apparatus comprising a frame, carriage means reciprocally mounted on said frame for substantially horizontal movement above the stack between a retracted position adjacent to the stack and an advanced position in front of the stack, carriage operating means having a housing operatively secured to said frame and having a moving element operatively coupled to said carriage means for movement thereof between said retracted and advanced positions, pickup means reciprocally mounted on said carriage means for substantially vertical movement between raised and lowered positions with respect to the stack, pickup operating means having a housing operatively secured to said frame and having a moving element releasably engaging said pickup means for movement thereof between said raised and lowered positions, and control means for causing said pickup operating means to engage said pickup means for movement thereof to said lowered position to grasp the top sheet of material of the stack, thereafter causing said pickup operating means to move said pickup means to said raised position, thereafter causing said pickup operating means to be released from said pickup means, thereafter causing said carriage operating means to move said carriage means to said advanced position to deliver the top sheet to the given location, and thereafter causing said carriage operating means to retract said carriage until said pickup operating means is again in releasable engagement with said pickup means preparatory to a subsequent cycle.  
  12. The sheet feeding apparatus of claim 11, wherein said carriage operating means includes a pneumatic cylinder secured to said frame and a piston rod operatively coupled to said carriage means, and said pickup operating means includes a pneumatic cylinder secured to said frame and a piston rod releasably engaging said pickup means,  
  13. A sheet feeding apparatus for delivering sheets of material, one at a time, to a given location from a stack of such sheets at another location, said sheet feeding apparatus comprising a frame, carriage means reciprocally mounted on said frame for substantially horizontal movement above the stack between a retracted position adjacent to the stack and an advanced position in front of the stack, carriage operating means having a housing operatively secured to said frame and a moving element operatively coupled to said carriage means for movement thereof between said retracted and advanced positions, pickup means reciprocally mounted on said carriage means for substantially vertical movement between raised and lowered positions with respect to the stack, biasing means for urging said pickup means normally to the raised position thereof, pickup operating means having a housing operatively secured to said frame and having a moving element releasably engaging said pickup means for movement thereof between said raised and lowered positions. and control means for causing said pickup operating means to engage said pickup means and to move downwardly to move said pickup means against the urging of said biasing means to said lowered position to grasp the top sheet of material of the stack. thereafter causing said pickup operating means to move upwardly and thereby enable said pickup means to move to said raised position by the urging of said biasing means. thereafter causing said pickup operating means to be released from said pickup means. thereafter causing said carriage operating means to move said carriage means to said advanced position to deliver the top sheet to the given location. and thereafter causing said carriage operating means to retract said carriage until said pickup operating means is again in releasable engagement with said pickup means preparatory to a subsequent cycle.  
  14. The sheet feeding apparatus of claim 13, wherein said biasing means includes at least one compression spring intercoupling said pickup means to said carriage means.  
  15. The sheet feeding apparatus of claim 13, wherein said carriage means includes a frame and at least one block, said block having a vertical extending bore therethrough. and further comprising a headed bolt passing through said bore and engaging said pickup means. and said biasing means includes a spring between said retaining means and said block and encircling said bolt.  
  16. Sheet feeding apparatus for delivering sheets of material. one at a time, to a given location from a stack of such sheets at another location. said sheet feeding apparatus comprising a frame, carriage means reciprocally mounted on said frame for substantially horizontal movement above the stack between a retracted position adjacent to the stack and an advanced position in front of the stack, carriage operating means having a housing operatively secured to said frame and a moving element operatively coupled to said carriage means for movement thereof between said retracted and advanced positions, pickup means reciprocally mounted on said carriage means for substantially vertical movement between raised and lowered positions with respect to the stack and having keeper structure, biasing means for urging said pickup means normally to the raised position thereof, at least one arm for releasably engaging said keeper structure and being swingably mounted to move said pickup means between said raised and lowered positions, a motor having a housing operatively secured to said frame and having a moving element operatively coupled to said arm for causing swinging movement thereof. control means for causing said arm to engage said keeper structure and to swing downwardly to move said pickup means against the urging of said biasing means to said lowered position to grasp the top sheet of material of the stack. thereafter causing said arm to swing upwardly and thereby enable said pickup means to move to said raised position by the urging of said biasing means, thereafter causing said arm to be released from said keeper structure, thereafter causing said carriage operating means to move said carriage means to said advanced position to deliver the top sheet to the given location, and thereafter causing said carriage operating means to retract said carriage until said arm is again in releasable engagement with said keeper structure preparatory to a subsequent cycle.  
  17. The sheet feeding apparatus of claim 16, and further comprising at least one rod rotatably mounted on said frame for rotating about its longitudinal axis. said arm being anchored to said rod and being swingable as said rod is rotated.  
  18. The sheet feeding apparatus of claim l6, wherein said arm has a forked portion for engaging said keeper structure.  
  19. The sheet feeding apparatus of claim 16, wherein said arm has a slot in the free end thereof, said keeper structure including an upstanding bar and at least one laterally extending pin carried by said bar. said slot being longitudinally aligned with said bar and being movable to a position wherein said bar is disposed in said slot and has a portion disposed above said pin. said arm being swingable in a predetermined direction to press against said pin end thereby move said pickup means against the top sheet of the stack 20. The sheet feeding apparatus of claim 19, wherein said keeper structure further includes an upper pin carried by said bar and located above said lower pin, said upper pin being engaged by said forked portion to assist in raising said keeper structure when said arm is swinging in an opposite direction.  
  2l. Sheet feeding apparatus for delivering sheets of material, one at a time, to a given location from a stack of such sheets at another location, said sheet feeding apparatus comprising a frame, carriage means reciprocally mounted on said frame for substantially horizontal movement above the stack between a retracted position adjacent to the stack and an advanced position in front of the stack, carriage operating means having a housing operatively secured to said frame and a moving element operatively coupled to said carriage means for movement thereof between said retracted and ad vanced positions, pickup means including elongated support structure and a plurality of spaced-apart suction cup units suspended therefrom and a plurality of keeper structures attached to said support structure, said support structure being reciprocally mounted on said carriage means for substantially vertical movement of said suction cup units between a raised position with respect to the stack and a lowered position pressed against the top sheet of the stack, biasing means for urging said suction cup units normally to the raised position thereof, a plurality of arms respectively releasably engaging said keeper structures and being swing ably mounted to move said pickup means between said raised and lowered positions, a motor having a housing operatively secured to said frame and having a moving element operatively coupled to said arms for causing swinging movement thereof, control means for causing said arms to engage said keeper structures and to swing downwardly to move said pickup means against the urging of said biasing means to said lowered position to press against the top sheet of material of the stack for retention of such top sheet, thereafter causing said arms to swing upwardly and thereby enable said pickup means to move to said raised position by the urging of said biasing means, thereafter causing said arms to be released from said keeper structures, thereafter causing said carriage operating means to move said carriage means to said advanced position to deliver the top 23. The sheet feeding apparatus of claim 21, wherein each of said keeper structures includes an upstanding bar and a pin extending laterally outwardly therefrom. and each of said arms includes a forked portion for receiving said associated bar and for pressing down on said associated pin to cause the associated suction cup to be pressed upon the top sheet of the stack.