Patent GB-719851-A (GB)
Title: Improvements in apparatus for feeding and delivering sheets in screen stencil printing machines
IPC: B41F15/08
Filed: 1949-09-07
Abstract: 719,851. Stencilling; feeding and delivering sheets. FREEBORN, J. E., and TENAX. TRANSFERS, Ltd. Sept. 7, 1950 [Sept. 7, 1949; May 31, 1950], Nos. 23223/49 and 13652/50. Classes 100 (1) and 100 (2). In a stencil-screen printing- machine, registered sheets on a feed-table are fed therefrom to a printing table by a feed member comprising at least one gripper which draws the sheets over the surfaces of both tables, and into correctly registered position on the printing table, the front register stop associated with the feed table being automatically retractable to allow such feeding movement. As described, the machine comprises, in one form Figs. 1, 4, 6, 8, 13 and 14, a printing table 2 formed with a series of perforations leading to a vacuum chamber located below the table whereby a sheet may be retained on the table by suction, a feed table 3, and a delivery table 4. The main part of the feed table 3 carries a pile of sheets, and is raised progressively by known means to keep the top sheet substantially level with the printing table 2. A screen frame carrier 7, hinged at 8 a short distance above and adjacent the feed table 2, is raised and lowered by a rod 12 operated from the main drive-shaft 14. The construction shown in Figs. 16, 19 and 24 is generally similar as regards the printing-apparatus, but has modified means for feeding, delivering, and gripping the sheets. Sheet-carriers; gripper mechanism.-In the first form, the top sheet of the pile on the table 3 is slid by hand along the table to register against a front stop 80, Fig. 6, and a side register bar 77, the side edge of the sheet is then clamped automatically by grippers on a trolley 20 which conveys the sheet to the printing table 2, and a single gripper on a second trolley 25, Fig. 1, grips centrally the forward edge of the sheet before printing is effected, and conveys the sheet, after printing, to the delivery table. An air blast from a nozzle 150 assists the feed. The front stop 80 is lowered automatically by the action of the screen frame 7 on a plunger 81 which is interconnected with the front stop. The feed and delivery trolleys 20, 25 are reciprocated by endless chains 22, 26, respectively, driven by sprockets 23, 27 on a shaft 29 which is driven first in one direction, and then in the other by means described below. The feed trolley 20, Figs. 6 and 8, which is situated in a trough 21 extending along one side of the feed and printing tables 3, 2, comprises a pair of side-plates 70, 71 and a bottom bar 72, two pairs of wheels 73 which run in tracks in the trough, and a shaft 86 rotatably mounted on the plate 70 and carrying grippers 85 adapted to grip the side edge of sheet against rubber inserts in the upper edge of the plate 71. A fixed central bar 77 has an upstanding edge 78 which serves as a side register for the sheets. Each gripper 85 is rotatably mounted on the shaft 86, and is urged into gripping position by a spring 87. At the end of the feeding stroke of the trolley 20, a cross-piece 93, Fig. 6, on a collar secured to the shaft 86, engages in a slot 94 on a stub shaft 96, and the latter is rotated by rack-and-pinion means 98, 99 and bell-crank 103, against the action of a spring, whereby the gripper shaft 86 is rocked, and fixed collars 89 on the latter retract the grippers 85 to release the sheet. A spring-urged lever 109 then engages in a slot in a collar 107 on the opposite end of the gripper shaft, and retains the grippers in the open position until the trolley returns to the other end of the trough 21, whereupon the lever 109 is released by striking an abutment 112, and the grippers clamp a fresh sheet. Means to decelerate the feed trolley 20 and prevent rebound from the stops at each end of the trough 21, comprises key-hole shaped slots in each end of the trolley which engage over the ends of spring-urged levers. In the modified form shown in Fig. 19, for use with the machine shown in Fig. 16, a rackoperated feed-bar 300, described below, is used instead of a trolley, and is provided with a single sheet-gripper 330 housed in a recess in the bar, and comprising a pivoted radial arm 332 having a clamping finger 333. A spring-urged plate 336 acts on a roller 339 to hold the gripper in the clamping and retracted positions. The gripper is operated with a snap-action by known means. A similar single gripper is provided on the delivery bar. The delivery trolley 25, Figs. 1, 13 and 14, comprises a U-shaped frame 130, 131 provided with two pairs of wheels 132 which run on a guide-plate 133 extending beneath a central slot in the delivery table 4. A U-shaped bracket 134 secured to the trolley is attached at 135 to the lower lap of the drive chain 26. A pair of arms 136 on the trolley carry a rotatable anvil member 138 which is apertured to receive a pair of pins 141 carrying a clamp-plate 140, the anvil 138 and plate 140 forming a sheet gripper. The pins 141 bear on a stirrup 142 pivoted at 143 to two links 144 which are secured at their other ends to a transverse shaft 145 rotatably mounted in the arms 136. A cam 146 is secured at each end of the shaft 145. The clamp plate 140 is urged towards the anvil 138 by compression springs 147 interposed between the anvil and stirrup 142. The above arrangement provides a toggle action. In the inoperative position shown in Fig. 13, a cam 146, at the end of the delivery stroke of the trolley, has engaged a stop-pin 149, the shaft 145 has been rotated carrying the links 144, and the clamp 140 moved away from the anvil 138. In the operative position shown in Fig. 14, a cam 146, at the end of the stroke towards the feed-table, has engaged a lever 147a (not shown) to cause the toggle mechanism to snap back and the clamp 140 to grip the leading edge of a sheet 148 resting on the printing table 2. The sheet 148 is gripped before and during the printing operation by the clamp 140. The delivery trolley starts its delivery stroke as the screen corner 7 rises, and there is a progressive peeling of the printed sheet away from the screen. Trolley drive (first form).-The feed and delivery trolleys 20, 25 are reciprocated in opposite directions with a substantially harmonic motion, gathering speed progressively and then decelerating progressively. The feed trolley drive tends to urge this trolley beyond the limits of its travel, and incorporates a lostmotion or overdrive clutch. The delivery drive sprocket 27, Fig. 4, is keyed to the drive-shaft 29, but the feed drive sprocket 23 is rotatably mounted on this shaft and carries a pair of arms 33 which are urged together by springs to hug an abutment on a disc 31 keyed to the shaft 29 to provide a resilient coupling. The shaft 29 is driven by epicyclic gearing 50 operated from the main cam-shaft 14 by means comprising a pair of cams 41, 42, Fig. 1, engaging rollers 43, 44 on a bell-crank lever 45 to one arm 47 of which are pivoted two links 48, 49. The link 48 is pivoted at 60 to an internally-toothed annulus 55, and the link 49 is pivoted at 62 to an arm 63 extending from a planet carrier 57 which carries a pair of planet-wheels 58 engaging the teeth of the annulus 55. A sun-wheel 59, engaged by the planet-wheels, is keyed to the shaft 29. Small upward or downward movements of the bell-crank lever 45 cause several clockwise or counterclockwise revolutions of the shaft 29. Trolley drive (second form).-In the construction shown in Figs. 16, 19 and 24, the feed table 203, printing table 202, and delivery table 204 are similar to those described above, but the sheets are fed to, and delivered from, the printing table by bars 300, 301, respectively, provided with teeth meshing with pinions 306 rotatably mounted on a shaft 308. The feedbar 300 is mounted in a trough 304 beneath a slot in a side portion of the table 203, and the delivery bar 301 is of T-section mounted on rollers 305 beneath slots in the tables 202, 204. The pinions 306 are driven from the shaft 308 through a slipping clutch. The drive to the shaft 308 includes a crank disc 310, on the main drive shaft, having a slide-block 312 on a crankpin, the block engaging in a slot in a plate 314 slidably mounted in vertical guides 315. The plate 314 is reciprocated with a harmonic motion on rotation of the drive-shaft. Links 316, 321 are pivoted to the plate 314 and to a planetary spread-increasing gear means for driving the shaft 308. A link 317 (not shown), is secured to a gear-wheel freely rotatable on the shaft 308 within a casing 319 (not shown) also rotatable on this shaft. The link 321 is pivoted to the casing 319 which carries a countershaft on which are fixed a pinion and a gear-wheel, the gear-wheel engaging a pinion fixed to the shaft 308, and the pinion meshing with the first gear-wheel. Sheet-registering and guiding means.-As described above, in the machine shown in Fig. 1, the sheets are registered at the front by a vertically-movable stop 80, and at the side by the fixed bar 77. In the machine shown in Fig. 16 there is a fixed side register bar 360 (not shown) at the side of the feed table 203, and a vertically movable front stop 365 operated by a cam on the delivery bar 301. A retractable side guide 361, Fig. 24, is also provided at the side of the printing table 202, and comprises a plate hinged at 362 and urged upright by a spring 363. On the descent of the frame 209 carrying the stencil screen, a lateral arm 364 on the guide 361 is engaged by the frame to depress the guide and so allow the screen to contact the paper sheet. Specification 719,858 is referred to.