EP0089080B1

EP0089080B1 - Sheet-holding apparatus

Info

Publication number: EP0089080B1
Application number: EP19830200310
Authority: EP
Inventors: Leo Norbert Vackier
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1982-03-16
Filing date: 1983-03-04
Publication date: 1986-07-09
Also published as: CA1198751A; DE3364404D1; EP0089080A1; JPS58183555A

Description

This invention relates to sheet-holding apparatus comprising a rotatable sheet holder having a cylindrically curved surface for supporting a sheet of flexible material concentric with the axis of rotation of the holder and comprising mechanical fastening devices for engaging the leading and trailing margins of the sheet and holding it taut against said curved supporting surface.
Rotatable sheet holders are used in various kinds of reprographic processes in which it is necessary for successive zones along a sheet of flexible material to be brought successively to a site where a step in the formation of a graphic image on the supported sheet is performed. Examples of such apparatuses are described in US-A-1,967,266, US-A-2,966,848 and US-A-401,815.
In these prior art apparatuses the sheet holder comprises a rotatable, cylindrical surface to which the sheet is held by mechanical clamps associated and rotating with the cylindrical surface.
The cylindrical supporting surface is a two-part surface with segmental peripheral portions of comblike structure with intermeshing teeth, which are relatively adjustable thereby to accommodate different sheet formats. The application of a new sheet to the sheet holder requires the rotatable surface to be arrested in order that the operator be able to properly apply manually the new sheet onto the holder.
It is the object of the present invention to provide an apparatus which permits the automatic gripping of a new sheet by a rotatable sheet holder, without need for the holder to become temporarily arrested for the carrying out of such gripping of a sheet and for the application thereof onto the sheet supporting surface.
According to the present invention, a sheet-holding apparatus comprising a rotatable sheet holder having .a two-part cylindrically curved supporting surface with segmental peripheral portions of comblike structure with intermeshing teeth, said sheet holder being rotatably driven in a circular path about its axis for supporting a sheet of flexible material concentric with the axis of rotation of the holder and a mechanical leading clamp and trailing clamp associated with said supporting surface at the respective leading and trailing ends thereof for engaging the leading and trailing margins of the sheet and holding it taut against said curved support surface, is characterized in that it comprises arms supporting the leading clamp and secured to a shaft pivotally mounted in the sheet holder so that the leading clamp is connected to said holder for limited pivotal movement relative thereto, control means for controlling the angular position of said shaft in such a way that said shaft is rotated in the same direction of rotation as that of the holder during the last part of a revolution of the holder, whereby the leading clamp is advanced away from the leading end of the sheet supporting surface preparatory to the gripping of a sheet at a pick-up position, that said shaft is counter-rotated at the initial part of a revolution of the sheet holder for the gripping of the sheet whereby, as a consequence of the counter-rotation of said shaft about its axis during the continued angular displacement of said shaft about the axis of the sheet holder, the leading clamp remains at said pick-up position and moves slightly radially outwardly towards a sheet awaiting pick-up at that position, means which automatically opens and closes the leading clamp at said pick-up position so that such clamp can take hold of the margin of the sheet where this is held ready for pick-up, and means which automatically subsequently opens and closes the trailing clamp to permit it to take hold of the trailing margin of such sheet.
Apparatus according to the invention can be used in various kinds of printing machines including offset and xerographic printers.
A particularly important field of use for the invention is in printing plate-making machines, for holding the metal plates during transfer thereto of an image to be printed.
The sheet holder can for example easily be made to a size and with an adjustment range suitable for holding sheets ranging in size from A4 to A1. The holder can be infinitely adjustable within a given range for holding a sheet of any dimension within that range or the holder can be adjustable to suit two or more specific sheet dimensions in a given range.
In preferred embodiments of the invention there is a mechanism which causes the angular spacing of the leading and trailing fastening devices to increase slightly at a stage in the operating cycle such as to cause tensioning of a sheet held by such devices. In this manner tautness of a sheet on the cylindrically curved supporting surface can be ensured automatically and without need for a sheet to be rolled or otherwise progressively pressed against the supporting surface after the leading margin of the sheet has been secured to the holder and before its trailing margin is secured.
It is preferable to employ leading and trailing clamps each of which comprises a series of jaws distributed in spaced relation along the transverse dimension of the holder (the dimension parallel with its axis of rotation). This feature is conducive to firm holding of a sheet across its entire width. In certain apparatuses according to the invention the means for closing the jaws operates to close the inner jaws onto a sheet margin before the outer ones. By this means risk of any lateral displacement of a sheet margin during application of the clamping forces thereto, and the unsatisfactory clamping of sheets the leading and/or trailing margin of which is buckled in the transverse direction, is avoided or reduced.
The invention includes a printing apparatus comprising a sheet-holding apparatus with an automatic sheet pick-up facility as above referred to, means for supporting flexible sheets in a position near the path followed by the periphery of the sheet holder for take-up by the leading clamp of the holder and means for image-wise transferring material to a said sheet while it is held by and rotates with said sheet holder.
Reference will now be made to the accompanying drawings in which a particular sheet-holding apparatus according to the invention, selected by way of example, is shown incorporated in a xerographic platemaker.
In the drawings:

Fig. 1 is a perspective view of a multiple-size offset platemaker,
Fig. 2 is a side view of the sheet-holding apparatus according to the invention adjusted for holding large sheets,
Fig. 3 is a side view of the sheet-holding apparatus adjusted for holding smaller sheets,
Fig. 4 shows a detail of the rotatable holder,
Fig. 5 shows a detail of the cam mechanism for operating the sheet clamps and
Fig. 6 is a section, in various planes of the sheet-holding apparatus.

The platemaker 10 (Fig. 1) comprises a housing 11 in which the components of the apparatus are located. Inside the housing 11 is provided a holder 12 for an original to be copied. Through an objective 13 an image of the original is projected onto a charged photoconductor 14 which is held by a support 15a. The support 15a as illustrated stands upright in the image plane of the objective 13.
After exposure, the support 15a is swung into a horizontal position around axis 16 and when horizontally oriented, the support is held and fixed in a frame 17 by known means.
Frame 17 is supported by a plurality of air bearings 18 that move over and along slides 19 to have the photoconductor 14 submitted to a wet processing and image transfer.
Liquid toner development of the photoconductor 14 is carried out at processing station 20 comprising a plurality of slotted processing chambers 21. A rinsing chamber 22 is used to remove toner particles and other hydrophobic material which were not attracted by the electrostatic charge on the photoconductor but deposited on the non image-areas of the photoconductor 14. The processing station 20 comprises in addition a supply station 23 for processing liquid and rinsing solution and associated pump means (not shown).
Slidingly arranged upon slides 19 is a second support 15b which is analogous in structure to support 15a, in that it is likewise used to support a photoconductor (not shown) which is supplied by a photoconductor supply station 24. The supply is represented as being in the form of a roll, but a supply of photoconductors in sheet form may of course be used.
In order to permit both supports 15a and 15b to perform a to-and-fro motion on slides 19 without obstructing the passage to one another, a support lifting mechanism in the form of hooks 25, linked via rods 26 to eccentrics 27 is provided. This mechanism causes support 15b, carrying a photoconductor, to pass over support 15a carrying the exposed photoconductor 14 which is in the course of being processed.
After processing of photoconductor 14 on support 15a, support 15b is lowered and passed over a charging station 28 in order to charge the photoconductor on that support prior to its image-wise exposure.
The developed photoconductor 14 is subsequently transported through a transfer station which is in the region of the machine indicated by arrow 29. At this region there is a receptor sheet holding apparatus according to the present invention, including a rotatable sheet holder 30. Also at that region there are a number of aluminium plate storage racks 29a, and a plate take-up station 29b. A transfer mechanism comprising a set of sucker cups 29c operates in dependence on a plate size selector mechanism (not shown) to remove an aluminium plate of selected size from the corresponding storage rack 29a and to transfer that plate to the take-up station 29b where the sheet is held with one edge at a predetermined position (P-U in Fig. 2) ready for gripping and withdrawal by the sheet holder 30.
After an aluminium plate has become transferred to the holder 30 as hereafter described, transfer of toner image from the developed photoconductor 14 takes place. For this purpose the said photoconductor is transported through the transfer station along a path such that the surface bearing the liquid toner travels very close to but just out of contact with the surface of the receptor sheet on the sheet holder 30, the latter being rotated in synchronism with the advance of photoconductor 14 on support 15a. Actually, the surface of the receptor sheet just makes contact with the liquid toner on the photoconductor, but not with the photoconductor surface. The liquid toner image transfers to the aluminium plate held by the sheet holder 30. An electrical potential gradient is applied by a voltage source and electrical conductors (not shown) between the conductive backing of the photoconductor and the receptor sheet holder 30 in order to effect this transfer. Toner transfer in an electric field is well known in electrophotography and needs no further description. After transfer and fixing of the toner, the aluminium plate is released from the holder 30 and it can be received e.g. in a collecting tray. A further aluminium plate can then be taken up by the holder 30 ready for receiving a toner transfer image from a photoconductor, which may be another photoconductor transported to the transfer station or it can be the photoconductor 14 after it has been conditioned for reuse.
The sheet-holding apparatus according to the invention will now be described with reference to Figs. 2 to 6.
Referring particularly to Figs. 2 and 3: the rotatable sheet holder 30 comprises a shaft 31 which is rotated by a motor (not shown) and which carries two relatively angularly displaceable holder parts 32, 33 having segmental peripheral portions 34, 35. The parts 32, 33 have counter-weights 45, 46 to balance the weights of the segmental portions 34, 35.
As shown in Fig. 4, which is a diagrammatic plan view of the sheet supporting surface of the sheet holder 30, the said peripheral portions 34 and 35 of the sheet holder are of comb-like form with intermeshing teeth 96, 97 respectively and said portions together provide a cylindrically curved sheet- supporting surface 39 for supporting one of the aluminium plates indicated by the rectangle 99 in broken lines in Fig. 4. During rotation of the shaft 31 this surface 39 follows the circular path represented in broken line.
The teeth 96, 97 on each of the portions 34, 35 have a width of about 8 mm and are spaced by gaps 12 mm wide. Each of said portions has fourty- two teeth. The overal width of the support surfaces formed by the two segmental portions is about 1.24 metres and its radius of curvature is about 30 cm. In its setting providing the longest arc of sheet- supporting surface (Fig. 2) such surface is large enough to support a plate having the dimensions of e.g. a large opened newspaper.
In order to permit the relative angular displacement of the holder parts required for changing the sheet size setting of the apparatus, holder part 32 is rotatably mounted relative to the shaft 31 which part 33 is fixed relative to that shaft. The mechanism for bringing about such relative angular displacements of the holder parts will now be described.
The holder part 33 which is positively connected to the driving shaft 31 supports a motor 42 and the output shaft of this motor is connected by a chain and sprocket coupling 44 with the shaft 43 of a worm 40 which is rotatably supported in bearings which are also carried by the said holder part 33. The worm 40 is in meshing engagement with a worm gear 41 connected to the holder part 32. The worm gear 41 is capable of being immobilised relative to shaft 31 by a locking mechanism which will presently be described. Operation of the motor 40 while the worm gear 41 is locked to the shaft 31 causes the motor 40, the worm and the associated coupling to move bodily around the periphery of the worm gear 41, and the holder part 33 moves as a unit with such motor-worm assembly. Consequently the segmental comb-like portions receive a relative movement of recession or approach, according to the direction of rotation of motor 40, thereby lengthening or shortening the arcuate extent of the sheet supporting surface 39. Such size adjustments can be effected while the sheet holder is stationary, i.e. when the shaft 31 is not being driven, or while the holder is rotating.
For gripping opposed end margins of a sheet and holding it against the curved sheet supporting surface 39 the sheet holder is provided with leading and trailing clamps 58, 48 respectively. The trailing clamp 48 will be described first.
The trailing clamp comprises a series of jaws distributed in spaced relationship across the sheet holder. Each of these jaws comprises a fixed jaw member and a pivoted jaw member. A description of one of the jaws will suffice, the others being similar. Referring to Figs. 2 and 3, the fixed jaw member is a bar 47 which is secured to the holder part 32 and has a tapered outer end portion 102 (Fig. 4) for contacting the trailing margin of a sheet to be held. The pivoted jaw member is an arm 36 which is rotatable about shaft 49 supported by the bar 47 and which is shaped at one end to cooperate with the tapered end portion of the fixed jaw mem ber 47. Arms such as 51 are secured to the shaft 49 and carry rollers 50 which at certain positions in the course of one revolution of the sheet holder are contacted and depressed by fixed cams 52 and 53. The arms 51 are connected to a transverse bar 55 which covers all of the pivoted jaw members 36 so that the said displacements of the arms 51 by the cams 52 and 53 causes contacting ofthetrailing ends of each of the jaws 36 by the bar 55 and thereby simultaneous pivotal movement of all of the pivoted jaws 36 into their open positions. This opening movement takes place against the biasing force of springs such as 54. These springs are located by pins carried by a transverse bar 56 and the springs bear at one end against that bar and at their other ends against plates such as 57 connected to the pivoted jaw members.
The leading clamp 58 likewise comprises a series of jaws distributed in spaced relation across the sheet holder. One of these jaws will be described with reference to Figs. 2 and 3. The other jaws are similar. The jaw comprises co-operating jaw members 37 and 100. The jaw member 100 is a bar support between two arms 60a, 60b which are secured to a shaft 59 pivotally mounted in part 33 of the sheet holder. The outer end of shaft 59 carries an arm 61 with a roller 62 which is permanently engaged in a cam groove 91 (Figs. 5 and 6) in a cam plate 90 secured to the mounting frame (not shown) of the rotatable sheet holder. The jaw members 37 are pivotally connected to jaw member 100 by a shaft 66 fitted between the arms 60a and 60b. The ends of jaw members 37 remote from the jaw mouth extend below a transverse bar 67 which is fixedly connected to the outer pivoted jaw members of the leading clamp. The several pivoted jaw members are biased towards closed position by springs such as 63 guided on pins 64 which are secured to shaft 59. The springs bear against plates such as 65 connected to the pivoted jaw members.
In addition to the sheet holder parts 32, 33 the main shaft 31 carries near one end thereof a radial plate 69 which rotates with such shaft (see Fig. 2). Near its outer end the plate 69 carries a bar cam 70 which is pivotally connected to the plate by a pivot pin 71. The cam 70 is loaded by a spring 72 which urges the cam into its position shown in Fig. 2 in which one end of the cam bears against a stop 74. Cam 70 co-operates with the bar 67 on the pivoted jaw members 37 to cause opening of the leading clamp jaws at the commencement of an operating cycle of the apparatus as hereafter described.
When the shaft 31 commences to rotate (clockwise in the aspect of Figs. 2 and 3 at the commencement of the cycle to carry the sheet- supporting surface 39 towards the pick-up position P-U where the leading clamp 58 is located, the roller 62 on arm 61 is in the most radially inward region of cam groove 91 shown in Fig. 5 and as the shaft 31 rotates the steep portion of the cam groove, which is within the angle marked P in Fig. 5, causes the roller 62 to be pushed radially outwardly away from the main shaft 31. This displacement of roller 62 rotates shaft 59 anticlockwise in the aspect of Fig. 2. If this were the only movement imparted to that shaft, the leading clamp jaws would be rocked anticlockwise about shaft 59. However, because the shaft 59 is pivotally mounted in the holder part 33, the shaft 59 is driven bodily clockwise around the axis of the main shaft 31. The result of the combination of this bodily movement of shaft 59 with its anticlockwise rotation about its own axis is that the outer ends of the jaw members 37, 100 remain at the pick-up position P-U but move slightly radially outwardly towards the sheet 38 awaiting pick-up at that position.
During the said radially outward movement of the outer ends of the jaw members 37, 100 the cam 70 is being carried clockwise about the axis of shaft 31 by the radial plate 69 to which such cam is pivoted and the tapered end portion of the cam 70 moves into contact with the bar 67 and displaces it towards the shaft 31. The cam accordingly causes clockwise pivotal movement of the jaw members 37 into their open positions. In consequence of the above described radially outward movement of the outer ends of the jaw members 100 and 37 and the pivotal opening movement of jaw members 37 the open jaw mouths pass over the adjacent margin of the sheet 38 at the pick-up position P-U. As the high part of the cam 70, which in the aspect of Fig. 2 coincides with the bottom of the cam pivot pin 71, passed beyond the bar 67, the compression springs 63 cause the pivoted jaw members 37 to close onto the margin of the sheet 38. In this movement the bar 67 which is connected to such jaw members 37 presses against the cam 70 on what in the aspect of Fig. 2 is the left hand side of the cam pivot and swings this cam into a position in which its lefthand end portion abuts against a stop 73. In the course of this movement the bar 67 rides along the concave portion of the cam 70. Eventually the cam 70 is carried by the plate 69 out of range of the bar 67 and the cam springs back to its original position against stop 74 under the action of the cam return spring 72.
Movement of the bar 67 thereby to set free the jaw members 37 and to permit them to rock into their closed position by their biasing by the springs 63 causes the two outer jaw members to close last, since it is only after said inner jaw members will have been closed, that the bar 67 looses contact with said jaw members and the outer jaw members, upon further rocking over some angular degrees only, will close. A further advantageous step consists in extending the described differential operation of the jaw members over all the jaw members, for instance by providing the trailing ends of all but the two outer jaw members 37 with a small set screw that abuts against the bar 67 that withholds the members. Minimum height of a set screw ensures earlier engagement of a jaw member with the sheet on the sheet holder, whereas a greater height of a set screw demands a further displacement of the bar 67 before operative engagement of the jaw members occurs. By suitably increasing the set length of the set screws from the center towards the outer extremities of the bar 67, it may be obtained that engagement of the leading sheet margin occurs progressively from the central region towards the ends of such margin. The described procedure may be advantageous in the gripping of less flat sheet margins. The same arrangement as described hereinbefore may be used for controlling the clamps for gripping the trailing margin of the sheet.
Immediately after closure of the leading lamp 58 on the adjacent margin of sheet 38 at the pick-up position, the leading end of the segmental portion 35 reaches the pick-up position and abuts against the jaw member 100 of the leading clamp 58. As shown in Fig. 3, depicting a situation in which the sheet support has been adjusted to its minimum size and in which the sheet support has reached this point in its cycle, a shock absorber 81 may be provided, e.g. on one of the jaw members 60a or 60b in order to prevent metal to metal impact of the segmental portion 35 against such jaw members. Thereafter the leading clamp jaws 37, 100 rotates bodily around the axis of the driving shaft 31 together with the holder part 32, 33 and the sheet 38 is drawn from the pick-up position and becomes progressively supported by the rotating arcuate support surface 39.
Because the holder parts 32, 33 have been adjusted as hereinbefore described to make the support surface 39 of the correct length for the sheet 38 to be supported, the trailing margin of sheet 38 falls onto such support surface in the vicinity of the trailing clamp 48 and immediately thereafter the arms 51 operating the pivoted jaw members 36 of the trailing clamp are momentarily depressed by fixed cam 53 causing the trailing clamp jaws to open and then close onto the trailing margin of the sheet 38.
The next operation in the machine cycle is the tensioning of the sheet held on the support. This is effected by closing an electric circuit energising electromagnet 75. This causes a small displacement of link arm 76 secured to a shaft 77, and of a short link arm 78 which is also secured to such shaft. The arm 78 carries a roller 79 which engages in a fork 80 secured to the worm gear 41. It is this engagement which immobilises the worm gear 41 relative to shaft 31 during the size adjustments of the sheet support as hereinbefore described. The slight turning movement of shaft 77 caused by energisation of the electromagnet 75 effects a slight turning movement (anti-clockwise in the aspect of Fig. 2) of the worm gear 41 and consequently of the holder part 32 and the trailing clamp 48 which is secured to that part. This movement tensions the held sheet against the arcuate support surface 39. The electromagnet is kept energised, maintaining the tension on the sheet, until after the sheet has been carried through the image transfer station and is ready to be released. In order to ensure that the leading edge of the held sheet is not displaced during the tensioning step, the spring loading on the pivoted jaw members 37 of the leading clamp is made stronger than the spring bias on the pivoted jaw members 36 of the trailing clamp. If the tensioning force is sufficient to overcome the frictional grip exerted by the trailing clamp jaws the trailing margin of the sheet will slide relative to such jaws but the position of the leading edge of the sheet will remain unchanged.
The clamp jaws can have a high coefficient of friction with respect to the material of the sheets to be held.
After the sheet holder has been rotated through nearly 360° the clamping jaws are opened to release the sheet from the holder. The opening of the jaws is effected by fixed cams. At an appropriate position along the path followed by the transverse bar 67, this bar encounters a fixed cam which depresses that bar, thereby pivoting the jaw members 37 of the leading clamp into open position. This cam is not shown in Fig. 2 but it is shown in Fig. 3 which shows the apparatus parts at the moment the jaw members are opened. The opening of the trailing clamp is effected by the fixed cam 52 which by displacing arms 51 and bar 55 causes the jaw members 36 to swing open. The release of the leading and trailing margins of the sheet allows the sheet to fall into a receiving zone for collection and further processing, e.g. for treating the sheet to make it image-wise receptive of printing ink.
During the major part (about three-quarters) of the revolution of the sheet holder, the follower roll 62 is in that part of the cam groove 91 (Fig. 5) which is concentric with the main shaft 31 and during that part of the revolution the leading clamp is therefore in juxtaposition to the leading end of the segmental portion 35 of the sheet holder. Before the segmental portions 34, 35 arrive back at their position illustrated in Fig. 2 the follower roll 61 enters the sector corresponding with the angle marked a in Fig. 5 and during the last part of the revolution the said follower roll 62 is therefore displaced progressively inwardly towards the shaft 31. This displacement of the follower roll causes the entire leading clamp assembly to rock (clockwise in the Fig. 2 aspect) around the axis of shaft 59. The outer ends of the leading clamp jaws are thereby advanced away from the segmental portion 35 of the sheet support and they reach the pick-up position P-U as the revolution is completed and the apparatus is ready for a repetition of the cycle for handling a further sheet. The rotation of the sheet holder can be performed continuously, i.e. without stopping the rotation between successive revolutions or the holder can be rotated step-wise, one revolution or part of a revolution at a time.
The apparatus could be modified in regard to the control cam means to permit a held sheet to be retained on the rotatable support during more than one revolution, e.g. to permit take-up of the sheet by the support during one revolution and transfer of an image to the sheet during the following revolution.
If required two cam plates such as 90 can be provided, one at each end, of the sheet holder and one being a mirror image of the other. The or each cam plate can be secured to the supporting frame of the rotatable sheet holder by screws 92.
Fig. 6 shows the axial distribution of the various components already referred to. The left- hand side of the figure shows the apparatus parts involved in the clamping of the leading margin of a sheet whereas the right hand side of the figure shows parts involved in the clamping of the trailing margin of the sheet.
Fig. 6 also shows means which is provided for taking up any axial play of the main shaft 31. This means is provided at what in the aspect of Fig. 5 is the left-hand end portion of the shaft and comprises nested bushings 93, 94 housing a compression spring 96. The bushing 93 is secured in the fixed cam plate 90 and the outer race of a shaft bearing 95 fits slidingly in said bushing. The inner race of the bearing is secured to the shaft. The compression spring 96 bears at one end against the flange of the bushing 93 and at its other end via the bushing 94 against the outer race of the shaft bearing 95 so that the spring constantly urges the shaft 31 to the left, so ensuring that any axial play which might otherwise occur, e.g. as a consequence of wear, is automatically taken up. The shaft is axially withheld by the bearing 101 at the opposite side. It is thereby ensured that a sheet held by the rotatable sheet holder 30 will receive the transfer image at a predetermined position on the sheet. This is a particularly important factor if for example a plurality of toner images have to be transferred to the sheet in register.
While the sheet-holding apparatus has been illustrated as employed in a xerographic platemaker, the apparatus can be employed in other reprographic work. The sheet holder can be used not only for holding metal plates e.g. plates of aluminium or copper, but also for holding sheets of various other materials, e.g. sheets of paper, polymeric material, silver halide sheets materials, or sheets comprising inorganic or organic photo-conductors.
The leading clamp 58 may comprise jaws that are arranged for engaging the leading sheet margin through holes that are provided in said margin, rather than by friction as illustrated in the figures. In that way, a more positive hold of the leading sheet margin and a better registering of the sheet, may be obtained.
The angular adjustment of the sheet holder 30 in order to accommodate different sheet formats, may also occur otherwise than by a worm and wormgear. For instance, the interior of the sheet holder may be provided with two linear air- motors that are fitted together with their cylinders in back-to-back relationship, and the pistons of which have different stroke lengths. One piston rod is connected to one holder part 32, and the other piston rod is connected via suitable links to the other holder parts 33. By actuation of either one of the motors, or of both motors simultaneously, three different end-to-end rod lengths may be obtained, and thereby three corresponding relative positions of the plate holder.

Claims

1. A sheet-holding apparatus comprising a rotatable sheet holder (30) having a two-part (32, 33) cylindrically curved supporting surface with segmental peripheral portions (34, 35) of comb- like structure with segmental peripheral portions (34, 35) of comb-like structure with intermeshing teeth (96, 97), said sheet holder being rotatably driven in a circular path about its axis (31) for supporting a sheet (38) of flexible material concentric with the axis of rotation of the holder, and a mechanical leading clamp (58) and trailing clamp (48) associated with said supporting surface at the respective leading and trailing ends thereof for engaging the leading and trailing margins of the sheet and holding it taut against said curved supporting surface, characterized in that is comprises arms (60a, 60b) supporting the leading clamp (58) and secured to a shaft (59) pivotally mounted in the sheet holder (30) so that the leading clamp (58) is connected to said holder (30) for limited pivotal movement relative thereto, control means (61, 62, 91) for controlling the angular position of said shaft (59) in such a way that said shaft (59) is rotated in the same direction of rotation as that of the holder during the last part of a revolution of the holder, whereby the leading clamp (48) is advanced away from the leading end of the sheet supporting surface preparatory to the gripping of a sheet (38) at the pick-up position (P-U), that said shaft (59) is counter-rotated at the initial part of a revolution of the sheet holder for the gripping of the sheet (38) whereby, as a consequence of the counter-rotation of said shaft about its axis during the continued angular displacement of said shaft (59) about the axis (31) of the sheet holder, the leading clamp (48) remains at said pick-up position (P-U) and moves slightly radially outwardly towards a sheet (38) awaiting pick-up at that position, means (70, 67, 63) which automatically opens and closes the leading clamp (58) at said predetermined region so that such clamp can take hold of the margin of the sheet (38) where this is held ready for pick-up, and means (52, 53, 55) which automatically subsequently opens and closes the trailing clamp (48) to permit it to take hold of the trailing margin of such sheet.

2. Apparatus according to claim 1, wherein there is mechanism (75, 76, 78, 80, 41, 40) which causes the angular spacing of the leading and trailing clamps (58, 48) to increase slightly after the gripping of a sheet (38) such as to cause tensioning of the sheet gripped thereby.

3. Apparatus according to claim 2, wherein said mechanism for causing tensioning of a sheet comprises an electromagnet (75) which functions through mechanical coupling means (76, 78, 80, 41, 40) to cause partial rotation of one said holder part (32) together with the corresponding sheet clamp (58).

4. Apparatus according to claim 1, wherein each of the leading and trailing clamps (58, 48) comprises a series of jaws (36-47; 37-109) distributed in spaced relation across the holder (30).

5. Apparatus according to claim 4, wherein said jaws (36, 37) are independently biased by springs (63, 54) towards closed position.

6. Apparatus according to claim 4 or claim 5, wherein the means (55, 67) for closing the jaws operates to close the inner jaws of each series before the outer one.

7. Apparatus according to any of claims 1 to 6, wherein the size and adjustment range of said sheet holder (30) are such that the supporting surface is capable of holding sheets ranging in size from A4 to A1.

8. Printing apparatus comprising a sheet-holding apparatus according to any of claims 1 to 7, means (29a) for supporting flexible sheets in a position near the path followed by the periphery of the sheet holder (30) for take-up by the leading clamp (58), and means (15a, 15b) for conveying image-carrying material to the vicinity of said sheet holder (30) for image-wise transfer of the image to a sheet (38) while it is held by and rotates with said sheet holder (30).