Method for mounting a plate on a rotatable drum

Abstract: There is disclosed a method for mounting a plate (18) on the cylindrical surface (20) of rotatable drum (10) having circumferential grooves (12) cut into the surface of drum (10). Two groups of clamps (14,16) are provided and at least one group of clamps (16) is slidable in grooves (12). Both types of clamps (14, 16) are switchable by release means (66) from a release state to a clamp state. The method includes sliding the second group clamps (16) and aligning them along a line essentially parallel to the axis of drum (10) and at a distance from first group of clamps (14) which is greater than the length of plate (18) to be mounted on drum (10), switching the first group clamps (14) to the release state and feeding a plate (18) to drum (10) to meet the first group clamps (14), switching the first group clamps (14) to the clamp state thus clamping the leading edge of the plate (18) to the drum surface (20), rotating drum (10) in a first direction to cause the plate (18) to be attached to the drum surface (20), the rotation stops when the second group of clamps (16) is in position to be activated by release means (66), and switching the second group clamps (16) to the release state and rotating drum (10) in a second direction to bring the second group clamps (16) to the trailing edge of plate (18) where the clamps (16) are switched to the clamp state terminating the clamping process of plate (18) to the drum surface (20).

Description: The present invention relates to a method for attaching a plate to a rotatable drum and to a method for attaching plates of various sizes to the drum of a so-called external-drum platesetter that can operate at very high rotation speeds. Imagesetters, namely marking engines for plotting images on plates or sheets of film, of the so-called external-drum type, are known in the art. Typically, a sheet is attached to the outer surface of a rotating drum and held in contact with the surface by a vacuum system, which has orifices distributed over the surface. Often, such an imagesetter is required to handle sheets of various sizes. U.S. Pat. No. 5,383,001 to Bosy addresses such a need, by providing a suitable design of the vacuum system. External-drum platesetters are a particular category of external-drum imagesetters, wherin the image is plotted on a light-sensitive printing plate. Printing plates are generally made of metal and therefore they are stiffer than films and their mass per unit area is much higher. Thus, when a plate is attached to the drum, it requires a greater force to keep it in contact with the drum's surface at any given rotational speed, to counteract the centrifugal force, than is required by a film. Consequently, a vacuum system to hold plates on the rotating drain must be more efficient and more powerful than that used for holding films. However, even with a powerful vacuum system, there is a danger of failure, due to a bend or a bulge at the edge of the plate. Moreover, since in a vacuum system the forces holding the plate are exerted by atmospheric pressure, they have an upper limit. Above a certain rotational speed the centrifugal force per unit area exceeds atmospheric pressure and any plate attachment system solely based on vaccum would then totally fail. Such failures may cause the entire plate to fly off the drum by centrifugal action - which would be disastrous for the entire machine. Therefore platesetters require mechanical attachment means - either to assist the vacuum system and guard against drastic failure, or as the main holding means, possibly still aided by vacuum. One common and useful such means is clamps mounted on the drum and operative to hold down the edges of the plate. Clamps and grippers for holding a plate onto a rotatable drum have been known in the art. For example, clamps (also called grippers) are generally used in offset printing presses to hold the printing plate onto a cylinder. During operation, centrifugal forces act to cause a pulling force on each edge of the plate, such that tends to pull the edge from the clamp. Therefore in printing presses, clamps are designed to exert enough gripping force to exceed that required for countering the centrifugally induced pulling force, at maximum expected rotational speed, by a safety margin. Such clamps are disadvantageously cumbersome for mounting on platesetters and, moreover, difficult to activate (i.e. to open for accommodating the plate), because they must include either a heavy bolting or leveraging mechanism or a very strong spring-loading mechanism. A clamp on a printing cylinder must also exert a positive stretching force on the plate, in order to assure contact between the plate and the cylinder over its entire surface. U.S. Pat. No. 3,203,074 describes an improved clamp, in which the gripping and stretching forces increase by centrifugal action in proportion to the rotational speed. A clamp with such a centrifugal action (also termed dynamic clamp) may overcome the disadvantages discussed in the previous paragraph. U.S. Pat. No. 4,250,810 discloses a further improved clamp with centrifugal action, whereby the gripping and stretching forces can be independently adjusted. It is noted that in platesetters, there usually is also a vacuum system, which assures contact between the plate and the drum over its entire surface, and that therefore no stretching force, but only a gripping force, is required at the edges of an attached plate. U.S. Pat. No. 5,335,046 discloses a clamp for the drum of a platesetter with a mechanism that centrifugally induces a gripping force. There is a row of such clamps for the leading edge and another one for the trailing edge. The mechanism includes a weight, connected to the clamp by a rod, located inside the drum near the opposite surface. The clamp is opened by means of a piston and cylinder assembly, activated by supplied pressurized air. The clamp mechanism of the '046 patent has two serious drawbacks: (1) It is relatively complicated, with a relatively large number of parts, and thus is expensive to manufacture. (2) It does not conveniently accommodate plates of various sizes. The '046 patent does disclose the possibility of providing a row of trailing edge clamps for each plate size; however, such an arrangement is even more expensive and, furthermore, does not accomodate small variations in plate size. The secondly listed drawback also applies to other types of clamps of prior art, such as those discussed hereabove. There is thus a widely recognized need for, and it would be highly advantageous to have, a clamping system, for attaching a plate to a rotatable drum, that is dynamic, i.e. has a centrifugally induced gripping action, is adaptable to any plate size and easy to operate and is of simple design, having a small number of parts. US-A-3001472 discloses a printing roll assembly capable of carrying a variable number of printing plates. The assembly includes a printing roll having a plurality of parallel, circumferential grooves. The printing plates are provided with scarfs at each end of the plate and lying parallel with the axis of the cylinder and overlying one of the grooves. Hooks are disposed within the scarfs detachably connected to carriages independently moveable within the grooves. Adjustment of the hooks exerts tension forces on the plates when mounted at any place on the circumference of the printing roll. The invention provides a method for mounting at least one plate on the cylindrical surface of a rotatable drum having circumferential grooves cut into the surface of the drum and two groups of clamps where at least one group of clamps is slidable in grooves and both types of clamps are switchable by release means from a release state to a clamp state, characterized in the following steps:- a) sliding the second group clamps and aligning them along a line essentially parallel to the axis of drum and at a distance from first group of clamps which is greater than the length of plate to be mounted on drum; b) switching the first group clamps to the release state and feeding a plate to drum to meet the first group clamps; c) switching the first group clamps to the clamp state thus clamping the leading edge of the plate to the drum surface; d) rotating drum in a first direction to cause the plate to be attached to the drum surface, the rotation stops when the second group of clamps is in position to be activated by release means; e) switching the second group clamps to the release state by engaging with said release means so as to prevent the clamps from gripping, and f) rotating drum in a second direction to slide the second group clamps engaged with said release means to the trailing edge of plate where the clamps are switched to the clamp state terminating the clamping process of plate to the drum surface. The release means may switch the clamps to the release state by moving inward essentially perpendicularly to engage grooves of clamp body section and moves outward to switch the clamps to the clamp state. The force clamping the plate to the drum surface exerted by the clamps in the clamp state may be partly derived from centrifugal effect. The force clamping plate to the drum surface exerted by clamps in the clamp state may be partly derived from centrifugal effect and partly from spring action. According to a refinement of the method, any of the clamps in the second group is operative to grip an edge of the plate by means of pressure that is partly derived from action of a spring and partly - from centrifugal effect. BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is an isometric drawing, schematically showing a drum with the clamping system of the present invention; FIG. 2 is a cross-sectional view of the system of Fig. 1; FIG. 3 is an isometric drawing, schematically showing a portion of the drum of Fig. 1 with slidable clamps according to the present invention; FIG. 4A is a side view and cross-sectional view of a slidable clamp according to the present invention; FIG. 4B is a top view and sectional side view of the slidable clamp of Fig. 4A: FIG. 5 is an isometric view of a stationary clamp according to the present invention; FIG. 6 is a schematic isometric drawing of a platesetter illustrating operation of the clamping system of the present invention. FIG. 7 is an isometric drawing, schematically showing an alternative configuration of the clamping system of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of an inexpensive dynamic clamping system, for attaching a plate to a rotatable drum. Specifically, the present invention can be used to easily attach a plate of any size to the drum of a marking engine, such as a platesetter, so that the plate is firmly gripped at any rotational speed. The clamping system is characterized by simple design and a small number of parts. The principles and operation of a clamping system according to the present invention may be better understood with reference to the drawings and the accompanying description. Referring now to the drawings, Figure 1 is an isometric view, and Figure 2 - a cross-sectional view, of a drum with a preferred embodiment of the clamping system of the present invention. There is shown a drum 10 , with outer surface 20. Surface 20 is cut by two (generally - several) grooves 12 running circumferentially and extending over a considerable portion of the drum's circumference. Each groove 12 has an undercut profile, to be described herebelow. At one end of each groove 12 is a stationary clamp 14 , fixedly attached to drum 10. Stationary clamp 14 is attached preferably within a depression 22, cut into surface 20 , in order to keep the extent of the clamp above surface 20 to a minimum, so as to clear an optical assembly (not shown) suspended a small distance above the surface. In each groove 12 there is a slidable clamp 16 , to be described herebelow. Stationary clamp 14 and slidable clamp 16 are operative to grip a plate 18 near its end edges 19. Drum 10 is preferably provided with a vacuum system, operative to press the entire surface of plate 18 onto surface 20. Referring now to Figure 5 , there is shown the construction of stationary clamp 14. It consists essentially of two parts - a base 24 and a gripper 26 - mutually attached by a hinge 28, which, for example includes a cylindrical pin 30 , whose axis forms the pivot axis 32 of the hinge. Gripper 26 is formed so as to have a relatively large body 34 , extending to one side of pivot axis 32 , and a relatively short tongue 36 , extending to the other side of pivot axis 32. Body 34 , preferably has a flat top surface, with a notch 37 cut across it. Tongue 36 has a tip 40 . It is noted that base 24 serves to join hinge 28 to the drum and could also be regarded as part of the hinge structure; other structures and other means for such joining may be provided. The assembly of stationary clamp 14 also includes a spring, such as coiled spring 38 , disposed around hinge pin 30. Stationary clamp 14 is attached to drum 10 by fastening base 24 to the drum, for example - by means of screws, preferably inside a depression 22 (Fig. 1), so that tongue 36 preferably points in a direction opposite the normal direction of drum rotation and so that tip 40 lies beyond the depression (that is, in contact with surface 20 ). Spring 38 acts to lift body 34 away from the drum and thus press tongue 36 toward the drum. The tip 40 of tongue 36 preferably has a friction pad or one or more small protrusions thereon, facing the drum. Normally, tip 40 is pressed against drum surface 20. Spring 38 acts to make this pressure strong enough to grip the edge of a plate, which has been inserted between the tip and the drum surface, under static conditions. All stationary clamps 14 are attached to drum 10 so that they are aligned along a line that, preferably, is parallel to the drum's axis. When drum 10 rotates, centrifugal force on body 34 tends to lift it away from the drum. This force is translated by the pivoting action into a radially inward force of tip 40 , which thus increases its pressure against drum surface 20 and consequently also the gripping force on the edge of a plate therebetween. The centrifugal force is essentially - CF = M x ω 2 x R where M is the mass of gripper 26 . ω is the angular speed of the drum and R is the distance from the drum's s axis of rotation (not shown) to the center of mass 42 of gripper 26. The force at tip 40 is essentially - TF = CF x D1/D2 where D1 is the distance from pivot axis 32 to center of mass 42 and D2 is the distance from pivot axis 32 to the middle of tip 40 . Clearly, it is desirable, at any rotational speed, to make TF large enough to grip the edge of the plate firmly enough to prevent its being pulled out. This can be achieved, for any given design of tongue 36 and tip 40 (which also fixes D2), by making the moment of centrifugal force, CF x D1, to be of sufficient magnitude. This, in turn, is obtained, for a given drum radius R and any angular speed ω. by making the moment of mass of gripper 26 with respect to pivot axis 32, M x D1, to have sufficient magnitude. The moment of mass may be made sufficiently large by appropriate construction of body 34; for example, M may be made large enough by making the body's volume large and or fabricating it out of a high-density substance, while a sufficient complementary value for D1 may be obtained by shaping the body so as to extend sufficiently far from hinge 28. We now refer to figure 4A, which shows slidable clamp 16 in a side view and in a cross-sectional view and figure 4B, which shows the same in a length-sectional view and a top view. Slidable clamp 16 is, in principle, similar to stationary clamp 14 - both in its construction and in its mode of action, except for some essential details, to be described herebelow. It consists of base 44 and a gripper 46 , joined by a hinge, which includes, for example, pivot pin 48. Gripper 46 is formed to have, on one side of pivot pin 48, an elongated massive body 50 and, on the other side of that pin, a short tongue 52 , which ends with a tip 60. A spring 51 , preferably of the compression type, is disposed between base 44 and body 50 . When clamp 16 is mounted on drum 10 (as will be described herebelow), spring 51 acts to push body 50 radially outwards and, by the pivoting action, to press tip 60 radially inwards. When the edge of a plate lies under tip 60, the latter then presses against it. When the drum rotates, the centrifugal force virtually acting on body 50 is added to the push of spring 51 and thus the pressure of tip 60 on the plate underneath it is increased -- all in a manner similar to that described hereabove with respect to stationary clamp 14. Clearly, again, it is desirable to make the mass of body 50 of gripper 46 as large as possible. This can be done by making it of dense material and or by making any of its dimensions as large as possible. In the preferred embodiment described herein, body 50 is shaped to be narrow and relatively tall (i.e. with large dimension in the radial direction), but also other shapes are possible. According to this embodiment, base 44 has a central slot sunk into its upper surface over most of its length, the width of the slot being slightly greater than the width of body 50 . Thus base 44 can partially accommodate body 50 during the latter's motion about pivot 48. Referring now to Figure 3, which shows a section of drum 10 with corresponding segments of two grooves 12 (one of them shown cut open) and a slidable clamp 16 mounted inside each groove, it is noticed that each groove 12 is undercut into the wall of drum 10; that is, the groove is relatively narrow near surface 20 of the drum and wider under the surface, inside the wall. Preferably, the profile shape of the wide portion 54 of groove 12 is rectangular. Base 44 of slidable clamp 16 is formed to conform to wide portion 54 and to easily slide therealong. Body 50 of gripper 46 is formed to be slightly narrower than the narrow portion 56 of groove 12, so that it is slidable through that narrow portion whenever base 44 slides through wide portion 54 . Preferably, body 50 of gripper 46 is also formed, and clamp 16 assembled, so that in its normal position, body 50 protrudes from groove 12 beyond surface 20 . The top surface of body 50 preferably has a depression, such as notch 58 , cut into it (to be engaged by an activation pin, as described herebelow). Slidable clamps 16 are mounted each in a groove 12 so that tip 60 of tongue 52 of gripper 46 preferably points in the direction of normal drum rotation. As will be described herebelow, during normal operation, the (preferably) trailing edge of plate 18 , mounted on drum 10, lies underneath tip 60 and is pressed by the latter. Tip 60 is preferably substantially wider than narrow portion 56 , thus bridging the latter and pressing the plate directly against drum surface 20 . It is also preferably provided, on its side facing plate 18. with one or more small protrusions or with a friction pad. These are operative, under the aforementioned pressure by the tip on the plate against surface 20 , to firmly grip the edge of the plate. The bottom surface of base 44 is preferably coated with a glide pad, which operates to ease its sliding on the bottom surface of wide portion 54 of groove 12 whenever pushed against it. The top surface of base 44 has preferably attached thereto one or more friction pads 62 (figs. 4), positioned so as to be able to contact the top surface of wide portion 54 of groove 12. The gripping force of tip 60 on the plate (due to the action of spring 51 and due to centrifugal force on body 50 ) is countered by an upward pressure of the portion of base 44 that lies underneath (opposite) tongue 52 . The base is thus pressed against the top surface of wide portion 54 , preferably through friction pads 62, thereby inhibiting any sliding motion therebetween. During drum rotation, additional upward pressure is exerted by the entire base 44 , due to centrifugal forces on itself - which adds to its friction with the top surface of wide portion 54 . Looking at the action of slidable clamps 16 from a different point of view, the clamps may be regarded as forcefully clamping together the plate (at its edge) and the outer shell of the drum (being between the top surface of wide portion 54 and drum surface 20 ) - simultaneously anchoring the plate and base 44 to the drum. The clamping force is increased by centrifugal action on body 50 . In the case of stationary clamps 14 , the clamping action is on the plate alone - between tip 40 and drum surface 20. It is noted that the construction of the clamps of the present invention, of both types, is inherently simple and involves a very minimal number of components. Such clamps are therefore advantageously inexpensive to fabricate. Typical operation of the clamps will now be explained with reference to Figure 6. There is shown, by way of example, an activation mechanism, in relation to the drum and the clamps thereon. It includes a bar 64 , subtending the length of drum 10 and suspended, parallel to its axis, at a certain distance from surface 20 . In bar 64 are pins 66, one pin essentially coplanar with each groove 12 , which are movable radially. The pins may be moved by any of a variety of means, such as solenoids, pneumatic actuators or cam. In an alternative configuration, bar and pins may be replaced by a direct activation mechanism, such as a cam or a manually operable lever assembly. Mounting of a plate, to be subsequently plotted, onto drum 10 proceeds as follows: 1. Initially all slidable clamps are preferably aligned along a common line, parallel to the drum's axis and circumferentially just far enough from the stationary clamps to freely accommodate the full length of the plate; the latter condition can be arrived at by moving all slidable clamps in a manner similar to that described in steps 7 and 8 herebelow, whereas initial alignment along a line can easily be performed manually 2. The drum is rotated to a position (shown in the illustration) such that notch 37 on body 34 of each stationary clamp 14 is directly under a corresponding one of pins 66 . 3. Pins 66 are pushed inwards, pressing on bodies 34 against the force of spring 38 (Fig. 5) and causing tips 40 to lift off surface 20 . 4. A new plate (not shown) is introduced tangentially to the drum (from top right, in the illustration) and registered thereon (preferably by means of registration pins provided thereon) so that preferably its leading edge is positioned under tongues 36 of all relevant clamps 14 , a certain distance behind corresponding tips 40 . 5. Pins 66 are withdrawn, freeing all grippers 26 to grip the edge of the plate, in a manner explained hereabove. 6. The drum is slowly rotated (counter-clockwise in the illustration) while the plate is being gradually attached to its surface, for example by means of rollers (not shown); if a vacuum system is included, it becomes operational at this step. 7. Rotation stops when notches 58 on bodies 50 of slidable clamps 16 are positioned directly below pins 66; the pins are then pushed inwards, pressing on bodies 50 against the forces of springs 51 (Figs. 4 ) and causing tips 60 to lift off surface 20; they also cause bases 44 to be pushed against the bottom surfaces of grooves 12 8. The drum is slowly rotated backwards (clockwise in the illustration) while clamps 16 , which are held stationary by pins 66 engaging notches 58 , slide along their respective grooves 12 . 9. The backward rotation stops when the other (preferably trailing) edge of the plate becomes positioned under tongues 52 of the relevant clamps, a certain distance behind corresponding tips 60 . 10. Pins 66 are withdrawn, fleeing all grippers 46 to grip the trailing edge of the plate, in a manner explained hereabove. 11. The drum is accelerated to nonnal operational speed and plotting proceeds in the usual manner. Demounting of the plate, after plotting, proceeds in a similar manner, though essentially in a reverse order. For this operation, another bar, similar to activation bar 64 with pins 66, may be used. It is noted that during normal (fast) drum rotation, the clamps firmly grip the plate, through the pressure of the tips of the tongues against the drum surface, and simultaneously each slidable clamp is, in effect, anchored in its position, through friction (preferably by means of the friction pads) between the top surface of its base and the top surface of the wide portion of the groove -- all assisted by centrifugal effects, as explained hereabove. It is also noted that the position of each slidable clamp along its respective groove is infinitely variable, within a range determined by the circumferential length of a groove, and that therefore the length of a plate that can be gripped by it may assume an infinite number of values within a corresponding range. It is likewise noted that operating the clamps (both in gripping a plate and in positioning a slidable clamp) is relatively easy, whether done manually or by an activation mechanism such as described hereabove. It is further noted that even if only a single length of plates need ever be accommodated, the configuration with slidable clamps advantageously eases the process of gripping plates during mounting and releasing them during demounting (all -- by the use of reverse rotation). Furthermore it is noted that nothing has to be fed to the clamps in order for them to operate as described (that is - they do not require vacuum, or pressurized air or electric current, etc.). Such feeding on a rotating drum would require special provisions, which are relatively expensive, dispensing with them, as permitted by the present invention, is therefore advantageous. It will be appreciated that other means and procedures, including manual ones, for mounting a plate on the drum, or demounting it therefrom, and for operating the clamps and moving the slidable clamps along the grooves, are possible - all coming within the scope of the present invention. It will also be appreciated that a drum may be provided with any number of stationary clamps and any number of grooves, each groove with a corresponding slidable clamp, but not all clamps need be engaged; in particular, if a plate substantially narrower than the length of the drum is mounted, only a corresponding number of the clamps would be engaged in gripping it. Although, in the preferred embodiment, each stationary clamp is paired, and axially aligned, with a groove and a corresponding slidable clamp, it will be appreciated that such pairing and/or alignment is not essential and that the number of stationary clamps need not be equal to the number of slidable clamps. In such a case, the activating mechanism of Fig. 6 may include additional pins - for any stationary clamps that are not aligned with corresponding grooves. It will further be appreciated that slidable clamps can also be utilized in place of stationary clamps. In an altemative configuration, basically similar to that described hereabove, there may be additional clamps, preferably of the slidable type, disposed on the drum so that they can operate to grip a plate 18 along its side edges. A preferred embodiment of such a configuration is illustrated in Figure 7. Here a number of grooves 12' , similar to grooves 12, are cut into surface 20 of drum 10 in a generally axial direction. Pairs of slidable clamps 16' , similar to 16 , are mounted one to a groove 12' and facing its middle. A plate 18 is mounted on the drum and held, at one end, by stationary clamps 14 and, at the other end, by slidable clams (not shown). Slidable clamps 16' are then operated, in a manner similar to that described hereabove, to grip the side edges of a mounted plate and thus offer additional safety and holding power; they may be particularly advantageous in holding down a plate on a drum that lacks a vacuum system. It is noted that plates of any width (up to nearly the length of the drum) may thus be gripped.

Claims: A method for mounting at least one plate (18) on the cylindrical surface (20) of a rotatable drum (10) having circumferential grooves (12) cut into the surface of the drum (10) and two groups of clamps (14,16) where at least one group of clamps (16) is slidable in grooves (12) and both types of clamps (14, 16) are switchable by release means (66) from a release state to a clamp state, characterized in the following steps:- a) sliding the second group clamps (16) and aligning them along a line essentially parallel to the axis of drum (10) and at a distance from first group of clamps (14) which is greater than the length of plate (18) to be mounted on drum (10); b) switching the first group clamps (14) to the release state and feeding a plate (18) to drum (10) to meet the first group clamps (14); c) switching the first group clamps (14) to the clamp state thus clamping the leading edge of the plate (18) to the drum surface (20); d) rotating drum (10) in a first direction to cause the plate (18) to be attached to the drum surface (20), the rotation stops when the second group of clamps (16) is in position to be activated by release means (66); e) switching the second group clamps (16) to the release state by engaging with said release means (66) so as to prevent the clamps from gripping, and f) rotating drum (10) in a second direction to slide the second group clamps (16) engaged with said release means (66) to the trailing edge of plate (18) where the clamps (16) are switched to the clamp state terminating the clamping process of plate (18) to the drum surface (20). The method of claim 1 where the release means (66) switches the clamps (14,16) to the release state by moving inward essentially perpendicularly to engage grooves (37,58) of clamp body section (34,50) and moves outward to switch the clamps (14,16) to the clamp state. The method of claim 1 where the force clamping the plate (18) to the drum surface (20) exerted by clamps (14,16) in the clamp state is partly derived from centrifugal effect. The method of claim 1 where the force clamping plate (18) to the drum surface (20) exerted by clamps (14,16) in the clamp state is partly derived from centrifugal effect and partly from spring action.

Inventors: BERES MOSHE, SOLOMON YEHUDA BARNES, BERES, MOSHE, SOLOMON, YEHUDA BARNES
Applicants: CREO IL LTD, CREO IL. LTD.
Application year: 1998
Main classification: B41F27
IPC codes: H04N1, B41F27