WO1999042290A1

WO1999042290A1 - Sheet grippers

Info

Publication number: WO1999042290A1
Application number: PCT/IL1998/000086
Authority: WO
Inventors: Shmuel Douvdevani; Assaf Bernstein; Alex Feygelman; Aron Shmaiser
Original assignee: Indigo N.V.
Priority date: 1998-02-23
Filing date: 1998-02-23
Publication date: 1999-08-26
Also published as: AU6227998A

Abstract

A gripper mechanism (10) for mounting a sheet (22) on a drum (20) in a printing machine, the mechanism comprising: a holding element (16) movable between a holding position at which the holding element secures the sheet to the drum and a release position at which the holding element does not secure the sheet; and a magnetic holder (24, 26) which provides a relatively large force to hold the holding element (16) at the holding position and which provides a relatively small force to urge the holding element to the holding position when it is removed from the holding position.

Description

SHEET GRIPPERS

FIELD OF THE INVENTION

The present invention is related to the field of printing machines and the like and especially to the gripping of sheets of paper and the like for printing thereon. BACKGROUND OF THE INVENTION

In many types of printing machines, paper is fed to a drum or the like to which is temporarily attached. The paper, while mounted on the drum, is brought into contact with a plate, intermediate transfer member or other similar element, from which an image is transferred to the paper. In multi-color printing machines, and especially in high speed presses, the placement of the paper on the drum must be secure, fast and accurate.

In general, the paper is secured on the drum using a series of "grippers" spaced along the height of the drum. When a sheet of paper is to be attached to or removed from the drum, the grippers are lifted off the surface of the drum. A sheet is inserted under the lifted grippers and the grippers are returned to the drum surface, trapping the sheet and securing it to the drum.

Many mechanisms are available for raising and lowering the grippers. One such mechanism mounts a row of grippers on a rotatable shaft. When the grippers are to be raised, the shaft is rotated in one direction and when the grippers are to be lowered, the shaft is rotated in the other direction. In order to better secure the sheet of paper to the drum, springs are provided which provide an additional force pressing the grippers to the drum. Thus, the rotatable shaft must have enough torque to overcome the force of the springs. Furthermore, the torque increases as the springs are stretched increasing the torque further and the torque must be applied during the entire time that the grippers are raised.

Generally, the presence of such springs and the resultant extra torque requirement are not problematic. However, under certain circumstances, for example in electrostatic printing machines in which a number of elements are time synchronized, the extra torque which is necessary to lift the grippers may cause small but significant changes in the timing of these elements.

SUMMARY OF THE INVENTION It is an object of some aspects of the invention to provide a gripper mechanism which has a sufficient force to hold a sheet of paper or the like on a drum but for which the force required to lift the grippers is reduced as the grippers are raised. It is an object of some aspects of the invention to provide a springless mechanism for holding sheet to a drum.

It is an object of some aspects of the invention to provide a mechanism which provides a substantial force urging the gripper mechanism to a hold position only when the mechanism is near or at the hold position.

It is an object of some aspects of the invention to provide a magnetic latch mechanism which provides a relatively high holding force when the gripper mechanism is in a holding position and a relatively low force when it is in a release position.

In a preferred embodiment of the invention, a gripper blade, and more generally a plurality of gripper blades, which may be of conventional design, are mounted on an element which is rotatable around a shaft. At a first rotation position of the shaft, the blades are rotated toward the surface of the drum holding a paper or other sheet on the drum. At a second rotation position of the shaft, the blades are rotated away from the drum such that the blade is lifted from the drum surface, releasing the sheet. When a new sheet is inserted beneath the blades, the shaft is rotated to bring the blades in contact with the sheet, securing it to the drum, in preparation for an additional printing cycle.

In order to more securing hold the sheet to the drum, the rotatable elemeήTto which the blade is attached or another element mounted on the shaft, is preferably provided with a magnetic latch mechanism. In one preferred embodiment of the invention, the rotatable element is provided with a magnet which mates with a second magnet attached to the drum. When the shaft is at the first rotation position, the magnets touch or are close to each other providing a restraining force which prevents rotation of the element away from the first position. This, of course, also prevents the release of the sheet of paper from the gripper blades. In a second preferred embodiment of the invention, one of the magnets is replaced by a keeper to which the remaining magnet is attached.

In a third embodiment of the invention, the magnets are replaced by an electromagnet which is activated when it is desired to hold a sheet and deactivated when a sheet is to be released or changed. However, and unlike the springs of the prior art which provide for an increasing torque requirement for increasing rotation away from the first position, the magnet holders of the present invention provide for a decreased torque as the shaft is rotated away from the first position. It should be noted that there is no utility to any force toward the first position when the shaft is not at that position. In the preferred embodiment the rotation of the shaft is provided by an external mechanism and the force on the sheet is provided by the magnets, substantially only when the gripper blades are in a position in which the sheet is being held. There is thus provided, in accordance with a preferred embodiment of the invention, a gripper mechanism for mounting a sheet on a drum in a printing machine, the mechanism comprising: a holding element movable between a holding position at which the holding element secures the sheet to the drum and a release position at which the holding element does not secure the sheet; and a magnetic holder which provides a relatively large force to hold the holding element at the holding position and which provides a relatively small force to urge the holding element to the holding position when it is removed from the holding position.

In a preferred embodiment of the invention, the magnetic holder comprises a magnet and a keeper which are closely spaced when the holding element is in the holding position and relatively separated when the holding element is in the release position. In a preferred embodiment of the invention, the keeper is a second magnet.

In a preferred embodiment of the invention, the mechanism includes means for sliding the magnet or the keeper relative to each other to reduce the force between them, prior to separating the magnets.

In a preferred embodiment of the invention, the magnet is a permanent magnet. Alternatively, in a preferred embodiment of the invention, the magnet is an electromagnet which is activated when holding is desired and deactivated when release is desired. There is further provided, in accordance with a preferred embodiment of the invention, a gripper mechanism for mounting a sheet on a drum in a printing machine, the mechanism comprising: a holding element movable between a holding position at which the holding element secures the sheet to the drum and a release position at which the holding element does not secure the sheet; and a electro-magnetic holder which when activated, provides a relatively large force to hold the holding element at the holding position and which provides substantially no force to urge the holding element to the holding position when it is deactivated.

In a preferred embodiment of the invention, the electromagnetic holder is deactivated prior to movement of the holding element away from the holding position.

In a preferred embodiment of the invention, the electromagnetic holder is activated prior to arrival of the holding element at the holding position when a sheet is to be secured.

In a preferred embodiment of the invention, the holding element is mounted on a shaft and wherein the holding element is moved between the holding position and the release position by rotation of the shaft.

Preferably, a plurality of holding elements are mounted on the shaft. Preferably a plurality of magnetic holders are mounted on the shaft. In a preferred embodiment of the invention, a magnetic holder is mounted together with a holding element on a rotatable element which is rotated by the shaft. In an alternative preferred embodiment of the invention, at least one magnetic holder is mounted on the shaft via an element which does not have a holding element attached thereto.

Preferably, the holding element comprises a blade that presses the sheet against the drum in the holding position.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more clearly understood from the following description of preferred embodiments thereof, taken together with the following drawings in which:

Figs. 1A and IB are cross-sectional views of a gripper in accordance with a preferred embodiment of the invention in respective holding and releasing positions; and

Fig. 2 shows typical curves of "spring" force for a conventional spring mechanism and for a magnet mechanism in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figs. 1A and IB show cross-sectional views of a gripper mechanism 10, in accordance with a preferred embodiment of the invention. Gripper mechanism 10 comprises a rotatable element 12 rotatably mounted on a shaft 14. A gripper blade 16 is mounted on element 12 and is designed to operatively interact with a holding surface 18 of a drum 20. Preferably, shaft 14 is rotated with respect to drum 20 by a motor, cam and follower or other mechanism (not shown). Preferably a number of gripper mechanisms 10 are situated along the length or height of drum 20 and are mounted on shaft 14.

In the position shown in Fig. 1A, referred to herein as a "holding position," gripper blades 16 hold a sheet of material 22, such as paper, onto drum 20. In order to provide a holding force to reliably secure sheet 22 to drum 20 a pair of magnets 24 and 26 are provided, preferably for each mechanism 10 mounted on shaft 14. One of these magnets is secured to element 12 and the other is secured to drum 20. In the position shown, magnets 24 and 26 are contacting (or nearly contacting) such that they provide a maximal force to secure sheet 22 to drum 20. In the position shown in Fig. IB, referred to hereinafter as a "release position," shaft 14 and element 12 have been rotated slightly to release blades 16 from sheet 22, allowing for the removal of the sheet from the drum. Magnets 24 and 26 are seen to be separated such that a very small torque is required to hold element 12 in the release position.

Fig. 2 shows graphs comparing the force applied by a spring of the prior art (A) and a pair of magnets (B) of the present invention. As can be seen from the curve, after only a small rotation, the force of the magnets has been reduced practically to zero, which the force of the springs has remained nearly unchanged, increasing by a few percent.

It should be understood that many variations, within the scope of the invention as defined by the following claims, are possible. In particular, the pair of magnets can be replaced by a magnet and a keeper, where the magnet may be either on element 12 or on drum 20.

Furthermore, the permanent magnets which are described above may, in a further preferred embodiment of the invention, be replaced by an electromagnet and a keeper. A power supply activates the electromagnet when a holding force is desired and deactivates the electromagnet when no such force is desired. Such a mechanism can provide essentially zero force even in the holding position, when the sheet is to be replaced. In some preferred embodiments of the invention, the electromagnet may be activated prior to the element reaching the holding position to provide increased torque to bring the element to the holding position.

Furthermore, while a magnetic holder of the invention is preferably present for each of the grippers, in some preferred embodiments of the invention, a magnetic holder may be present for only some of the grippers. Alternatively, the magnetic holders may be associated with elements, similar to element 12 which are mounted on shaft 14 and which do not have associated blades. In this embodiment of the invention, the magnetic elements provide force to the blades via shaft 14. This last embodiment of the invention is especially adaptable for use with electromagnetic holding, for which large holding forces may be provided.

In the above described preferred embodiments of the invention, the magnets (or the magnet and keeper) are fixed on element 12 and on drum 20. In a further preferred embodiment of the invention, element 26 (a magnet of a keeper) can be mounted for movement toward and/or away from the center of the drum or axially on the drum. In this preferred embodiment of the invention and prior to rotation of shaft 14 to move element 12 from the holding position, element 26 is moved radially or axially on the drum such that its area of contact with element 24 is reduced. This motion will reduce even further the force which must be overcome in rotating shaft 14. In practice the elements 26 for all the grippers may be mounted on an elongate element which slides parallel to the axis of the drum. This element may then be moved axially (for example utilizing a cam and follower) prior to rotation of shaft 14 to release sheet 22. A similar arrangement may be used for a single gripper or for axial movement of element 26.

Furthermore, while the invention is described as utilizing only a magnetic holder in the absence of any springs, the magnetic holder may, in some aspects of the invention, be used in conjunction with conventional spring mechanisms. In general, such spring mechanisms would provide part of the holding force and the magnetic holder would provide part of the holding force.

Claims

1. A gripper mechanism for mounting a sheet on a drum in a printing machine, the mechanism comprising: a holding element movable between a holding position at which the holding element secures the sheet to the drum and a release position at which the holding element does not secure the sheet; and a magnetic holder which provides a relatively large force to hold the holding element at the holding position and which provides a relatively small force to urge the holding element to the holding position when it is removed from the holding position.

2. A mechanism according to claim 1 wherein the magnetic holder comprises a magnet and a keeper which are closely spaced when the holding element is in the holding position and relatively separated when the holding element is in the release position.

3. A mechanism according to claim 2 wherein the keeper is a second magnet.

4. A mechanism according to claim 2 or claim 3 wherein the magnet is a permanent magnet.

5. A mechanism according to any of claims 2-4 and including means for sliding the magnet or the keeper relative to each other to reduce the force between them, prior to separating the magnets.

6. A mechanism according to claim 2 wherein the magnet is an electromagnet which is activated when holding is desired and deactivated when release is desired.

7. A gripper mechanism for mounting a sheet on a drum in a printing machine, the mechanism comprising: a holding element movable between a holding position at which the holding element secures the sheet to the drum and a release position at which the holding element does not secure the sheet; and a electro-magnetic holder which when activated, provides a relatively large force to hold the holding element at the holding position and which provides substantially no force to urge the holding element to the holding position when it is deactivated.

8. A mechanism according to claim 6 or claim 7 wherein the electromagnetic holder is deactivated prior to movement of the holding element away from the holding position.

9. A mechanism according to any of claims 6-8 wherein the electromagnetic holder is activated prior to arrival of the holding element at the holding position when a sheet is to be secured.

10. A mechanism according to any of the preceding claims wherein the holding element is mounted on a shaft and wherein the holding element is moved between the holding position and the release position by rotation of the shaft.

11. A mechanism according to claim 10 wherein a plurality of holding elements are mounted on the shaft.

12. A mechanism according to claim 10 or claim 11 wherein a plurality of magnetic holders are mounted on the shaft.

13. A mechanism according to any of claims 10-12 wherein a magnetic holder is mounted together with a holding element on a rotatable element which is rotated by the shaft.

14. A mechanism according to any of claims 10-12 wherein at least one magnetic holder is mounted on the shaft via an element which does not have a holding element attached thereto.

15. A mechanism according to any of the preceding claims wherein the holding element comprises a blade that presses the sheet against the drum in the holding position.