METHOD OF UNSTRESSING A SHEET

METHODE D'APLANISSEMENT D'UNE FEUILLE

English Abstract

ABSTRACT OF THE DISCLOSURE


The present invention concerns a method of relaxing
or unstressing a sheet which is transferred from one bank
of grippers in a rotary offset printing machine to the
next bank of grippers. The invention reduces or eliminates
stress in the front edge region of the sheet after the
sheet has been transferred, yet before the next printing
operation. This is accomplished by briefly opening or
relaxing at least some of the grippers of the bank of
grippers that are guiding the sheet. This invention reduces
image imperfections caused by creasing, buckling or twisting
of the sheet.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:


1. Method of unstressing a sheet which is transferred
from one bank of grippers in a rotary offset printing
machine to the next bank of grippers wherein, after the
sheet has been transferred yet before the start of printing
and before the next sheet is transferred, at least some of
the grippers of the bank of grippers guiding the sheet are
briefly relaxed.


2. Method according to Claim 1, wherein all grippers
of the bank of grippers are briefly opened and closed again
and wherein, during this period, the sheet is guided by a
guide element working in conjunction with the cylinder of the
bank of grippers.


3. Method according to Claim 1, wherein two or more
grippers of the bank of grippers remain constantly closed
from the time the sheet is transferred up to the start of
printing or up to the transfer of the next sheet whereas
the other grippers are opened for a brief period.


4. Method according to Claim 1, wherein the bank of
grippers is divided into two separately driven groups of
grippers controlled in such a way that the groups of
grippers open and close alternately.



5. Method according to Claim 1, wherein every other
gripper is opened.

Description

3~

The invention concerns a method of unstressing a
sheet which is transferred from one bank of grippers in a
xotary offset printing machine to the next bank of grippers.
In multi-colour printing incorrect sheet transfer
often leads to a reduction in printing quality. Such
difficulties can be expected particularly in the case of
twisted paper which is not lying flat~ The following defects
have been found: creasing of the sheetr reduced spacing in
the case of multi-colour printing or register differences
arising from printing unit to printing unit or when the same
machine is passed through twice. Even in the case of a
sheet which is lying flat a reason for a reduction in
printing quality can be found in the fact that the sheet is
transferred in a slightly crinkled condition or, in the case
of a gripper transfer system with closely juxtaposed grippers r
buckling occurs during transfer from a first to a second
cylinder. Such incorrect sheet transfer causes stresses in
the front edge region of the sheet which in turn lead to a
change in the sheet during subsequent printing. This then
~esults r for example, in the above-mentioned image imperfections.
The object of the invention consists in eliminating or
reducing the stresses in the front edge region of the sheet
after the sheet has been transferred yet before the next
start of printing.
The invention achieves this goal by the fact that,
after the sheet has been transferred yet before the start
of printing and before the next sheet is transferredr at
least some of the grippers of the bank of grippers guiding
the sheet are briefly relaxed or opened.
This brief opening and closing of all or some grippers

frees the front edge of the sheet sufficiently in order
to achieve the desired unstressing. Since the sheet is
wrapped around the cylinder, the very fact of its curvature
means that the front edge of the sheet has an inherent
tendency to strai~h-ten itself anyway. This straightening
force supports the unstressing of the front edge of the sheet
as soon as it is released by some or all grippers~
If all the grippers of the banks of grippers are opened,
it is necessary to provide an additional guide element in
order to ensure that the exact register of the sheet is
retained. This guide element may consist of two or more
rollers which act, Eor example, on the sheet transported by
the cylinder in question.
It is also possible to leave two or four grippers
closed and to unstress the sheet by opening the remaining
grippers. It has even been shown in tests that, in many
cases, the opening merely of every other gripper permits
sufficien-t unstressing of the front edge of the sheet.
In order to achieve the above-described unstressing of
the sheet by means of the opening of the grippers it is
merely necessary for the gripper opening cam to be of
appropriate design. If the bank of grippers consists of two
separately controlled groups of grippers, then each of these
groups of grippers must be provided with a special control
~5 cam. It is then best for the two groups of grippers to be
opened and closed once or twice alternately. This alternate
opening and closing permits very effective unstressing of the
front edge of the sheet.
A notably simple embodiment of the invention is
explained below with the help of the drawings.

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~ig. 1 shows a section through a diagrammatically
presented bank of grippers according to the invention and
Figs. 2 and 3 show a section through a similar bank
of grippers in the area of their control elements.
The gripper 1 shown in Fig. belonging to a bank of
grippers 2 of an impression cylinder 3 is fixed on a gripper
housing 4 which is rotatably mounted on the gripper shaft 5.
A stop block 6 is rigidly attached to the gripper shaft 5
and connected via a compression spring 7 to the g~ipper
housing 4. The compression spring 7 constantly forces the
gripper 1 in the direction of the gripper contac-t shoulder 18.
Screwed into an adjustment tongue 9 of the stop block
6 is an adjusting bolt 10 whose end 11 in conjunction with
a stop face 12 of the gripper housing 4 when the gripper 1
lifts off from the gripper contact shoulder 18.
The gaps 13 of the individual grippers 1 may be
different so that the unstressing of the previously gripped
front edge of the sheet is achieved employing an appropriately
designed gripper shaft control cam 14 (see Fig. 2) by the
fact that only some grippers 1 lift off while the others
with a slightly larger gap 13 do not lift off yet. In the
case oE a positively closing gripper system the gripper
shaft control cam 14 is provided with a recess 15 whereas
in the case of a non-positively closing gripper system the
gripper shaft control cam 14 exhibits a prominence 16.
This measure represents a simple and low-cost way of
unstressing a sheet which, for example, has been transerred
to an impression cylinder 3.
After the sheet (not shown) has been transferred to
the bank of grippers 2 of the impression cylinder 3, the

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cam roller 17 of the gripper shaft 5 reaches the
above-mentioned slight recess 15 or prominence 16 of the
gripper shaft control cam 14 and causes the l.ift-off from
the gripper contact shoulder 8 oE all those grippers 1
which have a small gap 13 whereas the other grippers 1 with
the larger gap 13 continue to hold the sheet in conjunction
with the gripper contact shoulder 8 due to the force of
the compression spring 7. The first-mentioned group of
grippers 1 must have finished lifting off and reclosing
before the start of printing.




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