Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method of Maynetic Cylinder Assembly
Description
This application relates to a method of
assembling a magnetic cylinder as for use in rotary
offset printing.
In rotary offset printing, ink is applied
to a plate mounted on one cylinder. The ink is trans-
ferred to a resilient blanket on a second cylinder.
A paper web is imprinted with the ink on the blanket.
The plate and blanket cylinders have to accommodate
a mechanism to hold the plate or blanket on the cyl-
inder surface. This mechanism is typically located
in a gap extending axially of the cylinder and having
a peripheral dimension of the order of three-eights
inch. That portion of the web which passes the blanket
cylinder gap is not imprinted and represents scrap.
This results in a waste of paper and a significant
expense. Moreover, the cylinders in a rotary offset
press rotate at a high speed and with substantial
pressure between cylinders. The gaps described above
cause shock and vibrations which degrade printlng
quality and contribute to press maintenance problems.
The gaps also destroy the symmetry of the cylinders,
an undesirable condition in high speed rotation.
Cylinders have been proposed to which a
plate is held magnetically. Magnetic cylinders which
have been available do no~ have sufficient holding
capability for reliable operation in rotary web off-
set printing.
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Summary of the Invention
..
In accordance with the invention, the method
of assemblinq the magnetic cylinder, includes the
steps of providing a cylinder with two spaced apart
helical pole pieces defining two magnet receiving
slots on the cylinder surface and winding two flexible
magnets on the cylinder. One magnet is wound in one
slot and the other magnet is wound in the adjacent
slot. The magnets have like poles facing each other.
The magnets enter the slots with an angular displace-
ment such that one magnet leads the other magnet, to
minimize demagnetization of each of the magnets by
proximity to the field of the other magnet.
Further features and advantages of the in-
vention will readily be apparent from the followingspecification and from the drawings.
Brief Description of the Drawings
Figure 1 is a perspective view of a cylinder
and plate incorporating the invention, with a section
cut away;
Figure 2 is an enlarged fragmentary view
showing a portion of the cylinder surface with the
pole pieces in elevation and the magnets in section;
Figure 3 is an enlarged fragmentary section
of the magnetic structure and plate of a prior art
cylinder;
Figure 4 is an enlarged fragmentary section
of the magnetic structure and plate illustrating the
invention, taken along line 4-4 of Figure l;
Figure 5 is a perspective showing the plate
as it is mounted on the cy~inder~
Figure 6 is a view similar to Figure 4 with
an offset blanket on the surface of the cylinder; and
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Figure 7 i5 a diagrammatic end view of the
cylinder, with a portion broken away, illustrating
the winding of the magnets into the gaps between the
pole pieces.
The printing roll 10, Figure 1, has a
cylindrical body 11 with stub shafts 12 extending from
each end. The cylindrical body is preferably of the
general construction shown in Wright U.S. patent
3,810,055. On the surface of the cylindrical body,
two helical pole pieces 14, 15, Figure 2, are spaced
apart defining helical slots 17, 18. Magnets 20, 21
are located in the slots establishiny a magnetic field
through the pole pieces. The field holds a plate 23
on the surface of the cylinder. The plate 23 is not
shown in Figure 2.
The magnets have a radial dimension less
than the pole pieces. Annular mem~ers 25, 26 overlie
the magnets, filling the outer portion of the sl~ts
17, 18.
A typical printing cylinder is of the order
of 40 inches in length and has a diameter of the order
of 7.5 inches. The magnetic structure on the cylinder
surface has, as will appear, a radial dimension of
less than one-half inch.
The prior art magnetic structure of Figure 3
may be compared with the magnetic structure ~f the
present cylinder and plate in Figure 4. Elements in
the prior art construction of Figure 3 ~orresponding
with the elements of Figures l, 2 and 4 will be
identified with the same reference numeral and a prime
mark. Differences in geometry and in the characteris-
tics of the materials afford a substan~ial increase in
the force holding the printing plate on the cylinder.
The dimensions shown in Figures 3 and 4 and referred
to in the specification provide examples of ~he prior
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art and the invention ~or pur,poses of comparison,
While individual dimensions are not critical, the
relati~e dimensions of the elements of Figure 4 con-
tribute to the increased plate holding force achieved
by the invention.
The cylinder body 11, which may be of steel,
has a sleeve 28 of a nonmagnetic material thereon to
isolate the magnetic structure from the body.
Typically, the sleeve is of brass and has a radial
lQ dimension of .050 inch.
The magnets 20, 21 are preferably a flexible
ru~ber-like material impregnated with magnetic
particles. Minnesota Mining and Manufacturing Company
sells such magnets under the trademark Plastiform.
The magnets are wound into the slots 17, 18 between
the pole pieces 14, 15 during assembly of the cylinder,
as will appear. The fields of the magnets are oriented
with like poles of adjacent magnets facing each other,
as indicated in the drawing. The magnets have an
axial dimension of .051 inch and a radial dimension
of 0.250 ~nch. The pole pieces, 14; 15 are of a low
reluctance material, preferably a stainless steel.
AISI No. 430 ferritic stainless steel is suitable.
This material resists corrosion by the inks, solvents
and cleaners used in printing so that the peripheral
surfaces of the pole pieces maintain the desired
dimension and cylindrical configuration. The axial
dimension of the pole pieces, here 0~032 inch, is
determined by the coercive ~orce and axial dimension
of magnets 20, 21 and the parmeability of the pole
piece material so that a condition of substantial
saturation i5 achieved at the peripheral faces of
the pole pie~es with ~he printing plate mounted on
the cylinder.
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The printing plate 23 is of a magnetic
material and has ~ thickness related to its reluctance
such that substantial saturation ~s achieved in the
annular plate sections between adjacent pole pieces
14, 15. In the example illustrated in Figure 4, the
plate has a thickness of 0.D15 inch. This thickness
permit~ easy cutting, handling and preforming of the
end sections to conform with the cylinder surface as
will be described below.
It is preferred that the magnetic field
through plate 23 not exceed saturation. The existence
of a stray field outside the plate would attract
particles of magnetic material to the plate surface.
This would result in poor printing quality and could
damage the plate or the blanket. Furthermore, such a
stray magnetic field does not contribute to the force
holding the plate on the cylinder but rather detracts
therefrom. The force re~uired to lift an end of the
plate from the cylinder, sometimes referred to as the
"peel-of force", is directly related to the three-
quarter power of the plate thickness. This relation-
ship exists throughout the range in which the curve
of the hold down force per unit area as a function of
the gap between the plate and the ~ole pieces is sub-
stantially linear. Based on both measured and calcu~lated data, the curve is substantially linear until
the peel-off orce is reduced to about 40 to 50~ of
its initial value. The plate must be thick enough
that ~he peel-off force is sufficient that the plate
is not peeled from the ~ylinder by tacky inX~ Rn
excessive plate thickness, however, increases the
difficulty o~ cutting, handling and forming the plate
and of mounting it on a cylinder.
The term ~substantial saturation" as used
herein means a condition of saturation of th~ order of
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90-95~. A design to achieve a higher level of satura-
tion requires an excessive increase in magnet ~oer-
cive force and/or axial dimension for a minimal in-
crease in flux. Moreover, at such a high l~vel of
saturation a stray field begins to appear outside the
plate, diminishing the gain in the peel-off and hold
down forces. A flux level much below 90% saturation
represents inefficient utilization of the materia:l in
the pole pieces and plate.
The annular members 25, 26 overlying the
magnets 20, 21 between the outer portions of the
pole pieces 14, 15 are of a high reluctance material
to minimize the flux path in shunt with plate 23. An
austenitic stainless steel, AISI No. 310, has been
found satisfactory.
The outer peripheral surface of the pol~
pieces and the inner surface of plate ~3 are pre-
ferably in intimate contact. This minimizes the
occurrence and si~e of air gaps in the magnetic
circuit. Any air gap greatly increases the circuit
reluctance and reduces the hold down force acting on
the plate.
The advantages of the construction of Figure
4 will be appreciated from a consideration of the prior
art construct~on of Figure 3. Here, the magnets 20',
21' have an axial dimension of 0.093 inch. The coer-
cive force is such that the pole pieces 14', 15' are
saturated and the magnetic hold down force potentially
available is not effectively used. ~he annular magnet
cover members 25', 26' have a xadial dimension of
0.100 inch and are o a stainless steel, AISI Nv. 304,
which t~pically has a lower reluctance than ~hat of
the AISI No. 310 material. The cover members pr~vide
a significant magnetic shllnt path reducing the flux in
plate 23' and thus redueing the hold down force.
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The hold down f~rce i~ localized at the pole
pieces 14' r 15'. With the prior art magnet width of
Figure 3, there are four sets of magnets and pole
pieces per axial inch of the cylinder. The ratio of
pole piece to magnet area on the outer surfaGe of the
cylinder is 0.34 to 1. With the construction of
Figure 4, there are six sets per axial inch. The
ratio of p~le piece to magnet area is 0.63 to 1.
These differences in geometry and magnetic charac-
teristics of the elements provide an increase of theorder of 50~ in the peel-off force and of the order
of 80% in the hold down force exerted on the plate
23 as compared with the forces on page 23'.
The peel-off force required to lift an end
of plate 23 is proportional to the hold down force
when the plate is in contact with the pole pieces and
is inversely proportional to the fourth root of the
proportionality constant between the hold down force
and the gap between the plate and the pole pieces.
This measure of the ability to resist a peel-off
force is accurate when the relation between the hold
down force and the gap is linear over the first 46%
of the peel off foxce decrease, and when the bending
of the plate as the end is lifted does not exceed the
mechanical yield strength of the plate material.
Tacky ink will exert such a peel-off force. If the
plate end is lifted too far, the plate will shift in
position on the cylinder or may come off. As dis-
cussed above, it is desirable that the gap between
the plate and pole pieces be minimized and that the
inner surface of the plate have intimate contact with
the ~ole pieces throughout the circumference and the
length of the plate.
An additional factor to enhance the intimate
contact bPtween the plate and pole pieces is precurving
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the leading and trailing ends 30, 31 respectively of
the plate as shown in Figure 5. The curvat~e is
preferably on a radius substantially equal to or
slightly less th n the radius of the cylinder surace.
This aids in establishing and in maintaining an
intimate contact between the plate and the pole
pieces at the ends of the plate where it is most im-
portant.
Figure 5 also illustrates a preferred con-
struction for locating the plate-23 on the cylinder.
Positioning pins 33, 34 extend radially outwardly
from the cylinder surface. A semi-circular comple-
mentary notch and an elongated notch in the edge of
the plate receive the pins and locate the plate on
the cylinder while allowing for manufacturing
tolerances. After positioning the plate end 30 as
shown, the remainder of the plate is wrapped around
the cylinder surface.
The plate 23, Figures 1, 4 and 5 is a
printing plate. An image of the material to be
printed is suitably formed on the outer surface to
pick up ink from an inker in an offset printing
operation. As illustrated in Figure 6, the magnetic
cylinder may also be used for the resilient blanket~
Here, the cylinder and magnetic structure may be the
same as in Figure 4. The plate 36 has the resilient
blanket sheet 37 suitably affixed to its outer sur-
face. As in Figure 4, plate 36 is o~ a magnetic
material with a reluctance and radial dimension such
that it is substantially saturated by the flux be-
tween pole pieces 14, 15. The inner surface of plate
36 has intimate contact with the peripheral outer
surface of the pole pieces.
Magnetically mounted blankets have sometimes
exhibited a tendency to creep or shift peripherally
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on the cylinder surface. The cause of this movement
is not fully understood but it is ~elieved to be due
to a localized separation of the plate 36 from the
cylinder surface. Maximi~ation of the hold down force
is one factor in eliminating this movement. An in-
crease in the pole piece to magnet area ratio above
the 0.63 to 1 ratio of the cylinder construction cle-
scribed above has been found to provide a higher hold
down force at a very small gap dimension, e.g., less
than 0.0005 inch. With such a higher area ratio,
however, the hold down force drops rapidly as the gap
increases. With an area ratio between about 0.45 to
1 and about 0~65 to l a high hold down force at very
small gap is achieved, with an acceptable decrease as
the gap increases. As described above, the peel-off
force required to pull the plate off from its end is
thereby maximized.
Figure 7 illustrates the method of assembly
of the flexible magnets 20, 21 with the cylinder. The
cylindrical body 11 has the sleeve 28 and pole pieces
14, 15 mounted thereon. The flexible magnets are then
wound into the slots 17, 18 between the pole pieces as
by rotating the cylinder in the direction of arrow 39.
The magnets are oriented with like pole pieces adja-
cent. If the magnets are brought into close proximitywith this orientation, the field of each magnet tends
to demagnetize the other. In accordance with the in-
vention, the magnets are angularly displaced with Gne
magnet 20, which may be considered the leading magnet,
completely in the slot Setween adiacent pole pieces
before the trailing magnet 21 enters the adjacent
slot. Thus, the pole pieces are interposed hetween
the magne~s before the magnets come into clo~e proxi-
mity~ ~emagnetization of the magnets during assembly
is minimized.
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If the pGle pieces have been work hardened
during manu~acture, the reluctance may increase.
Annealing will reduce the reluctance, maximizing the
holding capability. A combination of annealing and
the magnet asse~bly method described above has in-
creased the holding forces by a factor of about 1.2
over thos~ of the prior art cylinder of Figure 3.
