Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2145458
AIR FOIL WING EXTENSION FOR AN AIR TURN
FIELD OF THE INVENTION
The present invention relates to devices :for contactlessly
drying and guiding traveling webs, and more particularly, an
improved air flotation turning device which minimizes or
eliminates web flutter and/or wrinkling.
BACKGROUND OF THE INVENTION
In web printing and drying operations, it is often desirable
that the web be contactlessly supported in order to avoid damage
to the web itself or to the coating (such as ink) previously
applied to one or more surfaces of the web. One conventional
arrangement for contactlessly supporting a web includes
horizontal upper and lower sets of air bars between which the web
travels. Hot air issuing from the air bara~ both dries and
supports the web. Occasionally it becomes necessary to change
the direction of web travel while maintaining the contactless
environment . This can be accomplished using air' turns, which are
devices that support a flexible web on a cushion of air pressure
as the web travels around a curved path. Emir turns have a
generally partially cylindrical surface through which pressurized
a:ir is introduced through various slots, holes or apertures, or
other designs or patterns. Typical air i~urns which are
commercially available are a 95° turn, which carries the web
around a 95° arc, and a 20° "shallow wrap" turn, which carries
the web around an arc of 20°.
Such air turns replaced grater rollers . Grater rollers were
a means to turn the web utilizing frictional contact with the
1
CA 02145058 2002-10-30
76407-12
web. As a result, web marking problems often arose. Although
the use of air turns eliminated marking problems, the absence of
the additional frictional restraint provided by the rollers led
to web tracking problems, especially in the case of "baggy" or
non-uniform webs. To compensate for tracking problems, the air
turn was used as a steering device. By tilting one edge of the
air turn in a direction perpendicular to and toward the web, a
force was provided tending to push the web away from that side.
Conversely, if that end of the air turn were moved away from the
web, the resulting air pressure forces pulled the web toward that
end. Optical sensors are used to monitor web drift and send a
signal to the steering drive motor controlling the position of
the air turn. The drive motor moved the operator end of the air
turn. Alternatively or additionally, the air turn could be
tilted manually.
One example of an air turn is that disclosed in U.S. Patent
No. 4,182,472.
Specifically, a guide for contactless support of a
running web as the latter changes directions is provided. The
guide is formed as a drum-like member having an arcuately curved
surface which can be variable as to the length of its arc,
depending on the degree of turn or change of direction desired
for the running web. A series of parallel grooves extending in
the direction of web travel are formed in the arcuate surface of
the drum-like member. An air nozzle extends along the length of
the drum-like member and at each end of the groves, and
pressurized air is fed through the nozzles so as to form a
pneumatic cushion between the web and the arcuate surface and
2
CA 02145058 2002-10-30
76407-12
thereby float the web. The grooves in the arcuate surface act
as labyrinth seals in inhibiting the transverse air flow out of
the cushion and towards the edges of the running web.
A further example of an air turn is provided in U.S. Patent
No. 2,689,196, wherein a series of holes are formed in the
cylindrical surface for the passage of pressurized air
therethrough to support and guide a web passing over the drum.
Similarly, U.S. Patent No. 3,097,971 discloses a device having
a series of slits in the curved supporting surface and which
extend longitudinally and/or transversely to the web. Air under
pressure is passed through these slits to form a cushion between
the drum and the web.
An important aspect of any flotation system is the stability
of the web as it passes over the air bar. Airflow instabilities
near the web can induce web flutter and subsequent web contact
with mechanical parts of the drying, resulting in coating or web
damage. Web flutter can be manifested in a multitude of forms,
ranging from a violent flapping of the web to a high frequency
drumming.
Excessive web flutter has been encountered in conventional
air turn applications. Where a plurality of air turns are used
together so that the web follows a sinusoidal path, web flutter
has been encountered as the web leaves the lower air turn and
before it reaches the upper air turn.
It is therefore an object of the present invention to
minimize or eliminate web flutter at the exit and/or entry point
of an air turn.
3
CA 02145058 2002-10-30
76407-12
SUMMARY OF THE INVENTION
The present invention provides in a contactless
web support having an elongated arcuate surface and over
which a running web is floatingly supported, said web
support having a pair of elongated nozzles extending along
the length of said arcuate surface, each of said nozzles
having discharge slots configured to discharge pressurized
air over said arcuate surface, said web support including
means for supplying pressurized air to said nozzles, a first
air foil wing extension fixed to the leading or trailing
longitudinal edge of said air turn assembly, said first wing
extension comprising a first flat portion extending away
from said arcuate surface, and a second perforated section
adjacent said first flat portion, such that pressurized air
can escape through perforations in said perforated section.
The invention also provides in a contactless web
support having an elongated arcuate surface and over which a
running web is floatingly supported, said surface of said
web support having a plurality of substantially parallel
grooves forming labyrinth seals extending around said
arcuate surface in a direction in which the web passes
thereover, said web support having a pair of elongated
nozzles extending along the length of said arcuate surface,
each of said nozzles having discharge slots configured to
discharge pressurized air over said arcuate surface, said
web support including means for supplying pressurized air to
said nozzles, a first air foil wing extensions fixed to the
leading or trailing longitudinal edge of said air turn
assembly, said first wing extension comprising a first flat
portion extending away from said arcuate surface, and a
second perforated section adjacent said first flat section.
4
CA 02145058 2002-10-30
76407-12
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of a conventional
air turn which can be used in accordance with the present
invention;
Figure 2 is a cross-sectional view of a
conventional air turn which can be used in accordance with
the present invention;
Figure 3 is an isometric view of an air turn
having air foil wing extensions in accordance with the
present invention;
Figure 4 is a cross-sectional view of an air turn
having air foil wing extensions showing the theoretical air
flow pattern;
Figure 5 is a top view of an air foil wing in
accordance with the present invention; and
4a
~~4~0~~
Figure 6 is a side view of the air foil wing of Figure 5.
DETAILED DESCRIPTION OF THE INVENTION
The web support provided by the present invention can
support a moving web through various degrees of turning movement,
but the present invention has been illustrated in Figure 1 as
showing a web support for an approximate 90° l.urn, wherein the
web W passes over and without contact with the support S. It
should be understood that the following detaila_d description of
the web support is for purposes of illustration and should not
be construed as limiting of the present invention. The support
S includes a member 1 formed generally with a 90° arcuate surface
2 which extends across the width of the web W to be supported and
preferably beyond the edges of the web. Shaft means SM are
provided at each side of the support so that t:he device can be
suitably mounted in the drier apparatus. A series of
substantially parallel or parallel grooves 3 optionally can be
formed in the surface of the arcuate member 1 and extend in the
direction of web travel as disclosed in the aforementioned U.S.
Patent No. 4,182,472. The grooves preferably are generally
rectangular in cross-section, but may assume other configurations
so long as the effect thereof is to create .a labyrinth seal
whereby the flow of air out from the pressurized cushion is
inhibited. The grooves 3 form circumferential ribs 4 and also
define the upper surface 2 of arcuate surface. Alternatively,
the arcuate surface can be smooth, although it :is preferred that
labyrinth seals be formed therein.
Air nozzle N is located along each of the longitudinal ends
of the support S and extend the full length thereof transversely
across the width of the web being supported. The nozzles are
generally U-shaped and have a sharp nozzle edge 6 (Figure 2)
spaced a distance from the member 1 so as to define an elongated
slot or nozzle 8 through which pressurized a.ir is discharged
towards the web. The size of the nozzle opening can be adjusted
by a series of screws 12 or the like threadab7Ly engaged in the
frame F of the device and extending freely through the nozzle N.
The nozzle is secured to the support by any suitable means, such
as screws or the like.
The sides of the support S include end plates 16 secured by
cap screws to the ends of the arcuate member 1. The
aforementioned shaft means SM are secured to the end plates such
as by welding. The shaft means are adjustably mounted in the
frame F of the apparatus so that the angular position of the
support can be changed by rotating the support on the shafts.
The frame F has a series of holes 20 pas:~ing therethrough
and through which pressurized air is fed from the chamber 24.
Chamber 24 is also defined by members 2 and 28 (which can be
formed out of sheet metal) secured to a central duct 30 such as
by welding. Pressurized air is supplied to the end of the duct
30 by a supply conduit 31 from a suitable air supply AS. The
duct 30 has a longitudinal opening 32 which allows air to feed
into chamber 24 and be discharged through the nozzle N and to
each of the grooves in the arcuate surface.
A barrier 34 may be optionally provided extending
transversely to the direction of the web travel and across the
grooves 3. The barrier 34 blocks the grooves intermediate their
6
length to form a barrier to the air flow in the grooves and
eliminate flow instability and prevent one slot from dominating
the other.
Turning now to Figure 3, air foil wing sections 60, 61 are
shown, preferably fabricated from stainless steE~l. Each includes
a flat, non-perforated portion 63 closest to the air supply slot
of the air turn. The discharge air coming from the supply slot
increases in velocity as it moves across the flat portions 63 of
the wing, which is arranged with respect to the air turn so as
to be parallel and in close proximity to the web. To that end,
each air foil wing extension can be secured to the air turn by
any suitable means, such as by screws attached to the frame of
t:he device through brackets 65, 65'. The increased air velocity
creates a pressure drop between the flat portions 63 and the web,
which tends to draw the web towards the flat portions 63. A
large reduction of web flutter, in the range of 80o to 1000, has
been demonstrated. Although wings preferably are used at both
the web entry and web exit ends of the web support, if web
flutter occurs at only one end in a particular application, only
one wing could be used at that end.
The air foil wing extensions also each inc:Lude a perforated
section 64 adjacent the flat section and remote from the air
turn. As the discharge air moves onto the perforated sections
64, its velocity is gradually reduced. Preferably the perforated
sf~ctions 64 are bent away from the web at a gradual angle, which
increases its distance from the web the further- it extends away
from the air turn. An angle of about 11° is especially
preferred, as best seen in Figure 6. A plurality of perforated
7
214~a~g
bleed ports shown generally at 66 in Figure 3 of the perforated
section 64 allow ambient air to be drawn into the gradual
increasing area between the web and this sectic>n 64 of the wing.
This also causes the air velocity and turbulence to diminish.
The net result is a smooth discharge of air e:~cpelling from the
edges of each air foil wing 61, as shown in Figure 4.
The amount and location of the bleed ports 66 is important
to achieve a gradual diffusion or slow down in air velocity
coming off the flat, non-perforated portion 63 of the wing.
Preferably the amount and size of the ports increases as the wing
extends further from the non-perforated section 63. In a
preferred embodiment in which the flat, non-perforated section
63 is 6.0 inches long and 57.75 inches wide (not including
brackets 65, 65' ) , the perforated section 64 is 2.028 inches long
and includes three generally parallel rows of ports, as best seen
in Figure 5. The first row 67 closest to flat:, non-perforated
section 63 includes 62 0.25 inch diameter ports spaced 0.875
inches from each other, beginning and ending 1.75 inches from the
sides of the wing. The middle row 68 includE=s 126 0.25 inch
diameter ports spaced 0.437 inches from each other, beginning and
ending 1.312 inches from the sides of the wing. The third row
6~~ furthest from the flat, non-perforated section 63 includes 64
0.375 diameter ports spaced 0.875 inches from each other,
beginning and ending 0.875 inches from the sides of the wing.
The perforated section 64 of the wing preferably terminates
in a flange portion 70, as can be best seen in Figure 6. In the
preferred embodiment, the flange portion 70 is 1 inch long, and
includes a row of 0.312 inch diameter ports 71..
8
