Note: Descriptions are shown in the official language in which they were submitted.
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1 BLADE FOR A SCREENING APPARATUS
2
3
4
BACKGROUND OF THE INVENTION
6
7 The present invention refers to a blade for a screening apparatus,
8 especially for fiber suspensions during pulping.
9
European Pat. No. EP-A-O 275 921 describes a blade for a screen with
11 drum-like base body. The blade includes in the meridian section a steadily
12 increasing cross section in direction of rotation and terminates in a steep
edge.
13 Joined to the steep edge in direction of rotation is a prolongation which
has a
14 cross section decreasing in direction of rotation and exhibits a flute. The
area of
increasing cross section forms a suction zone in conjunction with a screen
wall
16 that surrounds the drum-shaped base body, and the area of decreasing cross
17 section forms a pressure zone in conjunction with the screen wall. The
18 contoured outer surface of the blade exhibits a steep edge at the
transition
19 between the area of increasing cross section and the area of decreasing
cross
section, resulting in a sudden directional change that can cause a pressing of
21 impurities in the area of the pressure zone through the perforations of the
screen
22 wall so that the screening quality is adversely affected. The profile of
the blade
1
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1 conducts the material stream directly towards the perforations which are
thus
2 subject to a considerable hydraulic load, risking a slight shift of the
screen. The
3 provision of a flute and the sudden directional change cause strong
pulsation
4 which results in swirling in the material stream, and may lead especially at
increasing material density to fiber accumulations at the blade. This may lead
to
6 malfunction of the screen, especially when the screening surface becomes
7 plugged. The sharp edge between the increasing cross sectional area and the
8 decreasing cross sectional area leads to a relatively high flow residence in
the
9 material stream through the blade so that this conventional screening
apparatus
has to run at relatively high speeds, resulting in a relatively high energy
11 consumption during operation.
12
13 U.S. Pat. No. 4,676,903 discloses a screening apparatus for screening
14 and purification of pulp, and includes a rotor which has a contoured
surface with
grooves which are formed by a first plane parallel to the envelope surface, an
16 inclined plane, an upper plane and a side plane, with the side plane
extending
17 essentially perpendicular to the first plane. This profile of the groove
also results
18 in the formation of a steep edge between the pressure zone and the suction
19 zone during rotating rotor. This conventional screening apparatus thus has
basically the same drawbacks as the screening apparatus according to
21 European Pat. No. 0 275 921. Fiber bundles and slivers may form on the
22 protruding areas as a result of the steep edges, leading to malfunction of
the
2
CA 02186328 2006-11-30
1 screening apparatus and/or to a decreasing efficiency of the screening
2 operation. Further disclosed in this patent is a blade, often referred to as
3 hydrofoil, which is so constructed that a maximum pressure pulse is gener-
4 ated by the leading surface of the blade at the narrowest clearance point
towards the screening basket. This patent desires to generate and to sustain
6 over a long period very high frequency pulses. This, however, creates
7 undesired pressure pulsations.
8 The publication "Weekly Gazette for Paper Manufacturing, No. 17,
9 1988, Pages 733-737 discloses an article by D. Egelhof, entitled "The
1o Vertical sifter pulsating arms - bases, development, results", setting
forth
11 that optimization of the blade profile can produce an attenuation of the
12 pressure oscillations emanating from a vertical sifter. This publication
fails
13 to present particulars and only refers to the arrangement of the blades at
the
14 rotor to effect a reduction of the pressure oscillations.
German Pat. No. 28 30 386 C2 and International patent specifica-
16 tion W090/05807 describe screening apparatuses which include a rotor
17 carrying on its outer circumference cleaning and screening elements in
18 protruding disposition. This protruding attachment also results in
formation
19 of fiber bundles and slivers as well as dead zones and shives because the
cleaning elements exert an adverse effect upon the area of the material
21 stream. The annular gap
-3-
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2186328
1 between the screen wall and the rotor is narrowed by the jutting height of
the
2 cleaning elements and engagement elements. When exhibiting only a slight
3 clearance between the apex of the cleaning element and the screen wall
4 surface, the material being treated may be subject to a significant
thickening at
this greatly constricted annular gap. This results to malfunction of the
screening
6 apparatus. If, on the other hand, the rotor is positioned at a greater
clearance
7 from the screen wall, then, the rotor speed must be raised so that the
energy
8 consumption of the screening apparatus is increased.
9
SUMMARY OF THE INVENTION
11
12 It is an object of the present invention to provide an improved blade for a
13 screening apparatus, obviating the afore-stated drawbacks.
14
In particular, it is an object of the present invention to provide an
16 improved blade for use in a screening apparatus, to keep the screen free
from
17 impurities, to prevent thickening effects and pulsations while effecting a
18 sufficient fluidization of the material been screened mainly through
creation of
19 turbulences.
21 It is yet another object of the present invention to provide an improved
22 blade for a screening apparatus, which is particularly suitable for fiber
4
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zi863*213
1 suspensions during pulping and accomplishes an efficient and high-quality
2 screening operation in a cost-efficient manner.
3
4 These objects and others which will become apparent hereinafter are
attained in accordance with the present invention by providing a blade
exhibiting
6 a screen-proximal outer contour profile which in conjunction with the screen
wall
7 forms in material flow direction a suction zone section of increasing blade
width
8 and a pressure zone section of decreasing blade width, with the length of
the
9 suction zone section and the length of the pressure zone section defining
the
overall length of the blade, whereby the screen-proximal outer contour profile
of
11 the blade is smooth and continuos also in a transition area between the
area of
12 increasing width and the area of decreasing width of the blade.
13
14 The blade according to the present invention thus exhibits at the
screen-proximal outer contour a profile without edges, interruptions or flutes
and
16 thus has a smooth configuration from the zone of increasing width to the
zone
17 with decreasing width, including the transition area. Thus, the area of the
18 pressure zone section is characterized by a steady pressure increase so
that a
19 careful material transport towards the perforations of the screen wall is
effected.
This prevents impurities from surging in the pressure zone section through the
21 perforations of the screen wall so that the screening quality is enhanced.
At the
22 same time, turbulences required to free and clean the perforations and a
5
CA 02186328 1996-10-25
1 sufficient fluidization of the material being screened are created as a
2 consequence of the changing blade width in the suction zone section.
Further,
3 pulsation and ensuing beating are eliminated by the steady and continuous
4 contour profile in the suction zone section so as to attain a more efficient
material flow for increasing the efficiency of the screening operation. The
outer
6 contour profile of the blade also reduces its inherent flow resistance in
the area
7 of the material flow so that the screening operation can be executed more
8 efficiently and the screening apparatus can be operated in energy-saving
9 fashion since the speed of the rotor can be reduced while yet achieving the
same productivity and thus operate at enhanced current consumption.
11
12 A screening apparatus equipped with a blade according to the present
13 invention is particularly suitable for effecting a screening in the high
consistency
14 zone as well as a screening for use with rather coarse impurities because
of the
significantly improved screening quality. The reduction of the pulsations as a
16 result of the continuous and steady outer contour profile prevents the risk
of
17 damages and disturbances during subsequent processing steps, and the strain
18 upon the screen walls, which are typically formed by screen baskets, can be
19 reduced.
21 According to a feature of the present invention, the geometry of the blade
22 has been shown particularly suitable when being constructed as follows:
6
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1 - the pressure zone section of the blade has a length which is about 0.1-
0.45
2 times the overall length of the blade, preferably about 0.3 times the
overall
3 length;
4
- the blade in the pressure zone section has a radius of curvature of about
0.3-
6 0.6 times the overall length of the blade, preferably 0.4-0.5 times the
overall
7 length;
8
9 - the leading end of the blade in the suction zone section is defined by a
radius
of curvature which is in the order of 3-25 mm, preferably 6-20 mm.
11
12 In the area of the suction zone section, the performance of the blade can
13 be optimized when satisfying the following profile characteristics:
14
- the blade is defined in the suction zone section by a radius of curvature of
16 about 0.8-1.5 times the overall length of the blade, preferably 0.9-1.4 of
the
17 overall length;
18
19 - the trailing end of the blade is preferably defined by a radius of
curvature
in the order of 1-5 mm, preferably 1.5-3 mm.
7
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21863,28
1 Further parameters to be considered include the overall length of the
2 blade in relation to the inside screen diameter as well as the greatest
blade
3 width. The overall length in relation to the inside screen diameter is
4 approximately in the order of 0.05-0.3 times the inside screen diameter,
preferably from 0.08-0.2 times the inside screen diameter. The greatest blade
6 width is about 0.15-0.35 times the overall length of the blade, preferably
0.2-0.27
7 times the overall length.
8
9 When constructing the blade and in particular the outer contour profile of
the blade that is adjacent to the screen by observing the indicated ranges for
the
11 parameters, pressure pulsations can be eliminated and current consumption
can
12 be reduced because the width of the blade and the leading blade radius in
the
13 area of the pressure zone section result in a highly siender configuration
of the
14 blade.
16 With regard to the entrance area of the pulp, the blade according to the
17 present invention results in a careful advance of the pulp towards the
18 perforations of the screen wall. As a result of the steady pressure
increase in the
19 area of the pressure zone, a pressing of impurities through the
perforations can
be avoided so that a separation of impurities is effected. Moreover, this
section
21 is also dimensioned of suitable length to prevent thickening effects.
8
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1 The outer contour profile of the blade according to the present invention
2 also imparts the suction zone section with a sufficient length to ensure a
reliable
3 cleaning of the perforations in the screen wall. A suction zone section of
too
4 short a length would result in a premature breakdown of the stream and
causes
swirling that adversely affects the cleaning action.
6
7 The careful material flow towards the screen wall optimizes the interaction
8 with the screen wall even in those situations in which the screen wall is
e.g. of
9 contoured configuration so that a higher throughput is effected. Moreover,
the
rotor speed can be reduced, thereby diminishing the energy consumption.
11
12 Preferably, the smallest distance between the outer contour profile of the
13 blade and the adjacent screen wall is in the range of about 0.5-10 mm,
14 preferably 1.5-6 mm. This minimum clearance is substantially smaller than
in
conventional constructions of such screens so that high-quality screening
results
16 can be accomplished without experiencing any matfunction. Moreover, the
rotor
17 speed can be reduced because the blade positively effects the area of the
18 material flow with respect to cleaning and material conduction as a
consequence
19 of the reduced distance.
21 The screen-distant side of the blade which in case of a freely protruding
22 blade also effects the performance of the screening apparatus, may also be
so
9
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1 constructed as to essentially correspond to the configuration of the
2 screen-proximate outer contour profile of the blade. This results in a
symmetric
3 configuration of the blade. On the other hand, the profile of the screen-
distant
4 side of the blade may however also deviate from the profile of the
screen-proximate side. For example, the screen-distant side may be constructed
6 as a straight or flat section.
7
8 In particular, the blade is of a shape enhancing flow dynamics so as to be
9 enable to reduce the inherent flow resistance of the blade and to further
reduce
the energy consumption. Also interfering streams in the area of the material
flow
11 can be suppressed in order to enhance the material flow on the cleaning
side as
12 well as of the material supply side.
13
14 If necessary, the blade may be constructed of multiple parts. Whether the
blade is constructed in form of a singular part or multiple parts depends
16 essentially on the size of the blade as well as on its function and
arrangement,
17 e.g. upon a base body.
18
19 Preferably, the section of increasing blade width and the section of
decreasing blade width may point in material flow direction or in opposition
21 thereto. Thus, the blade according to the present invention positively
affects the
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1 material flow regardless of the direction of relative movement between the
2 screen wall and the blade.
3
4 The disposition of the blade or several blades may vary. For example, the
blade may be secured directly onto the outer circumference of a base body,
6 such as for example a rotor, preferably a rotor of essentially cylindrical
or conical
7 configuration, or may be an integral part of the base body, or may also be
8 secured to the base body via a support arm. When the blade is placed and
9 secured to the outer circumference of the base body, the connection can be
effected through welding or soldering or may be screwed thereon. When being
11 placed and secured directly upon the outer circumference of the rotor, no
12 protruding components are created so that dead zones, slivers, shives, or
like
13 rejects are avoided. This improves the operational safety of a screening
14 apparatus when equipped with a blade according to the present invention.
The
elimination of protruding components results also in a reduction of the
inherent
16 flow resistance of the assembly in the area of the material stream so as to
17 experience not only a more efficient operation of the screening apparatus
but
18 also an energy-saving operation.
19
According to another feature of the present invention, the blade can be
21 secured to the base body via spacers such as shims or the like, by which
the
22 distance between the screen-nearest point of the outer blade contour and
the
11
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2 186
1 screen wall can be modified and varied. In this manner, a fine-tuning to the
2 respective screening task can be effected.
3
4 Moreover, the blade can be arranged at an inclination relative to the
generatrix of the base body. Thus, transitional areas between individual
6 screening steps can be realized.
7
8 The performance of the blade can be further improved by orienting the
9 blade upon the base body in relation to the tangent at an angle which may
range
up to 100, or preferably up to 4 .
11
12 According to another embodiment of the present invention, the screening
13 apparatus may comprise a plurality of such blades which are arranged in
various
14 axial planes of the base body in offset disposition in circumferential
direction.
This results in a blade assembly for a screening apparatus by which the action
16 of the blades can be best suited to the screening tasks as assigned to the
17 respective screening planes, without encountering any relevant mutual
18 influence.
19
Thus, the blade according to the present invention can be so configured
21 and constructed for use with a screening apparatus for fiber suspensions,
in
22 particular for pulping, in an optimum manner as to exhibit an outer contour
12
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Ir1C3U3w8
1 profile that enhances the flow dynamics, without formation of sharp edges.
This
2 reduces the inherent flow resistance and accomplishes a significant energy
3 saving. Moreover, through the section of steadily increasing blade width and
the
4 section of steadily decreasing blade width, with both sections being
smoothly
connected to one another, a suction zone is formed, on the one hand, in
6 conjunction with the facing screen wall that generates a sufficient suction
to
7 create an effective cleaning of the screen, and a pressure zone is formed,
on the
8 other hand, which enables a careful material transport towards the
perforations
9 of the screen wall without experiencing a sudden pressure increase. This
considerably improves the screening quality.
11
12 BRIEF DESCRIPTION OF THE DRAWING
13
14 The above and other objects, features and advantages of the present
invention will now be described in more detail with reference to the
16 accompanying drawing in which:
17
18 FIG.1 is a fragmentary, schematic top view of a screening
19 apparatus using a blade according to the present invention;
21 FIG. 2 is a fragmentary, schematic top view of a screening
22 apparatus using a plurality of blades according to the present invention;
and
13
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~18 630948
1 FIG. 3 is a fragmentary, schematic top view of a screening
2 apparatus for use with a modified blade assembly according to the present
3 invention.
4
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
6
7 Throughout all the Figures, the same or corresponding elements are
8 generally indicated by the same reference numerals.
9
Turning now to the drawing, and in particular to FIG. 1, there is shown a
11 fragmentary, schematic top view of a screening apparatus intended for
12 screening a material stream, e.g. a fiber suspension, and equipped with a
blade
13 or hydrofoil 5 according to the present invention. The screening apparatus
14 includes a screen, generally designated by reference numeral 2, and shown,
for
ease of illustration, only by way of a section of a screen wall 2a. The screen
2 is
16 formed with generally vertically oriented individual perforations 3 which
extend
17 through the screen wall 2a transversely to the flow direction of the
material
18 stream, as indicated by arrow A.
19
It will be appreciated by persons skilled in the art that the screening
21 apparatus has more mechanical apparatus which does not appear in the
22 foregoing Figures. For example, the screening apparatus typically includes
a
14
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1 vessel for accommodating the screen, a rotor, such as e.g. rotor 1
schematically
2 shown in FIG. 2, for carrying the blade 5 that advances the fiber suspension
3 towards the screen perforations 3. However, this apparatus, like other
necessary
4 apparatus, is not part of the invention, and apart from the rotor 1 has been
omitted from the Figures.
6
7 The screen 2 is defined by an inner diameter designated by reference
8 character Di. The blade 5 can be secured in a manner as best suited for
various
9 tasks and different rotor types. As the present invention is concerned
primarily
with the relative movement between the screen wall 2a and the outer contour
11 profile of the blade 5, the following description is primarily directed
towards the
12 interaction between the screen wall 2a and the blade 5, on the one hand,
and
13 the material flow in direction of arrow A, on the other hand. It will be
understood
14 by persons skilled in the art that the material flow direction can
certainly be
reversed in order to attain the desired effects of freeing the perforations 3
of
16 impurities, prevention of pulsations and effecting a sufficient
fluidization of
17 material being screened.
18
19 The blade 5 exhibits adjacent to the screen wall 2a an outer contour
profile 6 which is so configured as to interact with the screen wall 2a to
form in
21 material flow direction A a suction zone section C of increasing blade
width and
22 a pressure zone section D of decreasing blade width, whereby the suction
zone
CA 02186328 1996-10-25
Z '18fi3~8
1 is designated by reference character P- and the pressure zone is designated
by
2 reference character P+. The blade 5 is defined by an overall blade length Fl
3 which equals the sum of the length L1 of the pressure zone section D and the
4 length L2 of the suction zone section C. In relation to the inside screen
diameter Di of the screen 2, the overall blade Fl is approximately 0.05-0.3 x
Di,
6 preferably 0.08-0.2 x Di. The length L1 of the pressure zone section D is
7 approximately 0.1-0.45 x Fl, preferably 0.3 x Fl. The greatest blade width
is
8 designated by reference character Fb and is measured between the
9 screen-distant side of the blade 5 and the apex of the outer contour profile
6 of
the blade 5. The greatest blade width Fb is for example in a range of 0.15-
0.35 x
11 Fl, especially from 0.2-0.27 x Fl. In the pressure zone section D, the
outer
12 contour profile 6 of the blade 5 is preferably formed by a circular section
at a
13 radius R2 which is approximately in a range of 0.3-0.6 x Fl, preferably 0.4-
0.5 x
14 Fl. The leading end of the blade 5 is preferably formed by a directly
following
circular section at a radius R3, or phases out at the radius R3 which is in a
16 range approximately of 3-25 mm, especially 6-20 mm.
17
18 The outer contour profile 6 of the blade 5 in the suction zone section C is
19 formed preferably by a circular section at a radius R1 which is in the
order of
0.8-1.5 x Fl, especially 0.9-1.4 x Fl . At the entrance into the suction zone
P- the
21 trailing end of the outer contour profile 6 is formed by a circular section
at a
22 radius R4 which is in the range of 1-5 mm, preferably in a range of 1.5-3
mm.
16
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1 The smallest distance between the screen wall 2a and the outer contour
2 profile 6 is designated by reference character s which is in a range of
3 0.5-10 mm, preferably from about 1.5 to about 6 mm.
4
As schematically illustrated in FIG. 1, the screen-distant side of the
6 blade 5 extends approximately perpendicular to a not shown center axis of
the
7 screening apparatus. As further shown schematically in FIG. 1, the
8 screen-distant side of the blade 5 may however also extend at an angle FW in
a
9 range of up to 100, preferably up to 4 . At such disposition, the blade 5
extends
in relation to the tangent upon a base body 1 which may be e.g. a rotor base
11 body 1, as shown in FIG. 2.
12
13 The blade 5 interacts with the perforations 3 of the screen 2 regardless of
14 the configuration of the perforations 3. In the area of the suction zone P-
(suction
zone section C), turbulences are generated in zone of the perforations 3 for
16 freeing and cleaning of the perforations 3 as well as for advancing the
material
17 stream through the perforations 3. The increasing width of the blade 5 in
the
18 suction zone section C effects a careful material transport to the nearest
19 perforation 3 in direction of the material flow. Starting from the point of
greatest
width of the blade 5, a steady pressure build-up is effected in the pressure
zone
21 section D so as to force the material through the perforations 3 in an
efficient
22 manner in the pressure zone P+.
17
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218630948
1 As a consequence of the steady pressure build-up created by the outer
2 contour profile 6 of the blade 5 in the area of the pressure zone P+ up to
the
3 smallest distance s, the screening quality is improved because impurities
are
4 prevented from being pressed through the perforations 3 as a result of a
sudden
pressure increase. The performance of the blade 5 according to the present
6 invention is more efficient as the particular configuration of the outer
contour
7 profile 6 enhances the flow dynamics so that the energy consumption of the
8 overall screening apparatus is reduced. Also, the inherent flow resistance
of the
9 blade 5 is thereby reduced and dead zones, shives, slivers or like rejects
which
adversely effect the efficiency of the screening apparatus or may even lead to
11 malfunction and plugging of the perforations 3. As the suction zone section
C
12 and the pressure zone section D are joined together without interruption by
a
13 smooth transition, pulsation is reduced that previously subjected the
screen 2 to
14 considerable stress, especially alternating loads. Thus, the blade 5
according to
the invention results in a highly efficient screening operation to achieve
16 high-quality results as well as in an energy-saving, reliable operation of
the
17 screening apparatus.
18
19 Turning now to FIG. 2, there is shown a fragmentary, schematic top view
of a preferred embodiment of a screen 2 using a plurality of blades 5
according
21 to the present invention. The screening apparatus includes a rotor with a
rotor
22 base body 1 which rotates clockwise in correspondence to the material flow
18
CA 02186328 1996-10-25
1 direction as indicated by arrow A. The rotor base body 1 may be of
substantially
2 cylindrical or conical or truncated cone shaped configuration and is
surrounded
3 by the screen 2 forming the screen wall 2a with perforations 3, such as
4 openings, slots or the like. The geometry of the perforations 3 may be
selected
in any suitable manner, for example as shown in U.S. Pat. No. 5,073,254 which
6 illustrates screen openings leading into groove-shaped recesses extending
7 essentially transversely to the flow direction of a fiber suspension. The
recesses
8 are of arcuated configuration and define a downstream flank and an upstream
9 flank, with the downstream flank having a sharper curvature than the
upstream
flank. Both flanks are connected smoothly and continuously by an arched
11 transition.
12
13 As shown in FIG. 1, the rotor base body 1 has an outer circumference 4
14 for supporting several blades 5 spaced from each other in circumferential
direction and secured to the rotor base body 1 e.g. through welding or
soldering.
16 Persons skilled in the art will understand that it is certainly within the
scope of
17 the present invention to secure the blades 5 on the rotor base body 1 in a
18 different manner, e.g. through a screwed connection or via support arms 7
for
19 mounting the blades 5 to the rotor base body 1, as shown for example in
FIG. 3.
It is also possible to provide in addition spacers, such as shims or the like,
to
21 secure the blade 5 to the rotor base body 1 in order to modify the smallest
19
CA 02186328 1996-10-25
~186328
1 distance s between the screen wall 2a and the outer contour profile 6 of the
2 blade 5, if necessary.
3
4 Although not shown in detail, the screening apparatus may also be so
constructed that the rotor base body 1 with the attached blades 5 is
stationary,
6 while the screen 2 rotates about a common center axis of the screen 2 and
the
7 base body 1. Moreover, the blades 5 may be arranged on different axial
planes
8 of the base body 1. For example, each blade 5 may be inclined in relation to
the
9 generatrix of the base body 1. Preferably, the blades 5 are arranged in
offset
relationship towards each other in different axial planes of the base body 1.
11
12 As described above, a substantially similar effect and same advantages
13 are attained when reversing the direction of rotating of the rotor base
body 1 or
14 upon reversal of the material flow direction to effect a relative movement
between the blades 5 and the screen 2 in opposite direction of arrow A.
16
17 While the invention has been illustrated and described as embodied in a
18 blade for a screening apparatus, it is not intended to be limited to the
details
19 shown since various modifications and structural changes may be made
without
departing in any way from the spirit of the present invention.
21
22 What is claimed as new and desired to be protected by Letters Patent is
23 set forth in the appended claims:
