Note: Descriptions are shown in the official language in which they were submitted.
1 BACKGROUND OF THE INVENTION
This invention relates to elevator buckets, and
more particularly to a cantilevered bucket for a mill duty
elevator.
A mill duty bucket elevator is an equipment system
for lifting dense, flowable substances from a low elevation
to a higher elevation. It is designed to withstand the
severe service conditions imposed by the handling of materials
such as cement, rock, fertilizer, lime, gypsum, pyrites,
slag, coal, and fine ore.
A bucket elevator is made of a long endless chain
loop trained around an upper traction wheel and a lower
sprocket wheel. A series of buckets is attached to the
chain at regular intervals for conveying material from one
end of the elevator to the other end. The elevator is
usually vertically oriented and operates in a casing having
a feed chute for filling the buckets near the bottom or boot
of the casing, and a discharge chute into which the bucke~s
di~charge their contents near the top or the head of the
casing. The maximum height of an elevator is determined by
the ~7eight and strength of the chain, the density of the
material to be carried, and the size and weight of ~he
bucket~.
Prior art elevator buckets have, in some circumstances,
experienced a short fatigue life. We have concluded, after
analysis and testing, that the cause of fatigue failure in
elevator buckets is due to resonant vibration of the buckets
caused by i~pact of the chain bushings with the traction
,whee:L.
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J
1 The resonant vibration in the bucket, as it passes
over the traction wheel, causes internal stresses in the
bucket which are concentrated at several regions in the
bucket. These stresses result in the initiation of fatigue
cracks at these points of high stress, and these cracks
propagate through the joints and corners of the bucket
adjacent to its attachment point on the chain. Although no
liability attaches to the manufacturer because of the use in
conditions exceeding the specified limits for which the
elevator was designed, these situations can create bad will
for the company. Accordingly, it is considered desirable to
design the buckets in such a way that they are resistant to
fatigue cracking under all conditions o use.
One technique for lengthening the service life of
elevator buckets is to provide vibration isolation mounting
devices between the bucket and the chain. This scheme could
probably be made to work, bu~ suitable vibration isolation
mounts for this application are excessively costly considering
the number of buckets in an elevator, which can amount to as
many as 400 buckets on a single chain. Therefore, we decided
that a less costly and more durable solution to the fatigue
cracking problem It70uld be a re~esign of the bucket itself to
better enable it to withstand the vibration without developing
fatigue cracks at points of ~tress.
One problem with bucket redesign for an existing
elevator configuration is that the buckets must be compatible
with the existing elevator design. This is because it would
be extremely costly to redesign an elevator for a new bucket
con~iguration More importantly, the new bucket should be
3~ compatible rtJith the thou~ands of existing elevators, so that
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1 ~/hen the buckets of the present design in these existing
elevators wear out, they may be replaced with the improved
buckets to impro~e the performance and operation of the old
elevators.
In addition to improving the fatigue life, it
would be desirable to improve the buckets in other ways.
For example, the redesigned bucket should be easier and
faster to ~abricate, use lighter gauge materials, have fewer
different pieces, and more easily meet critical dimensions.
The improved bucket should be lighter, thereby making it
possible to build elevators of greater height. Finally, if
possible, the improved bucket should provide a larger capacity
bucket while remaining dimensionally compatible with the
existing elevators, and thereby improve the capacity of the
elevator.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to
provide a method for manufacturing a bucket for a mill duty
elevator that is resistant to the fatigue effects of vibration,
is light weight, and is inexpensive to produce. Another
object of this invention is to provide a light weight bucket
for a mill duty elevator that is resistant to the fatigue
effects of vibration and is inexpensive to produce. Yet
another object of this invention is to provide a bucket
that can be abricated of few different parts and whose
critical dimensions are more easily attained.
These and other objects of the invention are
attained in an elevator bucket having laterally extending
braces from the front lip to the back wall, and a longitudinally
extending bar alon~ the top rear lip to provide torsional
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1 stiffness. A preferre-3 embodiment made according to the
invention has a trough shaped wrapper member for~ing the
front, bottom and back walls of the bucket and also forming
the top stiffening bar. A hole cut in the wrapper member in
the portions which form the back and bottom walls provides
an opening which defines the edges of a chain channel. Four
identical plates form two outside walls of the bucket and
two inside partitions extending across the cross-sectional
U-shape of the channel adjacent the hole in the wrapper
plate. The inside partitions function as sides of the chain
channel and as the laterally extending front lip braces. A
back plate is fastened to the inside partitions and to the
wrapper plate adjacent the top and bottom edges of the hole
in the wrapper plate to rorm the front wall of the chain
channel.
DESC~IPTION OF THE DRAWINGS
The invention and its many attendant objects and
advantages will become better understood by reading the
following description of the preferred embodiment in conjunction
with the following drawings, wherein:
Figure 1 is a schematic of a mill duty bucket
elevator for which the buckets of this invention were designed;
Figure 2 is an elevation of the top portion of the
bucket elevator shown in Figure 1 showing some of the details
of the buckets and the traction wheel over which the chain
is trained;
Figure 3 is a perspective view of a prior art
bucket and a portion of the chain of the elevator shown in
Figure l;
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1 Figure 4 is an enlarged perspective view of the
bucket of this invention;
Figure 5 is a front elevation o~ the bucket shown
in Figure ~;
Figure 6 is a sectional elevation along lines 6-6
in Figure 5.
Figure 7 is a rear perspective view of the bucket
shown in Figure 4; and
Figure 8 is a flat developed view o~ the wrapper
plate for the bucket of this invention.
DE5CRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings wherein like reference
characters designate identical parts, and more particularly
to Figure 1 thereo~, a mill duty bucket elevator for which
the buckets of this invention were designed is shown having
a casing 10 enclosing the moving components of the elevator
which include a top traction wheel or sprocket 12 mounted on
a supporting shaft 14 journaled in end bearings (not shown)
on the casing head lS, and a bottom sprocket wheel 16 mounted
on a supporting shaft 18 also journaled in end bearings in a
gra~ity take-up ~not shown) in the casing boot 19. A long
continuous loop of chain 20 i5 supported vertically at its
top by the traction wheel 12, and is guided at its bottom by
the spr~cket wheel 16. The traction wheel, shown more
clearl~ in Figure 2, is formed of a hub 22 to which rim
segments 24 of hardened steel are fastened by bolts 26.
As shown in Figures 2 and 3, the chain 20 is
formed of parallel chain links 28 connected together by pins
30 and held i~ parallel spaced relationship by chain bushings
3~ 32. Alternate pairs of chain links 28a are disposed on the
1 outside of every other pair of chain links 28~ and the
outside links 28a are provided with mounting flanges 34.
The mounting flanges 34 each have three holes 36 disposed in
parallel, vertically aligned axes to receive bolts 38 which
are used to secure the buckets 21 to the flanges 34.
In general, the operation of the bucket elevator
is as follows: the shaft 14 of the traction wheel 12 is
driven by an electric motor thru a gear box and chain drive,
not shown, to rotate the traction wheel 12 in the clockwise
direction as shown by arrow 40. The material to be elevated
is fed into the elevator through an entry chute 42 and falls
into the open topped buckets 21. The buckets are formed
with a forwardly sloping front wall to facilitate catching
the incoming material from the entry chute 42 and to facilitate
centrifugal discharge of the material at the exit chute 44
as explained below. The buckets 21 are spaced very closely
together as shown. A bucket is attached to every other pair
of chain links as shown in Figure 2, and the chain bushings
are typically six inches apart, so the buckets can be spaced
about one every foot of the chain. As shown, this produces
a very close spacing of the buckets, but the forwardly
extending front wall and the inward velocity of the ~aterial
falling into the casing through the entry chute 42 enables
the buck.ets to fill despite the close vertical spacing along
the chain 20.
As the buckets are carried by the chain over the
top of 'che traction wheel 127 centrifugal force propels the
contents ou~ of the bucket and through the exit chute 44.
The empt~ bucket then continues on down the rear flight of
3~ the chain loop and around the sprocket wheel 16 of the
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1 gravity takeup, which maintains correct chain tension and
proper alignment of the chain in the casing boot 19.
Our theory to explain the short fatigue life that
has sometimes been seen in the prior art bucket shown in
Figure 3 is as follows When the bushings 32 of the ascending
chain 12 reach the traction wheel 12, they strike the wheel
with an impact that is a function of the chain speed, the
chain tension, the chain pitch, and the hardness of the
bushings and the wheel. This impact is transmitted through
the chain mounting flanges 34 to the back wall 46 of the
bucket 21 which is excited to a condition of resonance by
the impact. The front wall 50 of the bucket 21 is supported
by the bucket end walls 52 and bottom wall 54, but is unsupported
along the entire length of the leading edge or lip 56
bet~een the side walls 52. At the resonant condition of the
bucket, the front wall 50 resonates like a drum head at its
natural frequency. The inertial forces of the vibrating
mass of the wall 50 tend to be high and must be carried by
the attaehment to the end and bottom walls. The resulting
stres~ can be concentrated at upper corners of the chain
ehannel and other high stress regions. Cracks can initiate
at certain points in the bucket, typically at regions which
are fixed again~ flexing, and these cracks can propagate
through the channel members, sometimes resulting in loose or
dislodged Guckets. If the condition is not corrected~
damage to the elevator casing and the remaining buckets
could re5ul~.
'rhe bueket of this invention, shown in Figures 4-
7, includes a ~,7rapper plate 60 whieh is ~ent in a trough or
3~ U shaped section alony bend lines ~ parallel to a longitudinal
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1 a~is 61. The formed wrapper plate 60 includes an out~ardly
sloping front wall 62, a lower floor 64, a vertical rear
wall 66, and a forwardly sloping top wall 68. The wrapper
plate 60 terminates in a narrow top rear lip 70 which lies
parallel to and spaced above the ~loor 64. A hardened steel
wear bar 72 is welded to the front leading edge of the
sloping front wall 62 and provides an abrasion resistant
leading edge 73 to resist abrasive wear at the region of
maximum ~ear inflicted by the incoming material from the
entry chute ~2.
~ rectangular opening 74 is formed in the wrapper
plate 60 which, when bent in~o its trough shape as shown in
Figures 4-7, provides the rearward opening for a vertical
channel 75 in the bucket to receive the chain as shown in
Figure 3. A set of three mounting holes 76 i5 formed in a
vertical line on each side of the opening 74 for receiving
the mounting bolts by which the chain flanges 34 are fastened
to the bucket.
Four side pieces, including two outer side pieces
78 and two inner side pieces 80, extend laterally across the
trough shaped wrapper plate 60 perpendicular to the longitudinal
aY.is 61 to close the ends of the trough and divide the
interior into three compartments. The outer side pieces 78
and the inner side piecea 80 are all the same shape and all
have identically dimensioned outside edges so that they can
be cut from stacked sheet steel by digitally controlled
flame cutting or plasma arc cutting equipment. These pieces
could also he cut by shearing, stamping, or any other cutting
technique. The inner side pieces 80 each have an additional
openi.n~ 82 formed in their front central portion to allow
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1 material falling into the bucket from the casing entry chute
42 to flow from the center compartment 84 into the two side
compartments 86. This is desirable because the inlet configuration
is often about two thirds as wide as the bucket to prevent
flooding the elevator boot, so the majority of the material
initially floY7s into the center compartment 84.
The outer side pieces 78 are spaced slightly in
from the extreme end edges of the wrapper plate 60 to provide
a protruding lip on the wrapper plate 60 which forms an
inside corner with the outer side pieces 78 to facilitate
the formation of a secure and sound weld around the edge of
the outer side pieces 78. Likewise, the inner side pieces
80 are spaced slightly outward from the edges of the opening
74 to provide an inside corner at which the inner side
pieces 80 can be welded to the edges of the opening 74.
A rectangular back plate 88 is welded in a nearly
vertical position between the top lip 70 and the floor 64 of
the wrapper 60 adjacent the top and bottom edges of the
opening 74. The sides of the rectangular back plate 88 are
welded to the sides of the inner side pieces 80. In this
manner, the rear face of the back plate 88 forms the front
ace of the vertical chain channel 75, and the rear portions
of the inner side pieces 80 form the side faces of the chain
channel 75 7
The inner side pieces 80 also provide an important
~tifening function for the sloping front ~7all 6~ of the
bucket and for the bucket as an entirety. The long cantilevered
leading ront edge 73 and indeed the entire front wall 62 of
~he bucket is no~7 supported and s~iffened b~ the inner side
pieces 80 ,7hich ~horten the effective length of the unsupported
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1 front edge to one-third. Therefore, the amplitude of the
vibrations of the front wall 62 will be reduced considerably,
and the stress experienced by the bucket at critical areas
as a consequence of resonance will be less. The rear top
lip 70 provides an important torsional stif~ness to the
bucket to prevent torsional flexing and the accompanying
stress cycling during loading of the bucket and also at
chordal impact of the chain bushing with the traction wheel.
A bearing plate 90 is provided on the inside of
the upright rear wall 66 of the wrapper plate 60 in the
region of the mounting holes 76 to provide a thicker and
stronger backing for the attachment bolts 38 which hold the
bucket to the chain mounting flanges. The bearing plates 90
distribute the forces exerted on the rear wall 66 by the
attachment bolts 38 over a wide area so the maximum stress
is held within limits which provide an acceptable fatigue
life.
The manufacture of the bucket is accomplished by
cutting the wrapper plate 60 to its correct outside dimensions,
cutting the opening 74, and punching the mounting holes 76
The rectangular plate thus formed is bent to the trough
shaped form shown in Figures ~-7 by bending the edges along
parallel lines in the order indicated by the let~ers A-D~
It is important that the lower rear corner bend ~ be made
last, otherwise the sides of the trough shaped member ~0
will inter~ere with the press structure when the other bends
are formed. The stops on the press must be set up carefully
so that the bends are all parallel, otherwise the side
pieces 78 and gO will not all fit snuggly in the trough, and
the buc~et will not lie horizontal across the chain.
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1 Once the wrapper member is properly bent, and the
back plate 88 and side pieces 78 and 80 a~e cut, the welding
can proceed. Automatically controlled submerged arc welding
is preferred because of its speed and the quality oE the
welds but other forms of welding can be used if the production
~uantities do not warrant the set up and equipment costs of
the automatic welding operation.
Al~hough the length of welding involved in the
fabrication of buckets according to this invention is somewhat
longer than the welding on prior art buckets, because of the
need to weld the inner side pieces 80 in place, the additional
cost of this additional welding can be substantially reduced
by the use of automatic welding equipment mentioned. The
savings from the use of lighter gauge material, the improved
fatigue life, the higher speeds, and the greater elevator
height made possible by the use of a lighter bucket and
reduced sensitivity to chain bushing impact wi~h the traction
wheel more than make up for the additional cost of the
additional welding.
The internal bracing provided by the inner side
pieces 80 and the rear top lip 70 of this invention provide
a bucket of great rigidity, so that lighter gauge material
can be used. Prior art buckets for use in mill du~y elevators
were normall~ formed of seven gauge steel plate, but it was
~ound that, because of the strength of this configuration,
a much lighter gauge material can be used. ~ bucket of this
configuration fabricated of ten to fourteen gauge steel
sheet is stronger and more rigid than the prior art bucket
design, and yet the weight o a bucket of this configuration
3~ is less than the ~7eight of a prior art bucket o the same
1 capacity. Therefore, the elevator can be extended to a
greater height. The greater rigidity and lighter weight of
this bucket enables it to tolerate the vibration and stresses
incurred by high speed operation. The overall result from
using this new bucket in the existing elevator design is a
significant improvement in durability and operational efficacy.
The bucket of this invention also facilitates
manufacturing efficiency, in that there are a fewer number
of different parts used in this bucket design. Because of
the use of a single piece wrapper plate 60 and the identical
outside dimensions and shapes of the outer and inner side
pieces 78 and 80, this bucket is formed of only four parts
of different shapes. These are the wrapper plate 60, the
inner and outer side pieces 78 and 80, the back plate 88,
and the bearing plates 90. The wrapper plate 60, in the
bearing plates 90, and the back plate 88 are rectangular
pieces and so the waste involved in the cutting of these
pieces is minimal.
The bucket of this invention achieves the internal
bracing of the front wall 62 and the torsional stiffening
about the longitudinal axis 61 with the use of stiffeners
~hich also provide other structural functions in the bucket.
The top lip 70 is merely the terminal edge portion of the
wrapper plate 60. It requires no additional fabrication or
in~entorying of parts, but provides an important attachment
for the top edge o~ the back plate 88 and top edge of the
inner side pieces 80, and even more importantly, it lends
substantial torsional stiffness to the bucket against flexing
around ~he axis 61. Similarly, the inner side piece~ 80
provide plural functions. They serve as the side walls for
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1 the chain channel 75, they stiffen the back wall 66 and
sloping back wall 68 of the wrapper plate 60 in the region
of attachment to the chain, and they stiffen the front wall
62 to reduce the amplitude of resonant vibrations during
filling with material and upon impact of the chain bushings
with the traction wheel.
Obviously, numerous modifications and variations
of the invention disclosed herein are possible and will
occur to those skilled in the art in light of this disclosure.
Accordingly, it is expressly to be understood that these
modifications and variations, and the equivalents thereof,
may be practiced while remaining within the spirit of and
scope of the invention as defined in the following claims,
wherein I claim:
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