CA1198998A - Elevator bucket for high speed operation - Google Patents

Abstract

ABSTRACT OF THE INVENTION
A bucket for attachment to the chain in a mill duty bucket elevator, includes a one piece wrapper member bent into a U-shaped trough defined by a sloping front wall, a horizontal bottom wall, an upright rear wall, a forwardly sloping rear wall, and a horizontal top lip. A rectangular hole in the wrapper plate defines the outside edges of a chain channel in which the chain lies when the bucket is fastened to the chain. Four side pieces having identical shapes and outside dimensions lie transversely across the trough to provide two end walls and two inside partitions.
The partitions brace the front wall and act as side walls of the chain channel. A back plate fastened to the partitions and to the top lip and bottom wall adjacent the rectangular hole acts as the front face of the chain channel. The top lip, fastened to the two end walls and the two partitions, provides torsional stiffness to the bucket.

Description

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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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

-2-1 ~/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 ~3--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;

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.

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 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 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 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 _g_ 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.

~ 3 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 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:

Claims (20)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An elevator bucket for attachment to an elevator chain, comprising:
a one piece wrapper (60), bent along lines parallel to a longitudinal axis into a generally U-shaped trough configuration to provide front (62), bottom (64) and rear walls (66), and a top rear lip (70);
means defining a hole (74) in said wrapper, opening in said bottom and rear walls;
two inner side pieces (80) and two outer side pieces (78), all four of said side pieces being of identical outside dimensions and shape, said shape being such as to fit laterally across the U-shaped trough of said wrapper, all of said side pieces being fastened to said front, bottom, and rear walls and said top rear lip of said wrapper;
said two inner side pieces extending transverely across said trough adjacent two sides of said hole, respectively, and said two outer side pieces extending laterally across said trough adjacent the longitudinal ends thereof;
a back piece (88) fastened to said wrapper adjacent the top and bottom edges of said hole and fastened to said inner side pieces along opposite sides of said back piece;
said back piece and said inner side pieces closing said hole from said trough and defining a vertically extending channel (75) for receiving a chain (20) to which said bucket may be attached;
said top rear lip acting as a torsion bar to provide torsional stiffness about said longitudinal axis, and said inner side pieces acting as lateral braces to provide lateral stiffness to said front wall, whereby the amplitude of resonant vibrations of said bucket is minimized.

2. The elevator bucket defined in claim 1, further comprising a wear bar (73) fastened to said front wall along the top edge thereof and also fastened to all four side pieces, said wear bar being thicker and harder than the material of said wrapper.

3. The elevator bucket defined in claim 1, wherein said outer side pieces are spaced slightly inwardly from the longitudinal ends of said wrapper to provide an inside corner to facilitate welding said outer side pieces to said wrapper.

4. The elevator bucket defined in claim 1, further comprising openings (82) in both of said inner side pieces through which material flowing into the region between said inner side pieces can flow to the two laterally outside regions between said outside pieces and the inner side pieces adjacent thereto, said opening defining means including edge surfaces in each of said inner side pieces spaced inwardly from the outside edges of said inner side piece and parallel thereto.

5. An elevator bucket, comprising:
two side compartments and one center compartment arranged in line defining a longitudinal direction;
a vertical channel opening in the back of the bucket for receiving a chain to which the bucket can be attached in a bucket elevator;

one side of one of said side compartments, and one side of said channel being formed by a first single inner side piece;
one side of the other of said side compartments, and the other side of said channel being formed a second single inner side piece;
said center compartment having two end sides extending across said longitudinal direction, said center compartment end sides being formed by portions of said two inner side pieces, respectively;
said channel having a front wall, and said center compartment having a back wall, said front and back walls both being formed by a single back piece fastened at opposite longitudinal edges thereof to said inner side pieces, respectively;
said two side compartments having outside side walls formed by two outer side pieces, respectively;
said outer side pieces and said inner side pieces all being of the same outside shape and dimensions;
said two side compartments having front, bottom and rear walls, and said center compartment having front and bottom walls, all formed by a trough-shaped wrapper plate attached to the edges of said inner and outer pieces.

6. The bucket defined in claim 5, further comprising a rear top lip extending the entire longitudinal length of said bucket and connected to the top edge of said rear walls, to said outer and inner side pieces, and to said back piece.

7. The bucket defined in claim 6, further comprising an opening in each of said inner side pieces providing communication between said center compartment and each of said side compartments.

8. The bucket defined in claim 6, further comprising a wear bar welded to the front top edge of said wrapper plate, to said outer side pieces, and to said inner side pieces; said wear bar being harder and thicker than the material of said wrapper plate.

9. A method of forming an elevator bucket, comprising:
cutting a wrapper piece of sheet metal to size, cutting an opening in said wrapper piece, and punching a set of mounting holes in said wrapper for mounting the bucket to the chain;
cutting four identical side plates;
bending said wrapper piece into a trough shape;
welding two of said side plates in the two outer ends of the trough shaped wrapper to close the ends, thereof;
welding the other two side plates, one on either side of said opening, to the inner portion of said trough shaped wrapper to divide said trough into two end compartments and one center compartment, and form the sides of a rearwardly opening chain channel;
welding a back plate to the top and bottom edges of said opening and to the sides of the inner side plates to close the back of the center compartment and the front of said chain channel.

10. An elevator bucket for a mill duty elevator, comprising:
a trough shaped structure forming front, bottom and rear walls extending parallel to a longitudinal axis;
attachment means on regions of said rear wall for attachment of said bucket to an elevator chain;
two outer side pieces extending transversely of said longitudinal axis and closing the ends of said trough shaped structure;
means in said bottom and rear walls defining an opening which constitutes the edges of a chain channel;
two inner side pieces on either side of said opening extending laterally in said trough shaped structure and attached to said bottom and rear walls adjacent said opening to provide side walls for said chain channel;
a back plate extending between and fastened to said inner side pieces and to said bottom wall to provide the forward wall for said chain channel;
a torsion bar extending the full longitudinal length of said bucket along the top of said rear wall and attached thereto, and attached at the ends of said torsion bar to said outer side pieces, respectively; and two laterally extending braces connected to said front wall near the leading lip thereof and operatively connected to the rear wall for providing lateral support for said front wall.

11. The bucket defined in claim 10, wherein said torsion bar is attached also to the top portions of said two inner side pieces and to the top portion of said back plate.

12. The bucket defined in claim 11, wherein said torsion bar is attached along the back edge thereof to the top edge of said rear wall.

13. The bucket defined in claim 10, wherein said laterally extending braces are operatively connected to said rear wall adjacent said attachment means.

14. The bucket defined in claim 13, wherein said inner side pieces and said braces are formed by two unitary side pieces attached along the front edge thereof to said front wall and attached along the rear edge to said rear wall.

15. The bucket defined in claim 13, wherein said torsion bar is attached to the top portions of said two inner side pieces and to the top portions of said back plate.

16. The bucket defined in claim 15, wherein said inner side pieces and said braces are formed by two unitary side pieces attached along the front edge thereof to said front wall and attached along the rear edge to said rear wall; and said torsion bar is attached along the back edge thereof to the top edge of said rear wall and to the top edge of said back plate.

17. The bucket defined in claim 16, wherein said front, bottom and rear walls, and said torsion bar are formed as a unitary member cut from a single steel plate and bent into said trough shape.

18. The bucket defined in claim 17, wherein said inner and outer side pieces all have the same shape and the same outside dimensions and all fit within said trough shaped unitary member and, when oriented laterally thereacross, contact said rear, bottom and side walls.

19. The bucket defined in claim 18, wherein said inner and outer side pieces are welded in position in said trough shaped unitary member to the front, bottom and rear walls thereof and to the torsion bar thereof.

20. The bucket defined in claim 19, wherein said outer side pieces are spaced inward slightly from the lateral ends of said trough shaped member to provide an inside corner to facilitate welding of said outer side pieces to said trough shaped member.