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The bucket
This dissertation analyses the strength and durability of a bucket elevator head manufacturedby Downfields Engineering Pty Ltd. The head is the top most component of a vertical bucket elevator and such contains the complete drive assembly. This means that all start-up and running torques are confined to this area. This analysis utilises computer simulation software with a finite element approach to predict head behaviour under worst-case scenario loadings.
Downfields is a privately owned company. The owner, Keith Schelberg, is also the founder and managing director. It was founded in 1976 on a grain property 20 km north west of Dalby. The company grew and relocated to Toowoomba in 1980. Now in 2005, Downfields employees 30 staff and continue to manufacture a wide range of grain handling and aeration equipment. A list of handling equipment includes bucket elevators, screw conveyors, drag conveyors，to just name a few.
Not only does Downfields build equipment but it also designs complete storage, processing facilities and is committed to always improving design and efficiency.
Downfields have only one manufacturing plant located in Toowoomba but manufacture for clients all over Australia and also some overseas, such as China. They have found that quality and durability are an important part of having a good product and reputation.
One of the most important items of equipment for Downfields is the bucket elevator. This is because it is their most prominent article, fewer competitors and
Figure 1-1 Bucket Elevator
many years of experience. At Downfields elevators are economically manufactured by profiling, pressing and welding sheet metal. The sheet metal assemblies create both the structure and sealed enclosure of an elevator. See Figure 1-1 for a typical configuration.
Figure 1-1 Bucket Elevator
The head is one of the major structural elements of the overall elevator. It supports the weight of buckets and belt, and also accommodates the drive and anti-runback back device. Corrosion and wear can also affect the durability depending on the environment in which the elevator is operated.
The author of this dissertation had been an employee of Downfields for a period of seven and a half years. During the course of employment it was noticed that an increase of client demands in areas of the following:
· Higher elevators. This allows grain to be spouted over greater distances, from elevator or fill taller silos. Increasing the height also increases loads on head bearings and brackets due to longer belt and more buckets.
· Larger Capacity elevators. To increase capacity either more buckets or larger sized buckets are used. A larger drive power is required to assist in this increase.
· Larger drives. The larger drive can be because an increase in hight, capacity or capability to start with fully loaded buckets. A larger drive transmits greater reactions back to mounts welded to the head assembly.
· Corrosive environments. Feedlots use elevators for conveying reconstituted (moistened) grain. The moisture in the grain allows grain, husk and dust to stick to the inside of the sheet metal structure. This rapidly increases the rate of corrosion inside the elevator.
Also other areas of the elevator noticed:
· Wear inside the head. The continual ‘rubbing’ of the grain against the steel slowly wears it away. The wearing occurs in known regions of the structure. These regions are the nose and/or throat of the head where the grain strikes because it is centrifugally discharged from the buckets.
· Optimum bracket design. There are a varying number of different styles used to support bearings, backstop or drive unit. They can be made from either a thicker profile or thinner pressed section. Brackets can also be attached by having a thickening plate, between the bracket and head structure, or an extended bracket that continues to the base flange.
Bucket elevators are one of Downfields most important pieces of equipment. The author had chosen the elevator head then after discussions with Downfields a specific model was chosen.
1.1 Project Objectives
1) Research
· Background of bucket elevators
· Materials used
· Methods of manufacture
· Styles of brackets used
2) Test materials used
3) Conduct analysis of each style of bracket
4) Conduct analysis of elevator head as a unit
5) Conduct analysis of head with effects of corrosion and wear
6) Conclude on overall safety factors of current design
Elevator Background
2.1 Elevator History
Bucket elevators are no new invention, according to (Colijn 1985, p. 330)
The bucket elevator is probably the oldest known form of conveyor, Its history can be traced back to the days of Babylon where wicker baskets lined with a natural pitch and fastened to ropes operating over wooden sheaves turned by slaves, were used for the elevating of water into irrigation ditches.
The Concept of the elevator has been around for many years. Variations that have changed to the elevator are its method of manufacture and types of materials used. As new materials are developed and quality materials become more readily available, then changes in design have been made to adapt to these materials. New technology has improved both design and manufacturing procedures. Computer technology has helped reduce design time, reduce rework and understanding the behaviour of materials under different loadings. New technologies in manufacture have reduced manufacturing time, costs, weight and increased tolerances. These changes have allowed increase discharge height and greater capacities to be obtained.
2.2 Elevator uses
An elevator is ideally used where the product needs to be elevated and consume only a small amount of ground area. Conveyed products are mostly granular solids, which range from powders to rocks. Limitations depend on how easily product can be loaded into and discharged from the bucket. Large granular products create difficulty loading and sticky products discharging. For products that are fragile and easily crushed, slower conveyance speeds are used.
2.3 Parts of an elevator
The elevator has five main areas being the head, legging, boot,belt & buckets, elevator access and elevator support. Each of these areas can be broken into smaller areas as listed below.
The head consists of:
· Main structural sheet metal frame
· Covers with or without wear liners
· Throat wiper
· Head pulley
· Drive Shaft
· Bearings
· Gearbox, coupling (belts, chain, etc.) and motor
· Backstop
Legging consists of
· Flange connections
· Inspection windows and access doors
Boot consists of
· Main Structural sheet metal frame
· Inlet chute with or without wear liner
· Boot pulley and shaft
· Bearings
· Pulley take-up for belt tensioning
Belt & buckets consist of
· Belt with holes
· Buckets
· Bolts, spacer washers
The Elevator Head
3.1 Assembly
The head of an elevator has a greater number and some of the most important functions for conveying the material. These functions consist of
· Supporting the weight of buckets, belt and material.
· Restraining the drive so electrical energy can be converted into mechanical energy to elevate material.
· Allow access and/or inspection to throat wiper, pulley lagging, belt and buckets.
· Enclose the conveying system so elements of the environment cannot contaminate the material.
· Enclose the material so waste isn’t created and dust cannot escape to the outside environment.
· Remove and separate the material from the buckets.
· Absorb impact of the material as well as converge it towards its final destination.
· Due to intermittent stopping, the head has to stop the belt running backwards.
All these functions are combined into the one main head sheet metal structure. Most individual functions are accomplished by components that attach to the main frame. See Figure 3-1.
Figure 3-1 Elevator Head Assembled
3.2 Head Selection
Selecting the size and model of elevator was done with Downfields assistance. The elevator chosen was one of the largest models made using a 3mm sheet metal structure. This choice was to be a model that was expected to have high stress concentrations. The general specifications of the chosen elevator are as follows.
Downfields model No.: SPS300
Motor power: 15 kW
Gearbox ratio: 20:1
Capacity: 20 tonnes per hour
Conveyed material: Wheat @ 750 Kg/m3
Discharge height: 26.5m
The Design of the head
4.1 Materials
Elevators can be made using different types of materials. Materials of the structure and brackets also can be different types as well.
The materials used to construct the head structure vary depending on conveyed material, cost and appearance, which are specified by the customer. These materials can range from galvanised, mild steel or stainless steel sheet. Galvanised is the most common material used by Downfields. Mild steel is used when a painted finish is required and stainless steel for when high moisture or corrosive materials are being conveyed. Since galvanised sheet was the most common it was used in the analysis for the head structure. Mechanical properties of the galvanised sheet were unavailable so testing strips were compared with mild steel ones to check their comparison. For more information on testing refer to section Chapter 6.0 Sheet Metal Joint Test Samples.
Bracket material normally matches that of the head structure, but for a galvanised head, mild steel is used. They are constructed by using standard plate and structural sections, which are unavailable in a galvanised finish. During the fabrication process a large amount of welding is done in the bracket areas, which melts and burns away the galvanised coating. Due to the unavailability and fabrication process brackets are made from standard mild steel plate and structural sections then painted, for a galvanised head structure construction.
A list of the materials used in this analysis is,
· 3.0mm galv – AS1397/G2 Z275
· 3.0mm HR – 250 grade AS1594/ GD HA1
· 5.0mm plate – 250 grade AS1594 GR HA250
· Channel & Angle – 300PLUS
These material specification types were given by Downfields (refer to appendix D.1 Material Grades on page 62). Note sheet refers to 3mm or less and plate refers to 4mm or greater.
4.1.1 Galvanised Sheet
Galvanised sheet is a mild steel sheet coated with a layer of zinc both sides. These layers are applied using a hot dip process. This material has the advantages of reduced fabrication time and durability. According to Abbott (1997, p. 3) extreme Australian weather conditions from hot sun, heavy wind and rain conditions encourage the use of galvanised sheet. The analysis of the sheet metal structure in this dissertation primarily used the galvanised coated sheet.
The specification for this sheet is 3.0mm galv – AS1397/G2 Z275 and the code refers to the following,
G – indicate that mechanical properties have been achieved or modified by in-line heat treatment prior to hot dipping.
2 – Commercial forming.
Z – It is a zinc coating.
275 – the mass of zinc in grams per m2 for both sides.
This grade doesn’t specify the minimum mechanical properties of the galv sheet because it is a formability grade not a structural grade. The formable G2 grade has less carbon as shown in Table 1 than the structural G250 grade, which gives it more flexibility. For this reason test were conducted to compare the galv with mild steel sheet.
Table 1 Galv Sheet Required Chemical Composition
(Adapted from AS 1397 – 2001, Table 2.1, p 9. &
AS 1594 – 2002, Table 2.2, p 11.)
Since the galv material is made with a mild steel centre and layered with zinc this can change the overall mechanical properties, but according to (AS 1397 – 2001, p 10.)
It is international practice to tensile test zinc-coated sheet and strip with the coating intact, and to calculate the strength using the cross-sectional area of the steel base metal only, since the contribution made by the zinc coating is so small that, for practical purposes, it can be ignored. The strength value obtained is close to the strength of the base material itself.
For this reason the contribution of the zinc layers were also ignored in the head structure
models.
4.1.2 Mild Steel Sheet
The purpose of considering mild steel in this analysis was for comparison with the galvanised sheet. Mild steel specification was 3.0mm HR – 250 grade AS1594/ GD HA1 which refers to the following,
H – indicates that it has been hot rolled
A – indicates the reoxidation practice is aluminium killed
1 – indicates that it is for commercial forming
(AS 1594-2002, pp 6-8)
The specification of HA1 is a formable grade and not a structural grade, but the supplier had specified that it is a 250 grade, which shows similar chemical compositions, as shown in Table 1, to the structural grade 250. This indicates that its mechanical properties are 250Mpa for the minimum yield strength and 350Mpa for minimum tensile strength (AS 1594 – 2002, Table 3.1, p 15.).
中文譯文
斗式提升機
本文分析了力和斗式提升機頭部manufacturedby Downfields工程私人有限公司。耐久性頭部是最上面的一個組成部分，垂直斗式提升機等包含完整的驅(qū)動裝置。這意味著所有啟動和運行轉(zhuǎn)矩只限于這一地區(qū)。這項分析利用了有限元方法的計算機模擬軟件來預(yù)測在最壞的情況下負荷頭的行為。
Downfields是一家私人擁有的公司。店主人Keith Schelberg，也是創(chuàng)辦人兼董事總經(jīng)理。它成立于1976年的具有糧食屬性的公司，它位于多爾比20公里的西北部。該公司在1980年不斷的發(fā)展，并搬遷到了圖沃柏?，F(xiàn)在，在2005年，Downfields已經(jīng)擁有30名工作人員和雇員，他們繼續(xù)生產(chǎn)的糧食處理和通風(fēng)設(shè)備被運用到廣泛范圍。而處理設(shè)備中包括斗式提升機，螺旋輸送機，帶式輸送機，以此僅舉幾例。
不僅Downfields生產(chǎn)建設(shè)設(shè)備，而且它也設(shè)計完整的存儲和處理設(shè)施，并一直致力于改進設(shè)計和提高設(shè)備的效率。
Downfields只有一個制造工廠，但在位于圖沃柏的工廠為客戶制造全澳及海外一些所需要的產(chǎn)品，如中國。他們發(fā)現(xiàn)，質(zhì)量和耐用性是一個好的產(chǎn)品和聲譽的重要組成部分。
Downfields生產(chǎn)的設(shè)備中最重要的項目之一是斗式提升機。這是因為這是他們最突出的一個產(chǎn)品，不僅他擁有很少的競爭對手，而且還積累了多年的經(jīng)驗。在Downfields升降機是最經(jīng)濟的制造產(chǎn)品，它們通過沖壓及焊接薄板。鈑金組件是提升機的結(jié)構(gòu)和密封外殼所必須的構(gòu)件。參見圖1-1為一個典型的配置。
圖1-1斗式提升機
頭部是整體結(jié)構(gòu)提升機主要因素之一。它支持著提升機和鏈條的重量，同時還包含了驅(qū)動器和反回轉(zhuǎn)工況的回轉(zhuǎn)設(shè)備。腐蝕和磨損也會影響提升機的耐久性，這種耐久性取決于該提升機的工作環(huán)境因素。
本論文的作者已為七年半的Downfields雇員。在工作的過程中，他注意到了在下列領(lǐng)域的產(chǎn)品客戶需求量增加：
高度大的提升機，這使得糧食能提升到更大的距離，從提升因素或填充系數(shù)講。增加的高度也增加頭部軸承和支架負荷，這歸咎于鏈條的長度和更多的料斗。
容量大的提升機，為了提高容量則需要用更多或更大尺寸的料斗。一個更加大的驅(qū)動力裝置是協(xié)助提升機提升容量增長所必須的因素。
大驅(qū)動的提升機，較大的驅(qū)動器可以使高度增加，更加有能力提升滿載的提升料斗。一個更大的驅(qū)動器傳輸，會產(chǎn)生更大的反饋到頭部焊接組裝處。
腐蝕性環(huán)境，飼養(yǎng)場使用提升機運送混合的（潮濕的）糧食。這些糧食水分允許的谷物、稻殼和灰塵粘貼在板材金屬結(jié)構(gòu)里面。這也大幅度的加快了電梯內(nèi)部的腐蝕速率。
在其他地區(qū)也發(fā)現(xiàn)提升機：
位于里面的頭部，這種不間斷“糧食對鋼鐵的摩擦”慢慢使得頭部磨穿而泄漏。在這些已知的結(jié)構(gòu)區(qū)段經(jīng)常發(fā)生，這些區(qū)段是出料口和/或頭部，因為糧食是通過離心力從提升機的出料口而排出的。
支撐架的優(yōu)化設(shè)計。有許多用于支撐軸承，逆止或驅(qū)動裝置，不同的型號支撐架的數(shù)目不同。他們不是被制造成較厚的側(cè)板，就是被制造成較薄的壓型部分。這些部分也可以有一個附加的加厚板，支架和頭部之間的結(jié)構(gòu)，或可以繼續(xù)擴展支架的基礎(chǔ)法蘭。
斗式提升機是Downfields最重要的設(shè)備的一個部分。筆者選擇了提升機的Downfields頭部，然后討論一個具體的模型選擇。
1.1 項目目標(biāo)
1）研究
斗式提升機的背景
使用的材料
制造方法
提升機運用的場合
2）用于測試材料
3）每種類型提升機支撐架的分析
4）電梯的頭部作為一個單元的分析
5） 就腐蝕和磨損對頭部的影響分析
6） 對設(shè)計的總體安全系數(shù)分析
提升機的背景
2.1提升機的歷史
斗式提升機不是什么新發(fā)明，根據(jù)（科萊恩1985年，第330頁）
斗式提升機的可能是已知的最古老的輸送方式，其歷史可追溯到巴比倫的地方，在那奴隸用鑲有天然瀝青和拴繩的柳條筐完成每天的作業(yè)，分別用于水的提升，使水進入灌溉溝渠。
該提升機的概念已流傳了許多年。提升機的變化是它的制造方法和使用的材料種類的變化。隨著新材料的開發(fā)和材料的質(zhì)量變得更加容易獲得，然后在設(shè)計變更已作出適應(yīng)這些材料。新技術(shù)提高了設(shè)計和制造程序。計算機技術(shù)幫助減少設(shè)計時間，減少返工和掌握不同載荷下的材料特性。新的制造技術(shù)已經(jīng)降低了制造時間、成本、重量和公差的加大。這些變化允許獲得更大的提升的高度和能力。
2.2提升機的使用
提升機是一種理想的使用產(chǎn)品，它用于生產(chǎn)只需要占據(jù)少量的地面面積。傳輸?shù)奈锪现饕罟腆w，從巖石粉末顆粒范圍。決定了提升機是否可以輕松地將物料加載到料斗里面。大顆粒粘性的物料制造裝卸困難。對于脆弱的，容易破碎的物料運送時，運送速度應(yīng)該較慢使用。
2.3提升機的部件
電梯有5個主要部件，分別是頭部，底座，鏈條，料斗，提升機檢修門和提升機的支持。這些區(qū)段每一個都可以被分解成較小的部分具體如下。
該機構(gòu)頭部包括：
主要結(jié)構(gòu)板鋼架
蓋板
管道刷
頭滑輪
驅(qū)動軸
軸承
變速箱，連接（皮帶，鏈條等）和制動器
支撐架
下部結(jié)構(gòu)包括：
法蘭連接
檢驗窗戶和門卡
啟動結(jié)構(gòu)包括:
主要結(jié)構(gòu)板材架
灣槽
啟動滑輪和軸
軸承
滑輪采取活動的拉緊帶
鏈條和料斗的組成:
皮帶的孔
料斗
螺栓，墊片墊圈
提升機的頭部
3.1概述
一部提升機的頭部有一個更大的數(shù)目和最重要的功能是輸送物質(zhì)。這些功能包括：
1.料斗的重量，鏈條和物料。
2.制約的驅(qū)動器，以便可以將電能轉(zhuǎn)換為機械能提高物料。
3.允許通過或檢查的管道雨刮器，滑輪滯后，皮帶和料斗。
4.附上的輸送系統(tǒng)，以便環(huán)境的元素不能污染物質(zhì)。
5.附上的材料使不創(chuàng)造廢物和使灰塵無法逃逸到外部環(huán)境。
6.使材料從料斗移出和分開。
7.吸收材料的沖擊使其更好的運送，達到最終的運輸目的。
8.由于間歇性停止，頭部結(jié)構(gòu)必須避免鏈條打滑回轉(zhuǎn)。
所有這些功能都合并成一個金屬板結(jié)構(gòu)的主要頭部結(jié)構(gòu)。大多數(shù)個別構(gòu)件職能完成的組件，附加到主框架。見圖3-1。
3.2 頭部的挑選
挑選提升機的尺寸和模型是Downfields所輔助的工作，提升機的選擇是首先通過3mm大的金屬支架設(shè)備制成的最大模型。這些選擇的模型是通過高壓壓型而成的。傳統(tǒng)選擇提升機的特征值如下：
Downfields的型號No：SPS300
提升功率：15Kw
變速箱減速比：20:1
輸送量：20 t/h
輸送物料：小麥，750Kg/
提升機頭部的設(shè)計
4.1材料
提升機可使用不同類型的材料。管道內(nèi)的結(jié)構(gòu)和材料也可以是不同的類型，以及。所用的材料建造的頭部結(jié)構(gòu)取決于所要提升的物料，成本和外觀，這是由客戶指定。這些材料可以從鍍鋅，低碳鋼或不銹鋼板。鍍鋅是作為最常見的材料被Downfields經(jīng)常使用。低碳鋼被使用于完成后所是必要的地方，和當(dāng)輸送高水分或腐蝕性物質(zhì)時，不銹鋼用于鍍鋅板上時，最常見的是在它的頭部結(jié)構(gòu)。鍍鋅板的力學(xué)性能與低碳鋼相比進行了測試，檢查他們的比較，有時候鍍鋅板是不可用的。欲了解更多有關(guān)測試的信息，請參閱第6.0章鈑金聯(lián)合測試樣品。
支架的材料通常匹配的頭部結(jié)構(gòu)，但對于鍍鋅頭，則用低碳鋼。他們通過使用標(biāo)準(zhǔn)的構(gòu)造板塊和結(jié)構(gòu)部分，這是一個不可用鍍鋅完成。在制造過程中大量的焊接完成管道內(nèi)地方，融化和燒傷外鍍鋅涂層。由于無法獲得和制造過程中管道內(nèi)是標(biāo)準(zhǔn)的軟鋼制成的板材和結(jié)構(gòu)部分，然后畫了一個鍍鋅頭結(jié)構(gòu)示意圖。
在這個分析中使用的材料清單:
3.0mm galv – AS1397/G2 Z275
3.0mm HR – 250 grade AS1594/ GD HA1
5.0mm plate – 250 grade AS1594 GR HA250
Channel & Angle – 300PLUS
這些材料規(guī)格種類給予Downfields（參閱附錄D.1中第62頁材料等級）。注意：表是指3毫米或以下，板指為4mm或更大。
4.1.1鍍鋅板
鍍鋅板是一個溫和鋼板鋅層與雙方涂層。這些應(yīng)用層采用熱浸過程。這種材料具有降低制造時間和耐用的優(yōu)點。據(jù)雅培（1997，第3頁）從極端炎熱的太陽，大風(fēng)和降雨條件，澳大利亞的天氣條件鼓勵鍍鋅板的使用。工作表的金屬結(jié)構(gòu)分析，在此論文主要用于鍍鋅彩色涂層鋼板。
此表規(guī)格3.0毫米鍍鋅 - AS1397/G2 Z275和代碼是指以下，
G - 表明，機械性能已達到或修改的在線熱處理前的熱浸。
2 - 商業(yè)的結(jié)構(gòu)。
Z - 這是一個鋅涂層。
275 - 在每平方米克鋅雙方的質(zhì)量。
這個級別不是指鍍鋅板的力學(xué)性能最低，因為它是一個成形等級而不是結(jié)構(gòu)等級。 G2級的與表1中的G250級相比較成型性碳少，這給它帶來更多的靈活性。基于這個原因，測試進行比較輕鋼板鍍鋅。由于鍍鋅材料是用低碳鋼中心和鋅層構(gòu)成這可以改變整體力學(xué)性能。（如1397年至2001年，10頁。）
這是國際慣例，與涂層完好拉伸試驗鍍鋅鋼板和鋼帶，并計算強度使用的鋼鐵基本金屬截面積，因為在鋅層發(fā)揮的功能是很微小的，實際上，它可以被忽略。獲得的力量值是接近的基礎(chǔ)材料本身的力量.因為這原因?qū)τ阡\層的作用也忽略的頭部結(jié)構(gòu)。
4.1.2低碳鋼板
在審議這一分析的目的是為軟鋼與鍍鋅板的比較。低碳鋼規(guī)范是3.0mm HR – 250 grade AS1594/ GD HA1是指以下，
H - 表示它已被熱軋
A - 表示鋁再氧化的做法
1 - 表示它是商業(yè)的結(jié)構(gòu)（如1594至2002年，頁6-8）
該規(guī)范是一個成型性HA1區(qū)級，而不是一個結(jié)構(gòu)性的等級，但供應(yīng)商已明確指出這是一個250級，這表明類似的化學(xué)成分，如表1所示，在結(jié)構(gòu)級250。這表明，其力學(xué)性能的最低屈服強度和拉伸強度最低（如1594年至2002年，表3.1，磷15 350Mpa 250Mpa。）。
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