電動(dòng)絞肉機(jī)的設(shè)計(jì)

電動(dòng)絞肉機(jī)的設(shè)計(jì),電動(dòng)絞肉機(jī)的設(shè)計(jì),電動(dòng),絞肉機(jī),設(shè)計(jì) 江西農(nóng)業(yè)大學(xué)畢業(yè)設(shè)計(jì)(論文)任務(wù)書 設(shè)計(jì)（論文） 課題名稱 電動(dòng)絞肉機(jī)的設(shè)計(jì)與研究 學(xué)生姓名 李帝 院（系） 工學(xué)院 專 業(yè) 機(jī)械設(shè)計(jì)制造及其自動(dòng)化 指導(dǎo)教師 肖懷國(guó) 職 稱 副教授 學(xué) 歷 畢業(yè)設(shè)計(jì)（論文）要求： 1. 要求在完成論文期間，積極主動(dòng)，查閱大量文獻(xiàn)，獨(dú)立創(chuàng)新； 2. 按時(shí)完成畢業(yè)設(shè)計(jì)內(nèi)容，方案切實(shí)可行； 3. 獨(dú)立繪制裝配圖和零件圖； 4. 圖紙量不少于1.5張A0； 5. 獨(dú)立完成畢業(yè)設(shè)計(jì)說明書，格式正確，要求字?jǐn)?shù)不少于5000字； 6. 完成電子文檔，并打印裝訂成冊(cè)。 畢業(yè)設(shè)計(jì)（論文）內(nèi)容與技術(shù)參數(shù)： 1. 熟悉電動(dòng)絞肉機(jī)的整個(gè)設(shè)計(jì)過程 2. 電動(dòng)絞肉機(jī)可實(shí)現(xiàn)對(duì)不同肉種的加工. 畢業(yè)設(shè)計(jì)（論文）工作計(jì)劃： 1. 了解本機(jī)工作原理，明白設(shè)計(jì)意義； 2. 查閱資料并畫出機(jī)械機(jī)構(gòu)草圖； 3. 通過計(jì)算確定零件尺寸，并掌握零件主要參數(shù)及材料熱處理方式； 4. 寫設(shè)計(jì)說明書； 5. 根據(jù)設(shè)計(jì)說明書的計(jì)算尺寸，畫出各零件圖； 6. 畫出機(jī)械總裝配圖。 接受任務(wù)日期 2012 年 2 月 7 日 要求完成日期 2012 年 5 月 5 日 學(xué) 生 簽 名 李帝 2012年 5 月 1日 指導(dǎo)教師簽名 年 月 日 院長(zhǎng)（主任）簽名 年 月 日 編 號(hào) 20080970 江西農(nóng)業(yè)大學(xué) 工學(xué)院 畢業(yè)設(shè)計(jì)材料 題 目 電動(dòng)絞肉機(jī)設(shè)計(jì) 專 業(yè) 機(jī)械設(shè)計(jì)制造及其自動(dòng)化 學(xué)生姓名 李帝 材 料 目 錄 序號(hào) 附 件 名 稱 數(shù)量 備注 1 畢業(yè)設(shè)計(jì)論文 1 2 零件圖 15 3 總裝圖 1 4 設(shè)計(jì)任務(wù)書 1 二〇一二年 五月 電動(dòng)絞肉機(jī)的設(shè)計(jì) 目錄 摘要 .I Abstract .II 1緒 論 .1 1結(jié) 構(gòu) 及 工 作 原 理 .2 1.1絞肉機(jī)的結(jié)構(gòu) .2 1.2絞肉機(jī)的工作原理 .2 3絞龍的設(shè)計(jì) .3 3.1絞龍的設(shè)計(jì) .3 3.1.1絞龍的材料 .4 3.1.2絞龍直徑 .4 3.1.3絞龍的轉(zhuǎn)速 .4 3.2絞筒的設(shè)計(jì) .4 4 傳動(dòng)系統(tǒng)的設(shè)計(jì) .4 4.1電機(jī)的選擇 .5 4.2 帶輪的設(shè)計(jì) .5 4.3齒輪傳動(dòng)的設(shè)計(jì)計(jì)算 .8 4.4齒輪軸的設(shè)計(jì)與校核 .11 4.4.1各軸的轉(zhuǎn)速計(jì)算 .12 4.4.2各軸輸入功率計(jì)算 .12 4.4.3各軸輸入扭矩計(jì)算 .12 4.4.4 按彎扭合成強(qiáng)度校核軸徑 .12 5絞 刀 設(shè) 計(jì) .13 5.1絞刀的設(shè)計(jì) .13 5.1.1刀刃的起訖位置 .14 5.1.2刀刃的前角 .14 5.1.3刀刃的后角 .14 5.1.4刀刃的刃傾角 .15 5.1.5刀片的結(jié)構(gòu) .15 6生產(chǎn)能力分析 .16 6.1絞刀的切割能力 .16 6.2 絞肉機(jī)的生產(chǎn)能力G .16 6.3功率消耗N .16 7絞肉機(jī)的使用與日常維護(hù)簡(jiǎn)介 .16 8設(shè)計(jì)小結(jié) .18 參考文獻(xiàn) .19 I 摘要 絞肉機(jī)是肉類加工企業(yè)在生產(chǎn)過程中將原料肉按不同工藝要求加工規(guī)格不 等的顆粒狀肉餡,廣泛適用于各種香腸、火腿腸、午餐肉、丸子、咸味香精、寵 物食品和其他肉制品等行業(yè)。本文論述了絞肉機(jī)的結(jié)構(gòu)、工作原理、主要技術(shù) 參數(shù)、傳動(dòng)系統(tǒng)、典型零件的結(jié)構(gòu)設(shè)計(jì)及生產(chǎn)能力分析。希望對(duì)本次設(shè)計(jì)的探 索和研究能設(shè)計(jì)出實(shí)用經(jīng)濟(jì)的家用絞肉機(jī)，更好的方便人民的生活食品工業(yè)的 現(xiàn)代化水平，在很大程度上依賴于食品機(jī)械的發(fā)展及其現(xiàn)代化水，離開現(xiàn)代儀 器和設(shè)備，現(xiàn)代食品工業(yè)就無從談起。 食品工業(yè)的發(fā)展是設(shè)備和工藝共同發(fā)展的結(jié)果，應(yīng)使設(shè)備和工藝達(dá)到最佳 配合，以設(shè)備革新和創(chuàng)新促進(jìn)工藝的改進(jìn)和發(fā)展，以工藝的發(fā)展進(jìn)一部促進(jìn)設(shè) 備的發(fā)展和完善。兩者互相促進(jìn)、互相完善，是使整個(gè)食品工業(yè)向現(xiàn)代化邁進(jìn) 的必要條件。 關(guān)鍵詞：絞肉機(jī)；擠肉樣板；絞刀；絞龍；螺旋供料器；齒輪強(qiáng)度 II Abstract Is meat processing enterprise in stage production of raw meat will according to different process request processing specifications vary granular meat, widely used in all kinds of sausage, ham bowel, lunch meat ball, salty, essence, pet food and other meat products, etc.Many practical machines in our life are from mechanical design, this paper elaborates on the meat choppers structure,operating principlemain ,technical parameter, transmission system andstructural design and production capacity analysis of typical parts are mannyintroduced.It is hoped that a practical and economical household meat chopper could be designed according to the authors exploration, which would be convenient for the life of people. instruments and equipment the modern food industry cannot survive. The food industry is the development of the equipment and the result of the development of common technology should make the equipment and technology reach the best with equipment and innovation to promote the innovation process improvement and development, in order to promote the development of the technology into a equipment of development and perfection. Both promote each other, mutual perfect, is to make the whole food industry on the necessary conditions to modernization. Key words: meat chopper;crowded meat model;reamer;stranding cage;screw feeder;gear strength 1 1緒 論 絞 肉 機(jī) 是 肉 類 加 工 企 業(yè) 在 生 產(chǎn) 過 程 中 ， 將 原 料 肉 按 不 同 工 藝 要 求 加 工 規(guī) 格 不 等 的 顆 粒 狀 肉 餡 ， 以 便 于 同 其 它 輔 料 充 分 混 合 來 滿 足 不 同 產(chǎn) 品 的 需 求 絞 肉 機(jī) 為 系 列 產(chǎn) 品 ； 工 作 時(shí) 利 用 轉(zhuǎn) 動(dòng) 的 切 刀 刃 和 孔 板 上 孔 眼 刃 形 成 的 剪 切 作 用 將 原 料 肉 切 碎 ， 并 在 螺 桿 擠 壓 力 的 作 用 下 ， 將 原 料 不 斷 排 出 機(jī) 外 。 可 根 據(jù) 物 料 性 質(zhì) 和 加 工 要 求 的 不 同 ， 配 置 相 應(yīng) 的 刀 具 和 孔 板 ， 即 可 加 工 出 不 同 尺 寸 的 顆 粒 ， 以 滿 足 下 道 工 序 的 工 藝 要 求 。 隨著國(guó)民經(jīng)濟(jì)的發(fā)展和人民生活水平的 提高，人民對(duì)食品工業(yè)提出了更高的要求?，F(xiàn)代食品已朝著營(yíng)養(yǎng)、綠色、方便 、功能食品的方向發(fā)展，且功能食品將成為新世紀(jì)的主流食品。食品工業(yè)也成 為國(guó)民經(jīng)濟(jì)的支柱產(chǎn)業(yè)，作為裝備食品工業(yè)的食品機(jī)械工業(yè)發(fā)展尤為迅猛。 在肉類加工的過程中，切碎、斬拌攪拌工序的機(jī)械化程度最高，其中絞肉 機(jī)、斬拌機(jī)、攪拌機(jī)是最基本的加工主械.幾乎所有的肉類加工廠都具備這3種 設(shè)備。國(guó)內(nèi)一些大型肉類加工廠先后從西德、丹麥、瑞士、日本等引進(jìn)了先進(jìn) 的加工設(shè)備，但其價(jià)格十分昂貴。目前中、小型肉類加工企業(yè)所使用的大部分 設(shè)備為我國(guó)自行設(shè)計(jì)制造的產(chǎn)絞肉機(jī)是為中、小型肉類加二企業(yè)所設(shè)計(jì)的較為 理想的、絞制各種肉餡的機(jī)械，比如生產(chǎn)午餐肉罐頭和制造魚醬、魚圓之類的 產(chǎn)品，它將肉可進(jìn)行粗、中、細(xì)絞以滿足不同加工工藝的要求，該機(jī)亦可作為 其他原料的擠壓設(shè)備。 絞 肉 機(jī) 零 部 件 主 要 采 用 優(yōu) 質(zhì) （ 鑄 鐵 件 ） 或 不 銹 鋼 制 造 ， 對(duì) 加 工 物 料 無 污 染 ， 符 合 食 品 衛(wèi) 生 標(biāo) 準(zhǔn) 。 刀 具 經(jīng) 特 殊 熱 處 理 ， 耐 磨 性 能 優(yōu) 越 ， 使 用 壽 命 長(zhǎng) 。 該 機(jī) 操 作 簡(jiǎn) 單 、 拆 卸 組 裝 方 便 調(diào) 節(jié) 或 更 換 。 2 1結(jié) 構(gòu) 及 工 作 原 理 1.1絞肉機(jī)的結(jié)構(gòu)（下圖） 絞肉機(jī)主要由動(dòng)力機(jī)構(gòu)以及傳送機(jī)構(gòu)和切割機(jī)構(gòu)組成，其中動(dòng)力機(jī)構(gòu)主要 為電動(dòng)機(jī)，皮帶輪，減速器。被切割物料的傳送機(jī)構(gòu)主要是進(jìn)肉斗，絞筒，絞 龍。而切割機(jī)構(gòu)主要為絞刀，擠肉樣板，以及旋蓋。 1.2絞肉機(jī)的工作原理 工作時(shí)，先開機(jī)后放料，由于物料本身的重力和螺旋供料器的旋轉(zhuǎn)，把物 連續(xù)地送往絞刀口進(jìn)行切碎。因?yàn)槁菪┝掀鞯穆菥嗪竺鎽?yīng)比前面小，但螺旋 軸的直徑后面比前面大，這樣對(duì)物料產(chǎn)生了一定的擠壓力，這個(gè)力迫使已切碎 的肉從格板上的孔眼中排出。 用于午餐肉罐頭生產(chǎn)時(shí)，肥肉需要粗絞而瘦肉需要細(xì)絞，以調(diào)換格板的方 式來達(dá)到粗絞與細(xì)絞之需。格板有幾種不同規(guī)格的孔眼，通常粗絞用之直徑為8 10毫米、細(xì)絞用直徑35毫米的孔眼。粗絞與細(xì)絞的格板，其厚度都為101 2毫米普通鋼板。由于粗絞孔徑較大，排料較易，故螺旋供料器的轉(zhuǎn)速可比細(xì)絞 時(shí)快些，但最大不超過400轉(zhuǎn)/分。一般在200400 轉(zhuǎn)/ 分。因?yàn)楦癜迳系目籽劭?3 面積一定，即排料量一定，當(dāng)供料螺旋轉(zhuǎn)速太快時(shí)，使物料在切刀附近堵塞， 造成負(fù)荷突然增加，對(duì)電動(dòng)機(jī)有不良的影響。 絞刀刃口是順著切刀轉(zhuǎn)學(xué)安裝的。絞刀用工具鋼制造，刀口要求鋒利，使 用一個(gè)時(shí)期后，刀口變鈍，此時(shí)應(yīng)調(diào)換新刀片或重新修磨，否則將影響切割效 率，甚至使有些無聊不是切碎后排出，而是由擠壓、磨碎后成漿狀排出，直接 影響成品質(zhì)量，據(jù)有些廠的研究，午餐肉罐頭脂肪嚴(yán)重析出的質(zhì)量事故，往往 與此原因有關(guān)。 裝配或調(diào)換絞刀后，一定要把緊固螺母旋緊，才能保證格板不動(dòng)，否則因 格板移動(dòng)和絞刀轉(zhuǎn)動(dòng)之間產(chǎn)生相對(duì)運(yùn)動(dòng)，也會(huì)引起對(duì)物料磨漿的作用。絞刀必 須與格板緊密貼和，不然會(huì)影響切割效率。 螺旋供料器在機(jī)壁里旋轉(zhuǎn)，要防止螺旋外表與機(jī)壁相碰，若稍相碰，馬上 損壞機(jī)器。但它們的間隙又不能過大，過大會(huì)影響送料效率和擠壓力，甚至使 物料從間隙處倒流，因此這部分零部件的加工和安裝的要求較高。 絞肉機(jī)的生產(chǎn)能力不能由螺旋供料器決定，而由切刀的切割能力來決定。 因?yàn)榍懈詈笪锪媳仨殢目籽壑信懦?，螺旋供料器才能繼續(xù)送料，否則，送料再 多也不行，相反會(huì)產(chǎn)生物料堵塞現(xiàn)象。 3絞龍的設(shè)計(jì) 3.1絞龍的設(shè)計(jì) 絞龍又名螺旋供料器其作用是向前輸送物料，并在前端對(duì)肉塊進(jìn)行擠壓。 如圖31所示，設(shè)計(jì)上采用一根變螺距、變根徑的螺旋，即螺距后大前小，根 徑后小前大，這樣使其絞龍與絞龍之間的容積逐漸減小實(shí)現(xiàn)了對(duì)物料的擠壓作 用。絞龍的前軸分為兩個(gè)部分的連接，分別是與軸承配合的圓形軸和與絞刀連 接的方形軸，后軸與凸緣聯(lián)軸器連接傳遞動(dòng)力以實(shí)現(xiàn)對(duì)被絞物料的輸送與擠壓 的雙重作用。 4 3.1.1絞龍的材料 絞龍為鑄件所以其材料可選灰色鑄鐵可選為HT150。 3.1.2絞龍直徑 5.2CGKD G生產(chǎn)能力 K物料綜合特性系數(shù) -物料得填充系數(shù) 物料的堆積密度 C與螺旋供料器傾角有關(guān)的系數(shù) 3.1.3絞龍的轉(zhuǎn)速 由于絞龍只有一種工作轉(zhuǎn)速，則從電機(jī)至絞龍的運(yùn)動(dòng)路線為定比傳動(dòng)，其 總的傳動(dòng)比可利用帶傳動(dòng)、齒輪傳動(dòng)等構(gòu)機(jī)逐級(jí)減速后得到。絞籠的轉(zhuǎn)速不易 太高，因?yàn)檩斔湍芰Σ⒉皇请S轉(zhuǎn)速增加而增加。當(dāng)速度達(dá)到一定值以后，效率 反而下降，且速度過高，物料磨擦生熱，出口處的壓力升高，易引起物料變性 ，影響絞肉質(zhì)量，因此絞籠的轉(zhuǎn)速一般在200一400r/min比較適宜。經(jīng)查食品加 工過技術(shù)裝備綜合考慮本機(jī)選用326r/min。 3.2絞筒的設(shè)計(jì) 由于肉在絞筒內(nèi)受到攪動(dòng)，且受擠壓力的反作用力作用，物料具有向后倒流的 趨勢(shì)，因此在絞籠的內(nèi)壁上設(shè)計(jì)了8個(gè)止推槽.沿圓周均勻分布，絞筒內(nèi)壁與絞 籠之間的間隙要適當(dāng)，一般為2- 3mm。間隙太大會(huì)使物料倒流;間隙太小絞籠與絞筒內(nèi)壁易碰撞。 絞筒的材料可選用鑄鐵，選HT150 4 傳動(dòng)系統(tǒng)的設(shè)計(jì) 由于絞龍只有一種工作轉(zhuǎn)速，所以無需變速裝置，然而考慮到絞龍的轉(zhuǎn)速 5 與被加工肉的加工質(zhì)量，本設(shè)計(jì)選擇兩級(jí)傳動(dòng)，第一級(jí)由電動(dòng)機(jī)輸出的帶輪傳 動(dòng)，并實(shí)現(xiàn)一次減速，第二級(jí)傳動(dòng)為齒輪傳動(dòng)，因?yàn)閷?duì)生產(chǎn)衛(wèi)生要求較嚴(yán)格， 故齒輪需要采用閉式傳動(dòng)型，所以采用一級(jí)減速箱的傳動(dòng)，則從電機(jī)至絞龍的 運(yùn)動(dòng)路線為定比傳動(dòng)，其總的傳動(dòng)路線為：電動(dòng)機(jī)高速軸小皮帶輪 大皮帶輪 齒輪軸大齒輪絞龍。 4.1電機(jī)的選擇 由上面的介紹可知，整個(gè)傳動(dòng)路線是電動(dòng)機(jī)輸出然后經(jīng)過帶輪與齒輪的兩級(jí)減 速傳遞到絞龍與絞刀的， N= 4(KW)WG G絞肉機(jī)的生產(chǎn)能力，1000kg/h W切割1kg物料耗用能量，其值與孔眼直徑有關(guān)，d小則w大，當(dāng)d 3mm， 取w0.0030kw.h/kg。 傳動(dòng)效率，取0.75 所以根據(jù)N4kw，查機(jī)械設(shè)計(jì)課程設(shè)計(jì)表2- 239，選擇Y系列的三相鼠籠電動(dòng)機(jī)其同步轉(zhuǎn)速n1500r/min，滿載時(shí)為1440r/m in，型號(hào)為Y112M- 4。多以整個(gè)傳動(dòng)系統(tǒng)的傳動(dòng)比為 4.41 ，取為4.4，所以依據(jù)前面的 總 =1440326 傳動(dòng)設(shè)計(jì)，初定傳動(dòng)比分配為 =4.4= x 其中 為皮帶輪的傳動(dòng)比，先設(shè)為1.總 12 1 76， 為減速箱的減速比，初定為2.5。,2 4.2 帶輪的設(shè)計(jì) 選用普通V帶傳動(dòng)，動(dòng)力機(jī)位Y系列三相異步電動(dòng)機(jī)，功率P=4kw，轉(zhuǎn)速n= 1440，計(jì)算步驟參照機(jī)械設(shè)計(jì)帶輪設(shè)計(jì)步驟。 計(jì)算項(xiàng)目 計(jì)算內(nèi)容 計(jì)算結(jié)果 定V帶型號(hào)和帶輪直徑 工作情況系數(shù) 由表 =1.2 計(jì)算功率 = P=1.24 =4.8kw 選帶型號(hào) 查機(jī)械簡(jiǎn)明設(shè)計(jì)手冊(cè) A型 小帶輪直徑 由機(jī)械設(shè)計(jì)表 取 =100mm1 6 大帶輪直徑 =ix =176mm 2 1 查機(jī)械設(shè)計(jì)簡(jiǎn)明手冊(cè)選 =180mm（為滑動(dòng)率，取=1%）2 計(jì)算項(xiàng)目 計(jì)算內(nèi)容 計(jì)算結(jié)果 計(jì)算帶長(zhǎng) 求 = = =140 1+22 100+1802 求 = = =40 2-12 1801002 初取中心距 a=2X( )280mm1+2 帶長(zhǎng) L= +2a+ = 199+2500+ L=886mm 2 1092500 基準(zhǔn)長(zhǎng)度 由機(jī)械設(shè)計(jì)圖 11.4 基準(zhǔn)長(zhǎng)度 選用 900 求中心距和包角 中心距 a= + -4 14（ -） 2-82 = + 900- 1404 14 （ 900- 140） 2-8402 =287mm 小輪包角 = - 601180 2-1 = - 60 180 180100577.28 =163.271 120 求帶根數(shù) 7 帶速 v= = D1n1601000 90960601000 v=7.54 傳動(dòng)比 i= = i=1.76 1 2 1440818.18 帶根數(shù) 查機(jī)械設(shè)計(jì)表11.7、11.8，,11.10 =1.32kw；0 =0.95； =0.87； =0.15kw；0 計(jì)算項(xiàng)目 計(jì)算內(nèi)容 計(jì)算結(jié)果 Z= （ 0+0） = 4.8（ 1.32+0.15） 0.950.87 =3.96 取z=4根 求軸上載荷 張緊力 500 （ ）+q0= 2.5 2 500 （ ）+0.10 4.87.544 2.50.950.95 7.542 =135.4N 0 （由機(jī)械設(shè)計(jì)表 11.4 q=0.10 ） 軸上載荷 =2z 0sin12 =24135.4 =1071.3N sin163.22 8 帶輪結(jié)構(gòu)設(shè)計(jì) 由于帶速v 30 ，帶輪用 HT200制造。小帶輪采用整體式結(jié)構(gòu)，大帶輪采用腹 板式結(jié)構(gòu)，見下圖。 綜上整理帶傳動(dòng)參數(shù)如表： 小帶輪直 徑 1 大帶輪直徑2 傳動(dòng)比 i 帶基準(zhǔn)長(zhǎng)度 根數(shù)Z 中心距a 100mm 180mm 1.76 900mm 4 287mm 4.3齒輪傳動(dòng)的設(shè)計(jì)計(jì)算 查機(jī)械設(shè)計(jì)簡(jiǎn)明手冊(cè)，選擇兩齒輪的材料均為40 ,并經(jīng)調(diào)質(zhì)及表面淬火， 齒面硬度為45- 50HRC。閉式傳動(dòng)的齒輪，主要失效形式是彎曲疲勞折斷和磨粒磨損，磨損尚無 完善的計(jì)算方法，故只進(jìn)行彎曲疲勞強(qiáng)度計(jì)算。 校核步驟按機(jī)械設(shè)計(jì)齒根彎曲疲勞強(qiáng)度計(jì)算： 先初定兩齒輪間的中心距： ， 根據(jù) 321)(HaamuKTAC 式中：配對(duì)材料修正系數(shù)Cm1 螺旋角系數(shù)Aa476 載荷系數(shù)K1.6 9 小齒輪額定轉(zhuǎn)矩 )(7.46819541 MNnPT 齒寬系數(shù) 0.4（查簡(jiǎn)明機(jī)械設(shè)計(jì)手冊(cè)表 10-3）a 由前面初定，再查簡(jiǎn)明機(jī)械設(shè)計(jì)手冊(cè)表10-2（GB1357-87）齒數(shù)比u=i=2.5 許用接觸應(yīng)力 則PaH10829.0.lim a80mm（查機(jī)械設(shè)計(jì)表12.4）,.6185.24076)15.2(47632a 計(jì)算項(xiàng)目 計(jì)算內(nèi)容 計(jì)算結(jié)果 1.初步計(jì)算 轉(zhuǎn)矩 由前表查得 =46.7N.M1 1 齒寬系數(shù) 由表12.13，取 =1.0 =1.0 彎曲疲勞極限 由圖 12.23 =500Mpa 1 =350Mpa2 初步計(jì)算的許用 彎曲應(yīng)力 0.7 1 1 =0.7 500 =350Mpa1 0.72 2 =0.7350 =245Mpa2 值 由表，取 =1.45 =1.45 初取齒輪齒數(shù) 取小齒輪齒數(shù) =30 =30 1 1 2=75 齒形系數(shù) 由圖機(jī)械設(shè)計(jì)圖12.21 =2.6 1FY =2.2 2 應(yīng)力修正系數(shù) 由 機(jī)械設(shè)計(jì) 圖 12.22 10 6.1SaY ， 7.2 初步計(jì)算的齒輪模數(shù)m m 3 121 =1.41 查機(jī)械設(shè)計(jì)表12.3取 m=1.5 初步齒寬b b= 40mm 1取 32mm2取 計(jì)算項(xiàng)目 計(jì)算內(nèi)容 計(jì)算結(jié)果 2.校核計(jì)算 圓周速度v v= = =1.95m/s 1160100045.7818.18601000 v=1.08 精度等級(jí) 由表 選9級(jí)等級(jí) 齒數(shù)z和模數(shù)m 由前計(jì)算，m=1.5； =30， =i =75 =301 2 1 1 =752 使用系數(shù) 查表 =1 動(dòng)載系數(shù) 查表 =1.2 重合度 = 1.883.2（ 11+12） cos =1.88-3.2 （ ）=1.9 =1.9 130+175 重合度系數(shù) =0.25+ =0.25+ =0.77 0.75 0.751.72 齒間載荷分配系數(shù) 由表， = = 載荷系數(shù)K 110.69 K= =1.251.11.41.25 K=2.4 11 彎曲最小安全系數(shù) 由表 12.14 =1.2 應(yīng)力循環(huán)次數(shù) =60 =601274.3 11 14400 =2.41 108 =60 =60168.5722 14400 =0.61 108 彎曲壽命系數(shù) 由機(jī)械設(shè)計(jì)圖 12.24 =0.92 1 =0.982 尺寸系數(shù) 由機(jī)械設(shè)計(jì)圖 12.25 =1.0 許用彎曲應(yīng)力 = = 1 11 5000.9211.25 （式12.19） =368Mpa 1 = = 2 22 3500.9811.25 =274.4Mpa2 驗(yàn)算 =1 21 111 = 2.631.580.69 22.412968075753 =94.8Mpa1 1 =2 12222 12 =94.8 2.181.812.631.58 88.3Mpa2= 2 綜上可知本齒輪設(shè)計(jì)符合要求。 4.4齒輪軸的設(shè)計(jì)與校核 齒輪軸的結(jié)構(gòu)如下： 4.4.1各軸的轉(zhuǎn)速計(jì)算 =1440高 = =齒 14401.76818.18 = =327.27低 818.182.5 4.4.2各軸輸入功率計(jì)算 = =4 高速 軸 1 = 0.960.97=3.7248kw齒輪軸 高速 軸 = 0.90.97=3.25kw低速 軸 齒輪軸 4.4.3各軸輸入扭矩計(jì)算 =9550 =955041440Nm=26.5 Nm高 高 高 =9550 =95503.7248818.18 Nm=43.8 Nm齒 齒 齒 =9550 =95503.25327.27 Nm=94.9 Nm低 低 低 4.4.4 按彎扭合成強(qiáng)度校核軸徑 （1）畫出齒輪軸的的受力圖（如下） （2）作出水平內(nèi)的彎矩圖（如下） 13 支點(diǎn)反力為 = = =2069/2=1030,齒輪左側(cè)軸承截面彎矩為： 支 1支 2 22 1 =1030X =6077Nmm，齒輪右側(cè)軸承處截面彎矩 =1030X29Nmm 2 2 （3）作出垂直面內(nèi)彎矩圖（如下） 以相同的方法求出支點(diǎn)反力 =-73.56 =837.5N1 ,1 故 = X =-4345Nmm，同理 =49410Nmm11 2 2 (4) 合成彎矩為 M= 2+2 （5）對(duì)齒輪截面經(jīng)行安全校核 ，當(dāng)量彎矩M= =181510Nmm,齒輪截面 = =181500/ =19.9M 2右 +()2 0.14533 pa,許用彎曲應(yīng)力=60Mpa,故危險(xiǎn)截面滿足，設(shè)計(jì)的齒輪軸有足夠的 強(qiáng)度。 5絞 刀 設(shè) 計(jì) 絞刀的作用是切割物料。它的內(nèi)孔為方形，安裝在絞籠前端的方軸上隨其 14 一起旋轉(zhuǎn)，刀刃的安裝方向應(yīng)與絞籠旋向相同。絞刀的規(guī)格有2刃、4刃、8刃 。 絞刀用ZG65 Mn材料制造，淬火硬度為HRC55 - 60，刃口要鋒利，與樣板配合平面應(yīng)平整、光滑。 5.1絞刀的設(shè)計(jì) 絞刀的幾何參數(shù)對(duì)所絞出肉的顆粒度以及產(chǎn)品質(zhì)量有著很大的影響，現(xiàn)對(duì) 十字刀片的各主要幾何參數(shù)進(jìn)行設(shè)計(jì)。 十字刀片其每一刃部的絞肉(指切割肉的)線速度 分部其刃部任一點(diǎn)位置上只有法向速度 。 v 再?gòu)娜我蝗~刀片的橫截面上來看，其刃部后角 較大，而前角 及刃傾角 都為零。 因此，該刀片的幾何參數(shù)(角度)不盡合理。故再將以一葉刀片的與網(wǎng)眼扳 相接觸的一條刀刃為對(duì)象，分析刀片上各參數(shù)的作用及其影響，設(shè)計(jì)各參數(shù)。 5.1.1刀刃的起訖位置 絞肉時(shí)，絞肉機(jī)的十字刀片作旋轉(zhuǎn)運(yùn)動(dòng)。從式I可以看出，在轉(zhuǎn)速一定的 條件下，刀刃離旋轉(zhuǎn)中心點(diǎn)越遠(yuǎn)，則絞肉(指切割肉的)線速度越快。并且在螺 桿進(jìn)科速度也一定的條件下，假定絞肉時(shí)刀片所消耗的功全部轉(zhuǎn)化為熱能，則 任一與網(wǎng)眼板相接觸的刀刃，在單位時(shí)間內(nèi)產(chǎn)生的熱量為： VFQ 式中：Q單位時(shí)間內(nèi)任一與網(wǎng)眼板相接觸的刀刃切割肉所產(chǎn)生的熱量（Js） F鉸肉時(shí)任一與網(wǎng)眼板相接觸的刀刃上的切割力（N） 任一刀刃切割肉的線速度（ms） 所以，絞肉(切割肉)的線速度越快，則所產(chǎn)生的熱量也越大，因此絞肉的 線速度不能很高。 根據(jù)經(jīng)驗(yàn)，我們知道一般絞肉時(shí)刀刃切割肉的錢速度處在30一90mmin之間最 為理想，因此由這些數(shù)據(jù)可估算出刀刃的起訖位置，即刃的起點(diǎn)半徑 和終點(diǎn) 半徑R。 5.1.2刀刃的前角 在刀片旋轉(zhuǎn)速度以及螺桿進(jìn)料速度都一定的情況下，前角大，切割肉所需 的力和切割肉所產(chǎn)生的熱都小；反之，則大。但前角很大時(shí)，則因刀具散熱體 積小而使切割肉時(shí)所產(chǎn)生的溫度不能很快冷卻。因此，在一定的條件下，前角 有一合理的數(shù)值范圍： 15 一般?。?(肉質(zhì)軟取大值，反之取小值)4025 5.1.3刀刃的后角 刀刃后角的目的：一是減小后刀面與網(wǎng)眼板(包括三眼板)表面的摩擦；二 是在前角不變的情況下，增大后角能使刀刃鋒利。 刀片磨損后將使刀刃變鈍，使肉在絞肉(切割)過程中變形能增加，同時(shí)由 于磨損后刀片的后角基本為零，加大了刀片與網(wǎng)眼扳的摩擦，兩者都使絞肉過 程中產(chǎn)生的熱量增多。 另外，在同樣的磨鈍標(biāo)準(zhǔn)V B下，后角大的刀片由新用到鈍所磨去的金屬體積較大。這說明增大后角可提高 刀片的耐用度，但同時(shí)也帶來的問題是刀片的N B磨損值大(反映在刀體材料的磨損過大這一方面)，并且刀刃極度也有所削弱， 故后角也有一合理的數(shù)值范圍： 一般?。?(肉質(zhì)軟取大值反之取小值)53 5.1.4刀刃的刃傾角 從分析由前刀面和后刀面所形成的刀刃來得知刀傾角 對(duì)刀片性能的影響 情況。 在任一葉刀片的法剖面內(nèi)，當(dāng)把刀刃放大看時(shí)，可以把刀刃看成是一段半 徑為 的圓弧，由于刀刃有刃傾角 ，故在線速度方向剖面內(nèi)的刀刃將變成橢r 圓弧(斜剖刀刃圓柱所得) 橢圓的長(zhǎng)半徑處的曲率半徑，即為刀刃實(shí)際純圓半徑 。 er0 其關(guān)系為： cos0ner 由此可見，增大刀傾角 的絕對(duì)值，可減小刀刃的實(shí)際鈍圓半徑 ，這就er0 說明增大刃傾角就可使刀刃變得較為鋒利。 一旦刀刃的起訖半徑r及R確定后，其最大初始刃傾角 就可確定了max0 r/acsinmx0 初始刃傾角按下式計(jì)算： )/(20 bRractg 式中：r刀刃起始點(diǎn)半徑(mm)； R刀刃終止點(diǎn)半徑(mm)； b葉刀片外端寬度（mm）; 初始刃傾角；0 16 5.1.5刀片的結(jié)構(gòu) 根據(jù)以上對(duì)絞刀各個(gè)幾何參數(shù)的分析，具體結(jié)構(gòu)可參照零件圖，此絞刀的 特點(diǎn)：后角取4 ，刀片的壽命較長(zhǎng)；前角取30 ，以減小絞肉所需的力及功增 加刃傾角，以提高刀刃的鋒利度；采用全圓弧形的前刀面結(jié)構(gòu)，以改善刀刃的 強(qiáng)度；采用可換式刀片結(jié)構(gòu)，以節(jié)約刀體材料并可選用不同幾何參數(shù)刀片。 6生產(chǎn)能力分析 6.1絞刀的切割能力 根據(jù)需要被絞的肉質(zhì)不同可以選用不同類型的絞刀，一般來說絞刀的刃越多其 切割效率越好，但磨損強(qiáng)度也隨之提升。絞刀的切割能力與多因素有關(guān)。 6.2 絞肉機(jī)的生產(chǎn)能力G 生產(chǎn)能力G（kg/h）： AF1 式中： 被切割1kg物料的面積，其值與孔眼直徑有關(guān)（ ）；1 hcm/2 A絞刀切割能力利用系數(shù)，一般為0.70.75； 6.3功率消耗N 功率消耗N可用下式計(jì)算： (kw)WG 式中：W切割1kg物料耗用能量，其值與孔眼有關(guān)（kw h/kg）； 傳動(dòng)效率； 由生產(chǎn)能力計(jì)算可知，在N、D一定的條件下，絞刀的刃數(shù)越多，生產(chǎn)能力 越大。但是不同刃數(shù)的絞刀應(yīng)與不同孔徑的擠肉樣板相匹配，才能得到較為合 理的生產(chǎn)量和功率消耗。 7絞肉機(jī)的使用與日常維護(hù)簡(jiǎn)介 每 次 使 用 絞 肉 機(jī) 前 ， 得 簡(jiǎn) 單 沖 洗 一 下 。 一 般 而 言 ， 絞 肉 機(jī) 在 上 次 用 完 后 都 是 及 時(shí) 清 洗 過 的 ， 使 用 前 的 清 洗 ， 主 要 是 沖 掉 機(jī) 器 內(nèi) 外 的 浮 塵 等 。 另 一 個(gè) 17 好 處 是 ， 使 用 前 的 沖 洗 會(huì) 使 絞 肉 比 較 變 得 輕 松 流 暢 ， 也 會(huì) 使 工 作 結(jié) 束 后 的 清 洗 變 得 比 較 省 事 。 絞 肉 機(jī) 正 常 使 用 的 情 況 下 一 年 內(nèi) 不 需 重 新 加 油 ； 絞 肉 機(jī) 潤(rùn) 滑 油 品 類 為 黃 油 ； 加 油 孔 位 置 ： 機(jī) 身 頂 部 兩 個(gè) 螺 栓 孔 后 部 （ 背 向 絞 肉 部 件 的 方 向 ） 的 一 個(gè) 螺 栓 孔 可 方 便 加 油 （ 一 定 要 加 注 黃 油 ， 不 能 加 液 體 機(jī) 油 ） 。 絞 肉 機(jī) 機(jī) 箱 部 分 正 常 情 況 下 不 需 做 維 護(hù) 保 養(yǎng) ， 主 要 是 防 水 和 保 護(hù) 好 電 源 線 、 避 免 電 源 線 破 損 及 做 好 清 潔 等 。 絞 肉 部 件 的 日 常 維 護(hù) ： 每 次 使 用 完 畢 后 ， 需 將 絞 肉 三 通 、 螺 桿 、 刀 片 孔 板 等 拆 卸 下 來 ， 清 除 殘 留 物 后 再 按 原 次 序 裝 回 。 這 樣 做 的 目 的 一 方 面 保 證 機(jī) 器 及 加 工 食 物 的 衛(wèi) 生 ， 另 一 方 面 可 保 證 絞 肉 部 件 拆 裝 靈 活 ， 方 便 檢 修 和 更 換 ， 刀 片 和 孔 板 是 易 損 件 ， 使 用 一 段 時(shí) 間 后 可 能 需 要 更 換 。 18 8設(shè)計(jì)小結(jié) 在大四即將畢業(yè)之前，通過這幾十天的學(xué)習(xí)，我覺得自己的專業(yè)知識(shí)和獨(dú) 立思考問題的能力有了很大的提高，對(duì)我走向社會(huì)從事專業(yè)工作有著深遠(yuǎn)的影 響。現(xiàn)在談?wù)剬?duì)本次畢業(yè)設(shè)計(jì)的認(rèn)識(shí)和體會(huì)。 首先，經(jīng)過這么多天的繁瑣的機(jī)械設(shè)計(jì)，我懂得了堅(jiān)持的重要性，因?yàn)槲?們的動(dòng)手能力相對(duì)比較薄弱，平時(shí)這方面的訓(xùn)練量又少，多以剛開始做起來特 備不順手，感覺很浮躁，但最終我還是堅(jiān)持下來了，并懂得了堅(jiān)持的可貴之處 。這次設(shè)計(jì)中我做了許多重復(fù)性的工作，耽誤了很多的時(shí)間，但是這些重復(fù)性 的工作卻增強(qiáng)了我的實(shí)踐能力和動(dòng)手能力，積累了設(shè)計(jì)經(jīng)驗(yàn)。同時(shí)也得到一條 經(jīng)驗(yàn)，搞設(shè)計(jì)不能只在腦子里想它的結(jié)構(gòu)，必須動(dòng)手，即使你想的很完美，但 是到實(shí)際的設(shè)計(jì)過程時(shí)，會(huì)遇到許多意不到的問題。 其次，我學(xué)會(huì)了查閱資料和獨(dú)立思考。當(dāng)開始拿到畢業(yè)設(shè)計(jì)題目時(shí)，心里 真的是一點(diǎn)頭緒也沒有，根本不知道從那里下手。在肖老師的細(xì)心指導(dǎo)下，我 開始查閱相關(guān)書籍，借鑒他人的經(jīng)驗(yàn)，結(jié)合自己的構(gòu)想，再利用自己所學(xué)過的 專業(yè)知識(shí)技能。設(shè)計(jì)師一個(gè)嚴(yán)謹(jǐn)?shù)倪^程，他們的邏輯性非常的強(qiáng)，不能有半點(diǎn) 的馬虎與大意。 最后真誠(chéng)的感謝肖懷國(guó)老師的諄諄教導(dǎo)，還有各位同學(xué)不厭其煩的幫助，因?yàn)?有你們的幫助，我才能夠完成這次繁雜的設(shè)計(jì)。再次誠(chéng)摯的向你們說聲謝謝！ 19 參考文獻(xiàn) 【1】 邱宣懷、郭可謙、吳宗澤等.機(jī)械設(shè)計(jì)第4版.北京：高等教育出版社 ，2010. 【2】 中國(guó)機(jī)械工程學(xué)會(huì)中國(guó)機(jī)械設(shè)計(jì)大典，江西科學(xué)技術(shù)出版社 【3】 唐金松主編簡(jiǎn)明機(jī)械設(shè)計(jì)手冊(cè)，上?？萍技夹g(shù)出版社 【4】 劉鴻文.材料力學(xué).4版.北京：高等教育出版社.2010. 【5】 .何銘新、錢可強(qiáng).機(jī)械制圖.5版.北京：高等教育出版社.2008. 【6】 陳于萍，周兆元主編互換性與測(cè)量技術(shù)基礎(chǔ)第二版 機(jī)械工業(yè)出版社 【7】 張?jiān)Ｖ兄骶幨称芳庸ぜ夹g(shù)裝備M第一版北京：中國(guó)輕工業(yè)出版社 【8】 蔣曉、沈培玉、苗青.AutoCAD2008中文版機(jī)械設(shè)計(jì)標(biāo)準(zhǔn)實(shí)例教程.北京： 清華大學(xué)出版社.2008. 【9】 鄭文緯、吳克堅(jiān).機(jī)械原理.7版.北京：高等教育出版社.2010. A simplified twin screw co-rotating food extruder: design, fabrication and testing S.A.M.A.N.S. Senanayake a, B. Clarke b,* Division of Agricultural and Plantation Engineering, The Open University of Sri Lanka, Nawala, Nugegoda, Sri Lanka Department of Postharvest Technology, School of Agriculture, Food and Environment, Silsoe Collage, Cranfield University, Silsoe, Bedfordshire MK45 4DT,UK Received 6 July 1998; accepted 10 February 1999 Abstract A simplified co-rotating twin screw food extruder was designed, fabricated and tested in England, followed by extensive testing in Sri Lanka. It was built as a model to meet the specific product and financial constraints of less developed countries and was expected to be used in those countries to widen the production capabilities of extruded foods. The machine had an estimated delivery of 10 kg/h and was made mainly with mild steel. Two types of screw were made, one with a constant pitch of 14 mm and the other with varying pitch in segments of 14, 12 and 10 mm. The machine was powered by a 2.2 kW electric motor with electronic speed control .The machine also had electrical heating with a temperature controller and a pressure sensing device. The cost of fabrication of the machine was estimated at ￡2000 with most of the parts built in a fairly simple workshop. A mixture of rice and dried banana was successfully extruded as a potential snack food and on the basis of maximum expansion the best results was obtained from a barrel temperature of 120°C, screw speed 125 rpm, feed moisture 15% and with a die orifice size of 3 mm. When the alternative compress ion screw was tested very similar results were achieved with no significant improvement in product expansion. ? 1999 Elsevier Science Ltd. All rights reserved. Keywords: Twin screw extruder; Design; Low cost; Snack food; Continuous cooker; Local construction; Cereal mixtures Nomenclature a Die diameter (mm) B Channel width (mm) C Screw circumference (mm) d Screw core diameter D Outer diameter of screws (mm) H Flight depth (mm) M Moisture content (% wet basis) n Number of fight turns N Speed angular (rev/min) p Pitch (mm) Q Delivery rate (mm3/min) S Total helical length of screws (mm) t Temperature (℃) T Residence time (min) a Overlap angle of screw fights (degrees) d Calender gap (mm) e Side clearance (mm) q Product density (g/mm3 ) / Helix angle (degrees) * Corresponding author. Fax: +01525-863277; e-mail: b.clarke@cran- ?eld.ac.uk 0260-8774/99/$ ± see front matter?1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 0 4 9 – 7 1. Introduction Extrusion cooking is finding ever increasing applications in the food process industry. Apart from providing a means of manufacturing new products, it has successfully revolution is many conventional manufacturing processes (Harlow, 1985, Frame, 1994). Today, extruders come in a wide variety of sizes, shapes and method of operation. There are three types of food extruder found in industry: hydraulic ram, roller and screw type extruders (Frame, 1994). The screw extruders are very different to the other two having special features such as continuous processing and mixing ability. Single and twin screw types are both widely used in the food process industry. Unfortunately, most of the food extruders available in the market are either so costly that less developed countries cannot afford to buy them except by some form of assistance or outside investment or else are not appropriate for the wide variety of materials that need to be processed. As a result the growth of extrusion technology of food into these countries has been hindered despite its many advantages. Fig. 2. Plan drawing of the twin screw extruder with drive system. 1-V belt pulley, 2-gear box, 3-food seal, 4-ˉange clamp bolt, 5-die plate, 6-die, 7-two segments of the extruder chamber, 8-extruder screw. were made so that they could be externally screwed to the die plate.2.5. Drive system The machine was driven by an electric motor of 2.2kW using a twin belt drive between the motor and a gearbox shown in Fig. 2. The speed reduction in the box was2.08 while an electronic speed controller was used to control the speed continuously over the range required. Fig. 3. Front portion of barrel showing provision for heaters, temperature and pressure sensors. 1-slots for heaters, 2-end flanges, 3-side flanges to barrel, 4-hole for pressure sensor, 5-twin holes to form the barrel. 2. Motor power In twin screw extruders the motor power is utilized mainly to compress and shear the food dough that squeezes through various gaps in the intermeshing screws and the gap between the screws and the barrel. When dealing with a wide range of foods under different process conditions the shear resistance can vary widely because of changes in the rheological behaviour which would prevent accurate estimate of the motor power. Owing to the unknown character therefore of the novel materials a motor power was selected based on that used for similar materials in similar sized extruders with a safety margin and from exploratory trials in the Brabender extruder. Rossen and Miller (1973) give a range of specific energy consumption figures for different extruders which ranged from 0.02 to 0.10 kWh/kg. At 10kg/h throughput this gave a maximum power requirement of 1 kW while the Brabender trials tended to indicatea power requirement of about half of this value. The 2.2 kW, 3 phase AC motor used was amply capable of supplying this power plus all other drive friction losses. 3. Gear box In the co-rotating extruder the two screw shafts are driven at the same speed in the same direction. The main problem is that they are very close together. The gearbox was designed to drive two pinions, coupled to the shafts by shear pins, by using a gear wheel of more than double the width of the pinions. In this way the two pinions could ?t side by side driven simultaneously and maximise their diameter space as shown in Fig. 2. Lubricated phosphor bronze thrust bearings were used to resist the axial load generated by the material along the shaft. 2.6. Heating and temperature control Heating of the barrel to give necessary thermal input for cooking the food was done by two sets of cartridge heaters having capacities of 800 and 1200 W. The heaters were positioned in the grooves made on the top and bottom of the barrel towards the die end as shown in Fig. 3. A single temperature controller was set up together with a thermocouple to sense the temperature inside the barrel very close to die plate. Owing to the shortness of the barrel only one thermocouple was considered necessary. In an early design heaters were also used near to the feed hopper but were not used as they tended to cause premature gelatinization of the starch and blockage of the feed. 4. Pressure sensor Pressure measurements are not so important in the commercial production processes as it cannot be directly controlled to monitor the product characteristics. Neither was such a device needed as a safety measure as this was covered by an overload cut out on the electrical supply. However, in experimental work the measurement of pressure is useful to ascertain the relationship between the pressure and the other controllable parameters such as die size, temperature, moisture content and speed. In this study, a device was built using strain gauges mounted on a small cantilever beam in order to measure the pressure inside the extruder barrel (Fig. 4). A four arm strain gauge bridge was fixed at the point of maximum bending moment. The pressure was tapped from a small hole made in the die end of the barrel in which a plunger, sealed by an O-ring, actuated the cantilever beam to transmit the pressure force. The strain in the beam was detected as a voltage difference. This feature could have been used as an automatic safety cut-out but reliance was placed instead on belt slip in the initial drive stage and the motor itself had an overheating cut-out. Fig. 4. Position of pressure and temperature sensors on the extruder barrel. 1-location of strain gauges on the pressure sensor, 2-cantilever support to plunger, 3-temperature sensor. 5. Testing and evaluation A range of rice and banana mixtures were selected as being both novel yet having high potential as processed foods in Sri Lanka. These materials are cheap and common crops in most developing countries and represent an opportunity to produce an attractive, nutritious and tasty snack food. This would provide labour, utilisation of excess perishable fruits in season and a means of storing them for at least one year in appropriate packages. The main product qualities were assessed as part of the same programme and shown to be satisfactory by Gamlath (1995). The rice was prepared in the form of grits (<800 lm) and the banana was dried and milled to a similar sized powder which was mixed and flood fed from the feed hopper. Extrusion trials were carried out as given below. Sixteen combinations of variable levels were studied in two sets of experiments. In both sets the throughput was measured when the flow became stable. Initial trials indicated no significant difference in performance due to the variable pitch screws as a means of compressing the feed so all subsequent trials and the results quoted in this paper are for the fixed pitch screws. The extrudate diameter was measured using a vernier calliper immediately after extrusion and before any further drying took place which could cause some further reduction in ratio but not to affect the general result. All tests were replicated three times making 48 individual trials carried out in a fully randomised format Experiment 1 Fixed settings: Speed (N) 125 rev/min Die size (a) 5 mm diameter Variables: Barrel temperature (t) two levels (100°C and 120°C) Feed moisture content (M) four levels (15%, 20%, 25%, 30%) Experiment 2. This experiment was carried out using fixed settings of barrel temperature and the feed moisture determined in experiment 1 on the basis that maximum product expansion represented the best quality. Fixed settings: Barrel temperature (t).120°C Feed moisture content (M).15% 6. Testing and evaluation A range of rice and banana mixtures were selected as being both novel yet having high potential as processed foods in Sri Lanka. These materials are cheap and common crops in most developing countries and represent an opportunity to produce an attractive, nutritious and tasty snack food. This would provide labour, utilisation of excess perishable fruits in season and a means of storing them for at least one year in appropriate packages. The main product qualities were assessed as part of the same programme and shown to be satisfactory by Gamlath (1995). The rice was prepared in the form of grits (<800 lm) and the banana was dried and milled to a similar sized powder which was mixed and flood fed from the feed hopper. Extrusion trials were carried out as given below. Sixteen combinations of variable levels were studied in two sets of experiments. In both sets the throughput was measured when the flow became stable. Initial trials indicated no significant difference in performance due to the variable pitch screws as a means of compressing the feed so all subsequent trials and the results quoted in this paper are for the fixed pitch screws. The extrudate diameter was measured using a vernier calliper immediately after extrusion and before any further drying took place which could cause some further reduction in ratio but not to affect the general result. All tests were replicated three times making 48 individual trials carried out in a fully randomised format Experiment 1 Fixed settings: Speed (N) 125 rev/min Die size (a) 5 mm diameter Variables: Barrel temperature (t) two levels (100°C and 120°C) Feed moisture content (M) four levels (15%, 20%, 25%, 30%) Experiment 2. This experiment was carried out using fixed settings of barrel temperature and the feed moisture determined in experiment 1 on the basis that maximum product expansion represented the best quality. Fixed settings: Barrel temperature (t).120°C Feed moisture content (M).15% Table 1 Results of Experiment 1 (Die orifice diameter=5 mm, screw speed=125 rpm) Temperature (°C) Feed moisture (%) Throughput (g/s) Expansion ratio Pressure (MN/m2) 100 15 3.76 1.01 2.97 100 20 2.56 1.00 2.38 100 25 2.04 1.00 1.83 100 30 1.25 1.00 1.38 120 15 2.16 1.06 2.91 120 20 2.00 1.05 2.07 120 25 1.18 1.01 1.59 120 30 1.02 1.00 1.38 Variables: Die orifice diameter (a) two levels (3, 4 mm) Speed (N) four levels (100, 125, 150, 175 rev/min) 4. Results and discussion 4.1. Machine performance Generally the extruder performed very satisfactorily.The extrudates produced by the machine were fairly well expanded. During extrusion operations it did not become necessary to dismantle the barrel lengthways by splitting into two halves as it never seized up. In order to clean the screw and barrel the latter barrel was very easily pulled o. from the screws within a few minutes after extrusion. This was in part due to a shorter than usual barrel length. This suggests that the horizontal splitting of the barrel was not essential which would make the machining process of the barrel far easier. No serious difficulties were encountered as far as the operation of the machine is concerned, except initial feeding problems due to a temperature rise close to the feed hopper. This happened because some heaters were installed a little too close to the feed point so these were later removed and the difficulties were overcome as mentioned earlier. Many extruders have cooling facilities in this region but these were not found to be necessary. Those heaters further from the feed point and close to the die end proved to be sufficient to gelatinize the rice grits. The extrudate was observed to change from a powder at feed to a continuous, expanded extrudate at exit although quantitative assessments of the degree of gelatinization were not carried out. 7. Extruder settings and product characteristics It can be seen from Table 1 and Fig. 5 that the throughput dropped with each increase of feed moisture content at both the barrel temperatures used. When the feed moisture was increased from 15% to 30%, the throughput was reduced by 66.8% and 52.7% at 100℃ and 120℃barrel temperatures, respectively. This effect was probably caused by an increase in backflow allowed by the reduced viscosity which the increase in moisture produced. Another important observation made was the variation of product expansion with the pressure and feed moisture content. The expansion was found to be highest at the lowest moisture content with associated highest pressures (Fig. 6) and a steady reduction in both expansion ratio and pressure as moisture content increased. The product was well gelatinised but with low expansion ratio. The second series was designed to test a wider range of parameters and if possible increase the expansion ratio which was thought to depend on the die diameter. The results of Experiments 2 are tabulated in Table 2 below. Fig. 5. Throughput as a function of feed moisture content with die diameter 5 mm and screw speed 125 rev/min. Fig. 6. Pressure and expansion ratio as a function of feed moisture content at feed moisture 15%, die diameter 5 mm and screw speed 125 rev/min Fig. 7 and Table 2 show that the throughput increased with the speed due to increased rate of material conveyance. The pressure changes with screw speed was not found to be significant. The product expansion, however, showed a downward trend with the increase of speed as evident from Fig. 8. This reduction can be attributed to the reduction of pressure and lower degree of gelatinization due to reduced residence time. At settings of 125 rpm, feed moisture 15%, temperatures 120°C, die size 3 or 4 mm diameter a very acceptable product was achieved. The overall performance of the machine was found to be quite satisfactory in achieving all the parameter settings and measurements required. Each trial only lasted a few minutes in running time which was mainly spent in reaching equilibrium conditions indicated by the temperature reading but after 48 trials no significant wear was observed even though the prototype was in mild steel. Cleaning and maintenance was quick and simple and in the event of a complete seizure of the screws the barrel could be split on this machine. The gearbox was of a bolted construction to permit modifications but future designs should be welded together. The 2.2 kW motor was found to be amply capable and most of the time it only consumed about 0.5kW. No mechanical breakdowns were experienced. The prospects for use of this design in developing countries seem to be good from these experiments. Scale up to a higher capacity would bring some difficulties as discussed by Levine (1989); Singh, Smith and Frame (1998) and Yacu (1992) and although these issues were not addressed they are not considered to be insurmountable. Fig. 7. Throughput as a function of speed with feed moisture 15% and barrel temperature 120°C. Fig. 8. Pressure and expansion ratio as a function of speed with 3 mm die size, feed moisture 15% and barrel temperature 120°C. 8. Conclusions The following conclusions were made from this study. · Simplified extruders for specialised applications can successfully be made and operated in less developed countries to process local food materials. · All components can be made in an unsophisticated workshop except gears, seals, motor, temperature sensor and heaters. · Simple machining processes such as drilling and boring can be used to produce twin holed barrels to accommodate the intermeshing screws. Horizontal splitting of the barrel is not essential in this type of machine so that fabrication of the barrel for these machines can be simple enough for developing country manufacture. · A simple construction of gear box, using straight spur gears driven by a single large gear wheel is quite adequate to run the twin screws in the same direction. · An attractive and acceptable snack food was produced from the prototype machine from mixture of cereals and fruits. References Frame, N. D. (1994). 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R., & Harper, J. M. (1980). Applications of Low cost ex