機(jī)械擊打式核桃破殼機(jī)的設(shè)計
機(jī)械擊打式核桃破殼機(jī)的設(shè)計,機(jī)械擊打式核桃破殼機(jī)的設(shè)計,機(jī)械,擊打,核桃,破殼機(jī),設(shè)計
12 屆畢業(yè)設(shè)計 機(jī)械擊打式核桃破殼機(jī)的設(shè)計 設(shè)計說明書 學(xué)生姓名 : 學(xué) 號 : 所屬學(xué)院 : 機(jī)械電氣化工程學(xué)院 專 業(yè) : 農(nóng)業(yè)機(jī)械化及其自動化 班 級 : 12-2 指導(dǎo)教師 : 日 期 : 2012.06 塔里木大學(xué)教務(wù)處制 前 言 核桃又名胡桃,系胡桃科核桃屬。與扁桃、腰果、榛子并列稱為世界四大干果 1。核桃在我 國大部分地區(qū)都有分布,并且我國已經(jīng)具有幾千年的栽培歷史,近年來在核桃的種植、生產(chǎn)、加 工、出口等方面,都具有很大的增長趨勢。核桃果實內(nèi)的核仁含有很高營養(yǎng)價值,其蛋白質(zhì)含量 達(dá) 15 左右,最高可達(dá) 29.7 ,其含脂肪約 63,最高可達(dá) 76.34,且核桃油中飽和脂肪酸 即豆寇酸、棕桐酸和硬脂酸總量小于 10,而不飽和脂肪酸中的油酸、亞油酸、亞麻酸和花生烯 酸總量高達(dá) 90,為人體必需的脂肪酸。此外,核桃仁還含有人體必需的多種氨基酸、微量元素 和維生素,其中微量元素鋅和錳是腦垂體的重要組成部分,常食有益于腦的營養(yǎng)補(bǔ)充,同時對大 腦神經(jīng)很有益處,對神經(jīng)衰弱,頭昏健忘等癥有輔助治療作用 2。 核桃是林果業(yè)中最具市場競爭力的特色果品,也是近年來南疆林果業(yè)中發(fā)展速度最快的 樹種。隨著核桃產(chǎn)量的日益增加,市場上對核桃深加工產(chǎn)品的需求也越來越迫切,將核桃破殼后 深加工不僅可以增加核桃的附加值,而且還能帶動整個核桃產(chǎn)業(yè)的發(fā)展。核桃破殼是核桃深加工 中必須首先解決的重要工序。 核桃是主要的名特優(yōu)林果樹種之一,種植歷史悠久,種質(zhì)資源豐富。隨著西部大開發(fā)戰(zhàn) 略的貫徹落實,自治區(qū)將農(nóng)業(yè)產(chǎn)業(yè)結(jié)構(gòu)調(diào)整的著力點,放在提高農(nóng)產(chǎn)品的質(zhì)量、效益和產(chǎn)品競爭 力上,將林果業(yè)作為農(nóng)業(yè)增效、農(nóng)村經(jīng)濟(jì)發(fā)展、農(nóng)民增收的支柱產(chǎn)業(yè)培育,使核桃生產(chǎn)向著基地 化、規(guī)?;?、產(chǎn)業(yè)化方向發(fā)展。核桃是林果業(yè)中最具市場競爭力的特色果品,也是近年來南 疆林果業(yè)中發(fā)展速度最快的樹種。 據(jù)統(tǒng)計,2009 年全疆核桃種植面積已達(dá) 300 萬畝,比 2000 年(40 萬畝)增加了 260 萬 畝,平均每年增加 24.5 萬畝核桃面積。約占全疆林果業(yè)總面積(1600 萬畝)的近五分之一,約 占全國核桃栽培總面積(3000 萬畝)的十分之一,為第二大果樹。全疆 300 萬畝核桃,其中 投產(chǎn)面積約 100 萬畝,年產(chǎn)核桃堅果 8 萬噸,總產(chǎn)值約 20 億元。核桃年產(chǎn)值位于自治區(qū)干、 堅果樹種之首,僅次于葡萄、杏,名列全疆林果樹種年產(chǎn)值第三位 3。 但是我們不能只靠賣原料來增收,而要走農(nóng)產(chǎn)品深加工,一是取仁或取仁后加工飲料等,二 是制油來提高核桃的附加值使農(nóng)民增收。核桃破殼取仁是核桃深加工的第一步,必須首先解決。 破殼后的核桃殼有著廣泛的用途,它可以制成活性碳、過濾器中的濾料、堵漏材料等等。核桃破 殼取仁將大大提高核桃的附加值。 的核桃以個大、皮薄、質(zhì)優(yōu)聞名全國,由于薄殼和中殼核桃的橫隔膜退化或呈膜質(zhì)、革 質(zhì),內(nèi)褶壁退化或不發(fā)達(dá),較易于用機(jī)械剝殼取仁,核桃品種性狀的改良也為機(jī)械化破殼取 仁提供了有利條件。 目 錄 1.緒論 .1 1.選題的意義和目的 .1 1.1 本課題所涉及的問題及國內(nèi)(外)研究現(xiàn)狀及分析 .1 1.2 核桃破殼機(jī)的技術(shù)現(xiàn)狀及存在的問題 .3 1.3 方案的確定 .4 2.設(shè)計方案的選 擇 .4 2.2 核桃破殼部分的設(shè)計 .5 2.3 核桃破殼機(jī)箱體設(shè)計 .5 2.4 軸的設(shè)計 .7 2.4.1 凸輪軸的設(shè)計 .7 2.4.2 軸的校核 .9 2.4.3 軸系零件的定位 .10 2.5 軸承的選擇 .10 2.6 鍵聯(lián)結(jié)的選擇與校核 .11 2.6.1 鍵的選擇 .11 2.6.2 鍵的安裝 .11 2.6.3 校核鍵聯(lián)接的強(qiáng)度 .11 2.7 軸承端蓋的設(shè)計 .11 3.電動機(jī)的選擇 .12 4.減速器的設(shè)計 .13 4.1 確定總的傳動比 .13 4.2 減速器的選擇 .13 4.2.1 選擇蝸桿的傳動類型 .13 4.2.2 選擇材料 .13 5 .聯(lián)軸器的選擇 .13 6.總結(jié) .13 致 謝 .14 參考文 獻(xiàn) .15 塔里木大學(xué)畢業(yè)設(shè)計 1 1.緒論 1.選題的意義和目的 核桃,又稱胡桃,為胡桃科植物。核桃仁含有豐富的營養(yǎng)素,每百克含蛋白質(zhì) 1520 克, 脂肪 6070 克,碳水化合物 10 克;并含有人體必需的鈣、磷、鐵等多種微量元素和礦物質(zhì),以 及胡蘿卜素、核黃素等多種維生素。核桃中所含脂肪的主要成分是亞油酸甘油脂,食后不但不會 使膽固醇升高,還能減少腸道對膽固醇的吸收,因此,可作為高血壓、動脈硬化患者的滋補(bǔ)品。 此外,這些油脂還可供給大腦基質(zhì)的需要。核桃中所含的微量元素鋅和錳是腦垂體的重要成分, 常食有益于腦的營養(yǎng)補(bǔ)充,有健腦益智作用。 科學(xué)家們發(fā)現(xiàn),每 100 克核桃肉中含有 20.97 個單位的抗氧化物質(zhì),它比柑桔高出 20 倍, 菠菜的抗氧化成份為 0.98 個單位,胡蘿卜為 0.04 個單位,西紅柿為 0.31 個單位。 核桃營養(yǎng)豐 富,含有豐富的蛋白質(zhì)、脂肪,礦物質(zhì)和維生素。每 100 克中含蛋白質(zhì) 15.4 克,脂肪 63 克,碳 水化物 10.7 克,鈣 108 毫克,磷 329 毫克,鐵 3.2 毫克,硫胺素 0.32 毫克,核黃素 0.11 毫克, 尼克酸 1.0 毫克。脂肪中含亞油酸多,營養(yǎng)價值較高,此外,還含有豐富的維生素 B、E。核桃含 有豐富的維生素 B 和 E,可防止細(xì)胞老化,能分健腦、增強(qiáng)記憶力及延緩衰老。核桃中還含有特 殊的維生素成分,不但不升高膽固醇,還能減少腸道對膽固醇的吸收,適合動脈硬化、高血壓和 冠心病人食用。核桃仁含有亞麻油酸及鈣、磷、鐵,是人體理想的肌膚美容劑,經(jīng)常食用有潤肌 膚、烏須發(fā),及具有防治頭發(fā)過早變白和脫落的功能。核桃仁還含有多種人體需要的微量元素, 是中成藥的重要輔料,有順氣補(bǔ)血,止咳化痰,潤肺補(bǔ)腎等功能。當(dāng)感到疲勞時,嚼些核桃仁, 有緩解疲勞和壓力。 目前我國核桃面積約 667 公頃,年產(chǎn)量約 20 多萬噸。核桃出仁率達(dá) 50左右,優(yōu)質(zhì)的核 桃仁為淡黃色或琥珀色,營養(yǎng)豐富而味美,可生食,是很好的滋補(bǔ)品,也是制作糕點的原料。許 多國家有消費核桃的習(xí)慣,美國的膳食指南將其與大豆列為同類食物。近年來,核桃除銷售干果 或核桃仁外,核桃乳、核桃速食粉、核桃精等加工品也已進(jìn)入市場,另有少量的核桃油產(chǎn)品銷售, 但是核桃的深加工產(chǎn)品較少見,隨著核桃生產(chǎn)的發(fā)展,其后續(xù)產(chǎn)品的開發(fā)和加工也迫在眉 4 。 我國的核桃栽培面積約 130 萬 hm2以上,主要種植區(qū)域在西南和西北。在國際市場上,核桃 與杏仁、腰果、榛子一起并列為世界 4 大干果,核桃作為保健食品早已被國內(nèi)外所認(rèn)識。我國核 桃總產(chǎn)量約 1 萬 t,全國人均占有 0.24kg。這與國際上一些國家相比相差甚遠(yuǎn),如美國人均占有 核桃 1.5kg,是我國的 6 倍。 核桃是主要的名特優(yōu)林果樹種之一,種植歷史悠久,種質(zhì)資源豐富。隨著西部大開發(fā)戰(zhàn) 略的貫徹落實,自治區(qū)將農(nóng)業(yè)產(chǎn)業(yè)結(jié)構(gòu)調(diào)整的著力點,放在提高農(nóng)產(chǎn)品的質(zhì)量、效益和產(chǎn)品競爭 力上,將林果業(yè)作為農(nóng)業(yè)增效、農(nóng)村經(jīng)濟(jì)發(fā)展、農(nóng)民增收的支柱產(chǎn)業(yè)培育,使核桃生產(chǎn)向著基地 化、規(guī)模化、產(chǎn)業(yè)化方向發(fā)展。核桃是林果業(yè)中最具市場競爭力的特色果品,也是近年來南 疆林果業(yè)中發(fā)展速度最快的樹種。據(jù)統(tǒng)計,2009 年全疆核桃種植面積已達(dá) 300 萬畝,比 2000 年(40 萬畝)增加了 260 萬畝,平均每年增加 24.5 萬畝核桃面積。約占全疆林果業(yè)總面積 (1600 萬畝)的近五分之一,約占全國核桃栽培總面積 (3000 萬畝)的十分之一,為第二大 果樹。全疆 300 萬畝核桃,其中投產(chǎn)面積約 100 萬畝,年產(chǎn)核桃堅果 8 萬噸,總產(chǎn)值約 20 億 元。核桃年產(chǎn)值位于自治區(qū)干、堅果樹種之首,僅次于葡萄、杏,名列全疆林果樹種年產(chǎn)值第三 位 5。 針對核桃加工存在的問題和市場的需求,確定核桃加工工藝,除脫青皮、分級、清洗、脫水、 烘干、去殼、仁殼分離與包裝外,還可進(jìn)一步深加工。在加工中,存在的問題是核桃脫殼比較困 難,主要由人工完成。人工剝殼難以滿足生產(chǎn)發(fā)展的要求,故研制高效剝殼機(jī)已成當(dāng)務(wù)之急 6。 塔里木大學(xué)畢業(yè)設(shè)計 2 1.1 本課題所涉及的問題及國內(nèi)(外)研究現(xiàn)狀及分析 目前,國內(nèi)外一些企業(yè)和科研院所已研制出了一些剝殼加工設(shè)備,就其機(jī)具的結(jié)構(gòu)特點與工 作原理大致可分為以下幾種: (1)擠壓法破殼是利用一對直徑相同轉(zhuǎn)速相等但轉(zhuǎn)向相反的圓柱輥子,圓柱輥子之間的間隙可調(diào), 當(dāng)物料通過適當(dāng)?shù)妮佔娱g隙時受到輥子的擠壓力作用而達(dá)到破殼效果 7。 (2)撞擊法破殼是利用物料在高速運動過程中突然受到阻礙時受到?jīng)_擊力作用,使物料的外殼破 碎從而達(dá)到破殼目的。撞擊法破殼常用的設(shè)備主要是由一個作高速回轉(zhuǎn)運動的甩料盤和固定在甩 料盤周圍的內(nèi)壁組成。高速回轉(zhuǎn)運動的甩料盤使物料產(chǎn)生一個很大的離心力使其撞擊周圍的內(nèi)壁, 當(dāng)離心撞擊力大于物料外殼的極限承載力時,物料外殼將會破裂,從而實現(xiàn)破殼。 (3)剪切法破殼是指將物料放在固定的刀架和相對旋轉(zhuǎn)的轉(zhuǎn)鼓之間,在相對運動著的刀具的剪切 力作用下,物料的外殼被切裂從而實現(xiàn)殼仁分離的。剪切法破殼裝置主要是由刀板座和以刀板轉(zhuǎn) 鼓為工作部件組成的破殼機(jī)。該裝置的刀板轉(zhuǎn)鼓和刀板座上均裝有凹形刀板,并且可以根據(jù)物料 的大小調(diào)節(jié)刀板轉(zhuǎn)鼓和刀板座之間的間隙以適應(yīng)于不同尺寸大小的物料。工作時,物料受到旋轉(zhuǎn) 的刀板與固定的刀板間產(chǎn)生剪切力作用,使其外殼破裂從而實現(xiàn)破殼。 (4)碾搓法破殼是將物料放在固定的磨片和運動著的磨片之間,利用定磨片和動磨片產(chǎn)生的碾搓 作用,迫使物料的外殼被撕裂,從而達(dá)到破殼目的。碾搓法破殼的主要裝置是由一個固定的圓盤 和一個相對轉(zhuǎn)動的動圓盤組成。當(dāng)物料由喂入裝置進(jìn)入定圓盤和動圓盤之間的間隙中時,動圓盤 轉(zhuǎn)動產(chǎn)生的離心力時物料沿著沿徑向方向向外運動的同時,也使物料與定圓盤之間產(chǎn)生方向相反 的摩擦力;與此同時,圓盤上的齒形結(jié)構(gòu)不斷地對物料外殼進(jìn)行切割作用。物料在摩擦力和剪切 力的雙重作用下,最終使外殼產(chǎn)生裂痕直至完全破裂,從而達(dá)到破殼的目的。 (5)摩擦法破殼是指對于本身質(zhì)量很大的物料,可以利用物料與設(shè)備之間直接接觸相互摩擦的方 法破殼,而對于本身質(zhì)量很小的物料,即使是調(diào)節(jié)設(shè)備間隙也不能使物料與設(shè)備全部都直接接觸, 這時主要就是靠物料與物料、物料與機(jī)構(gòu)之間的摩擦力作用使其破殼。 (6)搓撕法破殼是指利用兩個轉(zhuǎn)速不同的相對轉(zhuǎn)動的橡膠輥筒轉(zhuǎn)動時對物料進(jìn)行搓撕作用而實現(xiàn) 破殼的。搓撕法破殼的主要部件是由兩個水平放置的摩擦系數(shù)較大的彈性橡膠輥筒組成。工作時, 兩個橡膠滾筒分別以不同速度做相對轉(zhuǎn)動,兩輥面之間存在著一定的線速度差,當(dāng)物料進(jìn)入橡膠 滾筒的工作區(qū)時,與兩輥面相接觸,當(dāng)嚙入角小于摩擦角時,物料就可以順利進(jìn)入兩輥之間的間 隙。物料在進(jìn)入兩輥間的間隙同時受到兩個不同方向的摩擦力撕搓作用和兩輥面的法向擠壓力的 作用,當(dāng)物料到達(dá)兩個輥子中心連線附近時法向擠壓力最大,物料受壓產(chǎn)生塑性變形,與此同時, 擠壓力也會將物料外殼擠破,從而完成物料的破殼。 (7)真空法破殼指將物料放在真空爆殼機(jī)中,在真空狀況下將物料加熱到一定的溫度,利用真空 泵抽吸水分;同時,真空作用使得物料殼外部壓力降低,而殼內(nèi)部則相對處在高壓狀態(tài),當(dāng)內(nèi)外 壓力差到達(dá)一定程度時,就會使得物料外殼破裂到達(dá)破殼效果。 (8)壓力膨脹法破殼是指預(yù)先將具有一定壓力的氣體壓入物料的殼內(nèi),保持一定的時間后,當(dāng)物 料的內(nèi)外氣壓達(dá)到平衡,然后突然瞬間減壓,此時物料的內(nèi)外壓力平衡被打破,在高壓作用下, 殼體內(nèi)部的氣體產(chǎn)生非常巨大的爆破力從而沖破殼體,以實現(xiàn)破殼效果。 (9)激光法破殼是利用激光逐個切割物料的外殼以實現(xiàn)破殼目的的。雖然采用這種方法能夠達(dá)到 幾乎 100%的整仁率,但是由于該方法費用十分昂貴、效率比較低等原因,很難得到應(yīng)用和推廣。 (10)能量法破殼是指將物料放置在一個高溫高壓的環(huán)境中,經(jīng)過一段時間的高溫高壓作用,使 得物料殼體內(nèi)部聚集大量的熱量,然后瞬間將物料脫離高溫高壓的環(huán)境,這時,物料的殼與仁之 間的壓力瞬時爆破,從而達(dá)到破殼效果。 (11)超聲波法破殼是指采取超聲波發(fā)生器產(chǎn)生大于 20kHz 的超聲波作用于物料的外表面,經(jīng)碰 撞、摩擦等多種力綜合作用實現(xiàn)破殼的。 國外研究現(xiàn)狀及分析 塔里木大學(xué)畢業(yè)設(shè)計 3 國外早在 20 世紀(jì) 60 年代初,就著手研制堅果剝殼機(jī)具,至 80 年代初,美國、意大利、法國等 已相繼推出了各種堅果剝殼機(jī),如夏威夷果剝殼機(jī)、杏仁剝殼機(jī)等。經(jīng)過數(shù)十年的發(fā)展,堅果剝殼 機(jī)具已日趨成熟,目前,正朝著機(jī)電一體化方向發(fā)展 8。 目前,核桃破殼取仁方法有離心碰撞式破殼法、化學(xué)腐蝕法、真空破殼取仁法、超聲波破殼 法和定間隙擠壓破殼法。離心碰撞方法碎仁太多,所以應(yīng)用很少;化學(xué)腐蝕方法由于在實際操作 中不好控制,仁易受到腐蝕,處理不好還會對環(huán)境造成污染;真空破殼和超聲波破殼方法設(shè)備昂 貴,破殼成本高,且破殼效果不夠理想;定間隙擠壓破殼方法值得探索。核桃破殼裝置是核桃破 殼取仁機(jī)的核心裝置。機(jī)械剝殼常用方法有借助粗糙表面碾搓作用的碾搓剝殼、借助撞擊作用撞 擊剝殼、利用剪切作用的剪切剝殼和利用成對軋輥擠壓作用的擠壓剝殼。常見的破殼裝置有圓盤 剝殼裝置、齒輥剝殼裝置、離心剝殼裝置、錘擊式剝殼裝置、軋輥式剝殼裝置、對輥窩眼式開口 裝置、沖壓式破殼裝置、核桃鋸口破殼裝置、核桃破殼挖核裝置及平板擠壓式破殼裝置。 國內(nèi)研究現(xiàn)狀及分析 我國堅果剝殼機(jī)具發(fā)展緩慢,遠(yuǎn)遠(yuǎn)落后于種植業(yè)的發(fā)展,在一些生產(chǎn)應(yīng)用的機(jī)具中,存在如下幾 個突出的問題,因而,難以推廣應(yīng)用 9。 (1)剝殼率低。不少剝殼機(jī)漏剝或剝殼不完全,果仁去凈率不高,有些剝殼機(jī)剝殼率只有 50%。這 是堅果剝殼機(jī)推廣使用的最大障礙。 (2)損失率高。由于參數(shù)選擇不合理,造成剝殼不完全現(xiàn)象嚴(yán)重,碎仁夾帶在碎殼中難以回收而被 棄除。有些機(jī)具果仁損失率高達(dá) 20%。 (3)果仁完整性差。有些機(jī)具的設(shè)計,為了減少漏剝或剝殼不完全現(xiàn)象,一味追求剝殼率的提高, 導(dǎo)致高的破碎率,從而降低了產(chǎn)品的商品價值。 (4)通用性差。一般剝殼機(jī)僅能用于某一品種堅果的剝殼作業(yè),對于不同品種的堅果,不能通過更 換主要零部件來實現(xiàn)一機(jī)多用。 (5)機(jī)具性能不穩(wěn)定,適應(yīng)性差。為某類堅果專門開發(fā)的專用機(jī)型,在該堅果品種、大小規(guī)格、外 殼形狀和含水量等因素出現(xiàn)變化時,剝殼機(jī)具剝殼性能就變差。 (6)作業(yè)成本偏高。我國堅果剝殼機(jī)具尚未形成規(guī)模和系列,多數(shù)是單機(jī)制造,制造的工藝水平低、 成本高、也因為通用性差,不能一機(jī)多用,使得生產(chǎn)企業(yè)設(shè)備配置的成本高,致使加工堅果的作業(yè)成 本增加。 本課題重點研究核桃破殼機(jī)的破殼部分,以改善現(xiàn)存的剝殼率低、損失率高、果仁完整性差、 通用性差、機(jī)具性能不穩(wěn)定、適應(yīng)性差、作業(yè)成本偏高等問題。 關(guān)鍵問題解決思路:對核桃破殼機(jī)的主要部件擠壓輥進(jìn)行了設(shè)計研究,確定了核桃在擠壓輥中 的導(dǎo)入條件, 給出了擠壓輥之間間隙、直徑、長度的確定方法。 1.2 核桃破殼機(jī)的技術(shù)現(xiàn)狀及存在的問題 目前,雖然我國已研制開發(fā)出了一些堅果破殼機(jī)械,但是核桃破殼機(jī)的發(fā)展相當(dāng)緩慢,并且 能進(jìn)行批量生產(chǎn)的成熟機(jī)型不多,遠(yuǎn)不能滿足實際生產(chǎn)需要。具有代表性的核桃破殼機(jī)主要有: 農(nóng)業(yè)大學(xué)史建新、喬園園、董遠(yuǎn)德等研究人員研制的新型核桃破殼機(jī)。該機(jī)主要由喂料斗、 喂料撥輪、機(jī)架、物料輸送圓盤、導(dǎo)向摩擦盤、觸點開關(guān)、氣缸、供氣組件、時間繼電器和電磁 閥等組成。工作時,物料輸送圓盤外圓周上均布的導(dǎo)向輥子在摩擦盤的驅(qū)動下推動核桃轉(zhuǎn)動實現(xiàn) 其導(dǎo)向,時間繼電器通過電磁閥控制活塞桿對核桃進(jìn)行擊打。該新型核桃破殼機(jī)結(jié)構(gòu)簡單、破殼 效率高,能實現(xiàn)核桃的機(jī)械化破殼取仁 10。王琦祥研制的核桃破殼機(jī),該機(jī)由一對保持一定夾 角的工作盤、儲料斗、出料斗、帶爪鉤鏈條等所構(gòu)成。通過鏈條運行,其上以一定間距分布的爪 鉤便將核桃從儲料斗一個一個抓起送入一對保持一定夾角的工作盤中,隨工作盤的旋轉(zhuǎn)將核桃外 殼壓碎而保持核桃仁的完整,被已壓碎的外殼和核桃仁一起落入有斜面的出料斗,借該斜面作用 使已破碎外殼和核桃仁在出料斗出口處分開。該機(jī)可對大小混在一起的核桃進(jìn)行破殼 11。南京 農(nóng)業(yè)大學(xué)農(nóng)業(yè)工程學(xué)院吳子岳通過對綿核桃物理機(jī)械特性的測定和內(nèi)力分析,提出了剝殼取仁原 塔里木大學(xué)畢業(yè)設(shè)計 4 理破裂綿核桃殼,并研制了雙齒盤弧齒板式剝殼裝置和綿核桃剝殼取仁機(jī),該機(jī)主要由機(jī)架、 喂入裝置、剝殼裝置、調(diào)速電機(jī)、出料斗等組成。該機(jī)的最佳運動參數(shù)是:齒盤轉(zhuǎn)速 7080r/min,喂入輪轉(zhuǎn)速 25r/min 左右,最大生產(chǎn)率為 180kg/h12。馮光旭、馮昱、李琳娜 等人員研制的核桃破殼機(jī)。該機(jī)包括底座、破殼組件,其中破殼組件包括活動塊和固定塊,固定 塊設(shè)置在底座一端,活動塊設(shè)置在蝸桿一端,蝸桿另一端外套彈簧,蝸桿與蝸輪配合,蝸輪軸下 端固定在底座上,蝸輪軸上端安裝手柄。該機(jī)省力效果明顯,破殼效果好,容易掌握施力的大小 13。 雖然許多科研人員對核桃破殼的機(jī)理及設(shè)備做了大量的分析和研究,也獲得了很多寶貴的經(jīng) 驗和成果,但是在有些方面依然存在著不足。主要存在以下幾方面的問題: 第一,在簡化核桃的幾何物理模型進(jìn)行理論分析時,并沒有從不同方位分別分析核桃殼的受 力情況。 第二,目前存在的核桃破殼機(jī)構(gòu)在進(jìn)行核桃破殼時,對加載力的方位、載荷大小以及加載行 程等綜合因素對破殼效果的影響卻少有研究。如果加載力太大,則很有可能將核桃殼打破的同時 把核桃仁也打爛甚至打碎,這樣就會影響破殼效果,降低一路仁和二路仁比率;若加載力很小, 就不足以將核桃殼打破,從而降低一次性破殼率,同時還會影響破殼后核桃殼、仁的順利分離。 第三,在輸送核桃過程中可能產(chǎn)生堆積現(xiàn)象,但是擊打裝置在擊打核桃破殼時只能一對一地 將核桃進(jìn)行擊打,因此如何保證將輸送裝置中的核桃單一化還沒有得到較好的解決。 第四,核桃在輸送的過程中可能出現(xiàn)跳動和偏出,不利于擊打裝置的擊打破殼,這也是一個 亟待解決的重要問題。 1.3 方案的確定 打擊方式破殼能得到較多的高路仁, 破殼后的核桃殼較碎, 有利于實現(xiàn)核桃的機(jī)械破殼, 破殼的綜合效果優(yōu)與擠壓破殼。該機(jī)主要由輸送機(jī)構(gòu)、 破殼機(jī)構(gòu)、傳動機(jī)構(gòu)、 電機(jī)及機(jī)架等組 成, 不受山核桃形狀和大小限制, 免除了分級步驟。 其結(jié)構(gòu)示意圖如下圖所示: 圖 11 核桃破殼機(jī)的結(jié)構(gòu)示意圖 工作過程:電機(jī) 1 通過帶輪鏈接將動力輸送給減速器 2,減速器經(jīng)過減速輸入適合的轉(zhuǎn)速, 由聯(lián)軸器 3 連接凸輪軸 4,推動打擊桿 5 帶動打擊板,將由輸送裝置輸送的預(yù)先分級好的核桃 (3335mm)破殼,再由輸送機(jī)將破殼的核桃送走。 2.設(shè)計方案的選擇 經(jīng)過查閱相關(guān)資料和同學(xué)探討,最終確定設(shè)計的部分包凸輪軸、打擊桿、打擊板、電動機(jī)、 減速器等等,其中電動機(jī)和減速器只要選用合適的就可以了,主要設(shè)計的是核桃破殼裝置的,破 塔里木大學(xué)畢業(yè)設(shè)計 5 殼后的由人工去殼。 2.2 核桃破殼部分的設(shè)計 其結(jié)構(gòu)示意圖如圖 21 所示: 圖 21 核桃破殼機(jī)的破殼部分的結(jié)構(gòu)示意圖 打擊板在凸輪軸的帶動下,由打擊桿傳動,打擊桿帶著打擊板將傳送帶上的核桃殼打破,完 成核桃破殼的目的。工作時先調(diào)整好打擊板與輸送裝置的距離,以便提高核桃破殼的質(zhì)量和效率。 2.3 核桃破殼機(jī)箱體設(shè)計 表 21 箱體設(shè)計 設(shè)計項目 計算與說明 結(jié)果 箱座壁厚 805.a 取 =8m 箱蓋壁厚 1 12.1 取 =81 箱蓋凸緣厚度 b=1.5 =1.5812b取 =12b 箱座凸緣厚度 =1.5 =1.5812 取 =12m 箱座底凸緣厚度 2=2.5 2.58202 取 =202 地腳螺釘直徑 fd=0.036a+120.036163.751217.9fd 取 =18fd 地腳螺釘數(shù)目 n=4n 取 =4n 軸承旁連接螺栓直徑 1=0.75 0.751813.5f 取 =141m 蓋與座連接螺栓直徑 2d=0.50.6 0.5180.618910.8f 取 =102d 連接螺栓 的距離2l=125200l 取 =160l 軸承端蓋螺釘直徑 3d=(0.40.5)3 取 =83dm 塔里木大學(xué)畢業(yè)設(shè)計 6 0.4180.5187.29fd 檢查孔蓋螺釘直徑 4=0.30.44 0.3180.4185.47.6f 取 =64dm 定位銷直徑 d=0.70.8d 0.7100.810782 取 =7.5 、 、 至外箱壁f12 距離 查表 4-7( 28) 取PmCf16042取 mCf16204 、 至凸緣邊距離fd2查表 4-7( 28) 取f42 取 f42 軸承旁凸臺半徑 1RmCf12 mRf12 凸臺高度 h根據(jù)低速軸座外徑確定,以便于扳手操作為 準(zhǔn) 取 外箱壁至軸承座端面距離 1l 561)0(2lC取 =541l 齒輪頂圓與內(nèi)箱壁距離 1 11.2 =1.289.6取 =16.51lm 齒輪與箱體內(nèi)壁距離 2 8 取 =102 箱蓋,箱座肋厚 、1m=0.85 0.8586.8 11 =0.85 0.8586.8 取 =6.8 ,1 =6.8m 軸承端面外徑 2D查表 4-9( ) ,3Pmd1502.取 =1802D1302 塔里木大學(xué)畢業(yè)設(shè)計 7 mdD18025.32d105.293mdD13085.23 軸承旁連接螺栓距離 Sd15484).2(1mDS2).(901 取 =150,Sm120 箱座深度 dH=(260+2.521))503(/sd 2+(3050)=162.5182.5 取 =170dHm 箱座高度 1708510)1(dH =183188 取 箱座寬度 aB由內(nèi)部傳動件位置結(jié)構(gòu)及壁厚確定 2.4 軸的設(shè)計 2.4.1 凸輪軸的設(shè)計 設(shè)計的軸長為 488mm,軸分為 8 段。其結(jié)構(gòu)示意圖如下: 塔里木大學(xué)畢業(yè)設(shè)計 8 圖 22 傳動軸結(jié)構(gòu)示意圖 第一段軸用軸承與箱體連接,其直徑為 30mm;第二段連接第一段與凸輪,其直徑為 32mm,第 三段是凸輪,也是與打擊桿連接的傳動部分,是凸輪軸的主要部分;第四段是連接第五段下一個 凸輪軸的,其直徑為 32mm,第六段直徑為 32mm,第七段裝軸承與箱體連接,其直徑為 30mm,最 后一段是連接聯(lián)軸器的,其直徑為 25mm。 軸的材料:軸的材料主要是碳剛和合金剛。由于碳剛比合金剛價格便宜,對應(yīng)力集中的敏感 性較低,同時也可以用熱處理或化學(xué)熱處理的辦法提高其耐磨性和抗疲勞強(qiáng)度,所以本設(shè)計采用 45 號剛作為軸的材料。調(diào)制處理。 凸輪的設(shè)計 首先應(yīng)該根據(jù)工作要求確定從動件的運動規(guī)律,然后按照這一運動規(guī)律設(shè)計凸輪軸的輪廓。 凸輪軸剖面圖如下所示: 圖 23 凸輪軸凸輪結(jié)構(gòu)示意圖 凸輪的設(shè)計原理:根據(jù)從動件的運動規(guī)律圖 2-4 設(shè)計合適的凸輪。 塔里木大學(xué)畢業(yè)設(shè)計 9 圖 24 凸輪從動件的運動規(guī)律 根據(jù)機(jī)械擊打式核桃破殼機(jī)的運動規(guī)律,以及運動的特點,從動件選用等速運動的規(guī)律,凸輪的 運動可以頂起挺桿的上下運動,挺桿的上下運動可以帶動行程開關(guān)的開閉;將余弦加速度運動規(guī) 律代入壓力角公式 (2-)cos1(2 inarct1hRb 1) 的 函 數(shù) 關(guān) 系和 bmR (2-21)(arctnhRbm 2) 令 ,求出mbR (221()146m2tanbh 3) 為簡化計算,對于對心移動從動件盤形凸輪機(jī)構(gòu),可按俠士公式計算基圓半徑 bR (假定 發(fā)生在推程的中點 處,且具有最大速度 ,這樣假定2tan1hvRmbm2hsmv 的誤差一般不會很大) 塔里木大學(xué)畢業(yè)設(shè)計 10 (21 46mtan2mbhvR 4) 2.4.2 軸的校核 經(jīng)過分析,主軸的受力最大,而且凸輪軸的凸輪部分受力是主要的,因此,對該軸進(jìn)行扭矩校核。 軸的結(jié)構(gòu)見圖 3-3 (1)軸的扭矩計算 電動機(jī)輸出轉(zhuǎn)矩: Td= m dnp950 = NM46.2510 (2 5) 式中: dp為電動機(jī)額定功率, 為電動機(jī)轉(zhuǎn)速 主軸輸入轉(zhuǎn)矩: 1235.4620.96.7084.5dTi N (2 6) 1 為聯(lián)軸器的傳動效率根據(jù)設(shè)計指導(dǎo)書參考表 1 初選 .2 為軸承的傳動效率初選 98.033 為齒輥的傳動效率初選 64 根據(jù)要求,軸要滿足下列條 (2)軸的強(qiáng)度條件: 053.2.047.533 MPadWT (2 7) 式中: 為軸的切應(yīng)力,MPa;T 為轉(zhuǎn)矩,N.mm; T為抗扭截面系數(shù), 3m; 為許用扭切應(yīng)力, MPa. 表 22 常用材料的 值和 C 值 軸的材料 Q235,20 35 45 40Cr,35SiMnMPa/ 12-20 20-30 30-40 40-52 C 160-135 135-118 118-107 107-98 該軸的材料為 45 號鋼,則滿足強(qiáng)度條件,軸是安全的。 11 (3)軸傳遞的轉(zhuǎn)矩 mNdFT.5203.2131 (2 塔里木大學(xué)畢業(yè)設(shè)計 11 8) mNdFT.52037.22 (2 9) (4)軸的剛度計算 689.035.10.84.7344 radGlIlP (2 10) 式中:T 為轉(zhuǎn)矩; l為受轉(zhuǎn)矩作用的長度, mm;G 為材料的切變模量,MPa;d 為軸徑,mm; PI為軸 截面的極慣性距。 1, ,故軸是安全的。 2.4.3 軸系零件的定位 (1)軸向定位 為了防止軸上零件發(fā)生沿軸向的移動,必須對其進(jìn)行定位,根據(jù)軸上零件的的安裝要求和對軸的 結(jié)要求,要選擇不同的定位方式,常用的定位方式主要有軸肩定位、套筒定位、軸端擋圈和彈性 擋圈,軸間定位方式在本設(shè)計中有用到,具體的結(jié)構(gòu)和參數(shù)見零件圖和明細(xì)表。 (2)周向定位 鍵主要是為了實現(xiàn)軸上零件的周向定位來傳遞轉(zhuǎn)距,鍵的形式用多種,因此要根據(jù)不同的要求來 選擇不同型號的鍵,根據(jù)傳動的要求,本設(shè)計全部采用圓頭普通平鍵(A 型) ,它的兩個側(cè)面是工 作面,上表面與輪轂槽底之間留有間隙,其主要特點是定心性好、拆裝方便。 2.5 軸承的選擇 主軸通過箱體內(nèi)腔,其兩端由軸承固定在箱體上。根據(jù)軸受力和軸徑的不同, ,本設(shè)計選用的軸承 是:深溝球軸承 已知此處軸徑 d=30mm,所以選內(nèi)徑為 30mm 的軸承,在機(jī)械設(shè)計手冊中選擇深溝球軸承;查表 6- 1,選擇型號為 61906 GB/T2761994 的軸承。 基本額定動載荷:C r =16.2KN 基本額定靜載荷:C 0=10.5KN 2.6 鍵聯(lián)結(jié)的選擇與校核 2.6.1 鍵的選擇 根據(jù)軸的直徑的不同,應(yīng)該選擇不同型號的鍵,另外,鍵的長度也有一系列的標(biāo)準(zhǔn),應(yīng)該優(yōu)先選 用第一系列,在以上的說明書中知道安裝鍵的軸有一處,直徑為 30mm。 GB/T1096 鍵32810 13 從機(jī)械設(shè)計手冊表 4-1 中查得鍵的截面尺寸為:寬度 mb10,高度 h8。由聯(lián)軸器的 標(biāo)準(zhǔn)并參考鍵的長度系列,可以確定取此鍵的長度 l32(比伸入到聯(lián)軸器的深度短一些) 。 2.6.2 鍵的安裝 鍵的安裝位置見零件圖。 2.6.3 校核鍵聯(lián)接的強(qiáng)度 軸和聯(lián)軸器的材料是鋼和鑄鐵,且屬于靜聯(lián)接由文獻(xiàn) 12 的表 6-2 查得許用擠壓應(yīng)力為p=120- 150MPa,取其平均值,p=135MPa。鍵的工作長度為 mbLl 321032,鍵與 輪轂的鍵槽的接觸高度為 mhk485.0.。由文獻(xiàn) 1 的式 6-1 可得 塔里木大學(xué)畢業(yè)設(shè)計 12 cpckldT1032 (2 11) MPa1354.354. M傳遞的轉(zhuǎn)矩(N.M) d軸的直徑(mm) l鍵的工作長度(mm);A 型,l=Lb k鍵與輪轂的接觸高度(mm);k=ht,h 為鍵的高度, b鍵的寬度(mm) t切向鍵工作面寬度(mm)c 鍵的許用切應(yīng)力(MPa)p 鍵連接的許用擠壓應(yīng)力,/ MPa 可見聯(lián)接的擠壓強(qiáng)度滿足,即該鍵可以正常工作。 2.7 軸承端蓋的設(shè)計 所選軸承外徑為 62mm,在 45-65 的范圍內(nèi),所以選擇螺釘直徑 d 3=6mm,螺釘數(shù) 4 個m7130 (2 12) 0Dd65.2.3 (2 13) m9.7.302 (2 14) D52106)51(4 (2 15) md937305 (2 16) D602)42(6 (2 17) mde.7.13 (2 18) e1 b=510 b 取 5mm h=(0.81)b=8mm 塔里木大學(xué)畢業(yè)設(shè)計 13 3.電動機(jī)的選擇 經(jīng)過查閱資料,確定凸輪軸的轉(zhuǎn)速為 70-80r/min,選擇 70r/min,齒輥所需功率為 3KW,符合這一 范圍的同步轉(zhuǎn)速為:查機(jī)械設(shè)計文獻(xiàn)第 155 頁表 12-1 可知min750r in10r min150r 根據(jù)容量和轉(zhuǎn)速,由設(shè)計手冊查出的電動機(jī)型號,因此有以下三種傳動比選擇方案,如下表 31 表 31 電動機(jī)的類型 方 方 案 電動機(jī)型 號 額定功 率 kw 同步轉(zhuǎn)速 inr滿載轉(zhuǎn)速 inr電動機(jī) 質(zhì)量 kg 參考價 格 傳動裝置傳動比 1 Y-160M1-8 4 750 720 118 5.00 10.21 2 Y132M1-6 4 1000 960 73 3.48 13.61 3 Y112M-4 4 1500 1440 43 2.22 20.42 本參考價格為 4 極,同步轉(zhuǎn)速為 1500rmin,功率為 4kw 的電動機(jī)價格為 1 計算,表中數(shù)值為相 對值,僅供參考。 綜合考慮電動機(jī)和傳動裝置的尺寸,質(zhì)量,價格以及傳動比,可見第三種方案比較合適,因此選 定電動機(jī)的型號是 Y112M-4。 該電動機(jī)的主要外型和安裝尺寸如下表 32: 表 32 電動機(jī)主要外形尺寸 其主要外形安裝尺寸如圖 31 圖 31 電動機(jī)主要外形安裝尺寸 4.減速器的設(shè)計 4.1 確定總的傳動比 由 選定的電動機(jī)滿載轉(zhuǎn)速 mn 和工作機(jī)的主軸的轉(zhuǎn)速 n,可得傳動裝置的總的傳動比是:42.071ia ( 41) i 在 1530 范圍內(nèi)可以選用雙頭閉式傳動。 中心高 外形尺寸 地腳安裝尺寸 地腳螺栓孔直 徑 軸伸尺寸 裝鍵部位尺寸 112 38265190 190140 12 2860 8 塔里木大學(xué)畢業(yè)設(shè)計 14 4.2 減速器的選擇 4.2.1 選擇蝸桿的傳動類型 根據(jù) GBT 10085-1988 的推薦,采用漸開線蝸桿(ZI) 4.2.2 選擇材料 根據(jù)蝸桿傳動傳遞的功率不大,速度只是中等,故蝸桿采用 45#鋼,因希望效率高些,采用 雙頭蝸桿。 剩余的計算省略 5 .聯(lián)軸器的選擇 本設(shè)計的聯(lián)軸器的選擇主要包括了一個聯(lián)軸器的選擇,電動機(jī)軸與減速器的輸入主軸的聯(lián)結(jié), 根據(jù)文獻(xiàn) 12 中的表 12-23Y 系列電動機(jī)的外型尺寸,本設(shè)計所選用的電動機(jī)的型號為 Y112M-4, 可知電動機(jī)的輸出主軸的外伸部分的長度 E 和直徑 D 分別是 60 和 28。又本設(shè)計的蝸輪軸的直徑 計算最小值為 36.91mm 和蝸桿的計算最小直徑為 14.69mm。又軸上都裝有鍵,要將尺寸擴(kuò)大 7%左 右。最終確定的蝸輪軸的直徑和蝸桿軸的直徑分別是 42mm 和 28mm,G 根據(jù)文獻(xiàn) 12 表 8-2 凸緣 聯(lián)軸器,最后確定電動機(jī)與減速器的輸入軸間的聯(lián)軸器選擇為 GY5 型。 6.總結(jié) 通過此次設(shè)計使我掌握了科學(xué)研究的基本方法和思路,為今后的工作打下了基礎(chǔ),在以后的 日子我將會繼續(xù)保持這份做學(xué)問的態(tài)度和熱情。 我所選設(shè)計題目是“機(jī)械擊打式核桃破殼機(jī)的設(shè)計” ,之所以選擇這個題目,是因為我對這個 課題比較的感興趣。在我的生活里,核桃破殼主要是在門縫里夾碎,這樣力道不容易把握,不是 夾得太碎就是破裂程度很小,同時對門也造成了一定程度的破壞。因此,就想設(shè)計一款既省力又 快速且破殼完整的機(jī)械。 通過這段時間的努立,我基本上按要求完成了機(jī)械設(shè)計課程設(shè)計中指定的各項任務(wù),通過這 次設(shè)計,進(jìn)一步提高了我的機(jī)械知識水平,鞏固了所學(xué)課程;無論是看圖能力還是畫圖能力都得 到了較大的提高,使我們對機(jī)械有了更深刻的理解和認(rèn)識,培養(yǎng)了我綜合運用所學(xué)知識解決工程 實踐問題的能力。 由于實踐經(jīng)驗和資料的缺乏,加之時間緊迫,在設(shè)計過程中遇到了許多問題,大部分問題在 老師的指導(dǎo)和同學(xué)們的幫助下下得以解決。但也有很多地方設(shè)計的不近人意,例如所繪制的圖紙 有些地方表達(dá)的不是很清楚,希望各位老師給予諒解。 致 謝 對于這次設(shè)計的完成,,首先感謝母校塔里木大學(xué)的辛勤培育,感謝學(xué)校給我提供了如此難 得的學(xué)習(xí)環(huán)境和機(jī)會,使我學(xué)到了許多新的知識、知道了知識的可貴與獲取知識的辛勤。 承蒙張宏老師的耐心指導(dǎo),我順利地完成了我的畢業(yè)設(shè)計。在此深深感謝我的指導(dǎo)老師給予 了我耐心的指導(dǎo)和幫助,表現(xiàn)了他對工作高度負(fù)責(zé)的精神。在我的設(shè)計過程中,還得到了眾多同學(xué) 的支持和幫助,在此,我對這些同學(xué)表示我衷心的感謝和永遠(yuǎn)的祝福! 對于這次畢業(yè)設(shè)計,還有許多美好的設(shè)想由于時間緊湊和自身因素?zé)o法得以實現(xiàn),這不能不 說是本次設(shè)計的遺憾之處。不過,至少它啟發(fā)了我的的思維,提高了我的動手能力,豐富了我為 塔里木大學(xué)畢業(yè)設(shè)計 15 人處世的經(jīng)驗,進(jìn)一步鞏固了所學(xué)知識,這為我在以后的學(xué)習(xí)過程當(dāng)中奠定了堅實的基礎(chǔ) 。同時 為以后在自己的工作崗位上發(fā)揮才能奠定了堅實的基礎(chǔ)。 最后,再一次衷心的感謝贈與我知識、給予我?guī)椭乃欣蠋熍c同學(xué),你們傳遞的知識使我 受用一生,你們的恩情我會銘記一生!雖然說謝謝二字不足以表達(dá)我的感情,但是仍然對你們說 聲“謝謝”。 參考文獻(xiàn) 1 吳子岳.綿核桃剝殼機(jī)的研制J包裝與食品機(jī)械,1995,(02):5456. 2 王高平.一種新型核桃加工設(shè)備的研究J南方農(nóng)機(jī),2002,(02:113115. 3 喬園園,史建新,董遠(yuǎn)德.影響核桃殼仁脫離的主要因素J農(nóng)機(jī)化研究,2008,(04:4344. 4 郗榮庭,劉夢軍.中國干果M.北京:中國林業(yè)出版社,2005.3437. 5 史建新,辛動軍.國內(nèi)外核桃破殼取仁機(jī)械的現(xiàn)狀及問題探討J農(nóng)機(jī)化,2001,(06): 127129. 6 辛動軍,史建新.核桃剝殼機(jī)導(dǎo)向裝置試驗研究J.農(nóng)業(yè)大學(xué)學(xué)報,2001,(03):230234. 7 袁巧霞.我國堅果脫殼機(jī)現(xiàn)狀及亟待解決的技術(shù)問題J.農(nóng)機(jī)化研究,2001,(03):133135. 塔里木大學(xué)畢業(yè)設(shè)計 16 8 史建新,趙海軍,辛動軍.基于有限元分析的核桃脫殼技術(shù)研究J.農(nóng)業(yè)工程學(xué)報,2005,(03): 4850. 9 吳子岳.綿核桃剝殼取仁機(jī)械的研究J.農(nóng)業(yè)工程學(xué)報,1995,(04):143145. 10 史建新,辛動軍.國內(nèi)外核桃破殼取仁機(jī)械的現(xiàn)狀及問題探討J.農(nóng)機(jī)化,2001,(06): 4951. 11 濮良貴,紀(jì)名剛主編.機(jī)械設(shè)計第六版:高等教育出版社,2001. 12 王少巖,郭 玲.機(jī)械設(shè)計基礎(chǔ)實訓(xùn)指導(dǎo)(第三版).大連:大連理工大學(xué)出版社,2009. 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). The technology of extrusion cooking. Blackie Academic and Professional, London.
Gamlath, G. G. S. G. (1995). Nutritional, Physico-chemical and sensory evaluation of extruded cereals with perishables. Ph.D. thesis, Cranfield University, Bedford, England.
Harlow, N. (1985). Revolutionising a cereal need. Food Processing, pp. 29-30.
Harper, J. M., & Jansen, G. R. (1985). Production of nutritious precooked foods in developing countries by low cost extrusion
Technology. Food Review International, 1, 27± 97.
Harper, J. M. (1979). Food extrusion: critical reviews in food science and nutrition. Florida: CRC press.
Harper, J. M. (1992). A comparative analysis of single and twin screw extruders. In J. L. Kokini, C.-T. Ho & M. V. Karwe, Food
extrusion science and technology. New York: Marcel Dekker.
Hauck, B. W. (1985). Comparison of single and twin screw extruders- 2. Food Trade Review (Suppl. 5-9).
Hauck, B. W., & Ben Gera (1987). Single and twin screw extruders. Milling, pp. 18±20.
Jansen, G. R., & Harper, J. M. (1980). Applications of Low cost ex
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