張有強，王 偉，廖結(jié)安

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采棉機摘錠磨損失效分析

張有強，王 偉，廖結(jié)安

（塔里木大學機械電氣化工程學院現(xiàn)代農(nóng)業(yè)工程重點實驗室，阿拉爾 843300）

新疆是中國棉花主產(chǎn)區(qū)，機械化收獲程度相對較高。采棉機是高端農(nóng)機裝備，摘錠是其核心零部件在采摘過程中與棉纖維、硬質(zhì)棉稈接觸摩擦導致表面涂層磨損，降低田間采凈率及摘錠使用壽命，故其性能好壞直接影響采棉機的運行效率和機采棉品質(zhì)。該文建立了采棉機摘錠的實體模型，通過有限元方法分析摘錠在脫棉過程中應力與應變的分布規(guī)律，旨在揭示摘錠失效的力學機制。數(shù)值結(jié)果表明表面涂層磨損是作用在鉤齒表面上的摩擦力合力在采摘過程中不斷撕裂涂層引起、摘錠斷裂是扭轉(zhuǎn)變形所致及錐齒輪磨損歸因于過載后產(chǎn)生的塑形變形造成。并基于掃描電子顯微鏡（scanning electron microscope, SEM）觀測失效摘錠的微觀結(jié)構(gòu)形貌進行驗證，結(jié)果表明該有限元分析結(jié)果與實際情形基本吻合。

機械化；有限元方法；模型；采棉機；摘錠；磨損

0 引 言

新疆生產(chǎn)建設兵團是中國最大的機采棉基地，擁有近2 000臺采棉機，機械化采收率達80%以上。摘錠是采棉機上使用最多的零部件，其表面質(zhì)量好壞直接影響到采棉機的作業(yè)性能。一方面采棉機摘錠在采摘過程中產(chǎn)生嚴重磨損，特別是摘錠在脫棉過程中承受來自脫棉盤施加的較大載荷，出現(xiàn)摘錠鉤齒齒頂斷裂，喪失抓取棉花的能力，導致采凈率下降而提前失效。另一方面一臺采棉機安裝2 000多根摘錠，一般情況下摘錠服役約400 hm2，更換周期短且價格較高增加采棉機的運營成本。

中國機采棉區(qū)推廣使用的采棉機多以進口機型為主，如凱斯、迪爾機型。從更換后的摘錠統(tǒng)計分析表明90%摘錠鉤齒頂端出現(xiàn)斷裂和80%鉤齒表面涂層出現(xiàn)磨穿現(xiàn)象，但現(xiàn)有文獻對其研究成果尚未報道。研究者大多從摘錠的采摘工作原理[1-3]、采摘系統(tǒng)結(jié)構(gòu)優(yōu)化與建模[4-5]展開探討。張有強等[6-7]從靜力學考察了單個摘錠軸向斷裂現(xiàn)象，分析了摘錠從根部起第8～11齒之間斷裂的原因；將摘錠的采摘過程簡化為干摩擦動力學系統(tǒng)，從非線性動力學角度出發(fā)分析了參數(shù)變化時摘錠的振動行為。另外，摘錠的表面磨損科學本質(zhì)是金屬與生物質(zhì)材料間的摩擦導致，屬于典型軟磨硬的磨損機制，這類摩擦副之間的摩擦磨損[8-16]已有研究。摘錠鉤齒的斷裂與機械結(jié)構(gòu)中齒輪傳動出現(xiàn)的齒根折斷[17-20]，機械加工過程中刀具的斷齒[21-24]等都具有相似之處，這些研究成果可為分析摘錠失效提供參考。

另外，電鍍鉻涂層具有突出的高硬度、耐腐蝕、表面處理成本低和良好的摩擦學特性被許多工程實際廣泛應用[25-31]。這些顯著的處理優(yōu)勢恰好是摘錠表面所需求的，因此目前市場化摘錠表面進行電鍍鉻涂層處理，提高耐磨、耐蝕的同時降低了運營成本。

本文通過建立摘錠結(jié)構(gòu)的實體模型，采用有限元數(shù)值方法分析摘錠在脫棉過程中鉤齒部位的應力與應變分布，探討摘錠在疲勞和扭轉(zhuǎn)共同作用下產(chǎn)生失效的力學行為。進一步借助于掃描電子顯微鏡（scanning electron microscope，SEM）觀測采棉機摘錠鉤齒斷裂和表面涂層磨損的微觀形貌及演化過程，以期為采棉機摘錠結(jié)構(gòu)優(yōu)化設計、動力分配及提高表面耐磨性等提供參考。

1 采棉機摘錠工作過程

1.1 摘錠整體結(jié)構(gòu)

采棉機的采摘系統(tǒng)如圖1所示。摘錠在一次完整的采摘過程中需完成將棉花接觸勾住并在其自身的高速旋轉(zhuǎn)下纏繞，在采摘頭滾筒的旋轉(zhuǎn)下將纏繞的棉花從棉鈴中拽出，然后經(jīng)脫棉盤將纏繞在摘錠桿身的棉花脫下，最后通過輸棉管道收集在集棉箱中。

以CASE620采棉機摘錠為例，摘錠采摘工作部分呈圓錐形，頭部球面直徑5.4 mm，根部直徑為12 mm，長度為120 mm，轉(zhuǎn)速4 000 r/min，質(zhì)量0.094 kg。圖2是摘錠橫截面的微觀結(jié)構(gòu)，由基體和涂層構(gòu)成?；w材料為低碳合金鋼，圖3a為摘錠基體材料的能譜分析。為了提高摘錠耐磨性，表面進行電鍍鉻涂層處理，圖3b為摘錠表面涂層材料的能譜，涂層厚度約為30m。

圖1 采棉機采摘系統(tǒng)

圖2 摘錠的橫截面結(jié)構(gòu)

注：KCnt為X射線計數(shù)。

1.2 摘錠單個鉤齒結(jié)構(gòu)

圖4是新摘錠單個鉤齒的真實結(jié)構(gòu)，每個鉤齒通過機械切削加工形成具有一定的傾斜角度，目前市場化摘錠圓錐表面上具有3排14齒的外部結(jié)構(gòu)。通常機械加工和電鍍過程中產(chǎn)生應力集中使鉤齒表面、亞表層產(chǎn)生微裂紋。從圖2可以清晰地看到涂層區(qū)域內(nèi)微裂紋錯綜分布、孔洞和亞表層缺陷。此外，受幾何結(jié)構(gòu)所限，摘錠鉤齒部位是應力最為集中的區(qū)域，即使是未使用的新摘錠也會出現(xiàn)齒頂斷裂情形，且其斷裂位置截面和形狀不一，如圖4右上所示。

摘錠基體表面經(jīng)電鍍鉻涂層后，表面電鍍鉻顆粒交錯堆積形成初始粗糙峰（粗糙度）。表面粗糙度過大會導致脫棉過程摩擦阻力較大、不利于脫棉；過小會降低采摘時抓取棉花的摩擦力、降低采凈率。一般新摘錠表面粗糙度為0.7m左右，磨損失效后降低到0.2m左右。

圖4 摘錠鉤齒的微觀形貌

2 摘錠建模及網(wǎng)格劃分

摘錠的結(jié)構(gòu)如1.1描述，基體材料以20CrMnTi為例，彈性模量205.67 GPa，泊松比為0.27，剪切模量為80.94 GPa，密度為7.8×103kg/m3，拉壓強度為1.1×109N/m2，屈服強度為8.5×108N/m2。為了不失一般性且便于計算，將摘錠尾端的錐齒輪簡化為一圓臺，同時摘錠套筒簡化為一圓柱筒。另外高硬度的電鍍鉻層主要目的在于提高耐磨性，但鍍鉻層和基體材料的其他物理特性參數(shù)差異不大，故在建模過程中未考慮鍍鉻層。在網(wǎng)格處理過程中整體采用六面體網(wǎng)格類型，摘錠鉤齒進行細化處理，摘錠實體建模及網(wǎng)格化處理如圖5a。

a. 摘錠的網(wǎng)格化

a. Grid partition of spindle unit

b. 摘錠采摘過程的約束與加載

如1.1節(jié)所述摘錠的采摘原理，假定摘錠與套筒之間形成邊界潤滑，套筒為固定約束（圖5b中已隱藏），取摘錠與套筒之間的滑動摩擦系數(shù)為0.1。采用轉(zhuǎn)動約束描述摘錠旋轉(zhuǎn)，脫棉過程中脫棉盤對摘錠施加的載荷采用壓力來描述，同時受到滾筒旋轉(zhuǎn)將棉花從摘錠根部移到頭部的力采用切向力描述；提供摘錠旋轉(zhuǎn)的動力采用主動扭矩來描述；采用負載扭矩來描述纏繞在摘錠表面上棉纖維滑動時形成的摩擦阻力矩。施加約束的具體參數(shù)值如圖5b所示。

3 有限元分析與驗證

3.1 摘錠鉤齒的應力與應變

圖6表明摘錠最大的應力與應變出現(xiàn)在摘錠圓柱面上，即與套筒接觸的位置，這是因為在其圓柱面的兩端施加了大小相等、方向相反的扭矩所致。

a. 摘錠桿的應力分布

a. Stress distribution of spindle rod

b. 摘錠桿的應變分布

摘錠的圓錐面為棉花采摘的主要工作部位，圖6表明摘錠鉤齒的應力和應變大于其余圓錐面，從單個鉤齒來看直徑較大的棱邊部位比直徑較小的棱邊部位承受較大的接觸應力，導致摘錠表面沿直徑大端棱邊開始磨損。數(shù)值模擬過程中由于只考慮了摘錠“自轉(zhuǎn)”未同時考慮隨滾筒“公轉(zhuǎn)”，所以在鉤齒上承受應力較大部位呈“矩形”分布（如圖7a所示）。但在實際采摘過程中摘錠既“自轉(zhuǎn)”又“公轉(zhuǎn)”，摩擦力合力方向是沿軸向摩擦力與切向摩擦力的合成，并且合力方向也隨滾筒旋轉(zhuǎn)而變化，因此真實應力表現(xiàn)并非規(guī)則的“矩形”分布。

a. 應變（圖6b）的局部放大

a. Local amplification of strain (Fig. 6b)

b. 實際磨損形貌

為了驗證摘錠在采摘過程中表面涂層磨損形貌的演化過程，在采棉季節(jié)實時跟蹤了一臺凱斯620采棉機的田間采摘工作，采摘時間為2016年10月1日至11月7日，地點為新疆生產(chǎn)建設兵團第一師十團七連。對選定的摘錠進行采樣，并進行超聲處理清洗表面污垢和雜質(zhì)。圖8為連續(xù)采摘100、200、300、400 hm2的摘錠鉤齒表面磨損形貌，清晰表明鉤齒表面涂層是從齒端邊緣開始磨穿，并逐漸向摘錠鉤齒表面直徑大端棱邊擴展延伸。隨著采摘工作的持續(xù)，發(fā)展為類似“掃把”型的磨損形貌，最終導致鉤齒表面涂層脫落，這與數(shù)值模擬結(jié)果（圖7a所示）基本吻合。

a. 采摘100 hm2的磨痕a. Wear scar after picking 100 hm2b. 采摘200 hm2后磨痕b. Wear scar after picking 200 hm2 c. 采摘300 hm2后磨痕c. Wear scar after picking 300 hm2d. 采摘400 hm2后磨痕d. Wear scar after picking 400 hm2

3.2 摘錠桿身的扭轉(zhuǎn)變形

圖9a為摘錠桿身的整體變形，表明摘錠圓錐面比圓柱面變形較大，變形量從根部到頭部逐漸變大；圖9b為摘定桿身總變形的矢量圖，其方向呈螺旋形分布，表明摘錠桿身在采棉過程中處于周期性扭轉(zhuǎn)狀態(tài)，可能引起材料內(nèi)部微觀結(jié)構(gòu)的滑移和塑性疲勞，在脫棉不順暢或外載荷突增的情況下可能導致扭轉(zhuǎn)斷裂，這與實際從根部起第8～11鉤齒之間斷裂較為吻合[6]。

a. 摘錠桿的扭轉(zhuǎn)變形

a. Torsional deformation of spindle rod

b. 摘錠桿扭轉(zhuǎn)變形的矢量分布

3.3 摘錠錐齒輪的變形

假定其他載荷條件不變，當摘錠錐齒輪主動扭矩和負載扭矩減小到100 N·mm，從圖10明顯可以看到摘錠的主要變形在錐齒輪上，其他部位變形很小。這表明提高摘錠主動扭矩有利于棉花采摘，在實際中從棉鈴中摘取棉花的力（扭矩）很小[5]，采棉機摘錠提供的主動扭矩足以順利完成采摘工作。但農(nóng)業(yè)機械運行工況復雜，外載荷突增時而發(fā)生，出現(xiàn)主動扭矩不足以克服外載荷增加時，摘錠錐齒輪將會承受較大應力，產(chǎn)生塑性變形導致疲勞磨損。

注：壓力為0.2 MPa，旋轉(zhuǎn)速度為147 rad·s-1, 力為50 N，主動扭矩與負載扭矩均為100 N·mm.

4 結(jié)論與討論

摘錠是采棉機的關鍵部件，摘錠的失效嚴重影響了采棉機的運行。本文通過有限元方法分析了摘錠在采摘過程中力學行為，結(jié)合觀測摘錠結(jié)構(gòu)的微觀形貌驗證了有限元分析的結(jié)果，為摘錠的優(yōu)化設計提供參考。

1）摘錠主要失效形式是鉤齒表面涂層磨損，數(shù)值結(jié)果表明在脫棉過程中單個鉤齒呈現(xiàn)為直徑較大的棱邊部位比直徑較小的棱邊部位承受較大的接觸應力，導致摘錠鉤齒表面涂層沿直徑大端棱邊開始磨穿，并逐漸擴展形成“掃把”型磨損形貌，與實際摘錠涂層表面的磨損形貌基本一致。

2）摘錠桿身的斷裂是由于扭轉(zhuǎn)變形導致，最大變形出現(xiàn)在從根部起第8～11鉤齒之間，與實際斷裂位置較為吻合。摘錠錐齒輪的磨損是由于主動扭矩不足以克服外載突增負載扭矩時，錐齒輪齒頂將承受較大應力，引起塑性變形導致錐齒輪磨損。

在本文的基礎上，下一步將基于多物理場耦合，建立摘錠殼體模型討論摘錠表面涂層結(jié)構(gòu)參數(shù)變化對其力學性能的影響，主要分析涂層的泊松比、涂層厚度及彈性模量變化對摘錠表面涂層結(jié)構(gòu)力學性能影響，為摘錠表面涂層改性提供參考。

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Wear failure analysis on spindle of cotton picker

Zhang Youqiang, Wang Wei, Liao Jiean

(,,,843300,)

Vigorously developing precision and advanced agricultural equipment is one of the ten major areas emphasized in the “Made in China 2025” blueprint, the core strategy of agricultural mechanization centered on improvement of the performance of key mechanical components. With the increase of cotton planting area, the application of modern mechanized cotton pickers becomes more and more attractive, especially in Xinjiang, China, where the mechanization degree in harvesting is relatively higher than the other regions in China. Cotton picker is agricultural machinery of mechanization of cotton harvesting, and spindles in cotton picking machinery are the key component. As the work condition of cotton picker spindle is complex and changeable during the picking process, the contacting interfaces of spindle with cotton fiber involve gradually in the aspects of the stress state, topography, and surface microstructure etc. Surface coating wear of spindle are common with the increase of picking time, leading to the degradation of the spindle and drop of harvesting rate. Accordingly, a lot of effort has been made on improving its wear resistance in the past decades due to the problem of inadequate lifetime of the spindles. However, the previous researches are lack of investigation on failure process of the spindle, especially the mechanical analysis on the spindle surface. The main objective of this study was to reveal the wear process of spindle by analyzing the wear topography of spindle in the cotton picking process. Furthermore, based on the numerical method of finite element analysis, the mechanical performance of spindle was investigated for normal operating parameters of the spindle in field work. Therefore, the solid model of cotton picker spindle in the picking process was established by SolidWorks software along with ANSYS software. The stress and strain distribution of the spindle in the removal cotton process were simulated by the finite element analysis. The numerical simulations results showed that the surface of spindle’s teeth sustained more severe stress than the other portions. In terms of a single spindle’s tooth, the larger diameter edge of spindle’s teeth bore a greater contact stress than smaller diameter edge, leading to the beginning of wearing in the surface coating along the larger diameter edge of spindle’s teeth. In cotton picking process, the spindle is rotated at high speed, at the same time, the spindle rotates with roller while rotating itself. Under the combined actions of tangential friction forces and axial friction forces on the rotating spindle surface, the resultant force direction of the friction force had a certain inclination angle to the axis of spindle, and the inclination angle changed in real time. The surface coating of spindle’s teeth was worn gradually and the characteristic “broom type” wear scar was formed as the picking time increased. Moreover, the fracture of spindle was due to torsion formation, and dangerous section occurred between 8th and 11th teeth from the root of spindle. When the spindle was installed correctly and the lubrication was adequate in the cotton picking process, the wear of the spindle’s bevel gear was caused by the plastic deformation. It was mainly because of spindle deformation was concentrated on the bevel gear when the external load increased instantaneously, such as branches, debris, and gravel was suddenly brought into picking room in cotton picking process. These numerical simulations results were consistent with the microstructure and morphological characteristics of the worn spindle’s surface with electroplated chromium coatings by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS). The research results will helpful for reducing failure of spindle (fracture and wear), which makes it possible to prolong the service life of spindle used, and has remarkable economic benefits and important social significance.

mechanization; finite element method; models; cotton picker; spindle; wear

10.11975/j.issn.1002-6819.2017.18.006

S225.91

A

1002-6819(2017)-18-0045-06

2017-03-24

2017-07-23

國家自然科學基金項目（11362020）

張有強，男，甘肅定西人，副教授，主要從事生物質(zhì)材料與金屬的摩擦磨損研究。阿拉爾 塔里木大學機械電氣化工程學院，843300。Email：zhangyqlzjd@126.com