2084 DTII（A）型帶式輸送機(jī)

2084 DTII（A）型帶式輸送機(jī),dtii,型帶式,輸送 河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯1外文資料：Design of High Speed Belt ConveyorsG. Lodewi jks, The Netherlands.SUMMARYThis paper discusses aspects of high-speed belt conveyor design. The capacity of a belt conveyor is determined by the belt speed given a belt width and troughing angle. Belt speed selection however is limited by practical considerations, which are discussed in this paper. The belt speed also affects the performance of the conveyor belt, as for example its energy consumption and the stability of its running behavior. A method is discussed to evaluate the energy consumption of conveyor belts by using the loss factor of transport. With variation of the belt speed the safety factor requirements vary, which will affect the required belt strength. A new method to account for the effect of the belt speed on the safety factor is presented. Finally, the impact of the belt speed on component selection and on the design of transfer stations is discussed.1 INTRODUCTIONPast research has shown the economical feasibility of using narrower, faster running conveyor belts versus wider, slower running belts for long overland belt conveyor systems. See for example I-5. Today, conveyor belts running at speeds around 8 m/s are no exceptions. However, velocities over 10 m/s up to 20 m/s are technically (dynamically) feasible and may also 河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯2be economically feasible. In this paper belt speeds between the 10 and 20 m/s are classified as high. Belt speeds below the 10 m/s are classified as low.Using high belt speeds should never be a goal in itself. If using high belt speeds is not economically beneficial or if a safe and reliable operation is not ensured at a high belt speed then a lower belt speed should be selected.Selection of the belt speed is part of the total design process. The optimum belt conveyor design is determined by static or steady state design methods. In these methods the belt is assumed to be a rigid, inelastic body. This enables quantification of the steady-state operation of the belt conveyor and determination of the size of conveyor components. The specification of the steady-state operation includes a quantification of the steady-state running belt tensions and power consumption for all material loading and relevant ambient conditions. It should be realized that finding the optimum design is not a one-time effort but an iterative process 6.This paper discusses the design of high belt-speed conveyors, in particular the impact of using high belt speeds on the performance of the conveyor belt in terms of energy consumption and safety factor requirements. Using high belt speeds also requires high reliability of conveyor components such as idlers to achieve an acceptable component life. Another important aspect of high-speed belt conveyor design is the design of efficient feeding and discharge arrangements. These aspects will be discussed briefly.2 BELTSPEED2.1 BELT SPEED SELECTION河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯3The lowest overall belt conveyor cost occur in the range of belt widths of 0.6 to 1.0 m 2. The required conveying capacity can be reached by selection of a belt width in this range and selecting whatever belt speed is required to achieve the required flow rate. Figure 1 shows an example of combinations of belt speed and belt width to achieve Specific conveyor capacities. In this example it is assumed that the bulk density is 850 kg/m3 (coal) and that the trough angle and the surcharge angle are 35 and 20 respectively.Belt speed selection is however limited by practical considerations. A first aspect is the troughability of the belt. In Figure 1 there is no relation with the required belt strength (rating), which partly depends on the conveyor length and elevation. The combination of belt width and strength must be chosen such that good troughability of the belt is ensured. If the troughability is not sufficient then the belt will not track properly. This will result in unstable running behavior of the belt, in particular at high belt speeds, which is not acceptable. Normally, belt manufacturers expect a sufficiently straight run if approximately 40% of the belt width when running empty, makes contact with the carrying idlers. Approximately 10% should make tangential contact with the center idler roll.A second aspect is the speed of the air relative to the speed of the bulk solid material on the belt (relative airspeed). If the relative airspeed exceeds certain limits then dust will develop. This is in particular a potential problem in mine shafts where a downward airflow is maintained for ventilation purposes. The limit in relative airspeed depends on ambient conditions and bulk material characteristics.河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯4A third aspect is the noise generated by the belt conveyor system. Noise levels generally increase with increasing belt speed. In residential areas noise levels are restricted to for example 65 dB. Although noise levels are greatly affected by the design of the conveyor support structure and conveyor covers, this may be a limiting factor in selecting the belt speed.2.2 BELT SPEED VARIATIONThe energy consumption of belt conveyor systems varies with variation of the belt speed, as will be shown in Section 3. The belt velocity can be adjusted with bulk material flow supplied at the loading point to save energy. If the belt is operating at full tonnage then it should run at the high (design) belt speed. The belt speed can be adjusted (decreased) to the actual material (volume) flow supplied at the loading point. This will maintain a constant filling of the belt trough and a constant bulk material load on the belt. A constant filling of the belt trough yields an optimum loading-ratio, and lower energy consumption per unit of conveyed material may be expected. The reduction in energy consumption will be at least 10% for systems where the belt speed is varied compared to systems where the belt speed is kept constant 8.3 ENERGY CONSUMPTION3.1 TRANSPORT EFFICIENCYThere are a number of methods to compare transport efficiencies. The first and most widely applied method is to compare equivalent friction factors such as the DIN f factor. An advantage of using an equivalent friction factor is that it can also be determined for an empty belt. A drawback of using an 河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯5equivalent friction factor is that it is not a pure efficiency number. It takes into account the mass of the belt, reduced mass of the rollers and the mass of the transported material. In a pure efficiency number, only the mass of the transported material is taken into account.The second method is to compare transportation cost, either in kW-hr/ton/km or in $/ton/km. The advantage of using the transportation cost is that this number is widely used for management purposes. The disadvantage of using the transportation cost is that it does not directly reflect the efficiency of a system.The third and most pure method is to compare the loss factor of transport 10. The loss factor of transport is the ratio between the drive power required to overcome frictional losses (neglecting drive efficiency and power loss/gain required to raise/lower the bulk material) and the transport work. The transport work is defined as the multiplication of the total transported quantity of bulk material and the average transport velocity. The advantage of using loss factors of transport is that they can be compared to loss factors of transport of other means of transport, like trucks and trains. The disadvantage is that the loss factor of transport depends on the transported quantity of material, which implies that it can not be determined for an empty belt conveyor.The following are loss factors of transport for a number of transport systems to illustrate the concept:3.2 INDENTATION ROLLING RESISTANCEIt is important to know how the indentation rolling resistance depends on the belt velocity to enable selection of a proper belt velocity, 11.河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯6load on the belt decreases with a factor 2 then the indentation rolling resistance decreases with a factor 2.52 (2 4/3). The bulk load decreases with increasing belt speed assuming a constant capacity. Therefore, the indentation rolling resistance decreases more than proportionally with increasing belt speed.Secondly, the indentation rolling resistance depends on the size of the idler rolls. If the roll diameter increases with a factor 2 then the indentation rolling resistance decreases with a factor 1.58 (2 2/3). In general the idler roll diameter increases with increasing belt speed to limit the bearing rpms to maintain acceptable idler life. In that case the indentation rolling resistance decreases with increasing belt speed.Thirdly, the indentation rolling resistance depends on the visco-elastic properties of the belts cover material. These properties depend on the deformation rate, see Figure 3. The deformation rate in its turn depends on the size of the deformation area in the belts bottom cover (depending on belt and bulk load) and on the belt speed. In general the indentation rolling resistance increases with increasing deformation rate (and thus belt speed), but only to a relatively small account.Fourthly, the indentation rolling resistance depends on the belts bottom cover thickness. If the bottom cover thickness increases with a factor 2 then the indentation rolling resistance increases with a factor 1.26 (2 1/3). if a bottom cover is increased to account for an increase in belt wear with increasing belt speed, then the indentation rolling resistance increases as well.It should be realized that the indentation rolling resistance, although important, is not the only velocity dependent resistance. The rolling 河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯7resistance of the idler rolls for example depends on the vertical load as well as on their rotational speed. The effect of the vertical load, which directly depends on the belt speed, is large. The effect of the rotational speed is much smaller. Another resistance occurs due to acceleration of the bulk solid material at the loading point. This resistance increases quadratically with an increase in belt speed assuming that the bulk material falls straight onto the belt. This will affect smaller, in plant belt conveyors in particular.3.3 RUBBER COMPOUNDSThe indentation rolling resistance depends on the visco-elastic properties of the belts bottom cover as discussed in the preceding section. This implies that the rolling resistance can be decreased by selecting a special low indentation rolling resistance (rubber) compound that is available on the market today. A small premium has to be paid for this special compound, but costs can be limited by applying it for the bottom cover only and using a normal wear-resistant compound for the carrying top cover. In that case turnovers are required to fully use the energy saving function of the bottom compound.A Quantitative indication of the level of indentation rolling resistance is the indentation rolling resistance indicator tan/E 1/3, where tan is the loss angle and E the storage modulus of the compound. Compounds with a reasonable indentation rolling resistance performance have indicators below 0.1. Figure 8 shows these indicators for typical medium to good performing rubbers. As can be seen in that figure, the choice for a specific rubber compound affects the energy consumption of the belt conveyor, in particular as a function of the ambient temperature.河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯8One comment (warning) must be made. A special belt with low indentation rolling resistance compound should never be selected if only one conveyor belt manufacturer offers it. In that case the conveyor system can only perform in accordance with its design specifications when that specific belt is used. It is much better, also cost wise, to specify the upper limit of the resistance indicator as given above that can be met by more than one conveyor belt manufacturer.Figure 1: Indentation rolling resistance indicators for four different rubbers as a function of temperature.4. SAFETY FACTOR REQUIREMENTSFor design purposes, standards like DIN 22101, ISO 5048 and CEMA provide safety factors (SF) that limit the permissible belt loads. Two types of safety factors can be distinguished: safety factors on the steady-state running tensions and safety factors on the non-stationary tensions. standards like the DIN standard base the recommended safety factor on reduction factors. DIN 22101 uses three reduction factors. The first (r0) generally accounts for the reduction of the strength of the belt (splices) due to fatigue. The second (r1) accounts for the extra forces that act on the belt in transition zones and on pulleys etc. The third (r2) accounts for the extra 河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯9dynamic stresses in the belt during starting and stopping. The required minimum safety factor can be calculated as follows:SF=1/(1-(r0+r1+r2) (1)The DIN standard also gives values for the three reduction factors. For example, for a steel cord conveyor belt under normal operational conditions the values are as follows: r00.665, r10.15, r20.06, which yields a safety factor SF8.Although much can be said about the applicability of the safety factor determined with the DIN standard for the design of long belt conveyor systems, the major drawback, keeping the belt speed selection in mind, is that the conveyor systems operational data and the real fatigue properties of the belt are not taken into account.It is possible to account for these factors and to achieve a tailor-made safety factor by taking the belts operational data into account. The reduction factors r1 and r2 are independent of the fatigue properties of the belt and thus constant with increasing number of load cycles. Lets assume that the reduction factor r0 varies linearly with the log1O of the number of load cycles (revolution of the belt through the total belt conveyor) from 0 to 0.665 at 10,000 load cycles (approximation of DIN standard):r0= 0.166 log10(N) (N0.665， r10.15， r20.06，產(chǎn)生安全因素 SF8。依據(jù)DIN標(biāo)準(zhǔn)設(shè)計(jì)的長距離帶式輸送機(jī)系統(tǒng)設(shè)計(jì)是完全可以應(yīng)用的。但主要缺點(diǎn)是傳送帶速度選擇所依據(jù)的輸送機(jī)系統(tǒng)操作的數(shù)據(jù)和傳送帶的真正的疲勞性質(zhì)沒有被考慮到。河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯24這些因素應(yīng)該被考慮，為了達(dá)到傳送帶定制安全系數(shù)的要求，輸送機(jī)系統(tǒng)操作的數(shù)據(jù)應(yīng)該被考慮到。 隨著裝載周期的增加，因數(shù)r1和r2是獨(dú)立于傳送帶的疲勞特性減少的。 假設(shè)，因數(shù)r0在10,000個(gè)載荷循環(huán)周期內(nèi)隨裝載周期log1O線性地(傳送帶的發(fā)展隨著皮帶輸送機(jī)發(fā)展的)從0到0.665減少 (DIN標(biāo)準(zhǔn)的略計(jì)) ：r0= 0.166 log10(N) (N10.000) (2)式中N為載荷的循環(huán)次數(shù)。 超過10,000載荷次數(shù)周期r0增加。 現(xiàn)在假設(shè)，在設(shè)計(jì)的輸送機(jī)的長度10,000 m之下，輸送機(jī)一年工作5000個(gè)小時(shí)，有5年的使用壽命。 載荷循環(huán)周期的總數(shù)可以用下式計(jì)算：N=(3600 V)/(2L)HY (3)式中V是輸送帶速度、L輸送機(jī)長度， H每年工作的小時(shí)的數(shù)和Y是預(yù)計(jì)工作年限。因數(shù)ro的減少值可以用式(2) 確定，并且載荷循環(huán)周期在圖1已給定。 結(jié)果如圖3顯示。圖3：給定例子的DIN 22101標(biāo)準(zhǔn)中r0帶式輸送機(jī)的安全系數(shù)可以根據(jù)公式（1）和圖2確定，結(jié)果如圖4所示。圖4 ： 所舉的例子所需安全因素的極小值從圖4可以知道要保證以2 河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯25m/s速度穩(wěn)定運(yùn)行的輸送機(jī)的設(shè)計(jì)所需的最小的安全系數(shù)約為7.5，在傳送帶運(yùn)行速度為20 m/s.安全系數(shù)擴(kuò)大為10。在安全系數(shù)允許范圍內(nèi)考慮輸送帶的速度可以有效的防止高速設(shè)計(jì)時(shí)估價(jià)過低和低速設(shè)計(jì)時(shí)估計(jì)過高(也取決于輸送機(jī)系統(tǒng)的長度)。以上給定的圖表和數(shù)據(jù)僅說明這個(gè)過程。 這個(gè)過程可以通過考慮被測量的傳送帶拉伸運(yùn)載的各組件(鋼繩子或織品)和橡膠的疲勞特性的優(yōu)化而被改善，還需要考慮傳送帶的實(shí)際載荷循環(huán)周期(空載，滿載，穩(wěn)定運(yùn)行，開始和停止，夏天和冬天工作環(huán)境等)。5 皮帶輸送機(jī)動(dòng)力學(xué)實(shí)質(zhì)上皮帶輸送機(jī)的動(dòng)力學(xué)性能不隨著傳送帶速度改變。 然而，隨著傳送帶速度的增加動(dòng)力學(xué)的變化率增加，這也導(dǎo)致傳送帶連續(xù)運(yùn)行穩(wěn)定性的降低。 本文不打算充分談?wù)撈л斔蜋C(jī)動(dòng)力學(xué)。 在參考文獻(xiàn)7中這個(gè)題目被廣泛地討論。 無論怎樣，關(guān)于高速輸送機(jī)的動(dòng)力學(xué)的一些注釋在這里需要提到。當(dāng)二個(gè)托輥組之間的傳送帶被一個(gè)托輥碾壓或在臨近固托輥的固有頻率運(yùn)行時(shí)將產(chǎn)生振動(dòng)，這將引起共鳴現(xiàn)象的發(fā)生。 共鳴產(chǎn)生將增加的滾筒軸承的磨損和使得傳送帶能量消耗的增加，隨著橫向振動(dòng)振幅的增加，輸送帶引起顫動(dòng)，所以必須避免共振。 在高速傳送機(jī)系統(tǒng)中共鳴的作用將對輸送機(jī)的結(jié)構(gòu)產(chǎn)生很大的破壞，例如共振將引起輸送帶速度的降低、毀壞以及軸承的磨損。因此設(shè)計(jì)皮帶輸送機(jī)時(shí)，應(yīng)該避免共振的，并且要最大的利用現(xiàn)有的靜態(tài)設(shè)計(jì)方法，以便最經(jīng)濟(jì)的設(shè)計(jì)帶式輸送機(jī)。高速輸送機(jī)的輸送帶的運(yùn)行軌跡必須良好。如果傳送帶沒有合適的河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯26運(yùn)行軌跡輸送帶將會(huì)隨著傳送帶速度的增加而跑偏，輸送帶向旁邊位移和向旁邊位移率隨著速度的增加而增2.1部分。 并且傳送帶制造商應(yīng)盡最大的努力做平直的傳送帶和制造更好的輸送帶接頭。 另外，制作長距離的傳送帶會(huì)減少接頭的數(shù)量而增加輸送帶平直性。對于水平運(yùn)輸?shù)膫魉蛶€設(shè)計(jì)部分可以做出相似的結(jié)論。 隨著托輥改變的傳送帶的位置的變化主要依賴于由于裝貨程度引起的皮帶張力變化。在(被中止的)起飛和(緊急狀態(tài))停止時(shí)，特別是在大張力變化期間傳送帶將向側(cè)面發(fā)生移動(dòng)。 隨著傳送帶速度的增加，皮帶張力在開始和停止的期間的變化將增加。利用低速輸送機(jī)靜態(tài)設(shè)計(jì)方法可以確定最大的邊位移。 然而對于高速的輸送機(jī)，動(dòng)態(tài)設(shè)計(jì)方法能符合要求的準(zhǔn)確地估算邊位移。除了開始和停止將花費(fèi)更多時(shí)間外，正常操作的開始的和停止的過程不會(huì)對高速輸送機(jī)造成改變，。 然而緊急剎車操作將會(huì)產(chǎn)生本質(zhì)的改變。 一般來說，緊急剎車的過程可以通過停止驅(qū)動(dòng)系統(tǒng)的運(yùn)行，這樣可以使輸送機(jī)在短的時(shí)間內(nèi)停止，并且，這樣不會(huì)損壞皮帶輸送機(jī)。 一臺(tái)長的陸上輸送機(jī)的典型的緊急剎車所需時(shí)間是30秒，也許時(shí)間對防止傷亡來說已經(jīng)足夠短了。 然而高速輸送機(jī)的大量能量（二次方地增加隨著傳送帶速度的增加)不得不通過制動(dòng)系統(tǒng)來轉(zhuǎn)化，所以這要求消耗更多的時(shí)間。所以，萬一緊急狀態(tài)發(fā)生，傷亡的機(jī)會(huì)更大。因此對于高速輸送機(jī)適當(dāng)增加安全防護(hù)設(shè)備是非常重要的。河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯276 托輥的選擇高速皮帶輸送機(jī)托輥選擇的重要標(biāo)準(zhǔn)是托輥的直徑。 一般認(rèn)為，與低速輸送機(jī)相比，高速輸送機(jī)的托輥的直徑將被增加，具體原因如下： 低速旋轉(zhuǎn)托輥軸承可以應(yīng)用在滿足使用壽命的低速皮帶輸送機(jī)。 這暗示可以按著安全使用維護(hù)規(guī)程操作。托輥的直徑對托輥的性能有很重要的影響。它與傳送帶速度相同，依據(jù)滾柱軸承的旋轉(zhuǎn)確定。 另外可允許操作溫度限制滾柱軸承速度。低摩擦的軸承類型和相應(yīng)的低熱的軸承最適用于高速輸送機(jī)的操作。 當(dāng)受純徑向載荷時(shí)深溝球軸承可以滿足輸送機(jī)的最高的速度要求，當(dāng)受到復(fù)合載荷時(shí)選用角接觸軸承。 托輥提供的滾動(dòng)摩擦抵抗力和制動(dòng)的轉(zhuǎn)矩將隨著托輥直徑的增加而減小。增加托輥的直徑唯一缺點(diǎn)是托輥價(jià)格和托輥的慣性隨之增加。托輥軸承的壽命將隨著托輥軸承的旋轉(zhuǎn)速度(和傳送帶速度有關(guān))減少而增加。 軸承壽命與傳送帶速度成反比。它隨著載荷的增加成為第三力。 然而軸承壽命的限制因素，是潤滑脂的使用壽命而不是托輥的固有壽命。7 中轉(zhuǎn)站高速帶式輸送機(jī)的設(shè)計(jì)的一個(gè)重要方面是高效率的供料和卸料。這減小了輸送帶覆蓋層的磨損和穩(wěn)定了輸送物料的流動(dòng)，在實(shí)際操作中高速輸送機(jī)將設(shè)計(jì)一個(gè)能對物料提供與輸送帶速度相同并且與傳送帶傳送河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯28方向相同的裝料裝置。為了完成加速輸送帶的安裝可以采用這個(gè)方法。 加速輸送帶可以使用低成本織品或固體編織物。 因而要考慮加速輸送帶和大塊粒狀物料之間的摩擦。 另高速傳送帶裝載的方法是使用一個(gè)利用重力的彎曲的滑道系統(tǒng)迫使粒狀材料在傳送帶上以極小速度在適當(dāng)?shù)姆较蚵涞絺魉蛶稀Ｄ壳?，基于離散元素的設(shè)計(jì)方法(DEM)是可以模仿粒狀物料在傳送帶中轉(zhuǎn)站上的流動(dòng)的 12。 這些方法的應(yīng)用使設(shè)計(jì)師可一依據(jù)粒狀物料的大小和物料流動(dòng)的方向變化確定輸送帶速度的變化，也可以計(jì)算物料在滑槽和輸送帶上的力。并且可以優(yōu)化滑槽的設(shè)計(jì)安排使得傳送帶磨損達(dá)到最小，并且可以防止粒狀材料的分解。同樣，高速傳送帶放電也應(yīng)引起注意。 高速輸送帶鋼架的變形將使輸送物料分解破碎，造成塵土和細(xì)化了材料。 一個(gè)一體化的接收滑道上的灰塵和細(xì)小物料特殊的收集貯倉或箱應(yīng)該被設(shè)計(jì)。8 結(jié)論本文探討了高速皮帶輸送機(jī)的設(shè)計(jì)。 基于上述討論可以總結(jié)如下：給定輸送帶寬度，則輸送機(jī)的輸送量可以通過選擇符合要求的帶速來實(shí)現(xiàn)物料的流動(dòng)率。 然而傳送帶速度選擇又受到實(shí)際應(yīng)用條件的限制。疏忽這些考慮將導(dǎo)致實(shí)際操作中出現(xiàn)問題，包括不穩(wěn)定運(yùn)行和塵土量超標(biāo)和高噪聲。不容易確定輸送帶速度和能量消耗之間的關(guān)系，部分是因?yàn)闈L筒受壓河南理工大學(xué)萬方科技學(xué)院畢業(yè)設(shè)計(jì)外文資料與中文翻譯29的復(fù)雜計(jì)算，其中很大一部分是滾筒的抵抗力，這也涉及到很多橡膠化合物黏彈特性的詳細(xì)的知識(shí)。另外輸送帶的速度選擇與具有庫侖摩擦力的各組件、鋼和托輥軸承都有很大的關(guān)系。并且這種抵抗力也發(fā)生在中轉(zhuǎn)站，特別是高速帶式輸送機(jī)大塊物料的加速裝載段，這個(gè)抵抗力作用的很明顯。在輸送帶使用壽命允許的應(yīng)力循環(huán)周期內(nèi)，依據(jù)穩(wěn)定運(yùn)行時(shí)輸送帶的張力和其他操作數(shù)據(jù)選擇的安全系數(shù)將能有效的防止對輸送帶的疲勞壽命估計(jì)不足或過高估計(jì)。輸送機(jī)的速度對輸送機(jī)各組件像托輥，以及平曲線段和中轉(zhuǎn)站都有重要的影響。確切的說，動(dòng)態(tài)設(shè)計(jì)可以設(shè)計(jì)出符合使用壽命和磨損特性要求的輸送機(jī)的組件和區(qū)段，包括托輥，輸送帶和槽形襯板。