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BEM Precision angular contact ball bearing Precision angular contact ball bearing BEM produces special precision angular contact ball bearings for machine tool and other applications where requirements regarding accuracy and speed are very high BEM precision angular contact ball bearings are nonseparable and are essentially single row angular contact ball bearings. In all such bearings the load is transmitted from one raceway to another at an angle to the bearing axis. Therefore carry axial loads acting in one direction in addition to radial loads. Axial forces produced in the bearing when subjected to a radial load must be counteracted by an opposing force applied externally. The bearings are therefore adjusted against a second bearing. The internal design of BEM precision angular contact ball bearings differs appreciably from that of standard single row bearing and reflects the latest state of the art where machine tool bearings are concerned. Only one flange on one ring has reduced height; the contact angles are small, and lightweight one-piece cages with a large number of balls are incorporated. To meet the requirements of modern machine tool applications as fully as possible, BEM precision angular contact ball bearings are made in several series and designs. They can be supplied singly or in matched bearing sets. BEM precision angular contact ball bearings are available in three dimension series-bearing series 719,70 and 72-with a contact angle of 15(designation suffix C)or 25 (designation suffix E). Bearings with the larger contact angle are recommended for applications where high axial stiffness and high axial load carrying capacity are required. The bearings have recently been redesigned as part of the continuous BEM product development program. This has resulted in bearings of C and AC, which have appreciably improved performance. Modified ball/raceway contact conditions, a new outer ring centered cage and more or larger diameter balls have enabled stiffness and load carrying capacity of the bearings to be enhanced, while permitting the bearings to operate an even higher speed. If there is little radial space available, or if very high speeds occur, bearings of series 719 or 70 should be chosen. For heavy loads at relatively moderate speeds then bearings of series 72 are more appropriate. Where stiffness requirements are paramount, bearings of series 719 have the advantage. They also incorporate a large number of balls. Both these factors contribute to a high stiffness of the spindle system: spindle stiffness increases with increasing spindle diameter and bearing stiffness is more strongly influenced by the number than by the size of balls. In Fact the stiffness of these light-series bearing is greater than that of bearings from the heavier series. Preload For single bearings, preload is obtained first after mounting and depends on adjustment against a second bearing which can accommodate axial loads acting in the opposite direction to those acting on the first bearing. Matched sets of two bearings arranged back-to-back of face-to-face are supplied in three preload classes in order to meet different demands in respect of speed, stiffness etc. Class L: light preload Class M: medium preload Class H: heavy preload The degree of actual preload depends on the bearing series, the contact angle and the bearing size. These quoted preloads are nominal values for bearings rranged back-to-back or face-to-face before mounting. Matched sets of three, four or five bearings in tandem/back-to back or tandem/face-to-face arrangements have greater preload than bearing pairs. Preload of mounted bearings When mounted the bearing sets will always have a higher preload. This increase is mainly determined by the fits and the stiffness of the bearing seating on the shaft and in the housing. Cage All BEM precision angular contact ball bearings are fitted with an outer ring centered cage of fabric reinforced phenolic resin. The cages are of a particularly lightweight design in order to keep centrifugal force at a minimum, and are designed to allow free passage of lubricant to the ball/raceway contacts. Designations of precision angular contact ball bearing The complete designation of a single bearing identifies the series, bore diameter, contact angle and design as well as the suffix indicating the tolerance class e. g. 71906 C/P4A. The designations of bearing sets also include suffixes indicating the number of bearings in the set, their arrangement and preload. Limiting speeds The limiting speeds quoted are guideline values and are valid provided that the bearings are lightly loaded (P 0.06C),that they are lightly preloaded by means of springs and that the transport of heat away from the bearing position is good. The values under oil spot lubrication are maximum values and should be reduced for certain other methods of oil lubrication. The values under grease lubrication are maximum values which can be attained using a good quality grease of soft consistency. If matched sets of two, three, four our five bearings are to be used, the limiting speed values given in the tables must be reduced. The appropriate limiting speeds should be multiplied by reduction factors . If the limiting speeds obtained from the above for matched bearing sets are inadequate, a simple design change-the inclusion of intermediate rings between the bearings-will allow appreciable increases to be made. For sets of three bearings, for example, it should then be possible to run at the limiting speeds for paired bearings. Springs to preload the bearings may be beneficial. This type of preload is generally used for high speed operation in order to obtain an even preload over the whole operating range of the machine. Matched bearing sets Depending on requirements, BEM precision angular contact ball bearings may also be supplied as complete sets of two, three, four or five bearings These bearing sets are matched in manufacture so that when the bearings are mounted immediately adjacent to each other, the predetermined value of the preload will be obtained, or the load will be evenly distributed. The bore and outside diameters of bearings belonging to a matched set will not differ by more than half the permissible diameter tolerance The load lines of bearings arranged back-to-back diverge towards the bearing axis. Axial loads can be accommodated in both directions. The back-to-back arrangement is relatively stiff and can also take up tilting moments .The load lines of bearings arranged face-to-face converge towards the bearing axis. Axial loads can be accommodated in both directions. The arrangement is not so stiff as the back-to-back arrangement and is less suitable for tilting moments.In a tandem arrangement, the load lines of the bearings are in parallel. Radial and axial loads are equally distributed over the bearings, but axial loads can only be carried in one direction. A set of bearings in tandem is therefore generally adjusted against another bearing which can take the axial loads acting in the opposite direction. Combinations of tandem and back-to-back or tandem and face-to-face are normally used when the design makes it impossible to adjust a further bearing or bearing set against the tandem set. Selection of bearing type BEM produces several types of high-precision bearings for machine tool and other applications where high demands are placed on accuracy and speed capability. Each of these bearing types has characteristic properties which make it particularly suitable for given applications. When designing a bearing arrangement it is necessary to consider a number of different factors, for example -accuracy, -available space, -loads, -Stiffness, -accommodation of axial displacements, -speed, and -heat generation. Depending on the application, one or other of these factors will have a dominant influence. It is not therefore possible to set down general rules for the selection of bearing type or bearing series. The following remarks should serve to highlight the properties of the different bearing types and to ease bearing selection. Accuracy Running accuracy The running accuracy of a bearing arrangement is governed by the accuracy of all the component parts of the arrangement. Where the bearings are concerned, it is primarily determined by the accuracy of form and position of the raceways on the bearing rings When selecting the appropriate tolerance class for a particular bearing, the maximum radial runout of the inner ring (Kia) is generally the determining factor for most applications Most BEM precision bearings are manufactured to tolerance class P4, SP or P5 specifications. P4 is the standard tolerance class for the angular contact ball bearings, SP the standard for the cylindrical roller bearing and double direction angular contact thrust ball bearings, and P5 the standard class for the taper roller bearings. Normally, the maximum values of Kia given in the table are much higher than the actual values. This means, for example, that if bearings with class SP tolerances are used, running accuracies of under 3 can be achieved. Dimensional accuracy The dimensional accuracy of a bearing is important with respect to the fit between bearing ring and shaft or housing. As the fit influences the clearance or preload of mounted bearings, the tolerances for the bearing and its seatings lie within narrow limits. Where cylindrical roller bearings with a tapered bore are concerned, slightly larger dimensional deviations are permissible than, for example, for angular contact ball bearings with similar running accuracy. This is because the clearance or Preload of the bearing is determined by driving up the inner ring on its tapered seating. Available space Precision bearing arrangements generally call for bearings with a low cross section because of the space available and the high requirements in respect of stiffness and running accuracy of the arrangement. These bearings generally have a large number of rolling elements and consequently have a high stiffness. They also enable relatively large diameter spindles to be used for a given housing bore diameter and therefore exhibit all the advantages which are of importance for both the stiffness and the running accuracy of a bearing arrangement, e.g. spindle bearing arrangementAlmost all of the angular contact ball bearings, cylindrical roller bearings, taper roller bearings and angular contact thrust ball bearings used for machine tool applications belong to the ISO Diameter series 0. It is thus possible, by selecting suitable combinations of bearing, to achieve an optimum bearing arrangement for any particular requirements within the same radial space. For bearing arrangements where less radial space is available, the angular contact ball bearings and cylindrical roller bearings belonging to ISO Diameter Series 9 can be used. Even less radial space is required if bearings belonging to ISO Diameter Series 8 are used. Bearings having the dimensions of this Diameter Series can be supplied on request. Angular contact ball bearings of series 72 belong to the ISO Diameter Series 2 and call for correspondingly larger space. Loads In machine tools-the main application for precision bearings-the load carrying capacity of a bearing is usually of much less importance when determining bearing size than in engineering applications in general. Other criteria such as stiffness, size of the requisite bore in the spindle, machining speeds and accuracy, are the decisive factors. When selecting the bearing type for a given bearing arrangement, however, the magnitude as well as the direction of action of the load do play an important part. As a general rule, roller bearings can carry heavier loads than ball bearings having the same envelope dimensions. Radial loads Cylindrical roller bearings having one ring without flanges, i. e. Some ofl the cylindrical roller bearings shown in this catalogue, can only accommodate purely radial loads. Angular contact ball bearings and Bearings are able to support combined radial and axial loads. Axial loads The double direction angular contact thrust ball bearings are designed to take loads which are purely axial in both directions. For large bearing arrangements, or those subjected to particularly heavy axial loads, single direction thrust ball bearings or cylindrical roller thrust bearings are recommended. They can be supplied with the degree of accuracy required for machine tool applications. Combined loads Combined loads are made up of a radial load and a simultaneously acting axial load. This type of load can be accommodated by bearings having raceways in inner and outer rings, situated at an angle to the bearing axis. Where precision bearings are concerned, the angular contact ball bearings, taper roller bearings fall into category. The ability to carry axial loads is determined by the angle of contact; the larger this is, the greater the axial load which can be accommodated. Stiffness The stiffness of a bearing, which is characterized by the magnitude of its elastic deformation under load, is of particular importance where bearing arrangements are required to have high accuracy. Roller bearings are stiffer than ball bearings because of the contact conditions between the rolling elements and raceways. Stiffness can be enhanced by preloading the bearing. Accommodation of axial displacements Generally a machine component is supported in a locating and a non-locating bearing. BEM精密角接觸球軸承 精密角接觸球軸承 BEM為機床和其它應用專門生產精密角接觸球軸承，這些應用對軸承和轉速的要求是非常高的。BEM精密角接觸球軸承是不可分離的，并必有一接觸角，在這類軸承中，載荷是沿軸向，以角接觸的形式由一個軸承套圈傳遞到另一個軸承套圈上，所以除承受徑向載荷外，還可以承受單方向作用的軸向載荷。徑向負荷所派生的軸向符合須由一個外加軸向力來抵消。所以軸承要與第二個軸承相對安裝。 BEM精密角接觸球軸承的內部設計明顯地不同與普通單列球軸承，體現(xiàn)出現(xiàn)代機床主軸軸承的風格，兩個套圈中有一個套圈只有一個有擋邊，減輕了軸承的重量；接觸角小，保持架輕，剛球多形成其結構特點。 為盡可能滿足現(xiàn)代機床的需要，BEM制造出多個系列，多種結構的精密角接觸球軸承，他們可按單一和配對兩種方式供貨。 BEM精密角接觸球軸承有719、70和72三個尺寸系列，分別帶15度的接觸角（代號后綴C）或25度接觸角（代號后綴E），在需要高的軸向剛性和高的軸向承載能力的場合，建議用較大接觸角的軸承。 作為BEM持續(xù)發(fā)展計劃的一部分，對C和E兩種接觸角系列的軸承進行了重新設計，使其性能得到了根本的改善。改進后的球與滾道接觸更佳，外圈引導保持架，滾動體更多更大，使得軸承的剛性和承載能力大大提高，并允許軸承以更高的速度運轉。 假如只有很小的徑向空間可用，或要高速運轉，就應選擇719或70系列的軸承，對于重載荷，中等轉速72系列的軸承更適用。如果剛性要求是首要的，可用于大的主軸直徑的719系列軸承就有他的優(yōu)勢。 他們裝有剛球多，軸徑增加，球數(shù)增多這兩個因素都對提高主軸系統(tǒng)的剛度有益。事實上球數(shù)增多比軸徑增大更能提高其剛性，所以輕系列的軸承比重系列的軸承的軸承剛性要好。 預 緊 對于單套軸承來說，安裝后即獲得預負載，其大小取決于對第二套軸承的調整如何，第二套軸承能從相反的方向調節(jié)作用與第一套軸承的軸向負載 為了滿足有關轉速和剛性等方面的要求，背靠背，面對面配對安裝的兩套軸承可分三個等級加預載。 等級L：輕預載 等級M：中等預載 等級H：重預載 實際預負載的大小取決于軸承的系列，接觸角和軸承的大小，所列出的預負荷值作為背對背和面對面安裝的標準值。 三套、四套甚至五套軸承背靠背或面對面進行安裝，其預緊力要比成對組合的要大一些。 裝后軸承的預載荷 組合配對軸承的預負荷在安裝后，其值總有增加，這種增加取決于軸承與軸或座孔的配合以及軸或座孔的自身剛性。 保持架 所有BEM精密角接觸球軸承均配以外圈引導的增強型酚醛膠布保持架。這種保持架重量輕，離心力小，并且允許潤滑介質自如地進入剛球和袞道的接觸表面。 精密角接觸球軸承的代號 一個完整的單一軸承的代號將標識出軸承的系列，孔徑，接觸角和結構，且后綴表明軸承的精度等級，如719606C/p4A.配對組合軸承的后綴還表示出組合中的軸承套數(shù)，排列和預負荷。 極限轉速 樣本上提供的極限轉速是一個指標值，他的成立條件是軸承受輕載（P<0.06c）,彈簧預緊，散熱條件良好。 滴油潤滑形式的極限轉速取最大值，采用其它潤滑應在此值上縮減，脂潤滑的極限轉速取最大值，采用帶柔性粘度的優(yōu)質潤滑脂可獲得最大極限轉速， 若要采用兩套、三套、四套甚至五套組合配對軸承，則極限轉速肯定要較表中給出的值低，合適的極限轉速應將這些系數(shù)乘以表中給出的指標值。 如果從上面獲取的極限轉速不恰當，如一個簡單的結構改變——在軸承間加上中間隔圈，則允許適當增加極限轉速。例如三套組合的軸承可配對組合的軸承來計算出的極限轉速運轉，用彈簧來預緊軸承是有利的。這種預緊形式通常用在高速運轉時以獲得機床整個過程中的均勻預負荷。 配對軸承 BEM可按需求提供兩套、三套、四套甚至五套組合配對的軸承組件。 這些組合配對的軸承在制造時就進行了調配。當軸承毗鄰安裝時，就可獲取預定的預負荷值，載荷也由每套軸承均勻分擔，組合配對軸承的內孔和外徑公差略高于單套軸承的公差的一半。 背靠背安裝的軸承壓力線沿軸線發(fā)散，能承受雙向軸向負荷，背靠背的軸承的剛性相當好，并能承受傾覆力矩。 面對面安裝的軸承壓力線沿軸承線收斂，也能承受雙向軸向負荷，面對面的軸承的剛性不如背靠背軸承，且不宜用來承受傾覆力矩。 軸承的壓力線平行的串聯(lián)安裝，徑向負荷和軸向負荷均等地分布在軸承上，但只能承受單方向的軸向負荷，所以對這種串裝形式還要另配一套軸承承受反向的軸向負荷，在設計上不允許再配一套軸承來承受反方向的軸向負荷時，一般只采用背靠背或面對面的配對組合串裝。 軸承的選擇 BEM為機床和其它有高精度、高速度要求的應用場合制造各種類型的高精度軸承，這些軸承中的每一種都有其特殊性質，使得它能適應于某種專門應用。在設計軸承組合支撐時，需要考慮很多因素， 例如： ——精度 ——可用空間 ——載荷 ——剛性 ——軸向位移的調整 ——速度 ——發(fā)熱 根據(jù)應用的不同，這些因素中的一個或另一個將有決定性影響，所以不可能為軸承類型和軸承系列的選取制定出一個適用規(guī)則。下面這些說明的作用是使不同類型軸承的特性更加明了，使得軸承的選型變得容易。 精度 旋轉精度 軸承組合支撐的旋轉精度受組合中所有組成件精度的支配。對于軸承來說，主要是取決于軸承套圈滾道的形狀和位置精度。 在為專用軸承選擇合適的公差等級時，內圈的徑跳最大值（Kia）通常是大多數(shù)應用的決定性因素。 大多數(shù)BEM精密軸承都加工成P4、SP或P5等精度等級，P4是角接觸球軸承的標準精度等級。 SP是圓柱滾子軸承和雙向角接觸推力球軸承的標準精度等級，而P5是圓錐滾子軸承的標準精度等級。 正常情況下，表中給出的徑跳值Kia比實際徑跳值高得多，這就意味著若選用SP級公差的軸承就可獲得低于3μm的旋轉精度。 尺寸精度 軸承的尺寸精度是很重要的，因為它關系到套圈與軸承和座孔的配合，而配合又影響到所裝軸承的游隙和預負荷，所以將軸承及其安裝座附件的公差限制在一個較窄的范圍內。 帶錐孔的圓柱滾子軸承允許有稍大的尺寸偏差，這種情況根角接觸球軸承的旋轉精度公差可以稍大類似，因為軸承的游隙或預緊取決于把軸承的內圈在錐形軸上擰多緊。 可用空間 精密軸承組合一般要求軸承具有小的截面積，這是因為考慮到可用空間，大剛性和運轉精度方面的高要求。這些軸承通常具有較多的滾動體，從而有較高的剛度。相對而言，能在給定的座孔內使用大直徑的主軸，展現(xiàn)出高剛性和高運轉精度的兩大優(yōu)勢如主軸軸承。 幾乎所有用于機床的角接觸球軸承，圓柱滾子軸承，圓錐滾子軸承和角接觸推力球軸承都屬于ISO直徑系列0。這就提供了一種可能性，通過選擇合適的軸承組合，在同一徑向空間，獲取適應特殊要求的最佳軸承配置。 在尺寸空間較小的地方可采用ISO直徑系列9的角接觸球軸承和圓柱滾子軸承。若尺寸空間再小就用ISO直徑系列8的軸承了。BEM可按要求供應不同直徑系列的軸承。72系列的角接觸球軸承屬于ISO直徑系列2，并要求相對大一點的尺寸空間。 載荷 在精密軸承的主要應用場合——機床主軸上，軸承的承載能力往往比其它應用場合的軸承在決定軸承尺寸上的重要程度要小得多。其它判斷準則，諸如剛性，對應于主軸的孔的尺寸，機床的切削速度和精度是決定性因素。然而在對待特定的軸承組合選擇軸承類型時，載荷 的大小和方向卻是主要考慮的對象。通常同樣大小的滾子軸承比球軸承能承受更重的載荷。 徑向載荷 一個套圈無擋邊的圓柱滾子軸承，如本樣本所展示的所有的圓柱滾子軸承，僅能承受純的徑向負荷。角接觸球軸承和圓錐滾子軸承能承受徑軸向聯(lián)合載荷。 軸向負荷 雙向推力角接觸球軸承是專為承受兩個方向的推力負荷設計的。如果軸承組合特別大，軸向載荷特別重，則可考慮采用單向推力球軸承或推力圓柱滾子軸承。BEM可以根據(jù)機床要求的不同精度等級提供軸承產品。 聯(lián)合載荷 聯(lián)合載荷由一個徑向負荷和一個同時作用的軸向負荷組成。承受這類載荷宜采用在內外圈滾道上與軸線成一定角度接觸的軸承。若要用到精密軸承，則可考慮樣本列出的角接觸球軸承，圓錐滾子軸承。 承受軸向載荷的多少決定于軸承的接觸角，接觸角愈大，軸向承載能力也就愈高。 剛性 軸承的剛性表示為軸承在承載時，產生彈性變形的大小。它在軸承組合需要高的精度時有福為重要。滾子軸承因為其滾動體和滾道的不同，比球軸承的剛性要好，剛性可通過加預負荷的方法來提高。