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BEM Precision angular contact ball bearingPrecision angular contact ball bearingBEM produces special precision angular contact ball bearings for machine tool and other applications where requirements regarding accuracy and speed are very highBEM 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.PreloadFor 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 preloadClass M: medium preloadClass H: heavy preloadThe 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 bearingsWhen 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.CageAll 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 bearingThe 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 speedsThe 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 setsDepending on requirements, BEM precision angular contact ball bearings may also be supplied as complete sets of two, three, four or five bearingsThese 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 toleranceThe 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.AccuracyRunning accuracyThe 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 ringsWhen 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 applicationsMost 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 accuracyThe 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 spacePrecision 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.LoadsIn 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 loadsCylindrical 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 loadsThe 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 loadsCombined 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.StiffnessThe 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精密角接触球轴承精密角接触球轴承BEM为机床和其它应用专门生产精密角接触球轴承，这些应用对轴承和转速的要求是非常高的。BEM精密角接触球轴承是不可分离的，并必有一接触角，在这类轴承中，载荷是沿轴向，以角接触的形式由一个轴承套圈传递到另一个轴承套圈上，所以除承受径向载荷外，还可以承受单方向作用的轴向载荷。径向负荷所派生的轴向符合须由一个外加轴向力来抵消。所以轴承要与第二个轴承相对安装。BEM精密角接触球轴承的内部设计明显地不同与普通单列球轴承，体现出现代机床主轴轴承的风格，两个套圈中有一个套圈只有一个有挡边，减轻了轴承的重量；接触角小，保持架轻，刚球多形成其结构特点。为尽可能满足现代机床的需要，BEM制造出多个系列，多种结构的精密角接触球轴承，他们可按单一和配对两种方式供货。BEM精密角接触球轴承有719、70和72三个尺寸系列，分别带15度的接触角（代号后缀C）或25度接触角（代号后缀E），在需要高的轴向刚性和高的轴向承载能力的场合，建议用较大接触角的轴承。作为BEM持续发展计划的一部分，对C和E两种接触角系列的轴承进行了重新设计，使其性能得到了根本的改善。改进后的球与滚道接触更佳，外圈引导保持架，滚动体更多更大，使得轴承的刚性和承载能力大大提高，并允许轴承以更高的速度运转。假如只有很小的径向空间可用，或要高速运转，就应选择719或70系列的轴承，对于重载荷，中等转速72系列的轴承更适用。如果刚性要求是首要的，可用于大的主轴直径的719系列轴承就有他的优势。他们装有刚球多，轴径增加，球数增多这两个因素都对提高主轴系统的刚度有益。事实上球数增多比轴径增大更能提高其刚性，所以轻系列的轴承比重系列的轴承的轴承刚性要好。预 紧对于单套轴承来说，安装后即获得预负载，其大小取决于对第二套轴承的调整如何，第二套轴承能从相反的方向调节作用与第一套轴承的轴向负载为了满足有关转速和刚性等方面的要求，背靠背，面对面配对安装的两套轴承可分三个等级加预载。等级L：轻预载等级M：中等预载等级H：重预载实际预负载的大小取决于轴承的系列，接触角和轴承的大小，所列出的预负荷值作为背对背和面对面安装的标准值。三套、四套甚至五套轴承背靠背或面对面进行安装，其预紧力要比成对组合的要大一些。装后轴承的预载荷组合配对轴承的预负荷在安装后，其值总有增加，这种增加取决于轴承与轴或座孔的配合以及轴或座孔的自身刚性。保持架所有BEM精密角接触球轴承均配以外圈引导的增强型酚醛胶布保持架。这种保持架重量轻，离心力小，并且允许润滑介质自如地进入刚球和衮道的接触表面。精密角接触球轴承的代号一个完整的单一轴承的代号将标识出轴承的系列，孔径，接触角和结构，且后缀表明轴承的精度等级，如719606C/p4A.配对组合轴承的后缀还表示出组合中的轴承套数，排列和预负荷。极限转速样本上提供的极限转速是一个指标值，他的成立条件是轴承受轻载（P0.06c）,弹簧预紧，散热条件良好。滴油润滑形式的极限转速取最大值，采用其它润滑应在此值上缩减，脂润滑的极限转速取最大值，采用带柔性粘度的优质润滑脂可获得最大极限转速，若要采用两套、三套、四套甚至五套组合配对轴承，则极限转速肯定要较表中给出的值低，合适的极限转速应将这些系数乘以表中给出的指标值。如果从上面获取的极限转速不恰当，如一个简单的结构改变在轴承间加上中间隔圈，则允许适当增加极限转速。例如三套组合的轴承可配对组合的轴承来计算出的极限转速运转，用弹簧来预紧轴承是有利的。这种预紧形式通常用在高速运转时以获得机床整个过程中的均匀预负荷。配对轴承BEM可按需求提供两套、三套、四套甚至五套组合配对的轴承组件。这些组合配对的轴承在制造时就进行了调配。当轴承毗邻安装时，就可获取预定的预负荷值，载荷也由每套轴承均匀分担，组合配对轴承的内孔和外径公差略高于单套轴承的公差的一半。背靠背安装的轴承压力线沿轴线发散，能承受双向轴向负荷，背靠背的轴承的刚性相当好，并