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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 non separable 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.