Page 18 - needle bearings
P. 18

Load Rating and Life
                                                                                                              NTN



           2.  Load Rating and Life
           2.1  Bearing life
             Even when bearings are in running under normal         where,
           condition, the raceway surface and the surfaces of the  p= 10/3 ʜʜʜʜʜFor roller bearing
           rolling elements inevitably subjected to repetitive    p= 3. ʜʜʜʜʜʜFor ball bearings
                                                                                              6
           compression stress result in flaking due to fatigue of the     L10 : Basic rated life (10 revolutions)
           raceway an element materials and can resist no longer to       C : Basic dynamic rated load, (N) (kgf)
           further running even under normal condition. "Bearing life"        (radial bearings: Cr, thrust bearings: Ca)
           is defined as the total cumulative bearing revolutions until   P : Bearing load, (N) (kgf)
           flaking occurs thus on the raceway surface or the element          (radial bearings: Pr, thrust bearings: Pa)
           rolling surfaces.
             Further use of bearings becomes unavailable, caused    Furthermore, the basic rated life cab be determined by
           by various defects such as seizure, wear, crack, chipping,  formula (2.2), where it is expressed in the operating
           chattering, rusting, etc. in addition to the flaking   hours.
           phenomenon stated above.  However, these phenomena          L10h  ʹ 500 f h ʜʜʜʜʜʜʜʜʜʜʜʜ(2.2)
                                                                                  p
           should be deemed as defects of bearing itself and,
           therefore, be discriminated from bearing life.  And the     f h  ʹ f n  C     ʜʜʜʜʜʜʜʜʜʜʜʜ(2.3)
           defect causes are such as incorrect selection of bearing,             P
           improper mounting, improper lubrication, imperfect                  33.3 1 p
           sealing, etc.  Hence, bearing trouble can be avoided by     f n  ʹʢ  n  ʣ ʜʜʜʜʜʜʜʜʜʜʜ(2.4)
           removing the said causes.
                                                                    where,
                                                                        L10h : Basic rated life, h
           2.2  Basic rated life and basic dynamic load rating            fh : Life factor
                                                                          fn : Speed factor
             Even when a group of same bearings have been in
                                                                          n : Rotational speed,  r/ min
           running under the same condition, considerably great
           variation is found among the individual bearing lives. This
                                                                    The formula (2.2) can also expressed as formula (2.5).
           comes from variation in material fatigue itself. Hence, the       6
           basic rated life defined below considering statistically this   10    C   p
                                                                     L 10h ʹ  ʢ     ʣ ʜʜʜʜʜʜʜʜʜʜʜ(2.5)
           variation in material fatigue is applied as the bearing life.   60 n  P
             The basic rated life means the substantial total
           cumulative revolutions of the individual bearings 90%    Considering the life of either one, of several bearings
           (90% reliability) of which can revolve without resulting in  installed in a machine or an equipment, until it breaks
           flaking due to rolling fatigue when a group of same    down due to rolling fatigue as the total bearing life as a
           bearings have been put individually in continuous running  whole, it can be determined by formula (2.6).
           under same condition.  When bearings have been in
           running in constant revolution, the basic rated life is     L ʹ          1             ʜʜʜʜʜ(2.6)
           expressed as the total cumulative revolutions of the           ʢ  1  ʴ  1  ʴʜ   1  ʣ
                                                                                             1 e
           individual bearings.                                              L1 e  L2 e    Ln e
             On the other hand, the basic dynamic load rating
           represents the load capacity of rolling bearing, then   where,
           meaning a constant load under which the basic rated life  e = 9/8 ⋯⋯⋯For roller bearings
           of 1,000,000 revolutions is achieved. This constant load  e = 10/9 ⋯⋯⋯For ball bearings
           of radial bearing is expressed with net radial load, while  L : Total basic rated life of bwaring as a whole, h
           that of thrust bearing is expressed with net axial load.    L1   L2 ʜLn : Individual basic rated life of bearings,
             The bearing dimensions table in this catalogue                      1, 2,ʜn, h
           describes the respective basic dynamic load ratings of the
           bearings that were manufactured using NTN’s standard
           materials and manufacturing processes in current use.   2.3  Bearing-applied machines and requisite life
             For the respective basic rated lives of the bearings   In selecting a bearing, it is essential to set up the
           manufactured using the special materials and special   requisite life of the bearing under the intended operating
           manufacturing processes, feel free to contact NTN for  conditions, but the requisite life is mainly determined by
           inquiry.                                               the durability period and in-running reliability required for
             The relationship among the basic rated life, the basic  a machine to which the bearing is applied. In general, the
           dynamic load rating and the axial load can be expressed  requisite lifetime as a guideline is as shown in Table 2.1.
           in formula (2.1).                                        The fatigue life of bearing is an important factor to
             Basic Rated Life specified in ISO 281.               decide the bearing size to be applied. In addition to this
                                                                  factor, however, the strengths and rigidities of shaft and
                                                                  housing must be taken into consideration.
                      C  p
               L 10 ʹʢ  ʣ ⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯(2.1)
                      P


                                                             A-14
   13   14   15   16   17   18   19   20   21   22   23