Page 11 - NSK_CAT_E1102m_A7-141

SELECTION OF BEARING SIZE

n fn n fn Lh fh Lh fh If the bearing load P and speed n are 5.2.5 Correction of Basic Rating Life The life adjustment factor for operating conditions
(h) known, determine a fatigue life factor fh
(min–1) (min–1) (h) 4.5 As described previously, the basic equations for a3 is used to adjust for various factors, particularly
80000 4.0 appropriate for the projected life of the calculating the basic rating life are as follows:
60000 0.08 60000 0.105 80000 5.5 60000 3.5 lubrication. If there is no misalignment between
0.11 60000 5.0 machine and then calculate the basic ( )For ball bearings C 3 the inner and outer rings and the thickness of the
40000 0.09 40000 40000 L10 = P lubricating film in the contact zones of the bearing is
30000 0.1 30000 0.12 30000 load rating C by means of the following . . . . . . . . . . . . . . . . . (5.5)
sufficient, it is possible for a3 to be greater than one;
20000 0.12 20000 0.13 equation. 10 however, a3 is less than one in the following cases:
15000 15000 3
0.14 4.5 ( )For roller bearingsC . . . . . . . . . . . . . . . (5.6) • When the viscosity of the lubricant in the
10000 0.14 40000 4.0 C= fh ⋅ P L10 = P contact zones between the raceways and rolling
8000 10000 0.15 30000 fn . . . . . . . . . . . . . . . . . . . . . . . (5.3) elements is low.
6000 0.16
0.16 8000 0.17 20000 3.5 20000 3.0 A bearing which satisfies this value of The L10 life is defined as the basic rating life with • When the circumferential speed of the rolling
0.18 6000 0.18 15000 C should then be selected from the bearing a statistical reliability of 90%. Depending on the elements is very slow.
0.19 2.5 tables. machines in which the bearings are used, sometimes a
0.20 10000 reliability higher than 90% may be required. However, • When the bearing temperature is high.
8000 2.0 recent improvements in bearing material have greatly
15000 3.0 6000 1.9 extended the fatigue life. In addition, the developent of • When the lubricant is contaminated by water or
1.8 the Elasto-Hydrodynamic Theory of Lubrication proves foreign matter.
4000 0.20 4000 0.25 4000 1.7 5.2.4 Temperature Adjustment for Basic that the thickness of the lubricating film in the contact
3000 1.6 Load Rating zone between rings and rolling elements greatly • When misalignment of the inner and outer rings
3000 3000 10000 1.5 influences bearing life. To reflect such improvements is excessive.
8000 2000 1.4 If rolling bearings are used at high in the calculation of fatigue life, the basic rating life is
2000 0.25 2000 0.30 2.5 1500 1.3 temperature, the hardness of the bearing adjusted using the following adjustment factors: It is difficult to determine the proper value for a3 for
1500 0.3 1500 1.2 steel decreases. Consequently, the basic
0.35 6000 1000 1.1 load rating, which depends on the physical specific operating conditions because there are still
1000 0.4 1000 0.40 800 1.0 properties of the material, also decreases. many unknowns. Since the special bearing property
800 0.5 800 0.45 4000 2.0 600 0.95 Therefore, the basic load rating should be Lna = a1 a2 a3 L10 . . . . . . . . . . . . . . . . . . . . . . . . . . . (5.7)
600 0.6 600 0.5 3000 1.9 500 0.90 adjusted for the higher temperature using factor a2 is also influenced by the operating conditions,
1.8 400 0.85 the following equation: where Lna : Adjusted rating life in which reliability, there is a proposal to combine a2 and a3 into one
400 400 2000 1.7 300 0.80 material improvements, lubricating quantity(a2×a3), and not consider them independently.
300 300 1500 1.6 0.75 Ct = f t ⋅ C . . . . . . . . . . . . . . . . . . . . . . . (5.4) conditions, etc. are considered
1.5 200 In this case, under normal lubricating and operating
200 200 1000 1.4 where Ct : Basic load rating after L10 : Basic rating life with a reliability of 90%
150 0.6 800 temperature correction conditions, the product (a2×a3) should be assumed
600 1.3 (N), {kgf} a1 : Life adjustment factor for reliability
150 500 equal to one. However, if the viscosity of the lubricant
400 1.2 f t : Temperature factor a2 : Life adjustment factor for special bearing is too low, the value drops to as low as 0.2.
100 0.7 100 0.7 300 If there is no misalignment and a lubricant with high
80 1.1 (See Table 5.3.) properties viscosity is used so sufficient fluid-film thickness is
60 0.8 80 200
50 0.8 1.0 C : Basic load rating before a3 : Life adjustment factor for operating secured, the product of (a2×a3) may be about two.
0.9 0.95 temperature adjustment
40 60 0.90 (N), {kgf} conditions When selecting a bearing based on the basic load
1.0 50 0.9 0.85
30 40 0.80 If large bearings are used at higher The life adjustment factor for reliability, a1, is listed in rating, it is best to choose an a1 reliability factor
1.1 0.75 than 120oC, they must be given special
1.0 dimensional stability heat treatment to Table 5.4 for reliabilities higher than 90%. appropriate for the projected use and an empirically
20 1.2 30 prevent excessive dimensional changes. The life adjustment factor for special bearing
The basic load rating of bearings given such determined C/P or fh value derived from past results
15 1.3 1.1 special dimensional stability heat treatment properties, a2, is used to reflect improvements in
1.4 20 may become lower than the basic load for lubrication, temperature, mounting conditions, etc.
rating listed in the bearing tables. bearing steel. in similar machines.
10 1.5 1.2 NSK now uses vacuum degassed bearing steel, and
15 1.3 the results of tests by NSK show that life is greatly The basic rating life equations (5.1), (5.2), (5.5), and
improved when compared with earlier materials. The (5.6) give satisfactory results for a broad range of
10 1.4 basic load ratings Cr and Ca listed in the bearing tables bearing loads. However, extra heavy loads may cause
were calculated considering the extended life achieved detrimental plastic deformation at ball/raceway contact
Ball Roller Ball Roller by improvements in materials and manufacturing points. When Pr exceeds C 0r (Basic static load rating)
Bearings Bearings Bearings Bearings techniques. Consequently, when estimating life using or 0.5 Cr, whichever is smaller, for radial bearings or
Equation (5.7), it is sufficient to assume that is greater Pa exceeds 0.5 Ca for thrust bearings, please consult
Fig. 5.3 Bearing Speed and Fig. 5.4 Fatigue Life Factor than one. NSK to establish the applicablity of the rating fatigue
Speed Factor and Fatigue Life life equations.

Table 5.3 Temperature Factor f t Table 5.4 Reliability Factor a1

Bearing 125 150 175 200 250 Reliability (%) 90 95 96 97 98 99
Temperature oC 1.00 1.00 0.95 0.90 0.75
a1 1.00 0.62 0.53 0.44 0.33 0.21
Temperature

Factor ft

A 26 A 27