NSK introduces new life equation to more accurately predict bearing life under variable operating conditions

NSK introduces new life equation to more accurately predict bearing life under variable operating conditions

Keywords: NSK, Bearings

Introduction

The extremely competitive nature of modern manufacturing means that the scope for over sizing products to ensure acceptable levels of life under all types of operating conditions is severely circumscribed. Instead, the move is towards greater determinism in predicting product performance in order to enable closer matching of product operating parameters to each particular application. NSK is in the vanguard of this development with its advanced bearing life equation,integrating a new life modification factor. The new equation takes account of recent advances in bearing technology to predict, with higher accuracy, the service life of NSK bearings under the various and often harsh operating conditions in which they are used (see Plate 1).

  Plate 1An NSK bearing

Basic ISO model takes no account of recent improvements in bearing technology

In most rolling element bearing applications there is a requirement to estimate fatigue life. The usual mechanism for achieving this is the ISO model,which is based upon empirical research carried out in the 1940s and 1950s. Although providing a reasonable benchmark for the comparison of probable fatigue life between bearing sizes and designs, the basic ISO equation (Figure 1) does not take account of the vast improvements in steel quality, surface finish or lubrication made over recent years, or enable allowance to be made for the fatigue load limit of different materials.

  Figure 1 Basic ISO 281 - 1990 equation

The limitations of the basic ISO model are aptly illustrated when the latest bearing related technologies are combined. The result is a generally longer fatigue life for bearings used in relatively clean environments than those standardised for ISO life equations.To illustrate this point: while the actual life of a bearing under clean lubrication can be up to 20 times longer than the calculated life, actual life under contaminated lubrication can be as low as one-tenth of the calculated life. In view of this situation, a new approach was called for. This has been provided by NSK with a research programme involving the empirical testing of over 450 roller and 550 ball bearings under 40 different types of operating conditions. The culmination of this research is the advanced bearing life equation, which for the first time integrates a life modification factor (a_NSK) to provide a more accurate figure for the actual life of a bearing under different operating conditions. The advanced bearing life equation is a development from NSK's previous new life theory(Figure 2), which was an extension of the basic ISO model, based on empirical data gathered from a wide range of applications over a number of years. This data enabled new ground to be broken, in particular with the addition of adjustment factors for environment and fatigue limits, to provide better estimates of the life of NSK bearings than previous theories.

  Figure 2 NSK previous generation life theory

Calculation of life modification factor, a_NSK

In NSK's previous new life equation, the adjustment factors, a₂, a₃,a₄ and a₅, were assumed to be independent of each other. However, new research has confirmed that, in practice, they actually influence each other. Consequently, a new theory was called for – the advanced bearing life equation, which takes account of the interrelationships between these adjustment factors.

A product of the basic life equation (ISO 281:1990) multiplied by the correction factor a_NSK, the advanced bearing life equation is denoted by: _Lable = a₁.a_NSKL₁₀. In calculations using the advanced bearing life equation, the fatigue load limit P_u,is introduced, which is the bearing load below which the bearing does not fail after more than 10¹¹ revolutions. In addition, a contamination factor a_c is used (see Table I), and the load parameter (P–P_u)/C.1/a/_cis set on the abscissa (abscissa – the distance (measured parallel to the X axis) of a point from the Y axis) of the graphs shown in Figures 3 and 4.

  Figure 3 New life equation curves for ball bearings

  Figure 4 New life equation curves for roller bearings

Finally, the viscosity ratio (K) is used as a function of the lubrication parameter (a_L) to account for the fact that improved lubrication(higher K) results in longer life. This means that a_NSK is a function of K and increases with’it.

Figures 3 and 4 show the new life equation curves of ball bearings and roller bearings respectively. Based upon experimental results, the standard permissible range for the a_NSK life modification factor was set from 0.1 to 50(the latter representing ideal lubrication conditions). For bearings operated under heavily contaminated lubrication (a_c = 0.05), however, the new life correction factor was defined as a minimum of a_NSK = 0.1.

The accuracy of the advanced bearing life equation is put to the test in Figure 5, which compares actual endurance test data from ball bearings with bearing life calculated using the new equation. As is apparent from the graph,excellent correlation exists between actual test results and calculated data.

  Figure 5 Comparison of endurance test data and ball bearing life calculated with new equation

Conclusion

The results of NSK's test programme prove conclusively that the advanced bearing life equation is a valuable tool for providing greater accuracy in predicting fatigue life where the operating and lubrication conditions are well understood. The equation offers the advantage of being familiar, as it resembles the format of the current ISO equation, enabling engineers to compare and quantify life improvements. Furthermore, in some applications the theory enables the use of more compact bearings with a high degree of confidence in their suitability, thus reducing costs and improving competitiveness.

For further information contact: Michelle Reed, NSK Europe Ltd, European Bearings Business Unit, Northern Road, Newark, Nottinghamshire, NG24 2JF UK. Tel: +44 (0)1636 605123. E-mail: info-uk@nsk.com;Web site: www.nskbearings.com