The lubricant formulation: one driver for premature bearing failures and white etching cracks

Arnaud Ruellan¹, Kenred Stadler², Marc Ingram³

¹ SKF Research & Technology Development, SKF BV, Nieuwegein, The Netherlands

² SKF Research & Technology Development, SKF GmbH, Schweinfurt, Germany

³ Afton Chemical Ltd, Bracknell, UK (until end of 2017); Ingram Tribology Ltd

INTRODUCTION:

Premature bearing failures associated to White Etching Cracks (WECs) have been extensively studied in the past decade. Within SKF, it has been demonstrated that the extensive subsurface crack networks (WECs) leading to premature failures can be a consequence of several different root causes (Fig. 1) [1].

Several hypotheses have been formulated on the role of the lubricant and tribochemistry under mixed lubrication and high slip conditions, based on tests of 81212 bearings on FE-8 rigs [2-6]. It has been suggested that WECs may develop in points of high frictional energy accumulation [3, 5] but evidence suggests that it is very dependent on the presence of specific additives and/or reaction layers [3, 4]. Some authors suggest that certain oils will lead to hydrogen ingress and subsequent weakening of the bearing steel [2, 3, 6]. Others suggest that specific additives and/or reaction layer could induce high surface shear stresses promoting surface micro-cracks [4]. The aim of this study is to verify or falsify the assumption that certain additive and/or reaction layers can accelerate bearing failures and to better understand why.

Fig. 1 – The SKF approach of WEC associated bearing failures. In this work, the focus is the role of tribochemistry under mixed lubrication and high slip.

METHODS:

Tests have been performed on 81212 bearings in SKF LAD 100 rig in comparable conditions as the FE-8 set-up to confront the results to those reported in the literature [2-6]. The number of test variables has been reduced to enable a systematic and complete post analysis. Four tailored oils of same viscosity have been tested at 90C and 70C under conditions kept constant for all the tests and with a specific film thickness of l=0.6. The four oils A, B, C and D are expected to develop different reaction layers, each with a specific combination of properties. The bearing tests are suspended after 120% of the advanced estimated lifetime or stopped on vibrational level. Subsequently, the rollers and washers are systemically inspected as follows:

• ·optical microscopy of raceway;
• wear profiles assessment and surface roughness measurements in regions of negative, zero and positive slide to roll ratios;
• SEM-EDX analysis in the same areas;

Circumferential metallographic cross sections in specific points of negative, zero and positive slide to roll ratios to confirm or not the presence of WECs.

RESULTS:

All the tests with oils B, C and D have been suspended while all the tests with oil A have failed prematurely at 70C and 90C (30% of estimated life at 90C). WECs have been observed only in rollers tested with oil A regardless the position across the raceway, with a typical a top-down growth in the direction of over-rolling.

Fig. 2 – Typical results obtained for oil A and oil B at 90C: WECs only observed in rollers for oil A both at 90C and 70C.

DISCUSSION:

Wear assessments, roughness measurements and extra component tests suggest the rollers and washers are subject to comparable mechanical stresses and shear stresses both for oil A and B. This suggest that the oil formulation may reduce bearing performance by weakening the bearing steel. As expected, the topography and chemical composition of the reactions layers are different for each oil. However, no direct correlation could be identified between basic reaction layer properties and WECs, for example comparing reaction layers on rollers and washers for oil A (at 90C and 70C) (Fig. 2). Finally, no direct correlation with the frictional energy accumulation as proposed in [3] could be confirmed in those tests. Further tests and investigations are now being considered.

REFERENCES:

[1] Stadler, K., Vegter, R. H., and Vaes, D., 2018, “White Etching Cracks - a Consequence, Not a Root Cause of Bearing Failure,” SKF Evol., 1(January), pp. 21–29.

[2] Richardson, A. D., Evans, M.-H., Wang, L., Wood, R. J. K., Ingram, M., and Meuth, B., 2018, “The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 Steel,” Tribol. Lett., 66(1), p. 6.

[3] Franke, J., Carey, J. T., Korres, S., Haque, T., Jacobs, P. W., Loos, J., and Kruhoeffer, W., 2017, “White Etching Cracking—Simulation in Bearing Rig and Bench Tests,” Tribol. Trans., 0(0), pp. 1–11.

[4] Paladugu, M., Lucas, D. R., and Scott Hyde, R., 2018, “Effect of Lubricants on Bearing Damage in Rolling-Sliding Conditions: Evolution of White Etching Cracks,” Wear, 398–399, pp. 165–177.

[5] Gutiérrez Guzmán, F., Oezel, M., Jacobs, G., Burghardt, G., Broeckmann, C., and Janitzky, T., 2017, “Reproduction of White Etching Cracks under Rolling Contact Loading on Thrust Bearing and Two-Disc Test Rigs,” Wear, 390–391(May), pp. 23–32.

[6] Richardson, A. D., Evans, M.-H., Wang, L., Wood, R. J. K., and Ingram, M., 2018, “Thermal Desorption Analysis of Hydrogen in Non-Hydrogen-Charged Rolling Contact Fatigue-Tested 100Cr6 Steel,” Tribol. Lett., 66(1), p. 4.