White Etching Cracks
A damage phenomenon on the test bench
Cracks in the bearings of e.g. wind turbines can lead to expensive outages. These failures are increasingly indicative of the phenomenon of so-called White Etching Cracks (WEC). The causes of these cracks on or below surfaces, which are visible as white lines, are currently being intensively researched and attempts are being made to counter them with innovative test methods and materials. While WEC damage rarely occurs, it can lead to early failures as early as one to two years due to macroscopic damage in the raceway or even tearing of the ring. The exchange e.g. of a gearbox bearing in offshore installations costs more than one million euros.
The causes for the genesis of WEC are not yet fully understood. However, there are several hypotheses about their formation, of which the so-called hydrogen hypothesis is now regarded by experts as the most plausible one. It describes the following chain of events: In the vicinity of the bearings, for example in the lubricating oil, hydrogen is in molecular form. This can’t initially diffuse into the steel of the bearing. However, electric fields or chemical processes can split the hydrogen into atomic form. The hydrogen atoms are diffusible, allowing them to penetrate the steel and concentrate on non-metallic inclusions or other imperfections of the material.
As a result, the flow limit of the material is lowered. Another external load beyond the yield point causes structural changes in the previously damaged material. Compared to the base material, very hard zones with nanocrystalline ferrite are formed. These nanocrystalline microstructures are not attacked during a Nital etching and appear white in the etched finish - which is why they are referred to as white etching cracks. When external stress is applied to the modified material, tensions are created between the hard and less hard zones and eventually cracks that extend to the surface and cause the bearing to fail.
Test bench examinations at FUCHS
To what extent do certain additives and gear oil formulations prevent or promote the damage phenomenon of white etching cracks in rolling bearings and gears?
Based on this question, FUCHS investigated industrial gear oils and other lubricants on a special FE8 test stand designed for rolling bearings. Rolling bearings are, according to users, the machine elements that are particularly affected by WEC damage.
To investigate the damage phenomenon WEC the FE8 pitting test according to VW-PV-1483 was used. The test was carried out with cylindrical roller bearings with polyamide cages at an axial load of 60 kN and a speed of 350 and 750 rpm, with an oil temperature of 100 °C and an oil flow rate of 2 x 0.1 l / min. The focus was on the determination of the fatigue life of the bearing and the relevant influence of the oil formulation and the investigation of occurring WEC damage on the cylinder liner or cylindrical roller.
The low reference oil was defined as an API GL4 manual transmission fluid that has been proven to cause WEC damage to the bearing raceway.
RENOLIN CLP, RENOLIN UNISYN CLP, RENOLIN UNISYN XT, RENOLIN PG and PLANTOGEAR S were used as high reference oils.