An Experimental and Theoretical Study of Hybrid Bearing Micropitting Performance under Reduced Lubrication

Abstract

Hybrid bearings-that is, bearings with ceramic rolling elements and steel rings-are often used in applications with reduced (i.e., boundary or mixed) lubrication conditions. The mechanisms by which hybrid bearings perform significantly better than full-steel ones in these cases are so far unclear, although a number of published works have shown experimental results in which appreciable performance benefits were obtained by the use of hybrid bearings under boundary or mixed lubrication. In this article, the reduced lubrication performance of hybrid rolling contacts, versus full-steel ones, is studied in detail by means of rolling bearing fatigue experiments and a theoretical micropitting model. It is found that the large improvement in surface fatigue resistance of hybrid contacts cannot be explained solely on the basis of the unavoidable differences in some of the roughness parameters existing between the full-steel and hybrid contacts. It is also necessary to take into account a considerable reduction in the effective boundary friction coefficient of the hybrid contact. In the numerical micropitting simulations it was found that the boundary friction coefficient of a hybrid contact must be about two times lower than that for the corresponding full-steel contact, in order to be able to predict the experimental observations reasonably well. A similar ratio of the boundary friction coefficients was obtained in a number of dedicated tests, thus confirming the results of the micropitting model. The mechanisms of the strong micropitting resistance of hybrid bearings under reduced lubrication conditions are discussed in detail, shedding new light on the operational tribology and performance capabilities of bearings with rolling elements made of silicon nitride ceramics.

Rolling bearing seizure and sliding effects on fatigue life

“…Since the two friction coefficients have the same value ¼ 0:05, starvation and mixed-lubrication conditions will not affect the temperature rise of hybrid bearings. This is consistent with experimental observations reported in Brizmer et al 31 In contrast, for all-steel bearings, the friction coefficient doubles in case of boundary lubrication, and this has a significant effect on the thermal condition of the rolling contact.…”

“…As it can be seen, it appears that hybrid bearings can indeed take higher pressure under the same speed conditions, this could be due to the better tribological conditions, as explained in Brizmer et al. 31 but also to the fact that at the same speed the contact forces in the outer ring will be lower due to the lower ceramic density compared with steel. This is an important result because it highlights some advantages of hybrid bearings when it comes to the potential delay of seizure damage.…”

“…Figure 7 shows a plot of the last two columns of Table 4, in black the steel-steel cases and in blue the hybrid bearing cases (dots), superposed one can see sketched lines drawn by hand, indicating the possible location of the 'transition points' defining the onset of seizure. As it can be seen, it appears that hybrid bearings can indeed take higher pressure under the same speed conditions, this could be due to the better tribological conditions, as explained in Brizmer et al 31 but also to the fact that at the same speed the contact forces in the outer ring will be lower due to the lower ceramic density compared with steel. This is an important result because it highlights some advantages of hybrid bearings when it comes to the potential delay of seizure damage.…”

Progress in rolling bearing technology for refrigerant compressors

“…It has been verified by testing 14 that steel rolling bearings, which contain at least one rolling element made of ceramic (Si 3 N 4 ) improve substantially their performance under poor lubrication and high contamination conditions, therefore the next line of defence after nitriding or carbonitriding for lubrication with oil-refrigerant mixtures should be this type of rolling bearings so-called as semi-hybrids. The working mechanism is similar to hybrid bearings, explained already in Brizmer et al 41 and Vieillard et al 43…”

“…30,40 Another effective solution for poor lubrication problems is the use of full or semi-hybrid bearings (steel rings and all or a few rolling elements made of ceramic material, Si 3 N 4 ). The same model has been used 41 to explain how a hybrid bearing, by reducing its boundary friction coefficient, can reach much better performance under poor lubrication conditions than all-steel bearings. The predictions of the model were also confirmed by experiments.…”