Bi-Modal Failure Mechanism of Rolling Contact Bearings

The theory of failure of rolling contact bearings is based on fluctuating high level loading and material fatigue. This theory is unimodal, considering only the solid components of the bearing, and ignoring the liquid phase, which is the lubricant. Bearing life is rather dispersed, reaching a ratio of 20 between the extreme values. Since this theory was established, several exceptional phenomena were detected that could not be explained by it, such as: 1) Pit-ting damage beyond the contact path; 2) Detrimental effect of a minute quantity of water in the lubricant on bearing life. 25 ppm of water in the lubricant brought about shorter bearing life by over than 30%. The bimodal failure theory considers both solid and liquid bearing components. The damaging process of the lubricant evolves from its cavitation. During this process vapor filled cavities are formed in low pressure zones. When these cavities reach high pressure zones they implode exothermally. These implosions cause local high pressure pulses reaching 30,000 at accompanied by a temperature rise of about 2000 degrees K [1]. This paper includes cavitation erosion test results on stainless steel samples by vibratory and water tunnel test rigs. Various methods of lubricant dehydration are presented and evaluated. The main conclusion from this analysis is the use of water-free lubricants, for long life of RC bearings and more uniform service life thereof.

. This wide dispersion cannot be fully explained by material fatigue of the contacting elements. Accordingly, it is impossible to predict the service life of a single RCB. As a result this problem is treated by statistical methods.
A new approach to this problem considers the process occurring in the lubricating oil, namely cavitation. Between the rolling elements and the rings a contact path is formed. Along this path pressure fluctuates, causing the lubricant and the water in it to vaporize and condense in a cyclic manner. Vapor condensation is exothermal, accompanied by high pressure and high temperature rises [1]. This process is known as cavitation, exerting high local pressure within the lubricant and upon the surfaces of the bearing elements. Cantley [3] reported life tests he performed on full scale RC bearings lubricated with SAE-20 oil, containing a minute quantity of water of 25, 100 and 400 ppm. These small quantities of water were chosen to represent extreme conditions of relative humidity. He found that when tested with these mixtures of oil and water, bearing life was reduced by 32% -48%. He reached the conclusion that water absorption by lubricants can become an important factor in the determination of RCB life.
Water content of lubricants is not considered in bearing life tests. The unimodal failure theory is based on the stress distribution within the contact path, as caused by the metal to metal contact. Since the vapor pressure of water is larger than that of oil, water will cavitate first, thus causing damage to the bearing elements. It should be noted that most lubricant specifications do not contain the following data: 1) water absorption, 2) water content, and 3) oil vapor pressure.
Cavitation erosion of stainless steel specimens in lab tests is further presented. These tests were performed by the "International Cavitation Erosion Test", ICET. From this large reservoir two tests were chosen: one vibratory test and one as performed in a water tunnel. Both tests were performed by the same lab "Institute of Water Problems of the Bulgarian Academy of Sciences, Sofia, Bulgaria", IWP BASci. Both tests were performed on stainless steel specimens, 1H18N9T, which represents bearing materials. These tests simulate the erosion and space formed between bearing elements that will cause critical vibrations and bearing failure.   ( ) Validity test of this equation for the bearing curve in Figure 1 reveals a good fit, especially for values larger than T. Here the difference between the curves is about 4%. In any case this equation reasonably represents the actual bearing life dispersion.

General
Water is a polar liquid and lubrication oils are non-polar. Accordingly, these liquids cannot contact chemically. However, they can mix and form an emulsion. When the mixed liquid is under cavitation conditions the water will cavitate first, as it has a higher vapor pressure and is more volatile. The following two examples will illustrate cavitation erosion of stainless steel specimens in water. For this illustration it was assumed that all the erosion in the RCB is caused by cavitation only.
The common data for both tests are:

1)
Original test results up to erosion of 24 µm MDE, see Figure 2; 2) Expanded curve up to 13 µm MDE, see Figure 3; 3) Time to reach a critical erosion of 12.5 µm MDE: 815 min.

Water Tunnel Test Rig
Cavitation tunnels are traditional experimental rigs used to study cavitation phenomena ever since 1895. Various flow obstacles such as: cylinders, bolts, wedges, venturies, or barricades and counter barricades are used as cavitators. Usually, intensity of cavitation can be easily controlled by modifying the flow system geometry, and or changing the hydraulic circuit operating parameters.

1)
Original test results up to erosion MDE 290 µm, see Figure 4.
2) Expanded test results up to erosion of 20 µm, see Figure 5.
3) Time to reach the critical erosion of 12.5 µm: 9300 min.

Summary of Cavitation Erosion Test Results
The critical erosion time, as obtained from the vibratory CE tests, is 11.4 times shorter than that obtained by the water tunnel tests. This clearly indicates that the cavitation intensity of the vibratory CE tests is by far larger.
There is no possibility to determine the complete fit between the conditions in CE tests and those in the RC bearings. However, it is reasonable to assume that the cavitation is adequately simulated. In any case it is possible to apply the CE tests for material classification for RC bearings.

General
After proving the detrimental effect of water in lubrication oil on RC bearing life, Cantley [3] tried to dehydrate oil by an additive. This attempt failed. Later on this avenue was found to be counterproductive, since additives to lubricants were found to increase their hygroscopic property.
Water exists in lubrication oils in three forms: 1) Dissolved (up to saturation); 2) Dispersed (as in emulsions); 3) As free water.
In each form water is detrimental to the operation of RC bearings, and shortens their service life appreciably. Accordingly, lubrication oils should be dehydrated and kept water-free during their planned service life. Several dehydration methods are further listed [7]. 2) Centrifugal separation: By this method the oil is passed through a centrifuge, exerting forces of the order of 10,000 g. This method is best fit for low viscosity oils, such as turbine oils. The water forms separated here are the free and emulsified water. Separation efficiency increases with lower temperature.

Lubricant Dehydration Methods
3) Absorption removal: Cellulose based filters are here used to remove free and emulsified water.

Conclusions and Recommendations
In life tests of identical RC bearings, performed under controlled conditions, a large dispersion in life duration is evidenced. The ratio between the extreme results reaches the value of 20, see Figure 1 [2]. The curve in Figure 1 is the result of simultaneous operation of two failure mechanisms: 1) Metal fatigue due to repetitive high local loading.
2) Cavitation erosion of bearing components due to fluctuating high local pressures along the contact path.
The fatigue model is considered as the main failure mode, while the cavitation is yet disregarded.
Water is absorbed in lubrication oils and has a detrimental effect on RC bearing life. This effect is considerable. Even minute quantities as little as 25 ppm may shorten bearing life by nearly 30%. Since water has a higher vapor pressure it will cavitate first, causing severe damage in the bearing.
Cavitation erosion of stainless steel specimens can be easily performed in laboratory tests. These tests simulate the process in RC bearings and can be utilized for selection of adequate bearing materials.
It is highly recommended to keep lubrication oils water-free. The best method for dehydration of lubrication oils is by passing it through a membrane system, thus reducing the water content to 10 ppm and less.

Y. Meged Advances in Materials Physics and Chemistry
The bimodal failure theory for RC bearings renders a new aspect to the understanding of their operation and failure mechanism, and paves the way to improve their future design operation and maintenance.