Bearings in clean environments operate under very clean grease or highly-filtered oil lubrication, and will eventually fail due to subsurface origin-type fatigue if installed properly. Bearings subjected to contaminated environments fail due to surface origin-type fatigue. Cleaner steel has been proven to be effective in promoting longer life of bearings operating in clean environments, while “sealed and clean” bearings are effective in promoting longer bearing life under contaminated conditions.
Bearings that encounter debris are prevalent in the mining, aggregate, steel and cement industries. This debris generates surface origin fatigue and causes reduced bearing life. Sealed and clean steel bearings use a contact seal that excludes harmful debris from entering your bearings. Unfortunately, sealed bearings are not feasible for use in every application, including those in heavy industry. Seals also may not always keep all debris out of your bearings if they are available.
The Process of Surface Originating Flaking
Various types of debris can generate indentations in the raceways of your rotating bearings. Contact stress is extremely high at the indentation edges, and fatigue damage occurs at an accelerated rate.
The following equation describes the stress concentration at the shoulder of an indentation. The ratio of maximum shear stress, tc, at the indentation to the nominal contact pressure, po, in the case of no indentations is as follows:
tc/po = a1(Co)a2
Where a1 = 0.22
and a2 = -0.24
Co is called the furrow severity factor and is found by using the following equation:
Co = (π2 po/Eo)(r/c)
r = radius of furrow shoulder (mm)
c = half value of furrow width (mm)
po = nominal contact pressure (N/mm2)
Eo = reduced Young’s Modulus (N/mm2)
In the equation above, the shape of the indentation, in terms of r and c, applies to a strong influence on the fatigue life of the bearing. A higher r/c value promotes longer bearing life, because of the lower stress concentration at the edge of the indentation.
Indentation Contour & Material Factor
The following experiments were performed in order to clarify the relationship between indentation contour (ratio of r/c) and the percentage of retained austenite. This was done in a thrust bearing test machine used for evaluating rolling-contact fatigue life. The spindle section of the thrust bearing test machine with the test specimen is shown below.
- Washer-type pieces, which has a wide range of hardness and retained austenite, were prepared from an assortment of materials and heat treatment processes.
- A Vickers indentation was made on each test race.
- r and c created by the indentation were measured.
- Each test race was set in the test machine and submerged in a clean oil bath. Stress was applied by rolling steel balls over the indentation with specified load.
- The indentation contour was traced after removing the test piece from the test machine.
Analysis of the r/c versus cycle in this test showed r/c stabilize after 3,000 cycles. Results from this test show that, after one minute (3,000 cycles) of testing, the value of r/c increased with the increased volume percentages of the retained austenite. The upper limit of retained austenite is governed by the dimensional stability.
Next, the test was completed with three different sample sets. Set one has samples containing 32% retained austenite and a hardness value of Hv802. Set two has samples containing 33% retained austenite and a hardness value of Hv716. Set 3 has samples containing 10% retained austenite and a hardness value of Hv739. When comparing sets one, two and three, it was discovered that repeated stress produced a higher r/c value with harder materials compared to softer materials. It was also noted that the stress relaxation for the softer material with a lower austenite level was almost completed within a relatively short time of a few thousand cycles. The sample with a harder material and higher austenite levels continued in stress relaxation due to a continuous increase in r/c.
New Material Requirements
Since retained austenite is soft, it is difficult to produce a part with both high hardness and a high volume of retained austenite. Therefore, new steel specifications were required. Innovative heat treatment processes were created to overcome the special requirements set by this steel, which were completed by increasing the steel’s chromium content. This resulted in a greater number of fine carbides or carbo-nitrides within the material matrix.
Conventional heat treatment processes cannot attain these required material properties. Conventional processes are defined as those identified as being carburized or through-hardened bearings, since the life results are similar in a contaminated environment. New heat treatment processes were developed to refine the carbides or carbo-nitrides after carburizing or carbo-nitriding.
Based on the results of this study, new Tough Steel™ specifications were formulated. These specifications include the following element content:
C: 0.42 %
Fatigue Life Testing
Another thrust bearing test was performed under conditions of contaminated lubrication. The results indicate a controlled increase in the volume of retained austenite leads to a longer life. Furthermore, if the volume of retained austenite if maintained, but hardness is increased, even longer life is attainable. To prove the required material parameters, actual bearings were made from such materials and tested with contaminated lubrication.
The bearings used in this test were run in contaminated gear oil, with the contaminants mixed thoroughly into the oil, allowing the mixture to pass evenly through the test bearing. Oil temperature was controlled to maintain the viscosity. Tough Steel bearings ran successfully for a life that was seven to eleven times longer than that of conventional carburized or through-hardened taper bearings.
The life test results of deep groove ball bearings show Tough Steel bearings have 6 times more life than conventional ball bearings.
To analyze the longer life of Tough Steel bearings, the following tests were performed using the medium box test machine. During these life tests, the fatigue progress in the raceway surface was measured after a certain interval using “Fatigue Analysis”. This analysis is an original method developed by NSK and uses X-ray diffraction technology to determine fatigue progress in the material in a semi-quantitative manner.
As fatigue progresses, a change occurs in the martensite crystal lattice and retained austenite converts to martensite. Measuring both factors with X-ray, it is possible to determine the type of fatigue (surface origin or subsurface origin) and the sage in the fatigue progress. Either destructive or non-destructive inspection can be performed in Fatigue Analysis. After a certain interval in each test, a non-destructive inspection was conducted using X-rays only on the rolling contact surface. This evaluated the changes in the material structure. After the life test, a destructive inspection was made with X-ray analysis, in which very thin layers of the bearing material at the indentation zone were removed, and material structure changes were recorded within the bearing. This investigation revealed that under the same loading and environmental conditions, Tough Steel bearings exhibited slower progression of fatigue compared to conventional bearings. The test started with model indentation and was run until flaking occurred in the bearing raceway. It was done by pressing a Vickers indentor into the surface of the bearing and measuring the contour of the indentation.
Next, the bearing was loaded and a life test was performed with clean lubrication. At a certain interval, the bearings were removed from the medium box tester. The surface was then observed with a microscope and the contour of the indentation was measured. This process was repeated until flaking occurred. NSK then noted the starting point of the crack and the flaking area in both Tough Steel and conventional bearings. The appearance and progression of a crack in the Tough Steel bearing was delayed and slowed. Test results also revealed that Tough Steel bearing has a larger shoulder radius at indentations than that of conventional bearings. Results show that Tough Steel bearings have longer lives under contaminated conditions, since there is a lower stress concentration at the shoulder of the indentation. This difference in radius shape is the key reason for this type of bearing experiencing a longer life cycle in contaminated environments.
Wear Resistance & Seizure Limit
Tough Steel bearings have a large number of fine carbides and carbo-nitrides that give a higher resistance to wear and greater seizure resistance. The test results of the wear-amount rate and seizure limit were determined by a Sawin-type test machine, which is specifically used to evaluate wear resistance. The results show that the bearings with Tough Steel material experienced less wear and higher seizure limits than conventional, through-hardened and case hardened materials. Even though the load used in this experiment seems small, the maximum surface contact pressure is 98 N/mm2. The Sawin-type test machine was used in this particular test, because is can also simulate no-lubrication conditions, which is particularly valuable in helping to identify the change point from mild to severe wear, which is recognized as the seizure limit.
Dimensional stability is another important characteristic of bearing material. Several tapered roller bearings were tested using Tough Steel, through-hardened and case-hardened materials. The outside diameter of the L44610 bearing used in this test was 50.292 mm. Samples of five types of materials were kept in an oven at 130°C for 4,000 hours and then the outside diameters of the bearings were measured. Another set of samples of five types of materials were kept in an oven at 170°C for 1,000 hours. The outside diameters of the bearings were then measured. It was determined that the Tough Steel dimensional stability was between that of through-hardened and case-hardened bearings.
The results for case-hardened, carburized and carbo-nitriding bearings were also plotted in comparison to the Tough Steel bearing. Carburized bearings showed similar results to those of the Tough Steel bearings, but carbo-nitriding bearings showed very large outer ring expansion in both temperature ranges.
Estimated Reading Time: 8 minutes
- Optimizing Bearing Life with Lubrication - March 4, 2020
- Controlling Bearing Failure in Contaminated Environments - May 12, 2016