Contaminated circumstances

Contributing editor Rex Narraway looks at the implications in the bearing business of a newly developed steel from NSK-RHP

A newly developed steel from NSK can increase the longevity of rolling bearings running in contaminated environments. Using this steel, the life of a new range of bearings has been extended by a factor as high as eight when compared to those using conventional materials.

Known as HTF bearings, the range has been introduced to meet the requirements of designers in the automotive industry where mechanical assemblies are susceptible to lubricant contamination. This is especially critical where transmissions are concerned.

NSK acknowledges that there are other ways of combating this problem, including the use of its own ‘sealed-clean’ bearings which contain dynamic rubber seals to prevent ingress of solid contaminants while allowing oil to reach the bearing surfaces (Figure 1). However, in many roller bearing applications, there is insufficient space to provide the extra depth required in the housing to accommodate the length of the sealed bearing. In addition, when taper bearings are employed, the sliding surface often needs a lot of lubrication that cannot be provided when seals are used.

In contrast to bearings operating with very clean grease or using highly- filtered oil lubrication, which normally fail because of sub-surface fatigue, those running in contaminated conditions suffer from surface- originating fatigue. Wear resulting from this is often first revealed by the appearance of indentation on the bearing surface caused by wear particles and other solid debris contained in the lubricant. The increase in the life of HTF bearings has been made possible by reducing the effects of these indents on the bearing surface.

Research has shown that, when a bearing is under load, contact stress is concentrated on the edges of the indents resulting in surface flaking caused by accelerated fatigue damage. The point is graphically illustrated in Figure 2. If the shape of the indentation is given in terms of the radius of the shoulder dent, R, together with half the dent width, C, then the greater the value of the R/C ratio, the smaller the stress concentration and, hence, a longer bearing life.

Using this relationship in conjunction with a series of empirical tests, researchers at NSK have found that there is also a connection between the austenite retained in the bearing steel and the rolling fatigue life of a bearing in contaminated environments. Consequently, the basis of the HTF concept is to relieve the level of stress occurring on the edges of the indents by raising the level of the retained austenite. Achieving this has led to the development of an innovative heat treatment process called TF technology.

During the course of the development, a series of tests were conducted on a variety of test pieces which were produced from an assortment of materials to act as thrust washers. Heat treatment processes were applied to give a wide range of hardness and retained austenite levels. The object was to clarify the relationship between the ^indentation contour, as defined by the R/C ratio, and the percentage of retained austenite. To do so, a Vickers indentation was made on each test sample and the resulting R and C values created by the indents were measured.

Each washer was then set up in a thrust bearing test machine, normally used for evaluating rolling contact fatigue life, and submerged in a bath of clean oil. Stress was applied by rolling steel balls over the indentation under a specified load after which the test piece was removed from the machine and the indentation contour was traced.

Results from the test showed that the R/C ratio stabilises after 3000 cycles. Results from the test demonstrate that after 1min (3000 cycles) of testing, the value of the R/C goes up with increased volume percentages of the retained austenite. The upper limit of retained austenite is governed by the dimensional accuracy.

The test was repeated for three sets of samples as in Figure 4. A comparison of the results revealed that repeated stress produced a higher R/C value in the harder materials than in the softer ones. The stress relaxation for the softer material with a lower austenite level was almost completed within the short period of time of a few thousand cycles. In contrast, the harder material and higher austenite level continue with stress relaxation due to the increasing R/C value.

Another test designed to evaluate the dimensional accuracy of the bearing material, involved several tapered roller bearings treated with HTF, through- hardening, case hardening, carburising and carbo-nitriding, which were tested for comparison. From this, it was concluded that HTF bearings have a dimensional stability between that of through-hardened and case-hardened materials.

Since austenite is comparatively soft, it is difficult to produce a part that combines a sufficiently high level of retained austenite and the appropriate degree of hardness to render it suitable for bearing applications. Such properties cannot be attained using conventional heat treatment such as carburising or through hardening processes. As a result, NSK-RHP has developed a heat treatment in addition to a new steel with an increased level of chromium content, resulting in a higher level of fine carbides and carbo-nitrides within the material.

In addition to prolonging the life of the bearing surface in contaminated conditions, use of HTF technology has demonstrated a slowing down of the fatigue process, delaying the initiation of cracking and reducing the effects of crack propagation at the shoulder of the indentation. It also has further advantages in both wear resistance and seizure limit.

Gerald Rolfe, engineering manager at SKF Industrial Division believes that the HTF bearings are based on steel alloy and heat treatment methods designed to reduce the cost of carburising techniques which can provide increased wear resistance for rolling bearings under contaminated operating condition.

However, he points out this advantage is normally applicable to line contact (roller) bearings only, and offers little benefit for point contact (ball) bearing types. He also states that a high degree of retained austenite within the microstructure can result in severe instability of the material under certain conditions.

As far as SKF’s developments to solve the problem of use in arduous operating conditions, the company has adopted Bainitic through-hardening heat treatment for standard spherical roller bearings. This provides a tough, but stable microstructure, with very high static load capability. Gerald Rolfe says that other hi-tech hardening methods are under development for future use in rolling bearings generally, including high-vacuum carburising and carbo-nitriding techniques: ‘These advanced methods,’ he says, ‘outline surface modification heat treatments of modified steel alloys to overcome the current problems associated with conventional carburising techniques and provide tough, stable and wear-resistant products.’

There are other approaches to improving service life of bearings under contaminated conditions. SKF has recently patented and introduced a new type of ball bearing design, known as a Single Ball Hybrid Ceramic Bearing, in which only one ball in a conventional bearing is replaced with a ceramic ball. This has a ‘repairing effect’ on raceways, subject to indentation in contaminated conditions, greatly extending actual service life.

{{NSK-RHPTel: Ruddlington (01159) 366600Enter 500

SKFTel: Milton Keynes (01908) 838383Enter 501}}