Roulements - 2015-05-21
Various objectives are pursued when developing bearing technology, some of which are conflicting. Users want compact bearings with a high strength and a very long service life. The bearings should be insensitive to the entry of contamination (or should prevent this entry) and you will increasingly find the reduction of friction on the “wish list” of machinery and plant designers.
Just how these objectives are actually weighted depends largely on the field of application of the bearing. The service life is top of the list for wind power technology. Operators want and expect 180,000 hours with no servicing. Low-friction bearings are becoming increasingly popular in electric motors and the automotive industry. In the steel industry, bearings have to withstand high mechanical stresses, and in the field of food-processing machinery a long service life is required, even with frequent cleaning processes, i.e. in a damp environment.
NSK has developed dedicated materials, technologies, designs and series of bearings for all of the aforementioned industries. Synergies often arise between the individual fields of use. This is why it is worthwhile for users to check developments in other lines of industry – for example in the field of household appliances, or washing machines to be precise in this case.
Challenge: a high, one-sided load
A trends in this field is an increase in wash capacity and hence a larger diameter of drum. Up to 9 kg of textiles (when dry) can be washed, resulting in higher forces being applied to the shaft support bearings.
This is a great challenge for bearing developers, especially for the front-loading washing machines that are popular in Europe. In this design the drum weight is only borne on one side, which means that the bearings are subjected to high loading. This is exacerbated by unbalance of the rotating mass placing great stress on the bearing.
Designers cannot simply solve the problem of a higher stress with larger bearings due to cost and space constraints. Furthermore, larger bearings would generate higher friction and thus increase power consumption. But this is out of the question because household appliances are rated in terms of efficiency classes and low energy consumption is one of the most important criteria for high-quality machines.
Basic work: developing an optimised material
There are various approaches to optimising this kind of bearing in theory. Designers can develop a new material, optimise the bearing internal geometry or alter the performance through the choice of lubricant.
Having already developed the internal design and grease, NSK engineers next decided to work on material improvement. The primary goal was to improve the alloy of an existing NSK special steel so as to effectively prevent the appearance, and above all the propagation, of cracks and dents in the material. Particular attention was paid to rolling contact fatigue. This refers to defects deep in the microstructure that may lead to flaking on the surface.
The development work has resulted in BNEQARTET bearings. A comparison of the service lives of standard bearings and the new long-life bearings shows just how effective the NSK engineers’ development work was: the service life (L10) could be more than doubled.
Long service life even under adverse operating conditions
Even under adverse operating conditions, BNEQARTET bearings have a much longer service life. A significant improvement was achieved in tests in dirty environments where tests were aimed at generating flaking on the surface of the bearing raceways.
The new bearings were found to be more resistant to contamination. This is important since the entry of contaminants can never be completely avoided, even if users place great store in cleanliness during production.
Similar considerations apply when operating bearings at high temperatures. In tests at 150°C, approximately 1.5 times longer service life could be proven for BNEQARTET bearings.
Development goal: resilience to hydrogen
When it comes to developing long-life bearings, the resilience or insensitivity of the steel to hydrogen is another important goal, because even at room temperature, hydrogen molecules can penetrate deep into the material and cause its embrittlement. This in turn leads to bearing damage such as white structure flaking, currently a hot topic in the wind power industry, and to premature failures. This problem often, but not exclusively, occurs if the bearing is used in a damp environment.
The insensitivity of a bearing to hydrogen can be established via a hydrogen thrust test, where a material sample is charged with hydrogen and subjected to an axial thrust load. This test shows that BNEQARTET bearings have almost double the service life of standard bearings under the same conditions.
Optimised alloy, high-purity material
The improvements presented here were mainly achieved through an optimised alloy that enables, amongst other things, crack propagation inhibition. This means that the chemical composition of the steel was selected so as to effectively prevent the appearance and above all the propagation of cracks.
In addition, a particularly pure steel is used to minimise the occurrence of irregularities in the depth of the microstructure.
Compact bearings, low friction
The design improvements with a view to prolonging the service life also have positive effects on friction and thus on energy efficiency. The user does not have to install a larger bearing (and hence accept a lower energy efficiency) to double the bearing life. Or he can use a smaller, correspondingly more efficient bearing if the application does not require a doubling of the bearing life.
Wide range of use
With this property profile, the BNEQARTET bearings, which are available as deep-groove ball bearings, are ideally suited washing machines drum support. In addition, wherever the designer of a machine or plant wishes to avoid rolling contact fatigue and/or surface damage through contamination and thereby prolong bearing life, the BNEQARTET technology leads to significant improvements.
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