GEARS & GEARBOXES FEATURE


Erik Claesson, Ovako director, head of Industry


Solutions Development, looks at why fatigue is the enemy of gear


design, and how clean steel can offer superior fatigue properties that create new design opportunities


Bearing Quality (BQ) – Steels are a range of clean steels with reduced defect size. The effect is to help improve design life and/or increase torque on existing generations of gears and gearboxes


Gearing up to new design opportunities with clean steel T


here is growing demand for gears and gearboxes to be lighter, stronger


and capable of handling more power while subjected to ever greater and more complex loads. But while it is tempting to replace traditional steels with new, exotic and expensive materials, it is fatigue that accounts for a vast number of mechanical service failures – so this is a key factor that should be addressed. In many cases, the best alternative might be a clean steel of the same grade. That is, steel with carefully controlled inclusion sizes and, in some cases, isotropic properties. To understand the importance of clean


steel, it is best to start by looking at fatigue and how it can be mitigated by materials selection.


Figure 1: The relationship between inclusion size and fatigue strength


WHAT IS FATIGUE? Fatigue failure is when a metal component fails when subjected to a number of loading cycles, even though the peak load is much lower than its ultimate tensile stress (UTS) on a single loading cycle. The fatigue performance of a material is


evaluated by carrying out a large number of laboratory tests – including rotating bending fatigue (RBF) testing which provides data for high cycle fatigue. For gears, more specific tests, such as contact fatigue testing and tooth bending fatigue, are also performed. This is because different types of gears have different types of main failure modes. The vital requirement is for the


fatigue test data to be obtained under a test regime corresponding to the specific application. In the RBF test, a constant bending load is applied to a cylindrical sample of the material as it is rotated at high speed. The statistical nature of fatigue requires a number of tests to be carried out at different loads. The test program makes it possible to establish a ‘safe load’ or ‘fatigue load limit’, which is when the sample is able to survive the test beyond a certain number of cycles (typically 3–10 million). If the desire is to achieve ‘infinite life’ then the presence of inclusions that initiate failure becomes even more pronounced as a limiting factor.


STEEL CLEANNESS BENEFITS Ovako has built an extensive database of fatigue data. The clear conclusion from this experience is that defects in steel, such as non- metallic inclusions, can initiate fatigue failures (see Figure 1). These inclusions, which are typically larger sulfides for conventional steels and smaller oxides in more clean steels, act as stress raisers. In effect, in the area of the inclusion, they multiply the nominal load applied to above the safe fatigue limit. This causes the formation of incipient cracks





that propagate as the component continues to be subjected to load cycles. Eventually, the crack reaches a size that causes the component to fail. Steel cleanness can have a very


significant impact on the fatigue life of a component, especially for the high-hardness, high-strength, steels used for gears. A clean steel, containing smaller sized defects when compared with a conventional steel, offers a longer fatigue life. It is here that statistics become


important. This is because the key difference between conventional steel and clean steel is the probability of finding detrimental defects in the area where the component is subjected to a high cycling load. Using a clean steel decreases the probability of encountering these larger defects or inclusions. It is vital to understand that it is not


only the nominal stress level that is of importance when comparing the performance of different steel types. When using clean steel there is a much closer distribution of test results, reducing the need to use a high safety factor to take account of the potential scatter in performance with conventional steels.


PRODUCING CLEAN STEEL To exploit the design opportunities offered by clean steel, it is essential for steel manufacturers to focus on refining their production capabilities. It is the combination of all four main production stages (primary metallurgy; secondary metallurgy; casting; rolling) that determine the actual outcome. At each stage there are a number


of different parameters that are of importance. For the primary metallurgy


>> 16 DESIGN SOLUTIONS | NOVEMBER 2019 15


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