Clean steel keeps bearings rolling longer
29 November 2019
Mikael Östman, leader of the bearing segment at Ovako, looks at the new design possibilities offered by switching to clean steel.
Bearings are increasingly expected to be lighter, stronger and capable of handling ever greater and more complex loads. High mechanical strength is therefore a vital factor for the materials used to manufacture bearings. However, it is actually metal fatigue that accounts for the majority of all mechanical service failures. That is why Ovako has focused on the development of clean steels in which the small inclusions that give rise to premature fatigue failure are closely controlled.
Fatigue takes place when a metal component is subjected to a number of repeated loadings that cause it to fail. And this failure occurs even when the maximum load is well below the maximum load the component could easily sustain on a single loading cycle.
To determine the fatigue performance of a material we carry out a large number of laboratory tests on test specimens. Basic material data is obtained by rotating bending fatigue (RBF) testing which provides data for high cycle fatigue.
In the RBF test, a known, constant bending load (stress) representing a certain percentage of the material’s ultimate tensile stress (UTS) is applied to a cylindrical sample of the material. The fatigue test machine then rotates this sample at high speed, effectively subjecting it to fully reversed loading from tension to compression. The number of cycles to failure at this stress is recorded – the statistical nature of fatigue requires a number of tests to be carried out at each load.
A series of tests are carried out at varying percentages of UTS. Eventually it is possible to establish a ‘safe load’ or ‘fatigue load limit’ at which the sample will survive without failure beyond a certain number of cycles (typically 3-10 million).
Read the full article in the December issue of DPA.
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