March 2020

A new modelling approach for hybrid bearings gives a more accurate picture of their performance and helps engineers justify their use in the correct application, writes Guillermo Morales-Espejel, Principal Scientist at SKF Research and Technology Development. A pplications for hybrid bearings , which use ceramic rolling elements on steel raceways, continue to grow. Properties including low weight and good performance under demanding conditions have seen them used in niche applications such as high-speed machine tool spindles. More recently, their high electrical resistance has made them popular in applications such as variable speed drives, where electrical arcing can damage conventional bearings. Ceramic rolling elements have other advantages over all-steel bearings. They generally run at lower operating temperatures, resist surface damage from particulate matter and do not suffer from the potential risk of steel- to-steel surface welding. Because they have a lower boundary-lubrication coefficient of friction, this allows them to function more efficiently in applications with poor lubrication. However, unless the advantage of using a hybrid bearing is clear in advance, it can be difficult for engineers to justify their use. Will it outperform its steel equivalent in a particular application? Are the possible performance benefits worth the extra cost? The reason is because the equations used to calculate the rating life of a bearing do not account for the real-world performance of hybrid designs. Sub-surface fatigue The conventional bearing life model is based on sub-surface fatigue. Rotating bearing components are continually loaded and unloaded. Over many cycles, fatigue accumulates, leading to eventual failure. Fatigue behaviour is well-understood, so engineers can use data from their application, such as loads and speeds, in an equation. This helps them determine the rating life of a given bearing design. The dynamic load rating of a bearing, which is readily available, is mainly used to quantify its sub-surface performance. This model is commonly used and incorporated into international standards. However, it does not show hybrid bearings in their best light. Ceramic rolling elements are account for this, modern standards such as ISO 281 include correction factors in an attempt to accommodate these effects. However, these correction factors still do not represent the real behaviour of bearings in service. Improved model PWE spoke to Guillermo Morales-Espejel, principal scientist at SKF Research and Technology Development, who said that in 2012, SKF set out to remedy this by creating a Process, Controls & Plant Focus on:Seals, Bearings & Lubrication stiffer than their steel counterparts and deform less under load. This means loads are concentrated over a smaller area of material, which increases stress and accelerates sub- surface fatigue. More importantly, real-world experience does not always agree with this traditional model. In the field, most bearings fail due to problems at the surface, not in the body of the material. The underlying cause is usually down to poor lubrication or contamination. To Accurate modelling widens the use of hybrid bearings 34 | Plant & Works Engineering www.pwemag.co.uk March 2020

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