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Have A Tough Application For Bearings? Consider Going ‘Hybrid’

onv 17 28Have A Tough Application For Bearings? Consider Going ‘Hybrid’

Compared to their all-steel counterparts, hybrid bearings are all-round better performers. SKF’s Phil Burge, Marketing and Communications Manager, explains how a hybrid bearing differs from a conventional bearing and how these differences prove beneficial when the going gets tough. (Read More)

Hybrid bearings are constructed with steel rings and bearing grade silicon nitride rolling elements; in virtually every other detail, with the exception of some special materials of construction, they are no different from conventional all-steel rolling element bearings. The difference between the two types, however, becomes apparent when they are placed in service - particularly when the application throws up a host of challenges that would certainly compromise the performance and life expectancy of an all-steel bearing.

Like their all-steel counterparts, hybrid bearings come in a variety of formats, including single row deep groove ball, angular contact ball and single row cylindrical roller bearings in sealed and non-sealed versions. Standard hybrid bearings are constructed from regular carbon chromium bearing steels and the ceramic rolling elements are separated by conventionally designed cages constructed from standard cage materials. However, because of the severity of service that they are occasionally expected to endure some hybrid bearings make use of more exotic materials.

Extreme duty SKF hybrid bearings, for example, have rings made from high nitrogen stainless steel and various cage designs made from a glass fibre reinforced version of the high-performance polymer PEEK (PolyarylEtherEtherKetone). The combined properties of these various materials of construction greatly improve bearing performance, enabling these extreme duty hybrid bearings to run significantly longer than conventional hybrid bearings, especially in harsh conditions.

High nitrogen stainless steels are highly corrosion and wear resistant, have very high rolling contact fatigue strength and high impact toughness. Importantly, where high temperature, cryogenic or wide thermal cycling applications are concerned, high nitrogen stainless steels offer superior thermal dimensional stability and low coefficient of thermal expansion. Compared with regular bearing steels, high nitrogen stainless steels have been shown to have more than three times the fatigue life of the former.

The glass fibre reinforced PEEK cage material is tolerant of chemically aggressive media (for example, when hybrid bearings are used in sour gas compressor applications) and retains its integrity over wide operating temperature ranges. The material is particularly stable, having low moisture absorption and resistance to ageing, and being readily moulded, it provides opportunities for the development of novel cage designs.

As well as being suitable candidates for aggressive and highly contaminated applications, hybrid bearings have proven particularly successful under poorly lubricated conditions as well as being far less prone to surface distress and surface-initiated fatigue cracks than their all-steel alternatives. Moreover, since the rolling elements are made from an insulating ceramic, no electrical path can be formed between shaft and bearing housing, making this type of bearing suitable for use in electrical machines where arc currents arising from the use of variable speed drives might otherwise cause corrugation of the bearing raceways.

In sour gas compressors where lubricants are often contaminated by the pumped media, the life of a standard bearing under such conditions can be very short indeed. High nitrogen stainless steel hybrid bearings can provide between six and ten times the service life of conventional bearings in oil-flooded screw compressors used on such applications.

Cryogenic submersible pumps that transport liquefied gases need to withstand temperatures that range from –74 °C for liquefied petroleum gas down to –253 °C for liquefied hydrogen gas. In these cases conventional petroleum-based lubricants cannot be used and instead the bearings are lubricated by the pumped media. SKF has developed a hybrid bearing for cryogenic duties that uses a specially heat-treated variant of high nitrogen stainless steel for the rings and a specially designed flexible, single-piece glass fibre reinforced PEEK cage.

Modern chillers are equipped with centrifugal compressors that rely on hydrodynamic bearings lubricated with a mix of oil and refrigerant. To avoid diluting the refrigerant while still maintaining an oil-rich mixture to lubricate the bearings, these centrifugal compressors use oil injection and separation systems before and after the bearings. These systems can be eliminated altogether, thereby saving cost and complexity, by using pure refrigerant lubricated hybrid bearings made from high nitrogen stainless steel and using specially designed glass fibre reinforced PEEK cages.

It is easy to think that hybrid bearings have a relatively limited area of application – the prevention of electric arcing damage to raceways being most prominent. However, given the right choice of materials and designs these bearings have a much broader applications potential, particularly when those applications pose significant challenges that more conventional bearing styles would have difficulty to meet.

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