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Increasing machine life with hybrid bearings

Increasing machine life with hybrid bearings

Hybrid bearings have been shown to perform well under poorly lubricated and contaminated conditions and are much less prone to surface distress than their all-steel counterparts. Phil Burge of SKF explains the structure and materials of hybrid bearings and how these features help extend machine service life.

In simple terms, hybrid bearings are constructed with steel rings and bearing grade silicon nitride rolling elements; in virtually every other detail, with the exception of some metallurgical details and special cage materials and designs, 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.

Like their all-steel counterparts, hybrid bearings come in a variety of formats, including single row deep groove ball, single row cylindrical roller and angular contact ball bearings in sealed and non-sealed versions. They are generally categorised as ‘standard’ and ‘extreme duty’, the difference between these being largely down to their materials of construction.

The raceways of standard hybrid bearings are constructed from regular carbon chromium bearing steels and the rolling elements from a silicon nitride ceramic. 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. The combined properties of these various materials of construction greatly improve bearing performance, enabling extreme duty hybrid bearings to run significantly longer, especially in harsh conditions.

High nitrogen stainless steels are resistant to corrosion and wear, with excellent rolling contact fatigue strength; this is more than three times the fatigue life of regular bearing steels and has 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.

The glass fibre reinforced PEEK cage material is tolerant of chemically aggressive media and retains its integrity over wide operating temperature ranges. The material is particularly stable, having low moisture absorption and resistance to ageing, and as it is readily moulded it provides opportunities for the development of novel, problem-solving cage designs.

As well as being suitable candidates for aggressive and highly contaminated applications, hybrid bearings have proven particularly successful under poorly lubricated conditions; they are also 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.

So let’s consider the applications potential of hybrid bearings. 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 its VC4444 high nitrogen stainless steel for the rings and a flexible, single-piece glass fibre reinforced PEEK cage.

A rather more exotic application is high-performance motorsport – partic-ularly Formula 1. Ball and roller bearings with silicon nitride rolling elements are used almost universally by Formula 1 engineering teams, thanks to their light weight, low friction properties and resistance to high temperatures and vibration.

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 application pose significant challenges that more conventional bearing styles would have difficulty in meeting.

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