Hybrid bearings: Coping with extremes

When a particularly challenging application – such as a submersible pump that transports cryogenic liquids – requires a robust bearing, the designer is likely to specify a hybrid variety.

Although a hybrid bearing looks conventional, the materials used in its construction help it to withstand far more punishing treatment in service, and thrive in conditions of low lubrication or high contamination, for instance.

Hybrid bearings are made using steel rings and bearing-grade silicon nitride rolling elements. In almost every other way – apart from some metallurgical details and special cage materials and designs – they are identical to conventional, all-steel rolling element bearings.

Like their all-steel equivalents, 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 usually categorised as ‘standard’ and ‘extreme duty’ – the difference being largely down to their materials of construction.

The raceways of standard hybrid bearings are made from regular carbon chromium bearing steels, the rolling elements from a silicon nitride ceramic. The cage is also of conventional design. However, in extreme environments, more exotic materials may be needed.

Extreme environments

In extreme duty SKF hybrid bearings, for example, have rings that are made from high-nitrogen stainless steel, while cages use glass fibre-reinforced PEEK polymer. These materials ensure a longer lifetime than ‘standard’ hybrid bearings – especially in harsh conditions.

As well as resisting corrosion and wear, high nitrogen stainless steels have three times the fatigue life of regular bearing steels and higher impact toughness. For high temperature, cryogenic or wide thermal cycling applications, they offer superior thermal dimensional stability and a low coefficient of thermal expansion. The PEEK cage material is resistant to aggressive chemicals and retains its integrity over a wide temperature range. It has low moisture absorption and can easily be moulded – which allows for novel, problem-solving cage designs.

Hybrid bearings work well in aggressive, highly contaminated applications, cope with poor lubrication, and are less prone to surface distress and surface-initiated fatigue cracks than their all-steel alternatives. Because the rolling elements are made from an insulating ceramic, no electrical path can be formed between shaft and bearing housing – so are resistant to the arcing currents from variable speed drives that cause corrugation of the bearing raceways.

Hybrid applications

A number of punishing applications – from oil and gas to Formula 1 – have benefited from the use of hybrid bearings.

For instance, cryogenic submersible pumps that transport liquefied gases must work at –74°C (for liquefied petroleum gas) or even –253°C (for liquefied hydrogen). Here, conventional petroleum-based lubricants cannot be used – and the pumped media itself lubricates the bearings. SKF has developed a hybrid bearing to work at these low temperatures, using a heat-treated variant of its VC4444 high-nitrogen stainless steel for the rings, with a glass fibre-reinforced PEEK cage.

In sour gas compressors, where the pumped media often contaminates the lubricant, a standard bearing will not last long – so hybrid bearings are employed, as they provide up to 10 times the service life in oil-flooded screw compressors for this type of application.

Hybrid bearings are also widely used in Formula 1 racing. Ball and roller bearings with silicon nitride rolling elements are popular, thanks to their light weight – the ceramic has only 40% of the density of steel – low friction properties and resistance to high temperatures and vibration.

In fact, the needs of motor sport customers have driven SKF’s research into the design and manufacture of ceramic bearing components. The company has invested in larger scale production facilities to make these bearings more available, and affordable, to mainstream industries – whether it is for refrigerator compressors, electrical machines or gearboxes.