Plant & Works Engineering
Hybrid bearings boost machine lifetime
Published:  20 March, 2018

By withstanding contaminated and poorly lubricated conditions, hybrid bearings are less prone to surface distress than their all-steel counterparts. Phil Burge, marketing and communications manager at SKF explains the structure and materials of hybrid bearings and how these features help extend machine service life.

Hybrid bearings are designed to withstand tougher, harsher conditions than standard all-steel bearings, and they do it by using some rather unconventional materials – including ceramics, high temperature plastics and special grades of steel.

Other than these elements, a hybrid bearing looks almost identical to its conventional counterpart. However, it is when the two types of bearing are put into service that the hybrid version proves itself, as it can withstand the kinds of extreme conditions – such as low lubrication or high contamination – that would destroy a conventional product.

As well as being suitable for aggressive and highly contaminated applications, hybrid bearings have been successfully deployed under poorly lubricated conditions. Firstly, they are far less prone to surface distress and surface-initiated fatigue cracks than their all-steel alternatives.

Also, 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 from variable speed drives might otherwise cause corrugation of the bearing raceways.

Like conventional bearings, 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 classed as ‘standard’ and ‘extreme duty’ – the difference between them being largely down to their materials of construction.

In high-speed applications, silicon nitride bearings have two important benefits over their steel equivalents. Firstly, they have 40% of the density; and secondly, their modulus of elasticity is 50% higher than that of steel – making ceramic balls much stiffer than their steel counterparts. This greater stiffness, and reduced mass, results in much smaller contact areas between the ceramic ball and raceway or cage pocket. There is also lower adhesion between silicon nitride and steel when compared to steel against steel, which places a much lower demand on lubrication. This means that a hybrid bearing can continue working under a lubrication regime that, for a steel bearing, would be equivalent to dry running.

Material benefits

The raceways in standard hybrid bearings are made from regular carbon chromium bearing steels, while the rolling elements are constructed from a silicon nitride ceramic. Cages are of conventional design, and use standard cage materials. However, because of the conditions they must sometimes endure, some hybrid bearings use more exotic materials.

Extreme duty SKF hybrid bearings have rings made from high nitrogen stainless steel, while cage designs make use of a glass fibre reinforced version of the high-performance polymer PEEK (PolyarylEtherEtherKetone). The properties of these construction materials boost performance – enabling extreme duty hybrid bearings to run far longer than conventionally constructed standard hybrid bearings. This is especially true under harsh conditions.

High nitrogen stainless steels have high impact toughness, resist corrosion and wear, and have excellent rolling contact fatigue strength – giving them three times the fatigue life of regular bearing steels. 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

For example, SKF's high nitrogen stainless steel VC444 works at operating temperatures up to 150 °C. For extreme operating conditions, SKF high nitrogen stainless steel grade VC4444 has slightly less corrosion resistance than VC444 but will maintain its structural integrity and performance characteristics across an extremely wide range – from cryogenic to temperatures above 150 °C.

The glass fibre reinforced PEEK cage material tolerates chemically aggressive media (a good example is when hybrid bearings are used in sour gas compressor applications) and maintains its integrity over a wide range of operating temperatures. The material is highly stable, having low moisture absorption and resistance to ageing. Because it can be readily moulded – unlike conventional steel or brass cage materials – it can be used as the basis of novel, problem-solving cage designs.

Application potential

A number of punishing applications – which range from offshore to motorsport – have taken advantage of the benefits of hybrid bearings. Atlas Copco pioneered the use of hybrid deep groove ball bearings more than 20 years ago, when it incorporated them into its high-speed hand grinder. The SKF hybrid bearings were critical to the application – as without them it would not have been possible to create the product.

The combination of high speed performance and electrical resistance makes hybrid bearings highly suitable for high speed electrical motors – including air turbine and pneumatic motors. The bearings help to boost service life by extending lubricant life, resisting wear from particulate contamination (such as sand, pigments and abrasives) and resisting the effects of electric current.

Over time, more and more applications have begun to embrace the benefits of hybrid bearings. In sour gas compressors, for instance, where the lubricant is often contaminated by the pumped medium, the life of a standard bearing can be very short. High nitrogen stainless steel hybrid bearings provide up to 10 times the service life of conventional bearings in oil-flooded screw compressors used on such applications.

Cryogenic submersible pumps that transport liquefied gases must withstand temperatures that range from –74 °C (for liquefied petroleum gas) down to –253 °C (for liquefied hydrogen gas). Here, conventional petroleum-based lubricants cannot be used, instead, the pumped liquid itself lubricates the bearings. 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 with a 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, which helps to reduce cost and complexity, by using pure refrigerant-lubricated hybrid bearings. These are made from high nitrogen stainless steel, and also use specially designed glass fibre reinforced PEEK cages.

A more exotic application is seen in high-performance motorsport – particularly Formula 1. Almost every Formula 1 engineering team uses ball and roller bearings with silicon nitride rolling elements, thanks to their light weight (the ceramic material has only 40 percent of the density of steel), low friction properties and resistance to high temperatures and vibration.

In fact, it was the needs of motorsport customers that has driven SKF’s research into the design and manufacture of ceramic bearing components – and encouraged the company to invest in larger scale production facilities to make these bearings more available and affordable to mainstream industries.

Because of their specialist nature, it is easy to think that hybrid bearings have a limited area of application – such as the prevention of electric arcing damage to raceways. However, a wide choice of construction materials gives them a broad application potential, especially when these applications pose significant challenges – in which conventional bearings are unable to cope.