Materials drive bearing advances

Materials are at the heart of any bearing's success or failure. Paul Fanning looks at some recent developments.

More and more materials are being developed that allow bearings to cope with the issues facing these products. Of these issues, probably the most significant are friction and the resultant temperatures. Traditionally, of course, this problem has been overcome with the use of lubricants.

Keeping bearings properly lubricated in high temperature environments represents a serious challenge, however. Product surface temperatures can reach temperatures as high as 900 °C, while the effects of cooling water, scale, and chemicals all contribute to an extremely tough operating environment and present a lubrication challenge. Ultimately, the consequence of bearing failures often leads to equipment downtime, often damaging both equipment components and the end product. This all leads to increased costs in repair, affecting productivity and profitability.

To deal with high temperature applications, bearings are typically lubricated with a centralised lubrication system to counteract lubricant loss and prevent bearing failure. However, these systems drive up energy costs, consume excessive amounts of costly, high temperature greases, and are prone to pipe blockages caused by burnt grease. Even when working optimally, centralised lubrication systems must apply large amounts of lubricant to compensate for heat-induced grease degradation. This can pose significant fire and safety risks, as excess grease can accumulate outside the bearing housings, creating dangerous working conditions, including slippery floors and a higher risk of fire. Water cooling systems are also sometimes used, adding to high investment and operating costs.

Oil-air lubrication is sometimes used as an alternative to grease lubrication, but is also not ideal due to the difficulty to form a good lubricating film, especially at low speeds. It requires a significant capital investment and leads to increased costs for both energy and maintenance.

These problems have led to a rise in the popularity of self-lubricating bearings. One of the latest examples of this breed is SKF's DryLube, a bearing solution that contains a specially-engineered graphite mixture that eliminates the need for grease re-lubrication; delivering reliable, improved performance and allowing significant cost savings to be made in reduced maintenance and downtime. The SKF Drylube Bearings will benefit equipment, such as rollers, which are repeatedly subjected to extremely high temperatures in applications such as bakeries, food processing, mills, foundries, or other heavy industries.

By filling the empty space inside a bearing with an engineered graphite mixture, SKF provides an efficient and cost effective alternative to the use of grease in hostile environments. Almost any SKF bearing type can be supplied as an SKF DryLube Bearing, providing very low and constant friction, and an extremely high temperature limit of 350°C. SKF DryLube Bearings do not require re-lubrication, eliminating the cost and environmental consequences of conventional greasing programmes.

An even more fundamental use of materials to eliminate lubrication and maintenance issues can be found in the use of polymer bearings such as those offered by igus. Its iglidur range of polymer bearings are claimed to lower costs and raise service life by eliminating lubrication and lowering maintenance works. Many engineers persist in the belief that plastics cannot handle the tough applications or environmental elements. Plastic bearings, however, can endure extreme temperatures, heavy loads and high speeds.

Self-lubricating polymer bearings such as iglidur contain solid lubricants embedded in millions of tiny chambers of the fibre-reinforced material. During operation, the bearing transfers lubricant onto the shaft to help lower thecoefficient of friction (COF). Unlike a sintered-bronze bushing, polymer bearings release solid lubricants as soon as the bearing or shaft is set in motion. The fibre-reinforced materials inside the bearing withstand any high forces or edge loads.

Obviously, the major benefits of such plastic bearings are that they are lubrication and maintenance-free, but they are also designed to maintain a low COF consistently over the lifetime of the bearing and offer corrosion and chemical resistance. Some of the latest materials from igus offer properties such as high wear-resistance, low deformation characteristics and a temperature resistance from -40 to +200oC. But friction and temperature, of course, are far from being the only obstacles to effective bearing performance.

Another recent material advance for bearings comes from Federal-Mogul, which has developed an innovative polymer coated bearing shell that can reduce fuel consumption and CO2 emissions by withstanding mechanical loads produced by heavily boosted engines. Called IROX, the new technology addresses the lubrication challenges associated with frequent engine re-starts found in hybrid and other future stop-start engines by protecting both the crankshaft and the bearing shells from damage where metal-to-metal contact would otherwise occur. It is estimated that the IROX bearing overlay can help increase the life of crankshafts and bearing shells by more than five times in more extreme applications, such as direct-injected engines and engines with stop-start systems.

As engines are downsized but maintain their output through turbo-charging, the specific loads on the bearings increase. When hybrids operate in electric mode or when drivetrains using stop-start strategies switch off the engine, the crankshaft speed drops to zero. Without rotation, the crankshaft settles into contact with the bearing shells and the oil pump stops providing lubrication, allowing metal-to-metal contact and causing wear when the engine restarts.

The lubrication conditions at start up are very different from those that exist during high-speed, high-load operation. While solid lubricants or dry bearing materials are effective at preventing metal-to-metal contact at low running speeds, these conventional solutions are not suited to higher speeds, which require journal bearings with a generous lubricant supply. Federal-Mogul's new system combines the best features of both these established technologies by introducing a polymer coating for traditional metallic bearing shells, integrated with solid lubricants and wear inhibitors to produce a cost-effective, robust and production-ready solution.

NSK has developed TL (Tough and Long Life) bearing material to address the trend to higher operating temperatures in the dryer sections of paper making machines. Today, with the moves toward higher performance equipment, the temperatures in the hollow cylinder of the dryer section, through which steam or oil passes, can reach 180oC; and sometimes on newer machines, even 200-250oC. These temperatures cause high thermal stresses, which can lead to fracture of the inner rings of the spherical roller bearings used in the dryer sections. During the drying operation, thermal expansion of the cylinder takes place. This increases the tensile stress on the bearing inner ring due to the temperature difference between the cylinder and the ring. Under these conditions there is a greater risk of failure due to inner ring fracture.

NSK has analysed the mechanism of inner ring fracture and established a strength evaluation method that has resulted in the development of the TL specification material. The TL specification is a special surface treatment of an original steel type. It attains both high raceway surface hardness and dimensional stability under high temperatures (up to 200oC), while at the same time having the same level of compressive residual stress at the raceway surface as conventional carburised steel.

The latter feature increases the strength against fracture performance commonly experienced with conventional spherical roller bearings. In fact TL specification bearings have higher strength against inner ring fracture than bainitic steel (austempered high-carbon chrome bearing steel) and bearing steel (hardened high-carbon chrome bearing steel) units.

The raceway surface hardness of TL material also exceeds that of bainitic bearing steel and carburised steel. This means that TL specification steel achieves a longer life, when foreign debris is present, than any of the other bearing steels.

Author
Paul Fanning

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