Bearings and energy efficiency in machinery

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This article from NSK explains the steps it is taking to improve the energy-efficiency of its rolling-element bearings.

Bearings and energy efficiency in machineryBecause of their widespread use, rolling bearings are a key factor in the global industry drive to increase efficiency and reduce energy/power consumption. NSK is meeting these demands with its constant bearing improvement programme. The programme, a synergy of materials, design, lubrication and sealing technologies, has achieved a 30 per cent improvement in efficiency for the company's standard deep-groove ball bearings, and an even larger 47 per cent saving for domestic appliance bearings, both in comparison with conventional ball bearings of the same size. These major savings are helping to reduce energy use in high-consumption equipment as electric motors, air conditioners and domestic appliances.

The constant improvement programme is a graduated initiative, which started with standard deep-groove ball bearings. The first focus area in this improvement process was bearing material. Through its developments in the materials technology for ball bearings, NSK has proven that improvements in steel cleanliness (principally reducing the number of non-metallic inclusions in a steel) strongly correlates with an improved fatigue limit of bearing steel.

NSK acted upon these results, improving the steel making process and operating conditions to reduce impurities substantially, thereby achieving a decrease in oxides. The resulting long-life Z Steel is now the standard material for NSK's ball bearings. Bearings manufactured from Z steel have a significantly extended service life when compared to conventional vacuum degassed steel: up to 1.8 times longer. In addition, Z steel also benefits from more uniform response to heat treatment, a process that ensures good hardness and excellent wear resistance.

Further measures

Building upon the life and wear improvements achieved through new materials technology, NSK developed its surface optimisation process – a key factor in achieving increased efficiency. However, surface optimisation goes further by reducing the energy-consuming friction that impairs efficiency and ultimately leads to wear. A good surface geometry aids optimum oil film formation, while the machining processes ensure good compressive stress levels and high resistance to abrasion and excessive wear. The raceways of NSK's ball bearings are specially honed to reduce operating friction and to reduce noise. In addition, the better lubricant distribution that results from this operation also ensures longer operating life.

In addition to new materials technology and surface optimisation, NSK has also achieved incremental improvement in the efficiency of its ball bearings with improved low torque seals and lubricants - in particular, greases. NSK's experience with electrical motors, and the automotive industry, which has had a historic requirement for low-torque bearings to ensure efficient cold starts, has proved pivotal in the lubrication area.

Grease, of course, is retained within bearings by seals or shields; and these components also play a role in energy conservation: a seal that is too tight will lead to friction in a bearing and consequent energy loss. The challenge in the bearing market has been to develop high-speed seals that minimise bearing friction. This has led NSK to develop its V type seal. The V seal is a design innovation that seals effectively without an increase in torque or operating temperature. It has better sealing capability than a shield, and a speed capability comparable to that of a shielded bearing. The non-contact lip of the V design reduces drag in the bearing - which is an important advantage where power loss is critical, as in small electrical motors.

Stage 2

Following its improvement programme for general ball bearings, NSK implemented a two-stage second phase development focussed on improving the efficiency of grease-lubricated ball bearings used in domestic appliances.

Such appliances account for a vast amount of the world's energy consumption, and the market for them is growing fast; hence the urgency to provide lower-torque bearings that consume less power, yet sacrifice nothing in terms of operating life and noise and vibration reduction. In the Stage 1 development, NSK optimised the raceway radius of 6202 ball bearings and then tested a preloaded unit at 18,000rpm to determine the resultant reduction in friction. The test revealed a nearly five-times reduction in contact frictional power loss.

NSK then enhanced the design further, with a modified cage design, which improved lubrication flow, and an optimised grease fill. In subsequent verification and endurance tests, the bearings delivered up to 47 per cent reduction in power consumption compared to standard grease lubricated deep-groove ball bearings, while providing uncompromised life and 'silent' operation at 18,000rpm.

Following this development, Stage 2 realised further reductions in power consumption as a result equipping bearings with polyamide cages and lubricating them with special greases. Dynamic torque tests conducted on these bearings showed that the new grease reduced bearing torque by approximately 15 per cent, and the polyamide cage by over five times that amount. Further improvements were also registered in temperature running tests, the new grease reducing running temperature by 7 per cent and the polyamide cage by 40 per cent. In addition, the polyamide cage demonstrated its friction- and torque-reducing qualities by increasing motor run-down time by over 100 per cent.

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