Power Transmission
continuously variable transmissions, automatic transmissions and hybrid automobile transmissions. While the foregoing illustrates what manufacturers
can achieve by reviewing the bearing geometry, they are simultaneously working on new materials. For example, NSK has developed ES1 (Excellent Stainless Steel) for use in high-humidity, chemical and hygienic environments, where corrosion can impair the reliable long-term operating life of rolling bearings (Fig. 3). ES1 is claimed to provide superior corrosion resistance and rolling contact fatigue life compared with conventional AISI 440C steel in wet and aggressive environments; this improvement helps to reduce maintenance, downtime and replacement bearing costs.
steel that is based on the consistent refinement of Nirosteels. Through a combination of its composition and a new thermo- chemical surface treatment process, Cronitect offers extremely high hardness. Even after 600 hours of salt spray testing in accordance with DIN 50021 SS, the material is reported as showing no signs of corrosion.
Surface treatments improve tribological performance
Bearing manufacturers have also developed coatings and surfaces finishes to improve both corrosion resistance and tribological performance. Not surprisingly, in some cases it has been necessary to create specialist coatings for particular application areas. For instance, Schaeffler’s Durotect B is a coating for FAG cylindrical roller bearings used in wind turbine gearboxes. Field tests have demonstrated that significantly lower failure rates in wind turbine gearboxes are experienced if bearings with the Durotect B coating are used. This coating is a matt black, mixed iron oxide layer that is produced in a dip solution. After application, the coating is said to be highly resistant to bending and stress. As well as offering excellent anti-corrosion protection,
Durotect B also increases resistance to slippage and sliding, as well as performing well in the alternating, low-load conditions that are common in wind turbine (and tidal stream energy) applications. In a wind turbine, the rotor shaft bearings are subjected to highly dynamic loads and harsh conditions. This means that the design and specification of the rotor shaft, as well as the bearings that support it, are critical; typically a locating/floating arrangement of spherical, cylindrical and tapered roller bearings will be specified. For the spherical roller bearings, Schaeffler often recommends its Durotect P coating. This is a 10-20 micrometre PTFE coating that reduces friction and provides improved anti-wear protection.
Nitriding treatment for steel cages
Fig. 4. Schaeffler Cronitect high-performance stainless steel provides enhanced corrosion resistance under extreme operating conditions, including dry running applications.
Until the development of ES1, AISI 440C had been the general choice for use in bearing applications where corrosion resistance was a primary concern. Because this steel contains coarse eutectic carbides that act as crack initiators under rolling contact stress, its performance had not always been satisfactory. ES1, however, has no eutectic carbides, but instead consists of fine carbides, nitrides and strong martensite that improve resistance to corrosion and fatigue. NSK reports that ES1 outperforms other stainless steels in a five per cent aqueous sodium chloride immersion test, a salt-water spray test, 5-N sulphuric acid and 5-N hydrochloride solution immersion tests, and life tests in water. Schaeffler has also introduced a high-performance stainless steel, Cronitect, that provides enhanced corrosion resistance under extreme operating conditions, including dry running applications or when bearings come into contact with aggressive media such as water, salt spray, acids and cleaning chemicals (Fig. 4). Cronitect steel is a high-grade martensitic hardening
30
www.engineerlive.com
As well as surface treatments for bearings rings, NSK has also been working on treatments for cages. The company says it has applied a special nitriding surface treatment to the high-precision pressed steel cage of its HPS series spherical roller bearings, which can significantly reduce wear in harsh operating environments. Compared with conventional nitriding processes, NSK’s
treatment is said to form a finer and harder surface, with more uniform hardness. Since the special treatment is conducted at high temperatures, it might be expected that the dimensional accuracy of the cage would be affected. However, NSK has found a way of ensuring high accuracy of the cage after treatment. One of SKF’s recent developments is its NoWear bearings
that feature a wear-resistant diamond-like carbon (DLC) coating. This is claimed to extend the life expectancy of the bearings by between five and ten times compared with conventional components. DLC-treated bearings are therefore suitable for
applications with inadequate or low lubricant viscosity and where repetitive small movements or machine vibration could cause fretting corrosion in normal bearings. Applied to the bearings’ component surfaces using a chemical vapour deposition process, it is thermally stable up to 200˚C. l
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36