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Bearing Materials Provide Corrosion Resistance

Release time : 2015-06-16 08:08:22
The Schaeffler Group has developed a range of materials and corrosion-resistant coatings for rolling bearings. While standard steel bearing materials provide satisfactory corrosion resistance in many applications, highly corrosion-resistant, nitrogen-alloyed martensitic HNS (high-nitrogen) steels - such as Schaeffler's Cronidur and Cronitect steels - are also available. Cronitect, for example, is a high-performance stainless steel that provides maximum 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. Cronitect is a high-grade martensitic hardening steel based on the consistent refinement of high-grade Nirosteels. Through its composition and thermo-chemical surface-layer treatment process that optimises the surface zone and core properties of the base material, Cronitect achieves a high hardness and corrosion resistance. Even after 600hrs of salt-spray testing, the material shows no signs of corrosion. Cronitect is suitable for a range of applications, including food processing, materials handling, heavy machinery, power-transmission systems (gearboxes), hydraulic and pneumatic systems, consumer products and packaging. In the food industry, Cronitect can be used across most of Schaeffler's product range, including rolling bearings, linear guides and plain bearings. Here, the use of Cronitect increases the availability of machinery, providing extended maintenance intervals. Other food-processing applications that use Cronitect on rolling bearings and linear guides include meat processing, filling plants, packaging machines, fish processing and poultry production. On fitness machines, Cronitect eliminates the need for grease lubrication and complex, high-friction seals. Used predominantly to support the turbine rotor, these bearings are typically designed to have at least a 20-year operational service life. Schaeffler offers a range of rolling bearings with high static and dynamic-load safety factors, including tapered roller bearings, cylindrical roller bearings, spherical roller bearings, deep-groove ball bearings and four-point contact ball bearings. In developing European tidal-stream-energy systems, FAG double-row spherical roller bearings are used to support main turbine rotors with diameters up to 750mm. Schaeffler UK is also supplying spherical roller bearings and radial ball bearings for use on drive-train gearboxes for tidal-energy systems. Developed specifically for wind-turbine gearboxes, Schaeffler's Durotect B is a black-oxide coating for the FAG range of cylindrical roller bearings. Field tests have demonstrated that significantly lower failure rates in wind-turbine gearboxes are experienced if these units are fitted with Durotect B-coated cylindrical roller bearings. Durotect B is a matt black, mixed iron-oxide layer that is produced in a dip solution. The surface structure, magnetic and electrical characteristics of the base material are maintained, and the coating is highly resistant to bending and stress. As well as excellent anti-corrosion protection, Durotect B also offers increased resistance of the bearing during slippage and sliding, as well as during alternating, low-load conditions that are common in wind-turbine and tidal-stream energy applications. The coating also provides improved run-in behaviour of the bearings. Spherical roller bearings can now be protected with Schaeffler's Durotect P coating. This is a 10-20-micrometre-thick PTFE coating that reduces friction and provides improved anti-wear protection of main rotor bearings in wind turbines. The coating provides improved sliding capacity of the bearing outer ring, with a corresponding reduction in the friction factor to less than 0.1 under axial displacement of the bearing rings. Schaeffler has developed a surface coating that insulates rolling bearings from electrical-current damage. Isotect is an insulating layer for rolling-element bearings that is applied to the outer or inner ring external surfaces and acts like a resistor and capacitor connected in parallel. To allow for the capacitive resistance of bearings used with a frequency converter, Schaeffler has selected the appropriate material for the insulating layer, as well as the most suitable thickness.