Insulation cuts need for bearing maintenance

A range of insulated bearings combat electrical erosion in trains

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The problem occurs when stray electric current flows through motor bearings to earth. To solve the problem, NSK has developed two processes for insulating such bearings. One uses ceramics, the other, PPS (Polyphenylene sulphide). Both are used to coat the outer diameter and end faces of the bearing outer ring.

The ceramic coating for bearings for high speed trains is plasma sprayed onto the bearing outer ring. The coating is mainly alumina over a bond coat to improve its adhesion to the bearing steel.

Following the plasma spraying process, the bearings are coated with a special acrylic resin. The resin impregnates the pores of the ceramic coating and seals the surface to eliminate the effects of humidity. It also prevents significant deterioration of the electrical resistance when the bearings are exposed to strong pH 12.7 alkaline washing liquid at temperatures up to 80 degrees C. This occurs during disassembly and washing in some rolling stock maintenance shops.

The effectiveness of the process is proven daily on the Bullet 300 Series and later generation Shinkansen Bullet trains.

The glass fibre reinforced PPS process for new generation conventional trains is applied by injection moulding. While the physical properties of PPS are slightly inferior to alumina, the resin material is less expensive.

Train operators are looking for maintenance free running intervals of two million kilometres for future generations of conventional trains. This compares with intervals of 900,000 km for current generation trains equipped with AC traction motors. TS

Pointers
* New bearings for electric train traction motors are insulated to prevent erosion by stray current
* Bearings for high speed trains are coated with ceramic, mostly alumina over a bond coat, followed by acrylic resin
* Bearings for new generation conventional trains are coated with glass fibre reinforced PPS applied by injection moulding

Author
Tom Shelley

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