NSK has developed two processes, one using ceramics, the other a PPS (polyphenylene sulphide) resin, both of which are used to coat the outer diameter and end face surfaces of the bearing outer ring.
The ceramic coating is plasma sprayed sprayed onto the bearing outer ring and is mainly alumina (AL2O3) over a bond coat to improve its adhesion to the bearing steel.
Following the plasma spraying process, the bearings are coated with an acrylic resin.
The resin impregnates the pores of the ceramic coating and seals the surface to eliminate the effects of humidity.
Its qualities also prevent deterioration of electrical resistance when the bearings are exposed to strong alkaline washing liquid (pH 12.7) at high temperatures (80C).
This occurs during disassembly and washing in some rolling stock maintenance shops.
NSK's ceramic process is used on latest generation Shinkansen Bullet trains.
As a result of these successes, the ceramic-coated bearings are being supplied across the world for traction motors used in the latest generation of fast electric locomotives.
The second insulation material developed by NSK; a glass fibre reinforced PPS resin, is applied by injection moulding.
The material specification and mechanical design of the bearings ensures reliable and safe operation of railway vehicles.
All traction motor bearings (cylindrical roller and ball bearings) use high-purity steels to give improved fatigue life.
Tempering operations are undertaken to enable dimensional stability at the relatively high operating temperature experienced in some traction motor applications.
Advances in cage manufacturing technology have enabled a one piece, rolling element guided cage to be used in the roller bearings.
Finite element analysis has resulted in optimum roller pocket design giving higher strength and benefits for lubricant flow.
Unimax R No 2 lithium-complex soap grease is used extensively in traction motor bearings for Bullet 300 series trains, as well as newly manufactured trains for conventional railroad use.
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