chromex 40 steel for SNFA spindle bearings

A New Material for Spindle Bearings 

Over the years SNFA has developed and used many new bearing materials to meet the demands of the aerospace market. Until now there has been no need to transfer these materials to the machine tool sector but with modern spindle and electro-spindle designs that need is now there. To satisfy this need SNFA have decided to introduce bearings made using CHROMEX40. CHROMEX40 is a corrosion-resistant steel alloyed from nitrogen, molybdenum, chromium, vanadium, carbon and iron in such a way as to produce the fine grain structure which is so necessary to deliver the properties demanded.

This fine, clean structure gives the material , the ability to operate safely at the higher speed and loads required by modern spindles whilst giving an enhanced fatigue life. It even performs well under poor lubrication conditions and at  higher temperatures. These bearings will be supplied as standard with ceramic balls. Ceramic  material balls are essential components of high speed bearings and are an ideal solution where low density, high rigidity and life are major concerns. 

Snfa chromex 40 spindle bearings

The ceramic material normally used for balls is Silicon Nitride (Si3 N4) whose production process gives them high material  integrity and hardness. Other  important characteristics of silicon nitride are given in the Comparison Table below.

The advantage of hybrid bearings with steel rings and ceramic balls can be summarised as follow:


Material characteristics data Silicon nitride

Si3 N4

Bearing steel


Density  (g/cm3) 3,19 7,8
Linear thermal expansion

(10 -6/C) 20-1000C

3,2  -
Linear thermal expansion

(10 -6/C) 20-300C

 - 11
Modulus of elasticity (KN/mm2) 20C 315 210
Poisson's ratio 0,26 0,30
HV10 Hardness 20C 1700 700
Impact strength

 (MN/m 1,5) 20 C

6-8 25
Thermal conductivity (W/m C) 20C 30-40 40-50
Limiting Thermal 


ca.1000 ca.300
Hot hardness good bad
Dimensional stability good good
Corrosion resistence good bad
Magnetism absent present
Electrical insulation good bad

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