The heart of any ball bearing is represented by the balls themselves that, paired between inner and outer ring, allow the relative movement between the parts and determine most of bearing functional performances.
In a deep groove ball bearing as well as in an angular contact ball bearings, the ball is the component with the most stringent precision requirements. For instance, surface roughness of steel precision balls in a good bearing must reach values of Ra around 0.004 micron only, i.e. the equivalent thickness of a layer of few hundred thousands atoms put one over the other (the same roughness measured on the polished grooves of the best inner and outer rings shows values of at least 4 or 5 times higher).
The geometrical properties of balls are expressed by the international norm ISO 3290 (see below table) who defines the precision classes and, for each of them, expresses the tolerances in terms of max ball ovality, roundness, lot variation and gauge interval.
ICT offers steel balls in quality grades from G3 to G40, including the special Super-Premium grade designed for applications with high silence requirements. The chrome steel balls are made with clean 100Cr6 steel (i.e. without metallic and not metallic inclusions) produced only by well known Japanese, Korean and French steel mills. This is a guarantee of excellent resistance of balls to fatigue and wear, in such a way to avoid dangerous phenomena of balls spalling that might result into the same failure mode also on rings grooves.
Balls quality grades – ISO 3290 norm
International norm ISO 3290 fixes the tolerances of geometrical precision of balls used in bearings industry and in other applications. The quality grade is expressed by the letter G followed by one number who represents the millionths of an inch of tolerance of diameter variation of the ball, i.e. of ovality.
For instance, the G10 grade has a max tolerance of ovality equal to 10 millionths of an inch, i.e. a max ovality error of 0.25 micron. It derives that smaller is the number after letter G and more stringent are the geometrical tolerances of the balls, i.e. higher are their precision requirements. The best grade mentioned by ISO 3290 is G3, with max ovality of 0.08 micron.
Worldwide there are many companies who declare to be able to supply quality G10, indeed very probably only half of them are really capable to produce true G10 quality. While we move to quality grades more stringent like G5 and G3, the real number of suppliers capable to meet such levels of precision can be counted worldwide by not more than the fingers of one single hand.
The logo NHB Inside is the symbol of a superior quality, of an higher attention to all the aspects that can make the difference in the final bearing application. The steel balls, in fact, can make a big difference in terms of both performances and total cost, whatever the application is: ball bearings, constant-velocity joint, ball screws, steering column or any other one.
That’s why we incentivate our customers as well as the customers of our customers to use the logo NHB Inside on their finished products, as tool to communicate to the market “to be different”.