Why the edge of bearing is with black color?

Why the edge of bearing is with black color?

Apr 02, 2020

Why the edge of bearing is with black color?


How the black color formed?

The chamfered black spots on bearings are an inevitable result of one of the bearing processing process“Salt Bath Heat Treat”.

As long as the bearing have undergone the regular heat treatment process, the chamfers will have irregular black spots.

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Why only the edge of bearing keep black color?

1)     Not necessary

2)     Cost-Saving


1) Not necessary

The bearing chamfering have higher roughness than the outer ring. The black edge is the chamfered part which is not removed during the grinding process after the ferrule is heat-treated.

The dimensional accuracy and roughness requirements of the inner and outer chamfers of the bearing are NOT as high as those of the inner and outer rings and raceways. Therefore, it is NOT necessary to carry out fine grinding or polishing during mechanical processing.

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2)     Cost-saving

Each level of roughness increase will double the manufacturing cost.

There is a need to save processing costs when it is NOT necessary to do further fine grinding process of bearing edge.

Pure white is grinding all ferrules, so the price is expensive.


Attention: This is also one possibility of pure white surface that the bearing is without heat treatment. The bearing quality and hardness will be very poor. You need to recognise it carefully.

Features of bearings with black color edge

1)     Beautiful looking

2)     Anti-rust

3)     The black color cannot be wiped off. Certify the bearing is been go through heat treatment

P.S: If the black color can be scraped off, that certifies that black color is man-made and the bearing is not do heat treatment, the quality and hardness of bearing cannot be guaranteed.


For more bearing knowledge or bearing inquiry, freely contact the bearing specialist.