Bearing manufacturing technology – processing of steel balls

 

 

Abstract:

At present, the domestic steel ball technology level, equipment level, quality control level and tooling level have reached or approached the advanced level, management has become more refined and perfect, large-scale production and operation have emerged, and the total production volume has continued to expand. Steel ball quality level: batch production tolerance grade G5, complete specifications and materials, single particle vibration reaches 26 dB, life K value (the ratio of basic rated life test value L10t to basic rated life L10) reaches 12 or more.

 

Key words:Steel balls processing, ceramic balls, Si3N4 balls

 

 

As a key component of rolling bearings, steel balls play the role of carrying and transmitting loads and motion in bearings, and have a great impact on the life and vibration noise of bearings.

Any point on the surface of the steel ball is a working surface that bears loads and is subject to periodic variable loads. The contact area is small, the contact stress is large, and the stress conditions of the steel ball are relatively poor, which is very easy to produce fatigue damage. The entire surface of the steel ball is both a processing surface and a supporting surface. Therefore, the steel ball processing technology is also very unique and specialized in the machinery industry, and has very high requirements for raw materials, process technology, processing equipment, tooling and grinding tools, and grinding fluids.

At present, the domestic steel ball technology level, equipment level, quality control level and tooling level have reached or approached the advanced level, management has become more refined and perfect, large-scale production and operation have emerged, and the total production volume has continued to expand. Steel ball quality level: batch production tolerance grade G5, complete specifications and materials, single particle vibration reaches 26 dB, life K value (the ratio of basic rated life test value L10t to basic rated life L10) reaches 12 or more.

1) Processing technology

There are two main processing methods: water-based large-circulation processing technology and resin grinding wheel grinding processing technology.

2) Raw materials

GCr15 wire is widely used due to its moderate price and easy heat treatment, accounting for more than 85%. Other applications, such as applications where corrosion resistance is more important than durability, use stainless steel balls; special applications such as aircraft require the use of high-speed tool steel (tungsten steel) and heat-resistant steel (M50) with better heat resistance than GCr15. Steel balls for special purposes usually have higher material costs and processing costs due to the implementation of complex heat treatment processes (such as high-temperature quenching, multiple tempering, cold treatment, etc.).

3) Process equipment

A complete set of main steel ball processing equipment has been formed in China, including Z32, Z32G series steel ball cold heading machines and high-speed cold heading machines, 3M49, 3M79 series polishing machines, 3M46, 3M76 series grinding machines, 3M47, 3M77 series grinding machines, protective atmosphere heat treatment furnaces, steel ball surface strengthening machines, cleaning machines based on brush cleaning and high-strength sponge rotary extrusion cleaning, etc. The main inspection equipment includes steel ball surface quality appearance inspection instruments based on visual systems, and eddy current photoelectric appearance inspection instruments based on eddy current flaw detection principles with optical and vibration channels. The main polishing, grinding and lapping machine tools have grinding plate diameters of φ300~950(1000)mm, ball loading capacity of 20~1000 kg, and machine tool processing accuracy can meet the requirements of G5 steel balls.

4) Quality control methods

A complete set of methods has been formed for domestic steel ball quality control. The quality indicators and methods are mainly: raw material quality, size and dimensional accuracy, heat treatment hardness and metallographic structure, and surface defects. With the improvement of steel ball processing technology and processing equipment, various steel ball manufacturing companies can basically meet the standard requirements in terms of size and dimensional accuracy, heat treatment hardness, etc.

5) Contact fatigue failure

The mechanism of rolling fatigue can be roughly divided into two categories: internal origin spalling and surface origin spalling. Internal origin spalling is mainly caused by non-metallic inclusions under the rolling contact surface, which needs to be addressed by improving the purity of raw materials. Surface-origin spalling is directly related to the operating environment, lubrication status, and surface status of the rolling contact surface (surface roughness and scratches on the rolling surface) of the rolling bearing. It has a shorter lifespan than internal-origin spalling and accounts for a greater proportion of bearing damage. The countermeasures for surface-originated spalling include carbonitriding treatment on the surface of GCr15 steel balls. Since the cost of carbonitriding treatment is relatively high, a more important technology is to use ordinary overall quenching to increase the surface hardness of the steel balls, making it difficult to form indentations and improving durability in the presence of foreign matter.

6) Summary

Steel balls still need to further improve their quality as the primary goal, adopt a reasonable material purity evaluation method, and thoroughly manage quality from the blank. At the same time, in order to prevent unqualified products from leaving the factory, it is essential to develop a more functional automatic appearance inspection instrument. In addition, in order to improve durability, while requiring steel mills to further improve material purity, develop steel ball heat treatment technology suitable for application occasions, etc., to give traditional GCr15 steel balls various added values.

In recent years, with the increasing diversification of bearing application requirements, new material balls such as ceramic balls (such as Si3N4 balls) are used to replace steel balls under high-speed and high-temperature application conditions. The current production trend is increasing rapidly and the application demand is strong, but the issue of high prices needs to be resolved.