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(I) on Dimensional Stability of Rolling Bearings

2021-04-28 14:10

Abstract The factors affecting the dimensional stability of rolling bearings, as well as the methods and standards for evaluating the dimensional stability of rolling bearings at home and abroad are discussed.

Key Words bearing; tempering; dimensional stability; evaluation method

Rolling bearing is a kind of precision mechanical basic partsIt is widely used in rotating equipment. With the advancement of industrial technology, mechanical equipment has put forward higher requirements for the dimensional accuracy, rotation accuracy, reliability and stability of bearings, especially for high-speed and high-precision applications such as precision machine tool spindles and high-speed motors. In long-term high-speed rotation, maintaining dimensional stability is the key to ensure bearing accuracy. In this paper, the factors affecting the dimensional stability and its evaluation methods are discussed.

Factors Affecting the Dimensional Stability of Rolling Bearings

1.1 Influence of bearing manufacturing process

Rolling bearings are mainly composed of inner ring, outer ring, steel ball and cage, the typical production process is as follows:

(ferrule and rolling element) steel bar→ forging → spheroidizing annealing → turning → quenching and tempering (cold treatment) → grinding → assembly

                                                                   

Cage

Currently,study heat treatment(quenching, tempering and cold treatment) and grinding have more influence on the dimensional stability of bearing steel rings, and the main research direction is to reduce the residual austenite content and grinding stress to ensure the stability of steel in the service process.

1.1.1 Retained austenite in ferrule and steel ball

Under normal quenching and tempering conditions, the microstructure of conventional bearing steel is mainly composed of tempered martensite, carbide and residual austenite. The residual austenite is a metastable phase, with repeated stress, temperature changes and time, the residual austenite will undergo phase change, re-decomposed into martensite and carbide, this phase change leads to volume expansion, bearing rings and steel ball macro-dimensional accuracy loss. Therefore, reducing the residual austenite content in the bearing steel can effectively reduce the dimensional change in the bearing service process.

After years of research and practice, cold treatment after quenching can effectively reduce the content of residual austenite, this method has been widely used in the production of precision bearings. However, simply adding the cold treatment process cannot control the residual austenite content to a very low level. In engineering, some experienced technicians have achieved this goal by increasing the cold treatment process and appropriately increasing the tempering temperature. Figure1 shows the change in residual Auschst content with the increase in tempering temperature and quenching temperature (the material used in the Auschst experiment is.Fe-1.01C-1.45Cr-0.29Mnwt%, the austenitizing time is.20minutes,160

Tempering90minutes; the tempering experimental material isFe-1C-1.51Cr-0.4Mn wt%, the tempering time is90minutes, the austenitizing temperature is.860℃).

Figure1 The relationship between residual austenite content and quenching and tempering temperature.

Some domestic bearing manufacturers mistakenly believe that high hardness makes the fatigue life of the bearing longer, but there are many factors affecting the fatigue life of the bearing, and the influence of hardness on the wear resistance of the material should not be considered only. For example, the use of a lower tempering temperature to ensure high hardness will inevitably lead to insufficient decomposition of martensite and paralytic, and martensite will continue to decompose and precipitate during the service of the bearing.E-Carbides cause volume shrinkage, while residual austenite continues to transform into martensite causing volume expansion, so that the bearing loses dimensional accuracy. Therefore, for bearings with high dimensional stability requirements, the tempering temperature should be appropriately increased. In addition, increasing the tempering temperature does not necessarily lead to a significant decrease in hardness. A foreign company producesSUJ2Steel ferrule is used230The tempering process can still guarantee the hardness in60HRC. In fact, the adoptionThe tempering temperature of 232 ℃ can effectively transform the residual austenite into tempered martensite.. To a certain extent, the hardness difference between the ring raceway and the steel ball may be more worthy of the attention of domestic manufacturers.2 is shown as △Heffect on bearing life (deep groove ball bearings,52100Steel, raceway hardness63HRC,HFor the hardness difference between the raceway and the steel ball, the radial load5871N, mineral oil lubrication).

Figure2 △HEffect on bearing life

In the process of metallographic examination of bearings, the author found that the material segregation of domestic bearing rings is more serious. The segregation of the material will inevitably lead to uneven distribution of carbon content and alloying elements, andMsPoint andMfThere are also differences in points, which are right.Control of residual austenite, control of dimensional stability adversely affected. Therefore, the smelting of bearing steel and the forging process of bearing rings should also be paid attention to by bearing manufacturers. Table1 shows the bearing steel production process of some steel mills at home and abroad.It can be seen that there are great differences in the steelmaking and casting methods of various steel mills, and there are bound to be differences in the uniformity of materials, which will not be discussed in this paper.

Enterprise

Process

SwedenSKF

100tEAF-OBT-SKF-MR-IC/CC

Yamyang, Japan

90tUHP/EAF-EBT—LF-RH-CC

Japan Kawasaki

TBM—BOF-LF-RH-CC

Aichi Japan

80tEAF-VSC-LF-RH-CC

ItalyABS

80tEAF-LF-VD-CC

United StatesTimken

120tEAF-LF-CC

Krupp, Germany

UHP EAF-LF-RH-CC

POSCO, South Korea

BOF-LF-RH-CC

Nanjing Iron and Steel

BOF/UHP(EBT)-LF-VD-CC

Xingcheng Special Steel

100tEAF-LF-VD-CC

Xingtai Steel

80tBOF-LF-RH-CC

Xining Special Steel

EAF—LF/VD-CC

EAF—LF-VD-CC-ESR

Table1 Production Technology of Bearing Steel at Home and Abroad

 

1.2 Effect of grinding stress

The main reasons for the grinding stress are the elastic-plastic deformation of the workpiece surface caused by the grinding force, the thermal stress generated by the grinding heat and the tissue stress generated by the tissue transformation. The stress generated by the quenching and tempering process of the bearing ring has been redistributed after grinding, and grinding is the last processing process in the bearing manufacturing process. Therefore, controlling the grinding stress is very important to control the dimensional stability of the bearing ring.

Studies have shown that abrasives, grinding wheel dressing times, grinding feed, etc.ZGGCr15Dimensional changes in the rings of steel spindle bearings have an impactAnd the distribution of grinding stress along the layer depth can reach30μmAbove. The grinding force causes the elastic-plastic deformation of the surface of the workpiece and the thermal stress generated by the grinding heat. These two types of grinding stress are gradually released during the service of the bearing, which will inevitably lead to the deformation of the bearing ring, so that the bearing loses the dimensional accuracy. The residual stress caused by grinding force and grinding heat can be effectively released by additional tempering.

The grinding stress caused by the microstructure transformation not only affects the dimensional stability of the bearing, but also seriously affects the fatigue life of the bearing. Due to severe grinding heat, the surface of the workpiece has undergone structural transformation (I. e. quenching burns, tempering burns). Such defects cannot be improved even through additional tempering. During the service of the bearing, cracks are preferentially formed at such defects. Therefore, for precision bearings, the grinding process should be strictly controlled.

1.2.1 Dimensional stability of the cage

Most bearing manufacturers do not have the ability to produce cages, mostly after procurement directly with the ring, steel ball assembly into finished products, a small number of manufacturers on the structural design of the cage, the force situation and its impact on the lubrication of the study, little attention to the dimensional stability of the cage.

Taking nylon cages as an example, compared with ordinary steel cages, nylon cages have excellent properties such as light weight, low vibration, low noise, wear resistance, and low heat generation. They are widely used in the bearing industry. Commonly used materials for nylon cages arePA66-GF25andPA46-GF30. Nylon contains hydrophilic amide groups, has a strong water absorption, water absorption, the size of the cage will change. Zhou Lei is rightPA6When the material was studied, it was found that:① Moisture absorption expansion and internal stress release shrinkage combined effectPA6Dimensional changes in the material, hygroscopic expansion is the dominant factor, the greater the ambient humidity, the longer the time,PA6The greater the rate of dimensional change of the material;② As the amount of nucleating agent increases,PA6Dimensional change rate of the material decreases;③ Heat treatment makesPA6The internal stress of the material is released and the shrinkage size becomes smaller..

1.3 Influence of Bearing Design and Application Conditions on Dimensional Stability

From the perspective of bearing materials and bearing applications, it is not difficult to see that temperature is an important parameter in the bearing service process. The author believes that the ambient temperature of the bearing service and the temperature rise of the bearing itself will affect the dimensional stability of the bearing.

If the bearing tempering is not sufficient or improper material selection, the bearing is equivalent to artificial aging during operation under high ambient temperature conditions, and its mechanism is described in the previous section of this article.

In addition, how to reduce the operating temperature of the bearing has always been the direction of continuous exploration in bearing design and application. The main sources of heat during the operation of the bearing are as follows:① Rolling friction; ② Sliding friction; ③ Lubricant friction; ④ Seal friction. The heat generated by the above four kinds of friction causes the surface temperature of the ferrule and the steel ball to be much higher than the temperature monitored by the monitoring equipment, and this high temperature seriously damages the structure of the material surface. Therefore, the internal structure of the bearing should be optimized from the design point of view and application, the use of suitable rolling body materials and lubricants, the selection of reasonable tolerances, or the creation of good heat dissipation conditions. The elastic-plastic deformation caused by improper bearing installation should also be taken into account.

Evaluation method of dimensional stability

In recent years, competition in the domestic bearing industry has intensified, and the profits of low-end products have declined. More and more companies are trying to develop precision bearings. After a period of exploration, the industry'sThe statement "poor dimensional stability of domestic bearings" seems to have reached a consensus. However, there are few reports on the problem of "how much difference is the dimensional stability of domestic bearings", and it is difficult to find relevant test data. The author believes that the reason for this phenomenon may be due to the lack of relevant standard documents, and many domestic bearing manufacturers have not mastered the "evaluation"/Detection of bearing dimensional stability "method, the following will list several domestic and foreign public evaluation methods.

2.1 American Military Standard

U.S. military standards, in300149℃) Insulation2500After hours, the size change of the ferrule should not exceed0.010%, the size change of the steel ball should not exceed0.020%。

In addition, some US military standards stipulate that ferrules and steel balls should be able to withstand low temperatures.--62℃, high temperature150The change in the size of the ferrule shall not exceed0.0001mm/mm, the size change of the steel ball should not exceed0.0001mm/mm.

2.2 SAEStandard

SAEThe standard stipulates that the bearing rings and rolling elements should be able to withstand low temperatures.-62℃, high temperature196Temperature changes in the environment.

2.3 Domestic Standards

GB/T36222General Specification for Deep Groove Ball Bearings for Aircraft MotorsSimilar to the American standard, it is stipulated that the ferrule and rolling element should be in-62Placed in ℃25hours, then immediately put the bearing into196℃ environment, placed100Hours. The bearing is restored20The diameter shall be measured after ℃ and compared with the measured value before the temperature cycle, and the change in diameter shall not exceed the diameter.0.01%。

JB/T7048Rolling bearingsTechnical Conditions for Engineering Plastic CagesIt is stipulated in the regulation that the finished cage after injection molding should be humidified. When the finished cage has special requirements, other stable treatment methods can also be used. The rate of dimensional change of the cage is not explicitly limited in this standard.