Abstract: In view of the differences in radial clearance design and realization methods of four-row cylindrical roller bearings for rolling mills, which are widely used in metallurgical industry, when different bearing manufacturers leave the factory, this paper analyzes the reasons and puts forward solutions.

Key words: roll bearing; Four row cylindrical roller bearings; Interchangeability; analyze

1. Basic status and existing problems

The four-row cylindrical roller bearing used in rolling mill is a kind of bearing widely used in the rolling mill system in metallurgical industry. The code preparation method and the design of structural parameters of this type of bearing are not uniform in various countries. Since the 1970s ‚ began to replace bakelite shingles with this type of bearing [1]. In the method of compiling the bearing code, 672000 is used to represent four row cylindrical roller bearings for rolling mills. In the 1980s, FC00 000000 / PXY or FCD00000000 / PXY was used to indicate four rows of cylindrical roller bearings for rolling mills, and the type code used the prefix FC to indicate four rows of cylindrical roller bearings, and the first and second digits behind FC indicated the bearing's inner diameter size (nominal inner diameter mm / 5). The 3rd, 4th and 5th digits indicate the outer diameter size of the bearing (nominal outer diameter mm / 5), the 6th, 7th and 8th digits indicate the width size of the bearing, the PX behind the slash indicates the tolerance level of the bearing, and the Y indicates the radial clearance group of the bearing. The code preparation method of four row cylindrical roller bearings of foreign manufacturers is different from that of our country, and it is not used between countries, see Table 1.

For bearings of the same dimensions, there are differences in the selection of bearing materials and processing technology in various countries. There are also differences in the design of bearing structural parameters, mainly reflected in the inner diameter of the bearing roller group [2] (that is, the internal compound circle). Rolling mill uses four row cylindrical roller bearings as the main bearing unit of radial load of rolling mill, which has many advantages over the original bakelite shingle support, one of which is that the design takes into account the need of quick disassembly of rolling rolls and makes its inner rings interchangeable. In theory, the inner rings of four rows of cylindrical roller bearings of the same model with the same radial clearance group requirements, different manufacturers and different batches should be interchangeable, and the radial clearance group of the bearings after exchange should be the same. However, this is not the case in the current situation in our country. At the same user site, the same type of bearings of different bearing merchants occur incongruous phenomena (large or small radial clearance) when they are exchanged, resulting in premature bearing failure.

2. Conditions of bearing inner ring interchange

(1) The inner diameter Fw of roller group belongs to the structural design parameters of bearings. Designers from different countries and manufacturers have different ideas when designing the structural parameters of four columns of cylindrical roller bearings (involving raw materials, rated dynamic load, heat treatment process, working conditions and lubrication conditions, etc.). Even for the same type of bearings, the values of the inner diameter Fw of roller group may also be different. The FC4666206 / P64 roller group has two inner diameters: 257mm and 260mm. At present, only from the code of four columns of cylindrical roller bearings in China can not reflect the inner diameter of the roller group Fw. When ordering, if the roller group inner diameter Fw is not specifically indicated after the four columns of cylindrical roller bearing code, when several manufacturers provide the same type of bearing, if the user does not carry out technical coordination, there may be differences in the roller group inner diameter Fw of the same type of bearing provided by various manufacturers, resulting in bearings that cannot be exchanged.

(2) The radial clearance of four row cylindrical roller bearings is achieved by relying on the tolerance of the ring raceway. At present, there are two methods for designing and realizing radial clearance of four-row cylindrical roller bearings. One is the base shaft system method, in which the dimensional tolerance of the inner ring raceway is determined according to the bearing inner diameter size range. The radial clearance of different groups of the bearing is realized by the dimensional tolerance change of the outer ring raceway (that is, the required radial clearance is obtained by matching the outer ring raceway). This method is called one-way tolerance method (or single reduction). For example, the bearing of a certain size segment can be expressed as: the upper deviation of the inner ring raceway dimension tolerance is zero, and the lower deviation is -0.04mm. This method is not scientific enough, for large clearance group bearings, easy to cause the roller center circle diameter change, roller and cage hole outside side interference. The other is the base hole system method, that is, the dimensional tolerance of the outer ring and roller components is determined according to the bearing inner diameter size range, the radial clearance of different groups of the bearing is realized by the dimensional tolerance change of the inner ring raceway, that is, relying on the calculation data, grinding the inner ring raceway size to obtain the required radial clearance, this method is called bidirectional tolerance method (or double reduction). Taking FC3652168 / P64 as an example, it is expressed as follows: the upper deviation of the inner ring raceway dimension tolerance is -0.17mm, and the lower deviation is -0.205mm. This method is scientific and suitable for mass production. In the mechanical industry standard JB/T5389.1-1995 "four row cylindrical roller bearings for rolling bearing mill" is used in the base hole system method, most of the famous foreign bearing manufacturers use this method.

(3) In reality, there is another way to achieve radial clearance of different groups of bearings, that is, the mixed tolerance method. This method is caused by the designer's poor understanding of JB/T5389.1-1995 standard "Four row cylindrical roller bearings for rolling bearing mill". In the "Table A6" (inner raceway diameter deviation) given in JB/T5389.1-1995 standard, the diameter deviation of the inner raceway was originally given according to the basic group clearance of the bearing, if the designed bearing clearance is not the basic group clearance, it should be corrected and used again, otherwise, for bearings greater than the basic group clearance, In order to achieve the radial clearance required by the design of the finished bearing, the size of the outer ring raceway has to be ground again, resulting in the realization of the radial clearance of the bearing by the grinding of the inner and outer ring raceway.

Only when the bearing roller group inner diameter (inner compound circle size) Fw is the same, and the radial clearance design and realization method are the same, can the bearing inner ring interchange conditions be met.

3. Closing remarks

In view of the current chaotic situation of rolling mill bearing manufacturers in the above problems, we propose that the competent technical department of the industry can coordinate this problem, or make further explanation of the relevant standards. In addition, to strengthen the explanation and publicity of users on this issue, the most important thing is that users should make the following points clear when ordering four-row cylindrical roller bearings for rolling mills:

(1) Write the bearing model: FC3652168 / P64.

(2) Write down the inner diameter Fw of the roller group of the bearing.

(3) Write the radial clearance group requirements of the bearing.

(4) The realization method of bearing radial clearance is clearly proposed, that is, whether the outer raceway of the inner ring is single or double.

(5) Check the radial clearance and outer raceway dimensions of the four row cylindrical roller bearings of the same type rolling mill of various manufacturers that have entered the factory.

【 Reference 】

[1] Yang Xiaowei ∙ Design Points of Four-row cylindrical roller Bearings for Rolling mill [J]. Journal of Bearing, 1994 (3) : 2-4.

[2] WANG Shumei, HU Deyao ∙ Research on Bearing of Wire Rolling Mill [J]. Journal of Bearing, 1982 (5) : 12-16∙