Ball bearing

Abstract

To provide a ball bearing capable of actualizing good rotation even under an extremely low temperature where the dimensional shrinkage of bearing components is caused.SOLUTION: The ball bearing which is used in liquefied gas includes a pair of raceway rings consisting of an inner ring and an outer ring, a plurality of balls mounted between the raceway surfaces of the pair of raceway rings, and a cage for holding the plurality of balls between the pair of raceway rings. The cage is an inner ring guide where the cage is rotated by the slide contact of the inner diameter surface of the cage and the outer diameter surface of the inner ring with each other at a determined radial space. A guide space between the outer diameter surface of the inner ring and the inner diameter surface of the cage is 3-10% of a ball diameter as the diameter of the ball.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a ball bearing for use at extremely low temperatures. [Background technology]

[0002] There are rolling bearings for use in cryogenic environments, such as bearings used in submerged pumps that transport cryogenic liquefied gases (e.g., LNG, LN2, LNH3, and LH2) (Patent Documents 1 and 2). Because the rolling bearings used in such liquefied gas pumps are used while immersed in cryogenic liquefied gas, they are required to have wear resistance and low-temperature toughness at cryogenic temperatures.

[0003] In the technology of Patent Document 1, the linear expansion coefficient of the rolling element material is set to 70 to 105% of the linear expansion coefficient of the raceway material, and the cage material contains PTFE, a fibrous reinforcing material, and a solid lubricant, thereby improving wear resistance.

[0004] Patent Document 2 specifies an optimum numerical range for the gap ratio C = (rolling element diameter × guide gap) / (inner ring outer diameter × radial internal gap) for deep groove ball bearings. Here, the guide gap is the gap width between the outer diameter surface of the inner ring and the inner diameter surface of the cage. By specifying the gap ratio C, the bearing can withstand long-term use, with no deterioration in wear resistance or lubrication over time, and the rotation state can be stabilized. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2015-053348 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-150593 Summary of the Invention [Problem to be solved by the invention]

[0006] Rolling bearings used at extremely low temperatures experience dimensional shrinkage in their bearing components (race rings, rolling elements, and cages). Because the raceways, rolling elements, and cages are made of different materials, they shrink by different amounts at low temperatures, and each has a different linear expansion coefficient. Therefore, optimal design of the bearing components is necessary to ensure smooth rotation at low temperatures. Of these, the cage is the component that experiences the greatest difference in dimensional change and is therefore the most affected. Therefore, optimal design of the cage's guide clearance (radial clearance between the cage and the guiding raceway) and guide type is important to ensure proper function at extremely low temperatures.

[0007] In view of the above circumstances, an object of the present invention is to provide a ball bearing that can rotate well even at extremely low temperatures where dimensional shrinkage of bearing components occurs. [Means for solving the problem]

[0008] The ball bearing of the present invention is a ball bearing for use in liquefied gas, comprising a pair of raceways consisting of an inner ring and an outer ring, a plurality of balls interposed between the raceway surfaces of the pair of raceways, and a cage that holds the plurality of balls between the pair of raceways, wherein the cage is an inner ring guide that rotates by causing the inner diameter surface of the cage and the outer diameter surface of the inner ring to slide against each other with a predetermined radial gap between them, and the radial gap (guide gap) formed between the outer diameter surface of the inner ring and the inner diameter surface of the cage is 3% to 10% of the ball diameter, which is the diameter of the balls.

[0009] The ball bearing of the present invention defines the guide clearance of the cage (the radial clearance formed between the outer diameter surface of the inner ring and the inner diameter surface of the cage), which experiences a large difference in dimensional change at low temperatures, and the guide format of the cage. That is, the cage rotates by allowing the inner diameter surface of the cage and the outer diameter surface of the inner ring to slide against each other across a defined gap, a so-called inner ring guide, which allows the cage to rotate well even at extremely low temperatures where dimensional shrinkage occurs. In the case of an outer ring guide cage, the guide clearance expands due to dimensional shrinkage, and proper guidance is not possible. That is, with an outer ring guide cage, if the guide clearance is appropriate at low temperatures, it will be in an expanded state at room temperature, so will come into contact with the inner diameter of the outer ring and will not be able to be assembled. If the dimensions are such that assembly is possible, the cage will contract at low temperatures, causing the guide clearance to expand. This can cause the guiding to become unstable (this is particularly true for angular contact ball bearings with a counter-shaped outer ring), or rolling element guided ball bearings will be dominant.

[0010] Furthermore, if the cage is guided by the inner ring, if the guide clearance / ball diameter is too small, the outer diameter of the inner ring and the inner diameter of the cage will come into contact at low temperatures, resulting in poor rotation. Also, if the guide clearance / ball diameter is too large, the guide clearance will exceed the pocket clearance, resulting in rolling element guidance. Therefore, by setting the guide clearance to 3 to 10% of the ball diameter, an optimal guide clearance is achieved that prevents contact with the inner ring outer diameter at low temperatures. This ensures inner ring guidance while maintaining the guide clearance between the inner ring outer diameter and the inner diameter of the cage, allowing for normal operation even at extremely low temperatures.

[0011] In the above configuration, a tapered relief portion that expands in diameter from the inside to the outside in the axial direction can be provided on the axially outer side of the cage inner diameter portion. In this case, the inclination angle of the relief portion in the axial direction is preferably 10° to 45°. This makes it easier for liquefied gas to flow as a lubricant between the outer diameter of the inner ring and the inner diameter of the cage, forming an oil film and improving the lubrication state.

[0012] In the above configuration, the cage can be made of a special resin material containing polytetrafluoroethylene (PTFE) and glass fiber. Ball bearings used at extremely low temperatures are immersed in liquefied gases such as LNG, so lubricants such as oil and grease cannot be used. Liquefied gas itself is used as a lubricant, but liquefied gas has low viscosity and poor lubricant performance. Using a special resin with self-lubricating properties of PTFE and glass fiber as the cage material provides wear resistance in extremely low temperature environments where lubricants such as oil and grease cannot be used, thereby extending the bearing's life in liquefied gas. In this case, the raceway rings can be made of SUS440C and the rolling elements can be made of ceramic. [Effects of the Invention]

[0013] As described above, according to the present invention, it is possible to obtain a ball bearing that can rotate well even at extremely low temperatures where dimensional shrinkage of bearing components occurs. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view of a main portion of a ball bearing according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a cage that constitutes the ball bearing. [Figure 3] FIG. 2 is an enlarged view of a main part of FIG. 1. [Figure 4] 1 is a schematic diagram illustrating a submerged pump for liquefied natural gas, illustrating a state of use of an embodiment. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] A longitudinal cross section of a ball bearing according to one embodiment of the present invention is shown in Figure 1. This ball bearing 1 is equipped with a pair of raceways consisting of an inner ring 2 having an inner raceway surface 2b on its outer diameter surface 2a and an outer ring 3 having an outer raceway surface 3b on its inner diameter surface 3a, a plurality of balls 4 as rolling elements arranged to roll freely between the raceways 2b, 3b, and an annular cage 5 arranged between the inner ring 2 and the outer ring 3.

[0016] The ball bearing 1 of this embodiment is a so-called inner ring guide bearing in which the inner diameter surface 5a of the cage 5 and the outer diameter surface 2a of the inner ring 2 are in sliding contact with each other across a specified gap, thereby guiding the cage 5. Therefore, the outer diameter surface 2a of the inner ring 2, excluding the inner raceway surface 2b, serves as a cage guide surface 6 (hereinafter referred to as the guide surface 6) that is in sliding contact with the cage 5.

[0017] The cage 5 is inner ring guided. In the case of a so-called outer ring guided cage, dimensional shrinkage would expand the guide clearance (the gap between the outer diameter surface 2a of the inner ring 2 and the inner diameter surface 5a of the cage 5), preventing proper guidance. In other words, if the outer ring guided cage had an appropriate guide clearance at low temperatures, it would be in an expanded state at room temperature, coming into contact with the inner diameter of the outer ring and making assembly impossible. If the dimensions were set to allow assembly, it would contract at low temperatures, expanding the guide clearance. This could result in unstable guidance (particularly in the case of angular contact ball bearings with a counter-shaped outer ring), or rolling element guidance would become dominant. Therefore, by using inner ring guided as in this embodiment, good rotation is possible even at extremely low temperatures, where dimensional shrinkage occurs.

[0018] The raceways, that is, the inner ring 2 and the outer ring 3, are preferably made of stainless steel. Examples of martensitic stainless steel include SUS403, SUS420, and SUS440C, with SUS440C being particularly preferred.

[0019] The rolling elements 3 are preferably made of stainless steel or ceramics. The type of ceramic is not particularly limited, and ceramics based on silicon nitride, zirconia, silicon carbide, and alumina can be prepared. For example, rolling elements made of silicon nitride ceramics are preferred because they are particularly hard and have excellent wear resistance.

[0020] The cage 5 is made of a resin material primarily composed of polytetrafluoroethylene (PTFE), polypropylene, polystyrene, acrylonitrile styrene, acrylonitrile-butadiene-styrene plastic, polytrifluorochloroethylene, polycarbonate, polymethyl methacrylate, polyamide 6, polyamide 66, polysulfone, polyphenylene oxide, phenolic resin, epoxy resin, unsaturated polyester resin, urea resin, or melamine resin. For particularly good lubricity, a resin made of polytetrafluoroethylene (e.g., NTN Corporation's BEAREE FL3000) is recommended. This resin material transfers solid lubricant to the rolling element surface even at extremely low temperatures, providing excellent and stable solid lubrication. Furthermore, using a special resin material containing glass fiber in addition to PTFE is particularly preferable because it provides wear resistance in addition to lubricity.

[0021] Because the cage 5 experiences large dimensional changes at low temperatures, it is necessary to specify the guide clearance in the inner ring guide to ensure smooth rotation at low temperatures. As shown in Figure 1, the guide clearance Sr between the outer diameter surface 2a of the inner ring 2 and the inner diameter surface 5a of the cage 5 at room temperature is set to 3% to 10% of the ball diameter Da. That is, as shown in Figure 1, if the inner diameter of the cage is Hd and the outer diameter of the inner ring is d1, then Sr = Hd - d1 = 0.03 Da to 0.10 Da. Note that room temperature refers to an ordinary temperature without any particular cooling or heating, and specifically refers to a specified temperature in the range of 5°C to 35°C (based on JIS 8703).

[0022] If the cage 5 is guided by the inner ring, if the guide clearance / ball diameter (Sr / Da) is too small, the outer diameter of the inner ring and the inner diameter of the cage will come into contact at low temperatures, resulting in poor rotation. Furthermore, if the guide clearance / ball diameter (Sr / Da) is too large, the guide clearance Sr will exceed the pocket clearance, resulting in rolling element guidance. Therefore, by setting the guide clearance Sr to 3 to 10% of the ball diameter Da, an optimal guide clearance Sr will be achieved that prevents contact with the inner ring outer diameter at low temperatures. This ensures inner ring guidance while maintaining the guide clearance Sr between the inner ring outer diameter and the inner diameter of the cage, enabling normal operation even at extremely low temperatures.

[0023] As shown in FIG. 2, tapered relief portions 7, which expand in diameter from the inside to the outside in the axial direction, are provided on both axially outer sides of the cage inner diameter portion (i.e., on both axially outer edge portions of the portion where the inner diameter surface 5a of the cage 5 is present). The inclination angle TH of the relief portions 7, i.e., the inclination angle TH of the relief portions 7 relative to the cylindrical surface formed by the inner diameter surface 5a of the cage 5, is preferably 10° to 45°. As shown in FIG. 3, chamfered portions 8 are formed on both axially outer sides of the outer diameter surface 2a of the inner ring 2. If the axial length from the axially inner end 8a of the chamfered portion 8 to the axially inner end 7a of the relief portion 7 of the cage 5 is A, the relationship between A and the axial length HBA of the relief portion 7 (see FIG. 2) can be, for example, HBA = A > 0. The provision of such relief portions 7 facilitates the flow of liquefied gas as a lubricant between the inner ring outer diameter portion 2a and the cage inner diameter portion 5a, forming an oil film and improving lubrication.

[0024] The ball bearing of this embodiment can rotate well even at extremely low temperatures, which causes dimensional shrinkage of bearing components.

[0025] The ball bearing of the present invention is used in a submerged pump that transfers liquefied gases such as LNG, LN2, LNH3, and LH2 at extremely low temperatures. As shown in Figure 4, a submerged pump for liquefied natural gas (LNG) is designed to achieve airtightness within a pot (pressure vessel) 8 by immersing the entire pump in liquid, and the pump shaft 9 is integrally and coaxially connected to a motor shaft 10.

[0026] The pot 8 has an LNG suction port 11 that opens outward and a discharge port 12 that leads to external piping (not shown). A motor 13 installed inside the pot 8 has a motor shaft 10 that rotates using an external power source, and the upper and lower sides of the motor shaft 10 are supported by ball bearings A of the embodiment shown in Fig. 1. A multi-stage impeller 14 is attached to a pump shaft 9 that rotates integrally with the motor shaft 10.

[0027] In the illustrated pump, LNG flows into pot 8 from suction port 11 and flows downward along the inner surface of pot 8 due to impeller 14, which rotates integrally with pump shaft 9 when motor 13 is driven. LNG is then sucked in from the lowest stage of multi-stage impeller 14 and flows through piping 16 inside cylindrical inner wall 15 arranged around impeller 14 to discharge port 12. Some of the LNG flows through other piping 17 inside cylindrical inner wall 15 and inside motor 13 as a lubricating liquid, lubricating and cooling ball bearings A, before joining the downward flow along the inner surface of pot 8 and being sucked in again from the tip of multi-stage impeller 14.

[0028] Although the present invention has been described above in terms of an embodiment, it is not limited to the above embodiment and various modifications are possible, and the materials of the outer ring, inner ring, cage, and balls are not limited to those in the embodiment. The cryogenic environment ball bearing of the present invention may be used as a ball bearing for a liquefied gas pump, or as a ball bearing for supporting or driving a satellite antenna. [Example]

[0029] A ball bearing, model number 7204 in Table 1 and model number 7305 in Table 2, was assembled using inner and outer rings made of martensitic stainless steel (SUS440C), silicon nitride ceramic balls as rolling elements, and a ring-shaped cage, BEAREE FL3000 manufactured by NTN Precision Plastics Corporation, which is primarily composed of polytetrafluoroethylene (PTFE).

[0030] Table 1 shows the results of judging the operating performance of ball bearings at cryogenic temperatures (-190°C) for model 7204 ball bearings with a ball diameter of 7.9375 mm, with the guide clearance / Da (ball diameter) ratio at room temperature (approximately 20°C) changed to five levels: 0.02, 0.03, 0.07, 0.10, and 0.13. Operating performance was evaluated by measuring the guide clearance and pocket clearance at low temperatures (indicated by ◯, △, and × in the table).

[0031] As a result, when the guide clearance / Da (ball diameter) ratio was 0.07, the guide clearance was appropriately smaller than the pocket clearance, resulting in good operation (determined as ◯ in Table 1). Furthermore, when the guide clearance / Da (ball diameter) ratio was 0.10, the guide clearance was smaller than the pocket clearance, resulting in the second best operation after the guide clearance / Da ratio of 0.07 (determined as △ in Table 1). On the other hand, when the guide clearance / Da (ball diameter) ratio was 0.02, 0.03, or 0.13, the guide clearance was larger than the pocket clearance or the inner diameter of the cage came into contact with the outer diameter of the inner ring, resulting in poor operation (determined as × in Table 1). [Table 1]

[0032] Table 2 shows the results of judging the operating performance of ball bearings at cryogenic temperatures (-162°C) when the ratio of guide clearance / Da (ball diameter) for a model 7305 ball bearing with a ball diameter of 11.1125 mm at room temperature (approximately 20°C) was changed to five levels: 0.02, 0.03, 0.07, 0.10, and 0.13. Operating performance was evaluated by measuring the guide clearance and pocket clearance at low temperatures.

[0033] As a result, when the guide clearance / Da (ball diameter) ratio was 0.07, the guide clearance was appropriately smaller than the pocket clearance, resulting in good operation (determined as ◯ in Table 2). Furthermore, when the guide clearance / Da (ball diameter) ratio was 0.03, the guide clearance was smaller than the pocket clearance, resulting in the second best operation after the guide clearance / Da ratio of 0.07 (determined as △ in Table 2). On the other hand, when the guide clearance / Da (ball diameter) ratio was 0.02, 0.10, or 0.13, the guide clearance was larger than the pocket clearance or the inner diameter of the cage came into contact with the outer diameter of the inner ring, resulting in poor operation (determined as × in Table 2). [Table 2]

[0034] Table 3 shows the results of judging the operating performance of ball bearings at cryogenic temperatures (-50°C) when the ratio of guide clearance / Da (ball diameter) for model 7007 ball bearings, with a ball diameter of 7.9375 mm and at room temperature (approximately 20°C), was changed to five levels: 0.02, 0.03, 0.07, 0.10, and 0.13. Operating performance was evaluated by measuring the guide clearance and pocket clearance at low temperatures.

[0035] As a result, when the guide clearance / Da (ball diameter) ratio was 0.07, the guide clearance was appropriately smaller than the pocket clearance, resulting in good operation (determined as ◯ in Table 3). Furthermore, when the guide clearance / Da (ball diameter) ratio was 0.10, the guide clearance was smaller than the pocket clearance, resulting in the second best operation after the guide clearance / Da ratio of 0.07 (determined as △ in Table 3). On the other hand, when the guide clearance / Da (ball diameter) ratio was 0.02, 0.03, or 0.13, the guide clearance was larger than the pocket clearance or the inner diameter of the cage came into contact with the outer diameter of the inner ring, resulting in poor operation (determined as × in Table 3). [Table 3]

[0036] Table 4 shows the results of judging the operating performance of ball bearings at cryogenic temperatures (-250°C) for model 7010 ball bearings with a ball diameter of 8.7313 mm, with the guide clearance / Da (ball diameter) ratio at room temperature (approximately 20°C) changed to five levels: 0.02, 0.03, 0.07, 0.10, and 0.13. Operating performance was evaluated by measuring the guide clearance and pocket clearance at low temperatures.

[0037] As a result, when the guide clearance / Da (ball diameter) ratio was 0.07, the guide clearance was appropriately smaller than the pocket clearance, resulting in good operation (determined as "Good" in Table 4). Furthermore, when the guide clearance / Da (ball diameter) ratio was 0.10, the guide clearance was smaller than the pocket clearance, resulting in good operation similar to when the guide clearance / Da ratio was 0.07 (determined as "Good" in Table 4). On the other hand, when the guide clearance / Da (ball diameter) ratio was 0.02, 0.03, or 0.13, the guide clearance was larger than the pocket clearance or the inner diameter of the cage came into contact with the outer diameter of the inner ring, resulting in poor operation (determined as "Poor" in Table 4). [Table 4]

[0038] From the results in Tables 1 to 4, when the guide gap / Da was 0.07, good results were obtained in all environments. Also, when the guide gap / Da was 0.10, generally good results were obtained, but there were also cases where it was not good (Table 2), so it can be said that it operates well depending on the conditions. Furthermore, when the guide gap / Da was increased further to 0.13, it did not operate well in all cases, so it was found that 0.10 is an appropriate upper limit for the guide gap / Da.

[0039] On the other hand, when the guide clearance / Da is 0.03, there are cases where good results are obtained, but not always (Table 2), and it can be said that it operates well depending on the conditions.Furthermore, when the guide clearance / Da is further reduced to 0.02, it did not operate well in all cases, so it was found that 0.03 is an appropriate lower limit for guide clearance / Da.

[0040] 1. Bearings 2. Inner circle 2a Outer diameter surface 3 outer ring 4 balls 5 Cage 5a Inner surface 7 Relief Sr Guide Gap

Claims

1. A ball bearing for use in liquefied gas, comprising a pair of raceways consisting of an inner ring and an outer ring, a plurality of balls interposed between raceway surfaces of the pair of raceways, and a cage that holds the plurality of balls between the pair of raceways, The retainer is an inner ring guide that is guided by sliding contact between the inner diameter surface of the retainer and the outer diameter surface of the inner ring, and the guide clearance between the outer diameter surface of the inner ring and the inner diameter surface of the retainer is 3% to 10% of the ball diameter, which is the diameter of the balls.

2. 2. The ball bearing according to claim 1, wherein a tapered relief portion is provided on the axially outer side of the inner diameter portion of the cage, the relief portion expanding in diameter from the inner side toward the outer side in the axial direction.

3. 3. The ball bearing according to claim 2, wherein the inclination angle of the recess in the axial direction is 10° to 45°.

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