Shell type needle roller bearing

Abstract

Provided is a shell type needle roller bearing having rocking durability. In this shell type needle roller bearing (10), a shell-shaped outer ring (1) is made from an austenitic stainless steel material, and the average crystal grain size in a region up to 0.135 mm from a surface of a raceway surface (1d) of the shell-shaped outer ring (1) is 22 μm or less .

Description

Drawn cup needle roller bearing

[0001] The present invention relates to a drawn cup needle roller bearing, primarily for automobiles.

[0002] Automobiles are equipped with an electronically controlled throttle body (ETB) that adjusts the amount of air intake into the engine. The ETB is equipped with a throttle valve that opens and closes the flow path, and the bearing that supports the rotation of this throttle valve uses an outer ring made of iron, such as case-hardened steel.

[0003] A drawn cup needle roller bearing used for such applications is described, for example, in Patent Document 1. This drawn cup needle roller bearing is as follows: First, a strip steel made of chromium-molybdenum steel (e.g., SCM415) is prepared as the material for the outer ring. This strip steel is deep-drawn to the shape of the outer ring. During this process, only one edge of the outer ring is bent. A cage and needle rollers are assembled into this outer ring. After assembly, the other edge of the outer ring is bent to prevent the cage and needle rollers from coming apart. After assembly as described above, a heat treatment process is performed in which carbonitriding is performed, followed by quenching and tempering. The heat treatment hardens the surface of the chromium-molybdenum steel, resulting in case-hardened steel. Bending the edge of the outer ring after heat treatment can cause changes in hardness at the ends, resulting in inconsistent hardness. However, by performing heat treatment after assembly, the overall hardness of the outer ring, including both end portions, can be made uniform. Furthermore, a nitrogen-enriched layer is formed on the surface of the outer ring due to the carbonitriding process. The nitrogen-enriched layer contains a large amount of residual austenite, which undergoes plastic deformation to alleviate stress concentration and thereby extend the life of the steel.

[0004] Patent No. 3212880

[0005] In recent years, gasoline engines have begun to adopt exhaust gas recirculation (EGR) systems in order to improve fuel economy. As a result, throttle valve bearings in ETBs are now exposed to exhaust gas components circulated by the EGR. Case-hardened steel outer rings made of heat-treated chromium-molybdenum steel, such as those used in the drawn cup needle roller bearing described in Patent Document 1, are susceptible to corrosion caused by exhaust gas components. Drawn cup needle roller bearings used in such corrosive environments require the use of outer rings made of a material with superior corrosion resistance. Examples of such corrosion-resistant materials include austenitic stainless steel.

[0006] Drawn cup needle roller bearings applied to support parts of throttle valves and EGRs are used under conditions where a low load is applied and the valves oscillate when they open and close, and durability under such conditions (hereinafter referred to as "oscillation durability") is required.

[0007] In view of the above background, an object of the present invention is to provide a drawn cup needle roller bearing having an outer ring that uses a material with excellent corrosion resistance, has excellent oscillation durability, and can be manufactured at low cost.

[0008] In order to solve the above problems, the present invention employs a first configuration of a drawn cup needle roller bearing having a plurality of needle rollers arranged at intervals in the circumferential direction, a drawn cup outer ring having a cylindrical raceway surface formed on its inner diameter side with which the needle rollers roll and make contact, and a cage that holds the plurality of needle rollers at intervals in the circumferential direction, wherein the drawn cup outer ring is made of austenitic stainless steel and the average crystal grain size in a region from the raceway surface of the drawn cup outer ring to a depth of 0.135 mm is 22 μm or less.

[0009] In other words, in this invention, when the outer ring of a drawn cup needle roller bearing is formed from strip steel, it is formed by multi-stage deep drawing without heat treatment, which makes it possible to sufficiently fine the average crystal grain size near the raceway surface. Conventional drawn cup outer rings have to be heat-hardened at high temperatures of 800°C or more, which tends to make the crystal grain size coarse due to the heat. In the drawn cup needle roller bearing of this invention, the multi-stage deep drawing makes it possible to harden the surface area without the conventional heat hardening, and also makes the crystal grain size fine, resulting in a drawn cup needle roller bearing that is resistant to spalling, has wear resistance, and exhibits oscillation durability.

[0010] Furthermore, the drawn cup needle roller bearing according to the present invention may have the first configuration described above, but may also have a second configuration in which the outer diameter of the drawn cup outer ring is 12 mm or more and 30 mm or less.

[0011] Furthermore, the present invention may be configured as a bearing device in which the drawn cup needle roller bearing having the first or second configuration is press-fitted into an aluminum housing.

[0012] Furthermore, the present invention can be an electronic throttle body for an automobile that employs a drawn cup needle roller bearing having the first or second configuration.

[0013] Furthermore, the present invention may be an exhaust gas recirculation device that employs a drawn cup needle roller bearing having the first or second configuration.

[0014] The outer ring that constitutes the drawn cup needle roller bearing of this invention is formed by multi-stage deep drawing, which reduces the crystal grain size near the surface of the outer ring raceway, making it less susceptible to wear and peeling, and even if wear and peeling do occur, they are less likely to progress.

[0015] In exhaust gas recirculation systems used in gasoline engines, throttle valve bearings mounted thereon are now exposed to exhaust gas components. In this environment, exhaust gas components pose a problem for the bearing's outer ring. The outer ring of the drawn cup needle roller bearing of the present invention is made of austenitic stainless steel, which enhances corrosion resistance against exhaust gas components. Furthermore, drawn cup needle roller bearings used in support parts for throttle valves and exhaust gas recirculation systems oscillate, albeit at a low load, when the valve opens and closes. Therefore, they require oscillation durability to withstand such environments. The drawn cup needle roller bearing of the present invention achieves the oscillation durability required for such environments by reducing the crystal grain size near the outer ring raceway surface. The crystal grain size required to ensure oscillation durability can be achieved as part of the outer ring formation process using multi-stage deep drawing, without requiring any additional surface treatment, allowing for low-cost manufacturing.

[0016] 3B is an enlarged cross-sectional view of the state where a flange portion is formed after the outer ring used in the drawn cup needle roller bearing of FIG. 1 is deep-drawn; FIG. 3C is an enlarged cross-sectional view of the state where a through hole is provided in the bottom surface; FIG. 3C is an enlarged cross-sectional view of the state where a needle roller and a cage are assembled;

[0017] The present invention is a drawn cup needle roller bearing 10 characterized by a drawn cup outer ring 1. Figure 1 shows a schematic cross-sectional view of an embodiment of a drawn cup needle roller bearing according to the present invention. The illustrated embodiment is merely an example, and the present invention is not limited to the embodiment shown in the figure. The drawn cup outer ring 1 has a substantially cylindrical outer diameter surface 1a. Both axial ends of the outer diameter surface are bent radially inward to form width surfaces 1b, 1c.

[0018] A plurality of needle rollers 2 are arranged on the inner diameter side of the drawn cup outer ring 1, and roll in contact with a cylindrical raceway surface 1d formed on the inner diameter side of the drawn cup outer ring 1. The needle rollers 2 are arranged at circumferential intervals and rotatably supported by a cage 3. The cage 3 is approximately cylindrical with a smaller diameter than the drawn cup outer ring 1, and has a plurality of axially oriented window holes 4 formed at equal intervals in the circumferential direction according to the shape of the needle rollers 2. The needle rollers 2 accommodated in these window holes 4 rotate while maintaining equal spacing from one another. In a drawn cup needle roller bearing 10, the numerous needle rollers 2 support the load between the drawn cup outer ring 1 and the axis passing through the center.

[0019] In the drawn cup needle roller bearing 10 according to the present invention, the raceway surface 1d on the inner diameter side of the drawn cup outer ring 1 has an average crystal grain size of 22 μm or less in a region up to a depth of 0.135 mm from the surface. The depth of 0.135 mm from the surface is the depth at which there is a high possibility of flaking due to contact with the raceway surface, and by having the average crystal grain size sufficiently small up to this depth, oscillation durability is ensured.

[0020] JIS G 0551 can be used as a method for calculating the average crystal grain size from the surface that is the raceway surface 1d. Specifically, the bearing is cut perpendicular to the raceway surface 1d, and the cross section that is revealed by the cut is subjected to electrolytic corrosion using a nitric acid ethanol solution or an oxalic acid solution to expose the crystal grains. The state of the cross section is observed with a metallurgical microscope or a microscope, and the grain size is measured within 1 mm. ２ The number of grains (m) per unit area is counted, and the average crystal grain size (1 / √m) can be calculated.

[0021] The grain size of the raceway surface of the drawn cup needle roller bearing according to the present invention is preferably at least 8 in terms of grain size number. Outer rings made of SCM415, which have been used in the past, are manufactured by carbo-nitriding at around 850 to 950°C, which causes the grain size to increase due to the high heating temperature, resulting in a grain size of around 6 to 7. The drawn cup outer ring 1 of the drawn cup needle roller bearing according to the present invention is formed using the following materials in a multi-stage process, thereby eliminating the need for heat treatment and achieving a grain size finer than conventional methods.

[0022] The drawn cup outer ring 1 of the drawn cup needle roller bearing 10 according to the present invention is made of austenitic stainless steel. Austenitic stainless steel is stainless steel that has an austenitic structure as its main structure at room temperature. It contains chromium, which contributes to corrosion resistance, and nickel, which contributes to the austenitic structure. For example, SUS304 is a typical austenitic stainless steel, and can also be suitably used in the present invention.

[0023] The components of the austenitic stainless steel material used for the drawn cup outer ring 1 can be selected to contain, for example, 6.0 to 17.0 mass% Ni, 16.0 to 26.0 mass% Cr, with the balance being trace elements and Fe. Here, the trace elements may be, for example, 0.08 mass% or less C, 1.0 mass% or less Si, 2.0 mass% or less Mn, 0.045 mass% or less P, and 0.030 mass% or less S. In addition to these, Mo, Cu, and N may also be added to the extent that they do not impair the effects of the drawn cup needle roller bearing.

[0024] The manufacturing procedure for the drawn cup needle roller bearing 10 will be described with reference to Figures 2A and 2B. First, a raw material of austenitic stainless steel is prepared so as to have a predetermined blending ratio (S11). The raw material may contain stainless steel containing Cr and Ni, as shown in the example above. It is preferable that the hardness of the raw material prepared at the material stage be 200 HV or less.

[0025] The prepared material is first hot-rolled (S12) and roughly rolled to form a sheet while maintaining the structure of the austenitic stainless steel material. After cooling the sheet-shaped material, it is further cold-rolled (S13) to form a strip steel with a smooth surface. It is preferable to perform a dull finish (S14) on at least one side of this strip steel to roughen the surface, as this facilitates multi-stage deep drawing. Here, the one side refers to the surface that will become the outer diameter side surface 7a by deep drawing, as described below.

[0026] This steel strip (S21) is deformed into a cylindrical shape as shown in FIG. 3A by multi-stage deep drawing (S22). To ensure bearing precision and reduce the grain size near the raceway, it is preferable to perform the process in five or more stages. Among the deep drawing processes, the process of reducing the outer ring plate thickness to ensure the final bearing precision, known as ironing, is typically performed one to three times in bearing processing. However, ironing causes strong friction between the outer diameter of the outer ring and the inner diameter of the mold, smoothing the roughness of the outer diameter surface 1a of the cup-shaped outer ring 1, resulting in a lack of oil film during processing and prone to processing defects. This phenomenon is particularly pronounced with austenitic stainless steel, which is prone to work hardening and localized increases in contact surface pressure. Therefore, it is preferable to perform the ironing process fewer times. In this way, stepwise deep drawing enables the forming of austenitic stainless steel, a difficult-to-process material, while reducing the grain size. In this case, it is preferable from the viewpoint of ensuring bearing precision to process the outer ring so that the thickness of the outer ring raceway surface of the finished product is 0.6tmm or more and 0.9tmm or less relative to the outer ring material thickness tmm.

[0027] After forming into a cylindrical shape by multi-stage deep drawing, a through hole 6 is provided in the center of the bottom surface 5 and is then opened (S23). A partial cross-sectional view of the vicinity of the drawn cup outer ring 1 in this state is shown in Figure 3B. This configuration is for manufacturing an open-end type embodiment. However, the drawn cup needle roller bearing 10 according to the present invention is not limited to the open-end type and may also be a closed-end type. One end on the perforated side leaves a width surface 1b, which is a portion bent inward from the outer diameter surface 1a.

[0028] The needle rollers 2 are held by the cage 3 so that their ends catch on the bent width faces 1b, and are assembled into the inside of the drawn cup outer ring 1 (S24). A partial cross-sectional view of this assembled state is shown in Figure 3C.

[0029] After the needle rollers 2 and cage 3 have been fitted inside the drawn cup outer ring 1, the other end of the drawn cup outer ring 1 is bent to form a flange, preventing the needle rollers 2 and cage 3 from slipping out and completing the assembly (S25). A partial cross-sectional view of the state after this end has been bent to form the other width surface 1c is shown in Figure 3D.

[0030] The drawn cup needle roller bearing 10 according to the present invention achieves the required hardness without heat treatment after the completion of the above-described assembly, since the austenitic stainless steel material used for the drawn cup outer ring 1 is hardened. However, heat treatment may be performed if necessary. Furthermore, surface finishing may be performed to remove burrs that occur during deep drawing of the outer ring or through-hole drilling.

[0031] Furthermore, it is desirable from the standpoints of strength and ease of processing that the thickness of the drawn cup outer ring 1 be 0.4 mm or more and 1.0 mm or less.

[0032] Furthermore, the drawn cup needle roller bearing 10 according to the present invention may have a seal in addition to the above-mentioned parts, and may be filled with grease.

[0033] The hardness of the raceway surface 1d of the drawn cup outer ring 1 is preferably HV300 or higher, and more preferably HV400 or higher. Too low a hardness shortens the bearing's lifespan. Conventional steel outer rings have a surface hardness of approximately HV750. However, applications that support the sliding of valves that open and close, rather than high-speed rotation, require low rotational speeds and low radial loads, so such high hardness is not required. Examples of valves supported by the drawn cup needle roller bearing 10 of the present invention include valves for automotive electronic throttle bodies (ETBs) and exhaust gas recirculation systems (EGRs). Because the drawn cup needle roller bearing of the present invention is made of austenitic stainless steel with excellent corrosion resistance, it can also be used effectively for ETB and EGR valves exposed to automotive exhaust gases. While the loads experienced by the bearings supporting these valves when they open and close are small, the swinging conditions mean that large grain sizes are prone to wear and spalling. The drawn cup needle roller bearing 10 according to the present invention has a sufficiently small crystal grain size in the vicinity of the raceway surface, and can be suitably used while preventing wear and peeling.

[0034] An example of an embodiment in which an EGR valve is supported by a drawn cup needle roller bearing 10 is shown in Figure 4. An exhaust gas passage 21 is provided in a housing 20, and a valve 13 is provided within this exhaust gas passage 21, which opens and closes in response to the rotation of the valve 13. The valve 13 is connected and fixed to a rotating shaft 11. This rotating shaft is supported by a drawn cup needle roller bearing 10 together with a ball bearing 15. The drawn cup needle roller bearing 10 of this invention is resistant to deformation when assembled into the housing 20, and exhibits corrosion resistance even when exposed to exhaust gas flowing in from the exhaust gas passage 21. Furthermore, it exhibits oscillation durability, resisting wear and peeling even when subjected to low-load valve rotation and vibration.

[0035] 5 shows an example of an embodiment in which an ETB valve is supported by a drawn cup needle roller bearing 10. A throttle valve device 31 has a throttle body 32 with an intake passage formed therein. Two drawn cup needle roller bearings 10 are press-fitted into the housing that is this throttle body 32. The drawn cup needle roller bearings 10 support a shaft 33 and the rotation of a valve 34 fixed to the shaft. The drawn cup needle roller bearings 10 exhibit corrosion resistance even when they come into contact with exhaust gas components introduced into the intake passage. Furthermore, they also exhibit oscillation durability that makes them resistant to wear and peeling even when subjected to low-load valve rotation and vibration.

[0036] This section describes an example of an actual production of a drawn cup needle roller bearing according to the present invention. SUS304 austenitic stainless steel material was hot-rolled into a sheet material. After cooling, this sheet material was further cold-rolled (S13) to form a strip steel with a smooth surface. This strip steel was used to form a drawn cup outer ring having the shape shown in Figure 3A through multiple deep drawing steps. The grain size of this drawn cup outer ring was measured in accordance with JIS G0551 "Steel - Microscopic test method for grain size." Specifically, this drawn cup outer ring was cut radially and washed with cold water. The cross section was photographed using a microscope so that the area from the surface to a depth of 0.135 mm after washing was captured within the angle of view. The photograph is shown in Figure 6. The upper side is the surface of the raceway, and the depth increases toward the bottom. Within the observation field (24,300 μm ２ The number of particles observed was counted by the intercept method and was found to be 140. From this, the average crystal grain size was calculated to be 13.1 μm, which was 22 μm or less.

[0037] REFERENCE SIGNS LIST 1 Drawn cup outer ring 1a Outer diameter surface 1b, 1c Width surface 1d Raceway surface 2 Needle roller 3 Cage 4 Window hole 5 Bottom surface 6 Through hole 7a Outer diameter side surface 7b Inner diameter side surface 10 Drawn cup needle roller bearing 11 Rotating shaft 13 Valve 15 Ball bearing 20 Housing 21 Exhaust gas passage 31 Throttle valve device 32 Throttle body 33 Shaft 34 Valve

Claims

1. A drawn cup needle roller bearing comprising: a plurality of needle rollers (2) spaced apart in the circumferential direction; a drawn cup outer ring (1) having a cylindrical raceway surface (1d) on its inner diameter side with which the needle rollers (2) roll; and a cage (3) that holds the plurality of needle rollers (2) at circumferential intervals, wherein the material of the drawn cup outer ring (1) is austenitic stainless steel, and the average crystal grain size in a region from the surface of the raceway surface (1d) of the drawn cup outer ring (1) to a depth of 0.135 mm is 22 μm or less.

2. A drawn cup needle roller bearing according to claim 1, wherein the outer diameter of the drawn cup outer ring (1) is 12 mm or more and 30 mm or less.

3. A bearing device in which the drawn cup needle roller bearing according to claim 1 or 2 is press-fitted into an aluminum housing.

4. An electronic throttle body for an automobile, comprising the drawn cup needle roller bearing according to claim 1 or 2.

5. An exhaust gas recirculation system having the drawn cup needle roller bearing according to claim 1 or 2.

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