Boot for constant velocity universal joint

Abstract

Provided is a boot 30 for a constant velocity universal joint in which a large diameter cylindrical part 31, a small diameter cylindrical part 32, an elastic part 33 that has one valley section 33a curved in an arc shape and that extends from the small diameter cylindrical part 32 toward the large diameter cylindrical part 31, and a connection part 34 connecting the large diameter cylindrical part 31 and the elastic part 33 are integrally molded with a flexible material, and the elastic part 33 is elastically deformed following a relative angular displacement of two joint members 21, 24, wherein the connection part 34 is formed thicker than the elastic part 33.

Description

Boots for constant velocity universal joints

[0001] The present invention relates to a boot for a constant velocity universal joint that is attached as a sealing member to a constant velocity universal joint incorporated in a power transmission mechanism of an automobile or various industrial machines.

[0002] For example, automobiles equipped with a drive source such as an engine or an electric motor on their chassis are equipped with a power transmission device for transmitting the output of the drive source to the drive wheels. This type of power transmission device includes, for example, a drive shaft equipped with a fixed-type constant velocity universal joint that allows only angular displacement of the two connected shafts, a sliding-type constant velocity universal joint that allows angular and axial displacement of the two connected shafts, and a shaft member that connects the inner joint members of the two constant velocity universal joints that are spaced apart in the vehicle width direction so as to be able to transmit torque. The shaft member is also called an "intermediate shaft" or a "power transmission shaft."

[0003] In the drive shaft described above, a cylindrical boot (constant velocity universal joint boot) that functions as a sealing member is provided between the shaft member and the outer joint member of each constant velocity universal joint. A large-diameter cylindrical portion and a small-diameter cylindrical portion are provided at one axial end and the other axial end of the boot, respectively. The large-diameter cylindrical portion is attached to the outer joint member, and the small-diameter cylindrical portion is attached to the shaft member. This prevents leakage of lubricant filled inside the joint and the intrusion of foreign matter into the joint.

[0004] Between the large-diameter cylindrical portion and the small-diameter cylindrical portion of the boot, there is provided a portion (elastic portion) that elastically deforms in response to relative displacement between the outer joint member and the inner joint member (or the shaft member connected thereto). This elastic portion can be bellows-shaped or non-bellows-shaped, and an example of the latter is known from Patent Document 1 listed below.

[0005] The boot of Patent Document 1 is entirely made of a flexible (elastic) material such as resin or rubber, and its elastic portion has a generally J-shaped cross section with a single arc-shaped valley (fold) in a free state. The elastic portion is connected to the large-diameter cylindrical portion via a connecting portion (referred to as a "boot shoulder" in Patent Document 1), which is provided with a plurality of axially extending reinforcing ribs spaced circumferentially. This increases the rigidity (strength) of the connecting portion and suppresses expansion and deformation of the elastic portion due to centrifugal force during rotation of the constant velocity universal joint, thereby stably achieving the required sealing function of the boot. Furthermore, the boot of Patent Document 1, which is entirely made of a flexible material, has the advantage of being less expensive than boots whose portions corresponding to the connecting portion and large-diameter cylindrical portion are made of metallic tubular (annular) members.

[0006] Japanese Unexamined Patent Publication No. 11-190358

[0007] Constant velocity universal joints are used in two types of applications: high-speed rotation transmission applications in which a large centrifugal force is generated, causing the elastic portion of the boot to expand and deform; and relatively low-speed rotation transmission applications in which only a small centrifugal force is generated, so that the elastic portion of the boot does not expand and deform. In the latter case, in particular, it is considered that there is little need to take the rigidity (strength) improvement measures adopted in Patent Document 1, which lead to a complex boot shape and high costs. However, during transportation or carrying of a constant velocity universal joint (including a drive shaft) or when installing it on an object, the constant velocity universal joint may bend due to the weight of the shaft member (or the resulting bending moment), causing the two joint members to angularly displace relative to one another. In this case, if the boot's rigidity is insufficient, the elastic portion of the boot may be pinched (bitten) between the outer joint member and the shaft member, resulting in damage.

[0008] In view of the above-described circumstances, the main object of the present invention is to provide a boot for a constant velocity universal joint that has a simple structure yet can effectively prevent bending of the constant velocity universal joint due to the weight of the shaft member connected to the inner joint member, thereby contributing to simplification and efficiency of the transportation and handling of constant velocity universal joints and the assembly work into vehicles.

[0009] The boot for a constant velocity universal joint according to the present invention, which was invented to achieve the above object, comprises a large diameter cylindrical portion attached to the outer joint member of the constant velocity universal joint, a small diameter cylindrical portion attached to a shaft member that rotates together with the inner joint member of the constant velocity universal joint, an elastic portion having one arc-shaped valley portion and extending from the small diameter cylindrical portion toward the large diameter cylindrical portion, and a connecting portion connecting the large diameter cylindrical portion and the elastic portion, all integrally molded from a flexible material, and elastically deforms in response to the relative angular displacement between the outer joint member and the shaft member, and the connecting portion is formed to be thicker than the elastic portion.

[0010] In the above-described boot for a constant velocity universal joint according to the present invention (hereinafter also referred to simply as "boot"), the elastic portion is maintained in a non-contact state with the outer joint member against the moment load due to the weight of the shaft member, mainly by controlling the thickness of the connection portion by forming the connection portion thicker than the elastic portion. Therefore, the boot according to the present invention has a simple configuration that does not take measures that would complicate the shape, but it prevents the constant velocity universal joint from bending excessively due to the weight of the shaft member (excessively increasing the relative angular displacement of the two joint members), and can contribute to simplifying and streamlining the transportation and handling of constant velocity universal joints (including drive shafts, etc.) and the installation work on the vehicle body.

[0011] Here, an example of a method (procedure) for determining whether "the elastic portion is maintained in a non-contact state with the outer joint member against the moment load due to the weight of the shaft member" is shown below. First, in a drive shaft having a fixed constant velocity universal joint and a sliding constant velocity universal joint connected to one axial end and the other axial end of the shaft member, respectively, the sliding constant velocity universal joint is removed to expose the other axial end of the shaft member. Next, the outer joint member of the fixed constant velocity universal joint equipped with the boot according to the present invention is fixed in a horizontal position, and then the shaft member is released, and the shaft member (and the inner joint member connected to it) is tilted by its own weight. Finally, it is confirmed whether the elastic portion of the boot is in contact with the outer joint member.

[0012] As described above, the boot according to the present invention is configured so that the connection portion is thicker than the elastic portion, but if the connection portion is made too thick, various other problems may arise, such as impairing the deformability of the boot and reducing the operability of the constant velocity universal joint, increasing the cost and weight of the boot (and the constant velocity universal joint including the boot), reducing the moldability of the boot (making it more susceptible to molding defects), etc. Therefore, the inventors have conducted extensive research and found the following specific configuration that can achieve the above object without causing the various other problems mentioned above.

[0013] First, the connecting portion is composed of an annular portion having a thickness of 2 mm or more and 4 mm or less, extending radially outward from the end of the elastic portion on the large-diameter cylindrical portion side, and a tapered portion connecting the annular portion and the large-diameter cylindrical portion and gradually increasing in diameter from the annular portion side toward the large-diameter cylindrical portion side, and further, when the thickness x of the annular portion is set to 2 mm or more and less than 3 mm, the ratio δ1 (= y / x) of the thickness y of the tapered portion to the thickness x of the annular portion satisfies the following relational expression (1), and when the thickness x of the annular portion is set to 3 mm or more and 4 mm or less, the ratio δ1 satisfies the following relational expression (2): -0.21x + 1.24 ≦ δ1 ≦ -0.67x + 3.31 (1) -0.04x + 0.71 ≦ δ1 ≦ -0.33x + 2.33 (2)

[0014] When the thickness x of the annular portion is 2 mm or more and less than 3 mm, the ratio δ1 may satisfy the following relational expression (3), which is a partial modification of the above relational expression (1), and when the thickness x of the annular portion is 3 mm or more and 4 mm or less, the ratio δ1 may satisfy the following relational expression (4), which is a partial modification of the above relational expression (2): −0.21x + 1.24≦ δ1 ≦(−0.21x + 1.24)×1.2 (3) −0.04x + 0.71≦ δ1 ≦(−0.04x + 0.71)×1.2 (4)

[0015] Furthermore, when the thickness x of the annular portion is 2 mm or more and less than 3 mm, the ratio δ1 may satisfy the following relational expression (5), which is a partial modification of the relational expression (1), and when the thickness x of the annular portion is 3 mm or more and 4 mm or less, the ratio δ1 may satisfy the following relational expression (6), which is a partial modification of the relational expression (2): −0.21x+1.24≦ δ1 ≦(−0.21x+1.24)×1.1 (5) −0.04x+0.71≦ δ1 ≦(−0.04x+0.71)×1.1 (6)

[0016] As a second configuration that can achieve the above object without causing the various other problems mentioned above, the connecting portion is configured to include an annular portion having a thickness of 2 mm or more and 4 mm or less, extending radially outward from the end of the elastic portion on the large-diameter cylindrical portion side, and a cylindrical portion that connects this annular portion to the large-diameter cylindrical portion, and further, when the thickness x of the annular portion is set to be 2 mm or more and less than 3 mm, the ratio δ2 (= z / x) of the cylindrical portion thickness z to the annular portion thickness x satisfies the following relational expression (7), and when the annular portion thickness x is set to be 3 mm or more and 4 mm or less, the ratio δ2 satisfies the following relational expression (8): 0.6 ≦ δ2 ≦ -0.67x + 3.31 (7) 0.6 ≦ δ2 ≦ -0.33x + 2.33 (8)

[0017] As a third configuration that can achieve the above object without causing the various other problems mentioned above, the connecting portion is configured to include an annular portion having a thickness of 2 mm to 4 mm and extending radially outward from the end of the elastic portion on the large-diameter cylindrical portion side, and a cylindrical portion that connects the annular portion to the large-diameter cylindrical portion, and further, the ratio δ2 (=z / x) of the cylindrical portion thickness z to the annular portion thickness x satisfies the following relational expression (9): 0.6≦δ2 ≦0.6×1.2 (9)

[0018] As a fourth configuration that can achieve the above object without causing the various other problems mentioned above, the connecting portion is configured to include an annular portion having a thickness of 2 mm to 4 mm and extending radially outward from the end of the elastic portion on the large-diameter cylindrical portion side, and a cylindrical portion that connects the annular portion to the large-diameter cylindrical portion, and further, the ratio δ2 (=z / x) of the cylindrical portion thickness z to the annular portion thickness x satisfies the following relational expression (10): 0.6≦δ2 ≦0.6×1.1 (10)

[0019] Whether or not the above relational expressions (1) to (10) are satisfied is determined under room temperature conditions.

[0020] As described above, according to the present invention, it is possible to provide a boot for a constant velocity universal joint that has a simple structure and is capable of preventing bending of the constant velocity universal joint due to the weight of the shaft member connected to the inner joint member and further preventing the elastic portion from getting caught due to this. Such a boot can contribute to simplifying and streamlining the transportation and handling of constant velocity universal joints and the assembly work into vehicles.

[0021] 1 is a longitudinal sectional view of a power transmission device including, as a component, a constant velocity universal joint to which a boot for a constant velocity universal joint according to a first embodiment of the present invention is attached. FIG. 1 is a view showing a state in which the fixed constant velocity universal joint shown in FIG. 1 has an operating angle. FIG. 2 is a longitudinal sectional view of the boot shown in FIG. 1 in a free state. FIG. 3 is a scatter diagram showing the analysis results of a numerical analysis carried out in the process of completing the boot according to the first embodiment, the scatter diagram showing the analysis results for the first joint model. FIG. 4 is a scatter diagram showing the analysis results for the second joint model. FIG. 5 is a scatter diagram showing the analysis results for the third joint model. FIG. 6 is a scatter diagram showing the analysis results for the fourth joint model. FIG. 7 is a scatter diagram showing the analysis results for the fifth joint model. FIG. 8 is a scatter diagram created based on FIGS. 4 to 8 to show the relationship between the thickness of the annular portion of the boot according to the first embodiment and the ratio δ1. FIG. 9 is a view showing the analysis results confirming the effect of the inclination angle of the tapered portion of the boot on the elongation strain of the elastic portion of the boot. FIG. 10 is a longitudinal sectional view of a boot according to a second embodiment of the present invention in a free state. FIG. 11 is a scatter diagram showing the analysis results of a numerical analysis carried out in the process of completing the boot according to the second embodiment, the scatter diagram showing the analysis results for the sixth joint model. FIG. 12 is a view showing the analysis results for the seventh joint model. 15 is a diagram illustrating an analysis result for an eighth joint model. FIG. 16 is a scatter diagram illustrating the relationship between the thickness of the annular portion of the boot according to the second embodiment and the ratio δ2, created based on FIGS.

[0022] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0023] FIG. 1 shows a longitudinal cross-sectional view of a power transmission device including a fixed constant velocity universal joint to which a boot for a constant velocity universal joint (hereinafter simply referred to as a "boot") according to a first embodiment of the present invention is attached, and FIG. 2 shows the fixed constant velocity universal joint shown in FIG. 1 at an operating angle. The power transmission device shown in FIG. 1 is a drive shaft (rear wheel drive shaft) 1 that is arranged on the chassis of an automobile along the vehicle width direction and transmits torque output from a drive source such as an engine or an electric motor to wheels (particularly rear wheels). This drive shaft 1 includes a sliding type constant velocity universal joint 10 arranged on the drive source side (inboard side), a fixed type constant velocity universal joint 20 arranged on the wheel side (outboard side), and a shaft member 2 connected to the inner joint members of both constant velocity universal joints 10, 20 so as to be able to transmit torque (to rotate integrally). As used in the following description, the terms "axial direction," "radial direction," and "circumferential direction" refer to a direction parallel to the axis O of the shaft member 2 and a radial direction and a circumferential direction of a circle centered on the central axis O, respectively.

[0024] The sliding type constant velocity universal joint 10 shown in the figure is a so-called double offset type, and includes an outer joint member 11 having a cylindrical cup portion 12 with a bottom and a plurality of linear outer track grooves 13 formed on the cylindrical inner diameter surface of the cup portion 12, an inner joint member 14 having a convex spherical outer diameter surface on which a plurality of linear inner track grooves 15 are formed and one end of a shaft member 2 is spline-fitted into a shaft hole, a plurality of balls 16 interposed between the paired track grooves 13, 15 to transmit torque between the two joint members 11, 14, and a cage 17 that holds the balls 16 at intervals in the circumferential direction. Instead of the double offset type, other types of well-known sliding type constant velocity universal joints such as a tripod type or a cross groove type may be used for this sliding type constant velocity universal joint 10.

[0025] A resin boot 3 that functions as a sealing member and is molded into a cylindrical shape from a resin material whose main ingredient is a thermoplastic elastomer is provided between the outer joint member 11 and the shaft member 2. The boot 3 integrally includes a large-diameter cylindrical portion 4 attached to the cup portion 12 of the outer joint member 11, a small-diameter cylindrical portion 5 attached to the shaft member 2, and a bellows portion 6 that is provided axially between the cylindrical portions 4, 5 and serves as an elastic portion that elastically deforms in response to relative angular displacement and / or axial displacement of the outer joint member 11 and the inner joint member 14 (the shaft member 2 connected thereto).

[0026] The large-diameter cylindrical portion 4 of the boot 3 is attached to the cup portion 12 of the outer joint member 11 by fastening its outer peripheral surface with a boot band 7A, and the small-diameter cylindrical portion 5 of the boot 3 is attached to the shaft member 2 by fastening its outer peripheral surface with a boot band 7B. By attaching the boot 3 to the outer joint member 11 and the shaft member 2 in the above manner, leakage of the lubricant filled in the internal space of the cup portion 12 (inside the joint) and intrusion of foreign matter into the inside of the joint are prevented as much as possible.

[0027] 2 is a so-called Birfield type fixed type constant velocity universal joint, and includes an outer joint member 21 having a cup-shaped mouth portion 22 with a plurality of outer track grooves 23 formed on the concave spherical inner diameter surface of the mouth portion 22, an inner joint member 24 having a plurality of inner track grooves 25 formed on the convex spherical outer diameter surface and having the other end of the shaft member 2 spline-fitted into a shaft hole, a plurality of balls 26 interposed between the paired track grooves 23, 25 to transmit torque between the two joint members 21, 24, and a cage 27 that holds the plurality of balls 26 at intervals in the circumferential direction. Other types of fixed type constant velocity universal joints, such as an undercut-free type, may also be used for this fixed type constant velocity universal joint 20.

[0028] A cylindrical boot 30 that functions as a sealing member is provided between the outer joint member 21 and the shaft member 2, and is molded from a resin material whose main ingredient is a thermoplastic elastomer, similar to the above-mentioned boot 3. This boot 30 integrally includes a large-diameter cylindrical portion 31 attached to the mouth portion 22 of the outer joint member 21, a small-diameter cylindrical portion 32 attached to the shaft member 2, an elastic portion 33 that is provided between the two cylindrical portions 31, 32 and elastically deforms in response to relative angular displacement between the outer joint member 21 and the inner joint member 24 (the shaft member 2 connected thereto), and a connecting portion 34 that connects the elastic portion 33 to the large-diameter cylindrical portion 31.

[0029] The large-diameter cylindrical portion 31 of the boot 30 is attached to the outer joint member 21 (the mouth portion 22 thereof) by fastening its outer peripheral surface with a boot band 8A, and the small-diameter cylindrical portion 32 of the boot 30 is attached to the shaft member 2 by fastening its outer peripheral surface with a boot band 8B. By attaching the boot 30 to the outer joint member 21 and the shaft member 2 in the above manner, leakage of the lubricant filled in the internal space of the mouth portion 22 (inside the joint) and entry of foreign matter into the inside of the joint are prevented as much as possible.

[0030] Thermoplastic elastomers that can be used to mold the boots 3, 30 include polystyrene-based (TPS), polyvinyl chloride-based (TPVC), polyurethane-based (TPU), polyester-based (TPEE), and polyamide-based (TPA) materials. A single type of elastomer may be used, or multiple types may be mixed. The boots 3, 30 may also be molded using flexible materials other than the resin materials listed above, such as rubber. The boots 3, 30 can be molded using methods such as injection molding, blow molding, and extrusion molding. The optimal molding method is selected depending on the desired shape, but injection molding is preferred for the boot 30 of this embodiment.

[0031] It is particularly desirable to use a thermoplastic elastomer having a Shore D hardness of 35 or more and 60 or less according to ISO 48-4 for the boot 30 attached to the fixed type constant velocity universal joint 20. This is because if the Shore D hardness is 35 or less, the rigidity of the boot 30 itself is insufficient, and the elastic portion 33 is likely to come into contact with the outer joint member 21 when the fixed type constant velocity universal joint 30 forms a working angle, which may result in damage to the elastic portion 33. On the other hand, if the Shore D hardness is greater than 60, the fatigue strength of the boot 30 itself is insufficient, and therefore, there is a higher possibility of early damage when the fixed type constant velocity universal joint 30 rotates while forming a working angle.

[0032] The boot 30 of this embodiment is characterized by the elastic portion 33 and the connecting portion 34. The characteristic configuration employed in the boot 30 of this embodiment will be described in detail below with reference to the vertical cross-sectional view of the boot 30 in a free state (a state in which each part is a single component with no stress applied thereto) shown in FIG.

[0033] 3 , the elastic portion 33 of the boot 30 in a free state has a generally J-shaped cross section, integrally including a cylindrical portion extending from the small-diameter cylindrical portion 32 toward the large-diameter cylindrical portion 31 and one arc-shaped valley portion 33a. The thickness t of this elastic portion 33 is generally constant throughout the elastic portion 33 and can be set to a value that provides both flexible deformation behavior and desired durability. Specifically, the thickness t is preferably set to 2 mm or less, and the ratio of the thickness x of the annular portion 35 and the thickness y of the tapered portion 36, both of which will be described later, to each other is preferably set to a value within the range of 0.3 to 0.7 (0.3x≦t≦0.7x and 0.3y≦t≦0.7y).

[0034] The connecting portion 34 of the boot 30 is composed of a flange-shaped annular portion 35 extending radially outward from the end of the elastic portion 33 on the large-diameter cylindrical portion 31 side, and a tapered portion 36 that connects (the outer diameter end of) this annular portion 35 to the large-diameter cylindrical portion 31 and gradually increases in diameter from the annular portion 35 side toward the large-diameter cylindrical portion 31 side. The outer diameter surface of the tapered portion 36 is formed into a conical surface that is smooth in the radial direction, and the inner diameter surface of the tapered portion 36 is provided with an annular protrusion 37 that can be engaged in the axial direction with the open end surface of the mouth portion 22 of the outer joint member 21. The provision of this annular protrusion 37 makes it possible to easily and accurately position the large-diameter cylindrical portion 31 relative to the mouth portion 22 of the outer joint member 21 in the axial direction.

[0035] The inclination angle α of the tapered portion 36 with respect to the axial direction (here, the angle that the outer diameter surface of the tapered portion 36 makes with respect to the axial direction) is set mainly taking into consideration the rate of increase in strain (elongation strain) generated in the tapered portion 36 when a bending moment acts on the boot 30 in accordance with the relative angular displacement of both joint parts 21, 24 and the size of the boot 30, and is preferably set to 20° or more and 30° or less (20°≦α≦30°). The upper limit of the inclination angle α (=30°) is set based on the results of a test (numerical analysis) that confirmed the effect that the inclination angle α of the tapered portion 36 has on the rate of increase in strain generated in the tapered portion 36 when a bending moment acts on it.

[0036] 10 showing the analysis results, the larger the inclination angle α of the tapered portion 36, the larger the rate of increase in strain of the tapered portion 36 when a bending moment acts on the boot 30 (when the constant velocity universal joint 20 forms an operating angle), and the rate of increase in strain when a bending moment acts becomes particularly remarkable when the inclination angle α exceeds 30°. The results shown in Fig. 10 are the results of a numerical analysis performed using a model of a constant velocity universal joint 20 equipped with a resin boot 30 with an outer diameter of φ87.5 mm for the mouth portion 22 of the outer joint member 21, the outer diameter of the large diameter cylindrical portion 31 being φ89.5 mm, the outer diameter of the small diameter cylindrical portion 32 being φ33.7 mm, the thickness x of the annular portion 35 being 3 mm, the thickness y of the tapered portion 36 being 2 mm, and the thickness t of the elastic portion 33 being 1.2 mm.

[0037] 10 , for example, when a constant velocity universal joint 20 equipped with a boot 30 having a tapered portion 36 with an inclination angle α of 10° is subjected to a relative angular displacement between the two joint parts 21, 24 from an operating angle of 0° to an operating angle of 5°, strain increases by 9%. For these reasons, the upper limit of the inclination angle α is set to 30°. On the other hand, the smaller the inclination angle α, the greater the axial length of the tapered portion 36, and in turn the boot 30 including the inclination angle α. For these reasons, it is preferable to set the inclination angle α of the tapered portion 36 relative to the axial direction within a range of 20° to 30°.

[0038] The connecting part 34 (the annular part 35 and tapered part 36 constituting it) is thicker than the elastic part 33 to increase its rigidity so that, even if a moment load (bending moment) acts on the boot 30 and elastically deforms the elastic part 33 as a result of the shaft member 2 connected to the inner joint member 24 being angularly displaced relative to the outer joint member 21 due to its own weight or the like, the elastic part 33 can be maintained in a non-contact state with the mouth part 22 of the outer joint member 21 against this moment load (to prevent the elastic part 33 from being pinched between the shaft member 2 and the mouth part 22 and being damaged). Here, the thickness x of the annular part 35 is set to 2 mm or more.

[0039] The greater the thickness x of the annular portion 35, the more effectively it can prevent the elastic portion 33 from getting caught during angular displacement of the joint. However, if the thickness x is too large, it can cause a number of other problems, such as a decrease in the moldability of the boot 30 (making the boot 30 more susceptible to molding defects), a loss in the deformability of the boot 30, which reduces the operability of the constant velocity universal joint 20 (making it difficult to perform smooth angular displacement), and an increase in the cost and weight of the boot 30 (and the constant velocity universal joint 20 including it). For this reason, the thickness x of the annular portion 35 is set to 4 mm or less. In other words, the thickness x of the annular portion 35 is set within the range of 2 mm to 4 mm (2 mm≦x≦4 mm).

[0040] Furthermore, if the thickness y of the tapered portion 36 is too large, problems will arise in formability, etc., as in the case where the thickness x of the annular portion 35 is made large, so the upper limit of the thickness y is set to 4 mm, just like the thickness x of the annular portion 35.

[0041] However, in order to prevent the occurrence of the above-mentioned problems while preventing the elastic portion 33 from getting caught, the thickness x of the annular portion 35 and the thickness y of the tapered portion 36 are set to satisfy the relationship described below, which was discovered by the inventors through numerical analysis as a confirmation test. First, the analysis procedure will be described. The temperature condition during the analysis was room temperature.

[0042] [Step 1] A plurality of boot 30 models (boot models) were created in which the wall thickness x of the annular portion 35 was varied within the above-mentioned range of 2 to 4 mm. Here, a first boot model was created in which the wall thickness x of the annular portion 35 was 2.0 mm, a second boot model was created in which x was 2.5 mm, a third boot model was created in which x was 3.0 mm, a fourth boot model was created in which x was 3.5 mm, and a fifth boot model was created in which x was 4.0 mm. In each of the boot models, the outer diameter of the large-diameter cylindrical portion 31 was φ89.5 mm, and the outer diameter of the small-diameter cylindrical portion 32 was φ33.7 mm. [Procedure 2] When a predetermined bending moment was applied to a model (joint model) of the fixed constant velocity universal joint 20 equipped with each of the above boot models and set at a working angle of 0°, we confirmed how the relative angular displacement of the shaft member 2 with respect to the outer joint member 21 (the working angle of the constant velocity universal joint 20) changed and progressed with changes in the ratio δ1 (= y / x), which is the thickness y of the tapered portion 36 to the thickness x of the annular portion 35. The bending moment applied to the joint model was 1740 Nmm. Note that the bending moment of 1740 Nmm was significantly smaller than the bending moment due to the weight of the shaft member 2, approximately 1 / 4 to 1 / 2 of the bending moment due to the weight of the shaft member 2. This was done to confirm the behavior of the boot 30 until the elastic portion 33 of the boot 30 was engaged. For comparison, bending moments of 2000 Nmm and 3000 Nmm were also applied to the joint model. The 3000 Nmm bending moment was also smaller than the bending moment due to the weight of the shaft member 2. [Step 3] For each of the above joint models, a scatter plot was created, with the ratio δ1 on the horizontal axis and the amount of angular displacement on the vertical axis. FIG. 4 shows a scatter plot for the joint model equipped with the first boot model (first joint model), and FIGS. 5 to 8 show scatter plots for the joint models equipped with the second to fifth boot models (second to fifth joint models), respectively. It can be seen from FIGS. 4 to 8 that the greater the ratio δ1 (the greater the thickness y of the tapered portion 36), the greater the effect of suppressing angular displacement when a bending moment is applied.

[0043] Next, we consider the angular displacement behavior of each joint model based on Figures 4 to 8. In the first joint model, when the thickness y of the tapered portion 36 is changed so that the ratio δ1 is between 0.4 and 1.5, the trend in the amount of angular displacement changes relative to the ratio δ1:0.81, regardless of the magnitude of the bending moment applied to the model (Figure 4). In the second joint model, when the thickness y of the tapered portion 36 is changed so that the ratio δ1 is between 0.4 and 1.2, the trend in the amount of angular displacement changes relative to the ratio δ1:0.73, regardless of the magnitude of the bending moment applied to the model (Figure 5). In the third joint model, when the thickness y of the tapered portion 36 is changed so that the ratio δ1 is between 0.4 and 1.0, the trend in the amount of angular displacement changes relative to the ratio δ1:0.6, regardless of the magnitude of the bending moment applied to the model (Figure 6). In the fourth joint model, when the thickness y of the tapered portion 36 is changed so that the ratio δ1 is between 0.34 and 0.86, the tendency of change in the amount of angular displacement changes based on the ratio δ1:0.56, regardless of the magnitude of the bending moment applied to the model (FIG. 7).In the fifth joint model, when the thickness y of the tapered portion 36 is changed so that the ratio δ1 is between 0.3 and 0.8, the tendency of change in the amount of angular displacement changes based on the ratio δ1:0.56, regardless of the magnitude of the bending moment applied to the model (FIG. 8).

[0044] 4 to 8 show that for each thickness x of the annular portion 35, a line graph with a steep gradient indicating a large change in the amount of angular displacement and a line graph with a gentle gradient indicating a small change in the amount of angular displacement can be drawn. A scatter plot was then created as shown in FIG. 9, with the thickness x of the annular portion 35 on the horizontal axis and the ratio δ1 on the vertical axis. The ratio δ1 at the point where the trend in the amount of angular displacement changed was plotted in FIG. 9 for each of the joint models equipped with the first to fifth boot models, each having an annular portion 35 with a thickness x of 3 mm. It was found that the trend in the ratio δ1 changed with the thickness x of the annular portion 35 set to 3 mm as the base, and that the relationship δ1 = -0.21x + 1.24 held when the thickness x was between 2.0 mm and 3.0 mm, and the relationship δ1 = -0.04x + 0.71 held when the thickness x was between 3.0 mm and 4.0 mm. Therefore, when the thickness x is 2.0 mm or more and less than 3.0 mm and satisfies the relationship δ1≧−0.21x+1.24, and when the thickness x is 3.0 mm or more and 4.0 mm or less and satisfies the relationship δ1≧−0.04x+0.71, a boot 30 can be realized that has an improved effect of suppressing angular displacement when a bending moment acts on the constant velocity universal joint 20.

[0045] As described above, the upper limit of the thickness y of the tapered portion 36 is set to 4.0 mm (y≦4.0 mm) in consideration of formability, etc., and when y = 4.0, when the thickness x of the annular portion 35 is set to 2.0 mm, δ1 = (y / x) = 2.0, when the thickness x of the annular portion 35 is set to 2.5 mm, δ1 = 1.6, when the thickness x of the annular portion 35 is set to 3.0 mm, δ1 = 1.33, when the thickness x of the annular portion 35 is set to 3.5 mm, δ1 = 1.14, and when the thickness x of the annular portion 35 is set to 4.0 mm, δ1 = 1.0. The values ​​of δ1 corresponding to the thickness x are plotted in FIG. 9. In this case, too, it was found that the tendency of change in the ratio δ1 changed with the thickness x of the annular portion 35 being 3 mm as the reference, and that the relational expression δ1 = -0.67x + 3.31 was established when the thickness x was between 2.0 mm and 3.0 mm, and the relational expression δ1 = -0.33x + 2.33 was established when the thickness x was between 3.0 mm and 4.0 mm.

[0046] From the above, in the boot 30 of this embodiment, in which the connecting portion 34 is formed to be thicker than the elastic portion 33 and the connecting portion 34 is configured with the annular portion 35 having a thickness of 2.0 mm or more and 4.0 mm or less, and the tapered portion 36 connecting the annular portion 35 and the large-diameter cylindrical portion 31 and gradually increasing in diameter from the annular portion side toward the large-diameter cylindrical portion side, when the thickness x of the annular portion 35 is set to be 2 mm or more and less than 3 mm, the ratio δ1 (= y / x) of the thickness y of the tapered portion 36 to the thickness x of the annular portion 35 satisfies the following relational expression (1): When the thickness x of the annular portion is set to 3 mm or more and 4 mm or less, by setting the thickness y of the tapered portion 36 so that the ratio δ1 satisfies the following relational expression (2), it is possible to realize at low cost a boot 30 that has a simple structure, is excellent in formability and ease of deformation, and is capable of preventing pinching and damage to the elastic portion 33 caused by large bending of the constant velocity universal joint 20 (large angular displacement of the shaft member 2 with respect to the outer joint member 21). -0.21x + 1.24≦ δ1 ≦ -0.67x + 3.31 (1) -0.04x + 0.71≦ δ1 ≦ -0.33x + 2.33 (2)

[0047] When the thickness x of the annular portion 35 is set to be 2 mm or more and less than 3 mm, the ratio δ1 may be set to satisfy the following relational expression (3), which is a partial modification of the relational expression (1). When the thickness x of the annular portion 35 is set to be 3 mm or more and 4 mm or less, the thickness y of the tapered portion 36 may be set so that the ratio δ1 satisfies the following relational expression (4), which is a partial modification of the relational expression (2). -0.21x + 1.24≦ δ1 ≦ (-0.21x + 1.24) × 1.2 (3) -0.04x + 0.71≦ δ1 ≦ (-0.04x + 0.71) × 1.2 (4)

[0048] The case where the above relational expressions (3) and (4) are satisfied means that the value of the ratio δ1 is within the range of the lightly shaded area in Fig. 9. Therefore, in this case, the upper limit of the ratio δ1 is small, in other words, the range in which the thickness y of the tapered portion 36 can be set is narrow, but this is advantageous in ensuring the above-mentioned effects of the present invention.

[0049] Furthermore, when the thickness x of the annular portion 35 is set to be 2 mm or more and less than 3 mm, the ratio δ1 may be set to satisfy the following relational expression (5), which is a partial modification of the relational expression (1), and when the thickness x of the annular portion 35 is set to be 3 mm or more and 4 mm or less, the thickness y of the tapered portion 36 may be set so that the ratio δ1 satisfies the following relational expression (6), which is a partial modification of the relational expression (2). -0.21x + 1.24≦ δ1 ≦ (-0.21x + 1.24) × 1.1 (5) -0.04x + 0.71≦ δ1 ≦ (-0.04x + 0.71) × 1.1 (6)

[0050] In this case, the upper limit of the ratio δ1 becomes smaller than those of the above-mentioned relational expressions (3) and (4), and accordingly the range in which the thickness y of the tapered portion 36 can be set becomes narrower. However, this is advantageous in that the above-mentioned effects of the present invention can be more reliably obtained.

[0051] A boot 40 according to a second embodiment of the present invention will now be described with reference to the longitudinal cross-sectional view in a free state shown in Fig. 11, and also with reference to Figs. 12 to 15. Similar to the boot 30 described with reference to Figs. 1 to 3, this boot 40 is provided between the outer joint member 21 and shaft member 2 that constitute the fixed constant velocity universal joint 20 (drive shaft 1), and functions as a sealing member that seals the internal space of the mouth portion 22.

[0052] The boot 40 shown in FIG. 11 has a large-diameter cylindrical portion 41 attached to the mouth portion 22 of the outer joint member 21, a small-diameter cylindrical portion 42 attached to the shaft member 2, an elastic portion 43 provided between the cylindrical portions 41, 42 and elastically deforming in response to relative angular displacement between the outer joint member 21 and the inner joint member 24 (the shaft member 2 connected thereto), and a connecting portion 44 connecting the elastic portion 43 and the large-diameter cylindrical portion 41, and these are molded from a resin material (or rubber material) whose main raw material is a thermoplastic elastomer.

[0053] In a free state, the elastic portion 43 of the boot 40 has a generally J-shaped cross section, integrally including a cylindrical portion extending from the small-diameter cylindrical portion 42 toward the large-diameter cylindrical portion 41 and one arc-shaped valley portion 43a. The thickness t of this elastic portion 43 is generally constant throughout the elastic portion 43 and can be set to a value that provides both flexible deformation behavior and desired durability. Specifically, the thickness t is preferably set to 2 mm or less, and the ratio of the thickness x of the annular portion 45 and the thickness z of the cylindrical portion 46, both of which are described below, is preferably set to a value within the range of 0.3 to 0.7 (0.3x≦t≦0.7x and 0.3z≦t≦0.7z).

[0054] The connecting portion 44 of the boot 40 is composed of a flange-shaped annular portion 45 that extends radially outward from the end of the elastic portion 43 on the large-diameter cylindrical portion 41 side, and a cylindrical portion 46 that connects (the outer diameter end of) this annular portion 45 to the large-diameter cylindrical portion 41. The outer diameter surface of the cylindrical portion 46 is formed into a cylindrical surface of a constant diameter that is free of radial irregularities.

[0055] The connecting part 44 (the annular part 45 and the cylindrical part 46 constituting it) is formed thicker than the elastic part 43, thereby increasing its rigidity, so that even if the shaft member 2 connected to the inner joint member 24 is angularly displaced relative to the outer joint member 21 due to its own weight or the like, causing a bending moment to act on the boot 40 and elastically deforming the elastic part 43, the elastic part 43 can be maintained in a non-contact state with the mouth part 22 of the outer joint member 21 against this moment load (so that the elastic part 43 is not caught between the shaft member 2 and the mouth part 22).

[0056] The thickness x of the annular portion 45 is set within the range of 2 mm to 4 mm for the same reasons as for setting the thickness x of the annular portion 35 of the boot 30 according to the first embodiment within the range of 2 mm to 4 mm. Furthermore, the upper limit of the thickness z of the cylindrical portion 46 is set to 4 mm for the same reasons as for setting the upper limit of the thickness y of the tapered portion 35 of the boot 40 according to the first embodiment to 4 mm.

[0057] However, in order to prevent the occurrence of the above-mentioned problems while preventing the elastic portion 43 from getting caught, the thickness x of the annular portion 45 and the thickness z of the cylindrical portion 46 are set to satisfy the relational expressions described below, which were discovered by the inventors through numerical analysis as a confirmation test. The analysis procedure is described below. The temperature condition during the analysis was room temperature.

[0058] [Procedure 1] A plurality of boot 40 models (boot models) were created in which the wall thickness x of the annular portion 45 was varied within the above-described range of 2 to 4 mm. Here, a sixth boot model was created in which the wall thickness x of the annular portion 45 was 2.0 mm, a seventh boot model was created in which the wall thickness x of the annular portion 45 was 3.0 mm, and an eighth boot model was created in which the wall thickness x of the annular portion 45 was 4.0 mm. In each boot model, the outer diameter of the large-diameter cylindrical portion 41 was φ89.5 mm and the outer diameter of the small-diameter cylindrical portion 42 was φ33.7 mm. [Procedure 2] When a predetermined bending moment was applied to a model (joint model) of the fixed constant velocity universal joint 20 equipped with each of the above boot models and in a state where the working angle was 0°, it was confirmed how the relative angular displacement amount of the shaft member 2 with respect to the outer joint member 21 (the working angle of the constant velocity universal joint 20) changed and progressed with changes in the ratio δ2 (=z / x) of the wall thickness y of the cylindrical portion 46 to the wall thickness x of the annular portion 45. The bending moment applied to the joint model was 1740 Nmm. [Step 3] For each of the above joint models, a scatter diagram was created with the ratio δ2 on the horizontal axis and the amount of angular displacement on the vertical axis. Figures 12 to 14 show scatter diagrams for the joint models equipped with the sixth to eighth boot models (the sixth to eighth joint models), respectively. It can be seen from Figures 12 to 14 that the greater the ratio δ2 (the greater the thickness z of the cylindrical portion 46), the greater the effect of suppressing angular displacement when a bending moment is applied.

[0059] Next, the angular displacement behavior of each joint model will be considered based on Figures 12 to 14. In the sixth joint model, when the thickness z of the cylindrical portion 46 is changed so that the ratio δ2 is between 0.4 and 1.0, the trend in the amount of angular displacement changes relative to the ratio δ2:0.6 (Figure 12). In the seventh joint model, when the thickness z of the cylindrical portion 46 is changed so that the ratio δ2 is between 0.4 and 1.0, the trend in the amount of angular displacement changes relative to the ratio δ2:0.6 (Figure 13). In the eighth joint model, when the thickness z of the cylindrical portion 46 is changed so that the ratio δ2 is between 0.4 and 1.0, the trend in the amount of angular displacement changes relative to the ratio δ2:0.6 (Figure 14).

[0060] 12 to 14 show that for each thickness x of the annular portion 45, a line graph with a steep gradient indicating a large change in the amount of angular displacement and a line graph with a gentle gradient indicating a small change in the amount of angular displacement can be drawn. A scatter plot was then created as shown in Figure 15, with the thickness x of the annular portion 45 on the horizontal axis and the ratio δ2 on the vertical axis. The ratio δ2 at the point where the trend in the amount of angular displacement changed was plotted in Figure 15 for each of the joint models fitted with the sixth to eighth boot models, each with an annular portion 45 thickness x that varied in 1.0 mm increments. When the thickness x of the annular portion 45 was between 2.0 mm and 4.0 mm, δ2 was constant at 0.6. Therefore, when the thickness x of the annular portion 45 is 2.0 mm or more and 4.0 mm or less and satisfies the relationship δ2≧0.6 (the thickness z of the cylindrical portion 46 is set so that this is the case), a boot 40 can be realized that has an improved effect of suppressing angular displacement when a bending moment acts on the constant velocity universal joint 20.

[0061] As described above, the upper limit of the thickness z of the cylindrical portion 46 is set to 4.0 mm (z≦4.0 mm) in consideration of formability, etc., and when z = 4.0, the following occurs: - when the thickness x of the annular portion 45 is 2.0 mm, δ2 = (z / x) = 2.0; - when the thickness x of the annular portion 45 is 2.5 mm, δ2 = 1.6; - when the thickness x of the annular portion 45 is 3.0 mm, δ2 = 1.33; - when the thickness x of the annular portion 45 is 3.5 mm, δ2 = 1.14; - when the thickness x of the annular portion 45 is 4.0 mm, δ2 = 1.0. The values ​​of δ2 corresponding to the above thickness x are plotted in FIG. 15. It was then found that the trend in the ratio δ2 changed with the thickness x of the annular portion 45 being 3 mm as the reference, and that the relationship δ2 = -0.67x + 3.31 held when the thickness x was between 2.0 mm and 3.0 mm, and the relationship δ2 = -0.33x + 2.33 held when the thickness x was between 3.0 mm and 4.0 mm.

[0062] From the above, in the boot 40 of the present embodiment, in which the connecting portion 44 is formed to be thicker than the elastic portion 43 and the connecting portion 44 is configured with the annular portion 45 having a thickness of 2.0 mm or more and 4.0 mm or less, and the cylindrical portion 46 having a constant diameter connecting the annular portion 45 and the large-diameter cylindrical portion 31, if the thickness x of the annular portion 45 is set to be 2 mm or more and less than 3 mm, the ratio δ2 (= z / x) of the thickness z of the cylindrical portion 36 to the thickness x of the annular portion 45 satisfies the following relational expression (7), and if the thickness x of the annular portion 45 is set to be 3 mm or more and 4 mm or less, the thickness z of the cylindrical portion 46 is set so that the ratio δ2 satisfies the following relational expression (8), it is possible to realize a boot 40 at low cost that has a simple structure, is excellent in formability and ease of deformation, and is capable of preventing the elastic portion 43 from getting caught due to large bending of the constant velocity universal joint 20 (large angular displacement of the shaft member 2 with respect to the outer joint member 21). 0.6 ≦ δ2 ≦ -0.67x+3.31...(7) 0.6 ≦ δ2 ≦ -0.33x+2.33...(8)

[0063] The thickness z of the cylindrical portion 46 may be set so that the ratio δ2 satisfies the following relational expression (9): 0.6≦δ2≦0.6×1.2 (9) Satisfying this relational expression (9) means that the ratio δ2 is within the lightly shaded area in Fig. 15. Therefore, in this case, the upper limit of the ratio δ2 is small, in other words, the settable range of the thickness z of the cylindrical portion 46 is narrow, but this is advantageous in ensuring the above-described effects of the present invention.

[0064] Furthermore, the thickness z of the cylindrical portion 46 may be set so that the ratio δ2 satisfies the following relational expression (10): 0.6≦δ2≦0.6×1.1 (10) In this case, the upper limit of the ratio δ2 is smaller than when the relational expression (9) is applied, and the settable range of the thickness z of the cylindrical portion 46 becomes narrower accordingly, but this is advantageous in terms of more reliably enjoying the effects of the present invention described above.

[0065] Although the boots 30, 40 according to the embodiments of the present invention have been described above, the boots 30, 40 can be modified as appropriate without departing from the gist of the present invention.

[0066] The power transmission device including the boot 30 according to an embodiment of the present invention shown in Figure 1 is a rear wheel drive shaft, but the boots 30, 40 according to embodiments of the present invention can also be used as boots for a constant velocity universal joint (fixed constant velocity universal joint) that constitutes a front wheel drive shaft or a propeller shaft.

[0067] REFERENCE SIGNS LIST 1 Drive shaft 2 Shaft member 20 Fixed constant velocity universal joint 21 Outer joint member 22 Mouth portion 30 Boot (boot for constant velocity universal joint) 31 Large diameter cylindrical portion 32 Small diameter cylindrical portion 33 Elastic portion 34 Connection portion 35 Annular portion 36 Tapered portion 40 Boot (boot for constant velocity universal joint) 41 Large diameter cylindrical portion 42 Small diameter cylindrical portion 43 Elastic portion 44 Connection portion 45 Annular portion 46 Cylindrical portion t Wall thickness of elastic portion x Wall thickness of annular portion y Wall thickness of tapered portion z Wall thickness of cylindrical portion δ1, δ2 ratio

Claims

1. A boot for a constant velocity universal joint, comprising: a large-diameter cylindrical portion attached to an outer joint member of a constant velocity universal joint; a small-diameter cylindrical portion attached to a shaft member connected to an inner joint member of the constant velocity universal joint; an elastic portion having one arc-shaped valley portion and extending from the small-diameter cylindrical portion to the large-diameter cylindrical portion; and a connecting portion connecting the large-diameter cylindrical portion and the elastic portion, all integrally molded from a flexible material; the elastic portion elastically deforms in response to relative angular displacement between the outer joint member and the shaft member; and the connecting portion is formed to be thicker than the elastic portion, the connecting portion having a thickness of 2 mm to 4 mm, and comprising: an annular portion extending radially outward from the end of the elastic portion on the large-diameter cylindrical portion side; and a tapered portion connecting the annular portion to the large-diameter cylindrical portion, the diameter of which gradually increases from the annular portion side toward the large-diameter cylindrical portion side. A boot for a constant velocity universal joint, characterized in that when the thickness x of the annular portion is 2 mm or more and less than 3 mm, a ratio δ1 (=y / x) of the thickness y of the tapered portion to the thickness x of the annular portion satisfies the following relational expression (1), and when the thickness x of the annular portion is 3 mm or more and 4 mm or less, the ratio δ1 satisfies the following relational expression (2): -0.21x + 1.24≦ δ1 ≦-0.67x + 3.31 (1) -0.04x + 0.71≦ δ1 ≦-0.33x + 2.33 (2) 2. A boot for a constant velocity universal joint as set forth in claim 1, wherein when the wall thickness x of the annular portion is 2 mm or more and less than 3 mm, the ratio δ1 satisfies the following relational expression (3), which is a partial modification of the relational expression (1), and when the wall thickness x of the annular portion is 3 mm or more and 4 mm or less, the ratio δ1 satisfies the following relational expression (4), which is a partial modification of the relational expression (2): -0.21x + 1.24 ≦ δ1 ≦ (-0.21x + 1.24) × 1.2 (3) -0.04x + 0.71 ≦ δ1 ≦ (-0.04x + 0.71) × 1.2 (4) 3. A boot for a constant velocity universal joint as set forth in claim 1, wherein when the wall thickness x of the annular portion is 2 mm or more and less than 3 mm, the ratio δ1 satisfies the following relational expression (5), which is a partial modification of the relational expression (1), and when the wall thickness x of the annular portion is 3 mm or more and 4 mm or less, the ratio δ1 satisfies the following relational expression (6), which is a partial modification of the relational expression (2): -0.21x + 1.24≦ δ1 ≦(-0.21x + 1.24) × 1.1 (5) -0.04x + 0.71≦ δ1 ≦(-0.04x + 0.71) × 1.1 (6) 4. A boot for a constant velocity universal joint as set forth in any one of claims 1 to 3, wherein the angle formed by the tapered portion with respect to the axial direction is between 20° and 30°.

5. A boot for a constant velocity universal joint, comprising: a large diameter cylindrical portion attached to the outer joint member of a constant velocity universal joint; a small diameter cylindrical portion attached to a shaft member connected to the inner joint member of the constant velocity universal joint; an elastic portion having one arc-shaped valley portion and extending from the small diameter cylindrical portion toward the large diameter cylindrical portion; and a connecting portion connecting the large diameter cylindrical portion and the elastic portion, all integrally molded from a flexible material; the elastic portion elastically deforms in response to the relative angular displacement between the outer joint member and the shaft member; and the connecting portion is formed to be thicker than the elastic portion, the connecting portion having a thickness of 2 mm to 4 mm, and comprising an annular portion extending radially outward from the end of the elastic portion on the large diameter cylindrical portion side, and a cylindrical portion connecting the annular portion and the large diameter cylindrical portion; A boot for a constant velocity universal joint, characterized in that when the wall thickness x of the annular portion is 2 mm or more and less than 3 mm, the ratio δ2 (=z / x) of the wall thickness z of the cylindrical portion to the wall thickness x of the annular portion satisfies the following relational expression (7), and when the wall thickness x of the annular portion is 3 mm or more and 4 mm or less, the ratio δ2 satisfies the following relational expression (8): 0.6≦δ2≦−0.67x+3.31 (7) 0.6≦δ2≦−0.33x+2.33 (8) 6. A boot for a constant velocity universal joint, comprising: a large-diameter cylindrical section attached to the outer joint member of a constant velocity universal joint; a small-diameter cylindrical section attached to a shaft member connected to the inner joint member of the constant velocity universal joint; an elastic section having one arc-shaped valley section and extending from the small-diameter cylindrical section to the large-diameter cylindrical section; and a connecting section connecting the large-diameter cylindrical section and the elastic section, all integrally molded from a flexible material; the elastic section elastically deforms in response to relative angular displacement between the outer joint member and the shaft member; and the connecting section is thicker than the elastic section, the connecting section having a thickness of 2 mm to 4 mm and comprising an annular section extending radially outward from the end of the elastic section on the large-diameter cylindrical section side, and a cylindrical section connecting the annular section and the large-diameter cylindrical section; and the ratio δ2 (= z / x) of the thickness z of the cylindrical section to the thickness x of the annular section satisfies the following relational expression (9). 0.6≦δ2≦0.6×1.2...(9) 7. A boot for a constant velocity universal joint, comprising: a large-diameter cylindrical section attached to the outer joint member of a constant velocity universal joint; a small-diameter cylindrical section attached to a shaft member connected to the inner joint member of the constant velocity universal joint; an elastic section having one arc-shaped valley section and extending from the small-diameter cylindrical section to the large-diameter cylindrical section; and a connecting section connecting the large-diameter cylindrical section and the elastic section, all integrally molded from a flexible material; the elastic section elastically deforms in response to relative angular displacement between the outer joint member and the shaft member; and the connecting section is thicker than the elastic section, the connecting section having a thickness of 2 mm to 4 mm and comprising an annular section extending radially outward from the end of the elastic section on the large-diameter cylindrical section side, and a cylindrical section connecting the annular section and the large-diameter cylindrical section; and the ratio δ2 (= z / x) of the thickness z of the cylindrical section to the thickness x of the annular section satisfies the following relational expression (10). 0.6≦δ2≦0.6×1.1 (10)

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