Bidirectional axial prepressing structure of roller screw

Abstract

The utility model provides a roller screw bidirectional axial pre-pressing structure which comprises a screw, a nut, a roller set, a retainer set and two elastic pieces. The retainer group is provided with two retainers, the roller group is provided with a plurality of first rollers and second rollers which are arranged in a staggered manner, two ends of the first rollers and the second rollers are respectively connected with the two retainers, and first gaps and second gaps which are arranged in a staggered manner are axially formed between the first rollers and the retainers and between the second rollers and the retainers. The two elastic pieces apply force to the outer sides of the two retaining pieces respectively in the axial direction, so that the first pin roller and the second pin roller are stressed respectively to generate axial pre-pressing in the opposite directions, the staggered bidirectional pre-pressing effect is further formed, and the transmission precision and the movement stability of the pin roller screw are improved.

Description

TECHNICAL FIELD

[0001] The utility model relates to a transmission assembly, especially a roller screw bidirectional axial pre-pressing structure capable of pre-pressing rollers in the axial direction to improve the rolling stability. BACKGROUND

[0002] The planetary roller screw transmission assembly (also called planetary roller screw) has high load bearing capacity, high rigidity, excellent transmission efficiency and positioning accuracy, and is widely used in precision machine tools, servo drive systems, space control devices and high-performance robots that require fast response and high dynamic performance. The basic structure is to use a plurality of rollers arranged between the screw rod and the nut, which are arranged around the outer periphery of the screw rod, and the rotation is converted into linear motion by the rolling engagement of the rollers between the screw rod and the nut, thereby driving the external load.

[0003] As shown in Figure 1A , a general planetary roller screw transmission device includes a screw rod 11, a nut 12, a plurality of rollers 13, and a retaining member set 14. The screw rod 11 has an external thread structure, the nut 12 has an inner hole 120 capable of accommodating the plurality of rollers 13, and an annular tooth structure 12t is provided on the inner circumferential surface of the inner hole 120 of the nut 12 to engage with the external annular teeth of the rollers 13 to form rolling contact. The retaining member set 14 is arranged at both axial ends of the plurality of rollers 13 to position the plurality of rollers 13 between the screw rod 11 and the nut 12 and maintain the circumferential spacing.

[0004] However, in actual operation, the above-mentioned conventional device still has many problems. As shown in Figure 1B and Figure 1C , first, the geometric fit between the rollers 13, the screw rod 11 and the nut 12 is very precise, but due to manufacturing tolerances and assembly errors, a small gap G is generated between the rollers 13 and the screw rod 11 / nut 12, which causes the actual contact position of the rollers 13 to deviate from the designed pitch circle radius. If the plurality of fit gaps G are not effectively compensated, the rollers 13 and the nut 12 lack sufficient friction when actuated, which will cause sliding or idling, making it difficult to maintain pure rolling contact with sliding friction. Thus, the rollers 13 are subjected to greater wear, reducing the overall structure life. Similarly, the rollers 13 and the screw rod 11 can also slide due to insufficient contact, thereby destroying the planetary motion reduction and propulsion effect, resulting in a decrease in linear propulsion efficiency and positioning accuracy.

[0005] In addition, although the existing retaining member set 14 can control the angular distribution of the rollers 13, this component only has mechanical positioning function and cannot provide pre-pressing compensation or guide the direction of roller movement, nor can it effectively improve the sliding deviation problem of the rollers 13 due to the gap G.

[0006] Therefore, how to effectively improve the stability of the contact between the roller 13 and the nut 12 without increasing the structural complexity and manufacturing cost, and to strengthen the rolling engagement between the roller 13 and the screw rod 11, so as to ensure that the screw rod 11, the roller 13 and the nut 12 maintain stable rolling motion, is a important issue to be overcome in the field. Utility model content

[0007] The utility model discloses a kind of roller screw bidirectional axial pre-press structures that can solve the above technical problems, which can effectively compensate the fitting gap caused by processing error, and opposite direction axial pre-pressure is respectively applied by two kinds of rollers staggered arrangement, to realize stable bidirectional rolling contact effect, and then improve overall transmission efficiency and prolong service life with low abrasion.

[0008] To achieve the above object, the utility model provides a kind of roller screw bidirectional axial pre-press structure, it is characterized by comprising:

[0009] A screw rod, the outer circumferential surface of the screw rod is provided with a threaded structure;

[0010] A nut is sleeved on the screw rod, and at least one annular groove section is provided on the inner side of the nut;

[0011] A roller set is arranged between the screw rod and the nut, and has a plurality of first rollers and a plurality of second rollers arranged alternately, the outer circumferential surface of each first roller and second roller is provided with a plurality of annular teeth, and the plurality of annular teeth are engaged with the threaded structure and the annular groove section respectively;

[0012] A retaining member set is arranged in the nut, and has a first retaining member and a second retaining member located at both ends of the roller set, one end of the plurality of first rollers and the plurality of second rollers is connected with the first retaining member, the other end of the plurality of first rollers and the plurality of second rollers is connected with the second retaining member, wherein a first gap is provided between one end of the plurality of first rollers and the first retaining member, a second gap is provided between one end of the plurality of second rollers and the second retaining member, and the plurality of first gaps and the plurality of second gaps are arranged in an alternating manner at opposite ends of the roller set respectively;

[0013] Two elastic members are arranged in the nut and elastically abut on the outer side of the first retaining member and the second retaining member along an axial direction respectively, and a fastener is provided on the outer side of each elastic member, so that the two elastic members apply axial positive force on the plurality of first rollers and the plurality of second rollers respectively, and form an alternating bidirectional axial pre-press arrangement.

[0014] The roller screw bidirectional axial pre-press structure, wherein: a bearing is provided between the retaining member set and the two elastic members respectively.

[0015] The roller screw bidirectional axial pre-pressing structure, wherein the two elastic members respectively apply opposite positive pre-pressing forces to the plurality of first rollers and the plurality of second rollers via the first holder and the second holder.

[0016] The roller screw bidirectional axial pre-pressing structure, wherein the inner side of the nut is provided with two grooves for embedding the fasteners.

[0017] The roller screw bidirectional axial pre-pressing structure, wherein:

[0018] The first holder is provided with a plurality of first positioning grooves for accommodating one end of the plurality of first rollers and the plurality of second rollers, and the depths of the plurality of first positioning grooves are the same.

[0019] The second holder is provided with a plurality of second positioning grooves for accommodating the other end of the plurality of first rollers and the plurality of second rollers, and the depths of the plurality of second positioning grooves are the same.

[0020] The axial length of one end of the first roller accommodated in the first positioning groove is smaller than the axial length of the other end and the depth of the first positioning groove, so as to form the first gap; the axial length of one end of the second roller accommodated in the second positioning groove is smaller than the axial length of the other end and the depth of the second positioning groove, so as to form the second gap, and the plurality of first gaps and the plurality of second gaps are respectively formed at the two ends of the roller set and are staggered.

[0021] The roller screw bidirectional axial pre-pressing structure, wherein:

[0022] The first holder is provided with a plurality of first positioning grooves for accommodating one end of the plurality of first rollers and the plurality of second rollers, and the depths of the first positioning grooves for accommodating the first rollers and the second rollers are different.

[0023] The second holder is provided with a plurality of second positioning grooves for accommodating the other end of the plurality of first rollers and the plurality of second rollers, and the depths of the second positioning grooves for accommodating the first rollers and the second rollers are different.

[0024] The roller screw bidirectional axial pre-pressing structure, wherein a gasket is arranged between the elastic member and the fastener.

[0025] By means of the staggered gap structure and the double-sided elastic members, opposite axial pre-pressing forces are respectively generated on the first rollers and the second rollers, so that the roller set always maintains stable and sufficient rolling contact during movement.

[0026] In summary, the utility model discloses the structure design of pre-press compensation to roller through the bi-directional pressure and staggered gap guide, not only can effectively solve the problem of roller sliding and idling caused by tolerance increase structural motion stability, also can improve the overall rolling efficiency and positioning accuracy of roller screw, with high-precision transmission and practical application benefit. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1A It is the structural schematic diagram of existing planetary roller screw transmission device;

[0028] Figure 1B It is Figure 1A The meshing contact relation schematic diagram between the middle roller and the nut;

[0029] Figure 1C It is Figure 1A The meshing contact relation schematic diagram between the middle roller and the screw;

[0030] Figure 2 It is the three-dimensional exploded schematic diagram of the utility model embodiment;

[0031] Figure 3A It is the combined cross-sectional schematic diagram of the utility model embodiment;

[0032] Figure 3B It is Figure 3A The local enlarged schematic diagram;

[0033] Figure 3C It is Figure 3A The right side cross-sectional schematic diagram;

[0034] Figure 4 It is the schematic diagram that the axial length of the first roller and the second roller is different in the utility model embodiment;

[0035] Figure 5 It is the schematic diagram that the first plurality of rollers and the second plurality of rollers generate the axial pre-pressure of opposite direction through the staggered gap structure and the force of double-sided elastic piece in the utility model embodiment;

[0036] Figure 6A And Figure 6B It is the schematic diagram that the second roller is pushed to the right direction in the utility model embodiment;

[0037] Figure 7A And Figure 7B It is the schematic diagram that the first roller is pushed to the left direction in the utility model embodiment;

[0038] Figure 8A And Figure 8B It is the schematic diagram that one bearing is respectively arranged between the retaining member group and the two elastic pieces in the utility model.

[0039] Explanation of reference numerals: screw rod 21; threaded structure 211; nut 22; inner hole 220; ring groove section 221; ring groove 221g; recess 222; retreat groove section 223; roller set R; first roller 23; ring tooth 23t; first engagement section 231; second engagement section 232; ends 233, 234; second roller 24; ring tooth 24t; first engagement section 241; second engagement section 242; ends 243, 244; retainer set K; first retainer 25; first positioning groove 251; second retainer 26; second positioning groove 261; elastic members 271, 272; spacers 281, 282; fasteners 291, 292; first gap 31; second gap 32; bearing 33; axial lengths L1, L2, L3, L4. DETAILED DESCRIPTION

[0040] The structure and functional characteristics of the bidirectional axial pre-pressing structure of the roller screw of the utility model will be described in accordance with the preferred embodiments of the accompanying drawings.

[0041] Please refer to Figure 2 , Figures 3A-3C , Figure 4 and Figure 5 , the bidirectional axial pre-pressing structure of the roller screw of a preferred embodiment of the utility model comprises: a nut 22, a screw rod 21, a roller set R, a retainer set K and two elastic members 271, 272.

[0042] The nut 22 can be a hollow cylindrical member, having an inner hole 220 penetrating through both ends in the axial direction. The inner circumferential surface of the inner hole 220 of the nut 22 is provided with two ring groove sections 221 for forming rolling engagement with the roller set R. The plurality of ring groove sections 221 provide circumferential guidance and contact restriction, so that the roller set R can stably perform planetary rolling in the nut 22, and prevent it from being pulled out in the axial direction. Each ring groove section 221 has a plurality of circumferentially distributed ring grooves 221g, and a recess 222 is provided adjacent to the ungrooved area at both ends.

[0043] In addition, the inner circumferential surface of the nut 22 is provided with a radial retreat groove section 223 between the two ring groove sections 221, and the inner diameter of the retreat groove section 223 is greater than that of other parts, so as to provide sufficient space to avoid interference between the roller set R and the inner wall. The specific effect will be described later.

[0044] The screw rod 21 is a long rod member, for example, a long cylindrical shaft, which is disposed in the inner hole 220 of the nut 22 coaxially along the axial direction of the nut 22 and extends to the outside of the nut 22 for the connection of a driving device or installation on other external mechanisms. The outer circumferential surface of the screw rod 21 is provided with a threaded structure 211 (also referred to as a helical groove or helical tooth), which can be in the form of multi-start thread. In actual operation, the screw rod 21 can be driven to rotate by an external power source (not shown), and through the double-sided meshing action between the threaded structure 211 and the roller set R, the roller set R simultaneously generates revolution and revolution around the screw rod 21 and the nut 22, forming a planetary rolling motion. Through this motion mechanism, the rotational motion of the screw rod 21 can be converted into its axial linear motion opposite to the nut 22, thereby realizing high-efficiency and high-precision linear pushing effect.

[0045] Referring to Figure 2 , Figures 3A-3C and Figure 4 , the roller set R is disposed in the inner hole 220 of the nut 22 and located between the nut 22 and the screw rod 21. It includes a plurality of first rollers 23 and second rollers 24 arranged alternately. The plurality of first rollers 23 and second rollers 24 are circumferentially arranged uniformly and spaced apart around the outer circumference of the screw rod 21. In this embodiment, the number of first rollers 23 and second rollers 24 is three, which are correspondingly arranged at different circumferential angle positions of the outer circumference of the screw rod 21 and form rolling engagement with the corresponding threaded structure 211.

[0046] The first roller 23 and the second roller 24 are cylindrical members extending along the axial direction, and the outer circumferential surface is provided with a plurality of ring teeth 23t, 24t (as shown in Figure 4 ), which are used to mesh with the corresponding structures on the screw rod 21 and the nut 22. Specifically, each first roller 23 and second roller 24 is divided into at least a first engagement section 231, 241 and at least a second engagement section 232, 242 along the axial direction, wherein the diameter of the first engagement section 231, 241 is greater than the diameter of the second engagement section 232, 242. The ring teeth 23t, 24t of the first engagement section 231, 241 are used for rolling engagement with the threaded structure 211 of the screw rod 21, and the ring teeth 23t, 24t of the second engagement section 232, 242 are used for rolling engagement with the ring groove 221g (as shown in Figure 3B ) of the ring groove section 221 of the nut 22.

[0047] As shown in Figure 3A and Figure 4As shown, the first engagement sections 231, 241 are disposed at the axial middle sections of the first and second rollers 23, 24, and each of the two ends thereof is provided with a second engagement section 232, 242, so as to form a double-sided rolling engagement with the screw 21 and the nut 22, respectively. Furthermore, in order to avoid interference between the first engagement sections 231, 241 and the inner wall of the nut 22, the sections are correspondingly arranged in the avoiding groove sections 223 of the nut 22, so as to provide sufficient radial space for contacting and participating in rolling only with the screw 21.

[0048] The two ends 233, 234, 243, 244 of the first and second rollers 23, 24 are not provided with ring-shaped teeth 23t, 24t, which are used to connect with the retainer set K. The axial length L1 of one end 233 of the first roller 23 is smaller than the axial length L2 of the other end 234. The axial length L4 of one end 244 of the second roller 24 is smaller than the axial length L3 of the corresponding end 243. The design purpose will be further described in the retainer set K and the pre-pressing structure below.

[0049] Referring back to Figure 2 and Figure 3A and Figure 4 As shown, the retainer set K includes a first retainer 25 and a second retainer 26, which are respectively arranged at the two ends of the roller set R and are fixed inside the nut 22. The first and second retainers 25, 26 are respectively provided with positioning grooves corresponding to the roller set R, so as to fix the angular interval arrangement of the plurality of first and second rollers 23, 24 in the circumferential direction. Specifically, the first retainer 25 is provided with a plurality of first positioning grooves 251 for accommodating one ends 233, 243 of the plurality of first and second rollers 23, 24, and the groove depths of the plurality of first positioning grooves 251 are the same as each other. The second retainer 26 is provided with a plurality of second positioning grooves 261 for accommodating the opposite other ends 234, 244, and the groove depths of the plurality of second positioning grooves 261 are the same as each other.

[0050] As Figure 4 and Figure 5As shown, one end 233 of the first roller 23 pivoted to the first positioning slot 251 has an axial length L1 which is less than the depth of the first positioning slot 251 and also less than the axial length L2 of the other end 234 of the second roller 24. That is, the shorter axial length L1 of the one end 233 of the first roller 23 cannot reach the bottom of the first positioning slot 251 and forms a first gap 31 with the first retainer 25. In addition, the one end 244 of the second roller 24 pivoted to the second positioning slot 261 has an axial length L4 which is less than the depth of the second positioning slot 261 and also less than the axial length L3 of the other end 243 of the first roller 23. That is, the shorter axial length L4 of the one end 244 cannot reach the bottom of the second positioning slot 261 and forms a second gap 32 with the second retainer 26. The two gaps 31, 32 are staggered at opposite ends of the roller set R so that the force directions of different rollers are opposite to each other, achieving the effect of bidirectional axial pre-pressing.

[0051] In an alternative embodiment, the axial lengths of both ends of the first roller 23 and the second roller 24 can be set to be the same, and the depths of the first and second positioning slots 251, 261 of the first and second retainers 25, 26 are designed to be different to achieve the same staggered pre-pressing configuration. For example, the depths of the first positioning slots 251 of the first retainer 25 corresponding to the first roller 23 and the second roller 24 are different. So that the one end 233 of the first roller 23 does not contact the bottom of the deeper first positioning slot 251, forming a first gap 31. The depths of the second positioning slots 261 of the second retainer 26 corresponding to the first roller 23 and the second roller 24 are different. So that the one end 244 of the second roller 24 does not contact the bottom of the deeper second positioning slot 261, forming a second gap 32.

[0052] Referring back to Figure 2 and Figure 5 The two elastic members 271, 272 (for example, disc springs, wave springs, or elastic washers) are arranged inside the nut 22 and elastically abut the outer sides of the first retainer 25 and the second retainer 26 in the axial direction, respectively. The outer side of each elastic member 271, 272 is provided with a gasket 281, 282 and a fastener 291, 292 (for example, a C-shaped or O-shaped buckle), wherein the fastener 291, 292 is embedded in the groove 222 corresponding to the inner circumferential surface of the nut 22, so as to fix the elastic member 271, 272 at the default axial position and prevent it from sliding axially due to force.

[0053] Through the above configuration, the two elastic members 271, 272 can stably apply axial direction positive pre-pressing force to their adjacent first retainer 25 and second retainer 26. Further, through the staggered configuration design of the first and second gaps 31, 32, the selective control of the pre-pressing force transmission direction is achieved.

[0054] As Figure 5As shown, the first roller 23 has a first gap 31 between one end 233 and the first retainer 25; the second roller 24 has a second gap 32 between one end 244 and the second retainer 26.

[0055] Therefore, as Figure 6A and Figure 6B As shown, in Figure 5 The preload applied by the elastic element 271 on the left side is transmitted via the first retainer 25. Due to the first gap 31 between the first roller 23 and the first retainer 25, the preload does not act on the first roller 23, but is directly transmitted to the second roller 24, which is in close contact with the first retainer 25. Thus, the annular teeth 24t of the second roller 24 abut against the left side of the thread structure 211 of the screw 21 and the left side of the annular groove 221g of the nut 22, creating a stable meshing contact.

[0056] Similarly, such as Figure 7A and Figure 7B As shown, the preload generated by the elastic element 272 on the right side of the figure is transmitted through the second retainer 26. Because there is a second gap 32 between the second roller 24 and the second retainer 26, this pressure does not act on the second roller 24, but instead pushes directly against the first roller 23, which is in close contact with the second retainer 26. Thus, the annular teeth 23t of the first roller 23 abut against the right side of the threaded structure 211 of the screw 21 and the right side of the annular groove 221g of the nut 22, forming a stable rolling engagement and effectively preventing slippage and freewheeling.

[0057] This design utilizes two elastic elements 271 and 272, respectively positioned at both ends of the nut 22. Through the staggered design of the first and second gaps 31 and 32, the transmission path of the axial preload can be precisely guided. This ensures that each elastic element 271 and 272 acts only on specific first and second rollers 23 and 24, thereby applying positive preload in opposite directions to the first roller 23 and the second roller 24. This design ensures that all first and second rollers 23 and 24 are tightly pressed against the corresponding rolling contact surfaces of the screw 21 and the nut 22, eliminating gaps caused by manufacturing tolerances. This effectively reduces sliding friction, decreases backlash, improves the pure rolling stability of the rollers and the overall transmission accuracy, and extends the system's service life.

[0058] Please continue to refer to this. Figure 8A and Figure 8BAs shown, the utility model also can further set up a bearing 33 (such as ball, roller bearing or needle bearing) between the first retaining member 25 and the elastic member 271, and between the second retaining member 26 and the elastic member 272 respectively. The two bearings 33 are arranged between the elastic members 271, 272 and the first and second retaining members 25, 26, forming a rotatable contact surface. In this way, the contact friction between the elastic members 271, 272 and the first and second retaining members 25, 26 during the force exertion process is reduced, thereby reducing the energy loss and non-linear deviation when the pressure is applied. Not only does it maintain stable pre-pressing effect, but also improves the consistency and stability of the overall pre-pressing response.

[0059] In summary, the utility model discloses a roller screw bidirectional axial pre-pressing structure. The elastic components 271, 272 are arranged at both ends of the roller group R, and the first and second rollers 23, 24 have different axial lengths and / or the first and second retaining members 25, 26 have different groove depths. The first and second gaps 31, 32 are formed between the first and second rollers 23, 24 and the first and second retaining members 25, 26, thereby guiding the axial pre-pressing force to selectively act on specific rollers, and establishing a staggered bidirectional force exertion mechanism. This structure not only effectively compensates for the gap caused by the processing tolerance, eliminates the initial back gap, but also stabilizes the movement trajectory of the first and second rollers 23, 24, so that they always maintain a pure rolling state, effectively suppressing sliding friction and idling phenomenon.

[0060] In addition, the utility model also improves the pressure transmission efficiency and stability with the bearing 33. The overall structure is simple, and the manufacturing and assembly tolerance is high, which is suitable for high-precision, high-rigidity, high-frequency roller screw transmission systems.

Claims

1. A roller screw bidirectional axial preloading structure, characterized by, Comprising: a screw rod, the outer circumferential surface of which is provided with a threaded structure; a nut, which is sleeved on the screw rod, and the inner side of which is provided with at least one annular groove section; a roller group, which is arranged between the screw rod and the nut, has a plurality of first rollers and a plurality of second rollers arranged in an interlaced manner, and the outer circumferential surface of each first roller and second roller is provided with a plurality of annular teeth, which are respectively engaged with the threaded structure and the annular groove section; a retaining member group, which is arranged in the nut, has a first retaining member and a second retaining member located at the two ends of the roller group, the first retaining member is connected with one end of the plurality of first rollers and the plurality of second rollers, the second retaining member is connected with the other end of the plurality of first rollers and the plurality of second rollers, wherein a first gap is provided between one end of the plurality of first rollers and the first retaining member, a second gap is provided between one end of the plurality of second rollers and the second retaining member, and the plurality of first gaps and the plurality of second gaps are respectively arranged in an interlaced manner at the opposite ends of the roller group; two elastic members, which are arranged in the nut and respectively elastically abut against the outer side of the first retaining member and the second retaining member along an axial direction, and the outer side of each elastic member is provided with a fastener, thereby enabling the two elastic members to respectively exert an axial positive force on the plurality of first rollers and the plurality of second rollers and form an interlaced bidirectional axial pre-pressing arrangement.

2. The roller screw bidirectional axial preloading structure of claim 1, wherein: A bearing is respectively provided between the retaining member group and the two elastic members.

3. The roller screw bidirectional axial preloading structure of claim 1, wherein: The two elastic members respectively exert opposite-direction positive pre-pressing forces on the plurality of first rollers and the plurality of second rollers through the first retaining member and the second retaining member.

4. The preloaded bidirectional roller screw assembly of claim 1, wherein: The inner side of the nut is provided with two grooves for embedding the fasteners.

5. The roller screw bidirectional axial pre-pressing structure according to claim 1, wherein: the first retaining member is provided with a plurality of first positioning grooves for accommodating one end of the plurality of first rollers and the plurality of second rollers, and the groove depths of the plurality of first positioning grooves are the same as each other; the second retaining member is provided with a plurality of second positioning grooves for accommodating the other end of the plurality of first rollers and the plurality of second rollers, and the groove depths of the plurality of second positioning grooves are the same as each other; the axial length of one end of the first roller accommodated in the first positioning groove is smaller than the axial length of the other end of the first roller and the groove depth of the first positioning groove, so as to form the first gap; the axial length of one end of the second roller accommodated in the second positioning groove is smaller than the axial length of the other end of the second roller and the groove depth of the second positioning groove, so as to form the second gap, and the plurality of first gaps and the plurality of second gaps are respectively formed at the two ends of the roller group and arranged in an interlaced manner.

6. The roller screw bidirectional axial pre-pressing structure according to claim 1, wherein: the first retaining member is provided with a plurality of first positioning grooves for accommodating one end of the plurality of first rollers and the plurality of second rollers, and the groove depths of the first positioning grooves for accommodating the first rollers and the second rollers are different from each other; the second retaining member is provided with a plurality of second positioning grooves for accommodating the other end of the plurality of first rollers and the plurality of second rollers, and the groove depths of the second positioning grooves for accommodating the first rollers and the second rollers are different from each other.

7. The roller screw bidirectional axial preload structure as described in claim 1, characterized in that: a gasket is provided between the elastic member and the fastener.

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