Torsion beam hydraulic bushing

Abstract

The invention provides a torsion beam hydraulic bushing, and belongs to the technical field of automobile parts. The torsion beam lining solves the problem that an existing torsion beam lining cannot adjust the curve interval of axial rigidity. The torsion beam hydraulic bushing comprises a hydraulic assembly, an elastic assembly, a lower cover plate, an upper cover plate, a rubber body, a rigid block and a preset distance. In the invention, when a vehicle runs and the shaft body drives the upper cover plate to move towards the lower cover plate along the central axis of the bushing, the rubber body is stressed and deformed, the upper cover plate is subjected to the counter-acting force of the rubber body, and when the upper cover plate extrudes the rigid block, the axial rigidity of the rubber body is suddenly increased, namely, the axial rigidity of the rubber body is suddenly increased. The ratio of the counter-acting force to the downward moving distance of the upper cover plate is increased, the rigidity curve is steeped, the upper cover plate enters a non-linear section, in other words, the upper cover plate is not prone to descending, the rubber body is not prone to deformation, and therefore the rubber body can brake the moving distance of the shaft body more rapidly.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of automobile parts, and relates to a torsion beam hydraulic bushing. BACKGROUND

[0002] The torsion beam suspension is one of the key components of an automobile, and mainly balances the up and down bouncing of the left and right wheels to reduce the shaking of the vehicle and keep the vehicle stable. The torsion beam suspension comprises a torsion beam and a bushing, and the bushing is used to connect the torsion beam and the vehicle body. Specifically, the mandrel (or mounting bolt / axle pin) on the vehicle body passes through the middle hole of the bushing, and the torsion beam is connected with the side surface of the bushing. When the vehicle turns, the bushing will be deformed to adapt to the change of the suspension geometry, so that the vehicle has good operation stability and smoothness. However, the conventional bushing design usually only focuses on the connection relationship between the bushing and the torsion beam, and often ignores the connection relationship between the vehicle body and the bushing. Thus, when the vehicle body moves axially relative to the bushing, the curve interval of the axial stiffness of the bushing cannot be adjusted, thereby affecting the dynamic performance of the vehicle. SUMMARY

[0003] The purpose of the present application is to provide a torsion beam hydraulic bushing capable of adjusting the curve interval of the axial stiffness to solve the above problems existing in the prior art.

[0004] The purpose of the present application can be achieved by the following technical scheme: a torsion beam hydraulic bushing comprising: a hydraulic assembly; an elastic assembly arranged at one end of the hydraulic assembly, wherein the elastic assembly comprises a lower cover plate connected with the hydraulic assembly, an upper cover plate arranged on the lower cover plate, a rubber body connecting the upper cover plate and the lower cover plate, a plurality of rigid blocks arranged around the side surface of the rubber body for improving the rigidity of the rubber body, the rigid blocks being connected with the lower cover plate, and a preset distance being arranged between the rigid blocks and the upper cover plate.

[0005] In the above-mentioned torsion beam hydraulic bushing, the vertical section of each rigid block is triangular, and the rigid blocks are integrally formed with the rubber body.

[0006] In the above-mentioned torsion beam hydraulic bushing, the bottom surface of the lower cover plate is surrounded by a plurality of partition plates around the center thereof, and a plurality of positioning grooves are arranged between adjacent two partition plates.

[0007] In the above-mentioned torsion beam hydraulic bushing, the hydraulic assembly comprises an inner tube body, the side surface of the inner tube body is surrounded by a vulcanization body, the outer surface of the vulcanization body is provided with a pair of cavities, the two cavities are communicated through a flow guide body, and the flow guide body is clamped on the outer surface of the vulcanization body.

[0008] In the torsion beam hydraulic bushing, two chambers are symmetrical to the central axis of the inner tube body, the flow guide body is provided with a communication hole communicating with the corresponding chamber, and the outer surface of the flow guide body is provided with a flow channel communicating the two communication holes.

[0009] In the torsion beam hydraulic bushing, the flow guide body is provided with a clamping block at each end, the vulcanization body is provided with a clamping groove for mounting the corresponding clamping block, the flow guide body is provided with a number of protrusions corresponding to the number of chambers, and each protrusion is embedded in the corresponding chamber.

[0010] In the torsion beam hydraulic bushing, the outer surface of the vulcanization body is provided with a pressure relief assembly separating the two chambers, the pressure relief assembly does not contact the flow guide body, the outer surface of the vulcanization body is provided with two channel groups, one of which communicates with one of the chambers, and the other of which communicates with the other chamber, and the two channel groups are located on the two sides of the pressure relief assembly.

[0011] In the torsion beam hydraulic bushing, the pressure relief assembly includes an upper pressure relief sheet and a lower pressure relief sheet, the upper pressure relief sheet is inclined relative to the vulcanization body and faces one of the chambers, and the lower pressure relief sheet is inclined relative to the vulcanization body and faces the other chamber.

[0012] In the torsion beam hydraulic bushing, the side of the upper pressure relief sheet facing the corresponding chamber and the side of the lower pressure relief sheet facing the corresponding chamber are both provided with a support block, and the other side of the upper pressure relief sheet and the other side of the lower pressure relief sheet are both provided with a curved surface.

[0013] In the torsion beam hydraulic bushing, the upper cover plate is provided with a through hole, the end of the vulcanization body facing the lower cover plate is provided with a plurality of positioning blocks, each positioning block is inserted into a corresponding positioning groove, one end of the inner tube body can be inserted into the rubber body, the central axis of the inner tube body is coaxial with the through hole, the outer surface of the vulcanization body is wrapped with an outer sleeve, the edge of one end of the outer sleeve is provided with a surrounding edge, and the surrounding edge presses the lower cover plate in the outer sleeve.

[0014] Compared with the prior art, the present application has the following advantages: In the present application, when the vehicle is in the process of driving and the axle body moves along the central axis of the bushing towards the direction of the lower cover plate with the upper cover plate, the rubber body is deformed under force, the upper cover plate is subjected to the reaction force of the rubber body, and when the upper cover plate extrudes the rigid block, the axial stiffness of the rubber body suddenly increases, that is, the ratio of the reaction force to the distance of the upper cover plate moving downward increases, the stiffness curve becomes steep, enters the nonlinear section, in other words, the upper cover plate is less likely to drop, and the rubber body is less likely to deform, so that the rubber body can brake the moving distance of the axle body more quickly, thereby enhancing the dynamic performance of the vehicle. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a structural schematic diagram of a preferred embodiment of the present application.

[0016] Figure 2 is a structural schematic diagram of the elastic assembly.

[0017] Figure 3 is a combined view of the hydraulic assembly and the sleeve.

[0018] Figure 4 is a structural schematic diagram of the hydraulic assembly.

[0019] Figure 5 is Figure 4 is a structural schematic diagram after removing the flow guide.

[0020] Figure 6 is Figure 5 is a structural schematic diagram from another perspective.

[0021] Figure 7 is Figure 3 is a structural schematic diagram from another perspective.

[0022] Figure 8 is a structural schematic diagram of the flow guide.

[0023] Figure 9 is Figure 1 is a sectional view at A-A. DETAILED DESCRIPTION

[0024] The following is a specific embodiment of the present application and further describes the technical solutions of the present application in combination with the drawings, but the present application is not limited to these embodiments.

[0025] As Figure 1 — Figure 2 and Figure 9 shown, the present application is a torsion beam hydraulic bushing which includes a hydraulic assembly 100, an elastic assembly 200, a lower cover plate 210, an upper cover plate 220, a rubber body 230, and a rigid block 231.

[0026] The elastic assembly 200 is arranged at one end of the hydraulic assembly 100, and the elastic assembly 200 comprises a lower cover plate 210 connected with the hydraulic assembly 100, the lower cover plate 210 is provided with an upper cover plate 220, the upper cover plate 220 and the lower cover plate 210 are connected through a rubber body 230, the side of the rubber body 230 is surrounded by a plurality of rigid blocks 231 for improving the rigidity of the rubber body 230, the rigid blocks 231 are fixedly connected with the lower cover plate 210, and a preset distance 232 is arranged between the rigid blocks 231 and the upper cover plate 220. During installation, the shaft body on the vehicle frame needs to pass through the entire bushing, and the end of the shaft body is abutted against one side of the upper cover plate away from the lower cover plate, so that the hydraulic assembly 100 is installed on the twist beam. When the vehicle is running and the shaft body moves along the central axis of the bushing to the lower cover plate 210 with the upper cover plate 220, the rubber body 230 is deformed under stress, and the upper cover plate 220 is subjected to the reaction force of the rubber body 230. Because the preset distance 232 exists between the upper cover plate 220 and the rigid blocks 231, the upper cover plate 220 will preferentially complete the preset distance 232. In this process, as the displacement increases, the reaction force of the rubber body 230 on the upper cover plate 220 will linearly increase, that is, the reaction force is proportional to the distance of the lower movement of the upper cover plate 220. The rigid blocks 231 are stationary relative to the lower cover plate. When the upper cover plate 220 contacts the top end of the rigid blocks 231, the rigidity of the rubber body 230 is increased due to the rigid blocks 231. Therefore, when the upper cover plate 220 presses the rigid blocks 231, the axial rigidity of the rubber body is suddenly increased, that is, the ratio of the reaction force to the distance of the lower movement of the upper cover plate increases, the rigidity curve becomes steep, and enters the nonlinear section. In other words, the reaction force of the rubber body 230 on the upper cover plate 220 increases, the upper cover plate 220 is less likely to move downward, and the rubber body 230 is less likely to deform. Compared with the case without the rigid blocks 231, the distance of the lower movement of the upper cover plate 220 is reduced, so that the rubber body 230 can more quickly brake the movement distance of the shaft body.

[0027] Further, people can appropriately adjust the height of the rigid blocks 231 according to actual conditions to increase or decrease the value of the preset distance 232, thereby effectively controlling the curve interval of the axial rigidity of the bushing in the movement process of the upper cover plate 220 to the lower cover plate 210.

[0028] The upper cover plate and the lower cover plate are coaxially arranged, and the vertical section of each rigid block 231 is triangular, which passes through the central axis where the upper cover plate is located. Through the shape of the rigid block 231, the stability of the rigid block 231 can be further enhanced, and the rigidity of the rubber body 230 can also be effectively enhanced. The rigid blocks 231 and the rubber body 230 are integrally formed, specifically, the rigid blocks 231 and the rubber body 230 are integrally vulcanized.

[0029] As Figure 2 — Figure 9As shown, the hydraulic assembly 100 comprises an inner tube body 110, the side of which is surrounded by a vulcanized body 120, which is coaxially arranged with the inner tube body 110, and the outer surface of which is provided with a pair of chambers 121, which are symmetrically arranged with the central axis of the inner tube body 110, and are communicated through a flow guide 130, which is clamped on the outer surface of the vulcanized body 120. When the side of the hydraulic assembly 100 is subjected to extrusion force, that is, the side of the vulcanized body 120 is subjected to extrusion force, one of the chambers 121 will be extruded or both chambers 121 will be subjected to different extrusion forces. Since the two chambers 121 are communicated through the flow guide 130, the extruded chamber 121 or the chamber 121 subjected to greater extrusion force will move the liquid in it to the other chamber 121 through the flow guide 130 until the hydraulic pressure in the two chambers 121 continues to balance. In this way, the impact force on the hydraulic assembly 100 can be effectively buffered; further, the inside of the vulcanized body is solid, which can prevent external water from entering the inside of the vulcanized body, effectively preventing the risk of water-related abnormal noise after the bushing is installed on the vehicle.

[0030] The flow guide 130 is provided with a communication hole 131 communicated with the corresponding chamber 121, and the outer surface of the flow guide 130 is provided with a flow channel 132 communicated with the two communication holes 131. When the hydraulic pressure in one of the chambers 121 is greater than that in the other chamber 121, the chamber 121 with greater hydraulic pressure will discharge the liquid in it to the flow channel 132 through the communication hole 131 communicated with the chamber 121. Since the flow channel 132 is communicated with the two communication holes 131, the liquid in the flow channel 132 can flow into the chamber 121 with smaller hydraulic pressure through the other communication hole 131. In this way, the hydraulic pressure of the two chambers 121 can be balanced; further, when designing the communication hole 131, people can change the position of the communication hole 131 according to actual needs, that is, move the communication hole 131 transversely relative to the flow guide 130. In this process, the flow channel 132 is always communicated with the communication hole 131, and the two communication holes 131 are always communicated with the corresponding chambers 121. In this way, the length of the flow channel 132 can be increased or decreased to increase or decrease the resistance of the liquid flowing through the flow guide 130.

[0031] Further, the flow guide 130 is provided with a clamping block 133 at each end, the vulcanizing body 120 is provided with a clamping groove 122 for installing the corresponding clamping block 133, the flow guide 130 is provided with a number of protrusions 134 equal to the number of cavities 121, the protrusions 134 are embedded in the corresponding cavities 121, and each communication hole 131 is located on the corresponding protrusion 134. During installation, the flow guide 130 needs to be clamped on the vulcanizing body 120, specifically, the two protrusions 134 need to be clamped into the corresponding cavities 121, and a certain space is left between the cavity 121 and the protrusion 134 for filling liquid. Then, the clamping block 133 needs to be clamped in the corresponding clamping groove 122. In this way, the flow guide 130 can be clamped on the vulcanizing body 120.

[0032] The outer surface of the vulcanizing body 120 is provided with a pressure relief assembly 123 that separates the two cavities 121, the pressure relief assembly 123 does not contact the flow guide 130, and the outer surface of the vulcanizing body 120 is provided with two channel assemblies 124, one of which is in communication with one of the cavities 121, and the other is in communication with the other cavity 121. The two channel assemblies 124 are located on the two sides of the pressure relief assembly 123, and the pressure relief assembly 123 is located between the two cavities 121. When the hydraulic pressure of one of the cavities 121 exceeds the delivery capacity of the flow guide 130 for liquid, the liquid in the cavity 121 will overflow into the channel assembly 124 that is in communication with the cavity 121. Then, the liquid in the channel assembly 124 will exert pressure on the hydraulic assembly 100, and when the pressure exerted on the pressure relief assembly 123 reaches a predetermined value, the hydraulic assembly 100 is opened, and the liquid in the channel assembly 124 will flow through the pressure relief assembly 123 into the other channel assembly 124, and then flow into the other cavity 121, to quickly balance the two cavities 121. In this way, the pressure relief function of the cavity 121 can be achieved.

[0033] Further, the pressure relief assembly 123 comprises an upper pressure relief sheet 1231 and a lower pressure relief sheet 1232, the upper pressure relief sheet 1231 is arranged obliquely relative to the vulcanizing body 120 and faces one of the chambers 121, the lower pressure relief sheet 1232 is arranged obliquely relative to the vulcanizing body 120 and faces the other chamber 121, each of the channel groups 124 comprises a first channel 1241 and a second channel 1242, the heights of the two first channels 1241 on the vulcanizing body 120 are different, the heights of the two second channels 1242 on the vulcanizing body 120 are different, the height of the first channel 1241 and the height of the second channel 1242 are at the same height, the other first channel 1241 and the other second channel 1242 are at the same height, wherein one of the first channel 1241 and one of the second channel 1242 are respectively located on both sides of the upper pressure relief sheet 1231, the other first channel 1241 and the other second channel 1242 are respectively located on both sides of the lower pressure relief sheet 1232; two bridge blocks 125 are arranged on the vulcanizing body 120, the two bridge blocks 125 are respectively located on both sides of the pressure relief assembly 123 and are connected with the connection positions of the upper pressure relief sheet 1231 and the lower pressure relief sheet 1232, specifically, the two chambers 121 can be respectively named as a first chamber 121 and a second chamber 121, wherein the first chamber 121 is located on the left side of the pressure relief assembly 123, the second chamber 121 is located on the right side of the pressure relief assembly 123, the upper pressure relief sheet 1231 faces the second chamber 121, the lower pressure relief sheet 1232 faces the first chamber 121, the two first channels 1241 are in communication with the first chamber 121, and the two second channels 1242 are in communication with the second chamber 121, therefore, when the hydraulic pressure of the first chamber 121 is greater than the hydraulic pressure of the second chamber 121 and the flow guide 130 cannot guide the flow in time, the liquid in the first chamber 121 flows into the first channel 1241 with a higher height and presses the upper pressure relief sheet 1231, and when the pressing force of the upper pressure relief sheet 1231 reaches the pressure value that the upper pressure relief sheet 1231 can withstand, the upper pressure relief sheet 1231 deforms in the direction of the second chamber 121 due to the oblique arrangement of the upper pressure relief sheet 1231, so that the liquid in the first channel 1241 with a higher height can smoothly pass through the upper pressure relief sheet 1231 and enter the second channel 1242 with a higher height, and then enter the second chamber 121 through the second channel 1242; similarly, when the hydraulic pressure of the second chamber 121 is greater than the hydraulic pressure of the first chamber 121 and the flow guide 130 cannot guide the flow in time, the hydraulic pressure in the second chamber 121 sequentially passes through the second channel 1242 with a lower height, the lower pressure relief sheet 1232, the first channel 1241 with a lower height, and finally enters the first chamber 121, so that the first chamber 121 and the second chamber 121 can be quickly balanced.

[0034] Further, the upper pressure relief piece 1231 and the lower pressure relief piece 1232 are provided with support blocks 126 on the side of the corresponding chamber 121, and the other side of the upper pressure relief piece 1231 and the lower pressure relief piece 1232 are provided with curved surfaces 127. The support blocks 126 can effectively enhance the connection strength between the upper pressure relief piece 1231 and the vulcanized body 120 and between the lower pressure relief piece 1232 and the vulcanized body 120. The curved surfaces 127 can buffer the impact of the liquid on the upper pressure relief piece 1231 or the lower pressure relief piece 1232.

[0035] The upper cover plate 220 is provided with a through hole 221, and one end of the inner tube body 110 can be inserted into the rubber body 230, and the central axis of the inner tube body 110 is coaxially arranged with the through hole 221. The outer surface of the vulcanized body 120 is wrapped with an outer sleeve 140, and the edge of one end of the outer sleeve 140 is provided with a surrounding edge 141. The surrounding edge 141 presses the lower cover plate 210 in the outer sleeve 140. During assembly, the vulcanized body 120 is first installed in the outer sleeve 140. Then, the elastic assembly 200 is positioned on the top of the vulcanized body 120, and the top end of the inner tube body 110 is inserted into the rubber body 230 until the bottom surface of the lower cover plate 210 abuts against the top surface of the vulcanized body 120. At this time, the top end of the outer sleeve 140 exceeds the height of the lower cover plate 210. Then, the top end of the outer sleeve 140 is folded inward by the riveting process. In this way, the lower cover plate 210 is pressed in the outer sleeve 140, so as to connect the elastic assembly 200 and the hydraulic assembly 100 together, and the upper cover plate 220 can move relative to the lower cover plate 210. Secondly, the elastic assembly and the hydraulic assembly are independently designed, so that the height of the rigid block 231 can be adjusted to adjust the axial stiffness of the bushing during the manufacturing process.

[0036] Further, when the vulcanized body 120 is installed in the outer sleeve 140, the upper pressure relief piece 1231 and the lower pressure relief piece 1232 are tightly attached to the outer sleeve 140, so as to prevent the liquid on the left and right sides of the pressure relief assembly 123 from directly passing through the upper pressure relief piece 1231 or the lower pressure relief piece 1232. In addition, the outer side of the flow guide 130 is tightly attached to the inner wall of the outer sleeve 140, so that the liquid in the flow channel 132 cannot overflow through the side of the flow channel 132.

[0037] The bottom surface of the lower cover plate 210 is surrounded by a plurality of partitions 211 around the center thereof, and a plurality of positioning grooves 212 are arranged between two adjacent partitions 211. The vulcanizing body 120 is provided with a plurality of positioning blocks 128 on the end surface thereof facing the lower cover plate 210. When the lower cover plate 210 is placed on the upper end surface of the vulcanizing body 120, each positioning block 128 is inserted into the corresponding positioning groove 212, until the lower cover plate 210 abuts against the end surface of the vulcanizing body 120, so that the elastic assembly 200 can be mounted on the vulcanizing body 120.

[0038] It should be noted that all directional indications, such as upper, lower, left, right, front, rear, etc., are only used to explain the relative position relationship, movement condition, etc. between components in a certain specific posture (as shown in the drawings), and if the specific posture changes, the directional indications will also change accordingly. In addition, the descriptions such as "first", "second", "one" and the like in the present application are only for the purpose of description, and cannot be understood as indicating or implying the relative importance of the technical features indicated, or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0039] In the present application, unless otherwise specifically defined and limited, the terms "connection", "fixing" and the like should be understood in a broad sense, for example, "fixing" can be fixed connection, or detachable connection, or integral; can be mechanical connection, or electrical connection; can be direct connection, or indirect connection through an intermediate medium; can be the internal communication of two elements or the interaction relationship between two elements, unless otherwise specifically limited. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0040] In addition, the technical solutions of each embodiment of the present application can be combined with each other, but it must be based on the fact that a person skilled in the art can realize it, and when the combination of technical solutions appears contradictory or unachievable, it should be considered that the combination of technical solutions does not exist, nor is it within the scope of protection required by the present application.

Claims

1. A torsion beam hydraulic bushing, characterized in that, include: Hydraulic components; An elastic component is disposed at one end of a hydraulic component. The elastic component includes a lower cover plate connected to the hydraulic component. An upper cover plate is provided on the lower cover plate. The upper cover plate and the lower cover plate are connected by a rubber body. A plurality of rigid blocks for improving the rigidity of the rubber body are arranged around the side of the rubber body. The rigid blocks are connected to the lower cover plate. A preset distance is provided between the rigid blocks and the upper cover plate.

2. The torsion beam hydraulic bushing according to claim 1, characterized in that, Each rigid block has a triangular vertical cross-section, and the rigid block is integrally formed with the rubber body.

3. A torsion beam hydraulic bushing according to claim 1, characterized in that, The bottom surface of the lower cover plate is provided with multiple partitions around its center, and multiple positioning grooves are provided between two adjacent partitions.

4. A torsion beam hydraulic bushing according to claim 1, characterized in that, The hydraulic assembly includes an inner tube body, the side of which is surrounded by a vulcanized body. The outer surface of the vulcanized body is provided with a pair of chambers, which are connected by a fluid guide. The fluid guide is fixed to the outer surface of the vulcanized body.

5. A torsion beam hydraulic bushing according to claim 4, characterized in that, The two chambers are symmetrical about the central axis of the inner tube. The guide tube is provided with a connecting hole that communicates with the corresponding chamber. The outer surface of the guide tube is provided with a flow channel that connects the two connecting holes.

6. A torsion beam hydraulic bushing according to claim 5, characterized in that, The guide fluid is provided with a locking block at both ends, the vulcanized body is provided with a locking groove for installing the corresponding locking block, the guide fluid is provided with a number of protrusions the same as the number of chambers, the protrusions are embedded in the corresponding chambers, and each of the connecting holes is located on the corresponding protrusion.

7. A torsion beam hydraulic bushing according to claim 4, characterized in that, The outer surface of the vulcanized body is provided with a pressure relief assembly that separates the two chambers. The pressure relief assembly does not contact the fluid. The outer surface of the vulcanized body is provided with two channel groups, one of which is connected to one of the chambers and the other is connected to the other chamber. The two channel groups are located on both sides of the pressure relief assembly.

8. A torsion beam hydraulic bushing according to claim 7, characterized in that, The pressure relief assembly includes an upper pressure relief plate and a lower pressure relief plate. The upper pressure relief plate is inclined relative to the vulcanized body and faces one of the chambers. The lower pressure relief plate is inclined relative to the vulcanized body and faces the other chamber.

9. A torsion beam hydraulic bushing according to claim 8, characterized in that, Support blocks are provided between the side of the upper pressure relief plate facing the corresponding chamber and the vulcanized body, and between the side of the lower pressure relief plate facing the corresponding chamber and the vulcanized body. Curved surfaces are provided between the other side of the upper pressure relief plate and the vulcanized body, and between the other side of the lower pressure relief plate and the vulcanized body.

10. A torsion beam hydraulic bushing according to claim 4, characterized in that, The upper cover plate has a perforation, and the vulcanized body has multiple positioning blocks on one end face facing the lower cover plate. Each positioning block is inserted into a corresponding positioning groove. One end of the inner tube is inserted into the rubber body, and the central axis of the inner tube is coaxial with the perforation. The outer surface of the vulcanized body is covered with an outer sleeve, and a rim is provided at one edge of the outer sleeve. The rim presses the lower cover plate inside the outer sleeve.

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