Drive device, liquid ejection head

Abstract

To provide a driving device capable of suppressing variation of contact resistance and stabilizing a driving operation when electricity is conducted via a conductive elastic member.SOLUTION: A driving device of an embodiment comprises: an operation unit; a driving unit; a conductive housing; a conductive reference member; and a conductive elastic member. A driving element is disposed in the operation unit. The driving unit includes an electronic component for driving the driving element. The conductive housing stores the driving unit. The conductive reference member is spaced apart from the housing. The conductive reference member defines a positional relationship with the operation unit. The conductive elastic member biases a first part to the reference member, and a second part to the housing. The reference member and the housing are conducted to each other via the elastic member, and then a current pulse is applied.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] Embodiments of the present invention relate to a drive device and a liquid ejection head.

Background Art

[0002] Liquid ejection heads that eject liquid are known. Liquid ejection heads are mounted, for example, on inkjet printers, 3D printers, dispensing devices, and the like. An inkjet printer ejects ink droplets from an inkjet head to form an image or the like on the surface of a recording medium. A 3D printer ejects droplets of a modeling material from a modeling material ejection head and cures them to form a three-dimensional object. A dispensing device ejects droplets of a sample and supplies a predetermined amount to a plurality of containers or the like.

[0003] An inkjet head drives an actuator to eject ink from a nozzle. A drive circuit including a drive IC and electronic components that drive the actuator is housed in a conductive housing of the inkjet head. The housing serves as a heat sink that releases heat from electronic components and the like that generate heat during driving, and also serves as a shield box that suppresses the emission of radiation noise.

[0004] The inkjet head includes a reference plate that defines the positional relationship with the nozzle. The reference plate also serves as a fixing member when attaching the inkjet head to an inkjet printer. Therefore, if the reference plate is fixed to the inkjet printer, the nozzle is located at a predetermined coordinate position within the printer.

[0005] The inkjet head electrically connects the housing and the reference plate via a conductive member so that it can discharge to the main body of the inkjet printer when static electricity is applied. However, there is a concern that if a force is applied to push the reference plate through the conductive member, the positional relationship between the nozzle and the reference plate may change. Therefore, a conductive member having elasticity such as a spring is used, but if an oil film, an oxide film, or the like intervenes at the contact portion between the spring and the housing or between the spring and the reference plate, the contact resistance increases. As a result, the discharge path of static electricity and the state of radiation noise vary from inkjet head to inkjet head, and the printing quality may not be stable.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] The problem to be solved by the present invention is to provide a drive device and a liquid ejection head that can suppress variations in contact resistance when a conductive elastic member is energized and conduct, and stabilize the drive operation.

Means for Solving the Problems

[0008] An embodiment of the present invention Liquid ejection head includes an operating unit, a driving unit, a conductive housing, a conductive reference member, and a conductive elastic member. Nozzle for ejecting liquid and actuator is arranged in the operating unit. The driving unit drives the Actuator IC (Integrated Circuit)It includes. The conductive housing houses the drive unit. The conductive reference member is arranged separately from the housing. The conductive reference member It is a fixing member when attaching to the device and the of the nozzle position Coordinates defines Do . The conductive elastic member has a first part biased against the reference member and a second part biased against the housing. The housing, the reference member, and the elastic member are the elastic member By which the housing and the reference member are is conductive Doing , Connect the current pulse source in the assembly process and between the housing and the reference member a current pulse Is is applied Done.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Modes for Carrying Out the Invention

[0010] Hereinafter, a drive device and a liquid ejection head according to an embodiment will be described in detail with reference to the accompanying drawings. In each figure, the same components are denoted by the same reference numerals.

[0011] A driving device according to an embodiment will be described in detail using a liquid ejection head as an example. The liquid ejection head is, for example, an inkjet head mounted on an inkjet printer 10 that forms an image on a recording medium. FIG. 1 shows a schematic configuration of the inkjet printer 10. The inkjet printer 10 includes, inside the housing 11 of the inkjet printer, a cassette 12 that stores a sheet S, which is an example of a recording medium, an upstream conveyance path 13 of the sheet S, a conveyance belt 14 that conveys the sheet S taken out from the cassette 12, a plurality of inkjet heads 100 to 103 that eject ink droplets toward the sheet S on the conveyance belt 14, a downstream conveyance path 15 of the sheet S, a discharge tray 16, and a control board 17. An operation unit 18, which is a user interface, is arranged on the upper side of the housing 11 of the inkjet printer.

[0012] Image data to be printed on the sheet S is generated, for example, by a computer 200, which is an external connection device. The image data generated by the computer 200 is sent to the control board 17 of the inkjet printer 10 through a cable 201 and connectors 202, 203.

[0013] A pickup roller 204 supplies the sheets S from the cassette 12 to the upstream conveyance path 13 one by one. The upstream conveyance path 13 is composed of a pair of feed rollers 131, 132 and sheet guide plates 133, 134. The sheet S is sent to the upper surface of the conveyance belt 14 via the upstream conveyance path 13. The arrow 104 in the figure indicates the conveyance path of the sheet S from the cassette 12 to the conveyance belt 14.

[0014] The conveying belt 14 is a mesh endless belt with a large number of through holes formed on its surface. Three rollers, namely the driving roller 141, and the driven rollers 142 and 143, rotatably support the conveying belt 14. The motor 205 rotates the conveying belt 14 by rotating the driving roller 141. In the figure, 105 indicates the rotation direction of the conveying belt 14. A negative pressure container 206 is arranged on the back surface side of the conveying belt 14. The negative pressure container 206 is connected to a decompression fan 207. The fan 207 creates a negative pressure inside the negative pressure container 206 by the airflow formed, and adsorbs and holds the sheet S on the upper surface of the conveying belt 14. In the figure, 106 indicates the flow of the airflow.

[0015] The inkjet heads 100 to 103, which are examples of liquid ejection heads, are arranged to face the sheet S adsorbed and held on the conveying belt 14 with a slight gap of, for example, 1 mm therebetween. The inkjet heads 100 to 103 eject ink droplets toward the sheet S respectively. The inkjet heads 100 to 103 print an image when the sheet S passes below. Each of the inkjet heads 100 to 103 has the same structure except that the color of the ejected ink is different. The colors of the ink are, for example, cyan, magenta, yellow, and black.

[0016] The inkjet heads 100 to 103 are respectively connected to ink tanks 315 to 318 and ink supply pressure adjusting devices 321 to 324 via ink flow paths 311 to 314. Each of the ink tanks 315 to 318 is arranged above each of the inkjet heads 100 to 103. During standby, each of the ink supply pressure adjusting devices 321 to 324 adjusts the pressure inside each of the inkjet heads 100 to 103 to a negative pressure with respect to the atmospheric pressure, for example, -1.2 kPa, so that ink does not leak from the nozzles 23 (see Figure 2) of the inkjet heads 100 to 103. During image formation, the ink in each of the ink tanks 315 to 318 is supplied to each of the inkjet heads 100 to 103 by the ink supply pressure adjusting devices 321 to 324.

[0017] After image formation, the sheet S is sent from the conveyance belt 14 to the downstream conveyance path 15. The downstream conveyance path 15 is composed of the feed roller pairs 151, 152, 153, 154 and the sheet guide plates 155, 156 that define the conveyance path of the sheet S. The sheet S is sent from the discharge port 157 to the discharge tray 16 via the downstream conveyance path 15. The arrow 107 in the figure indicates the conveyance path of the sheet S.

[0018] Subsequently, the configurations of the inkjet heads 100 to 103 will be described. The following describes the inkjet head 100 with reference to FIGS. 2 to 5, but the inkjet heads 101 to 103 also have the same structure as the inkjet head 100.

[0019] FIG. 2 is a perspective view of the inkjet head 100. FIG. 3 is an internal configuration diagram of the inkjet head 100. FIG. 4 is a side view of the inkjet head 100 viewed in the X direction. FIG. 5 is a configuration diagram of the actuator and control system that are driven when ink is ejected. As shown in FIGS. 2 to 4, the inkjet head 100 includes a head portion 2 which is an example of an operating portion. The head portion 2 includes a nozzle plate 21 and an actuator substrate 22. The nozzle plate 21 is a rectangular plate formed of a resin such as polyimide or a metal such as stainless steel. The nozzles 23 for ejecting ink are arranged along the longitudinal direction (X direction) of the nozzle plate 21. The nozzle density is set, for example, within the range of 150 to 1200 dpi. The nozzles 23 are not limited to one row and may be arranged in two or more rows. Note that FIGS. 2 and 3 illustrate a reduced number of nozzles 23 for convenience of drawing.

[0020] The actuator substrate 22 is a rectangular substrate formed of, for example, insulating ceramics. The actuator 4 that operates when ink is ejected is arranged on the actuator substrate 22 (see FIG. 5). The detailed configuration of the actuator 4 will be described later. The actuator 4 is an example of a driving element arranged in the operating portion.

[0021] The actuator substrate 22 is disposed on one surface of an ink supply manifold 24 formed, for example, in a rectangular shape. The actuator substrate 22 and the ink supply manifold 24 are fixed, for example, with an adhesive or the like. The ink supply manifold 24 is connected to the ink supply pressure adjusting device 321 in FIG. 1 via an ink flow path 311. When ink is circulated and supplied to the ink supply manifold 24, two ink flow paths, a supply flow path and a discharge flow path, are connected to the ink supply manifold 24.

[0022] The ink supply manifold 24 is formed of a material having, for example, insulation properties. The material having insulation properties is, for example, a resin. The ink supply manifold 24 forms a common ink chamber (not shown) inside. The ink ejected from the nozzles 23 is supplied from the common ink chamber.

[0023] Particularly as shown in FIG. 3, the actuator substrate 22 is connected to a printed circuit board 31 via a flexible printed wiring board 3. The printed circuit board 31 is further connected to an FPC (Flexible Printed Circuits) cable 32. The other end side of the FPC cable 32 is electrically connected to the control board 17 of the inkjet printer 10. The flexible printed wiring board 3 mounts a driving IC (Integrated Circuit) 33 which is a driver chip (hereinafter referred to as a driving IC). A preferred example of the flexible printed wiring board 3 is a COF (Chip on Film). The printed circuit board 31 is, for example, a rigid through-hole board in which a glass fiber-containing epoxy resin layer and a copper wiring layer are laminated multiple times. The print data sent from the control board 17 of the inkjet printer 10 is input to the printed circuit board 31 via the FPC cable 32. The driving IC 33 temporarily stores the print data sent via the printed circuit board 31 and outputs a driving signal for ejecting ink at a predetermined timing to apply it to the actuator 4.

[0024] Here, the configuration and control system of the actuator 4 will be described with reference to FIG. 5. As shown in FIG. 5, a plurality of pressure chambers 41 and air chambers 42 are alternately arranged on the actuator substrate 22 along a first direction, for example, the X direction. The pressure chamber 41 communicates with the nozzle 23. The pressure chamber 41 communicates with a common ink chamber (not shown) of the ink supply manifold 24. On the other hand, the air chamber 42 arranged adjacent to the pressure chamber 41 is, for example, a closed space that does not communicate with the nozzle 23 and the common ink chamber (not shown).

[0025] The pressure chamber 41 and the air chamber 42 are formed by notching two piezoelectric members 43 and 44 laminated on the actuator substrate 22 in a direction where the polarization directions are opposite (for example, the opposing direction) along a second direction, for example, the Y direction, in a rectangular groove shape. That is, between the pressure chamber 41 and the air chamber 42, the piezoelectric members 43 and 44 are used as side walls for partitioning.

[0026] The electrode 45 is integrally formed on the bottom surface and both side surfaces of the pressure chamber 41 notched in a groove shape. The electrode 45 of the pressure chamber 41 is connected to an individual wiring 46 which is a wiring electrode. The electrode 47 is integrally formed on the bottom surface and both side surfaces of the air chamber 42. The electrode 47 of the air chamber 42 is connected to a common wiring 48 which is a wiring electrode. The electrodes 45 and 47 are formed of, for example, a nickel thin film.

[0027] The individual wiring 46 is connected to the driver D (i.e., the drive circuit) of the drive IC 33 mounted on the flexible printed wiring board 3. The drive IC 33 applies a drive voltage V1 as a drive signal to the actuator 4 of the channels (#1ch, #2ch ··· #Nch) that eject ink. On the other hand, the common wiring 48 is connected to, for example, ground (GND). With this configuration, the actuator 4 to which the drive voltage V1 (for example, a positive voltage) is applied has an electric field applied in a direction that intersects (preferably, is orthogonal to) the polarization axis of the piezoelectric members 43, 44, and the side walls on both sides in the X direction of the pressure chamber 41 deform symmetrically in the X direction in a shear mode. That is, the volume of the pressure chamber 41 expands, and ink is supplied from the ink supply manifold 24. Next, when a ground (for example, 0V) voltage is applied to the actuator 4, the side walls on both sides in the X direction return to the state before deformation in a shear mode. That is, the volume of the pressure chamber 41 contracts, the ink pressure increases, and ink droplets are ejected from the nozzle 23.

[0028] The flexible printed wiring board 3 and the printed circuit board 31 are an example of a drive unit. In particular, as shown in FIGS. 3 and 4, the housing 5 houses the flexible printed wiring board 3 and the printed circuit board 31 inside. The housing 5 is formed of a material having conductivity such as metal, for example. An example of the metal is aluminum. The housing 5 has a configuration that can be opened and closed by joining a first box-shaped member 51 and a second box-shaped member 52 so as to face each other. The first box-shaped member 51 and the second box-shaped member 52 are fixed by fixing members 53 arranged at the four corners, for example. The fixing member 53 is, for example, a screw. By forming the fixing member 53 of a material having conductivity such as metal, the electrical continuity between the first box-shaped member 51 and the second box-shaped member 52 is ensured, and the contact resistance of the contact surface is reduced.

[0029] The housing 5 is connected to the ground (GND) through the circuit of the printed circuit board 31 housed inside it. As an example, a conductive fixing member 54 such as metal conducts the inner peripheral surface of the housing 5 and, for example, the common wiring 48 of the printed circuit board 31 (see FIG. 5). The fixing member 54 is, for example, a screw. The common wiring 48 of the printed circuit board 31 is connected to the ground through the FPC cable 32. In this way, by surrounding the flexible printed wiring board 3, which is an example of the driving unit, and the printed circuit board 31 with the conductive housing 5 and further connecting a part of the inner peripheral surface of the housing 5 to the ground of the circuit housed inside, the emission of radiated noise can be suppressed. That is, the housing 5 functions as a shield box that suppresses the emission of radiated noise.

[0030] The housing 5 is formed with openings 55 and 56 at both end faces in the third direction, for example, the Z direction. The flexible printed wiring board 3 is connected to the actuator substrate 22 through the opening 55. The FPC cable 32 is drawn out from inside the housing 5 through the opening 56.

[0031] The housing 5 is fixed to one surface of the ink supply manifold 24 via an insulating member 25, for example, with a fixing member 57. The fixing member 57 is, for example, a screw. The insulating member 25 is, for example, a plate-shaped rubber. The ink supply manifold 24 has a metal plate 27 disposed via an insulating member 26 on the surface opposite to the side connecting to the housing 5. The insulating member 26 is, for example, a plate-shaped rubber. The metal plate 27 is, for example, an aluminum plate. The fixing member 57 integrally fixes the housing 5, the insulating member 25, the ink supply manifold 24, the insulating member 26, and the metal plate 27.

[0032] The reference plate 6, which is an example of a reference member, is disposed on one surface of the ink supply manifold 24 on the side opposite to the nozzle plate 21, and is fixed, for example, with an adhesive or the like. The reference plate 6 is aligned so that the positional relationship of the three-dimensional coordinates with the nozzle 23 becomes a predetermined positional relationship. That is, when assembling the inkjet head 100, the nozzle 23 and the reference plate 6 are aligned with each other so as to have a predetermined positional relationship set in advance. Since a plurality of nozzles 23 are formed on the nozzle plate 21, for example, alignment may be performed with the nozzle 23 used as a reference among them. The reference plate 6 also serves as a fixing member when attaching the inkjet head 100 to the inkjet printer 10. Therefore, if the reference plate 6 is attached to the inkjet printer 10, the nozzle 23 can be positioned at a predetermined coordinate position. Thereby, it is possible to suppress a decrease in printing quality due to misalignment of the nozzle 23.

[0033] The reference plate 6 is formed of a material having conductivity such as metal. An example of the metal is stainless steel. The reference plate 6 is disposed at a distance so as not to contact the housing 5. That is, for example, in the manufacturing process of the inkjet head 100, when attaching the housing 5, the reference plate 6 is pushed to prevent the positional relationship between the nozzle 23 and the reference plate 6 from shifting. Although the reference plate 6 and the housing 5 are arranged so as not to be in direct contact, the reference plate 6 and the housing 5 are electrically connected via a conductive elastic member so that even if static electricity is applied to the inkjet head 100, it can be discharged to the main body of the inkjet printer 10. A preferred example of the conductive elastic member is the coil spring 61. The material of the coil spring 61 is, for example, phosphor bronze (C5191). The coil spring 61 is disposed, for example, in a guide hole 62 formed in the second box-shaped member 52. The guide hole 62 is formed in the outer peripheral wall of the second box-shaped member 52 facing one surface of the reference plate 6. The guide hole 62 penetrates the outer peripheral wall of the second box-shaped member 52 in the second direction, for example, the Y direction. The coil spring 61 is disposed in a compressed state in the guide hole 62. That is, the coil spring 61 in the compressed state biases the reference plate 6 at one end side, which is the first part, and biases the housing 5 (the first box-shaped member 51) at the other end side, which is the second part.

[0034] The guide hole 62 is, for example, cylindrical. The size of the guide hole 62 is, for example, a diameter of 2 mm and a length of 5 to 6 mm. As the coil spring 61, one with a diameter smaller than that of the guide hole 62 and a length longer than the length of the guide hole 62 in the Y direction is used. Thereby, the coil spring 61 is in a compressed state within the guide hole 62. The coil spring 61 biases the reference plate 6 and the housing 5 respectively by its restoring action, and conducts the reference plate 6 and the housing 5. However, if a coil spring 61 with a large elastic force is adopted, the force pushing the reference plate 6 becomes stronger. Therefore, a coil spring 61 with as small an elastic force as possible is adopted. For this reason, if an oil film, an oxide film, or the like intervenes at the contact portion between the coil spring 61 and the housing 5, or between the coil spring 61 and the reference plate 6, the contact resistance changes accordingly. That is, variations in contact resistance are likely to occur for each inkjet head 100. Therefore, in this embodiment, after the housing 5 and the reference plate 6 are conducted through the coil spring 61 in the manufacturing process of the inkjet head 100, a current pulse is applied.

[0035] FIG. 6 shows the assembly process of the inkjet head 100. FIG. 7 is a configuration diagram when a current pulse is applied between the housing 5 and the reference plate 6. As shown in FIG. 6(a), the prefabricated head portion 2, the ink supply manifold 24, the reference plate 6, and the ink flow path 311 are fixed to each other. At this time, alignment is performed so that the positional relationship between the nozzle 23 and the reference plate 6 becomes the predetermined positional relationship, and then they are fixed. Subsequently, as shown in FIG. 6(b), the second box-shaped member 52 of the housing 5 is fixed to the ink supply manifold 24 with the fixing member 57. The flexible printed wiring board 3 and the printed circuit board 31 are arranged in advance within the second box-shaped member 52, and are fixed to the second box-shaped member 52 with, for example, a fixing member 54 that connects the housing 5 to the ground. Subsequently, as shown in FIG. 6(c), the coil spring 61 is placed into the guide hole 62, and the first box-shaped member 51 is used as a lid. The coil spring 61 is pushed by the first box-shaped member 51 and is in a compressed state. Thereby, the housing 5 and the reference plate 6 are conducted through the coil spring 61. Subsequently, the first box-shaped member 51 and the second box-shaped member 52 are fixed with the fixing member 53.

[0036] When the inkjet head 100 is thus assembled, as shown in FIG. 7, the current pulse source 7 and the voltmeter 71 are connected to the inkjet head 100. For the sake of convenience in drawing, FIG. 7 omits a part of the configuration of the inkjet head 100. The current pulse source 7 electrically connects, for example, the positive pole to the fixing member 53 of the housing 5 and the negative pole to the reference plate 6. Similarly, the voltmeter 71 electrically connects, for example, the positive pole to the fixing member 53 of the housing 5 and the negative pole to the reference plate 6.

[0037] The current pulse source 7 applies a current pulse between the housing 5 and the reference plate 6. The voltmeter 71 measures the voltage between the housing 5 and the reference plate 6 when the current pulse is being applied. That is, the resistance value between the housing 5 and the reference plate 6 is obtained. The application of the current pulse is to apply a constant current pulse a plurality of times at regular intervals. The current to be applied is selected as a current value that can break the oil film or oxide film. For example, it is set to 1 A as the current value for breaking the oil film or oxide film. As a preferred example, a current pulse is applied with an amplitude of 1 A, a pulse width of 1 second, and 5 cycles at 2-second intervals. Thereby, even if there are an oil film or an oxide film at the contact portion between the coil spring 61 and the housing 5 and at the contact portion between the coil spring 61 and the reference plate 6, they can be broken by passing a large current.

[0038] FIG. 8(a) shows the respective resistance values when six inkjet heads 100 are manufactured and a current pulse is applied with an amplitude of 1 A, a pulse width of 1 second, and 5 cycles at 2-second intervals. In this way, by applying a predetermined current pulse, the variation in the resistance value becomes small (σ = 0.05). On the other hand, FIG. 8(b) shows the respective resistance values when six inkjet heads 100 are manufactured and a current pulse is applied with an amplitude of 10 mA, a pulse width of 1 second, and 5 cycles at 2-second intervals. In this case, since the oil film and the oxide film cannot be broken and there is a variation in the resistance value (σ = 0.30), it is desirable to increase the current value to reduce the variation in the resistance value. The current value is preferably 1 A, but may be 1 A or more. When the current pulse is applied, there may also be welding marks left at the contact portion between the coil spring 61 and the housing 5 and at the contact portion between the coil spring 61 and the reference plate 6.

[0039] According to the above-described embodiment, after the housing 5 and the reference plate 6 are electrically connected via the coil spring 61, by applying a current pulse, it is possible to suppress the influence of an oil film, an oxide film, etc. on the contact resistance. As a result, it is possible to suppress variations in the electrostatic discharge path and the state of radiation noise for each inkjet head 100, and the printing quality is stabilized.

[0040] Although the coil spring 61 has been cited as a preferable example of the conductive elastic member, as a modification, other elastic members such as a leaf spring may be used.

[0041] The inkjet head 100 is not limited to the shear mode actuator 4 in which the pressure chambers 41 and the air chambers 42 are alternately arranged. It may be a shear mode - shared wall actuator 4 in which the pressure chambers 41 are continuously arranged. Further, it may be a drop - on - demand piezo actuator or the like.

[0042] Further, in the above - described embodiment, the inkjet head 100 of the inkjet printer 10 has been described as an example of the liquid ejection head, but the liquid ejection head may be a modeling material ejection head of a 3D printer or a sample ejection head of a dispensing device.

[0043] The embodiments of the present invention are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope.

Explanation of Reference Numerals

[0044] 10 Inkjet printer 100 - 103 Inkjet head 22 Actuator substrate 23 Nozzle 3 Flexible printed wiring board 31 Printed circuit board 33 IC 4 Actuator 5 Housing 6 Reference plate 7 Current pulse source 71 Voltmeter D Driver (drive circuit)

Claims

1. A liquid ejection head comprising: a nozzle for ejecting a liquid; an operating unit in which an actuator is disposed; a driving unit including an IC (Integrated Circuit) for driving the actuator; a conductive housing for housing the driving unit; a conductive reference member which is disposed separately from the housing and is a fixing member for attachment to an apparatus and defines the position coordinates of the nozzle; and a conductive elastic member having a first portion biased against the housing and a second portion biased against the reference member. The housing, the reference member, and the elastic member are electrically connected by the elastic member, and a current pulse is applied between the housing and the reference member by connecting a current pulse source in an assembly process.

2. The application of the current pulse is characterized in that a pulse of constant current is applied a plurality of times at regular intervals, as claimed in claim 1.

3. The application of the current pulse is a current value that breaks an oil film and an oxide film at a contact portion between the elastic member and the housing and / or between the elastic member and the reference member, as claimed in claim 1 or 2.

4. The elastic member is housed in a guide hole formed in a surface of the housing facing the reference member in a compressed state, as claimed in any one of claims 1 to 3.

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