Actuator

Abstract

The present invention provides an actuator including: a housing having an igniter housing and a cylinder, an igniter accommodated inside the igniter housing and a piston provided inside the cylinder to be slidable, the piston having a rod and an enlarged surface portion formed on one end surface or an outer peripheral surface of the rod, and part of the piston, which includes the other end of the rod, protruding outside the housing before activation, the igniter disposed such that the central axis direction of the igniter and the central axis direction of the rod of the piston are different from each other, wherein, during the activation of the igniter, the piston slides inside the cylinder by pushing the enlarged surface portion by a combustion product generated due to activation of the igniter, and the protruding portion of the piston is retracted inside the housing due to the sliding of the piston.

Description

[0001] This nonprovisional application claims priority under 35 U.S.C. .sctn. 119(a) on Patent Application No. 2006-110760 filed in Japan on 13 Apr. 2006 and Patent Application No. 2007-028154 filed in Japan on 7 Feb. 2007, and 35 U.S.C. .sctn. 119(e) U.S. Provisional Application No. 60/792,625 filed on 18 Apr. 2006, which are incorporated by reference.

BACKGROUND OF INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to an actuator that can be used in a human body restraining device of an automotive vehicle such as a steering retractor or an engine hood lifting device.

[0004] 2. Description of Related Art

[0005] In addition to air bag type restraining devices used in passenger restraining devices and pedestrian protection devices for vehicles, devices for lifting the hood of a vehicle during a collision to protect pedestrians and devices for retracting the steering wheel (toward the front of the vehicle), for example, also exist.

[0006] In these devices, a pyrotechnic actuator is used, and known actuators include a type that ejects a pin upon activation and a type (known as a pin puller) that retracts a protruding pin inward upon activation. A pin puller is structured such that prior to activation, a tip end portion of the pin protrudes from a cylinder (housing), and upon activation, the pin is retracted into the interior of the cylinder (housing), and includes an igniter for moving the pin. The igniter is disposed parallel or at a right angle to the movement direction of the pin.

[0007] U.S. Pat. No. 4,412,420 discloses a pin puller type actuator. In an actuator shown in FIG. 1, a bore 14 accommodating a piston P is formed in a housing H serving as a cylinder, and the piston P is provided to slide within the bore 14. A cartridge C is screwed into a screw portion formed in the housing H and disposed at a right angle to the axis of the piston P.

[0008] With the actuator structure shown in FIG. 1, an annular space 27 is formed around the piston P, and gas generated from the cartridge C is charged into the annular space 27, thereby raising the pressure in the interior of the annular space 27 such that the piston P moves in a leftward direction of FIG. 1. As a result, a right end portion of the piston P is retracted into the interior of the housing H.

SUMMARY OF INVENTION

[0009] The present invention provides an actuator including:

[0010] a housing having an igniter housing and a cylinder,

[0011] an igniter accommodated inside the igniter housing and a piston provided inside the cylinder to be slidable,

[0012] the piston having a rod and an enlarged surface portion formed on one end surface or an outer peripheral surface of the rod, and part of the piston, which includes the other end of the rod, protruding outside the housing before activation,

[0013] the igniter disposed such that the central axis direction of the igniter and the central axis direction of the rod of the piston are different from each other,

[0014] wherein, during the activation of the igniter, the piston slides inside the cylinder by pushing the enlarged surface portion by a combustion product generated due to activation of the igniter, and the protruding portion of the piston is retracted inside the housing due to the sliding of the piston.

BRIEF DESCRIPTION OF DRAWINGS

[0015] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein:

[0016] FIG. 1 shows a longitudinal sectional view of an actuator;

[0017] FIG. 2 shows a longitudinal sectional view of an actuator according to another embodiment;

[0018] FIG. 3 shows a longitudinal sectional view of an actuator according to another embodiment; and

[0019] FIG. 4 shows a longitudinal sectional view of an actuator according to another embodiment.

DETAILED DESCRIPTION OF INVENTION

[0020] The present invention provides a pin puller type actuator which retracts a piston, which is capable of retracting the piston reliably without damaging the piston.

[0021] In the actuator according to the present invention, upon activation of the igniter, combustion products (a flame, high-temperature gas, a shock wave, and so on) generated when the igniter is activated collide with and push the enlarged surface portion of the piston, but do not collide with the rod. The enlarged surface portion is disposed in the flow direction of the combustion products generated by the igniter.

[0022] In the actuator of the present invention, the igniter is disposed such that the central axis direction of the igniter and the central axis direction of the piston rod are different (preferably orthogonal) to each other, and therefore, as described above, the combustion products can be caused to collide with the enlarged surface portion of the piston by guiding the combustion products generated upon activation of the igniter to the enlarged surface portion of the piston. A method of forming a combustion product guidance passage in the housing or a method of providing an opening portion, in the direction of the enlarged surface portion, in the igniter housing accommodating the igniter, for example, may be applied to guide the combustion products to the enlarged surface portion of the piston.

[0023] In so doing, the shock wave is prevented from colliding directly with the rod, as occurs in the invention of U.S. Pat. No. 4,412,420, and instead the combustion products collide with the enlarged surface portion. Hence, the entire piston is retracted into the housing reliably without damaging the rod. Moreover, in comparison with a case in which the igniter and piston are disposed such that respective central axis directions thereof are parallel to each other, the entire actuator can be reduced in size. Note that the entire piston does not have to be retracted into the housing, and only a part thereof need be retracted.

[0024] The enlarged surface portion of the piston is formed integrally with the rod end surface, and there are no particular limitations on the shape and size thereof. However, a plane or curved surface having a surface area that is equal to or greater than at least the cross-section or end surface of the rod is preferably employed.

[0025] The igniter may employ a well-known electric igniter used widely in air bag gas generators and the like (using an ignition charge including zirconium and potassium perchlorate), and where necessary, an auxiliary charge may also be used.

[0026] The present invention further provides an actuator being the above shown invention actuator, wherein the combustion product generated when the igniter is activated changes direction by impinging on a collision surface portion provided in the housing, and then pushes the enlarged surface portion.

[0027] The collision surface portion is disposed in a position facing (preferably directly facing) the ejecting direction of the combustion products generated from an activation portion upon activation of the igniter, and therefore the combustion products collide with the collision surface portion, change direction, and then collide with the enlarged surface portion. Thus, the combustion products are caused to collide with the enlarged surface portion reliably such that the protruding part of the piston can be retracted.

[0028] The collision surface portion may be a wall surface separating the igniter housing from the cylinder. When the collision surface portion is a wall surface separating the igniter housing from the cylinder, the wall surface may form part of a wall surface of the cylinder. Further, the collision surface portion may be a cap member disposed to surround the activation portion of the igniter, which is capable of ejecting the combustion products in the direction of the enlarged surface portion.

[0029] Furthermore, by having the collision surface portion form a closed space including the igniter together with other members (for example, the igniter housing, the cylinder, and so on) including the enlarged surface portion, all of the combustion products generated upon activation of the igniter can be used to push the enlarged surface portion, which is preferable in terms of efficiency.

[0030] Here, the term "closed space" may signify a sealed space formed by press-contacting separate members with each other, or a state in which separate members abuts against each other or are disposed with a slight gap therebetween. In other words, any closed space capable of ensuring by its existence that the combustion products generated upon activation of the igniter can be caused to collide with the enlarged surface portion reliably without escaping (without entering a gap between the piston and cylinder, for example) may be used.

[0031] Note that when the cap member is used as the collision surface portion, a cap member having an opening or a fragile portion in the ejecting direction of the combustion products may be used. The fragile portion is a part that is formed thinly or has a notch inserted therein so that it ruptures and opens easily upon reception of the pressure of the combustion products.

[0032] The present invention further provides the actuator being the above shown invention actuator, wherein the enlarged surface portion of the piston is a portion which extends from the one end surface of the rod or an axial central portion of the rod either unidirectionally outward in the radial direction of the rod or in flange form.

[0033] The shape and size of the enlarged surface portion may be set appropriately in accordance with the structure of the actuator. For example, when the actuator is to be made as small as possible, an enlarged surface portion that extends unidirectionally outward in the radial direction of the rod from the one end surface or the central portion (including the central portion and its vicinity) of the rod may be used, and when the piston is large (having large mass), an enlarged surface portion that extends in flange form around the periphery of the one end surface or the central portion (including the central portion and its vicinity) of the rod may be used, whereby the enlarged surface portion is increased in size and the piston is retracted with greater force.

[0034] With the actuator of the present invention, the combustion products including a shock wave that are generated upon activation of the igniter collide with the enlarged surface portion rather than colliding directly with the piston rod, and therefore the piston can be retracted into the cylinder reliably without damaging the piston rod.

[0035] The invention actuator can be used in a human body restraining device of an automotive vehicle, such as a steering retractor or an engine hood lifting device.

Embodiment of Invention

(1) Actuator of FIG. 1

[0036] FIG. 1 is a cross-sectional view of an actuator.

[0037] An outer shell of an actuator 10 is formed by a housing 11 by combining an igniter housing 11a and a cylinder 11b. The igniter housing 11a and cylinder 11b share a part of the outer shell member. The housing 11 (igniter housing 11a and cylinder 11b) is made of metal, and the overall shape and size thereof are determined according to the attachment subject and attachment position of the actuator 10.

[0038] An igniter accommodating space 12 is formed in the interior of the igniter housing 11a. The igniter accommodating space 12 is formed, in advance, with an interior shape that corresponds to the exterior shape of an electric igniter 14 serving as an attachment subject. The electric igniter 14 is fitted into the igniter accommodating space 12 through a first opening portion 21 of the housing and then fixed by crimping a first opening peripheral edge portion 21a.

[0039] An activation portion 14a of the igniter is positioned within a closed space 12a remaining after the igniter 14 is fitted, and does not contact an inner wall surface of the housing 11. The closed space 12a serves as a passage for combustion products generated by activation of the igniter 14. 19 is an O-ring for maintaining airtightness.

[0040] A metal piston 15 having an L-shaped axial cross-section is accommodated slidably in a cylinder sliding space 13 inside the cylinder 11b. The piston 15 has a rod 16 and an enlarged surface portion 17 formed on an one end surface 16a of the rod 16, and a part thereof including the other end surface 16b of the rod protrudes to the outside of the housing 11 (cylinder 11b). The protruding portion including the other end surface 16b is retracted into the cylinder 11b upon activation of the actuator 10, and a length (L.sub.1) thereof is determined in accordance with the operating performance required by the attachment subject.

[0041] The enlarged surface portion 17 is a surface enlarged unidirectionally from the one end surface 16a of the rod 16 in a perpendicular direction to the axial direction of the rod 16 (i.e. in a radial direction), and has a surface area that is equal to or greater than the surface area of the one end surface 16a or the other end surface 16b of the rod 16. Annular grooves are formed in the circumferential direction in the rod 16 and the enlarged surface portion 17 respectively, and O-rings 18a, 18b for maintaining airtightness are fitted into the respective annular grooves.

[0042] A part of a surface of the enlarged surface portion 17 on the closed space 12a side abuts against a stepped surface 26 of the cylinder 11b, and therefore the other end surface 16b of the rod does not protrude any further to the outside of the housing 11 (cylinder 11b).

[0043] The piston 15 is fitted into the cylinder sliding space 13 through a second opening portion 22 in the housing. A partition wall 23 is fitted into an annular stepped surface in the vicinity of the second opening portion 22 and fixed by crimping the a second opening edge portion 22a to seal the second opening portion. The partition wall 23 may be formed by any member capable of keeping piston movement within a predetermined range upon activation, and need not seal the second opening portion 22 completely.

[0044] A movement space 25 allowing the piston 15 to slide is defined, between the enlarged surface portion 17 and the partition wall 23, by the enlarged surface portion 17, the partition wall 23, and the housing 11. A gap L.sub.2 between the enlarged surface portion 17 and partition wall 23 is equal to the length L.sub.1 of the protruding portion. By adjusting the lengths of L.sub.1 and L.sub.2, the length by which the piston 15 is retracted into the cylinder 11b can be adjusted.

[0045] The igniter housing 11a and the cylinder 11b are partitioned by a collision surface portion 20 serving as a common outer wall surface forming both the igniter housing 11a and cylinder 11b. The collision surface portion 20 forms a part of a cylindrical wall surface, and is the surface with which the combustion products generated upon activation of the igniter 14 collide.

[0046] The igniter 14 and piston 15 are disposed such that a central axis direction of the igniter 14 and a central axis direction of the piston rod 16 are orthogonal to each other. Thus, the activation portion 14a of the igniter 14 directly faces the collision surface portion 20.

[0047] The closed space 12a is surrounded by the igniter 14 (activation portion 14a), the collision surface portion 20, the igniter housing 11a, and the enlarged surface portion 17 (a pressure receiving surface 17a). A slight gap (of approximately 1 mm or less, for example) may be formed between a tip end surface 20a of the collision surface portion 20 and the pressure receiving surface 17a of the enlarged surface portion 17 within a range that does not impede normal operations, or the tip end surface 20a and pressure receiving surface 17a may be abutted against each other or pressed against each other. When a gap exists between the tip end surface 20a of the collision surface portion 20 and the pressure receiving surface 17a of the enlarged surface portion 17, the dimensional precision of the components can be managed easily.

[0048] Next, an operation of the actuator 10 shown in FIG. 1 will be described. Upon activation of the igniter 14, combustion products (a flame, high-temperature gas, a shock wave, and so on) are generated from the activation portion 14a. The generated combustion products collide with the collision surface portion 20, which is positioned inside the closed space 12a to face the activation portion 14a directly, and then change direction.

[0049] The outer peripheral surface of the rod 16 is isolated from the closed space 12a by the collision surface portion 20 and the enlarged surface portion 17, and therefore the combustion products, having changed direction, collide with the pressure receiving surface 17a of the enlarged surface portion 17 rather than the rod 16, thereby pushing the pressure receiving surface 17a. The pressure receiving surface 17a is also pushed by an increase in the internal pressure of the closed space 12a (change in the flow of the combustion products is illustrated by an arrow in FIG. 1). At this time, the combustion products collide with the collision surface portion 20 and the enlarged surface portion 17 (pressure receiving surface 17a), causing the rod 16 to move axially at the moment of collision, and airtightness is maintained by the O-rings 18a, 18b. Therefore, normal operations are not affected even when a slight gap exists between the tip end surface 20a and the pressure receiving surface 17a, and the operation progresses in a similar manner to a case in which the tip end surface 20a and pressure receiving surface 17a are abutted against each other or pressed against each other.

[0050] The enlarged surface portion 17 pushed by the combustion products moves through the movement space 25 towards the partition wall 23, and hence the piston rod 16 slides through the cylinder 11b towards the partition wall 23 such that the protruding portion of the piston rod 16 is retracted into the cylinder 11b (housing 11). As described above, L.sub.1=L.sub.2, and therefore the piston 15 is retracted completely into the cylinder 11b (housing 11).

(2) Actuator of FIG. 2

[0051] FIG. 2 is a longitudinal sectional view of an actuator according to another embodiment. An actuator 100 shown in FIG. 2 is identical in its basic structure to the actuator 10 shown in FIG. 1, and therefore only different parts will be described. Note that identical reference numerals to those used in FIG. 1 denote identical components.

[0052] A cap member 30 made of stainless steel or the like, which corresponds to the collision surface portion 20 shown in FIG. 1, is fitted onto the activation portion 14a of the igniter. When the thin cap member 30 is used in place of the collision surface portion 20 shown in FIG. 1, the axial dimension of the actuator 100 becomes smaller than the axial dimension of the actuator 10 shown in FIG. 1, and therefore the actuator 100 is reduced in size.

[0053] The cap member 30 is fixed such that a flange portion 31 thereof is sandwiched between the igniter housing 11a and the igniter 14. The cap member 30 has a single or a plurality of ejection ports 32 in a part of a side surface thereof for ejecting the combustion products. The ejection port 32 faces the enlarged surface portion 17 of the piston directly.

[0054] When the igniter 14 is activated, the combustion products generated by the activation portion 14a are ejected through the ejection port 32 alone due to the action of the closed space 12a defined by the activation portion 14a and cap member 30. The combustion products ejected through the ejection port 32 collide with and push the pressure receiving surface 17a of the enlarged surface portion 17, which directly faces the ejection port 32 (change in the flow of the combustion products is illustrated by an arrow in FIG. 2), and as a result, the piston 15 is retracted into the cylinder 11b.

(3) Actuator of FIG. 3

[0055] FIG. 3 is a longitudinal sectional view of an actuator according to another embodiment. An actuator 200 shown in FIG. 3 is identical in its basic structure to the actuator 10 shown in FIG. 1, and therefore only different parts will be described. Note that identical reference numerals to those used in FIG. 1 denote identical components.

[0056] The piston 15 accommodated in the cylinder 11b has a T-shaped axial cross-section, and has the piston rod 16 and an annular enlarged surface portion 27 extending in flange form from the one end surface 16a of the piston rod 16.

[0057] The closed space 12a remaining after the igniter 14 is fitted into the igniter accommodating space 12 is a substantially cylindrical space, and substantially the entirety of a pressure receiving surface 27a of the enlarged surface portion 27 faces the closed space 12a.

[0058] The collision surface portion 20 takes a substantially cylindrical form, similarly to the substantially cylindrical closed space 12a, and when the igniter 14 is activated, the combustion products generated from the activation portion 14 collide with a surface thereof directly facing the activation portion 14a.

[0059] Upon activation of the igniter 14, the combustion products generated from the activation portion 14a change direction due to the action of the collision surface portion 20, and fill the entirety of the substantially cylindrical closed space 12a, thus raising the internal pressure thereof. The outer peripheral surface of the rod 16 is isolated from the closed space 12a defined by the collision surface portion 20, the cylinder 11b, and the annular enlarged surface portion 27, and therefore the combustion products, having changed direction, collide with the pressure receiving surface 27a of the enlarged surface portion 27 rather than the rod 16, thereby pushing the pressure receiving surface 27a (change in the flow of the combustion products is illustrated by an arrow in FIG. 3). The pressure receiving surface 27a is also pushed by the increase in the internal pressure of the substantially cylindrical closed space 12a. As a result, the piston 15 is retracted into the cylinder 11b.

[0060] In the actuator 200, the enlarged surface portion 27 is larger than the enlarged surface portion 17 shown in FIGS. 1 and 2, and therefore the actuator 200 becomes larger than the actuators shown in FIGS. 1 and 2. However, the entire surface of the annular enlarged surface portion 27 is pushed evenly, and therefore the operation to retract the piston 15 becomes easier.

(4) Actuator of FIG. 4

[0061] FIG. 4 is a longitudinal sectional view of an actuator according to another embodiment.

[0062] An outer shell of an actuator 300 is formed by joining together an igniter collar 320 and a cylinder 330, which are provided as separate members. The igniter collar 320 includes a main body portion 321 contacting an igniter 323, and a first cylindrical joint portion 322 extending therefrom. The cylinder 330 includes a cylindrical piston accommodating portion (cylinder main body) 331, and a second cylindrical joint portion 332 protruding from a peripheral surface thereof. The igniter 323 is sandwiched between a tip end of the second cylindrical joint portion 332 and the igniter collar 320, and connected to the cylinder 330.

[0063] The igniter collar 320 and cylinder 330 are connected by fitting the first cylindrical joint portion 322 onto a step (a groove 332a) formed on an outer surface of the second cylindrical joint portion 332 and performing crimping from the outside of the first cylindrical joint portion 332.

[0064] The cylinder 330 has a first opening portion 333 and a second opening portion 334 on the respective end sides thereof, and has a communication hole 336 communicating with an igniter accommodating space 325 on a side surface thereof.

[0065] A disk-shaped enlarged surface portion 351 is formed at a desired position, in a length direction, of a piston 350 that is accommodated inside the cylindrical 330. The enlarged surface portion 351 has a pressure receiving surface 352 on the second opening portion 334 side, and abuts against a first stepped portion 337 formed in the interior of the cylinder main body 331.

[0066] The piston 350 has a first rod 358 existing on the first opening portion 333 side of the enlarged surface portion 351, and a second rod 359 existing on the second opening portion 334 side of the enlarged surface portion 351. The interior space of the cylinder 330 is divided into a first space 341 and a second space 342 by the enlarged surface portion 351. The second space 342 forms a single space (closed space) integrally with the communication hole 336 and the igniter accommodating space 325.

[0067] A cylindrical collision surface portion 360 is formed in the second space 342 in the cylinder 330 so as to cover an outer peripheral surface of the second rod 359. The cylindrical collision surface portion 360 is formed integrally with a wall surface of the cylinder 330 on the second opening portion 334 side, and a tip end surface 360a thereof abuts against the pressure receiving surface 352 of the enlarged surface portion 351. Note that upon activation, the combustion products generated from the activation portion 324 collide with a part of the surface of the cylindrical collision surface portion 360 directly facing the activation portion 324, and then change direction, rather than colliding with the entire collision surface portion.

[0068] A sealing member 345 and a thin annular stopper 347 are disposed in a laminated fashion at the end portion of the first rod 358. The sealing member 345 has an opening portion 346 having an inner diameter that is slightly larger than the outer diameter of the first rod 358, while the inner diameter of the annular stopper 347 is slightly smaller than the outer diameter of the first rod 358. The sealing member 345 and annular stopper 347 are fitted into a second stepped portion 338 formed in the cylinder main body 331, and fixed by crimping a side end portion 330a of a first opening portion in the cylinder main body 331. The thin annular stopper 347 prevents movement of the piston 350 prior to activation.

[0069] Next, an operation of the actuator shown in FIG. 4 will be described. When combustion products are generated from the ignition portion 324 upon activation of the igniter 323, the combustion products pass through the igniter accommodating space 325 and the communication hole 336 and flow into the second space 342. The igniter accommodating space 325, communication hole 336, and second space 342 together form a closed space, and therefore the disk-shaped pressure receiving surface 352 is pushed by the combustion products. The pressure receiving surface 352 is also pushed by an increase in the internal pressure of the second space 342, and as a result, a driving force acts on the piston 350.

[0070] At this time, the cylindrical collision surface portion 360 covers the outer peripheral surface of the second rod 359, and the tip end 360a thereof abuts against the pressure receiving surface 352. Therefore, the combustion products flowing into the second space 342 are unlikely to enter the gap between the collision surface portion 360 and the second rod 359, and the entire amount of the generated combustion products is used to drive the piston.

[0071] Then, from a state in which movement is blocked by the stopper 347 at the end portion of the first rod 358, the stopper 347 is ruptured and the piston 350 moves axially.

[0072] Note that in the embodiment shown in FIG. 4, the clearance between the outer peripheral surface of the enlarged surface portion 351 and the inner peripheral surface of the cylinder main body 331 and the clearance between the second opening portion 334 and the second rod 359 are managed strictly, and therefore the O-rings used in FIGS. 1 to 3 are not required.

[0073] The structures described above illustrate specific examples of the present invention, but the present invention is not limited to these structures. The sectional form of the piston, the shape of the collision surface portion, the formation method of the housing and cylinder, and so on may be modified arbitrarily to the extent that the effects of the present invention are exhibited.

[0074] The invention thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. An actuator comprising: a housing having an igniter housing and a cylinder, an igniter accommodated inside the igniter housing and a piston provided inside the cylinder to be slidable, the piston having a rod and an enlarged surface portion formed on one end surface or an outer peripheral surface of the rod, and part of the piston, which includes the other end of the rod, protruding outside the housing before activation, the igniter disposed such that the central axis direction of the igniter and the central axis direction of the rod of the piston are different from each other, wherein, during the activation of the igniter, the piston slides inside the cylinder by pushing the enlarged surface portion by a combustion product generated due to activation of the igniter, and the protruding portion of the piston is retracted inside the housing due to the sliding of the piston.

2. The actuator according to claim 1, wherein the combustion product generated when the igniter is activated changes direction by impinging on a collision surface portion provided in the housing, and then pushes the enlarged surface portion.

3. The actuator according to claim 2, wherein the collision surface portion is a wall surface separating the igniter housing from the cylinder.

4. The actuator according to claim 2, wherein the collision surface portion is a wall surface separating the igniter housing from the cylinder, and the wall surface form part of a wall surface of the cylinder.

5. The actuator according to claim 2, wherein the collision surface portion is a cap member disposed to surround an activation portion of the igniter, which is capable of ejecting the combustion products in a direction of the enlarged surface portion.

6. The actuator according to any one of claims 1 to 5, wherein the collision surface portion forms a closed space including the igniter together with other members including the enlarged surface portion.

7. The actuator according to any of claims 1 to 5, wherein the enlarged surface portion of the piston is a portion which extends from the one end surface of the rod or an axial central portion of the rod either unidirectionally outward in a radial direction of the rod or in flange form.

8. The actuator according to claims 6, wherein the enlarged surface portion of the piston is a portion which extends from the one end surface of the rod or an axial central portion of the rod either unidirectionally outward in a radial direction of the rod or in flange form.