Actuator with hollow piston rod

Abstract

An actuator for a vehicle includes a housing having a central axis, an inner surface, a distal end, and a proximal end. The distal end defines a distal end wall that extends at least partially radially inwardly toward the central axis. A hollow piston rod is slidably engaged with the inner surface of the housing. The piston rod includes a proximal end and a distal end and defines an inner volume extending from the proximal end to the distal end. The piston rod is movable axially from a retracted position to an extended position. The actuator further includes a piston rod cap that seals the distal end when the piston rod is in either the retracted position or the extended position and seals a space between the distal end wall of the housing and the piston rod when the piston rod is in the retracted position.

Description

Attorney Docket No.10213-293WO1 ACTUATOR WITH HOLLOW PISTON ROD ^ CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 640,541 filed on April 30, 2024, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0002] The present disclosure relates to safety devices for passenger vehicles. In particular, the disclosure relates to an actuator with a hollow piston rod. Passenger vehicles may include, for example, automobiles, boats, trains, aircraft, and spacecraft. BACKGROUND

[0003] Actuators in vehicles have many potential uses. In one example, an actuator can be used to lift a portion of the hood of an automobile (e.g., a rear portion of the hood proximate the windshield) in a situation during which the vehicle collides with a pedestrian. When actuated, the actuator raises the hood portion from its normal rest position to an elevated position to increase the space between the hood and the engine. When a pedestrian impacts the elevated hood, the extra space helps to “cushion” the pedestrian, lowering the likelihood the pedestrian impacts the hard engine components beneath the hood.

[0004] Depending on the vehicle environment (e.g., size, shape, engine type), manufacturers may have different performance requirements necessary for actuators to meet. In some cases, actuators need to be able to extend very quickly, while in others they may need to extend relatively slowly. To achieve these requirements, manufacturers often supply multiple actuator designs. However, this increases cost and complexity in manufacturing which could lead to a variety of issues. Therefore, there is a need for a standardized actuator design capable of a wide range of performance while minimizing cost and complexity. SUMMARY

[0005] Various implementation include an actuator comprising a housing comprising a central axis, an inner surface, a distal end, and a proximal end, the distal end defining a distal end wall extending at least partially radially inwardly toward the central axis. A gas generator is coupled to the proximal end of the housing and a piston rod is slidably engaged with the inner surface of the housing, the piston rod comprising a proximal end and a distal end and defining an inner volume extending from the proximal end to the distal end. A piston rod cap is coupled to the piston rod. The piston rod is movable axially from a retracted position to an extended position, wherein theAttorney Docket No.10213-293WO1 distal end of the piston rod is adjacent the distal end of the housing in the retracted position and the proximal end of the piston rod is adjacent the distal end of the housing in the extended position. The piston rod cap seals the distal end of the piston rod when the piston rod is in either the retracted position or the extended position and seals a space between the distal end wall of the housing and the piston rod when the piston rod is in the retracted position.

[0006] In some implementations, the piston rod cap comprises an integrally formed cover portion, piston rod seal portion, and housing seal portion, the piston rod seal portion and the housing seal portion extending away from the cover portion at least partially in the same direction and parallel to the central axis. In some implementations, the piston rod seal portion seals the distal end of the piston rod and the housing seal portion seals the space between the distal end wall of the housing and the piston rod when the piston rod is in the retracted position. In some implementations, the piston rod has a length, as measured from the proximal end of the piston rod to the distal end of the piston rod, and the piston rod seal portion has a length, as measured from a proximal end of the piston rod seal portion to a distal end of the piston rod seal portion. In some implementations, the length of the piston rod seal portion is 90% or less of the length of the piston rod. In some implementations, the length of the piston rod seal portion is 75% or less of the length of the piston rod. In some implementations, the length of the piston rod seal portion is 50% or less of the length of the piston rod. In some implementations, the length of the piston rod seal portion is 25% or less of the length of the piston rod.

[0007] In some implementations, the actuator further comprises an energy damper disposed around the piston rod. In some implementations, the energy damper is disposed adjacent the distal end of the piston rod in the retracted position.

[0008] In some implementations, the housing further comprises an outer surface, wherein the housing defines gas relief openings extending from the inner surface to the outer surface. In some implementations, the actuator further comprises a sleeve, the sleeve covering the gas relief openings. In some implementations, the sleeve comprises acrylonitrile butadiene styrene (ABS). In some implementations, the sleeve defines a shoulder, wherein the proximal end of the piston rod abuts the shoulder in the retracted position.

[0009] In some implementations, the proximal end of the piston rod has a first diameter, wherein the piston rod defines a protrusion having a second diameter and a valley having a third diameter, the valley disposed between the proximal end of the piston rod and the protrusion, and wherein the third diameter is less than the first diameter and the second diameter. In some implementations,Attorney Docket No.10213-293WO1 the first diameter and the second diameter are equal. In some implementations, the actuator further comprises a seal disposed around the piston rod and in the valley, the seal sealing a space between the piston rod and the inner surface of the housing. In some implementations, the seal is a rubber o-ring.

[0010] In some implementations, the piston rod cap comprises an elastic material. In some implementations, the elastic material comprises rubber.

[0011] In some implementations, the housing further comprises an outer surface and the cover portion comprises a cover portion wall extending from the cover portion in a direction at least partially parallel to the central axis and around at least a portion the outer surface of the housing.

[0012] In some implementations, the cover portion comprises a weakened portion.

[0013] In some implementations, the piston rod seal portion extends into the inner volume of the piston rod and the piston rod further comprises a crimp around the piston rod seal portion.

[0014] In various implementations, a method comprises providing a housing defining a central axis, coupling a gas generator to the housing, slidably engaging a piston rod with the housing parallel to the central axis, the piston rod defining an inner volume, selecting a length of an axially extending piston rod seal portion of a piston rod cap based on a desired actuation volume within the inner volume of the piston rod, coupling the axially extending piston rod seal portion of the piston rod cap with the piston rod, and coupling an axially extending housing seal portion of the piston rod cap with the housing to seal a space between the housing and the piston rod.

[0015] In some implementations, the piston rod defines a length, wherein selecting the length of an axially extending piston rod seal portion includes selecting the length of the axially extending piston rod seal portion of 90% to 75% of the length of the piston rod. In some implementations, the piston rod defines a length, wherein selecting the length of an axially extending piston rod seal portion includes selecting the length of the axially extending piston rod seal portion of 75% to 50% of the length of the piston rod. In some implementations, the piston rod defines a length, wherein selecting the length of an axially extending piston rod seal portion includes selecting the length of the axially extending piston rod seal portion of 50% to 25% of the length of the piston rod. In some implementations, the piston rod defines a length, wherein selecting the length of an axially extending piston rod seal portion includes selecting the length of the axially extending piston rod seal portion of 25% or less of the length of the piston rod.Attorney Docket No.10213-293WO1 BRIEF DESCRIPTION OF DRAWINGS

[0016] The drawings are merely exemplary to illustrate steps, structure, and certain features that can be used singularly or in combination with other features. The disclosure should not be limited to the implementations shown.

[0017] FIG.1 is a perspective view of one implementation of an actuator with a hollow piston rod in a retracted position.

[0018] FIG. 2 is a cross-sectional view of the actuator of FIG. 1 along the plane A-A as shown in FIG.1.

[0019] FIG.3 is an isolated cross-sectional view of a housing of the actuator of FIG.2.

[0020] FIG.4 is an isolated cross-sectional view of a piston rod of the actuator of FIG.2.

[0021] FIG.5 is a zoomed view of a portion of the piston rod of FIG.4.

[0022] FIG.6 is a zoomed view of a portion of the actuator of FIG.2.

[0023] FIG.7 is a zoomed view of another portion of the actuator of FIG.2.

[0024] FIG.8 is a perspective view of a sleeve of the actuator of FIG.2.

[0025] FIG.9 is a perspective view of a piston rod cap of the actuator of FIG.2.

[0026] FIG. 10 is a cross-sectional view of the piston rod cap of FIG. 9 along the plane B-B as shown in FIG.9 having a piston rod seal portion having a length L1.

[0027] FIG. 11 is a cross-sectional view of the piston rod cap of FIG. 9 along the plane B-B as shown in FIG.9 having a piston rod seal portion having a length L2.

[0028] FIG. 12 is a cross-sectional view of the piston rod cap of FIG. 9 along the plane B-B as shown in FIG.9 having a piston rod seal portion having a length L3.

[0029] FIG. 13 is a series of cross-sectional views of the actuator of FIG. 2 showing a piston rod cap having a piston rod seal portion with varying lengths.

[0030] FIG.14 is a partial cross-sectional view of the actuator of FIG.2 in an extended position.

[0031] FIG.15 is a perspective view of piston rod cap having a cover portion wall.Attorney Docket No.10213-293WO1

[0032] FIG. 16 is a zoomed view of a portion of the actuator of FIG. 2 with the piston rod cap of FIG.15. DETAILED DESCRIPTION

[0033] The devices, systems, and methods disclosed herein provide for an actuator having a hollow piston rod. The actuator can be included in a hood lifting mechanism for lifting the hood of a vehicle, for example. The actuator includes a housing and a piston rod slidably engaged with an inner surface of the housing, wherein the piston rod defines an inner volume extending from a proximal end of the piston rod to a distal end of the piston rod. A piston rod cap is coupled to the piston rod to seal the distal end of the hollow piston rod. The piston rod cap also seals a space between the piston rod and the housing when the actuator is in a retracted position. As used herein, the verb “seal” means to limit, including fully prevent, fluid communication between two or more locations, structures, surfaces, components, etc., as well as to limit, including fully prevent, the movement of solid material (e.g., dirt or liquids) between two or more locations, structures, surfaces, components, etc.

[0034] Referring to FIGS.1-3, an actuator 100 comprises a housing 101 comprising a proximal end 105, a distal end 106, and a wall 102 extending from the proximal end 105 to the distal end 106. The wall 102 comprises an inner surface 103 and an outer surface 104. The housing 101 further comprises a central axis 110 extending through the proximal end 105 and the distal end 106. The distal end 106 defines a distal end wall 107 extending at least partially radially inward toward the central axis 110. An opening 108 is defined by the distal end wall 107 and is configured for allowing a piston rod 112 to extend from the housing 101, as described below. An inner volume 109 is defined within the housing 101, bounded by the inner surface 103, the proximal end 105, and the distal end 106. In some implementations, the housing 101 may be formed from metal, such as stainless steel, and be formed by a deep drawing process.

[0035] Referring now to FIG.4, the piston rod 112 comprises a wall 113 extending from a proximal end 116 to a distal end 117 of the piston rod 112. The wall 113 comprises an inner surface 114 and an outer surface 115. An inner volume 118 is defined within the piston rod 112, bounded by the inner surface 114, the proximal end 116, and the distal end 117. Therefore, the piston rod 112 takes on the form of a hollow tubular structure. In the retracted position, the distal end 117 of the piston rod 112 is adjacent the distal end 106 of the housing 101. In an extended position, the proximal end 116 of the piston rod 112 is adjacent the distal end 106 of the housing 101. In some implementations, the piston rod 112 may be formed from metal, such as stainless steel, and be formed by a deep drawing process.Attorney Docket No.10213-293WO1

[0036] The proximal end 116 comprises a first diameter 121. Adjacent the proximal end 116, the piston rod 112 defines a protrusion 119 and a valley 120, having a second diameter 122 and a third diameter 123, respectively. As shown in FIG. 5, the valley 120 is disposed between the proximal end 116 and the protrusion 119, and the third diameter 123 is less than the first diameter 121 and the second diameter 122. In some implementations, the first diameter 121 and the second diameter 122 are equal. In other implementations, the first diameter and the second diameter are not equal. A seal 124 is disposed around the valley 120 and in contact with the inner surface 103 of the housing 101. The seal 124 may be a rubber o-ring, for example. The first diameter 121, second diameter 122, third diameter 123, and seal 124 are all sized to allow the piston rod 112 to be slidably engaged with the inner surface 103 of the housing 101, allowing the piston rod 112 to move from the retracted position to an extended position. In some implementations, the piston rod 112 is movable axially between the retracted position and the extended position.

[0037] The piston rod 112 is disposed within the inner volume 109 of the housing 101 in the retracted position of the actuator 100, for example as shown in FIG.2. In the extended position, as shown in FIG. 14, the piston rod 112 extends out from the housing 101 to lift a hood of a vehicle (not shown), for example. A portion of the piston rod 112 adjacent the proximal end 116 of the piston rod 112 remains within the inner volume 109 in the extended position. A damper 125 may be disposed around the piston rod 112 within the inner volume 109. In some implementations, the damper 125 may be disposed adjacent the distal end 117 of the piston rod 112 in the retracted position. In other implementations, the damper may be disposed at any point along the piston rod in the retracted position. The damper 125 may be formed from an elastic material, for example rubber, or from any other material capable of compressing to absorb energy from the movement of the piston rod 112, such as a woven fabric cushion. As shown in FIG.14, the damper is compressed between the distal end wall 107 of the housing 101 and the protrusion 119 of the piston rod 112 in the extended position.

[0038] To seal the piston rod 112 and the housing 101 from the outside environment, a piston rod cap 126 is coupled to the piston rod 112 adjacent the distal end 117 of the piston rod 112. The piston rod cap 126 is formed from an elastic material, such as rubber, and comprises an integrally formed cover portion 127, piston rod seal portion 128, and housing seal portion 129. In some implementations, the cover portion 127, piston rod seal portion 128, and housing seal portion 129 are not all integrally formed together but formed separately and coupled together, for example using an adhesive or other fastening mechanisms. In other implementations, the piston rod cap may be formed from an inelastic material, such as hard plastic.Attorney Docket No.10213-293WO1

[0039] The piston rod seal portion 128 and the housing seal portion 129 both extend away from the cover portion 127 at least partially in the same direction and parallel to the central axis 110 of the housing 101. The piston rod seal portion 128 extends into the inner volume 118 of the piston rod 112 from the distal end 117 of the piston rod 112 and forms an interference fit with the inner surface 114 of the piston rod 112. The interference fit causes compression of the elastic material of the piston rod seal portion 128 which causes a force to be exerted by the elastic material against the inner surface 114, thereby sealing the distal end 117 from the outside environment. As best seen in FIG.6, the outer diameter of the piston rod 112 is less than the diameter of the opening 108 in the housing 101, leaving a space between the distal end wall 107 and the piston rod 112. The housing seal portion 129 extends into the space between the distal end wall 107 and the piston rod 112 when the piston rod 112 is in the retracted position and is similarly compressed within the space, thereby sealing the space. When the piston rod is in the extended position, as shown in FIG.14, the housing seal portion 129 no longer seals the space between the distal end wall 107 and the piston rod 112.

[0040] The cover portion 127, as seen throughout the FIGURES, remains external of the housing 101 and the piston rod 112 in both the retracted and extended positions. The cover portion 127 is sized to cover the distal end wall 107 of the housing 101 so that, for example when used to lift a hood of a vehicle, the impact energy, and noise, of the actuator 100 can be absorbed and reduced when the piston rod cap 126 impacts the hood.

[0041] Referring now to FIGS. 10-13, the piston rod seal portion 128 has a length as measured from a proximal end 130 to a distal end 131 of the piston rod seal portion 128. The length of the piston rod seal portion 128 is measured in a direction parallel to the central axis 110. In some implementations, the piston rod seal portion 128 extends axially. FIGS.10-12 depict isolated views of the piston rod cap 126 having a piston rod seal portion 128 with lengths L1, L2, and L3, wherein L3 > L2 > L1. FIG.13 shows a series of actuators 100 having piston rod caps 126 with piston rod seal portions 128 having progressively longer lengths, including a piston rod seal portion 128 that extends along nearly the entire length of the piston rod 112, such that the proximal end 130 of the piston rod seal portion 128 is adjacent the protrusion 119.

[0042] To deploy the actuator 100, a gas generator 132 is coupled to the proximal end 105 of the housing 101. A lead wire 133 extends from the gas generator 132 to a vehicle electronic control unit (ECU, not shown) to form an electrical connection between the gas generator 132 and the ECU. In some implementations, a wrap 134, such as a heat activated shrink wrap or a hard plastic cap, may be disposed around the proximal end 105 to provide protection to the gas generator 132 and lead wire 133. In some implementations, the gas generator 132 may be a pyrotechnic gas generator,Attorney Docket No.10213-293WO1 such as an initiator or micro gas generator, both well known in the art. In other implementations, the gas generator may comprise a source of stored, pressurized gas that is released upon actuation.

[0043] When the ECU sends an actuation signal to the gas generator 132, the gas generator 132 introduces high-pressure gas into the inner volume 118 of the piston rod 112. The seal 124 prevents the high-pressure gas from passing around the piston rod 112 between the outer surface 115 of the piston rod 112 and the inner surface 103 of the housing 101. Because the piston rod cap 126 is permanently coupled to the distal end 117 of the piston rod 112 and seals the inner volume 118, the high-pressure gas will push against the proximal end 130 of the piston rod seal portion 128 within the inner volume 118, thereby pushing the piston rod 112 from the retracted position to the extended position.

[0044] By providing a piston rod cap 126 having a piston rod seal portion 128 with varying lengths, the actuator 100 can be tuned to different performance requirements while using otherwise standardized parts. In other words, a multitude of different performance characteristics for the actuator 100 can be achieved by simply using a different piston rod cap 126 having a piston rod seal portion 128 with a different length. For example, in some implementations, L3 = 21mm, L2 = 16mm, and L1 = 11mm. In some implementations, the length of the piston rod seal portion 128 may vary from just greater than 0% of the length of the piston rod 112 to as much as 90% of the length of the piston rod 112, for example 75%, 50%, or 25% of the length of the piston rod 112.

[0045] Changing the length of the piston rod seal portion 128 will therefore impact an actuation volume defined by the free space within the inner volume 118 of the piston rod 112, as best shown in FIG. 13. By decreasing the size of actuation volume within the inner volume 118 (e.g., a relatively longer piston rod seal portion 128), one can increase the speed and power with which the piston rod 112 moves from the retracted position to the extended position because the high-pressure gas from the gas generator 132 will generate a relatively higher pressure in the inner volume 118. Conversely, by increasing the size of the actuation volume within the inner volume 118 (e.g., a relatively shorter piston rod seal portion 128), one can decrease the speed and power with which the piston rod 112 moves from the retracted position to the extended position because the high-pressure gas from the gas generator 132 will generate a relatively lower pressure in the inner volume 118.

[0046] Referring now to FIGS. 7-8, the actuator 100 may further comprise a sleeve 135 disposed within the inner volume 109 of the housing 101 adjacent the proximal end 105 of the housing 101 and around the gas generator 132. The sleeve 135 covers gas relief openings 111 formed in the housing 101 extending from the inner surface 103 to the outer surface 104, preventing fluidAttorney Docket No.10213-293WO1 communication between the inner volume 109 of the housing 101 and the outside environment during normal operation of the actuator 100. The sleeve 135 defines a shoulder 136 adjacent to the proximal end 116 of the piston rod 112 in the retracted position. In some implementations, the proximal end 116 abuts the shoulder 136 in the retracted position. The shoulder 136 defines an opening 137 to allow high-pressure gas from the gas generator 132 to enter the inner volume 118 of the piston rod 112.

[0047] In the event the actuator 100 is in communication with an external heat source, such as a fire that heats the actuator 100 and causes the sleeve 135 to be heated above a threshold temperature, the sleeve 135 melts, allowing fluid communication between the outside environment and the inner volume 109 of the housing 101 through the gas relief openings 111. The threshold temperature is defined as the melting temperature of the sleeve 135. Therefore, in the event the gas generator 132 actuates in response to the external heat source after the sleeve 135 melts, high-pressure gases from the gas generator 132 will flow to the outside environment through the gas relief openings 111, reducing the pressure on the proximal end 130 of the piston rod seal portion 128. This event is one example of a “dry fire” event, wherein the actuator 100 is actuated without resistance from an external force, such as the hood of the vehicle. During a dry fire event, there is a risk that the piston rod 112 may be driven with too much energy and could fully exit the housing 101. Therefore, reducing the pressure by venting the high-pressure gas to the outside environment reduces this risk. In some implementations, the sleeve 135 may comprise a plastic material, for example acrylonitrile butadiene styrene (ABS) with a melting temperature of approximately 220 degrees Celsius, ensuring the sleeve 135 decomposes before the gas generator 132 actuates in response to the external heat source. In other implementations, the sleeve may be made from any other material capable of melting before the gas generator actuates automatically in response to the external heat source, such as other plastic materials and certain metals.

[0048] Referring now to FIGS.15-16, in some implementations the piston rod cap 126 may further comprise a cover portion wall 139 extending from the cover portion 127 in a direction at least partially parallel to the central axis 110, and around at least a portion of the outer surface 104 of the wall 102 of the housing 101. In some implementations, as shown, the cover portion wall 139 may abut the outer surface 104 of the wall 102. In some implementations, the piston rod cap 126 comprises a weakened portion 140 (e.g., a tear seam) in the area of the distal end 131 of the piston rod seal portion 128. When moving from the retracted position to the extended position, the piston rod 112 may break a portion of the cover portion 127 free from the rest of the cover portion 127 and the cover portion wall 139 at the weakened portion 140. In this case, the piston rod seal portion 128 and the broken portion of the cover portion 127 will move with the piston rod 112 such that theAttorney Docket No.10213-293WO1 piston rod cap 126 continues to seal the distal end 117 of the piston rod 112 when the piston rod 112 is in the extended position.

[0049] In addition to the interference fit discussed above, in order to ensure a tight fit of the piston rod seal portion 128 in the inner volume 118 of the piston rod 112, a crimp 138 in the wall 113 of the piston rod 112 may be used. The crimp 138 may surround an area of the wall 113 adjacent the proximal end 130 of the piston rod seal portion 128. As a result, the piston rod cap 126, whether using a weakened portion 140 or not, may stay permanently coupled to the piston rod 112 via the piston rod seal portion 128. The crimp 138 may be formed as known in the art, such as, for example, by using a crimping tool and applying pressure radially toward to the central axis 110 against the outer surface 115 of the piston rod 112.

[0050] The description in the present disclosure has been presented for purposes of illustration but is not intended to be exhaustive or limited to the implementations disclosed. It will be understood that various modifications and variations will be apparent to those of ordinary skill in the art and may be made without departing from the spirit and scope of the following claims. Accordingly, other implementations are within the scope of the claims. The implementations described were chosen in order to best explain the principles of the actuator and its practical application, and to enable others of ordinary skill in the art to understand the assembly for various implementations with various modifications as are suited to the particular use contemplated.

[0051] The terms “coupled,” “connected,” and the like as used herein to mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another.

[0052] The terminology used herein is for the purpose of describing particular implementations only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and / or groups thereof.

Claims

Attorney Docket No.10213-293WO1 CLAIMS ^ What is claimed is:

1. An actuator comprising: a housing comprising a central axis, an inner surface, a distal end, and a proximal end, the distal end defining a distal end wall extending at least partially radially inwardly toward the central^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^a piston rod slidably engaged with the inner surface of the housing, the piston rod comprising a proximal end and a distal end and defining an inner volume extending from the^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^wherein the piston rod is movable axially from a retracted position to an extended position, wherein the distal end of the piston rod is adjacent the distal end of the housing in the retracted position and the proximal end of the piston rod is adjacent the distal end of the housing in the^^^^^^^^^^^^^^^^^^^^^^^wherein the piston rod cap seals the distal end of the piston rod when the piston rod is in either the retracted position or the extended position and seals a space between the distal end wall of the housing and the piston rod when the piston rod is in the retracted position.

2. The actuator of claim 1, wherein the piston rod cap comprises an integrally formed cover portion, piston rod seal portion, and housing seal portion, and wherein the piston rod seal portion and the housing seal portion extend away from the cover portion at least partially in the same direction and parallel to the central axis.

3. The actuator of claim 2, wherein the piston rod seal portion seals the distal end of the piston rod and the housing seal portion seals the space between the distal end wall of the housing and the piston rod when the piston rod is in the retracted position.

4. The actuator of claim 2, wherein the piston rod has a length, as measured from the proximal end of the piston rod to the distal end of the piston rod, and wherein the piston rod seal portion has a length, as measured from a proximal end of the piston rod seal portion to a distal end of the piston rod seal portion.Attorney Docket No.10213-293WO1 5. The actuator of claim 4, wherein the length of the piston rod seal portion is 90% or less of the length of the piston rod.

6. The actuator of claim 4, wherein the length of the piston rod seal portion is 75% or less of the length of the piston rod.

7. The actuator of claim 4, wherein the length of the piston rod seal portion is 50% or less of the length of the piston rod.

8. The actuator of claim 4, wherein the length of the piston rod seal portion is 25% or less of the length of the piston rod.

9. The actuator of claim 1, further comprising an energy damper disposed around the piston rod.

10. The actuator of claim 9, wherein the energy damper is disposed adjacent the distal end of the piston rod in the retracted position.

11. The actuator of claim 1, wherein the housing further comprises an outer surface, wherein the housing defines gas relief openings extending from the inner surface to the outer surface.

12. The actuator of claim 11, further comprising a sleeve, the sleeve covering the gas relief openings.

13. The actuator of claim 12, wherein the sleeve comprises acrylonitrile butadiene styrene (ABS).

14. The actuator of claim 12, wherein the sleeve defines a shoulder, wherein the proximal end of the piston rod abuts the shoulder in the retracted position.

15. The actuator of claim 1, wherein the proximal end of the piston rod has a first diameter, wherein the piston rod defines a protrusion having a second diameter and a valley having a third diameter, the valley disposed between the proximal end of the piston rod and the protrusion, and wherein the third diameter is less than the first diameter and the second diameter.Attorney Docket No.10213-293WO1 16. The actuator of claim 15, wherein the first diameter and the second diameter are equal.

17. The actuator of claim 15, further comprising a seal disposed around the piston rod and in the valley, the seal sealing a space between the piston rod and the inner surface of the housing.

18. The actuator of claim 17, wherein the seal is a rubber o-ring.

19. The actuator of claim 1, wherein the piston rod cap comprises an elastic material.

20. The actuator of claim 19, wherein the elastic material comprises rubber.

21. The actuator of claim 2, wherein the housing further comprises an outer surface, and wherein the cover portion comprises a cover portion wall extending from the cover portion in a direction at least partially parallel to the central axis and around at least a portion the outer surface of the housing.

22. The actuator of claim 2, wherein the cover portion comprises a weakened portion.

23. The actuator of claim 2, wherein the piston rod seal portion extends into the inner volume of the piston rod and the piston rod further comprises a crimp around the piston rod seal portion.

24. A method comprising: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^slidably engaging a piston rod with the housing parallel to the central axis, the piston rod^^^^^^^^^^^^^^^^^^^^^^^^^^ ^selecting a length of an axially extending piston rod seal portion of a piston rod cap based^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^coupling the axially extending piston rod seal portion of the piston rod cap with the piston^^^^^^^^^coupling an axially extending housing seal portion of the piston rod cap with the housing to seal a space between the housing and the piston rod.

25. The method of claim 24, wherein the piston rod defines a length, wherein selecting the length of the axially extending piston rod seal portion includes selecting the length of the axiallyAttorney Docket No.10213-293WO1 extending piston rod seal portion of 90% to 75% of the length of the piston rod.

26. The method of claim 24, wherein the piston rod defines a length, wherein selecting the length of the axially extending piston rod seal portion includes selecting the length of the axially extending piston rod seal portion of 75% to 50% of the length of the piston rod.

27. The method of claim 24, wherein the piston rod defines a length, wherein selecting the length of the axially extending piston rod seal portion includes selecting the length of the axially extending piston rod seal portion of 50% to 25% of the length of the piston rod.

28. The method of claim 24, wherein the piston rod defines a length, wherein selecting the length of the axially extending piston rod seal portion includes selecting the length of the axially extending piston rod seal portion of 25% or less of the length of the piston rod.

Images