Patent Document

BACKGROUND OF THE INVENTION 
     Hydraulic struts have been used for many years for opening and holding open a door or other closure. Generally, a strut includes two telescoping members and is moveable between a compressed position when the door is closed and an extended position when the door is open. Some struts also provide for locking in the open or extended position. 
     The speed at which a conventional hydraulic strut extends or compresses is governed by the speed at which hydraulic fluid can enter a chamber and either apply pressure to a moveable piston or fill a vacuum created by the motion of the piston. This fluid flow is created from either the suction action of the retracting piston in a sealed strut, or by an injection system external to the strut. 
     One disadvantage of conventional hydraulic struts is that the rate of speed at which the strut extends and compresses is the same. 
     Applications exist where it is desirable to have a strut which opens at one predetermined rate of speed and closes at another. One example is a door which provides access to the personnel carrying area of an aircraft. Doors that can both open rapidly and close slowly allow for rapid and easy exit from the vehicle when the door opens and avoid injuring personnel or damaging items when closing the door. It is also desirable that the strut provide for a mechanism to lock it in the extended or open position. 
     There exist struts which have variable expansion and compression speeds. These are achieved by using variable flow rate valves which allow for fluid flow at two different speeds, depending on the direction of flow. Rapid opening struts operate by pressurized injection of hydraulic fluid into the strut from an outside source or an injection device. When the strut is compressed, the injected hydraulic fluid is returned to the outside source or discharged from the strut system. A disadvantage of this type of strut is that, because the strut is not sealed and self contained, the hydraulic fluid can become contaminated or leak onto surrounding structures. Another disadvantage is that the addition of an injection device makes the strut system bulkier and more cumbersome than conventional struts. 
     In view of the above, it should be appreciated that there is still a need for a dampening strut that can open and close at different speeds; is compact, sealed, and self-contained; and can automatically lock in an open position. The present invention satisfies these and other needs and provides further related advantages. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in a hydraulic strut which can open and close at different speeds; is compact, sealed, and self-contained; and can automatically lock in an open position. The hydraulic strut includes an outer telescoping member and an inner telescoping member. The strut has mounts on opposing ends so that it may be attached to a fixed structure and a pivotable door or closure attached to the fixed structure. 
     A feature of the present invention is that it includes both a second hydraulic reservoir, which is defined by an inner tubular metal cylinder, and a first hydraulic reservoir, which is defined by an outer tubular metal cylinder that circumferentially surrounds the inner cylinder. Thus, the first hydraulic reservoir surrounds the main hydraulic chamber, as opposed to being laterally or longitudinally situated from it. A valve connects the second hydraulic reservoir and the first hydraulic reservoir. Two distinct chambers, connected by a variable rate valve, avoids the need for an injection device and allows the strut to have a predetermined opening rate and a different predetermined closure rate, in a strut which is a self-contained, sealed unit. Furthermore, the struts are shorter and more compact than if the chambers were disposed laterally or longitudinally from one another. This is a significant advantage in areas where space is limited, such as, in aircraft passenger compartments. This feature also increases the strength of the strut. 
     Another feature is that the first hydraulic reservoir contains an auxiliary piston and the second hydraulic reservoir contains a main piston. The auxiliary piston creates hydraulic pressure which serves as a counter-force against which the main piston presses when the strut is compressing. The counter-force prevents a slippage or a jolt when the strut begins moving from a locked and extended position toward a compressed position, as would normally occur if no counter-force were present. This feature also reduces the volume of the first hydraulic reservoir as the strut extends and hydraulic fluid is drawn from the auxiliary hydraulic chamber into the second hydraulic reservoir by the movement of a main piston. By reducing the volume of the first hydraulic reservoir, the auxiliary piston prevents a suction force from building in the first hydraulic reservoir which would slow the transfer of hydraulic fluid and consequently slow the expansion rate of the strut. 
     The present invention also includes a unique and advantageous self-locking mechanism. A locking sleeve moveably retains a plurality of locking balls in the shaft of the main piston. The locking sleeve circumferentially surrounds the piston shaft and is spring biased to expose the locking balls when the strut reaches its fully extended position. The piston shaft includes an outer shaft, which is tubular, and an inner shaft, which is a solid rod and is moveably retained within the outer shaft. A trapezoidal shaped annular groove is formed in the inner shaft. A plurality of apertures are formed in the outer shaft. When the strut is not in an extended, locked position, the inner shaft and outer shaft are positioned so that the trapezoidal shaped annular groove is lined up with the plurality of apertures. Located within the space formed by the aligned inner and outer shafts is the plurality of locking balls. In this same position, the locking balls are retained by the spring biased locking sleeve. 
     A release sleeve circumferentially surrounds the outer shaft, and is accessible by hand from the outside of the strut. A connector pin attaches the release sleeve to the inner shaft and passes through opposed slots in the outer shaft. The release sleeve is spring biased such that when the strut reaches its fully extended position, the release sleeve moves the inner shaft to drive the locking balls into a plurality of spherical depressions formed in a portion of the outer telescoping member. In this position, the locking balls are in contact with both the inner telescoping member and the outer telescoping member, and effectively prevent movement of one relative to the other. To release the strut, an operator moves the release sleeve, and consequently the inner shaft, to a position which allows the locking balls to leave the spherical depressions and return to a retained position. 
     Several advantages of this release mechanism are that it automatically locks when the strut reaches the extended position, it minimizes the profile of the strut, and it reduces the danger of an accidental release of the lock. 
     Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a hydraulic strut that supports a door to the passenger carrying area of a helicopter. 
     FIG. 2 is a cross section of the hydraulic strut in a compressed position. 
     FIG. 2A is an enlarged cross section of an endcap of the hydraulic strut shown in FIG. 2, circle  2 A. 
     FIG. 3 is a front view of a valve for the hydraulic strut. 
     FIG. 4 is a cross section of the valve of FIG.  3 . 
     FIG. 5 is an enlarged cross section of the portion of the hydraulic strut shown in FIG. 2, circle  5 . 
     FIG. 6 is a cross section of the hydraulic strut of FIG. 2 in an extended position, before the locking mechanism is fully engaged. 
     FIG. 7 is an enlarged cross section of the portion of the hydraulic strut shown in FIG. 6, circle  7 . 
     FIG. 7A is an enlarged cross section of the portion of the hydraulic strut shown in FIG. 6, circle  7 A. 
     FIG. 8 is a cross section of the hydraulic strut in the extended position with the locking mechanism engaged. 
     FIG. 9 is an enlarged cross section of the portion of the hydraulic strut shown in FIG. 8, circle  9 . 
     FIG. 10 is a cross section of the hydraulic strut in an intermediate position. 
     FIG. 11 is an enlarged cross section of the portion of the hydraulic strut shown in FIG. 10, circle  11 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, a pair of identical dampening struts  20 ,  22  of the present invention are used to support a door  24  to the passenger area of a helicopter. Each strut includes an outer telescoping member  30  and an inner telescoping member  32 . Unless otherwise noted, all parts of the struts  20 ,  22  are preferably made from steel. 
     With reference to FIGS. 2 and 2A, the outer telescoping member  30  includes an endcap  33 , a tube-shaped outer cylinder  34 , a tube-shaped inner cylinder  36 , and a cylinder head  38 . The endcap  33  includes a mount portion  40 , a channel portion  60  and a valve portion  80 . With reference to FIG. 2A, the mount portion  40  includes a base  41 , which is preferably cylindrical in shape. A cylindrical bore  42  is located along the axial center of the base  41 . Preferably, the cylindrical bore  42  is threaded. Threadibly mounted to the base  41  at the cylindrical bore  42  is an eyelet mount  44 . The eyelet mount  44  includes a threaded shank  46 , a nut  48  and an eyelet  50 . The nut  48  is threadibly mounted on the threaded shank  46  and tightened securely against the base  41 . The eyelet  50  includes a solid ring  52 , a circular groove  54 , and an aperture  56  formed at the axial center of the ring  52 . This configuration of the eyelet  50  allows the strut to be mounted on a conventional axle swivel mount (not shown). 
     The channel portion  60  of the endcap  33 , also shown in FIG. 2A, includes a cylindrical base  62 , having an inwardly protruding cylindrical member  66 . The cylindrical base  62  is a solid, contiguous extension of the base  41  of the mount portion. The cylindrical base  62  has a greater diameter than the base  41  and the transition from the smaller diameter base  41  to the larger diameter cylindrical base  62  is accomplished by use of a sloping shoulder  64 , which extends radially outward from the base  41 . Transverse to the longitudinal axis of the cylindrical base  62 , a transverse bore  67  is formed through the inwardly protruding cylindrical member  66 . 
     Formed in the sloping shoulder  64  are two angular bores  68 . Each angular bore includes a small diameter channel portion  70  and a large diameter sealing portion  72 . The small diameter channel portion  70  connects to the transverse bore  67 . Located in the large diameter sealing portion  72  is a sealing ball  74  and a plug  76 . The sealing ball is seated where the angular bore narrows from the large diameter sealing portion  72  to the small diameter channel portion  70 . Adjacent to and firmly secured against the sealing ball  72  is the plug  76 . Additionally, the sloping shoulder includes a groove  78  circumferentially disposed around the sloping shoulder  64 . Located within the groove  78  is a shoulder O-ring  79 . 
     The valve portion  80 , as shown in FIGS. 2 and 2A, is preferably cylindrical and adjacent to and a solid contiguous extension of the cylindrical member  66  of the cylindrical base  64 . A longitudinal bore  84  is formed through the radial center of the valve portion  80  and connects to the transverse bore  67 . The longitudinal bore  84  includes a larger diameter portion  82  which forms a valve chamber  86 . At this point, the longitudinal bore  84  includes a valve seat  85 . The valve chamber  86  is cylindrical in shape and defines a cylindrical valve chamber side wall  88  and cylindrical valve chamber sloping wall  90 . A recessed, cylindrical groove  92  is formed at a terminal end  94  of the valve chamber  86 . Located within the valve chamber  86  is a valve  100 . The valve is omitted from FIG. 2 for purposes of clarity, but is shown in detail in FIGS. 3 and 4. Located within the groove  92  is a snap ring  95 , which moveably retains the valve  100 , as is described further below. 
     With reference to FIGS. 3 and 4, the valve  100  has a spherical end  102 , a cylinder end  104  and a sloping shoulder  106 . The cylinder end  104  is tubular and has an interior bore  105 . Extending outward from the cylinder end,  104  and positioned between the cylinder end  104  and the spherical end  102 , is the sloping shoulder  106 . The sloping shoulder  106  includes, as a radial extension of the sloping shoulder  106 , a protruding lip  107 . Bored through the sloping shoulder  106  are four large diameter fluid flow openings  108 ,  110 ,  112 ,  114 . These fluid flow openings  108 ,  110 ,  112 ,  114  open into the interior bore  105  of the cylinder end  104 . The spherical end  102  is a contiguous extension of the sloping shoulder  106  and curves inwardly to form a valve head  115  that is a portion of a sphere. Bored through the radial center of the spherical end  102  is a decreasing diameter bore  116  which connects with the interior bore  105  of the cylinder end  104 . The decreasing diameter bore  116  is formed such that it has a first narrowing point  118 , which connects to a large diameter channel  120 , which connects to a second narrowing point  122 , which in turn connects to a narrow diameter channel  124 . The narrow diameter channel  124  connects to the interior bore  105  of the cylindrical end  104 . 
     With reference to FIGS. 2, and  2 A, the valve  100  is located in the valve chamber  86  and oriented so that the spherical end  102  is facing the longitudinal bore  86 , and the cylinder end is directed toward the snap ring  95 . The protruding lip  107  engages the valve chamber side wall  88 . The valve  100  is movably retained within the valve chamber  86  by the snap ring  95  as is further described below. This allows the valve to move between a seated position where the valve head  115  of the spherical end  102  is firmly seated against the longitudinal bore  86 , and an unseated position where the protruding lip  107  is in contact with the snap ring  95 . 
     The endcap  33  is connected to the outer cylinder  34  at the cylindrical base  62  such that the O-ring  79  is between the outer cylinder  34  and the cylindrical base  62  to form a tight seal. The endcap  33  is connected to the inner cylinder  36  at the valve seat  80 . Preferably, the valve seat  80  and the inner cylinder  36  are each threaded and threadibly engaged to one another. Located within the inner cylinder and adjacent to the valve seat  80  is a short, tube-shaped stop sleeve  126 . 
     With reference also to FIGS. 2,  7  and  7 A, the cylinder head  38  is a solid cylinder of the same circumference as the outer cylinder  34  and has an outside end  150 , an inside end  152  and an outer surface  153 . A main aperture  154  is bored through the axial center of the cylinder head  38 . Extending from the inside end  152  and positioned radially outside of the main aperture  154  is an attachment sleeve  156 . The attachment sleeve is threaded at  158  for attaching the cylinder head  38  to the inner cylinder  36 . The cylinder head  38  also includes at least one longitudinal bore  160  radially offset from the main aperture  154  and extending through the cylinder head  38 . The cylinder head  38  is threadibly engaged to the outer cylinder  34  at the outer circumference  153  of the inside end  152 . Additionally, the cylinder head  38  includes an annular spherical groove  450  formed in the cylinder head  38 , a rounded end wall  157  and a shoulder slot  451 . 
     Braced against the inside end  152  of the cylinder head  38  is an auxiliary piston spring  170 . The auxiliary piston spring  170  is oriented such that it circumferentially surrounds the inner cylinder  36  and is circumferentially surrounded by the outer cylinder  34 . The auxiliary piston spring  170  is braced against an auxiliary piston  172 . The auxiliary piston  172  is ring-shaped and located such that it circumferentially surrounds the inner cylinder  36  and is circumferentially surrounded by the outer cylinder  34 . The auxiliary piston  172  includes a forward annular groove  174 , a rearward annular grove  176  and a face  177 . Located within the forward annular groove  174  is a forward auxiliary piston O-ring  178 , and located within the rearward annular groove  176  is a rearward auxiliary piston O-ring  180 . The forward auxiliary piston O-ring  178  contacts and forms a seal with the inner cylinder  36  and the rearward auxiliary piston O-ring  180  contacts and forms a seal with the outer cylinder  34 . 
     With reference to FIGS. 2,  5 ,  6 ,  7 , and  7 A, the inner telescoping member  32  includes a main piston  200 , a mount portion  202 , and a release mechanism  204 . The main piston  200  includes a main piston head  220 , an inner shaft  222 , an outer shaft  224 , and a locking mechanism  226 . 
     The main piston head  220  is cylindrical shaped and includes a face  230 , a main block  231 , an annular groove  232 , and an attachment sleeve  234 . The main piston head  220  is configured such that the main block diameter is just slightly less than the inner diameter  236  of the inner cylinder  36 . This allows the main piston head  200  to be moveably retained within the inner cylinder  36 . The face  230  is a flat, circular surface, which, when the strut is in operation, is in contact with the hydraulic fluid. Located within the annular groove  232  is a main piston O-ring  238 . The main piston O-ring contacts and forms a tight seal with an inner wall  240  of the inner cylinder  36 . The main piston head  200  also includes a sloping shoulder  242  which is adjacent to and a solid contiguous extension of the main block  231  opposite the face  230 . Extending from the sloping shoulder  242  is the attachment sleeve  234 . The attachment sleeve  234  has a rim  244  and an inner side  246 . Preferably, the inner side  246  is threaded. 
     The outer shaft  224  is tubular and includes a threaded head end  248  and at least two openings  250  through which at least two locking balls  304  move, as will be discussed further below. The outer shaft  224  is threadibly attached at the head end  248  to the inner side  246  of the piston head. The outer shaft  224  also includes a small bracing shoulder  449 . Moveably retained within the outer shaft  224  is the inner shaft  222 . The inner shaft  222  includes a trapezoidal groove  252  disposed circumferentially about the inner shaft  222  and an annular groove  254 . Located within the annular groove  254  is an inner shaft O-ring  258 . The inner shaft O-ring  258  contacts and creates a seal against an inner side  260  of the outer shaft  224 . protecting the locking mechanism  226  from contamination entering through the openings in the outer shaft  224  associated with the release mechanism  204 , which is more fully described below. 
     The locking mechanism  226  includes a locking sleeve  300 , a sleeve spring  302  and the two locking balls  304 . The sleeve spring  302  is braced against the rim  244  at one end of the spring and braced against the locking sleeve  300  at the other end of the spring. The locking sleeve  300  circumferentially surrounds the outer shaft  224 . Extending radially outward from the shaft  224 , is an annular small shoulder  303 . The locking sleeve  300  contacts and is urged against the small bracing shoulder  449  by the sleeve spring  302 . When the strut is not in the fully extended position, the locking mechanism is disengaged, as is shown in FIG.  5 . In this disengaged position, the locking sleeve  300  retains the locking balls  304  within the space formed by the alignment of the trapezoidal groove  252  and the openings  250 . 
     With reference to FIGS. 2 and 6, the release mechanism  204  includes a release sleeve  350 , a release sleeve spring  352  and a connector pin  354 . The release sleeve  350  is tube shaped and circumferentially disposed around the outer shaft  224 , outside of the outer telescoping member  32 . The release sleeve  350  has an outer surface  356  which is easily manipulable by hand, a spring retaining wall  358  and a spring bracing wall  360 . The outer surface  356  may be knurled or otherwise treated to improve its ability to be gripped and manipulated by hand. The release sleeve  350  is movably positioned by the release sleeve spring  352 . The release sleeve spring  352  is braced against the spring bracing wall  360  and positioned radially between the spring retaining wall  358  of the release sleeve  350  and the outside of the outer shaft  224 . The outer shaft  224  also has disposed circumferentially about it and firmly attached to it, a bracing ring  364 . The release spring  352  is braced against the bracing ring  364 . The release sleeve  350  also has a pair of radially extending, diametrically opposed apertures  362  through which a connector pin  354  is inserted. The connector pin  354  also passes through opposed longitudinal slots  368  in the outer shaft  224  and through an aperture in the inner shaft  222 . With this configuration, longitudinal movement of the release sleeve  350  will move the inner shaft  224  the same longitudinal distance as the release sleeve  350 . 
     The mount portion  202  includes an eyelet mount  380 , and a pin  384 . The eyelet mount  380  includes a threaded shank  386 , and an eyelet  388 . The threaded shank  386  is threadibly mounted on the outer shaft  224 . The eyelet  388  includes a solid ring  390 , a circular groove  392 , and an aperture  394  formed at the radial center of the ring  390 . This configuration of the eyelet  388  allows the strut to be mounted on a conventional axle swivel mount (not shown). The pin  384  passes through opposed apertures  387  in the outer shaft  224 , and through the threaded shank  386  thereby securing the mount portion  202  in place. 
     With reference to FIG. 2, when the strut is fully assembled and in operation, it contains hydraulic fluid (not shown). Preferably, the hydraulic fluid is military grade number MIL-H-83282. The hydraulic fluid is injected into the assembled strut  28  during manufacture through the two angular bores  68  formed in the sloping shoulder  64 . The angular bores  68  are then sealed by insertion of the sealing ball  74 , and then the plug  76 , into the large diameter sealing portion  72 . 
     Hydraulic fluid is retained in the transverse bore  67 , each of the small diameter channel portions  70  of the two angular bores  68 , the longitudinal bore  84 , the valve chamber  86 , a second hydraulic reservoir  400 , which is defined by the inner cylinder  222  and the face  230  of the main piston head  200  and in a first hydraulic reservoir  402 , which is defined by the space between the inner cylinder  36  and the outer cylinder  34  and limited by the auxiliary piston  172 . When the strut is in operation, a fluid flow path is defined by these members as follows. As the strut extends, hydraulic fluid flows from the first hydraulic reservoir  402  into the transverse bore  67 . Fluid flow follows the transverse bore  67  radially inward to the longitudinal bore  84 . Fluid flow follows the longitudinal bore along the radial center of the outer telescoping member  30  and into the valve chamber  86 . Fluid flows through the valve  100  and into the second hydraulic reservoir  400 . Fluid flow may also follow the same path in a reverse direction when the strut is being compressed. 
     With reference to FIGS. 2,  3 ,  4 , and  5  the strut is in a completely compressed position. As the strut extends, the main piston moves longitudinally in the second hydraulic reservoir  400  away from the valve  100 . This movement creates a suction force on the hydraulic fluid and draws fluid from the first hydraulic reservoir along the fluid flow path and into the main hydraulic chamber  400 . At the same time, fluid pressure builds up at the spherical end  102  of the valve. This forces the moveably retained valve  100  to move toward the snap ring  95 . This movement is halted when the protruding lip  107  contacts the snap ring. This position of the valve allows fluid to flow through both the narrow diameter channel  124  and the four large diameter fluid flow openings  108 ,  110 ,  112 ,  114 . This allows significantly increased fluid flow and therefore enables the strut to extend rapidly. 
     This rapid extension is also aided by the action of the auxiliary piston  172 . As the strut extends and hydraulic fluid is drawn from the first hydraulic reservoir  402 , the auxiliary piston  172  is pushed by the auxiliary piston spring  170  away from the cylinder head  38 . This motion both reduces the volume of the first hydraulic reservoir  402 , thus creating a variable reservoir for the hydraulic fluid, and helps inject the hydraulic fluid into the second hydraulic reservoir along the defined fluid flow path. The reduction in volume of the first hydraulic reservoir  402  prevents any suction force from building up within the first hydraulic reservoir  402  and counteracting the suction force created by the main piston  200 . Additionally, air from outside of the strut can flow through the offset longitudinal bore  160  in the cylinder head  38  and into the volume behind the moving auxiliary piston  172 . This action prevents a vacuum from forming behind the auxiliary piston  172  and therefore reducing the closure rate of the strut. These features allow the strut to open as rapidly as the main piston  200  can be moved and the valve  100  will allow. 
     With reference to FIGS. 6,  7 , and  7 A, as the strut  28  approaches its fully extended position the small bracing shoulder  449  passes into the shoulder slot  451 , and the locking sleeve  300  contacts the rounded end wall  157  of the cylinder head  38 . As the extending motion continues, the locking sleeve spring  300  is compressed. When the locking sleeve spring  300  is fully compressed, longitudinal motion of the main piston head  220  is arrested. In this position, the locking sleeve  300  no longer covers and retains the locking balls  304 . Longitudinal motion of the inner shaft  222  continues as is described below. Due to this continued longitudinal motion, the surface of the trapezoidal groove  252  applies a lateral force to the locking balls  304 , driving the locking balls  304  into the spherical groove  450  formed in the cylinder head  38  which lines up with the openings  250  formed in the outer shaft  224  when the strut  28  is in its extended position. Longitudinal motion of the inner shaft  222  ceases when the trapezoidal  252  has traversed completely past the locking balls  304 . 
     The continued longitudinal motion of the inner shaft  222  which is alluded to above is accomplished by the action of the release mechanism  204 . When the main piston head  220  and the outer shaft  224 , to which the main piston head  200  is attached, cease moving, the release sleeve  350  is urged further away from the cylinder head  38  by the release sleeve spring  352 . As the release sleeve spring extends and the release sleeve  350  moves away from the cylinder head  38 , the connector pin  354 , which is firmly attached to the release sleeve  350  and the inner shaft  222 , also moves, forcing the inner shaft  222  to move the same longitudinal distance as the release sleeve  350 . This action moves the trapezoidal groove  252  as is described above. The release sleeve  350  and inner shaft  222  stop moving when the connector pin  354  travels the entire extent of the opposed longitudinal slots  368 . 
     When the strut  20 ,  22  is in the position shown in FIGS. 8 and 9, the inner telescoping member  32  is locked into place in relation to the outer telescoping member  30  by the presence of the locking balls  304  in the spherical groove  450  and the openings in the shaft  250 . In this position, the weight of a load placed on the inner telescoping member is borne by the locking balls  304 . By this action, the door  24  attached to the strut  20 ,  22  can be locked into an open position as shown in FIG.  1 . 
     With reference to FIGS. 8,  9 ,  10  and  11 , the strut can be released from the locked position by actuating the release mechanism  204 . When a user moves the release sleeve  350  toward the cylinder head  38  as shown by the arrow, the connector pin  354 , which is securely attached to the release sleeve  350  moves the inner shaft  222  the same longitudinal distance that the release sleeve  350  is moved. As the release sleeve  350  is moved, the release sleeve spring  352  compresses, and the inner shaft  222  moves to the position where the trapezoidal groove  252  lines up with the opposed openings  250  in the outer shaft  224  and the spherical groove  450 . This allows the locking balls  304  to move back into the trapezoidal groove  252 . Because the locking balls  304  no longer maintain contact with the cylinder head  38 , the outer shaft  224  and the main piston head  200  may move longitudinally toward the valve  100  and the fully compressed position. 
     As the strut compresses toward this position, the main piston  200  traverses the second hydraulic reservoir  400  longitudinally. This motion of the main piston  200  creates pressure against the face  230  of the main piston  200  and is applied to the hydraulic fluid in the second hydraulic reservoir  400 , to force hydraulic fluid to flow from the main hydraulic chamber  400  toward the first hydraulic reservoir  402  along the defined fluid flow path. 
     In turn, this fluid flow is dampened by the valve  100 . As fluid flows from the second hydraulic reservoir  400 , pressure builds against the interior bore  105  of the cylinder end  104  of the valve  100 . Because the valve  100  is moveably retained within the valve chamber  86 , this pressure causes the valve to slide toward the valve seat  85 . The valve head  115  of the spherical end  102  of the valve  100  contacts the valve seat  85  and creates a seal. At the same time, the sloping shoulder  106  of the valve  100  contacts the valve chamber sloping wall  90 . This positioning of the valve  100  prevents hydraulic fluid from flowing through any of the four large diameter fluid flow openings  108 ,  110 ,  112 ,  114 . As a result, all fluid flow must pass through the single narrow diameter channel  124 . This restriction of hydraulic fluid flow serves to dampen the strut and slow the rate at which it compresses. The valve  100  may also be spring biased toward the longitudinal bore  86  to provide for a faster valve response (not shown). A strut can, of course, be manufactured such that the valve  100  and valve chamber  86  are oriented in the opposite direction of the one described, to enable the strut to close rapidly and open slowly, or a simple restrictor valve can be used when it is desired that the strut open slowly and close slowly. When the face  230  reaches and contacts the stop sleeve  126 , the hydraulic fluid flow stops and the strut has reached its fully compressed position. 
     As additional fluid enters the auxiliary chamber  402 , the fluid applies pressure against the face  177  of the auxiliary piston  172 . This pressure is opposed by the auxiliary piston spring  170 , which provides a minimal counter-force to the pressure created by the main piston as the strut compresses. The counter-force allows the strut to smoothly transition from the locked position to the beginning of the compression action, thereby avoiding an initial slippage or jolt as the release mechanism  204  is activated. Pressure applied to the face  177  of the auxiliary piston  172  then compresses the auxiliary piston spring  170 . As the auxiliary piston spring  170  compresses, the auxiliary piston  172  moves toward the cylinder head  38  and the volume of the first hydraulic reservoir  402  extends allowing more hydraulic fluid to exit the second hydraulic reservoir  400 , travel along the fluid flow path, and enter the first hydraulic reservoir  402 . 
     From the foregoing, it will be appreciated that the dampening strut of the present invention provides a strut which can open and close at two different speeds, is a compact, sealed, self-contained unit which requires no external fluid pump or injection device, and can be automatically locked in an open position. 
     While a particular form of the invention has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.

Technology Category: 2