Abstract:
The invention provides a telescopic member that includes a cylinder in which a rod carrying a piston is mounted to slide telescopically, the telescopic member including internal abutment mechanisms that are overridable over a predetermined force when the rod comes into abutment inside the cylinder in the shortening direction. The internal abutment mechanisms include an abutment piece extending in an open bore within the rod and projecting therefrom, the abutment piece being held to the rod via a pin.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to the benefit of and incorporates by reference in its entirety French Patent Application No. 06 09903 filed on Nov. 13, 2006. 
     FIELD OF THE INVENTION 
     The invention relates to a telescopic member having an overridable internal abutment. 
     BACKGROUND OF THE INVENTION 
     Numerous telescopic members are known in which the retracted position is defined by an internal abutment, e.g. actuators or undercarriage shock absorbers for aircraft. 
     In the more specific circumstance of shock absorbers, it is often necessary to define an abutment in shortening, since such shock absorbers are also used for retracting the undercarriage, the internal abutment thus serving to define the retracted position of the undercarriage. 
     Nevertheless, the working stroke defined by the abutment can be shorter than the stroke needed to absorb landing energy in the event of a hard landing. 
     Telescopic members are known that have an overridable abutments. In particular, it is known to use an abutment piece disposed in the end of the shock absorber with the rod of the shock absorber coming to bear thereagainst on reaching the abutment position. When the rod presses with a force above a predetermined threshold, that causes the abutment piece to buckle, thereby providing the rod with overtravel beyond its retracted position. 
     Nevertheless, such abutment pieces are of complex shape in order to enable buckling to be initiated, and because of manufacturing dispersion, it is very difficult to ensure that the buckling threshold is complied with. 
     OBJECT OF THE INVENTION 
     An object of the invention is to provide a telescopic member with an overridable abutment that is of a simple type. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to achieve this object, the invention provides a telescopic member comprising a cylinder in which a rod carrying a piston is mounted to slide telescopically, the telescopic member including internal abutment means that are overridable over a predetermined force when the rod comes into abutment inside the cylinder in the shortening direction. According to the invention, the internal abutment means comprise an abutment piece extending in an open bore within the rod and projecting therefrom, the abutment piece being held to the rod via a pin. 
     Thus, when the rod comes into abutment in the shortening direction with a force exceeding the shear threshold of the pin, the pin gives way and the abutment piece is pushed into the bore of the rod, thereby releasing overtravel for the rod in the shortening direction. The force causing the pin to shear can be well controlled, such that this force is much better determined than with prior art abutment pieces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       These aspects of the invention and other aspects can be understood more fully in the light of the sole FIGURE of the accompanying drawing, which is a section view through a shock absorber in a particular embodiment of the invention incorporating the extension/retraction function, and shown in the extended position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is illustrated herein for a shock absorber used for a helicopter main undercarriage of the type that comprises a rocker beam directly hinged to the fuselage of the helicopter, the shock absorber described herein being for hinge mounting between the rocker beam and a hinge point on the fuselage. 
     The shock absorber comprises a main cylinder  1  in which a rod  2  is mounted to slide telescopically. For this purpose, the rod  2  carries a sealing piston  3  that slides inside the main cylinder  1  and that defines therein an annular chamber  4  (interrupted by the rod  2 ), and an uninterrupted chamber  5  filled with hydraulic fluid, forming a supply of hydraulic fluid. The ends of the main cylinder  1  and of the rod  2  carry lugs  6 ,  7  for hinge connection of the shock absorber to the fuselage at one end and to the rocker beam of the undercarriage at the other end. 
     The shock absorber has a first auxiliary cylinder  10  rigidly mounted on the main cylinder, e.g. as shown by screw-fastening with an interposed sealing gasket, close to the end lug  6  of the main cylinder  1 . The first auxiliary cylinder  10  extends in a direction substantially perpendicular to a longitudinal axis of the main cylinder  1 . A first piston  11  is mounted to slide in sealed manner in the first auxiliary cylinder  10  to separate the inside thereof into a first pneumatic chamber  12  filled with gas under pressure and a first fluid chamber  13  in communication with the uninterrupted chamber  5  of the main cylinder  1  via a first throttle member  14 . 
     The shock absorber has a second auxiliary cylinder  20  rigidly mounted on the main cylinder, e.g. as shown by screw-fastening with an interposed sealing gasket, in the proximity of the end lug  6  of the main cylinder  1 . The second auxiliary cylinder  20  extends on the other side of the main cylinder  1  substantially on the same axis as the first auxiliary cylinder  10 . A second piston  21  is mounted to slide in sealed manner inside the second auxiliary cylinder  20  to subdivide the inside thereof into a second pneumatic chamber  22  filled with gas under pressure, and a second fluid chamber  23  in communication with the uninterrupted chamber  5  of the main cylinder  1  via a second throttle member  24 . 
     The throttle members  14  and  24  are well known to the person skilled in the art and they are not described in detail herein. 
     The fluid flow connection between the uninterrupted chamber  5  and the auxiliary cylinders  10  and  20  in this example is as simple as possible, since the auxiliary cylinders are directly in communication with the uninterrupted chamber  5  via the throttle members  14 ,  24  disposed at the inlet to the auxiliary cylinders, thus making it possible to avoid using any pipework. 
     The first pneumatic chamber  12  is inflated to a first pressure, while the second pneumatic chamber  22  is inflated to a second pressure that is greater than the first pressure. Typically, the first pneumatic chamber  12  is inflated to 20 bars, while the second pneumatic chamber  22  is inflated to 80 bars. 
     The operation of the shock absorber during a landing is as follows. Under drive from the rocker beam tending to shorten the shock absorber, the rod  2  is pushed into the main cylinder  1 , thereby forcing the hydraulic fluid contained in the uninterrupted chamber  5  to pass through the first and second throttle members  14 ,  24  to fill the first and second hydraulic fluid chambers  13 ,  23 , thereby pushing back the first and second pistons  11 ,  21  to compress the gas contained in the first and second pneumatic chambers  12 ,  22 . 
     Under normal conditions, the annular chamber  4  is connected via a first controlled valve  30  to a supply of hydraulic fluid or hydraulic tank of the aircraft (not shown) typically at a pressure of about 5 bars, such that the annular chamber  4  is filled progressively with hydraulic fluid as the rod  2  is pushed in, thereby avoiding any cavitation. 
     When the helicopter takes off, the gas under pressure in the pneumatic chambers  12 ,  22  pushes back the hydraulic fluid contained in the fluid chambers  13  and  23  through the throttle members  14  and  24  towards the uninterrupted chamber  5 , thereby pushing the rod  2  towards its extended position. The hydraulic fluid contained in the annular chamber  4  is pushed back to the pressurized hydraulic fluid supply of the aircraft. 
     Since the pneumatic chambers are thus placed beside the main cylinder, the shock absorber thus has an extended length L that is particularly compact. 
     In a particular aspect of the invention, the shock absorber is designed to enable the rocker beam to be retracted in order to limit its aerodynamic drag in flight. For this purpose, controlled shortening of the shock absorber is organized as follows. High pressure hydraulic fluid (typically at 206 bars) is injected into the annular chamber  4  via the first controlled valve  30 , thereby tending to retract the rod  2  into the main cylinder  1 . Simultaneously, a second controlled valve  31  enables the uninterrupted chamber  5  to be put into communication with a flask  40  such that the hydraulic fluid contained in the uninterrupted chamber  5  can be evacuated into said flask  40 . The hydraulic fluid received in the flask  40  pushes back a piston  41  slidably mounted in the flask  40 , which in turn pushes back the hydraulic fluid present under the piston  41  towards the supply of the aircraft. It should thus be observed that the hydraulic fluid from the uninterrupted chamber  5  does not become mixed with the hydraulic fluid of the aircraft. When the rod  2  comes into abutment against the end wall of the main cylinder  1 , the first controlled valve  30  is closed such that the hydraulic fluid present in the annular chamber  4  is held captive, thereby preventing the shock absorber from expanding. 
     It should be observed that during this retraction, the pneumatic chambers  12  and  22  are not compressed since their inflation pressures are higher than the pressure in the flask  40  (substantially the same pressure as the tank, i.e. 5 bars), such that the hydraulic fluid passes preferentially towards the flask  40 . 
     In order to cause the shock absorber to return to its extended position, it suffices to open the first controlled valve  30  to put the annular chamber  4  into communication with the supply of the aircraft, and simultaneously to open the second controlled valve  31 , if closed. Substantially the same pressure then exists on either side of the piston  3 , and because of the differences in section between the annular chamber  4  and the uninterrupted chamber  5 , the rod is driven outwards and the shock absorber extends, until it returns to the position shown in  FIG. 1 . 
     In the invention, the shock absorber is capable of overtravel in the event of a particularly hard landing. For this purpose, the rod  2  includes a hollow orifice opening out to the side of the piston  3  and receiving an abutment piece  50  in the bore in the rod  2  so as to project from the rod  2 . The abutment piece  50  is held to the rod by a pin  51 . The abutment piece  50  has an end wall, and a sealing gasket  52  is disposed between the rod  2  and the abutment piece  50 , thus enabling the bore of the rod to be isolated from the uninterrupted chamber  5 . 
     When the shock absorber is used for retracting the undercarriage, the abutment piece  50  comes into abutment against the end wall  53  of the main cylinder  1 , thereby defining the retracted position of the shock absorber. When the shock absorber is used for damping impacts on landing, the abutment piece  50  normally does not engage the end wall  53  of the main cylinder  1 . However, during certain hard landings, not only can the abutment piece  50  come into contact with the end wall  53 , but the energy that needs to be absorbed can be such that this contact does not suffice to stop the shock absorber being shortened. In the invention, the pin  51  is then designed to shear beyond a predetermined force threshold for the purposes firstly of absorbing a fraction of the surplus landing energy by rupturing the pin  51 , and also of allowing the rod  2  to perform overtravel in the main cylinder  1 , thereby overcompressing the pneumatic chambers  12 ,  22 , also contributing to absorbing another fraction of the surplus landing energy. 
     The shearing rupture of the pin propels the abutment piece  50  towards the bottom of the bore in the rod  2 , such that the abutment piece  50  no longer co-operates with the sealing gasket  52 . The hydraulic fluid contained in the uninterrupted chamber  5  can then flow (slowly) between the abutment member  52  and the wall of the bore in the rod  2  so as to empty out through a bleed orifice  54  located at the bottom of the rod, thereby contributing to reducing the high pressure that exists in the shock absorber because of the overcompression. 
     It should be observed that under normal circumstances, the bleed orifice  54  runs the risk of putting the annular chamber  4  into communication with outside air. To avoid any such air mixing with the hydraulic fluid of the aircraft, a floating piston  55  is disposed in sealed manner between the cylinder  1  and the rod  2  such that the arrival of hydraulic fluid via the first controlled valve  30  takes place under the floating piston  55 , while air coming from the bleed orifice  54  can only fill the space that extends between the floating piston  55  and the piston  3  of the rod  2 . There is thus no risk of the hydraulic fluid of the aircraft being polluted by outside air. 
     The invention is not limited to the above description, but on the contrary covers any variant coming within the ambit defined by the claims. 
     In particular, although the invention is shown herein in association with a shock absorber used for a helicopter main undercarriage of the type that includes a rocker beam directly hinged to the fuselage of the helicopter, the shock absorber shown being hinged between the rocker beam and a hinge point on the fuselage, the invention is not limited to an undercarriage of that type; for example it can be applied perfectly well to an undercarriage of the direct type in which the strut forms the main cylinder  1  and the rod  2  carries the wheel(s) directly. 
     Although the auxiliary cylinders are shown as being fastened directly to the main cylinder so that the throttle members are disposed at the inlet to the auxiliary cylinders, the auxiliary cylinders could be located elsewhere. For example, they could be secured to the side of the main cylinder, parallel thereto, the auxiliary cylinders being connected to the main cylinder via fluid flow connections, e.g. using rigid pipework. It is also possible to fix the auxiliary cylinders in the wheel bay of the aircraft, but that would require additional interfaces to be provided on the aircraft. Under such circumstances, provision should be made for a fluid flow connection with a flexible hose or a hose capable of accommodating movements of the shock absorber relative to the hose. Under all circumstances, it is preferable for the throttle members to be disposed at the inlet to the fluid flow connections such that said connections and the auxiliary cylinders are not subjected to the high pressures that exist in the uninterrupted chamber  5  while the shock absorber is being shortened. 
     Although the present description relates to a shock absorber having two pneumatic chambers, it would naturally be possible to apply the invention to a shock absorber having a single pneumatic chamber, or on the contrary having more than two pneumatic chambers. Finally, although the particular positioning of the member  50  in the bore of the rod  2  with retention by a pin is illustrated herein in an application to a shock absorber, this disposition applies more generally to any type of telescopic cylinder device.