Hydraulic Actuator Having Frangible or Deformable Components

The present invention relates to hydraulic actuators, primarily for aircraft, having frangible components to enhance survivability of the associated aircraft from ballistic action and more particularly to a unique construction of such actuator using piston and/or rod end glands of a suitable thermoplastic material.

There have been many designs of hydraulic actuators for dealing with damage caused by hostile gunfire. Normally such actuators are employed in pairs or greater numbers such that if one actuator is lost, another can continue to operate the control surface or other aircraft component. This redundant scheme will not be successful, however, if a damaged actuator is jammed such that the additional actuators or actuator cannot move it.

One approach to the problem has been to form the pistons of such actuators of brittle material which is strong enough for normal operation but having weak sections which break away if forced against an internally projecting deformation of the cylinder. This permits the rod and piston to be moved past the damaged part of the cylinder. Another design uses frangible layers of plastic material on the interior surface of the cylinder and/or the exterior surface of the rod such that inward deformation or "petalling" of the metal cylinder will, in most cases, be less than the thickness of the plastic layer. Hence the piston, which is sized to the interior of the plastic layer, will pass even though the actuator is inoperative. Should a projectile penetrate to the point of impacting on the rod, deforming the rod, it will also break away the plastic layer on the outside of the rod. Since the port in the rod end gland is sized to the plastic layer on the rod, the rod will pass through the end gland even if deformed substantially.

The above described designs are useful and effective if the actuator is sufficiently large to provide clearance for the broken away or deformed parts. Many small actuators are used, however, in which the clearance between the cylinder bore and the piston rod diameters is quite small, leaving insufficient space to accommodate broken away or deformed metal parts. The presence of such deformed or broken parts may result in requiring excessively high loads to unjam the actuator. There is, therefor, a need for a design for small actuators which will enable them to avoid jamming if hit by ballistic fire.

Because it is known that thermoplastic materials will deform plastically under local point contact this appeared to provide a possible solution. Certain engineering type plastics appear to have sufficient strength to withstand the operating pressures in an actuator under normal conditions but the placing of seal grooves on such a plastic piston has presented a problem. The use of conventional seal grooves, conventionally located, so weakens the already marginally strong plastic piston that it will experience structural failure at lower than normal operating pressures and/or at a sharp seal radius under a pressure impulse. Applicants have found that if the required seals can be incorporated into the thermoplastic components without using the usual centrally located deep groove and without the sharp seal radii, this will eliminate failures at this point. By locating such seals on the outside edge or edges of the piston, for example, the deep central groove is eliminated, most of the width of the piston is preserved for strength and different types of ring energized seals can be used. Larger seal fillet radii can be used providing better fatigue resistance. Since the thermoplastics are seal materials in themselves, a certain amount of compression of the sides of the components (piston or end gland) will often provide sufficient sealing of the piston against the rod, for example.

FIG. 1 is a cross sectional drawing of a typical installation of a frangible piston installed in an actuator wherein the clearance between the rod and cylinder is small such that piston fragments would be likely to cause jams interfering with movement of the rod.

FIG. 2 is a cross sectional drawing of an actuator having small clearance between the piston rod and the cylinder having a frangible or deformable rod end gland.

FIG. 3 is an enlarged cross sectional drawing of a portion of FIG. 1, showing the seal structure in greater detail.

FIG. 4 is a cross sectional drawing at approximately the same scale as FIG. 3, of a corresponding part of a conventional (prior art) piston.

Referring now to FIG. 1 a small diameter actuator 10 is shown including a cylindrical housing 12 having a plurality of ports for ingress and egress of hydraulic fluid and including a head end gland 13, a piston rod 14 passing through said head end gland and a rod end gland 15, and a piston 16 which is captured between a shoulder on rod 14 and a collar 18 which may be threadedly engaged or otherwise removably secured to said rod. Piston 16 is formed of a strong engineering plastic material with properties of high stiffness and low shear strength such as Arlon 1555 made by Greene, Tweed & Co. Located at the outer circumferentiated edges of piston 16 are a pair of dynamic seals 20 and 22, discussed in more detail below.

FIG. 2 shows a similar type of small diameter actuator 30 including a cylindrical housing 32 with fluid access ports, a piston 34 and a rod 36 in said housing, a separate generally annular rod end gland 38 abutting against a shoulder 39 on the inside of cylinder 32, and a spanner nut 40 threadedly engaged with the inside surface of the cylinder 30 at the end adjacent the gland, such that it secures gland 38 against shoulder 39. Rod end gland 38 is formed of an engineering plastic such as Arlon 1555, described above, or a plastic having similar properties. A dynamic seal 42 is formed on an inner circumferential edge of gland 38 such that it is adjacent to piston rod 36. Gland 38 may also include a static seal 44 on its outer circumferential surface sealing against the inside of the cylindrical housing. Seal 42 is, or may be essentially the same as seals 20 and 22 of FIG. 1, except that it seals to the inside against rod 36 rather than to the outside against the cylinder. Seal 44 may be a conventional O-ring in a groove.

FIG. 3 is an enlarged cross-sectional drawing of a portion of FIG. 1 showing the structure of piston 16 and seals 20 and 22 in greater detail. As set forth above, the seals are placed in grooves 24 and 26 on the outer circumferential edges of piston 16 thereby leaving an uninterrupted load carrying land width which in this embodiment is about 50% of the width of the entire piston. It could be made somewhat wider, if desired if it were thought necessary to provide more strength for normal operation. Each seal consists of a hollow annular seal jacket of polytetrafluorethylene (Teflon) carrying a rubber O-ring which is sized to provide a force tending to urge the upper surface of the seal against the inside surface of cylinder 12. Because of the flexibility of piston 16, it will tend to be compressed against piston rod 14 by the compression of collar 18, so an additional seal against the piston rod 14 is usually not necessary. The seal 42 of FIG. 2 is, or may be, essentially the same as seals 20 and 22 except that it is designed to exert a force inwardly against piston rod 36.

FIG. 4 is a cross-sectional drawing of a part of a conventional piston exemplary of the prior art, drawn at approximately the same scale as FIG. 3. In this piston 50, which is of metal, the seal groove 52 is centered on the circumferential face of the piston and is formed with a fairly small radius (typically such as 0.01") at the bottom of the groove. An O-ring 54 at the bottom of groove 52 exerts a force urging the Teflon ring seal 56 outwardly against the inside of the cylinder. Piston 50 will preferably also carry a static seal 46 on its inner diameter against the piston rod which seal may be a conventional O-ring in a groove. With a steel piston, for example, the groove location resulting in two small lands acting against the cylinder and the small radius fillets at the bottom of the groove do not normally result in weak spots such as could cause failure of the piston. Such a piston is normally very strong and not frangible as is the case of the piston of FIG. 3. Should a piston like that of FIG. 4 be formed of the plastic material described above, probable failure in normal operation would result because the two small radius fillets would be weak spots susceptible to fatigue as the narrow lands on each side of the seal are urged back and forth against the cylinder wall in normal operation.

Applicants' piston design, as shown in FIGS. 1 and 3 provides substantially greater width in the land contacting the cylinder and the seal grooves have much larger radii thereby avoiding the concentration of stress such as would occur in the structure of FIG. 4. Thus such a piston design is relatively stiff and will survive in normal operation, but if jammed into a petal deformation of the cylinder wall due to its low shear strength the piston will cut and pull through or will deform and pull around a dent in the cylinder wall.

Similarly, an actuator having a rod end gland 38 such as described in connection with FIG. 2, will also deform and extrude or split if a deformed rod 36 is forced through it. Yet this end gland has sufficient strength and stiffness to withstand normal operating forces.

While only a limited number of embodiments are described herein it is recognized that modifications will be apparent to those skilled in the art. Obviously, any particular cylindrical actuator may be formed with either or both of the frangible or deformable parts described above. The teachings herein are clearly useful with cylindrical actuators employed in tandem or in parallel.