Abstract:
A piston seal includes an inner surface configured to extend proximate a perimeter of a piston. Also included is an outer surface disposed proximate a chamber wall, wherein the chamber wall defines a chamber for the piston and the piston seal to translate within. Further included is a first axial side exposed to a first pressure portion of the chamber. Yet further included is a second axial side exposed to a second pressure portion of the chamber. Also included is at least one pressure differential reducing hole extending radially through the piston seal from the inner surface to the outer surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to pneumatic actuators, and more particularly to a low friction piston seal for pneumatic actuators, as well as a method of reducing frictional forces of the low friction piston seal. 
         [0002]    Pneumatic actuators have one or more pressure chambers that contain fluids, such as air, for example, at various pressures. The pressure in the chamber(s) generates forces on a piston and the forces cause the piston to move until the sum of all the forces on the piston equals zero. Piston seals are used to seal a designed gap present between an outer surface of the piston and a piston chamber wall. The piston seal prevents fluid disposed at a higher pressure on one side of the piston from flowing into fluid disposed at a lower pressure on the other side of the piston. The high pressure fluid forces the piston seal outward against the chamber wall, thereby providing the sealing function. The magnitude of the force that biases the piston seal against the chamber wall is dependent on the area that the pressure is applied on and the difference between the high and low pressure. This outward force results in friction as the piston strokes. The friction forces may vary drastically and cause difficulty in controlling the valve. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    According to one embodiment, a piston seal includes an inner surface configured to extend proximate a perimeter of a piston. Also included is an outer surface disposed proximate a chamber wall, wherein the chamber wall defines a chamber for the piston and the piston seal to translate within. Further included is a first axial side exposed to a first pressure portion of the chamber. Yet further included is a second axial side exposed to a second pressure portion of the chamber. Also included is at least one pressure differential reducing hole extending radially through the piston seal from the inner surface to the outer surface. 
         [0004]    According to another embodiment, a method of reducing frictional forces of a piston seal is provided. The method includes exposing a first axial side of the piston seal to a first pressure portion of a chamber. The method also includes exposing a second axial side of the piston seal to a second pressure portion of the chamber. The method further includes reducing a pressure differential between an inner surface and an outer surface of the piston seal by fluidly coupling the inner surface and the outer surface with at least one pressure differential reducing hole extending radially through the piston seal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0006]      FIG. 1  is a cross-sectional view of a piston assembly; 
           [0007]      FIG. 2  is a perspective view of a piston seal of the piston assembly; 
           [0008]      FIG. 3  is a cross-sectional view of the piston seal taken along line  3 - 3  illustrating a pressure differential reducing hole; and 
           [0009]      FIG. 4  is a flow diagram illustrating a method of reducing frictional forces of the piston seal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    Referring to  FIGS. 1 and 2 , a pneumatic actuator system  10  is generally illustrated. The pneumatic actuator system  10  may be employed in numerous contemplated applications, with one example being a bleed valve system of an aircraft. The pneumatic actuator system  10  includes a chamber wall  12  that at least partially defines a chamber  14  for a piston  16  to translate within. The piston  16  may be formed in numerous geometries. In the illustrated embodiment, the piston  16  is a cylindrical structure. Irrespective of the precise geometry, it is to be understood that the shape of the piston  16  substantially corresponds to the shape of the chamber  14 . The piston  16  is employed to separate and control portions of the chamber  14 . As one can appreciate, the pneumatic actuator system  10 , and more particularly the chamber  14 , may include a plurality of pistons for defining more than two chamber portions. 
         [0011]    In the exemplary embodiment, the piston  16  includes a first side  18  and a second side  20 . The first side  18  is exposed to a first pressure portion  22  of the chamber  14  and the second side  20  is exposed to a second pressure portion  24  of the chamber  14 . The pressures within each of the first pressure portion  22  and the second pressure portion  24  are distinct, with one pressure being higher than the other. For purposes of explanation, the first pressure portion  22  is at a higher pressure than the second pressure portion  24 . 
         [0012]    The piston  16  is dimensioned to provide a gap between a piston outer surface  26  and the chamber wall  12  to account for tolerances and positional variance issues associated with manufacturing, thereby ensuring translation of the piston  16  within the chamber  14 . A piston seal  28  is included to seal the above-noted gap. The piston seal  28  is formed to correspond to the geometry of the piston  16 . In the illustrated embodiment, the piston seal  28  is a substantially circular ring that corresponds to the substantially circular piston. Of course, other shapes are possible. The piston seal  28  may comprise a single, continuous structure that completely surrounds the piston outer surface  26 , or may comprise a plurality of segments that are spaced around the piston outer surface  26 . As illustrated in  FIG. 2 , a break  41  is provided to define an embodiment comprising a plurality of segments. The break facilitates expansion or contraction of the piston seal  28 , as required during operation of the pneumatic actuator system  10 . Alternatively, the piston seal  28  may include breaks to segment the overall structure, but overlap of the segments is provided to reduce leakage during operation. Irrespective of the precise configuration, the piston seal  28  is disposed within an annular groove  30  that extends proximate the piston outer surface  26 . Specifically, an inner surface  32  of the piston seal  28  is disposed in close proximity with an annular groove base wall  34 . 
         [0013]    The piston seal  28  comprises a first axial side  36  that is exposed to the first pressure portion  22  of the chamber  14  and a second axial side  38  that is exposed to the second pressure portion  24  of the chamber  14 . Additionally, the piston seal  28  includes an outer surface  40  that is disposed in close proximity to the chamber wall  12 . A slight gap is disposed between the outer surface  40  and the chamber wall  12 , as is the case with the interface between the inner surface  32  and the annular groove base wall  34 . The high pressure fluid imposes a force on the inner surface  32  and biases the piston seal  28  radially outwardly into contact with the chamber wall  12 , thereby causing friction as the piston  16  translates within the chamber  14 . The pressure differential between the first pressure portion  22  and the second pressure portion  24  also introduces a pressure force on the outer surface  40 , with the pressure profile along the outer surface  40  comprising a pressure gradient. The pressure gradient includes lower pressures proximate the second axial side  38  that is exposed to the second pressure portion  24 , relative to portions of the outer surface  40  that are proximate the first pressure portion  22 . The pressure gradient results in a pressure differential between the inner surface  32  and the outer surface  40 . 
         [0014]    At least one, but typically a plurality of pressure differential reducing holes  42  extends radially within the piston seal  28  from the inner surface  32  to the outer surface  40 . The plurality of pressure differential reducing holes  42  are circumferentially spaced from each other around the piston seal  28 . 
         [0015]    Referring to  FIG. 3 , a force diagram is illustrated with respect to a cross-sectional portion of a segment of the piston seal  28 . Specifically, the segment illustrates one of the plurality of pressure differential reducing holes  42 . As shown, the first pressure portion  22  of the chamber  14  is disposed along the first axial side  36 , while the second pressure portion  24  of the chamber is disposed along the second axial side  38 . The high pressure fluid effects the radial location of the piston seal  28  by interacting with the inner surface  32  and the outer surface  40 . The plurality of pressure differential reducing holes  42  allow fluid coupling, or communication, between the inner surface  32  and the outer surface  40 , such that the forces acting on these surfaces are more balanced, relative to a piston seal  28  without such holes. As illustrated, the plurality of pressure differential reducing holes  42  facilitates a majority of an axial width  44  of the outer surface  40  of the piston seal  28  having a substantially uniform pressure distribution  46 , with only a relatively small portion of the outer surface  40  experiencing a pressure gradient  48 . 
         [0016]    Advantageously, the plurality of pressure differential reducing holes  42  reduces frictional forces exerted on the piston seal  28  which provides benefits associated with stability and control. Additionally, a more reliable and predictable displacement model may be established by decreasing hysteresis in the actuator and valve system(s) that the piston is associated with. 
         [0017]    A method of reducing frictional forces of a piston seal  100  is also provided, as illustrated in  FIG. 4  and with reference to  FIGS. 1-3 . The pneumatic actuator system  10  and more specifically the piston seal  28  have been previously described and specific structural components need not be described in further detail. The method of reducing frictional forces of a piston seal  100  includes exposing  102  the first axial side  36  of the piston seal  28  to the first pressure portion  22  of the chamber  14 . The second axial side  38  of the piston seal  28  is exposed  104  to the second pressure portion  24  of the chamber  14 . The method  100  also includes reducing  106  a pressure differential between the inner surface  32  and the outer surface  40  by fluidly coupling the inner surface  32  and the outer surface  40  with at least one, but typically the plurality of pressure differential reducing holes  42 . 
         [0018]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.