Abstract:
The present invention is directed to a wobble piston and seal assembly for an oil free compressor. An upwardly directed curvature is provided on a piston surface which supports the seal. The curvature is located adjacent the perimeter of the surface to impart a slight dish shape to the surface. Preferably, the curvature has the same radius as the bend radius of the seal when the piston head is inserted into a cylinder. When the seal is initially clamped to the support surface, the seal is formed to take on the curvature of the support surface. Consequently, the seal is preformed into a shallow cup shape prior to final forming when the piston and seal assembly are inserted into a cylinder.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention is a Continuation of U.S. patent application Ser. No. 09/273,585 filed Mar. 22, 1999, said U.S. Patent Application is herein incorporated by reference in its entirety. The present application also incorporates U.S. patent application Ser. No. 09/247,705, filed Feb. 9, 1999 by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of compressors, and particularly to a wobble piston and seal assembly for an oil free compressor. 
     BACKGROUND OF THE INVENTION 
     One type of compressor for air and other gases is referred to as an oil free compressor. This is a reciprocating compressor in which lubricating oil is not required between a piston head and the adjacent walls of a cylinder in which the piston head is reciprocated. In an oil-lubricated compressor, the piston head is sized to only reciprocate in the cylinder. A connecting rod is connected to the piston head with a wrist pin which permits the piston head and connecting rod to rotate relative to each other. During operation of the compressor, oil is splashed or pumped from a sump onto the walls of the cylinder and onto bearing surfaces between the wrist pin and the connecting rod. At least one piston ring seal is provided in an annular groove around the perimeter of the piston to maintain a gas tight seal which prevents leakage of the compressed gas from a compression chamber and prevents most of the lubricating oil from flowing past the piston ring seals to the compression chamber. However, a small amount of lubricating oil may flow past the seal and into the compression chamber and contaminate the compressed gas. 
     In one common type of oil free compressor, the piston head is formed integrally with the connecting rod so that they do not rotate relative to each other. Since a driven end of the connecting rod is moved about a circle by an eccentric or a crank pin, the piston head will rock or wobble as it is reciprocated in a cylinder. The piston head is relatively thin and sufficient clearance must be provided between the piston head and the cylinder walls to allow the piston head to wobble. Because of the wobble or rocking motion of the reciprocating piston, greater demands are placed on a seal which must extend between the piston head and the cylinder walls. The seal is generally cup shaped when inserted into the cylinder and is formed from a resilient, low friction material which will press against and slide along the cylinder walls as the piston head wobbles during reciprocation. 
     One method used for forming a cup shaped seal on a wobble piston has been to clamp a flat ring or washer shaped piece of seal material to a flat surface on the piston head. The piston head and attached seal ring are forced into a cylinder. As the piston head enters the cylinder, the seal forms a 90° bend next to the cylinder wall to impart a cup shape to the seal. The fibers in the seal at the outside of the bend become highly strained as they are bent 90°, weakening the seal. In order to reduce the strain in the seal at the bend, the seal was formed from a softer material than otherwise would be preferred. The softer material is subject to greater wear and consequently has a shorter operating life than may be achieved with a harder seal material. 
     When the seal is bent into the cup shape, the region of the seal adjacent the bend tends to separate or pull away from the adjacent flat surface on the piston head. Consequently, the seal is not supported adjacent the bend. As the cylinder pressure increases during each cycle of compressor operation, the seal is forced downwardly toward the flat piston head surface, causing the cup bend radius to decrease. The smaller cup radius of the seal increases bending stress on the seal. Since the cylinder pressure varies over each stroke of the piston, the resultant seal bending stress is cyclic. At higher pressures, the unsupported portion of the seal in the region of the bend is forced towards the flat piston head surface, subjecting the seal material to bending fatigue and possible premature fatigue failure. While this problem may occur in a single stage compressor at moderate pressures, it is even more critical in a second stage high pressure cylinder of a two stage oil free compressor. Premature seal failure in the second stage has been an impediment to a successful, commercial two stage oil free wobble piston air compressor. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a wobble piston and seal assembly for an oil free compressor. An upwardly directed curvature is provided on a piston surface which supports the seal. The curvature is located adjacent the perimeter of the surface to impart a slight dish shape to the surface. Preferably, the curvature has the same radius as the bend radius of the seal when the piston head is inserted into a cylinder. When the flat annular seal is initially clamped to the support surface, the seal is formed to take on the curvature of the support surface. Consequently, the seal is preformed into a shallow cup shape prior to final forming when the piston and seal assembly are inserted into a cylinder. 
     When the piston and seal assembly are inserted into a cylinder, the seal is bent 90° from a plane through the piston head to form a cup shape. The lower surface of the seal remains in contact with and supported by the support surface on the piston head. Consequently, when the seal is subjected to high pressure during operation in a compressor, there is less flexing at the 90° bend radius on the seal due to the fact that the seal is supported by the curved top surface on the piston. When the piston head is subjected to high compressed gas pressure, the bend radius does not significantly change. This reduced the risk of fatigue failure of the seal. Further, since there is no significant reduction in the bend radius during operation of the compressor, there is less stress in the seal at the outside of the bend at high pressures. The reduced stress permits using a harder, more durable material for forming the seal. 
     It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying Figures in which: 
     FIG. 1 is a cross sectional view through a wobble piston according to the prior art; 
     FIG. 2 is an enlarged fragmentary cross sectional view as taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is an enlarged fragmentary cross sectional view of a corner of a piston head in a wobble piston assembly according to the invention with the seal attached prior to shaping the seal into a cup shape; 
     FIG. 4 is an enlarged fragmentary cross sectional view, similar to FIG. 2, showing details of an improved wobble piston and seal assembly according to the invention; and 
     FIG. 5 is an enlarged fragmentary cross sectional view, similar to FIG. 4, showing details of a wobble piston and seal assembly according to a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     Referring to FIG. 1 of the drawings, a cross sectional view is shown of a prior art wobble piston and seal assembly  10  for use in an oil free air compressor (not shown). The assembly  10  includes a wobble piston  11  having a head  12  and a connecting rod  13  formed as an integral unit. As used herein, “integral” is used to mean that the piston head  12  and the connecting rod  13  do not pivot or rotate relative to each other. The piston head  12  includes a plate  15  which is secured with a screw  16  for attaching a seal  14  to the piston head  12 . The assembly  10  is shown with the piston head  12  positioned within a cylinder  17 . The connecting rod  13  has a lower end  18  opposite the end attached to the head  12 . An opening  19  is formed in the connecting rod end  18  for pivotal attachment to either an eccentric, such as a crank pin on a crank shaft (not shown). As the eccentric is rotated, the piston head  12  will reciprocate and rock or wobble in the cylinder  17 . The area within the cylinder  17  above the piston head  12  forms a compression chamber wherein gas is compressed on upward strokes of the piston head  12 . 
     FIG. 2 is an enlarged fragmentary cross sectional view showing the seal  14 , its connection to the piston head  12 , and an adjacent portion of an interior wall  20  of the cylinder  17 . Prior to inserting the piston head  12  into the cylinder  17 , the seal  14  is a flat ring having an interior opening  21 . The piston head  12  has an annular flat top surface  22  against which the seal  14  is placed. Preferably, an annular flange  23  projects upwardly from the surface  22 . The flange  23  extends through the seal opening  21  to position the seal  14  on the piston head  12 . The plate  15  has a lower annular surface  24  which fits over the flange  23 . When the screw  16  (FIG. 1) is secured, the seal is clamped between the annular surface  24  and the flat piston head surface  22 . The plate  15  also has a perimeter  25  which is connected by a curved corner  26  to the lower surface  24 . As best seen in FIG. 2, there is a sufficient clearance between the cylinder wall  20  and the piston head  12  and the perimeter  25  of the attached plate  15  to provide for the seal  14  and to permit the piston head  12  to wobble or rock as it is reciprocated in the cylinder  17 . 
     After the flat seal  14  is clamped to the piston head  12 , it is formed into a cup shape by forcing the piston head  12  into the cylinder  17 . As the piston head  12  enters the cylinder  17 , an outer end  27  of the seal  14  is bent upwardly to form substantially a 90° bend  28  to the seal and to impart a cup shape to the seal. The bend is described as “substantially” 90° since the actual angle of the bend around the piston head will vary with any tilt of the piston head  12  relative to the axis of the cylinder  17 . When the plane of the piston head  12  is perpendicular to the axis of the cylinder  17 , the angle of the seal bend  28  will be 90° around the piston head  12 . When the piston head  12  is tilted in the cylinder  17 , the angle of the bend  28  on one side of the piston head  12  will be greater than 90° and the angle of the bend  28  on a diametrically opposite side of the piston head  12  will be less than 90°. The actual angle of the bend  28  at any location around the piston head  12  will depend on the amount of tilt and the to direction of the tilt. However, the average angle of the bend  28  will be 90°. The seal end  27  is maintained in contact with the cylinder wall  20  as the piston head  12  reciprocates and wobbles due to the resilience of the seal and due to air pressure pressing on the seal. As is shown in FIG. 2, the seal  14  lifts away from the flat piston head surface  22  in the region of the bend  28 . During operation of a compressor in which the piston and seal assembly  10  is installed, higher air pressures will tend to force the seal bend  28  towards the flat piston head surface  22 . Consequently, the radius of the bend  28  decreases at high pressure. This produces high stresses in the seal in the region of the bend  28 . A sufficiently soft material must be used to form the seal  14  in order to prevent seal failure at the bend. However, the softer material may be subject to greater abrasion due to friction with the cylinder wall  20  than a harder material. 
     FIGS. 3 and 4 show a fragmentary portion of a wobble piston and seal assembly  30  according to a preferred embodiment of the invention. The illustrated portion of the piston and seal assembly  30  is similar to that shown in FIG.  2 . The remaining portions of the wobble piston and seal assembly  30  are of conventional design. The assembly  30  includes a piston head  31  and a seal  32 . The piston head includes a seal retaining plate  33  which is secured to the piston head  31  with, for example, a screw (not shown). The seal  32  is clamped between an annular lower surface  34  on the plate  33  and an annular seal support surface  35  on the piston head  31 . Unlike the prior art piston  11  of FIGS. 1 and 2, the seal support surface  35  on the piston  34  has an upwardly curved portion  36  adjacent an outer perimeter  37  of the piston head  31 . Consequently, when a flat annular seal  32  is clamped to the piston head  31 , a partial curve or bend  38  is imparted to the seal  32  by the curved surface portion  36 . At this stage, the bend  38  is substantially less than 90°. This pre-shaping of the seal  32  before the piston head  31  is inserted into a cylinder  39  provides several advantages over the prior art. As the piston head  31  is inserted into the cylinder  39  and the bend  38  is formed to substantially 90°, the seal  31  is not lifted away from the support surface  35 , as it is lifted from the flat surface  22  in the prior art piston  11 . The seal  32  continues to be supported by the support surface  35  up to the perimeter  37  of the piston head  31 . Consequently, the radius of the bend  38  does not significantly decrease when the seal  32  is subjected to high pressure compressed air during operation of the assembly  30  in an air compressor. Since the seal is not subjected to the degree of fatigue as with prior art wobble piston and seal assemblies, the seal will have a longer operating life. Further, since there is a greater bend radius of the seal at the bend  38  at higher air pressures, the seal will have lower internal stresses than the prior art seal. This permits forming the seal from a harder material, which further increases the operating life of the seal. 
     FIG. 4 shows and describes a preferred piston construction with a specific way of securing the seal  32  to the piston head  31  using a plate  33  secured with a screw to the top of the piston head  31 . It will be appreciated that other means may be used for mounting the seal  32  on the piston head. For example, the plate  33  may be threaded to engage the top of the piston head  31  without the need for a separate screw. Alternately, as shown in FIG. 5, a seal  42  may be secured to a lower surface  43  on a piston head  44  with an annular member  45  which is secured to the piston head  44  with threads  46 . The annular member  45  has a curved seal support surface  47  similar to the seal support surface  35  with the curve  36 . However, the piston may be stronger if the seal support surface is integral with the connecting rod as in FIG. 4, where threads  46  are not required to take the load from the compressed air acting on the seal. 
     It is believed that the wobble piston and seal assembly for oil free compressor of the present invention and many of its attendant advantages will be understood by the forgoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.