Patent Application
20110265643
The present invention relates to a piston and cylinder assembly. It finds particular application in conjunction with a piston assembly used as a compressor unloader piston and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other applications.
A compressor unloader piston assembly includes sealing members (e.g., o-rings, quad rings, and/or cap seals) that provide a seal against the cylinder bore wall and may also guide the piston within a cylinder bore. Side forces exerted on the sealing members have caused excessive wear on the sealing members and, in addition, piston bore damage. The excessive wear requires replacement of the sealing members earlier than desired. In addition, the damage to the piston bore tends to accelerate wear on the sealing members.
Separate guide features have been used to shield the unloader piston o-rings from side forces. However, separate guides increase manufacturing costs and complexity.
The present invention provides a new and improved apparatus for a compressor unloader piston assembly which addresses the above-referenced problems.
In one embodiment, a piston and cylinder assembly includes a housing defining a bore. A piston is sized to reciprocate in the bore. A first sealing member provides a sealing engagement between the piston and a wall of the bore. A first bushing, of a first material, is around a first portion of the piston toward a first end of the piston. A second bushing, of a second material, is around a second portion of the piston toward a second end of the piston. The first and second bushings guide the piston within the bore. The first material is different than the second material.
In the accompanying drawings which are incorporated in and constitute a part of the specification, embodiments of the invention are illustrated, which, together with a general description of the invention given above, and the detailed description given below, serve to exemplify the embodiments of this invention.
With reference to
A first seal 20 (e.g., a lower sealing member) sealingly engages a first portion 22 (e.g., a lower portion) of the piston 16 and an inner wall 24 of the bore 14. Therefore, the lower seal 20 provides sealing engagement between the lower portion 22 of the piston 16 and the inner wall 24 of the bore 14. A second seal 26 (e.g., an upper sealing member) sealingly engages a second portion 30 (e.g., an upper portion) of the piston 16 and the inner wall 24 of the bore 14. Therefore, the upper sealing member 26 provides sealing engagement between the upper portion 30 of the piston 16 and the inner wall 24 of the bore 14. It is contemplated that the lower and upper sealing members 20, 26 are standard o-rings.
The lower portion 22 of the piston 16 is toward a first end 32 (e.g., a lower end) of the piston 16. The upper portion 30 of the piston 16 is toward a second end 34 (e.g., an upper end) of the piston 16.
A first bushing 36 (e.g., a lower bushing) is around the lower portion 22 of the piston 16. A second bushing 40 (e.g., an upper bushing) is around the upper portion 30 of the piston 16. In one embodiment, one or both of the lower and upper bushings 36, 40, respectively, are split (see, for example, the illustrated slanted cut 42 in the upper bushing 40) to facilitate flex installation around the lower and upper portions 22, 30, respectively, of the piston 16. As discussed in more detail below, the first and second bushings 36, 40, respectively, are different materials.
The lower and upper bushings 36, 40, respectively, are captured (retained) in respective grooves 44, 46 formed in the lower and upper portions 22, 30 of the piston 16. In the illustrated embodiment, the lower sealing member 20 is proximate the lower bushing 36, and the upper sealing member 26 is proximate the upper bushing 40.
As evident in the embodiment illustrated in
Since the lower and upper bushings 36, 40, respectively, provide the largest diameters at the lower and upper portions 22, 30, respectively, of the piston 16, the bushings 36, 40 act to guide the piston 16 in the bore 14. For example, the bushings 36, 40 act to maintain a vertical axis 50 of the piston 16 substantially parallel to the wall 24 of the bore 14 to maintain compression on the lower and upper sealing members 20, 26, respectively, while allowing the piston 16 to slide within the bore 14. In one embodiment, the piston 16 does not contact the bore 14.
By guiding the piston in the bore 14, side loads on the sealing members 20, 26 are reduced as the piston 16 reciprocates in the bore 14. For example, any side loads generated during reciprocation of the piston 16 in the bore 14 are resisted by the bushings 36, 40. Reduced side loads on the sealing members 20, 26 reduce excessive pressure between the sealing members 20, 26 and the wall 24 of the bore 14, which facilitates in extending the life of the sealing members 20, 26. In addition, reduced side loads during reciprocation of the piston 16 in the bore 14 help reduce abrasions on both the sealing members 20, 26 and the wall 24 of the bore 14. The bushings 36, 40 guide the piston 16 to protect the sealing members 20, 26 from side loads without requiring separate guide features.
In this manner, the bushings 36, 40 are a means for reducing side loads on the lower and upper sealing members 20, 26 during reciprocation of the piston 16 in the bore 14.
With reference to
In the illustrated embodiment, a first edge 60 is defined along the wall 24 of the bore 14. The first, lower bushing 36 reciprocates at the edge 60. In one embodiment, a majority of the first, lower bushing 36 passes by the edge 60 (and extends outside the bore 14) as the piston 16 reciprocates.
With reference again to
In one embodiment, the material of the lower bushing 36 has a hardness of about 85±5 Shore D, an elongation of about 25%, a tensile strength of about 13778 psi (95 MPa), and a coefficient of friction of about 0.30 to about 0.38. The material of the upper bushing 40 is an internally lubricated plastic having a hardness of about 60±5 Shore D, an elongation of about 260%, tensile strength of about 2900 psi (20 MPa), and a coefficient of friction of about 0.18. The internal lubrication of the upper bushing 40 and relatively softer material of the upper bushing 40 help avoid abrasion on the wall 24 of the bore 14.
In one embodiment, the material of the lower bushing 36 is Polyetheretherketon (PEEK), and the material of the upper bushing 40 is Polytetrafluoroethylene (PTFE). Optionally, the material of the lower bushing 36 and/or the material of the upper bushing 40 also includes fillers (e.g., Ekonol).
Since a majority of the lower bushing 36 extends outside the bore 14 as the piston 16 reciprocates past the edge 60, the relatively harder material of the lower bushing 36 helps the lower bushing 36 maintain its shape and not become damaged by the edge 60, which may be sharp. For example, if the lower bushing 36 becomes torn, loses its shape, and/or becomes damaged, the edge 60 may catch the lower bushing 36 and potentially prevent the piston 16 from reciprocating. The material chosen for the lower bushing 36 has relatively higher creep resistance, which helps the lower bushing 36 maintain its shape, and a relatively high tensile strength, which helps the lower bushing 36 resist tearing.
Since the upper bushing 40 is fully contained in the bore 14 as the piston 16 reciprocates, the relatively softer material is acceptable for avoiding abrasions on the wall 24 of the bore 14. The softer upper bushing 40 also makes it relatively easy to shape for facilitating installation.
With reference again to
In one embodiment, the wall 24 of the bore 14 (e.g., the housing 12) is aluminum, and the piston 16 is a relatively harder material (e.g., steel). In this embodiment, contact between the piston 16 and the wall 24 is minimized and/or prevented to avoid potential damage to the wall 24 caused by the relatively harder piston 16. The steel piston 16 seals against a steel plate (not shown) below the aluminum bore 14. If the steel piston 16 were made entirely of a softer material, such as PEEK, the seal against the steel plate would not be adequate. Therefore, the contemplated piston assembly 10 with guide bushings 36, 40 helps to protect the relatively softer aluminum bore wall 24 while providing any advantages gained by use of a steel piston 16.
While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention, in its broader aspects, is not limited to the specific details, the representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant's general inventive concept.
