Nonlubricated Compressor

Abstract

A small multistage nonlubricated compressor for air and other gases has an open crankcase cooled by a fan, the stages being arranged in cruciform for cooling. In each stage, the piston is guided by a cylindrical crosshead. The crossheads are guided in cylindrical sleeves of self-lubricating plastic such as polytetrafluoroethylene. The pistons are guided in cylindrical sleeves of graphite and have piston rings of self-lubricating plastic.

Description

The present invention relates to nonlubricated compressors of the multistage type for compressing air and other gases.

It is an object of the present invention to provide nonlubricated compressors with improved cooling means.

Another object of the present invention is the provision of nonlubricated compressors with improved bearing means.

Still another object of the present invention is the provision of nonlubricated compressors with improved discharge valves.

Finally, it is an object of the present invention to provide nonlubricated compressors which will be relatively simple and inexpensive to manufacture, easy to assemble, adjust, operate, maintain and repair, and rugged and durable in use.

Other objects and advantages of the present invention will become apparent from a consideration of the following description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a vertical cross-sectional view of a nonlubricated compressor according to the present invention, and showing the second and fourth stages in cross section;

FIG. 2 is a vertical cross-sectional view taken on the line 2--2 of FIG. 1, and showing the first and third stages in cross section; and

FIG. 3 is a perspective view of an outlet valve member according to the present invention.

Referring now to the drawings in greater detail, and first to FIG. 1 thereof, there is shown a multistage nonlubricated compressor 1 having an open crankcase 2 and four mounting pads 3, 4, 5 and 6 equally peripherally spaced apart at 90.degree. angles to each other. It will of course be understood that the number of compression stages may be other than four.

As will be evident from FIG. 2, the pads 4 and 6 are offset in the direction of the axis of the compressor, by a distance slightly greater than the width of the ball bearings of the connecting rods, and pads 3 and 5 are similarly offset. Pads 3, 4 and 5 each mount a crosshead guide 7 with a cylindrical liner 8, which are identical for all three pads. By contrast, pad 6, for the lowest pressure stage, mounts a crosshead guide 9 which has a larger flange and a slightly longer liner; and both the liner and the guide have a hole 10 in the front face.

Each liner 8 is of a self-lubricating plastic such as polytetrafluoroethylene, polytrifluoroethylene, polycarbonate, polysulfone, polyimide or polybenzamidazole. Among these, polytetrafluoroethylene, polyimide and polybenzamidazole are preferable, because they can stand higher temperatures to 800.degree.F. Particularly preferred is polytetrafluoroethylene. As polytetrafluoroethylene is too soft for many mechanical applications, it is often strengthened by the addition of one or more substances such as glass fibers, graphite and/or metal powders. Such compositions are well known in the art and are available commercially and so need not be described in greater detail.

Liner 8 has a flange which is secured to crankcase 2 by screws 13 passing through both the flange of cylinder 11 and the flange of crosshead guide 7. Liner 8 is prevented from rotating by pin 12 but is free to expand axially at its inboard end. The bore of liner 8 is cylindrical and crosshead 14 also has a cylindrical outer contour and reciprocates in this bore with a relatively close fit.

Crosshead 14 operates the fourth stage piston 15 by a connection comprising an enlarged head or disc on the inboard end of piston 15, which fits into a T-slot formed in boss 16 of crosshead 14. This T-slot has sufficient lateral clearance to allow piston 15 to line up in its liner 17 independently of the alignment of crosshead 14 in liner 8. This T-slot engagement ensures that piston 15 is securely engaged to crosshead 14 so long as their axes are coaxial but allows piston 15 to be disengaged by a short lateral motion after cylinder 11 has been removed and when crosshead 14 is at its point of maximum outboard travel. The enlarged head or disc of piston 15 is the solid of rotation of a figure whose outline is the same as that of the T-slot. Piston 15 is thus free to rotate in its bore after assembly to crosshead 14.

Fourth stage cylinder liner 17 is of graphite and is a tight fit in sleeve 18 which is retained in cylinder 11 by flange 19 and sealed at its inboard end by O-ring 20. By "graphite" is meant for example the wide range of generally commercially available compositions such as those made by mixing natural graphite with carbon powder, often with metal powders, pressing at high pressure in a mold, and then sintering at high temperature. Such products are often impregnated with various plastics after sintering so as to reduce any porosity present after sintering. Again, however, these graphite compositions and the fabrication of articles of any desired shape from them are well known in the art and so need no further discussion.

When screws 21 are removed, fourth stage cylinder head 22 can be removed from cylinder 11 complete with valve plates 23 and 24 and the associated valve parts, as plates 23 and 24 are fastened to cylinder head 22 by screws 25. The removal of this cylinder head assembly makes it possible to remove sleeve 18 with its liner 17 for easy inspection or replacement.

Piston 15 has self-lubricating plastic piston rings 26 and expanders 27 in the form of springs which continuously urge the piston rings 26 radially outwardly into slidable bearing contact with liner 17. The self-lubricating plastic of rings 26 may be the same as that of the crosshead liner 8. When the plastic piston rings 26 slide on the liner 17, a small amount of the graphite of liner 17 will embed itself in the surface of the self-lubricating plastic, whereupon the contact becomes essentially graphite-to-graphite, which provides a very advantageous bearing surface at the relatively high temperatures encountered along the skirts of the piston. By contrast, the crosshead is sufficiently far from the piston and sufficiently cooler than the piston that metal-to-plastic contact of the metal crosshead in its self-lubricating plastic sleeve is ideal for absorbing lateral thrust with minimum wear.

The fourth stage inlet valve assembly 28 is characterized by an annular flexible valve member of the type of my copending application Ser. No. 249,914, filed May 3, 1972. The fourth stage discharge valve 29 is shown in perspective in FIG. 3 and is generally disc-shaped but has a number of flats around its outer edge which allow the discharge air to bypass. The remaining cylindrical outer contour of valve 29 is a loose fit in self-lubricating plastic bushing 31 which serves as its guide, the valve 29 being held on its seat on valve plate 23 by spring 30. The plastic of bushing 31 may be the same as the liner 8.

At the base of cylinder 11 there is a peripheral series of holes 32. These holes serve to vent the annular space above crosshead 14, the flow of air in and out through these holes serving to cool directly the outboard section of the working bore of liner 8 and the inner surfaces of piston 15. The hole 32 which is lowermost in FIG. 1 has a lower left edge, as seen in FIG. 1, which is lower than any point on the inner periphery of liner 8. The position of that lowermost hole aids in expelling any foreign matter that gets into this cavity.

Second stage piston 33 has an inboard trunk section which is of the same diameter as crosshead 14 and reciprocates in an identical crosshead guide or liner. The upper or outboard section of piston 33 is reduced in diameter to match the diameter of second stage cylinder liner 34 of graphite as previously explained. Second stage piston rings 35 are of self-lubricating plastic as previously explained, and are provided with expanders 36. Several slots 37 are provided in the base of second stage cylinder 38 to vent the annular space defined by the two different diameters of piston 33, and some of these slots are positioned so that air from a cooling fan is forced through this annulus.

In FIG. 2, there is shown a connecting rod 39 which is typical of the four connecting rods and has a sealed grease-lubricated ball bearing 40 at its inner end and a sealed grease-lubricated needle bearing 41 at its outer end, this needle bearing being mounted on crosshead pin 46 which is secured in crosshead 48 by spring clip 47 which also prevents rotation of pin 46. Bearing 40 is mounted on double eccentric 42, which has two elements located 180.degree. apart. If the reciprocating parts attached to each side of this double eccentric are equal in weight, the reciprocating forces will be in dynamic balance and no counterweight will be necessary. Eccentric 42 is driven by key 43 on drive shaft 44, which may be a separate shaft or the extended end of an engine shaft or of an electric motor shaft, which in any event is provided with power means (not shown) for rotating it. The outboard end of shaft 44 is supported by sealed needle bearings in a bracket bolted to crankcase 2, seen in FIG. 1. A fan 49 is supported by its hub being mounted in a self-lubricating plastic bushing, the fan 49 being driven by being gripped between rubber discs 50 which prevent torsional vibrations from the compressor shaft being transmitted to the fan.

Third stage cylinder 51 and its associated parts are similar in all respects to those of the fourth stage cylinder 11 as shown in FIG. 1, except for the increase in dimensions due to the larger diameter of third stage piston 52. A sintered metal inlet strainer is shown in cross section at 53, and a similar strainer is provided for the fourth stage, both strainers being used to prevent foreign material from reaching the valves and valve seats. If a small particle were to lodge on a valve seating surface on the first or second stages of the compressor, the leakage would not be great because the pressure is low. But in the third and fourth stages, where the pressures may be for example 850 and 3,000 psi, respectively, even the smallest particle can cause a serious back leakage. It is for this reason that strainers 53 are provided on the third and fourth stages but not necessarily on the first and second stages. Third stage discharge valve 54 and its self-lubricating plastic guide bushing 55 can have the same characteristics as the corresponding fourth stage parts.

First stage piston 56 has an inboard trunk section which fits the liner 57 which has the same diameter as liner 8, as seen in FIG. 1, but is slightly longer and has a lateral hole 10, a snap ring 58 to retain the liner in crosshead guide 59, and a pin 60 to prevent rotation of liner 57. First stage piston 56 has a head section 61 which is considerably larger in diameter than the trunk section so that it is not possible to slide crosshead guide 59 and its associated parts over the piston after the piston has been attached to the connecting rod by means of the crosshead pin 46, as was the case with the other three cylinder assemblies. Accordingly, crosshead guide 59 must first be assembled on the crankcase, piston 56 then being inserted, and the crosshead pin and spring clip can then be inserted into the piston and connecting rod bearing through hole 10. First stage cylinder 62 has a graphite liner 63 while first stage piston 56 has a piston ring 64 of self-lubricating plastic which is held against liner 63 by expander 65. A rib 66 on crosshead guide 59 deflects the cooling air from fan 49 up through slot 67 into the annular space under the piston head and out through slot 68, cooling both the piston 56 and the working face of liner 63.

It will of course be understood that the inlets and outlets of the respective stages are interconnected as by finned tubing (not shown) for air cooling, or by coiled tubing inside cylindrical containers (not shown) through which water circulates for water cooling.

There is thus provided by the present invention an improved and simplified nonlubricated compressor in which the lateral thrust of the crossheads is borne by relatively long self-lubricating plastic liners, rather than by narrow plastic riding rings on the crossheads, and in which metallic pistons operate without side thrust in closely fitted graphite sleeves, the self-lubricating plastic rings operating without overhang in these sleeves, all pressure-sealing contact being plastic-to-graphite. The lateral thrust is thus relieved from graphite, which cannot bear it very well, and shifted to the plastic sleeves of the crosshead, which can. On the other hand, the high temperature encountered near the working face of the pistons is shifted from the plastic of the crosshead sleeve, which cannot bear it very well, to the graphite liner of the piston cylinders, which can.

In view of the foregoing disclosure, therefore, it will be seen that all of the initially recited objects of the invention have been achieved.

Although the present invention has been described and illustrated in connection with a preferred embodiment, it is to be understood that modifications and variations may be resorted to without departing from the scope of the invention, as those skilled in this art will readily understand. Such modifications and variations are considered to be within the purview and scope of the present invention as defined by the appended claims.

Claims

Having described my invention, I claim:

1. A compressor comprising a cylinder, a piston reciprocable in the cylinder, means for reciprocating the piston, a cylindrical sleeve of graphite within which said piston reciprocates, and at least one piston ring of self-lubricating plaster carried by said piston between the ends of the piston and in sliding contact with said graphite sleeve.

2. A compressor as claimed in claim 1, and a slidably guided cross head coaxial with and spaced from an secured to said piston for relieving lateral thrust from said piston.

3. A compressor comprising a cylinder, a piston reciprocable in the cylinder, means for reciprocating the piston, a cross head secured coaxially to the piston, a sleeve of self-lubricating plastic in which said cross head slides, and a sleeve of graphite within which said piston slides, said cross head being disposed between said piston and said reciprocating means and relieving the lateral thrust of said reciprocating means on said piston.

4. A compressor as claimed in claim 3, and a piston ring of self-lubricating plastic on said piston and in sliding contact with said graphite sleeve.

5. A compressor as claimed in claim 3, comprising an open, air cooled crank case carrying said cylinder and said sleeves and reciprocating means, both ends of said sleeve of self-lubricating plastic being open to the ambient atmospheric air.

6. A compressor as claimed in claim 5, and a fan carried by said crank case for increasing the circulation of ambient air around both ends of the last-named sleeve.