Abstract:
A compressor includes a passage in the sidewall of the cylinder for directing compressed gas against circumferential surfaces of piston rings. The pressure of the gas from the passage counteracts the pressure of gas between the piston ring groove and the piston ring, and urges the piston rings inwardly. The force with which the piston ring is urged into engagement with the cylinder walls is lessened, thereby reducing wear of the piston rings. The arrangement is especially useful for compressors having piston rings formed of low friction polymeric materials, such as fluorine-containing polymers.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to a reciprocating compressor for compressing gas such as air, and more particularly to a reciprocating compressor having a cylinder provided with a gas passage for passing compressed gas therethrough. 
     2. Description of the Related Art 
     FIG. 5 shows a conventional reciprocating compressor. Referring to FIG. 5, a piston 33 having a plurality of piston rings 36 on an outer circumferential surface thereof is adapted to be vertically reciprocated in a cylinder 31. When the piston 33 is lowered, gas is sucked from a suction valve 37 into the cylinder 31. Thereafter, when the piston 33 is lifted to come near a top dead center, the compressed gas in the cylinder 31 is discharged from a discharge valve 38 to the outside. 
     To reduce sliding friction between outer circumferential surfaces 47 of the piston rings 36 and an inner circumferential surface 41 of the cylinder 31 during vertical reciprocation of the piston 33 without using lubricating oil (i.e., in case of what is called an oil free type), the piston rings 36 are made of synthetic resin having a heat resistance and a low coefficient of friction, such as polytetrafluoroethylene (tetrafluoride resin) polychlorotrifluoroethylene (trifluoride resin). 
     The above-mentioned synthetic polymeric fluoride resins have a low coefficient of friction, but also have a low wear resistance. In particular, the amount of wear of the piston rings 36 greatly increases with an increase in sliding surface pressure of the outer circumferential surfaces 47 of the piston rings 36 against the inner circumferential surface 41 of the cylinder 31. 
     The piston rings 36 are engaged with ring grooves 35 formed on an outer circumferential surface 43 of the piston 33, and they are elastically projected from ring grooves 35 outwardly in a radial direction of the piston 33. During a gas compression stroke of the piston 33, the compressed gas of high pressure in the cylinder 31 penetrates from an upper surface of the piston 33 through a gap between the outer circumferential surface 43 of the piston 33 and the inner circumferential surface 41 of the cylinder 31 into the ring grooves 35. The compressed gas having entered the ring grooves 35 acts as a back pressure depicted by arrows 45 against inner circumferential surfaces 48 of the piston rings 36 so as to radially outwardly urge the piston rings 36. The back pressure against the inner circumferential surfaces 48 of the piston rings 36 is increased near the top dead center to further increase the sliding surface pressure of the outer circumferential surfaces 47 of the piston rings 36 against the inner circumferential surface 41 of the cylinder 31. Accordingly, the wear of the piston rings 36 made of the above-mentioned synthetic resin becomes remarkable. As a result, the durability of the piston rings 36 is reduced, and the synthetic resin powdered by the wear is mixed with the compressed gas in the cylinder 31 to contaminate the compressed gas. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to prevent wear of a piston ring due to the sliding friction between the inner circumferential surface of the cylinder and the outer circumferential surface of the piston ring during vertical reciprocation of the piston. 
     It is another object of the present invention to reduce the sliding surface pressure of the outer circumferential surface of the piston ring against the inner circumferential surface of the cylinder by applying the compressed gas from the inner circumferential surface of the cylinder to the outer circumferential surface of the piston ring when the piston is lifted up to near the top dead center at which a back pressure to the piston ring becomes maximum. 
     To achieve the above objects, the reciprocating compressor of the present invention comprises a cylinder having an inner circumferential surface and adapted to suck gas thereinto; a piston having an outer circumferential surface and adapted to vertically reciprocate along the inner circumferential surface of the cylinder; an annular groove formed on the outer circumferential surface of the piston; an annular piston ring engaged with the annular groove and having an outer circumferential surface contacting the inner circumferential surface of the cylinder so as not to allow the escape of gas compressed during a lifting stroke of the piston through a gap between the outer circumferential surface of the piston and the inner circumferential surface of the cylinder; and a gas passage provided in a wall portion of the cylinder for introducing the compressed gas in the cylinder to the outer circumferential surface of the piston ring so as to radially inwardly urge the outer circumferential surface of the piston ring. 
     With this construction, when the piston is lifted to compress the gas in the cylinder, the compressed gas is introduced through the gas passage formed in the wall portion of the cylinder to the outer circumferential surface of the piston ring so as to radially inwardly urge the outer circumferential surface of the piston ring, thereby canceling the back pressure radially outwardly urging the piston ring. 
     Accordingly, the sliding surface pressure of the outer circumferential surface of the piston ring against the inner circumferential surface of the cylinder can be reduced to thereby reduce the wear of the piston ring and accordingly prolong the service life. Consequently, a reciprocating compressor having a superior durability can be provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will be described in detail with reference to the following figures wherein: 
     FIG. 1 is a sectional side view of a reciprocating compressor according to a first preferred embodiment of the present invention; 
     FIG. 2 is a cross section taken along the line II--II in FIG. 1; 
     FIG. 3 is a vertical sectional view of an essential part illustrating a gas passage according to the first preferred embodiment; 
     FIG. 4 is a view similar to FIG. 3, illustrating a second preferred embodiment of the present invention; and 
     FIG. 5 is a vertical sectional view of an essential part of the reciprocating compressor in the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There will now be described some preferred embodiments of the present invention. Referring to FIG. 1 which is a sectional side view of a reciprocating compressor according to a first preferred embodiment of the present invention, reference numeral 1 denotes a cylinder made of metal such as aluminum alloy. In the cylinder 1, a piston 3 is adapted to be vertically reciprocated through a piston pin 4 rotatably engaged with an upper end of a connecting rod 2. A plurality of ring grooves 5 are annularly formed on an outer circumferential surface of the piston 3 at suitable positions below an upper end of the piston 3. With each ring groove 5 is engaged a piston ring 6 made of a material having a small coefficient of sliding friction, such as polymeric resins of polytetrafluoroethylene or polychlorotrifluoroethylene. The connecting rod 2 is connected with a crank shaft (not shown) adapted to be rotated by a prime mover (not shown), so that the vertical reciprocation of the piston 3 in the cylinder 1 as mentioned above is caused by vertical movement of the connecting rod 2. In a lowering stroke of the piston 3, a suction valve 7 provided on a cylinder head 14 is opened to suck the gas to be compressed, such as air, into a cylinder chamber 10. In a lifting stroke of the piston 3, the gas in the cylinder chamber 10 is compressed, and a discharge valve 8 provided on the cylinder head 14 is opened in the vicinity of a top dead center to discharge the compressed gas of high pressure to be introduced to an outside necessary place. 
     As shown in FIGS. 1 to 3, an orifice 9 is provided in an inner circumferential wall portion (thick wall portion) of the cylinder 1. The orifice 9 is comprised of an upper inlet 9a for admitting the compressed gas from the cylinder chamber 10, a vertical passage 9b continuing from the upper inlet 9a, and a plurality of lower outlets 9c continuing from the vertical passage 9b for discharging the compressed gas to outer circumferential surfaces 19 of the piston rings 6. When the piston 3 is lifted up to near the top dead center, the high-pressure gas compressed by the piston 3 in the cylinder chamber 10 is introduced from the upper inlet 9a of the orifice 9. The compressed gas introduced from the upper inlet 9a passes through the vertical passage 9b extending vertically in the inner circumferential wall portion of the cylinder 1 to reach the lower outlets 9c. Then, the compressed gas is discharged from the lower outlets 9c in such a manner as to radially inwardly urge the outer circumferential surfaces 19 of the piston rings 6 (see arrows in FIG. 3). Incidentally, the inner circumferential wall portion of the cylinder is provided with a cooling passage for passing a cooling water or a cooling air therethrough. 
     In operation, when the piston 3 is lifted to come near the top dead center, the gas pressure in the cylinder chamber 10 is rapidly increased. The high-pressure gas in the cylinder chamber 10 passes through a gap between an outer circumferential surface 16 of the piston 3 and an inner circumferential surface 17 of the cylinder 1 to enter the ring grooves 5 and radially outwardly urge inner circumferential surfaces 18 of the piston rings 6. As a result, a back pressure to the piston rings 6 is increased to accordingly increase a sliding surface pressure of the outer circumferential surfaces 19 of the piston rings 6 against the inner circumferential surface 17 of the cylinder 1. However, as mentioned above, the high-pressure gas compressed by the piston 3 also passes through the orifice 9 formed in the inner circumferential wall portion of the cylinder to reach the outer circumferential surfaces 19 of the piston rings 6 and radially inwardly urge the outer circumferential surfaces 19 of the piston rings 6. 
     In this manner, the compressed gas in the cylinder chamber 10 partly passes through the orifice 9 and acts in such a direction as to cancel the back pressure to the piston rings 6. Accordingly, the sliding surface pressure of the outer circumferential surfaces 19 of the piston rings 6 against the inner circumferential surfaces 17 of the cylinder 1 can be reduced to that extent. 
     FIG. 4 shows a second preferred embodiment of the present invention. As shown in FIG. 4, a porous member 12 is embedded in the inner circumferential wall portion of the cylinder 1 over a distance from near the top dead center of the piston 3 in association with its vertical reciprocation to a lower position by a distance approximately equal to the height H3 of the piston 3. That is, the porous member 12 is embedded over such a distance that when the piston 3 is lifted up to near the top dead center, the compressed gas is prevented from flowing through the porous member 12 to a lower side of the piston 3. 
     The porous member 12 is made of a porous ceramic material having a superior pressure resistivity, constituted of alumina, silicon carbide, silicon nitride, SnO2, or SiO2. 
     Further, as shown in FIG. 4, it is preferable that an unnecessary pore portion of the porous member 12 is sealed by a suitable sealing member 13, so as to effectively concentrate the high-pressure gas in the cylinder chamber 10 toward an inner circumferential surface of the porous member 12 facing the outer circumferential surfaces 19 of the piston rings 6 (see dashed arrows in FIG. 4). The sealing member 13 is provided along the inner circumferential surface 17 of the cylinder 1 over the entire circumference thereof or at circumferential intervals with a suitable height (H2) to extend below a lower surface of the cylinder head 14 by a distance at least equal to the height (H1). The sealing member 13 is made of thermosetting synthetic resin, metal, ceramic, or cermet (which is a sintered material of a mixture of metal powder and ceramic). The sealing member 13 is formed on an inner circumferential surface of the porous member 12 by a suitable method such as melt impregnation, thermal spraying, or plating, according to the material employed for the sealing member 13. 
     In embedding the porous member 12, it is preferable that a recess 15 is first formed by cutting the inner circumferential surface of the cylinder at an upper portion thereof to a suitable depth (L) and a suitable height (H3), and then the porous member 12 having numerous pores therein communicating with the outer circumferential surfaces 19 of the piston rings 6 is embedded in the recess 15. According to this method, the porous member 12 can be very easily provided in the inner circumferential wall portion of the cylinder 1 so as to be exposed to the inner circumferential surface 17 of the cylinder 1, so that manufacturing costs can be greatly reduced.