The present invention relates to a rotary compressor used for a refrigerator, an air-conditioner or the like and, more particularly, to the structure of a rolling piston/blade assembly in a rotary compressor, which structure prevents abnormal wear of the rolling piston and the blade so as to provide a highly reliable rotary compressor.
FIGS. 4 and 5 show the cross-sectional structures of a rotary compressor to which the present invention is applied. The rotary compressor, which is denoted by a reference numeral 1 as a whole, comprises a cylindrical hermetic shell 10, and a motor 20 and a compression unit 30 which are received in the hermetic shell 10. The motor 20 includes a stator 22, secured on the inner wall of the hermetic shell 10, and an armature 24. A rotary shaft 26 fixed in the center of the armature 24 is rotatably supported by two bearings 34, 36 which serve as side panels of a compression chamber of the compression unit 30. The rotary shaft 26 includes an eccentric portion referred to as a crank portion 28.
A cylinder member 32 is interposed between the two bearings 34 and 36. The cylinder member 32 has the same axis as the rotary shaft 26. As shown in FIG. 5, an inlet 33 and an outlet 35 for a refrigerant are formed in a peripheral wall of the cylinder 32.
An annular rolling piston 38 is provided in the cylinder 32. An inner peripheral surface 38b of the rolling piston 38 fits with an outer peripheral surface 28a of the crank portion 28, and an outer peripheral surface 38a of the rolling piston 38 contacts with lubricant films on an inner peripheral surface 32b of the cylinder 32.
A blade 40 is slidably mounted in a wall of the cylinder 32, and the tip of the blade 40 abuts against the outer peripheral surface 38a of the rolling piston 38. The blade 40 is urged toward the rolling piston 38 by spring 42, and the compressed refrigerant is supplied to the rear surface of the blade 40, thereby ensuring fluid tightness between the tip of the blade 40 and the rolling piston 38.
The blade 40, the rolling piston 38, the cylinder 32 and the bearings 34, 36 define a space of the compression chamber 50.
When the rotary shaft 26 rotates in a clockwise direction, as viewed in FIG. 5, the rolling piston 38 eccentrically rotates within the cylinder 32 so that the refrigerant fluid introduced from the inlet 33 is compressed and discharged from the outlet 35.
During the suction, compression and discharge process, a pressing force Fv is generated on a contact portion between the rolling piston 38 and the blade 40. Moreover, generally, the rotary shaft 26 and the rolling piston 38 are movably fitted to each other, so that a relative sliding velocity v between the rolling piston 38 and the blade 40 is affected by the balance of the loads and friction forces on these members and a drive force of the rotary shaft 26. FIGS. 6A and 6B respectively show the relationship between the pressing force Fv and the rotation angle of the rotary shaft 26 and between the relative sliding velocity v and the rotation angle of the rotary shaft 26, both during one rotation of the rotary shaft 26.
As shown in FIGS. 6A and 6B, the blade 40 exhibits the maximum value of the pressing force when the rotation angle of the rotary shaft 26 is about 90 degrees and about 270 degrees from a direction of reciprocation of the blade 40 as a reference line. The relative sliding velocity between the rolling piston 38 and the blade 40 changes in its direction and rate during one rotation of the rotary shaft 26 so that it is difficult to form an oil film on the contact portion between the rolling piston 38 and the blade 40.
FIGS. 7A and 7B show the contact portion between the rolling piston 38 and the blade 40 more specifically. Conventionally, a contact surface 40a of the tip of the blade 40 in contact with the outer peripheral surface 38a of the rolling piston 38 has been formed as an arcuate surface having a radius of curvature Rv. The radius of curvature Rv is substantially equal the a thickness t of the blade 40 and is approximately 1/10 to 1/3 of the radius of the rolling piston 38.
Mainly, cast iron or alloyed cast iron which has been quenched is used as a material for the rolling piston 38, and stainless steel or tool steel or such steel which has been subjected to surface treatment like nitriding is used as a material for the blade 40. Especially, the blade 40 is usually made of a material having high hardness and high toughness.
As shown in FIG. 8, a contact condition between the rolling piston 38 and the blade 40 can be considered as a problem of contact between cylinders having different curvatures. In this condition, a Hertz stress, i.e., a contact stress a expressed by the following formula is generated on the contact portion between the rolling piston 38 and the blade 40 due to the pressing force Fv of the blade 40. EQU .sigma..sup.2 ={(Fv/L).multidot.(E'/2 .pi.R)} (1) EQU 1/E'=(1/2).multidot.{(1-.upsilon.r.sup.2)/Er+(1-.upsilon.v.sup.2)/Ev}(2) EQU 1/R=1/Rr+1/Rv (3)
wherein reference symbol Rr is the radius of the rolling piston 38, Rv: the radius of the tip of the blade, L: the length of contact, Er: the elastic modulus of the material of the rolling piston, Ev: the elastic modulus of the material for the blade, .upsilon.r: a Poisson's ratio of the rolling piston material, .upsilon.v: Poisson's ratio for the blade material, and E': an equivalent elastic modulus.
Actually, the above-mentioned contact stress .sigma. between the rolling piston 38 and the blade 40 varies according to the change of the pressing force Fv of the blade during one rotation of the rotary shaft 26. However, if the pressing force Fv of the blade exceeds a certain level, the relative sliding velocity v between the rolling piston 38 and the blade 40 becomes 0, and the Hertz stress concentrates on one portion of the outer periphery of the rolling piston 38 so that this portion will repeatedly receive the stress. In general, fatigue rupture of a metallic material has a characteristic that if the stress exceeds a certain level, then the number of repetitions of the application for stress until fatigue rupture occurs decreases in accordance with the increase of the stress, as indicated by a so-called S-N curved line shown in FIG. 9.
However, no active research has been performed with respect to the above-described problem in the conventional structure of the contact portion between the rolling piston 38 and the blade 40 in a rotary compressor. When a system with a rotary compressor is used in a severe condition, abnormal wear between the rolling piston 38 and the blade 40 may be generated by a mechanism presumed to be as follows, and a deficiency of the cooling capacity may be caused by the following phenomenon:
Rising in an ambient temperature of the refrigeration system .fwdarw. rising in the discharge pressure .fwdarw. increase of the pressing force of the blade .fwdarw. the rolling piston/blade sliding velocity of 0 .fwdarw. repeated concentration of the Hertz stress .fwdarw. occurrence of fatigue wear of the outer periphery of the rolling piston .fwdarw. occurrence of severe wear of the outer periphery of the rolling piston .fwdarw. decrease of the cooling capacity.
For example, Japanese Utility Model Unexamined Publication No. 1-158589 discloses a blade with a tip that is formed of a surface having a plurality of curvatures so as to decrease the Hertz stress generated on the tip of the blade. Also, Japanese Patent Unexamined Publication No. 5-306693 discloses a shape of a blade whose edge is brought into plane contact with a rolling piston when the rolling piston is at such a rotation angle that a high-pressure chamber has the maximum pressure.
Moreover, Japanese Patent Unexamined Publication No. 4-314988 discloses having the hardness of the material of a blade a lower than that of the material of a rolling piston.