Abstract:
Disclosed is a compressor having a plate rotatable about an axis of a rotary shaft and a piston connected to the plate. The plate causes the piston to reciprocate between a top dead center and a bottom dead center of its stroke in accordance with the rotation movement of the plate. Cam surfaces are provided on the plate for actuating the piston. The cam surfaces have first portions for driving the piston toward the top dead center, and second portions for driving the piston toward the bottom dead center. Transmission members are interposed between the piston and the cam surface for transmitting the rotational movement of the plate to the piston. The first and second portions cause the transmission members to follow on the cam surfaces. At lease one of the first and second portions are arranged to have a normal line extending obliquely to the axis of the rotary shaft for constant contact between the transmission members and the one of the portions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wave plate type compressor in which a piston reciprocates in response to the rotation of a wave plate secured to a rotary shaft. 
     2. Description of the Related Art 
     In a conventional swash plate type compressor, one head of a double-headed piston completes a single compression cycle for every rotation made by the swash plate and the rotary shaft. On the other hand, with compressors using a wave plate, one head of the double-headed piston completes a plurality of compression cycles in accordance with the shapes of the cam surfaces or cam grooves on the wave plate for each rotation of the rotary shaft. The wave plate type compressors therefore have an advantage over the swash plate type compressor in that the discharge displacement per rotation is increased. 
     Conventional wave plate type compressors are disclosed in Japanese Unexamined Patent Publication No. 57-110783 and Japanese Unexamined Utility Model Publication No. 63-147571. In the compressor described in the Japanese Unexamined Patent Publication No. 57-110783, in particular, rollers 53 and 54 are provided between an associated double-headed piston 52 and the front and rear cam surfaces 51a and 51b of a wave plate 51 as shown in FIG. 13. The rollers 53 and 54 are rotatably fitted in the piston 52, and are capable of rolling on the wave plate 51. As the wave plate 51 rotates, its cam surfaces 51a and 51b engage and displace the rollers 53 and 54. These rollers then transmit this displacement to the piston 52, in turn, causing its reciprocation. 
     In the compressor described in the Japanese Unexamined Utility Model Publication No. 63-147571, cam grooves are formed on the front and rear surfaces of the wave plate instead of the cam surfaces. In this publication, balls rather than rollers are interposed between the cam groove and double-headed piston. 
     Although the rollers or balls may at first appear to be in line contact with the wave plate, a microscopic view reveals a plane contact exists between the contacting components due to their deformation under pressure. This deformation results in the occurrence of the so called &#34;Hertz&#34; contact which effectively increases the contact area shared between the rollers or balls and the wave plate. 
     To improve the durability of the compressor, it is important to reduce the contact pressure between the above contacting components. This can be done by increasing the length of the line contact or reducing the curvature of the contact portion (i.e., by increasing the radius of curvature). It is apparent, on a microscopic level, that a reduction in the curvature of the contact portion causes an increase in the contact area, and thus reduces the overall contact pressure. 
     Contact pressure can thus be reduced by increasing the contact area between the wave plate and either the length or diameter of the rollers or the diameter of the balls. Increases made to the length or diameter of the rollers and balls, however, are limited by the diameter of the piston, since each roller or ball is fitted to its associated piston. Such increases tend to increase the size of the piston as well as the compressor. Given the trend toward increasingly compact compressors, increases to the size of the compressor are distinctly disadvantageous. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a wave plate type compressor whose durability can be improved without enlarging the compressor. 
     To achieve the above object, according to a wave plate type compressor embodying this invention, the compressor has a plate rotatable about an axis of a rotary shaft and a piston connected to the plate. The plate causes the piston to reciprocate between a tope dead center and a bottom dead center of its stroke in accordance with the rotational movement of the plate. Cam means is provided on the plate for actuating the piston. The cam means has first portions for driving the piston toward the top dead center, and second portions for driving the piston toward the bottom dead center. Transmission means is interposed between the piston and the plate for transmitting the rotation movement of the plate to the piston. The first and second portions cause the transmission means to follow on the cam means. At lease one of the first and second portions are arranged to have a normal line extending obliquely to the axis of the rotary shaft for a constant contact between the transmission means and the one of the portions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which; 
     FIG. 1 is a cross-sectional side view of an entire compressor embodying the present invention; 
     FIG. 2 is a cross section taken along the line 2--2 in FIG. 1; 
     FIG. 3 is a cross section of a wave plate in the compressor shown in FIG. 1; 
     FIG. 4 is a cross-sectional view showing the wave plate turned 90 degrees from the position in FIG. 3; 
     FIG. 5 is a cross section of a wave plate in a modified embodiment; 
     FIG. 6 is a cross-sectional view showing the wave plate turned 90 degrees from the position in FIG. 5; 
     FIG. 7 is a cross section of a further example of the wave plate; 
     FIG. 8 is a cross-sectional view showing the wave plate turned 90 degrees from the position in FIG. 7; 
     FIG. 9 is a cross section of a still further example of the wave plate; 
     FIG. 10 is a cross-sectional view showing the wave plate turned 90 degrees from the position in FIG. 9; 
     FIG. 11(a) is a side cross-sectional view showing an entire compressor according to a modification of the present invention; 
     FIG. 11(b) is a perspective view of a shoe according to this modification; 
     FIG. 12 is a cross-sectional view taken along the line 12--12 in FIG. 11; and 
     FIG. 13 is a partially cross-sectional view of a conventional wave plate type compressor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the present invention will now be described referring to FIGS. 1 through 4. As shown in FIG. 1, a rotary shaft 3 is rotatably supported via bearings 4 and 5 in a pair of cylinder blocks 1 and 2 which are secured to each other. A plurality of bores 1a and 2a (five each in this embodiment) are respectively formed in the cylinder blocks 1 and 2 at equiangular distances on a plurality of axes L 1  located on an imaginary circumferential plane C 0  around the axis, L0, of the rotary shaft 3. Each bore 1a in the front cylinder block 1 is paired with the associated bore 2a in the cylinder block 2, thereby forming a plurality of cylinder bores. As shown in FIG. 2, a plurality of double-headed pistons 6 are reciprocally retained in the respective bores 1a and 2a. 
     A wave plate 7, secured to the rotary shaft 3, has can surfaces 7a and 7b formed with a predetermined width at the front and rear portions of the wave plate 7. A pair of shoes 8 and 9 are provided between the wave plate 7 and each piston 6. The piston 6 has a pair of recesses 6a and 6b at the center. The shoes 8 and 9 have first spherical surfaces 8a and 9a, which are fitted in the respective recesses 6a and 6b, and second spherical surfaces 8b and 9b, which slide on the respective cam surfaces 7a and 7b of the wave plate 7. As shown in FIG. 3, the radius of curvature R 1  of the second spherical surfaces 8b and 9b is larger than the radius of curvature R 2  of the first spherical surfaces 8a and 9a. The centers, Q 1  and Q 2 , of the first spherical surfaces 8a and 9a are located substantially at the centers of the second spherical surfaces 8b and 9b. 
     The cam surfaces 7a and 7b of the wave plate 7 are located on a displacement curve F on the circumferential surface C 0 . The displacement curve F is a 2-cycle displacement curve which has four first portions alternately protruding forward and rearward (leftward and rightward in FIG. 1) with respect to a plane perpendicular to the axis L 0  of the rotary shaft 3. In addition, second portions are provided that link the four first portions. Examples of the displacement curve F of the cam surfaces 7a and 7b include a sinusoidal displacement curve and a cycloidal displacement curve. 
     For each revolution of the wave plate 7, the piston 6 reciprocates twice. The reciprocation of the piston 6 causes the refrigerant gas in a suction chamber 10 to enter the bores 1a and 2a via inlet ports 12 and associated inlet valves 11. The refrigerant gas in the bores 1a and 2a is exhausted to a discharge chamber 15 via discharge ports 14 and associated discharge valves 13. 
     The cam surfaces 7a and 7b have cross sections on a plane containing the axis L 0  along an arc, which has the same radius of curvature as the radius of curvature R 1  of the second spherical surfaces 8b and 9b. Therefore, the second spherical surfaces 8b and 9b of the shoes 8 and 9 have a line contact with the cam surfaces 7a and 7b. Since the centers Q 1  and Q 2  of the first spherical surfaces 8a and 9a are located at the centers of the second spherical surfaces 8b and 9b, the displacement of the piston 6 accurately reflects the displacement of the cam surfaces 7a and 7b on the displacement curve F of the cam 7. 
     FIG. 4 illustrates the wave plate 7 turned 90 degrees from the position in FIG. 3. As shown in FIGS. 3 and 4, a pair of rightmost portions 7a 1  of the front cam surface 7a are arranged at an angular distance of 180 degrees from each other. A pair of leftmost portions 7a 2  are respectively separated from the pair of rightmost portions 7a 1  by 90 degrees. A leftmost portion 7b 1  of the rear cam surface 7b  is located at the back of the leftmost portion 7a 2  of the front cam surface 7a. A rightmost portion 7b 2  of the rear cam surface 7b is located at the rear of the rightmost portion 7a 1  of the front cam surface 7a. 
     The rightmost portion 7a 1  of the cam surface 7a is used for driving the piston 6 toward the bottom dead center on the side of the bore 1a. The leftmost portion 7a 2  of the cam surface 7a is used for driving the piston 6 toward the top dead center on the side of the bore 1a. The leftmost portion 7b 1  of the cam surface 7b is used for driving the piston 6 toward the bottom dead center of the piston 6 on the side of the bore 2a. The rightmost portion 7b 2  of the cam surface 7b is used for driving the piston 6 toward the top dead center of the piston 6 on the side of the bore 2a. 
     The leftmost portion 7a 2  (corresponding to the top dead center) of the cam surface 7a is located on a circle Ca 2  indicated by a chain line in FIG. 3. The leftmost portion 7b 1  (corresponding to the bottom dead center) of the cam surface 7b is located on a circle Cb 1  and is also indicated by a chain line in FIG. 3. The rightmost portion 7a 1  (corresponding to the bottom dead center) of the cam surface 7a is located on a circle Ca 1  as indicated by a chain line in FIG. 4. The rightmost portion 7b 2  (corresponding to the top dead center) of the cam surface 7b is located on a circle Cb 2  and is similarly indicated by a chain line in FIG. 4. The circles Ca 1 , Ca 2 , Cb 1  and Cb 2  have the same radius. 
     The centers, Pa 1  and Pb 1 , of the circles Ca 1  and Cb 1  lie outside the axis L 1  of the piston 6, and the centers, Pa 2  and Pb 2 , of the circles Ca 2  and Cb 2  lie on the axis L 1  of the piston 6. That is, a normal vector Va 1  on the displacement curve F at the rightmost portion 7a 1  (the bottom-dead-center portion, hereinafter referred to as the BDC portion) of the cam surface 7a is inclined outward with respect to the axis L 0  of the rotary shaft 3. A normal vector Va 2  on the displacement curve F at the leftmost portion 7a 2  (the top-dead-center portion, hereinafter referred to as the TDC portion) of the cam surface 7a is parallel to the axis L 0  of the rotary shaft 3. A normal vector Vb 1  on the cycle displacement curve F at the leftmost portion 7b 1  (the BDC portion) of the cam surface 7b is inclined outward with respect to the axis L 0  of the rotary shaft 3. A normal vector Vb 2  on the displacement curve F at the rightmost portion 7b 2  (the TDC portion) of the cam surface 7b is parallel to the axis L 0  of the rotary shaft 3. 
     A normal vector on the displacement curve F of the cam surface 7a is gradually inclined outward, with respect to the axis L 0  between the TDC portion 7a 2  and the BDC portion 7a 1  as the normal vector position is shifted toward the BDC portion 7a 1  from the TDC portion 7a 2 . Likewise, a normal vector on the displacement curve F of the cam surface 7b is gradually inclined outward with respect to the axis L 0  between the TDC portion 7b 2  and the BDC portion 7b 1  as the vector position is shifted toward the BDC portion 7b 1  from the TDC portion 7b 2 . 
     The radius of curvature R 1  of the second spherical surfaces 8b and 9b of the shoes 8 and 9 is restricted by the radius of curvature of the displacement curve F at the BDC portions 7a 1  and 7b 1  (indicated by r 0  in FIG. 4). If the normal vectors at the BDC portions 7a 1  and 7b 1  are parallel to the axis L 0 , therefore, the radius of curvature R 1  should be smaller than the radius of curvature r 0  of the displacement curve F at the BDC portions 7a 1  and 7b 1 . 
     Since the normal vectors Va 1  and Vb 1  at the BDC portions 7a 1  and 7b 1  are inclined outward with respect to the axis L 0  in this embodiment, the radius of curvature R 1  can be made greater than the radius of curvature r 0 . The radius R 1  of the BDC portion 7b 1  is in fact set larger than the radius of curvature r 0  as shown in FIG. 3. Given the above conditions, an arc crossing between the circumferential surface C 0  and the second spherical surface 9b and having a radius of curvature &#34;r&#34;, is smaller than the radius R 1 . As the inclination of the normal vector Vb 1  increases, the radius of curvature &#34;r&#34; becomes smaller than the radius R 1 . 
     If the radius of curvature &#34;r&#34; is larger than the radius of curvature r 0 , the second spherical surface 9b is lifted without contacting the BDC portion 7b 1 . If the radius of curvature &#34;r&#34; is equal to or smaller than the radius of curvature r 0  , the second spherical surface 9b comes in line contact with the BDC portion 7b 1 . By setting the radius of curvature &#34;r&#34; equal to or smaller than the radius of curvature r 0  and as close to this radius of curvature r 0  as possible, the radius of curvature R 1  of the second spherical surface 9b of the shoe 9 becomes greater than the radius of curvature r 0 . This would reduce the Hertz&#39;s pressure occurring between the second spherical surface 9b and the cam surface 7b. 
     The radius of curvature of the second spherical surface 8b of the shoe 8 can also be set greater than the radius of curvature r 0 , thus reducing the Hertz&#39;s pressure between the second spherical surface 8b and the cam surface 7a. The reduction in Hertz&#39;s pressure improves the pressure resistance characteristics of the shoes 8 and 9 as well as the wave plate 7. This pressure reduction thus improves the durability of the compressor. In this case, the radius of curvature R 1  of the second spherical surfaces 8b and 9b can be increased without increasing the diameter of the piston 6 or the diameter of the wave plate 7. It is therefore possible to improve the durability of the compressor without enlarging the compressor. 
     The present invention is not limited to the above-described embodiment. For example, normal vectors Vc 1  and Vd 1  at BDC portions 7c 1  and 7d 1  of cam surfaces 7c and 7d may be inclined inward with respect to the axis L 0  as shown in FIGS. 5 and 6. Normal vectors Vc 2  and Vd 2  at TDC portions 7c 2  and 7d 2  of the cam surfaces 7c and 7d are parallel to the axis L 0 . FIG. 6 illustrates the wave plate 7 turned 90 degrees from the position in FIG. 5. Even in the case where the normal vectors Vc 1  and Vd 1  are inclined inward with respect to the axis L 0 , the radius of curvature R 1  of the second spherical surfaces 8b and 9b can be set greater than the radius of curvature r 0  of the displacement curve F at the BDC portions 7c 1  and 7d 1 . 
     Further, normal vectors Vc 1  and Vf 1  at BDC portions 7e1 and 7f 1  of cam surfaces 7e and 7f may be inclined outward with respect to the axis L 0 . Normal vectors Ve 2  and Vf 2  at TDC portions 7e 2  and 7f 2  may be inclined inward with respect to the axis L 0 , as shown in FIGS. 7 and 8. FIG. 8 illustrates the wave plate 7 turned 90 degrees from the position in FIG. 7. 
     Furthermore, normal vectors Vg and Vh at all the points on the displacement curve F on cam surfaces 7g and 7h may be inclined outward with respect to the axis L 0  of the rotary shaft as shown in FIGS. 9 and 10. FIG. 10 illustrates the wave plate 7 turned 90 degrees from the position in FIG. 9. 
     As shown in FIGS. 11(a), 11(b) and 12, both the first surfaces 16a and 17a of shoes 16 and 17, which are to be fitted in a piston, and the second surfaces 16b and 17b of the shoes, which slide on the wave plate 7, may be designed with a cylindrical shape. In this case, both the cam surfaces 7i and 7j of the wave plate 7 that lie on a plane containing the axis L 0  of the rotary shaft, and the second surfaces 16b and 17b that lie on the same plane have respectively cross sections along a straight line. The first surfaces 16a and 17a slide in contact with the cylindrical inner walls of recesses 6a and 6b of the piston 6. The shoes 16 and 17 are rotatable within a plane containing the axis L 0  of the wave plate 7. 
     The second surfaces 16b and 17b are therefore always in line contact with the cam surfaces 7i and 7j, even if the normal vector Vj at the BDC portion 7j 1  is inclined with respect to the axis L 0  of the rotary shaft while the normal vector Vi at the TDC portions 7i 2  and 7j 2  are parallel to the axis L 0 . 
     The shoes 16 and 17, when cut along a plane perpendicular to the lengthwise direction of the second surfaces 16b and 17b, have semicircular cross sections. If the second surfaces 16b and 17b of the shoes 16 and 17 located at the BDC portion 7j 1  of the cam surface 7j, are cut at the circumferential surface C 0 , the cross sections are semi-elliptic. The curvature of this semi-elliptic cross section on the displacement curve F is greater than the curvature of the semicircular cross section. It is therefore possible to set the radius of curvature of the second surfaces 16b and 17b greater than the radius of curvature r 0  of the displacement curve F at the BDC portion 7j 1 . This reduces the Hertz&#39;s pressure between the shoes 16 and 17 and the cam surfaces 7i and 7j. 
     The cam surfaces may be formed in a convex shape, and the second surfaces of the shoes, which engage with the cam surfaces, may be formed in a concave shape.