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Title:
RECIPROCATING PISTON DRIVING SYSTEM
Document Type and Number:
WIPO Patent Application WO/1994/023202
Kind Code:
A1
Abstract:
Reciprocating piston driving system, of the type mounted inside a cylinder (2), defined in a respective cylinder block (1) and driven by a crankshaft (30) of a hermetic compressor, through a connecting rod (50), with a first end (51) mounted to the piston (3) and a second opposite end operatively connected with the crankshaft (30), in order to impart to the piston (3) an operative cycle having a compression stroke and a suction stroke, the connection between the crankshaft (30) and the connecting rod (50) comprising: a cam (40), mounted to the crankshaft (30), in order to rotate therewith and presenting a peripheral cam surface (41); a cam follower (51), mounted from the second end of the connecting rod (50); and a connecting rod-cam operative connecting means (51a, 41a), supported by the cam (40), keeping the cam follower constantly and operatively connected with the cam surface (41), the assembly of the connecting rod (50) being made in such a way as to avoid angular displacements thereof, at least on the plane containing the cam surface, said cam surface being designed in such a way as to define during said operative cycle at least a post-compression time and a post suction time, during which the piston remains stationary.

Inventors:
Lilie
Dietmar
Erich
Bernhard
Application Number:
PCT/BR1994/000013
Publication Date:
October 13, 1994
Filing Date:
March 24, 1994
Export Citation:
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Assignee:
Empresa, Brasileira Compressores S/a DE.
Embraco Lilie, Dietmar Erich Bernhard -.
International Classes:
F04B9/04; F04B39/00; (IPC1-7): F04B9/04; F04B39/00
Foreign References:
US4105371A
DE7305570U
EP0461943A2
DE433695C
DE2608664A1
FR344266A
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Claims:
CLAIMS
1. Reciprocating piston driving system, of the type mounted inside a cylinder (2) defined in a respective cylinder block (1) and driven by a crankshaft (30) of a hermetic compressor, through a connecting rod (50) , with a first end (51) mounted to the piston (3) and a second opposite end operatively connected with the crankshaft (30) , in order to impart to the piston (3) an operative cycle having a compression stroke and a suction stroke, characterized in that the connection between the crankshaft (30) and the connecting rod (50) comprises: a cam (40), mounted to the crankshaft (30), in order to rotate therewith and presenting a peripheral cam surface (41) ; a cam follower (51) , mounted from the second end of the connecting rod (50) ; and a connecting rodcam operative connecting means (51a, 41a, 60) , supported by the cam (40) , keeping the cam follower constantly and operatively connected with the cam surface (41) , the assembly of the connecting rod (50) being made in such a way as to avoid angular displacements thereof, at least on the plane containing the cam surface, said cam surface being designed in such a way as to define during said operative cycle at least a postcompression time during which the piston remains stationary.
2. Reciprocating piston driving system, according to claim 1, characterized in that the connecting rodcam connecting means (51a, 41a, 60) promotes the coupling between the cam follower (51) and the cam surface (41) .
3. Reciprocating piston driving system, according to claim 2, characterized in that the connecting rodcam connecting means (60) is an elastic element.
4. Reciprocating piston driving system, according to claim 3, characterized in that the coupling between the cam follower (51) and the cam surface (41) is obtained by an elastic guiding arm (60) , presenting a first end (61) mounted to the cam follower (51) and a second opposite end (62) , mounted to the center of rotation of the cam (40) , so as to constantly and elastically bias the cam follower (51) against the cam surface (41) of the cam (40) .
5. Reciprocating piston driving system, according to claim 3, characterized in that the connecting rodcam connecting means comprises a rail (41a) , which is provided along the cam surface (41) and which is in the form of a peripheral continuous groove of a cam (40) of circular contour, said cam surface being defined by the bottom of said groove.
6. Reciprocating piston driving system, according to claim 1, characterized in that the cam surface (41) is designed, in such a way as to define during the operative cycle of the piston (3) a post suction time during which the reciprocating piston (3) also remains stationary.
7. Reciprocating piston driving system, according to claim 6, characterized in that the post compression and post suction times are different from each other, corresponding, respectively, to a post compression radius and a post suction radius, each defining a corresponding region of post compression surface and post suction surface.
8. Reciprocating piston driving system, according to claim 7, characterized in that the post compression and post suction radiuses are constant during the respective post compression and post suction times.
9. Reciprocating piston driving system, according to claim 8, characterized in that the postcompression time is the longest of the times in each operative cycle of the reciprocating piston (3) .
10. Reciprocating piston driving system, according to claim 9, characterized in that each operative cycle of the reciprocating piston includes, between the post compression and post suction stop times, compression and suction times, which are different from each other.
11. Reciprocating piston driving system, according to claim 10, characterized in that the suction time is longer than the compression time.
12. Reciprocating piston driving system, according to claim 11, characterized in that the compression time is the shortest of the times, which define the total time of each operative cycle of the reciprocating piston (3) .
13. Reciprocating piston driving system, according to claim 12, characterized in that the post compression time corresponds to a region of the cam surface (41) , whose circumference arc presents a post compression radius that is larger than the radius of the suction, post suction and compression regions of said cam surface (41) .
14. Reciprocating piston driving system, according to claim 1, characterized in that it comprises a connecting rod guiding means (53) , mounted to the cylinder block (1) , in order to avoid said angular displacements of said connecting rod.
Description:
RECIPROCATING PISTON DRIVING SYSTEM

Field of the Invention

The present invention refers to a reciprocating piston driving system for hermetic compressors of the type used in small refrigerating appliances. Background of the Invention In the hermetic compressors using reciprocating pistons, the piston moves linearly during each operative stroke, in both directions and with a velocity that varies sinusoidally, as a result of the type of mechanical connection, i.e., the eccentric articulation between the connecting rod of the piston and the crankshaft of the compressor, said crankshaft rotating at a constant speed.

During each operative cycle, the piston starts from a first position of zero instantaneous velocity, towards a second position of zero instantaneous velocity, said second position being displaced from said first position in 180° of the crankshaft rotation. Said displacement is achieved, by initially submitting the reciprocating piston to an acceleration, which will result in a maximum instantaneous velocity, at a position of the stroke thereof that is intermediate relative to said first and second positions of zero velocity, when the motion of said piston begins to be braked, till reaching said second zero velocity, when the process is restarted, till the first zero velocity position is reached again. Said first and second positions of zero instantaneous velocity define, respectively, an upper dead point of compression end and a lower dead point of suction end of the piston operative cycle.

The piston operates during its linear movement inside a cylinder presenting an end, which is opposite to the

end whereby said piston is articulated to the connecting rod, and which is opened to a valve plate that separates the inner cavity of said cylinder from a cylinder head, inside which the discharge and suction chambers of the compressor are defined. The communication between the inner cavity of the cylinder and each of said chambers is achieved through discharge and suction orifices, which are periodically and selectively closed by respective discharge and suction valves, mounted to said valve plate.

During the suction stroke of the piston, the crankshaft drives said piston, in order to impart to said piston an increasing velocity, from its zero velocity position at the upper dead point, till the maximum velocity is reached, when said piston begins to be braked. This drive makes the piston move towards the lower dead point, thus gradually decreasing the pressure inside the inner cavity of the cylinder, till a pressure differential is reached, the sufficient to cause and maintain, while there is a certain minimum pressure differential, the opening of the suction valve, thereby allowing the gas to flow from the inside of the suction chamber towards the cylinder inside. The higher the valve stiffness, the longer the time required to reach the differential pressure condition that causes the opening of said valve during the suction stroke. Such longer time means that the opening will occur in a condition of the suction stroke closer to the lower dead point, i.e., closer to the moment of the piston stroke inversion.

The later the opening of the suction valve, the shorter the time available for the refrigerant fluid, that is leaking from the suction chamber to the cylinder inside, to fill up the volumetric capacity of said cylinder, before the piston reaches its moment of stroke inversion, i.e., the lower dead point, and

begins the compression stroke. This loss in the gas admission during the suction stroke will make the pressure differential between the cylinder inside and the discharge chamber, necessary to cause the opening of the discharge valve during the piston compression stroke, be reached later than it normally would, if the piston began said compression stroke with a maximum use of the volumetric capacity of the cylinder. As in the suction stroke, the piston will reach its stroke reversion, to the suction stroke, before the gas is completely drained to the cylinder inside. The short opening time of said discharge and suction valves result in losses during the suction and the discharge of the refrigerant fluid, said losses impairing the efficiency of the compressor. Moreover, due to the instantaneity of each stage of the piston operative cycle, resulting from the constant rotation of the crankshaft and to the cylindricity of said eccentric, said suction and discharge valves should be precisely dimensioned, so as to operate during the opening and closure of the respective gas passage orifices in precise times. Nevertheless, such stiffness increases the energetic consumption of the compressor. Some known solutions use flexible valves, which reach the opening condition earlier, both at the suction and at the compression strokes, thereby permitting a longer time for the suction and for the discharge of the refrigerant gas, before the subsequent inversion of the piston stroke takes place. Nevertheless, said valves, besides being fragile, allow the occurrance of gas back flow towards the inside of the suction chamber, during the suction stroke, and towards the cylinder inside, during the compression stroke of the piston. Disclosure of the Invention Thus, it is an object of the present invention to provide a reciprocating piston driving system for

hermetic compressors, which allows a maximum use of the cylinder volumetric capacity, during suction, and a maximum discharge of the refrigerant fluid to the discharge chamber, before the subsequent inversion of the piston stroke occurs.

A second object of the present invention is to provide a piston driving system, as stated above, which allows a better control of the times required for each operative stroke of said piston, particularly at the proximities of the lower dead point and at the upper dead point.

These objectives are achieved through a reciprocating piston driving system, of the type mounted inside a cylinder, defined in a respective cylinder block and driven by a crankshaft of a hermetic compressor, through a connecting rod, with a first end mounted to the piston and a second opposite end operatively connected with the crankshaft, in order to impart to the piston an operative cycle having a compression stroke and a suction stroke, wherein the connection between the crankshaft and the connecting rod comprises: a cam, mounted to the crankshaft, in order to rotate therewith and presenting a peripheral cam surface; a cam follower, mounted from the second end of the connecting rod; and a connecting rod-cam operative connecting means, supported by the cam, keeping the cam follower constantly and operatively connected with the cam surface, the assembly of the connecting rod being made in such a way as to avoid angular displacements thereof, at least on the plane containing the cam surface, said cam surface being designed in such a way as to define during said operative cycle at least a post-compression time during which the piston remains stationary. Description of the Drawings

The present invention will be described below, with

reference to the attached drawings, in which: Fig. 1 is a longitudinal sectional view of a portion of the compressor inside, in which the cylinder block assembly is mounted, according to the state of the art. Fig. 2 is an upper view of the piston-connecting rod - crankshaft assembly, according to the present invention; and

Fig. 3 is a cross sectional view of the crankshaft cam, according to the line III-III of figure 2. Best Mode of Carrying- Out the Invention

According to the illustration of figure 1, there is provided a reciprocating hermetic compressor, of the type comprising a cylinder block 1, which is lodged inside a hermetic case (not illustrated) , and which presents a cylindrical cavity, such as a cylinder 2, within which reciprocates a piston 3. The cylinder block 1 presents a pair of opposite faces, towards which the ends of the cylinder 2 are opened. Against one of said opposite faces of the cylinder block 1, there are attached, through non-illustrated gaskets, a valve plate 5 and a cylinder head 6, which forms together with said valve plate 5 two internal cavities, each defining a suction chamber 8 and a discharge chamber 9. The cylinder 2 is kept in selective fluid communication with said suction chamber 8 and discharge chamber 9, through respective gas suction orifices 5a and discharge orifices 5b, which are provided at the valve plate 5 and into which are mounted a respective suction valve 10 and a discharge valve 11, which operate synchronously during the suction and discharge strokes of the operative cycle of the piston 3. In this construction, the reciprocating piston 3 presents a first end 3a, facing the valve plate 5a and a second opposite end 3b, through which said reciprocating piston 3 is driven.

At said opposite end 3b, there is mounted a smaller eye

21 of a connecting rod 20, which is on its turn actuated by a crankshaft 30.. by mounting a larger eye

22 of said connecting rod 20 with a first eccentric end of said crankshaft 30. While rotating, the crankshaft 30 imparts to said larger eye 22 of the connecting rod 20 a circular movement that will cause a linear movement to the reciprocating piston 3 between points of minimum and maximum distance from the valve plate 5. Said points of minimum and maximum distance from the valve plate 5 represent the points of stroke inversion in the operative cycle of the reciprocating piston, respectively known as upper dead point and lower dead point. According to the driving system that is presently used in hermetic compressors, the reciprocating piston 3 presents a continuous linear movement, with a suction stroke and a compression stroke, which start, respectively, at the upper dead point and at the lower dead point.

Each operative cycle of the reciprocating piston is achieved within a total time T, which corresponds to the sum of a suction time plus a compression time, which are equal. Moreover, the piston stop at the upper dead point and at the lower dead point is instantaneous and of the same duration in both situations.

The equality between said suction and compression times results from the constant rotation of the crankshaft 30, from the cylindricity of the eccentric of said crankshaft 30 and from the connecting rod-eccentric connection.

In this known construction, the linear movement of the reciprocating piston 3, between the points of stroke inversion, is numerically equal to the diameter of the eccentric and corresponds to an angular displacement

from a central portion of the larger eye 22 of the connecting rod 20, according to a regular displacement curve, presenting a single radius that is equal to the eccentricity of the crankshaft 30, i.e., to the distance between the geometrical centers of the crankshaft 30 and said larger eye 22 of the connecting rod 20.

According to the present invention, the transmission of movement between the crankshaft and the reciprocating piston 3 is made through a connecting rod 50, having a first free end 51 and a second opposite end 52, attached to the piston. In this construction, as there is no angular movement of the connecting rod 50 during the operative cycle of the piston, the second opposite end 52 need not be articulated to said piston, as in the state of the art. Said first free end 51 carries a roller bearing 51a, the actuation of which will be described ahead. According to this construction, the linear movement of the piston, between the points of stroke inversion, corresponds to a linear movement of said first end 51 of the connecting rod 50 and is equal to the largest distance between two diametrically opposite points of a peripheral surface of a cam 40, mounted to the crankshaft 30, in order to rotate with the latter.

The roller bearing 51a, mounted to the first end 51 of the connecting rod 50 defines a cam follower, which is in permanent contact with the cam peripheral surface 41 of the cam 40, said construction being defined in such a way as to obtain different times of compression and suction, required for a better performance of the compressor, as described below.

The best performance of the compressor means the total filling up of the volumetric capacity of the cylinder 2, before the start of the compression stroke, as well as the total flowage of the refrigerant fluid from said

cylinder 2 to the discharge chamber 9, before the suction stroke of the reciprocating piston 3 starts, said conditions of total flowage and total filling up with refrigerant fluid to and from the inside of the cylinder 2 occurring even when the pressure differential that is needed to obtain the opening of the respective discharge valve 11 and suction valve 12 is achieved in a time interval close to the instant of the corresponding stroke end. In the present solution, the cam 40 is in the form of a cylindrical tubular body, which is peripherally provided with an annular groove of variable depth, defining a guide rail 4la on the cam surface 41. In this construction, the permanent contact between the free end 51 of the connecting rod 50 and the cam surface 41 is obtained by a connecting rod - cam operative connecting means 60, in the form of a "U" shaped elastic guiding arm, which is mounted upperly relative to the connecting rod-cam assembly and which has a first end 61, attached to the first cam follower end 51 of the connecting rod 50 and a second end 62, positioned at the center of rotation of the cam 40. The elasticity of the guiding arm 60 keeps a constant contact with the connecting rod 50 and the cam surface 40, during the operation of the reciprocating piston 3. The variable profile of the cam surface 40 is obtained in function of the time duration required for each time of the cycle of the reciprocating piston 3. In any of such situations, the irregular surface is defined, so that the total time of a cycle of the reciprocating piston 3 corresponds to the sum of the stop times at the end of compression and at the end of suction, i.e., of stroke inversion, as well as to the times of compression and suction motions, as described ahead. In another constructive option, the cam 40 is in the form of an irregularly shaped body, defined in function

of the required time characteristics mentioned above. In any of the solutions described above, the permanent contact between the first free end 51 of the connecting rod 50 and the cam surface 41 can be achieved by action of a cam follower, which is slidably connected with the guiding rail 41a. As in this constructive form the operative connecting means is stiff, in order to avoid that during each operative cycle of the piston the free end 51 of the connecting rod 50 provided with the cam follower 51a presents angular displacements during its displacement at least on the plane containing the cam surface 41, there is mounted to the cylinder block 1 a connecting rod guiding element 53 that, during the operation of the reciprocating piston 3, guides a portion of the rod of the connecting rod 50.

According to the present invention, the cam follower 51a, in the form of the roller bearing described above, maintains a permanent sliding contact with the cam surface, this contact being maintained by the guiding arm 60 which, for being elastic, automatically and constantly corrects any attempt of the connecting rod 50 to move away from the cam surface 41, thereby dispensing the use of the connecting rod guide 53. In order that the piston 3, when beginning to draw the refrigerant gas from the suction chamber 8, presents a minimum of residual pressure in its inside, the post- compression time, i.e., at the upper dead point, should be long enough to permit that a maximum residual gas, which is present inside said cylinder 2, escapes to the discharge chamber 9, before suction begins. During said stop time, the reciprocating piston 3 should remain at a stationary condition at its upper dead point position. As the crankshaft and, therefore, the cam 40 are still moving continuously, the lack of movement is obtained, by maintaining constant the radius of the cam surface

41 during the post compression time. Obtaining a long post compression time corresponds to the relative displacement of the cam follower 51 of the connecting rod 50, along a region of the cam surface 41 presenting an extension long enough to assure a desired stop time for the piston at the upper dead point during a corresponding rotation of the crankshaft. The circumference arc defined by said post compression region of the cam surface presents a radius that is larger than the radius of the suction, post suction and compression regions of said cam surface. After the end of the post compression time, the reciprocating piston starts the suction period, which corresponds to a respective suction region of the cam surface, whose curvature radius, or eccentricity, is dimensioned so as to allow a maximum entrance of refrigerant gas into the inside of the cylinder 2, during said suction stroke, until the pressure differential between the suction chamber and the cylinder inside does not maintain the suction valve 10 in an opening condition any more. The variation of the curvature radius during the suction results in a relatively long suction time, but not necessarily of the same duration as the post compression time, in order to allow the inside of the cylinder 2 to be filled up with gas, through small pressure differentials, although not causing turbulence inside said cylinder 2. The existence of turbulence during suction, due to the short suction time, impairs the gas admittance flow and decreases the compressor yield. A solution for this problem is to have a long post suction time. Nevertheless, in order not to alter the total time of a reciprocant piston cycle, the compression time should be extremely short. This means that the cam surface 41 should present a compression region, varying suddenly between the regions of post

suction and post compression surfaces. Nevertheless, sudden variations overload the points of the assembly where there is usually concentration of forces, thereby causing wear and stresses on said points. Thus, the compression time is calculated in such a way as to impart a quick initial linear displacement to the reciprocating piston 3, but said displacement being slower, from a certain instant, so as to impart an even movement to the assembly. Such initial acceleration and posterior brake should mean an even transition for the cam follower 51, said transition being achieved with the gradual increase of the radius, from the constant radius of the post suction circular region, till the constant radius of the post compression region is reached, presenting increases by unit of angular pitch, that are variable from said post suction radius up to the post compression radius. In order to avoid the brake to occur suddenly, said radius increase per unit of angular pitch, at the region of the cam surface 41 adjacent to the region thereof corresponding to the post compression radius, should be very small when compared with the cam surface accelerating region 41 of the reciprocating piston 3. The post suction radius that gives origin to a cam surface corresponding to a post suction stop should be smaller than the radius of the suction, compression and post compression regions. As the relation of the total time of rotation in a piston cycle should be maintained, said post suction time and, thus, the post suction radius, are calculated by the difference between said total time and the compression and suction times and the post compression time.