WO2003056178A1

WO2003056178A1 - Mounting arrangement for a reciprocating compressor

Info

Publication number: WO2003056178A1
Application number: PCT/BR2002/000186
Authority: WO
Inventors: Max Martins
Original assignee: Empresa Brasileira De Compressores S/A - Embraco
Priority date: 2001-12-21
Filing date: 2002-12-16
Publication date: 2003-07-10
Also published as: AU2002350297A1; BR0106614A

Abstract

A mounting arrangement for a reciprocating compressor of the hermetic type, comprising a cylinder (10) closed by a valve plate (20) and housing a piston (30), whose top face (31) defines a compression chamber (C) with the valve plate (20), said piston (30) being coupled to the smaller eye (41) of a connecting rod (40), whose larger eye (42) is mounted around a tubular bushing (60) which is journalled around the eccentric of a crankshaft (50). The bushing (60) has its external cylindrical face (61) and its internal cylindrical face (62) eccentric in relation to each other, so as to allow adjusting the position of the parts that form the compression chamber (C), in order to obtain a minimized dead volume of a predetermined value in the upper dead point position of the piston (30).

Description

MOUNTING ARRANGEMENT FOR A RECIPROCATING COMPRESSOR Field of the Invention

The present invention refers to a mounting arrangement of the piston-connecting rod assembly to the eccentric shaft of a reciprocating compressor, particularly a hermetic compressor for small refrigeration appliances, aiming at forming a dead volume in the interior of the cylinder when the piston reaches its upper dead point position. Prior Art

Reciprocating hermetic compressors generally used in small refrigeration systems comprise a cylinder, inside which reciprocates a piston driven by an eccentric shaft by means of a connecting rod projecting outwardly from the cylinder through an open end of the latter. The other end of the cylinder is closed by a valve plate, in which suction and discharge valves are mounted, selectively communicating the compression chamber, which is defined inside the cylinder between the piston top and the valve plate, with the low and high pressure sides of the refrigeration system to which the compressor is coupled.

In these known reciprocating hermetic compressors, the dimensioning and the assembly of the parts defined by the piston, connecting rod, and eccentric shaft are made so that to make the piston approximate as close as possible to the valve plate in the upper dead point position, that is, in the compression stroke end condition, in order to make minimum the dead volume of the refrigerant gas inside the compression chamber and thus minimize the efficiency losses of the compressor. However, the distance between the top of the piston, when the latter is in the upper dead point, and the valve plate should be such as to avoid the impact between both parts, which tends to damage the compressor. Thus, the mounting problem to be solved in this type of compressor is to provide, in a safe, economical and efficient manner, the lowest possible dead volume, with no risks of the piston hitting the valve plate in determined operational conditions resulting from dimensional variations of the assembly, as a function of alterations in the operational temperature . Due to the difficulty or even the impracticability of obtaining dimensional precision in the different parts involved and in the mounting thereof to the compressor assembly, the known solutions to obtain an acceptable dimension of the dead volume in the compression chamber comprise the use of gaskets having different thicknesses, which are mounted between the valve plate and the adjacent end of the compressor.

In an usual mounting arrangement shown in figure 1 of the enclosed drawings, the dimensioning of the component parts is made so that, in the upper dead point, the piston 30 has its top face 31 projecting slightly outwardly from the second end 12 of the cylinder 10, allowing the measurement of the projecting extension and the definition of the thickness of a gasket 70, which is necessary and sufficient to separate the adjacent face of the valve plate 20 from the top face 31 of the piston 30, thus forming an acceptable dead volume in the interior of the compression chamber C. This known type of mounting arrangement is usually achieved by employing gaskets 70 selected from tables that indicate the gaskets 70 that are adequate for each type of assembly, as a function of the projection reached by the top face 31 of the piston 30 in relation to the second end 12 of the cylinder 10. In order to allow a more precise adaptation of the thickness of the gasket to the ideal dead volume, those prior art gaskets are obtained by grouping (20 groups) gaskets 70 with different thicknesses, which are selected as a function of the dead volume to be produced.

Although being widely used, this prior art ^'solution presents some disadvantages, among which one can mention the frequent error in the selection of the gaskets, i.e., a grouping error, with the consequent obtainment of an overdimensioned dead volume, which impairs the efficiency of the compressor, or a dead volume that is insufficiently dimensioned, leading the piston to hit the valve plate, with the consequent reduction of the useful life of the compressor.

In addition to the above disadvantage, it should be also mentioned the necessity of storing gaskets of different thicknesses, and promoting, in the assembly line, the safe and correct groupings, and the positioning of the multiple gasket in its assembly site .

Considering the material from which they are produced, generally based on cellulose and supplied by a single supplier at a worldwide level, these gaskets have the application associated with different disadvantages that do not recommend their use.

Objects of the Invention

By reason of the above mentioned problems related to the mounting solutions presently used, it is an object of the present invention to provide an arrangement for mounting the piston-connecting rod assembly to the eccentric shaft of a reciprocating hermetic compressor, which is carried out in a safe, economically viable and efficient manner, assuring a minimum dead volume to be obtained in the compression chamber, by applying a gasket having a single thickness, and which arrangement is not affected by the thickness variations of said gasket at the closed end region of the cylinder. It is a further object of the invention to provide a mounting arrangement such as mentioned above, which allows suppressing the gasket between the cylinder and the valve plate. It is still a further object of the invention to provide a process for carrying out the above-mentioned mounting arrangement. Summary of the Invention

The mounting arrangement of the present invention is applied to a reciprocating compressor generally of the hermetic type used in refrigeration appliances, comprising: a cylinder with one of the ends closed by a valve plate and housing a piston, whose top face defines, with the valve plate and within the cylinder, a compression chamber; and a crankshaft having a cylindrical eccentric; a connecting rod having a smaller eye coupled around the piston, and a larger eye mounted around the eccentric; and a tubular bushing having an external cylindrical face tightly fitted inside the larger eye of the connecting rod, and an internal cylindrical face that is freely journalled around the eccentric.

According to the invention, the external and internal cylindrical faces of the tubular bushing present a determined eccentricity (E) therebetween, the tubular bushing being affixed inside the larger eye of the connecting rod, so that the projection of said eccentricity on a plane diametrical to the eccentric and containing the axis of the cylinder results in a predetermined distance, between the top face of the piston and the axis of the eccentric, which is slightly smaller than the distance between the latter and the valve plate, when the piston is in the upper dead point position.

According to the invention, the eccentricity between the external and internal cylindrical surfaces of the bushing is slightly superior to half the sum of the variation range of the dimension that defines the position of the piston top in relation to the cylinder top when the piston is found in the upper dead point. The bushing is mounted with interference into the larger eye of the connecting rod and slidingly journalled around the eccentric, so that the action of the eccentricity is null in relation to the displacement of the piston. In this situation, the piston top must be close to the ideal position for the formation of the chamber. The corrections required to reduce or increase the chamber are obtained by rotating the bushing clockwise or anti-clockwise, as needed. The present constructive arrangement allows eliminating the problem of determining the thickness of the gasket between the valve plate and the cylinder for each cylinder being mounted, and even eliminating the use of any gasket in this joint region, considerably simplifying the assembly of the compressors and making said assembly more precise and more reliable in terms of minimizing the dead volume in the compression chamber. Brief Description of the Drawings The invention will be described below, with reference to the enclosed drawings, in which:

Figure 1 is a schematically sectional view of a cylinder of a reciprocating hermetic compressor, illustrating the piston and the connecting rod in the upper dead point position and the prior art arrangement for mounting the connecting rod to the eccentric of the crankshaft and the valve plate to the cy1inder; and

Figure 2 is a view similar to that of the previous figure, but illustrating said mountings carried out according to the arrangement of the present invention. Description of the Illustrated Embodiment As already mentioned, the present mounting arrangement is applied to a reciprocating compressor, which in the illustrated example takes the form of a hermetic compressor of the type utilized in refrigeration appliances, such as refrigerators, freezers, and the like . The compressor considered herein comprises a cylinder 10 formed in a block suspended in the interior of a hermetic shell (not illustrated) , and presenting a first open end 11 and a second end 12 closed by a valve plate 20, in which are provided the usual suction and discharge valves (not illustrated) . Inside the cylinder 10 is mounted a piston 30 presenting a top face 31 turned to the valve plate 20 and defining, with the latter and inside the cylinder 10, a compression chamber C. The piston carries an articulating pin 32, around which is mounted a smaller eye 41 of a connecting rod

40, whose larger eye 42 is mounted around the eccentric 51 of the cylinder of a crankshaft 50 to be rotatively driven by a motor (not illustrated) . A tubular bushing 60 has an external cylindrical face 61, which is tightly fitted into the larger eye 42 of the connecting rod 40, and an internal cylindrical face 62, which is freely journalled around the eccentric 51 of the crankshaft 50.

In the prior art construction illustrated in figure 1, the external cylindrical face 61 and the internal cylindrical face 62 of the tubular bushing 60 are concentric, the thickness of the bushing being dimensioned to comply with the structural and functional requirements, including the provision of a radial gap located between the larger eye 42 of the connecting rod 40 and the eccentric 51, and which is sufficient to allow said larger eye 42 to be fitted around the eccentric 51, after the piston 30 has been introduced into the cylinder 10. After mounting the piston 30 in the cylinder 10, and the larger eye 42 of the connecting rod 40 to the eccentric 51, said radial gap between these parts is eliminated upon mounting the tubular bushing 60. By reason of the maximum tolerances accepted for the construction and assembly of the components involved in the formation of the compression chamber C of the compressor, each compressor will present, after mounting the piston-connecting rod-eccentric assembly, a distance between the top face 31 of the piston 30 and the adjacent face of the valve plate 20, which makes complex to determine a minimized and safe dead volume in the compression chamber C when the piston 30 reaches the upper dead point position illustrated in figure 1. As mentioned before in the discussion of the prior art, the use of a gasket 70 of different thicknesses between the valve plate 20 and the cylinder 10 has been the solution found for obtaining a minimum, but safe spacing between the valve plate 20 and the piston 30, when the latter reaches the upper dead point position.

The solution of the present invention is illustrated in figure 2 of the drawings and it is based on the fact that the external cylindrical face 61 and the internal cylindrical face 62 of the bushing 60 present a determined eccentricity E therebetween, which is dimensioned in order that, as a function of the angular positioning of the tubular bushing 60, the projection of said eccentricity E on a plane P diametrical to the eccentric 51 and containing the axis of the cylinder 10 results in a predetermined distance d, between the top face 31 of the piston 30 and the eccentric axis, which is slightly smaller than the distance D between said eccentric and the valve plate 20, when the piston 30 reaches the upper dead point position.

Thus, the ^' rotation of the tubular bushing 60 by 90° around the axis of its internal cylindrical face and in opposite directions, from the position illustrated in figure 2, in which the projection of the eccentricity E on the plane P is null, allows the top face 31 of the piston 30 in the upper dead point of the latter to present a spacing, in relation to the valve plate 20, ranging by a value that is twice the value of said eccentricity E.

Since the eccentricity E is dimensioned to correspond to at least half the sum of the maximum tolerances involved in the formation of the compression chamber C, the positioning of the tubular bushing 60 can be made so as to compensate the positioning deviations of the top face 31 of the piston 30 in relation to the valve plate 20, as a function of the dimensional deviations of both the construction and the assembly of the involved parts, allowing said top face 31 to be positioned always at a determined distance from the valve plate 20 affixed to the cylinder 10. The adjustment of said distance, that is, the dead volume in the compression chamber C is now effected by the angular positioning of the tubular bushing 60, allowing using a gasket 70 with a constant thickness, or even eliminating this gasket, with the valve plate 20 being seated directly against the cylinder 10. The mounting process with the tubular bushing 60 can follow different steps, other than those usually applied, until the end of the coupling of the piston- connecting rod-eccentric assembly, with the tubular bushing 60 being positioned, for example, to produce a null projection of the eccentricity E on the plane P and with the eccentric-piston assembly in the upper dead point position.

The positioning of the top face 31 of the piston 30 is then dimensioned and processed to indicate the direction and the angle of rotation of the tubular bushing 60, so that the eccentricity E, that is, its projection on the plane P, displaces the piston 30 to a position in which the top face 31 thereof defines a predetermined minimum spacing in relation to the valve plate 20, which will be seated, with or without the gasket 70, against the cylinder 10. As illustrated in the enclosed figures, the valve plate 20 receives, externally, another gasket 80, onto which the cover 90 of the cylinder 10 is seated and affixed.

After the assembly, and with the positioning of the tubular bushing 60 already adjusted, the latter can be affixed to the larger eye 42 of the connecting rod 40 by any adequate means .

The mounting arrangement proposed by the invention allows obtaining a constant minimized dead volume in the compression chamber of the compressors being mounted, regardless of the thickness of the gasket to be used in the compressor design, and guaranteeing the desired dimensional results with a minimum amount of errors .

Claims

1. A mounting arrangement for a reciprocating compressor, comprising a cylinder (10) having an end closed by a valve plate (20) , inside which is axially displaced a piston (30) having a top face (31) which defines, with the valve plate (20) and in the interior of the cylinder (10) , a compression chamber (C) ; a crankshaft (50) having a cylindrical eccentric (51) ; a connecting rod (40) having a smaller eye (41) coupled around the piston (30) , and a larger eye (42) mounted around the eccentric (51) ; and a tubular bushing (60) having an external cylindrical face (61) tightly fitted inside the larger eye (42) of the connecting rod (40) , and a internal cylindrical face (62) that is freely journalled around the eccentric (51), characterized in that the external cylindrical face (61) and the internal cylindrical face (62) of the tubular bushing (60) present a determined eccentricity (E) therebetween, the tubular bushing (60) being affixed inside the larger eye (42) of the connecting rod (40), so that the projection of said eccentricity

(E) on a plane (P) diametrical to the eccentric (51) and containing the axis of the cylinder (10) results in a predetermined distance (d) , between the top face (31) of the piston (30) and the axis of the eccentric

(51) , which is slightly smaller than the distance (D) between the latter and the valve plate (20) , when the piston (30) is in the upper dead point position.

2. The arrangement as set forth in claim 1, characterized in that said eccentricity (E) is at minimum equal to half the sum of the tolerances involved in the formation of the compression chamber (C) .

3. The arrangement as set forth in claim 1, characterized in that the valve plate (20) maintains a constant positioning in relation to the end of the cylinder (10) onto which it is seated and affixed.

4. The arrangement as set forth in claim 3 , characterized in that the valve plate (20) is seated on the respective end of the cylinder (10) by means of a gasket (70) with a predetermined thickness, which is placed between said parts.

5. The arrangement as set forth in claim 3, characterized in that the valve plate (20) is seated directly against the respective end of the cylinder (10) .