Field of the Invention

The present invention refers to an axial bearing for reciprocating hermetic compressors with a vertical shaft, of the type used in small refrigerating systems. Background of the Invention

Reciprocating hermetic compressors usually consist of a motor, mounted inside a hermetically sealed shell. These compressors comprise a cylinder block, supporting a main bearing, inside which rotates a vertical crankshaft, which is provided with a pump rotor at its bottom. During the compressor operation, the rotation of the crankshaft carries, through the pump rotor, lubricant oil from a sump, provided at a lower portion of the compressor shell, up to the bearing and other components. This lubrication of a hydrodynamic nature has the function of forming an oil film between the parts presenting a mutual relative movement, thereby avoiding the contact and, consequently, the wear therebetween. Nevertheless, in components that do not present a geometry that is favorable to the formation of an oil film, there occurs a marginal lubrication, the components being thus subjected to a higher friction, causing mechanical losses for the compressor. This type of marginal lubrication occurs mainly between the axial bearing and the crankshaft. This axial bearing is mounted to the cylinder block assembly, between the upper portion of the main bearing, in order not to present a relative movement between the latter and the supporting base or peripheral flange of the crankshaft eccentric. Said axial bearing is in the form of a supporting cylindrical ring, with its opposite end faces being parallel to each other. The loads exerted on said axial bearing refer to the weight of the electric motor and crankshaft.

Theoretically, parallel bearings do not generate a hydrodynamic support. Nevertheless, manufacture imperfections, thermal deformations or inclinations of the crankshaft during its rotation may give rise to an oil film between the annular surface of the axial bearing and the adjacent surface of the crankshaft, said oil film being insufficient to guarantee a good bearing performance. The small inclinations of the crankshaft result from the temporary load accumulations over the eccentric of said shaft during rotation. This effect is mainly noted during compression, when the forces on the shaft are much higher than the forces to which said shaft is subjected during suction. In order to obtain a convenient hydrodynamic force in parallel face axial bearings, it is necessary to provide said bearings with large surfaces, which fact, on the other hand, increases the friction losses. Disclosure of the Invention Thus, it is a general object of the present invention to provide an axial bearing for reciprocating hermetic compressors, which actuates together with the vertical crankshaft of the compressor, thereby minimizing the friction losses between the respective contact surfaces and, consequently, increasing the efficiency of the compressor.

It is a more specific object of the present invention to provide a hydrodynamic axial bearing, which allows to provide the region between the contact surfaces of the axial bearing and crankshaft with a fine lubricant oil film.

These and other objectives of the present invention are achieved with an axial bearing for reciprocating hermetic compressors of the type including: a hermetic shell, in which inside is mounted a cylinder block, provided with a main bearing, which carries at its upper portion an axial bearing, supporting a

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crankshaft, an oil sump being defined at the lower portion of said shell, said axial bearing presenting a contact face onto which is supported an adjacent lower face of a peripheral flange of the crankshaft, at least part of the contact face of the axial bearing being lowered in relation to a contact plane between said first end face and the adjacent lower face of the peripheral face and orthogonal to the crankshaft, defining at least one lowered portion, which is continuously supplied with the oil from the sump during the rotation of the crankshaft, in order to define a respective hydrodynamic wedge for supporting the peripheral flange. Brief Description of the Drawings The invention will be described below, according to the attached drawings, in which:

Fig. 1 is a median vertical sectional view of a compressor of the type used in the present invention,* Fig. 2 is a perspective view of an axial bearing according to the prior art; and

Figures 3a - 3f are perspective views of a different construction for an axial bearing, according to the present invention. Best Mode of Carrying Out the Invention According to the figures above, a reciprocating hermetic compressor comprises a hermetic shell 1, inside which is adequately supported a motor-compressor assembly, formed by a cylinder block 3, whereto is attached an electric motor 4, whose rotor 4a is attached to a crankshaft 5, supported by a bearing 6, which is machined to the cylinder block 3. The crankshaft 5 presents, at its upper portion, a peripheral flange 5a for supporting an eccentric end portion 5b of the crankshaft 5 and, at its lower portion, a pump rotor carrying oil from an oil sump 8 to the parts of the cylinder block 3 in need of

lubrication during the compressor operation. The peripheral flange 5a maintains a permanent contact with a secondary axial bearing 9 for supporting said crankshaft 5, according to a contact plane orthogonal to the crankshaft 5, as seen in fig. 2.

The axial bearing 9 is in the form of an annular cylindrical body, with a contact face 9a contacting a superficial portion of the peripheral flange 5a and an opposite attaching face 9b to be mounted and attached to a an upper portion of the main bearing 6. According to the prior art, said opposite surfaces of the axial bearing 9 are parallel to each other and orthogonal to the crankshaft 5. The rotation of the crankshaft 5 around its geometric axis causes a relative movement between the lower face of the peripheral flange 5a of said crankshaft 5 and the contact face 9a of the axial bearing 9. This relative movement may present severe frictional characteristics, due to the impossibility of forming an adequate lubricant oil film, thereby causing excessive power losses in this bearing. Due to the machining imperfections of the parts, the occurrance of thermal deformation or small inclinations of the crankshaft during its rotation cause the formation of a fragile oil film, which is not strong enough to substantially reduce the losses due to visco-mechanical friction in this bearing.

According to the present invention, the reduction of the compressor power losses, due to friction between the axial bearing 9 and the crankshaft 5, is achieved by using a hydrodynamic axial bearing 10, which allows the permanent formation of an oil film between the frictional surfaces of said axial bearing 10 and crankshaft 5. This oil film acts on said crankshaft 5, lifting the latter during its rotation and minimizing the contact between the frictional surfaces. The

formation of an oil film at this region results from a converging wedge shaped spacing between the contact face 10 of the axial bearing 10 and the lower face of the peripheral flange 5a. In an embodiment of the invention, this spacing in the form of a converging wedge results from the loss of parallelism between said contact face 10a and the lower adjacent face of the peripheral flange 5a. This loss of parallelism results from the modifications effected on the surface of the contact face 10a of said axial bearing 10. Such modifications mean that at least part of said first face 10a presents inclinations in relation to the crankshaft 5. In an preferred embodiment, ^' which is illustrated in figure 3a, the contact face 10a presents a progressive continuous inclination, along the circumferencial extension of said contact face 10a, relatively to the geometric axis of the axial bearing 10, from a region of said contact face 10a pertaining to the contact plane with the lower face of the peripheral flange 5a, forming a hydrodynamic wedge in the space located between the portion of said contact face 10a spaced from the adjacent face of the peripheral flange 5a of the crankshaft 5, caused by the filling of said space with the lubricant oil. This space forms a cavity which collects the oil and which is capable of retaining a volume of oil necessary to lubricate the two adjacent surfaces and create a hydrodynamic pressure acting under the peripheral flange 5a, lifting said flange during the compressor operation. During the rotation of the crankshaft 5, the oil located in this oil collecting cavity is conducted, by dragging, towards a wall of said cavity, in a direction opposite to that of the lubricant oil flow, thereby forming with said wall the hydrodynamic wedge.

Other solutions are also possible, in which the surface

of the contact face of the axial bearing 10 is modified, according to two or more inclination planes, which are parallel to each other and resulting in a saw toothed surface, figure 3b, or also according to planes that intercept in a diametral or approximately diametral line, figure 3c. Moreover, such solutions may further present stepped inclined surfaces. In another possible solution, illustrated in figures 3d and 3e, the converging wedge between the frictional surfaces of the axial bearing 10 and peripheral flange 5a is achieved by machining, at the contact face 10a of the axial bearing 10, superficial lowered portions 11, which are angularly distributed on said contact face 10a, in order to form on said face alternated steps, where the oil is deposited, said oil coming from the oil sump 8 during the rotation of the crankshaft 5. The amount of the superficial lowered portions 11, as well as the shape and dimensions thereof are defined in function of the desired operational characteristics, machining facility, structural strength of the part, etc..

In the illustrated solutions, the superficial lowered portions 11 present radial walls, orthogonal to the contact plane between the bearing 10 and the crankshaft 5, said walls being straight or arcuated radial walls, as illustrated respectively in figures 3d and 3e. The effect of the hydrodynamic wedge, when the solution uses superficial lowered portions 11, will be obtained by varying the thickness of the oil film, due to the presence of the end wall of said lowered portions 11, said end wall being in a direction opposite to that of the rotation of the crankshaft 5 and, consequently, of the lubricant oil deposited in said lowered portions 11. The pressure exerted by said lubricant fluid will act on the peripheral flange 5a, as described above.

Though not illustrated, said lowered portions 11 could

present distinct radial profiles for their end walls, said profiles not being exactly orthogonal to the plane of the internal surface of said lowered portions 11. In these solutions, the superficial lowered portions 11 present a respective internal face, preferably parallel to the contact plane with the crankshaft 5. In order that the oil deposited between the contact face 10a of the bearing 10 and the lower face of the peripheral flange 5a does not escape to the oil sump 8, by action of the centrifugal force acting at this region and caused by the rotation of the crankshaft 5, the machining of the first face 10a may also be made radially, beginning from the external peripheral edge of said contact face 10a and directed to the internal edge of the latter, so that the largest distance between the adjacent surfaces of the axial bearing 10 and peripheral flange 5a occurs at said internal edge of said axial bearing 10, figure 3f. This construction gives rise to a centrifugal hydrodynamic wedge, where the oil pressure also occurs close to the external peripheral edge of the first face 10a of the axial bearing 10, forcing the latter upwardly, i.e., temporarily lifting it during the time period in which the crankshaft 5 is rotating. With these modifications on the surface of the first face of the axial bearing 10, it is possible to substitute a large single supporting area for the crankshaft 5 by one or more supporting areas, which are reduced, but guarantee a continuous oil supply at the region between the axial bearing 10 and the peripheral flange 5a of the crankshaft 5, thereby reducing the shear stress of the lubricant fluid and, consequently, reducing the power losses of said bearing. It is possible to use combinations of the above cited solutions, as well as to invert the orthogonal face- machined face pair, by maintaining the ring of the

cylinder block orthogonal to the crankshaft and providing the above described geometrical variations in the flange 5a of the crankshaft.