Lapping of involute spiral scroll element

Abstract: The scroll elements (12) of a scroll-type compressor are treated employing a lapping tool (40) to achieve increased flatness of the base surface (34) and increased smoothness of the side walls (28, 30) of the involute wrap (24). This reduces flank leakage, tip leakage, and thrust friction losses. The lapping device (40) that is placed against the scroll element (12) has a radially extending base (42) and a generally spiral wrap (44), the wrap (44) generally matching that of the scroll element (12) workpiece. The lapping device wrap (44) has axially erect walls and a radially flat tip surface. After engaging the scroll element work piece, the lapping device (40) is moved relative to the scroll element (12) in an orbiting motion. A suitable lapping compound (48) is introduced at least between the lapping device wrap tip surface and the base surface (34) of the scroll element (12), and, if desired, also between the side walls of the lapping device wrap (44) and the side walls (28, 30) of the scroll element wrap (24). The lapping compound (48) can be introduced directly or in a gas flow.

Description: This invention relates to a method of finishing a scroll element for a scroll rotary machine. Specifically, this invention relates to scroll-type rotating machines, such as scroll compressors as used in air conditioning and refrigeration. The invention is more particularly directed to the achievement of superior finishing and surface smoothness in surfaces of moving parts that mate together in a scroll machine. Scroll type rotary machines are advantageously used to compress or pump a gas. These machines typically have two scroll members that face each other, each formed of a generally circular base plate and a spiral or involute wrap that is generally erect with respect to the base. That is, the wrap has side walls that are parallel to the axis of the scroll member. The scroll members maintain a fixed azimuth relative to one another, but are radially offset so that one can revolve about the other in an orbiting fashion. Relative orbiting motion is typically achieved by holding one scroll member fixed in a housing, and orbiting the other by rotating an eccentric shaft while holding the orbiting scroll member with an anti-rotation device, such as an Oldhman's ring. Orbiting motion can also be achieved by rotating both scroll members on parallel, offset axes. The walls of the involute wraps define crescent-shaped volumes which become smaller and smaller, and which move from the outside to the center of the mating scrolls with motion of the orbiting scroll element. A compressor fluid, such as a refrigerant gas, can be introduced at the periphery of the spiral wraps, and this fluid is compressed as it is moved under the orbiting motion of the device. The compressed fluid is then discharged at center. By introducing a compressed fluid into the center of the device and permitting it to expand, the scroll machine can be used as a motor. In order to achieve good efficiency characteristics, mating surfaces of the scroll elements must be as regular and smooth as possible, both to minimize frictional losses and to minimize gas leakage from the crescent-shaped volumes. Flank leakage can occur across the axial line where the walls of the two wraps are tangent, and tip leakage can occur where the tip of one scroll wrap contacts the flat base surface of the mating scroll element. Generally, tip leakage is a greater source of loss and inefficiency than flank leakage. Also, imperfections in flatness of the base surface of the scroll elements are responsible for thrust friction losses. Machining a scroll element so that the base wall is flat and the walls of the wrap are axially erect spirals is extremely difficult to accomplish. The presence of the wrap itself prevents standard grinding and machining techniques from being employed for this end, because the operative surfaces to be finished are recessed below the tip of the scroll wrap. In JP-A-59-208 187 there is described a method of finishing a scroll element for a scroll rotary machine according to the preamble of claim 1. The method of the type described in the aforementioned JP-A-59-208 187 utilizes a lapping device having a polishing layer contacting the scroll element to thereby polish the base surface and side walls of the scroll when the lapping device and scroll element are driven in orbiting motion in order to provide an optimum clearance between the scroll members. There has been no simple technique proposed previously to finish and flatten the scroll element surfaces so as to minimize leakage and frictional losses. Accordingly, the object of this invention is to provide a technique for increasing the efficiency of scroll compressors and other scroll type rotating machines by providing scroll elements with increased flatness on the base surface and by providing the wrap walls with increased smoothness. To achieve this, the method of the invention is characterized by the features set forth in the characterizing part of claim 1. In accordance with the principles of this invention, a flow of lapping compound is provided into the engaging surfaces of the lapping device and the scroll element, by introducing the lapping compound with a flow of a fluid directly into the crescent shaped volumes of the interfaced scroll element and lapping device and distributing the compound by orbiting action of the scroll element and lapping device. The elements are brought together before the lapping compound is brought therebetween. Advantageous embodiments of the invention are claimed in the subclaims. In a specific embodiment of the invention, the base and the wrap of the scroll element are finished by lapping them with the lapping device, the lapping device having a flat base and a generally spiral or involute wrap that mates with the wrap of the workpiece scroll element. The scroll element and the lapping devices are moved in an orbiting fashion with respect to one another, and a suitable lapping compound is introduced between the tip of the lapping device scroll and the base surface of the scroll element. Lapping compound can also be introduced between the side walls of the lapping device wrap and the side walls of the scroll element wrap. The eccentricity of rotation is controlled so that either the base surface only is lapped or both the base and wrap well surfaces are lapped, as required. Preferably, the lapping device has a significantly thicker wrap than a normal scroll element. This reduces the velocity of motion required for the lapping process. The lapping element is favorably made of a suitably hardened material, such as hardened steel, to avoid wear. The lapping compound can be introduced directly into the crescent shaped pockets, as a flow of gas and particles of the compound through the interface of the lapping device and scroll element workpiece. The principles of this invention can also be practiced by using the two facing scroll elements, i.e., the fixed and orbiting scroll elements, that are intended for the same compressor to lap each other. Fig. 1 is a cross section of a pair of scroll elements of a scroll compressor as viewed at 1-1 of Fig. 2. Fig. 2 is an axial section of the scroll elements as viewed at 2-2 of Fig. 1. Fig. 3 is a partial cross section showing a lapping element and a workpiece scroll element, for explaining the technique of this invention. With reference to the Drawing, and initially to Figs. 1 and 2 thereof, a scroll-type device for a scroll compressor or other rotary scroll machine has an upper, fixed scroll element 10 and a lower, orbiting scroll element 12. The lower scroll element 12 is driven by a rotary eccentric drive, but is held in a fixed azimuthal orientation by an anti-rotation mechanism, such as an Oldham's ring. The upper scroll element 10 has a flat, circular base member, and a spiral or involute wrap, the wrap spirals about an axis of the scroll element 10 and extends from the periphery of the element 10 to an outlet 18 at the center. The upper and lower scroll elements 10 and 12 engage one another so that a tip surface 20 of the fixed scroll wrap 16 contact a flat based surface 22 of the lower orbiting scroll element 12. The orbiting scroll element 12 also has an involute wrap 24 that conforms with the wrap 16, and extends from the periphery of the element 12 to its center. The wrap 24 spirals around the axis of the orbiting scroll element 12. The wrap 24 has an inner side wall 28 that engages the outside wall of the fixed element wrap 16, and an outer side wall 30 that engages the inside wall of the fixed scroll element wrap 16. The wrap 24 has a tip surface 32 that engages a flat base surface 34 of the base 14 of the fixed scroll element 10. A lapping tool 40, as shown in part in Fig. 3, has a base portion 42, which can be in the form of a disk, and a wrap 44 which spirals around the axis of the base portion 42. Here, the lapping tool 40 is intended for finishing the operative surfaces of the orbiting scroll 12. The wrap 44 conforms with the spiral of the wrap 24. As shown, the lapping tool wrap 44 is substantially thicker in the radial direction than the involute wrap 24 of the scroll element 12. The wrap 44 is preferably formed of a suitable hardened material such as hardened steel, with at least exterior surfaces being suitably treated so that the lapping tool 40 has a long useful life. In operation, the lapping tool 40 and the workpiece scroll element 12 engage one another and undergo orbital motion relative one another. That is, one or the other of the element 12 and tool 40 revolves about the axis of the other, but the element 12 and tool 40 are held in a fixed azimuthal relationship with each other. The eccentricity of the orbiting motion is controlled for proper lapping operation. A lapping compound 48 is introduced at least between a tip surface 46 of the wrap 44 and the base surface 14 of the scroll element 12. The lapping compound 48 can also be introduced, if desired, between the lapping tool wrap 44 and the side walls 28 and 30 of the scroll element wrap 24. The eccentricity of orbital motion can be controlled for precise lapping of these surfaces 28 and 30. The enlarged area of the tip surface 46, owing to the large relative thickness of the lapping tool wrap 44, creates a wider contact area with the workpiece scroll element base 14. This permits the relative velocity of motion to be kept low. A lapping tool for lapping the upper fixed scroll element 10 is similar to the lapping tool 40, except the wrap spiral direction is reversed to conform with the wrap 16. Lapping compound can be introduced directly into the crescent-shaped pockets or volumes that are formed between the wrap of the workpiece scroll element and the wrap of the lapping device. Then the compound is distributed by the orbiting motion of the scroll element and lapping device. Alternatively, lapping compound can be introduced with a flow of gas combined with particles of the lapping compound through the interface of the lapping tool 40 and the scroll element workpiece. While this invention has been described in detail with respect to a preferred embodiment, it should be understood that the invention is not limited to that precise embodiment.

Claims: A method of finishing a scroll element (12) for a scroll rotary machine in which the scroll element (12) has a base (14) with a radially flat surface (34) and a generally spiral wrap (24) about an axis of the element (12) with side walls (28, 30) that are axially erect comprising the steps of: placing against said scroll element (12) a lapping device (40) that has a radially extending base (42) and a generally spiral wrap (44) about an axis of the lapping device (40), the generally spiral wrap (44) matching the wrap (24) of the scroll element (12), the wrap (44) of the lapping device (40) having axially erect walls and a radially flat tip surface (46) that faces against the base flat surface (14) of the scroll element (12), said scroll element wrap (24) and lapping device wrap (44) defining a series of crescent shaped volumes, moving the scroll element (12) and the lapping device (40) against one another in an orbiting motion in which the axes of the device (40) and scroll element (12) revolve about one another but the device (40) and scroll element (12) maintain a constant azimuthal orientation relative to one another, and supplying a suitable lapping compound (48) at least between the tip surface (46) of the lapping device (40) and the base surface (34) of the scroll element (12) and between the lapping device wrap (44) and the side walls (28,30) of the scroll element wrap (24) for lapping at least the base flat surface (34) and side walls (28, 30) of the scroll element (12) while the lapping device (40) and scroll element (12) are being moved in said orbiting motion in contact with one another, characterized by bringing the scroll element (12) and the lapping device (40) together before the lapping compound (48) is brought in between the said scroll element (12) and lapping device (40), introducing said lapping compound (48) with a flow of a fluid carrying said lapping compound (48) directly into said crescent shaped volumes, the scroll element (17) and lapping device (40) being moved against one another and, by orbiting motion of the scroll element (12) and lapping device (40), the fluid including the lapping compound (48) is carried into the engaging surfaces of the lapping device (40) and the scroll element (12) and distributed. The method of claim 1, characterized by also comprising introducing said lapping compound between the side walls of the lapping device wrap (44) and the side walls (28, 30) of the scroll element wrap (32), and controlling eccentricity of the orbiting motion of said scroll element (12) and lapping device (40) to lap the side walls (28, 30) of the scroll element wrap (32). The method of claim 1, wherein said scroll machine has a second scroll element (10) having a base (14) with a radially flat surface (22) and a generally spiral wrap (16) similar to that of the first-mentioned element (12) but in which the direction of the spiral wrap (16) is reversed, characterized by the further steps of placing against the second scroll element (10) a second lapping device having a generally spiral wrap matching the wrap of the second scroll element (10) but which is substantially thicker radially than the second scroll element wrap (16), the wrap of the second lapping device having axially erect walls and a radially flat tip surface that faces against the base flat surface (22) of the second scroll element (10), moving the second scroll element (10) and the second lapping device against one another in an orbiting motion in which the axes of the device and scroll element (10) revolve about one another but the device and scroll element (10) maintain a constant azimuthal orientation relative to one another, and flowing the lapping compound between the tip surface of the second lapping device and the base surface (22) of the scroll element (10) and between the second lapping device wrap and the side walls of the second scroll wrap (16) while the lapping device and scroll element (10) are being moved in said orbiting motion in contact with one another.

Inventors: ETEMAD SHAHROKH, FRASER HOWARD HENRY JR, ETEMAD, SHAHROKH, FRASER, HOWARD HENRY, JR.
Applicants: CARRIER CORP, CARRIER CORPORATION
Application year: 1990
Main classification: F01C1
IPC codes: B24B19, F04C18, F01C21, B24B37, F01C1