Screw compressor

Abstract

To prevent a casing from contacting a rotor on a suction side of a screw compressor.SOLUTION: A screw compressor 1 comprises: a pair of male and female rotors 2; a rotor chamber 4 that houses the rotor 2; a casing 3 having a bore wall 8 that defines the rotor chamber 4; an opening part 11 provided at one axial end part of the bore wall 8; a discharge port 12 that is provided at the other axial end part of the casing 3, and discharges compressed fluid; and a step part 13 that is provided at one axial end part of an inner peripheral surface of the bore wall 8, and enlarges a gap δ between the rotor 2 and the inner peripheral surface of the bore wall 8.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a screw compressor. [Background technology]

[0002] Patent Document 1 discloses a screw compressor with different diameter rotors. To prevent the small diameter rotor from coming into contact with the bore wall near the discharge end face due to thermal deformation of the rotor and casing, the center of the bore wall of the small diameter rotor is eccentric from the axis of the small diameter rotor toward the low-pressure side intersection. The amount of eccentricity is set to equalize the female rotor tooth tip clearance near the discharge end face. The cross-sectional shape of the bore wall of the small diameter rotor, perpendicular to the axis, is constant from the suction side to the discharge side in the axial direction. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-159766 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventionally, efforts have been made to avoid contact near the discharge end face, but no consideration has been given to contact between the casing and the rotor on the suction side.

[0005] An object of the present invention is to prevent the casing from coming into contact with the rotor on the suction side of the screw compressor. [Means for solving the problem]

[0006] One aspect of the present invention provides a screw compressor comprising a pair of male and female rotors, a rotor chamber that houses the rotors, a casing having a bore wall that defines the rotor chamber, an opening provided at one axial end of the bore wall, a discharge port provided at the other axial end of the casing for discharging the compressed fluid, and a step portion provided at the one axial end of the inner surface of the bore wall that enlarges the gap between the rotor and the inner surface of the bore wall.

[0007] When a fluid is compressed in a screw compressor, the fluid becomes hot. For example, if the fluid is air, immediately after the screw compressor starts operating, the suction side of the casing is at atmospheric temperature, while the discharge side becomes hot due to the passage of compressed air, resulting in a large temperature difference between the suction side and the discharge side. As the screw compressor continues to operate, the heat from the discharge side is transferred to the casing, gradually raising the temperature of the suction side and eliminating the temperature difference. Under this transient condition, thermal deformation of the casing begins on the discharge side. Thermal deformation on the suction side occurs later. This causes large distortion on the suction side of the casing, which may come into contact with the rotor. The above configuration helps to solve this problem.

[0008] According to the above configuration, the opening is provided at one axial end of the bore wall of the casing, and a step is recessed at one axial end of the inner circumferential surface of the bore wall. This enlarges the gap between the rotor and the inner circumferential surface of the bore wall at one axial end of the casing. Therefore, even if a large distortion occurs on the suction side of the casing under transient conditions, the casing can be prevented from contacting the rotor. Furthermore, because the step is provided at one axial end, the gap between the rotor and the bore wall can be maintained small at the other axial end (i.e., the discharge side). In comparison with a case where the cross-sectional shape of the bore wall is constant from the suction side to the discharge side when the gap is enlarged, leakage of compressed air can be reduced, and a decrease in compression performance is suppressed.

[0009] The step portion may be provided on the inner circumferential surface of the bore wall on one axial end side of the opening. The opening may be provided on the radial side of the rotor.

[0010] The bore wall on the suction side of the opening has lower rigidity, and if contact occurs, the bore wall is more likely to crack or break. With the above configuration, contact can be avoided in areas where cracks or breakage are likely to occur. Furthermore, the expansion range of the gap between the inner circumferential surface of the bore wall and the rotor is reduced, making it easier to suppress a decrease in compression performance. Furthermore, if the opening is located on the radial side of the rotor, the bore wall on the suction side of the opening has even lower rigidity, making it more effective to avoid contact in areas where cracks or breakage are likely to occur.

[0011] The axially orthogonal cross section of the bore wall may include, at one axial end, a semicircular arc-shaped reference circular portion centered on the axis of the rotor, and a semicircular arc-shaped eccentric circular portion centered at an eccentric position eccentric to the axis of the rotor, and the eccentric circular portion may constitute the step portion.

[0012] According to the above configuration, the casing having the stepped portion can be easily processed.

[0013] The step portion is formed from one axial end of the casing to a predetermined step end position set between one axial end and the other axial end of the casing, and the step end position may be set at a position where the pressure in the space formed by the tooth grooves of the rotor and the inner surface of the bore wall begins to exceed the suction pressure, or further toward the one axial end than that.

[0014] According to the above configuration, it is possible to minimize leakage of compressed air and avoid contact between the casing and the rotor on the suction side.

[0015] The step end position may be set along a curved ridge line formed by the tooth crest of the rotating rotor.

[0016] With the above configuration, the pressure on one end side of the ridgeline is the suction pressure, and the pressure on the other end side of the ridgeline is a pressure higher than the suction pressure. When the step end position is set along the ridgeline, high compression performance can be maintained on the discharge side of the step, leakage of compressed air can be minimized, and contact on the suction side can be avoided.

[0017] The bore wall may include a partially cylindrical male bore wall that accommodates a male rotor and a partially cylindrical female bore wall that accommodates a female rotor, the female bore wall and the male bore wall intersect with each other at a pair of intersections on the opening side and the opposite side, and the intersections on the opening side may be included in the step portion.

[0018] In transient conditions, distortion increases at the intersection on the opening side. With the above configuration, the intersection on the opening side is included in the step portion, widening the gap between the intersection and the rotor. This effectively prevents contact between the casing and the rotor on the suction side.

[0019] An intake port for drawing fluid into the rotor chamber may be provided at one axial end of the bore wall, and the intake port and the discharge port may be positioned to face each other in the diametrical direction.

[0020] In the above configuration, since the suction port and the discharge port are diametrically opposed, the transfer of heat from the discharge side to the suction port is delayed, which increases the distortion of the casing near the suction port. By applying a step portion to a screw compressor of this type, it is possible to effectively prevent contact between the casing and the rotor on the suction side.

[0021] The screw compressor may be an oil-free machine.

[0022] According to the above configuration, since an oil-cooled structure is not adopted, the temperature difference between the discharge side and the suction side becomes large under transient conditions, and distortion on the suction side becomes large. By applying a step portion to such a screw compressor, contact between the casing and the rotor on the suction side can be effectively avoided. [Effects of the Invention]

[0023] According to the present invention, it is possible to prevent the casing from coming into contact with the rotor on the suction side of the screw compressor. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a cross-sectional view of a screw compressor according to an embodiment. [Figure 2] FIG. 2 is a perspective view of the screw compressor as seen from the suction side. [Figure 3] FIG. 2 is a perspective view of the screw compressor as seen from the discharge side. [Figure 4] FIG. [Figure 5] FIG. [Figure 6] FIG. [Figure 7] FIG. [Figure 8] FIG. 4 is a diagram showing a gap between the inner peripheral surface of the bore wall and the tip of the rotor tooth on the suction side. [Figure 9] 4 is a graph showing the pressure in the space formed by the tooth grooves of the rotor and the inner circumferential surface of the bore wall. [Figure 10] FIG. 4 is a perspective view of a screw compressor according to a first modified example. [Figure 11] FIG. 10 is a cross-sectional view of a screw compressor according to a second modified example. [Figure 12] FIG. 10 is a cross-sectional view of a screw compressor according to a third modified example. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments will be described with reference to the drawings. Note that the same or corresponding elements are designated by the same reference numerals throughout the drawings, and detailed descriptions thereof will be omitted.

[0026] 1, a screw compressor 1 according to an embodiment includes a pair of a male rotor 2m and a female rotor 2f (sometimes collectively referred to as rotors 2), and a casing 3. The casing 3 has a rotor chamber 4 that houses the rotor 2. The rotor chamber 4 includes a male rotor chamber 4m and a female rotor chamber 4f that spatially communicate with each other. The male rotor 2m is housed in the male rotor chamber 4m, and the female rotor 2f is housed in the female rotor chamber 4f.

[0027] The male rotor 2m has a rotor shaft 5m and a plurality of helical teeth 6m provided on the outer periphery of the rotor shaft 5m. A helical tooth space 7m is defined between each pair of adjacent teeth 6m. The female rotor 2f has a rotor shaft 5f and a plurality of helical teeth 6f provided on the outer periphery of the rotor shaft 5f. A helical tooth space 7f is defined between each pair of adjacent teeth 6f. The rotor shaft 5m of the male rotor 2m is rotatably supported about its own axis Am by bearings (not shown). The rotor shaft 5f of the female rotor 2f is also rotatably supported about its own axis Af by bearings (not shown).

[0028] A drive mechanism (not shown) including a motor is mechanically connected to the rotor shaft 5m of the male rotor 2m. When the male rotor 2m is driven to rotate by the drive mechanism, the teeth 6m of the male rotor 2m enter and mesh with the tooth grooves 7f of the female rotor 2f in a non-contact state, causing the male rotor 2m and the female rotor 2f to rotate synchronously. Instead of or in addition to the male rotor 2m, the female rotor 2f may also be driven to rotate by the drive mechanism.

[0029] 2 and 3, the rotor shafts 5m, 5f (see FIG. 1) are not shown. The casing 3 has a bore wall 8 that forms the rotor chamber 4. The bore wall 8 includes a male bore wall 8m that defines the male rotor chamber 4m and a female bore wall 8f that defines the female rotor chamber 4f. The tips of the teeth 6m of the male rotor 2m are radially adjacent to the inner circumferential surface of the male bore wall 8m (the cylindrical surface that defines the male rotor chamber 4m). The tips of the teeth 6f of the female rotor 2f are radially adjacent to the inner circumferential surface of the female bore wall 8f (the cylindrical surface that defines the female rotor chamber 4f).

[0030] The male bore wall 8m is partially cylindrical. The female bore wall 8f is also partially cylindrical. The male bore wall 8m and the female bore wall 8f intersect with each other at a pair of intersections 9a, 9b. Both the male bore wall 8m and the female bore wall 8f are C-shaped when viewed in the axial direction.

[0031] 2 and 3, one of the pair of intersections 9a, 9b is referred to as the upper intersection 9a, and the other of the pair of intersections 9a, 9b is referred to as the lower intersection 9b. In this case, the axial direction is oriented in the front-to-rear direction, and the male bore wall 8m and the female bore wall 8f are aligned left and right. One radial side is the upper side, where the opening 11 is located. The other radial side is the lower side, where the discharge port 12 is located.

[0032] An opening 11 is provided at one axial end of the casing 3. In this embodiment, the opening 11 is oriented in the radial direction and penetrates the upper part of the bore wall 8. The opening 11 spans the male bore wall 8m and the female bore wall 8f, and the upper intersection 9a is divided into two parts in the axial direction (front-rear direction) by the opening 11. The edge of one axial end of the opening 11 extends in the direction in which the male bore wall 8m and the female bore wall 8f are aligned (left-right direction). The bore wall 8 surrounds the opening 11, and a narrow wall portion 8a is provided at one axial end and upper part of the bore wall 8, defining the edge of one axial end of the opening 11. The width (dimension in the axial direction) of the narrow wall portion 8a is relatively small, and the rigidity of the narrow wall portion 8a is lower than that of other parts of the bore wall 8.

[0033] The discharge port 12 is provided at the other axial end of the casing 3. The casing 3 has an end wall 10 at the other axial end. The discharge port 12 penetrates this end wall 10 and faces the axial direction. The discharge port 12 opens to both the male rotor chamber 4m and the female rotor chamber 4f and is elongated in the direction in which the male rotor chamber 4m and the female rotor chamber 4f are aligned. In addition, the discharge port 12 is located on the other radial side (lower side) with respect to the axis Am of the male rotor 2m and the axis Af of the female rotor 2f. As described above, the opening 11 is formed on one radial side (upper side). The suction port (also called the "axial port") 11a and the discharge port 12 are positioned diametrically opposite each other.

[0034] In the screw compressor 1 configured as described above, when the rotor 2 is driven to rotate, a fluid such as air flows into the male rotor chamber 4m and the female rotor chamber 4f of the casing 3 through an inlet (not shown). The following description will be given assuming that the fluid is air. The inlet point is one axial end of the casing 3. The air flows into the tooth grooves 7m, 7f at one axial end of the casing 3. The air is compressed within the tooth grooves 7m, 7f and is discharged to the outside of the casing 3 through the discharge port 12.

[0035] When air is compressed, it becomes hot. Before the screw compressor 1 starts, the temperature of the casing throughout is the same as the surrounding temperature (for example, atmospheric temperature), i.e., room temperature. When the screw compressor 1 starts, the temperature on the discharge side of the casing 3 rises as it is exposed to the high temperature of the compressed air, whereas the temperature on the suction side of the casing 3 does not change significantly from room temperature because the air flowing in is at atmospheric temperature. As the screw compressor 1 continues to operate, the heat on the discharge side is transferred to the entire casing 3, and the casing temperature becomes almost uniform throughout, at a temperature higher than the surrounding temperature.

[0036] During the transient period from when the screw compressor 1 starts until the casing temperature stabilizes at a high temperature, a large temperature difference occurs between the suction side and the discharge side, especially immediately after startup. Thermal deformation of the casing 3 begins on the discharge side, with thermal deformation on the suction side occurring later. This causes large distortion on the suction side of the casing 3, which may come into contact with the rotor 2.

[0037] In particular, in this embodiment, the discharge port 12 is separated from the suction port 11a not only in the axial direction but also in the circumferential direction, making distortion more likely to occur on the suction side. In this embodiment, the screw compressor 1 is an oil-free machine, so a temperature difference is more likely to occur between the suction side and the discharge side under transient conditions than in the case where the screw compressor 1 is an oil-cooled machine, making the suction side more likely to be distorted. In this embodiment, the opening 11 is oriented in the radial direction, and a narrow wall portion 8a with relatively low rigidity is provided on the suction side. If distortion occurs in this narrow wall portion 8a and interference with the rotor 2 occurs, the casing 3 (particularly the narrow wall portion 8a) is more likely to be damaged.

[0038] In the screw compressor 1 according to this embodiment, in order to eliminate the above problem, i.e., the risk of contact between the suction side of the casing 3 and the rotor 2 under transient conditions, the cross-sectional shape of the bore wall 8 is made different between the suction side and the discharge side.

[0039] 4 is a schematic diagram of an opening at one axial end (suction side) of the casing 3. Referring to FIG. 4, a step 13 is provided at one axial end of the inner circumferential surface of the bore wall 8. The step 13 is recessed at one axial end of the inner circumferential surface of the bore wall 8. The step 13 thus provided increases the gap δ between the rotor 2 and the inner circumferential surface of the bore wall 8.

[0040] The cross section of the bore wall 8 perpendicular to the axis includes, at one axial end, a semicircular reference circle portion 14 centered on the axis lines Am, Af of the rotor 2, and a semicircular eccentric circle portion 15 centered at eccentric positions Em, Ef that are eccentric with respect to the axis lines Am, Af of the rotor 2. The reference circle portion 14 is directly continuous with the eccentric circle portion 15, or is indirectly continuous with the eccentric circle portion 15 via a continuous portion 16. The rotors 2 are a pair of male and female rotors. The reference circle portion 14 includes a male-side reference circle portion 14m centered on the axis line Am of the male rotor 2m, and a female-side reference circle portion 14f centered on the axis line Af of the female rotor 2f. The eccentric circular portion 15 has a male side eccentric circular portion 15m that defines the male rotor chamber 4m around an eccentric position Em that is eccentric with respect to the axis Am of the male rotor 2m, and a female side eccentric circular portion 15f that defines the female rotor chamber 4f around an eccentric position Ef that is eccentric with respect to the axis Af of the female rotor 2f.

[0041] The male side reference circle portion 14m and the male side eccentric circle portion 15m have the same diameter (radius Rm). The female side reference circle portion 14f and the female side eccentric circle portion 15f have the same diameter (radius Rf). The male side eccentricity Dm (the eccentricity of the male rotor 2m relative to the axis Am at the eccentric position Em) and the female side eccentricity Df (the eccentricity of the female rotor 2f relative to the axis Af at the eccentric position Ef) may be the same or different. In FIG. 4, the eccentricity D (the male side eccentricity Dm and the female side eccentricity Df) is exaggerated for ease of understanding only. The eccentricity D only needs to be large enough to avoid contact between the casing 3 and the rotor 2 due to distortion caused by thermal deformation. In reality, the eccentricity D is very small, as shown in FIGS. 6 to 8.

[0042] The male side eccentricity amount Dm is, for example, 200 μm to 500 μm. The same applies to the magnitude of the female side eccentricity amount Df. When the reference circle portion 14 and the eccentric circle portion 15 are continuous via a continuous portion 16, the continuous portion 16 may extend linearly in parallel to a line segment connecting the axis Am, Af of the rotor 2 and the eccentric positions Em, Ef.

[0043] The eccentric positions Em, Ef are eccentric relative to the axes Am, Af in the axis-orthogonal cross section in a direction approaching the opening 11 (in this embodiment, one radially upward side). Male side reference circular portion 14m forms approximately half of the discharge side of the inner circumferential surface of male bore wall 8m. Male side eccentric circular portion 15m forms approximately half of the suction side of the inner circumferential surface of male bore wall 8m. Female side reference circular portion 14f forms approximately half of the discharge side of the inner circumferential surface of female bore wall 8f. Female side eccentric circular portion 15f forms approximately half of the suction side of the inner circumferential surface of female bore wall 8f. Male side reference circular portion 14m and female side reference circular portion 14f intersect with each other at the lower (discharge side) intersection 9b of a pair of intersections 9a, 9b. Male side eccentric circular portion 15m and female side eccentric circular portion 15f intersect with each other at the upper (suction side) intersecting portion 9a of the pair of intersecting portions 9a, 9b.

[0044] 5 to 7, the step portion 13 is formed from one axial end of the casing 3 to a predetermined step end position P. The step end position P is set between the one axial end and the other axial end of the casing 3. On the other axial end side of the step end position P, the inner circumferential surface of the male bore wall 8m is a perfect arc formed by the male reference circular portion 14m, and the inner circumferential surface of the female bore wall 8f is a perfect arc formed by the female reference circular portion 14f. On the other axial end side of the step end position P, the male reference circular portion 14m and the female reference circular portion 14f intersect with each other at both of the pair of intersections 9a, 9b. In this respect, the casing 3 according to this embodiment differs from a configuration having a constant cross-sectional shape from one axial end to the other axial end.

[0045] At the step end position P, an arc-shaped step surface 13a facing one axial end is provided to connect the edge of the eccentric circle portion 15 and the edge of the reference circle portion 14. However, the width (radial dimension) of the step surface 13a corresponds to the amount of eccentricity D and is actually very small (see FIGS. 6 and 7).

[0046] In this embodiment, in both the male rotor chamber 4m and the female rotor chamber 4f, the step end position P is linear in the axially orthogonal direction. That is, the male side relief depth Lm (the distance from one axial end to the step end position P of the male rotor chamber 4m) is constant regardless of the circumferential position of the male bore wall 8m. The same is true for the female side relief depth Lf (the distance from one axial end to the step end position P of the female rotor chamber 4f). In other words, the step surface 13a is parallel to the axially orthogonal plane.

[0047] The center of Figure 8 is a view of the casing 3 housing the male rotor 2m and female rotor 2f, as viewed from the suction side in the axial direction. The two views at the top of Figure 8 are enlarged views of the eccentric circle portion 15 and the tooth tips of the rotor 2. The two views at the bottom of Figure 8 are enlarged views of the reference circle portion 14 and the tooth tips of the rotor 2. The two views at the left of Figure 8 are enlarged views of the inner circumferential surface of the male bore wall 8m and the tooth tips of the male rotor 2m. The two views at the right of Figure 8 are enlarged views of the inner circumferential surface of the female bore wall 8f and the tooth tips of the female rotor 2f.

[0048] The tooth tips of the rotor 2 form a tooth tip circle that is a perfect circle centered on the axis Am, Af of the rotor 2. The clearance (reference gap δr) between the reference circle portion 14 and the tooth tips of the rotor 2 is the difference in radius of the reference circle portion 14 of the tooth tip circle. The reference gap δr is set to an extremely small value in order to maintain the compression performance as high as possible while also achieving smooth rotation of the rotor 2.

[0049] In contrast, the clearance between the male side eccentric circular portion 15m and the tooth tip of the male rotor 2m (male side expanded gap δεm) corresponds to the sum of the reference gap δr and the male side eccentric amount Dm. The same is true for the female side expanded gap δεf (clearance between the female side eccentric circular portion 15f and the tooth tip of the female rotor 2f). In this way, the gap δ is expanded.

[0050] As described above, the screw compressor 1 according to this embodiment includes a pair of male and female rotors 2, a rotor chamber 4 that houses the rotor 2, a casing 3 having a bore wall 8 that defines the rotor chamber 4, an opening 11 provided at one axial end of the casing 3, and a discharge port 12 provided at the other axial end of the casing 3 for discharging compressed fluid. The screw compressor 1 further includes a step 13 recessed at one axial end of the inner circumferential surface of the bore wall 8, which enlarges the gap δ between the rotor 2 and the inner circumferential surface of the bore wall 8.

[0051] The presence of this step 13 prevents the casing 3 from coming into contact with the rotor 2 even if a large distortion occurs on the suction side of the casing 3 under transient conditions. The step 13 is provided at one axial end, and the gap δ between the rotor 2 and the bore wall 8 can be kept small at the other axial end (i.e., the discharge side). Compared to when the cross-sectional shape of the bore wall 8 is constant from the suction side to the discharge side when enlarging the gap δ, leakage of compressed air can be reduced and a decrease in compression performance is suppressed.

[0052] The bore wall 8 includes a partially cylindrical male bore wall 8m that accommodates the male rotor 2m and a partially cylindrical female bore wall 8f that accommodates the female rotor 2f, and the male bore wall 8m and the female bore wall 8f intersect with each other at a pair of intersections 9a, 9b on the suction side and the opposite side. Under transient conditions, distortion increases at the intersection 9a on the suction side. In this embodiment, the step portion 13 includes the intersection 9a on the suction side, widening the gap δ between the intersection 9a on the suction side and the rotor 2. Therefore, contact between the casing 3 and the rotor 2 on the suction side can be effectively avoided under transient conditions.

[0053] The opening 11 opens at one axial end of the bore wall 8. The bore wall 8 has a narrow wall portion 8a with relatively low rigidity on the suction side. This prevents contact between the narrow wall portion 8a and the rotor 2 and the resulting high probability of cracks or breakage, thereby improving the service life of the casing 3. In other words, applying the above-described step portion 13 to such a radial port screw compressor 1 is beneficial because it effectively prevents breakage of the casing 3.

[0054] The suction port 11a and the discharge port 12 are disposed at positions diametrically opposed to each other. If the suction port 11a and the discharge port 12 are spaced apart not only in the axial direction but also in the diametric or circumferential direction, the transfer of heat from the discharge side to the suction side is delayed, which increases distortion around the opening 11. Applying the above-described step portion 13 to a screw compressor 1 of this type is beneficial because it effectively prevents contact between the casing 3 and the rotor 2 on the suction side.

[0055] The screw compressor 1 is an oil-free machine. In oil-free machines, the temperature difference between the discharge side and the suction side is likely to increase under transient conditions, causing large distortion on the suction side. Applying the step portion 13 to such an oil-free machine is beneficial because it effectively prevents contact between the casing 3 and the rotor 2 on the suction side.

[0056] The step portion 13 is formed from one axial end of the casing 3 to a predetermined step end position P set between the one axial end and the other axial end of the casing 3. To avoid contact on the suction side, the gap δ between the inner circumferential surface of the bore wall 8 and the rotor 2 is enlarged on the suction side of the step end position P, while on the discharge side of the step end position P, the gap δ becomes the standard gap δr over the entire circumference of the rotor chamber 4. Therefore, compression performance can be maintained on the discharge side.

[0057] 9, the step end position P is set at a position where the pressure in the space defined by the tooth grooves 7m, 7f of the rotor 2 and the inner circumferential surface of the bore wall 8 (the so-called pressure chamber) begins to exceed the suction pressure, or at one axial end of that position. This makes it possible to minimize leakage of compressed air while avoiding contact between the casing 3 and the rotor 2 on the suction side.

[0058] FIG. 10 shows a first modified example of the above embodiment. Here, the curve formed by the apex of the tooth is referred to as the "ridge line RL." The step end position P may be set along the ridge line RL. The portion on the suction side of the ridge line RL is at suction pressure, and the portion on the discharge side of the ridge line RL is at a pressure higher than the suction pressure. Therefore, the step end position P may be set in a spiral shape along the ridge line RL. In this case, the step end position P is set in a spiral shape slightly on the suction side (one axial end side) of the ridge line RL. This makes it possible to suppress leakage of compressed air and avoid contact on the suction side.

[0059] 11 shows a second modified example of the above embodiment, and FIG. 12 shows a third modified example of the above embodiment. As shown in the second and third modified examples, the gap δ between the rotor 2 and the inner circumferential surface of the bore wall 8 at the stepped portion 13 may be gradually reduced from the suction side (one axial end side) toward the discharge side (the other axial end side). In this case, the gap δ may be reduced in a step-like manner from the suction side (one axial end side) toward the discharge side (the other axial end side) as shown in the second modified example, or may be reduced continuously as shown in the third modified example.

[0060] The present disclosure may include the following aspects. (Aspect 1) A pair of male and female rotors; a casing having a rotor chamber that accommodates the rotor and a bore wall that defines the rotor chamber; an opening provided at one axial end of the bore wall; a discharge port provided at the other axial end of the casing and configured to discharge the compressed fluid; a step portion provided at one axial end of an inner circumferential surface of the bore wall, the step portion expanding a gap between the rotor and the inner circumferential surface of the bore wall; A screw compressor comprising: (Aspect 2) the step portion is provided on the inner circumferential surface of the bore wall on one axial end side of the opening portion, 2. The screw compressor according to claim 1. (Aspect 3) The opening is provided on the radial side of the rotor. 3. The screw compressor according to claim 1 or 2. (Aspect 4) an axially orthogonal cross section of the bore wall at the one axial end portion includes a semicircular arc-shaped reference circular portion centered on the axis of the rotor and a semicircular arc-shaped eccentric circular portion centered on an eccentric position eccentric to the axis of the rotor, and the eccentric circular portion constitutes the step portion; A screw compressor according to any one of aspects 1 to 3. (Aspect 5) the step portion is formed from one end of the casing in the axial direction to a predetermined step terminal position set between the one end and the other end of the casing in the axial direction, the step end position is set at a position where a pressure in a space defined by tooth grooves of the rotor and the inner circumferential surface of the bore wall begins to exceed a suction pressure, or at one end side of the position in the axial direction. A screw compressor according to any one of aspects 1 to 4. (Aspect 6) the step end position is set along a curved ridge line drawn by the tooth top of the rotating rotor; 6. The screw compressor according to embodiment 5. (Aspect 7) the bore wall includes a partially cylindrical male bore wall that accommodates a male rotor and a partially cylindrical female bore wall that accommodates a female rotor, the female bore wall and the male bore wall intersecting each other at a pair of intersections on the opening side and the opposite side, The intersection on the opening side is included in the step portion. A screw compressor according to any one of aspects 1 to 6. (Aspect 8) an intake port for drawing fluid into the rotor chamber is provided at one axial end of the bore wall, and the intake port and the discharge port are positioned opposite each other in the diametrical direction; A screw compressor according to any one of aspects 1 to 7. (Aspect 9) The screw compressor is an oil-free machine. A screw compressor according to any one of aspects 1 to 8.

[0061] Although the embodiments have been described above, the above configurations can be appropriately added to, modified, or deleted within the scope of the spirit of the present invention. [Explanation of symbols]

[0062] 1. Screw compressor 2 rotors 2m male rotor 2f female rotor 3 Casing 4 Rotor chamber 4m male rotor chamber 4f Female rotor room 5m, 5f rotor shaft 6m,6f Teeth 7m,7f tooth groove 8 Bore Wall 8a Narrow wall part 8m male bore wall 8f Female Boa Wall 9a,9b intersection 10 End Wall 11 Opening 11a Suction port 12 Outlet 13 Step 13a Step surface 14 Reference circle 14m Male side reference circle 14f Female reference circle 15 Eccentric circle 15m male eccentric section 15f Female eccentric part 16 Continuous section Am, Af axis D Eccentricity Dm Male side eccentricity Df Female side eccentricity Em,Ef Eccentric position Lm Male side relief depth Lf Female side relief depth P Step end position Rm,Rf radius RL Ridgeline δ Gap δr Reference gap δεm Male side expanded gap δεf Female side expanded gap

Claims

1. A pair of male and female rotors; a casing having a rotor chamber that accommodates the rotor and a bore wall that defines the rotor chamber; an opening provided at one axial end of the bore wall; a discharge port provided at the other axial end of the casing and configured to discharge the compressed fluid; a step portion provided at one axial end of an inner circumferential surface of the bore wall, the step portion expanding a gap between the rotor and the inner circumferential surface of the bore wall; A screw compressor comprising:

2. the step portion is provided on the inner circumferential surface of the bore wall on one axial end side of the opening portion, The screw compressor according to claim 1.

3. The opening is provided on the radial side of the rotor. The screw compressor according to claim 1.

4. an axially orthogonal cross section of the bore wall includes, at the one axial end, a semicircular arc-shaped reference circular portion centered on the axis of the rotor, and a semicircular arc-shaped eccentric circular portion centered at an eccentric position eccentric to the axis of the rotor, and the eccentric circular portion constitutes the step portion; The screw compressor according to claim 1 or 2.

5. the step portion is formed from one end of the casing in the axial direction to a predetermined step terminal position set between the one end and the other end of the casing in the axial direction, the step end position is set at a position where a pressure in a space defined by tooth grooves of the rotor and the inner circumferential surface of the bore wall begins to exceed a suction pressure, or at one end side of the position in the axial direction. The screw compressor according to claim 1 or 2.

6. the step end position is set along a curved ridge line drawn by the tooth top of the rotating rotor; The screw compressor according to claim 5.

7. the bore wall includes a partially cylindrical male bore wall that accommodates a male rotor and a partially cylindrical female bore wall that accommodates a female rotor, the female bore wall and the male bore wall intersecting each other at a pair of intersections on the opening side and the opposite side, The intersection on the opening side is included in the step portion. The screw compressor according to claim 1 or 2.

8. an intake port for drawing fluid into the rotor chamber is provided at one axial end of the bore wall, and the intake port and the discharge port are positioned opposite each other in the diametrical direction; The screw compressor according to claim 1 or 2.

9. The screw compressor is an oil-free machine. The screw compressor according to claim 1 or 2.

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