Screw Fluid Machine

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a screw fluid machine including a male rotor and a female rotor that rotate while meshing with each other on parallel rotation axes.

2. Description of the Related Art

A screw fluid machine includes a pair of male and female screw rotors that rotate while meshing with each other, and a casing that houses both the rotors (the male rotor and the female rotor). Each of the male rotor and the female rotor includes helical teeth and tooth grooves. In the screw fluid machine, the volume of a plurality of working chambers formed by two tooth grooves of the screw rotors and an inner wall surface of the casing surrounding the two tooth grooves increases and decreases as both the screw rotors rotate, thereby suctioning and compressing gas. The space between the tooth tips of the screw rotors and the casing is designed to be a very small gap (outer-diameter gap), and prevents leakage of a working fluid.

FIG. 9 shows one example of a conventional screw fluid machine. A compressor 100 includes a motor 201 that supplies rotation power, and a screw fluid machine 101 that generates compressed air using the rotational force of the motor 201. The motor 201 is referred to as an interior magnet synchronous motor, and a rotor 205 and a stator 210 are accommodated in a motor housing 202. The motor housing 202 is composed of a body portion 202a having a cylindrical shape and having openings on a front side and a rear side, a front end bracket 202b attached to the opening on the front side of the body portion 202a, and a rear end bracket 202c attached to the opening on the rear side of the body portion 202a. A through-hole is formed in the rear end bracket 202c, and one end side of a shaft 204 to which the rotor 205 is fixed protrude from the inside of the motor housing 202 toward the outside on the rear side. The stator 210 includes a stator core 211 and a coil 220.

The screw fluid machine 101 includes a male rotor 120 and a female rotor 140. The male rotor 120 is connected to the shaft 204 of the motor 201 by connecting means (for example, spline, coupling, gear, or the like), and supplies rotation power of the motor 201 to a first shaft 105 of the screw fluid machine 101. Here, the shaft 204 and the first shaft 105 are directly connected to each other, and the rotation axis thereof is A1. A first bevel gear 110 is provided at a tip of the first shaft 105, and the first bevel gear 110 meshes with an adjacent second bevel gear 130 to rotate the second bevel gear 130 in an opposite direction at the same speed as the first bevel gear 110. Since the second bevel gear 130 is provided at a tip of a second shaft 135, as the second bevel gear 130 rotates, the female rotor 140 fixed to the second shaft 135 rotates in the opposite direction at the same speed as the male rotor 120 fixed to the first shaft 105. Incidentally, a screw fluid machine in which the male rotor 120 and the female rotor 140 mesh with each other to rotate the female rotor 140 without using the bevel gears 110 and 130 is also widely used.

As the motor 201 rotates, the male rotor 120 and the female rotor 140 rotate, and air is suctioned from a suction port (not visible in the figure) of the male rotor 120. Furthermore, when the male rotor 120 and the female rotor 140 rotate, the meshing of the tooth profiles of the rotors disengages, and the air is suctioned to fill a tooth profile space. Furthermore, when the male rotor 120 and the female rotor 140 rotate and the air is blocked by the wall of the casing 102, the suction is completed. The air trapped between the tooth profile space and the casing is compressed by the meshing of the rotors 120 and 140. In this manner, as the rotors 120 and 140 rotate, the air moves in an axial direction while being further compressed between a tooth profile space 145 and the casing 102, and reaches a discharge port (not shown in the figure), and the pressure of the air reaches a predetermined pressure. The compressed air is discharged from the discharge port on a discharge side formed in the casing 102.

In the screw fluid machine 101, when the male rotor and the female rotor rotate at high speed, a large speed difference occurs between the male and female rotor and the casing through a very small gap. Accordingly, a large shear force acts on a fluid existing in the gap, thereby generating frictional loss and becoming a factor that impairs the energy-saving performance of the screw fluid machine. In order to reduce frictional loss caused by a shear force, it is desirable to shorten the gap length along a tangential direction of rotation of the screw rotors. To achieve this, regarding the tooth profile in a cross-section perpendicular to the axis (hereinafter, referred to as the “cross-sectional tooth profile” or simply the “tooth profile”) may be made thinner near the tooth tip of the screw rotor. However, it is known that when the tooth profile in the vicinity of the tooth tip and on an advancing side is excessively thinned, a closed space called a “residual volume” is generated in which the volume of the working chambers becomes zero and a discharge passage from the working chambers is lost before the discharge from the working chambers is completed, and the fluid in the residual volume is excessively compressed, thereby causing power loss or vibration.

Japanese Unexamined Patent Publication No. 2008-133763 discloses a technique in which, in a screw fluid machine, the majority of a tooth profile curve on an advancing surface side from a radially innermost tooth root point to a radially outermost outer peripheral point in a cross-section perpendicular to the axis of a female rotor is formed in an elliptical arc shape so as to be located outside a pitch point-centered arc passing through the tooth root point. According to Japanese Unexamined Patent Publication No. 2008-133763, it is stated that the generation of a residual volume is prevented by forming the tooth profile curve of the female rotor so as to be located outside the pitch point-centered arc.

Japanese Unexamined Patent Publication No. 2022-69105 discloses a screw rotor which is formed such that the cross-sectional tooth profile varies depending on the axial position. Particular, by further thickening the tooth thickness of a female rotor in a cross-section perpendicular to the axis in the vicinity of a discharge side while satisfying specific conditions, tooth surface separation can be suppressed while reducing leakage loss in the vicinity of a discharge port where high pressure is generated. Incidentally, the “tooth thickness” as used in this specification refers to the thickness of a tooth in the tooth profile of the screw rotor in a cross-section perpendicular to a rotation axis direction.

SUMMARY OF THE INVENTION

In the screw fluid machine described in Japanese Unexamined Patent Publication No. 2008-133763, the generation of a residual volume at a discharge end face is prevented by locating the tooth profile of the female rotor outside the pitch point-centered arc; however, when this restriction is imposed, the tooth profile in the vicinity of the tooth tip of a male rotor cannot be thinned beyond a certain range, and it is difficult to further increase the effect of reducing frictional loss of the fluid in a very small gap at the tooth tip of the rotor. In Japanese Unexamined Patent Publication No. 2008-133763, any measures to address this issue are not particularly stated.

The inventors of the present application have focused on the fact that it is at the discharge end face of the rotor that the residual volume becomes a problem, and have devised means for solving the foregoing problem by changing the cross-sectional tooth profile orthogonal to the rotation axis depending on the axial position of the rotor, thereby forming the rod such that the tooth profile is allowed to be located inside the pitch point-centered arc upstream of the discharge end face, while the tooth profile is located outside the pitch point-centered arc (or on the arc) at the discharge end face as described in Japanese Unexamined Patent Publication No. 2008-133763.

In addition, the screw rotor itself in which the cross-sectional tooth profile changes in the axial direction has been devised in Japanese Unexamined Patent Publication No. 2022-69105; however, the avoidance of the generation of a residual volume and the positional relationship between the pitch point-centered arc and the tooth profile, which serves as a condition for the generation of a residual volume, are not taken into consideration. The inventors have particularly focused on avoiding the generation of a residual volume in the screw fluid machine, and have examined the positional relationship between the pitch point-centered arc and the tooth profile, which serves as a condition for the generation of a residual volume. The inventors have then found that, as will be described later, depending on the conditions under which the cross-sectional tooth profile is changed in the axial direction, a residual volume that is a closed space can also be generated upstream of the discharge end face and that there are predetermined conditions for avoiding such a situation.

An object of the present invention is to provide a highly efficient and reliable screw fluid machine that reduces frictional loss of a fluid in a gap between the tooth tips of screw rotors and a casing while avoiding power loss or vibration caused by the compression of the fluid in a residual volume.

In order to solve the foregoing problems, for example, configurations described in the claims are adopted.

According to one feature of the present invention, there is provided a screw fluid machine including a male rotor including twisted male teeth and rotating about a first rotation center; a female rotor including twisted female teeth and rotating about a second rotation center parallel to the first rotation center; and a casing including an accommodating chamber that rotatably accommodates the male rotor and the female rotor in a meshed state. The male rotor and the female rotor are rotated to convey a fluid between a suction port and a discharge port in a state where pressure of the fluid is changed. A cross-sectional shape of the male rotor at a predetermined position between a suction end and a discharge end is defined at a discharge-side end face of the male rotor and in the vicinity of the discharge-side end face. Namely, a cross-section orthogonal to the first rotation center is formed with a first cross-sectional shape in which a cross-sectional tooth profile in the vicinity of a tooth tip of each tooth is thickened such that a length from any point in a rotor advancing direction from the tooth tip of the tooth to a pitch point of the tooth is equal to or longer than a length from the tooth tip to the pitch point. In addition, when viewed in an axial direction, a cross-section orthogonal to the first rotation center at a predetermined position between a suction-side end face and a discharge-side end face is formed with a second cross-sectional shape in which a cross-sectional tooth profile in the vicinity of a tooth tip of each tooth is thinned such that a length from any point in the rotor advancing direction of the tooth tip of the tooth to a pitch point of the tooth is shorter than a length from the tooth tip to the pitch point.

According to another feature of the present invention, when viewed in the axial direction, the male rotor has a transition section between a section having the first cross-sectional shape and a portion having the second cross-sectional shape of the male rotor, in which the cross-section of the male rotor changes continuously from the second cross-sectional shape to the first cross-sectional shape. When viewed in the axial direction, the male rotor in a section from the suction-side end face to the transition section is formed with the same cross-sectional shape as the second cross-sectional shape and, when viewed in the axial direction, the male rotor in a section from the transition section to the discharge-side end face is formed with the same cross-sectional shape as the first cross-sectional shape. In addition, in the second cross-sectional shape of the male rotor, a tooth profile at a peripheral portion of the tooth tip, particularly, on an advancing surface side is set in a relationship in which a product of an angle and a lead, the angle being formed by sequentially connecting an intersection point where a normal line drawn from a point on an advancing surface intersects a pitch circle, a center of the male rotor, and the tooth tip with a straight line, is not larger than an axial length of the transition section.

According to the present invention, at a discharge end face of the male rotor and in the vicinity of the discharge end face, by thickening the cross-sectional tooth profile in the vicinity of the tooth tip of the screw rotor to create a shape that does not generate a residual volume, while thinning the cross-sectional tooth profile in the vicinity of the tooth tip in other portions, frictional loss of the fluid in a gap between the tooth tip of the screw rotor and the casing can be reduced. In addition, by changing the tooth profile to eliminate a residual volume at the discharge end face of the male rotor, a highly efficient and reliable screw fluid machine can be realized.

Problems, configurations, and effects other than those described above will become apparent from the following description of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a horizontal cross-sectional view showing a screw compressor 1 according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view of the screw compressor 1 in FIG. 1 taken along line B₁-B₁;

FIG. 3 is a cross-sectional view of the screw compressor 1 in FIG. 1 taken along line A-A;

FIG. 4 is an enlarged view of one tooth of a male rotor 2 of the screw compressor 1 in FIG. 2;

FIG. 5 is a view of the male rotor and seal lines of the screw compressor 1 shown in FIG. 1 when viewed in a direction from the top toward the bottom of FIG. 1;

FIG. 6 is a partial enlarged view of the male rotor of the screw compressor 1 shown in FIG. 3, and is a view showing the relationship between a cross-sectional tooth profile and a residual volume width;

FIG. 7 is a view showing seal lines of a screw compressor in a second embodiment of the present invention, when a transition section I_t is short;

FIG. 8 is a view showing seal lines of a screw compressor according to a third embodiment of the present invention; and

FIG. 9 is a cross-sectional perspective view showing a conventional screw fluid machine 101.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A screw fluid machine according to embodiments of the present invention will be described with reference to the drawings. In the following embodiments, a twin-rotor screw compressor will be described as one example of the screw fluid machine. Incidentally, in the following figures, the same reference signs are assigned to the same portions, and repeated descriptions will be omitted. In addition, the description will be made on the basis that the left side and the right side of FIG. 1 are a suction side and a discharge side of a screw compressor 1, respectively. Here, the “suction side” refer to a side of the screw compressor 1 in an axial direction from gas is suctioned, and the “discharge side” refers a side of the screw compressor 1 in the axial direction from which the gas is charged.

First Embodiment

FIG. 1 is a longitudinal cross-sectional view showing a screw compressor 1 according to the present embodiment. The screw compressor 1 includes a male rotor 2 and a female rotor 3 as a pair of screw rotors that rotate in mesh with each other, and a casing 4 (41, 42, and 43) that accommodates the male rotor 2 and the female rotor 3. The male rotor 2 is fixed to a shaft 22 and is rotatable in one direction around a rotation axis A1, and the female rotor 3 is fixed to a shaft 32 and is rotatable in the other direction around a rotation axis A2. The respective teeth of the male rotor 2 and the female rotor 3 are in mesh with each other, and as the male rotor 2 rotates, the female rotor 3 follows and rotates.

The casing 4 includes a main casing 41, a discharge-side casing 42, and a suction-side casing 43 that are joined to each other at split surfaces orthogonal to both the rotation axis A1 of the male rotor 2 and the rotation axis A2 of the female rotor 3. The main casing 41 has a joint surface 41a on the discharge side orthogonal to the rotation axes A1 and A2 of both the male and female rotors 2 and 3, and the discharge-side casing 42 is joined to the discharge side (the right side in FIG. 1) of the main casing 41.

The casing 4 includes an accommodating chamber 45 that accommodates the male rotor 2 and the female rotor 3 therein. In the accommodating chamber 45, a rotor tooth portion 21 of the male rotor 2 and a rotor tooth portion 31 of the female rotor 3 are disposed so as to be rotatable in mesh with each other. The accommodating chamber 45 is formed by closing an opening on one axial side (a protruding side on the right side of FIG. 1) of a space, which is surrounded by two cylindrical surfaces (bore inner peripheral surfaces) formed in the main casing 41 and partially overlapping each other, with the discharge-side casing 42. The discharge-side casing 42 is provided with a discharge-side bearing chamber 54 in which a discharge-side bearing 6 on a male rotor 2 side is disposed, and a discharge-side bearing chamber 55 in which a discharge-side bearing 8 on a female rotor 3 side is disposed.

The shaft 22 penetrating through the male rotor 2 and the shaft 32 penetrating through the female rotor 3 are disposed such that the rotation axes A1 and A2 thereof are parallel to each other. A suction side 22a of the shaft 22 is supported on the casing 4 (41) by one suction-side bearing 5 on the suction side in the axial direction, and a discharge side 22b of the shaft 22 is rotatably supported on the casing 4 (42) by two discharge-side bearings 6 on the discharge side in the axial direction. Similarly, a suction side 32a of the shaft 32 is supported on the casing 4 (41) by one suction-side bearing 7 on the suction side in the axial direction, and a discharge side 32b of the shaft 32 is rotatably supported on the casing 4 (42) by two discharge-side bearings 8 on the discharge side in the axial direction. The bearings 5 and 7 are ball bearings, and the bearings 6 and 8 are a combination of roller bearings and ball bearings. Incidentally, the types or numbers of bearings used in the bearings 5 to 8 are not limited to the example shown in FIG. 1.

The male rotor 2 includes the rotor tooth portion 21 including a plurality of male teeth (lobes) 21a having a helical shape, and the suction side 22a and the discharge side 22b of the shaft 22 extend outward from both axial end portions of the rotor tooth portion 21. The rotor tooth portion 21 has a suction-side end face 21b and a discharge-side end face 21c, which are orthogonal to the axial direction (rotation axis A1), at one end (a left end in FIG. 1) and the other end (a right end in FIG. 1) in the axial direction, respectively. Tooth grooves are formed between the male teeth 21a of the rotor tooth portion 21. For example, the suction side 22a of the shaft 22 is configured to penetrate through the suction-side casing 43 and extend outward, and is connected to an output shaft of a rotary drive source such as a motor (not shown). A shaft seal member 9 that seals a gap between the shaft 22 of the male rotor 2 and the casing 4 is disposed therebetween. For example, a known member such as an oil seal or a mechanical seal is used as the shaft seal member 9.

The female rotor 3 includes the rotor tooth portion 31 including a plurality of female teeth (lobes) 31a having a helical shape, the suction side 32a of the shaft 32 extends from both axial end portions of the rotor tooth portion 31 to the outside of the main casing 41, and the discharge side 32b of the shaft 32 extends to the opposite side. The rotor tooth portion 31 has a suction-side end face 31b and a discharge-side end face 31c, which are orthogonal to the axial direction (rotation axis A2), at one end (a left end in FIG. 1) and the other end (a right end in FIG. 1) in the axial direction, respectively. Tooth grooves are formed between the female teeth 31a of the rotor tooth portion 31.

The suction-side casing 43 is attached to a suction-side end portion of the main casing 41, a suction-side bearing chamber 51 in which the suction-side bearing 5 on the male rotor 2 side is disposed and a suction-side bearing chamber 52 in which the suction-side bearing 7 on the female rotor 3 side is disposed are formed in the main basing 41, and these spaces are isolated from an external space by the suction-side casing 43.

FIG. 2 is a cross-sectional view of the screw compressor 1 in FIG. 1 taken along line B₁-B₁. When the cross-section of a portion B₁-B₁ in FIG. 2 (a vertical cross-section perpendicular to the rotation axes A1 and A2) is viewed from the discharge side toward the suction side, the male rotor 2 includes four male teeth 21a having a convex shape in a circumferential direction, and the female rotor 3 includes six female teeth (lobes) 31a in the circumferential direction. When a cross-section B₁-B₁ in FIG. 1 is viewed from the suction side toward the discharge side, the rotation directions of the shafts 22 and 32 are as indicated by arrows in the figure. As will be described later, the tooth profile (cross-sectional tooth profile) represented by the contour of the male rotor 2 in a cross-section perpendicular to the rotation axis A1 changes in shape at different axial positions, for example, a cross-section B₃-B₃, a cross-section B₂-B₂, and the cross-section B₁-B₁ in FIG. 1, and the shape of the corresponding tooth grooves also changes.

FIG. 3 is a longitudinal cross-sectional view of portion A-A of the screw compressor 1 according to the embodiment shown in FIG. 1. As can be seen from FIGS. 1 to 3, a wall surface of the main casing 41 forming the accommodating chamber 45 is composed of a male-side bore inner peripheral surface 46 having a substantially cylindrical shape located outside the rotor tooth portion 21 of the male rotor 2 in a radial direction (see FIG. 1), a female-side bore inner peripheral surface 47 having a substantially cylindrical shape located outside the rotor tooth portion 31 of the female rotor 3 in the radial direction, a suction-side inner wall surface 48 on one axial side facing the suction-side end faces 21b (see FIG. 1) and 31b of the rotor tooth portions 21 and 31 of both the male and female rotors 2 and 3, and a discharge-side inner wall surface 49 on the other axial side facing the discharge-side end faces 21c (see FIG. 1) and 31c of the rotor tooth portions 21 and 31 of both the male and female rotors 2 and 3. Namely, a suction-side end of the accommodating chamber 45 in the axial direction is the suction-side inner wall surface 48, and a discharge-side end of the accommodating chamber 45 in the axial direction is the discharge-side inner wall surface 49. The casing 4 is configured such that the main casing 41 and the discharge-side casing 42 that are split at the position of the discharge-side inner wall surface 49 of the accommodating chamber 45 are joined to each other.

A plurality of working chambers C are formed by the rotor tooth portions 21 and 31 of both the male and female rotors 2 and 3 and the male-side bore inner peripheral surface 46 (see FIG. 1), the female-side bore inner peripheral surface 47, the suction-side inner wall surface 48, and the discharge-side inner wall surface 49 of an inner wall surface (accommodating chamber 45) of the casing 4 surrounding the rotor tooth portions 21 and 31. Each of the working chambers C suctions the gas from outside the casing 4 as the volume of each of the working chambers C increases with the rotation of the screw rotors (suction stroke), and compresses the gas as the volume decreases with the rotation of the screw rotors (compression stroke). When the screw compressor 1 is of a liquid supply type, the screw compressor 1 is configured such that a liquid such as oil or water is supplied to the working chambers C during the suction stroke or the compression stroke. The supply of the liquid to the working chambers C is for the purposes of cooling the compressed gas in the working chambers C, lubricating the male rotor 2 and the female rotor 3, and sealing gaps between both the male and female rotors 2 and 3 and the wall surfaces of the accommodating chamber 45 (inner wall surface of the casing 4), gaps at meshing portions between the male rotor 2 and the female rotor 3, or the like.

The casing 4 is provided with a suction passage 61 and a suction space 62 for suctioning external gas into the working chambers C. The suction passage 61 is, for example, a passage, one side of which opens to an outer wall surface of the main casing 41 and the other side (downstream side) of which is connected to the suction space 62. The suction space 62 is formed in the main casing 41 at a position facing the suction-side end faces 21b and 31b of the rotor tooth portions 21 and 31 of both the male and female rotors 2 and 3. The casing 4 is provided with a discharge passage 64 for discharging the compressed gas from the working chambers C to the outside of the casing 4. The discharge passage 64 allows communication between the accommodating chamber 45 (working chambers C) and the outside of the casing 4, and is formed in the discharge-side casing 42. Since a discharge port 65 serving as an inlet from the working chambers C to the discharge passage 64 is configured not to communicate with the discharge passage 64 before the working fluid in the working chambers C is compressed to a predetermined pressure, only a lower side with respect to a horizontal plane passing through the rotation axes A1 and A2 is opened and an upper side is closed.

In the screw compressor 1 configured as described above, as the male rotor 2 is driven by the drive source (not shown) such as a motor, the female rotor 3 meshing with the male rotor 2 also rotates. Accordingly, the working chambers C move in the axial direction as both the male and female rotors 2 and 3 rotate. At this time, the volume of the working chambers C gradually increases from a state in which the volume is zero, thereby causing the working chambers C to enter the suction stroke in which the gas is suctioned from the suction passage 61 of the casing 4 through the suction space 62 shown in FIG. 2. Thereafter, the communication of the working chambers C with the suction space 62 is blocked at the stage where the volume of the working chambers C approaches its maximum value, and the volume gradually decreases as both the male and female rotors 2 and 3 rotate, thereby causing the working chambers C to enter the compression stroke in which air is compressed. When the working chambers C communicate with the discharge passage 64, the compressed gas in the working chambers C is discharged to an external system (not shown) through the discharge passage 64.

As described above, the cross-sectional tooth profile of the male rotor 2 changes depending on the axial position along the rotation axis A1. FIG. 4 is an enlarged partial cross-sectional view of a portion (one tooth) of the teeth of the male rotor 2 of the screw compressor shown in FIG. 2. In FIG. 4, the tooth profiles of the male rotor 2 in different cross-sections perpendicular to the axis are shown in a superimposed manner by a solid line and a dashed line. A cross-sectional tooth profile 21s shown by the solid line is a cross-sectional tooth profile upstream (on the suction side) of a cross-sectional tooth profile 21d of the discharge-side end face 21c, for example, at a portion B₂-B₂ in FIG. 1. In addition, the cross-sectional tooth profile 21d shown by the dashed line is a cross-sectional tooth profile downstream (on the discharge side) of the cross-sectional tooth profile 21s, for example, at the portion B₁-B₁ in FIG. 1. A tooth tip 21t is a point on the cross-sectional tooth profile that is farthest from the rotation axis A1 of the male rotor 2 (rotor center). FIG. 4 is a view obtained by rotating the cross-sectional tooth profile 21s and the cross-sectional tooth profile 21d such that the positions of the tooth tips of the cross-sectional tooth profile 21s and the cross-sectional tooth profile 21d are aligned on a reference line L located on a plane connecting the rotation axes A1 and A2. A point O_cp is a pitch point, and a circle CP is a pitch point-centered circle having the pitch point O_cp as the center and passing through the tooth tip 21t. The difference in shape between the cross-sectional tooth profiles 21d and 21s is at its maximum in the vicinity of a predetermined angle θ₁ by which movement is made in an advancing direction from the reference line L. In the present embodiment, the angle θ₁ from the tooth tip 21t to a maximum deviation point of the cross-sectional tooth profile on an advancing side is approximately 30 degrees; however, if the number of teeth in the circumferential direction is four, the position of the maximum deviation point can be set, for example, within a range of approximately 20 to 40 degrees. In addition, the shapes of the cross-sectional tooth profiles 21d and 21s coincide with each other again in the vicinity of a predetermined angle θ₂ by which movement is made in the advancing direction from the tooth tip 21t. In the present embodiment, the angle θ₂ of the coincidence point is approximately 75 degrees.

One of the features of the present invention lies in the relationship between the cross-sectional tooth profile and the pitch point-centered circle CP. The cross-sectional tooth profile 21s is located inside the pitch point-centered circle CP in the vicinity of the tooth tip 21t and on an advancing surface side (above the reference line L in FIG. 4). The cross-sectional tooth profile 21d, which is located on the discharge side with respect to the cross-sectional tooth profile 21s, coincides with the pitch point center circle CP or is located outside of it near the tooth tip 21t and on the advancing surface side. Incidentally, here, the description of the shape of the female rotor 3 is omitted; however, the end face shape of the female rotor 3 corresponding thereto is determined by the cross-sectional shape of the male rotor 2.

FIG. 5 shows the state of seal lines formed in the male rotor 2 of the present embodiment, and is a view of the male rotor and seal lines of the screw compressor 1 shown in FIG. 1 when viewed in a direction from the top to the bottom of FIG. 1. Namely, FIG. 5 is a view of the male rotor 2 when viewed from the female rotor 3 side in a state in which the left side and the right side of FIG. 5 are set to be located on the suction side and the discharge side, respectively. In FIG. 5, the male rotor 2 is shown as being divided into three sections, I₂, I_t, and I₁ in the axial direction. Among these sections, the section I₂ indicates a portion that is shaped such that the cross-sectional tooth profile in the vicinity of the tooth tip 21t and on the advancing surface side, namely, the tooth profile in the range indicated by θ₁ in FIG. 4 is located inside the pitch point-centered circle CP. In addition, the section I₁ indicates a portion that is shaped such that the cross-sectional tooth profile in the vicinity of the tooth tip 21t and on the advancing surface side overlaps or is located outside the pitch point-centered circle CP. The length of the section I₂ in the rotation axis direction is the largest, and occupies one-half or more of the length of the male rotor 2 in the rotation axis direction. Incidentally, in the example here, it is assumed that in the section I₂, the tooth profile cross-section does not change except for the twist (rotation) of the tooth profile cross-section that changes depending on the axial direction. Therefore, in the cross-section B₂-B₂ that is a cross-section in the section I₂, the shape of the tooth profile corresponds to the shape of the cross-sectional tooth profile 21s shown in FIG. 4, and in the cross-section B₁-B₁ that is a cross-section in the section I₁, the shape of the tooth profile corresponds to the shape of the cross-sectional tooth profile 21d shown in FIG. 4.

Similarly to the section I₂, the boundary between the section I_t and the section I₂ on the suction side is shaped such that the cross-sectional tooth profile in the vicinity of the tooth tip 21t and on the advancing surface side is located inside the pitch point-centered circle CP. Then, in the section I_t, the tooth profile cross-section changes continuously from the shape at the point of connection with the section I₂, the cross-sectional tooth profile is changed to become thicker from the inside to the outside of the pitch point-centered circle CP, and at an end portion on the discharge side of the section I_t, the cross-sectional tooth profile changes to coincide with or be outside the pitch point-centered circle CP. Hereinafter, the section I_t in which the shape transitions continuously may be referred to as a “transition section I_t”. An end portion on the discharge side of the section I₁ opens to the discharge port 65. As will be described later, setting the width (the length when viewed in the axial direction) of the transition section I_t in FIG. 5 to a predetermined size or more is one of the features of the present invention.

Seal lines S1, S2, and S3 show the superimposed shapes of the same seal line at three different moments when the same seal line is moved from the suction side to the discharge side by the rotation of the male rotor 2. As the seal line formed by the male rotor 2 moves from S1 to S2 to S3, the working chambers are reduced in volume to compress the fluid with the rotation of the male rotor 2 and the female rotor 3 while moving from the inflow side to the discharge side in the direction of the rotation axis A1 (in a rightward direction in FIG. 3). Thereafter, the compressed fluid is discharged from the discharge port 65 facing the discharge-side end face 21c of the casing 4.

First, focusing on the seal line S1, in a region above the reference line L (see also FIG. 4), there is a portion that protrudes further toward the suction side from the intersection point on the suction side (left side) between the reference line L and the seal line S1 (a portion shown by hatching). In this specification, a space surround by this portion is denoted by V_res. If the space V_res moves toward the discharge side while the seal line S1 maintains its shape (corresponding to a case in which the tooth profile cross-section does not change in the axial direction), the space V_res reaches the end face of the discharge-side casing 42. Here, since a portion of the surface facing the discharge-side end face 21c, the portion being located above the reference line L, is closed, the space V_res becomes a closed space, the closed space V_res is further reduced by the rotation of the male rotor 2, and the pressure of the fluid trapped in the reduced space rises to a very high pressure, which becomes a factor that causes abnormal vibration of the rotor or power loss.

The condition under which the space V_res that can become a residual volume in this manner is generated is that the cross-sectional tooth profile in the vicinity of the tooth tip of the male rotor 2 and on the advancing surface side is located inside the pitch point-centered circle CP. This point will be further described with reference to FIG. 6. FIG. 6 is a cross-sectional tooth profile of the male rotor 2 in a certain cross-section, and shows only the male rotor 2. The pitch point O_cp is a position determined by internally dividing the distance between the rotation axes A1 and A2 by the ratio of the number of teeth of the male rotor 2 to the number of teeth of the female rotor, and a pitch circle C_pch shown by a dashed line is an imaginary circle having a rotation center O (rotation axis A1) and passing through the pitch point O_cp.

FIG. 6 shows the relationship between the cross-sectional tooth profile of the male rotor 2 and a residual volume width. Here, focus is given to one point (point P) indicated by a black circle located in the vicinity of the advancing surface side with respect to the tooth tip of one tooth located on the left side of FIG. 6. A normal line extending in a direction from the point P on the tooth profile toward the male rotor center O is defined as L1, and the intersection point between L1 and the pitch circle C_pch is defined as Q. Furthermore, a line connecting the intersection point Q and the rotor center O is defined as L2. In this case, in a portion of the cross-sectional tooth profile (including the point P) in the vicinity of the tooth tip 21t and on the advancing surface side (above the tooth tip 21t in FIG. 6), when the cross-sectional tooth profile is located inside the pitch point-centered circle CP shown in FIG. 4, the radius of curvature in the vicinity of the tooth tip 21t becomes smaller than that of the pitch point-centered circle CP, and as shown in FIG. 6, the normal line L1 drawn from the point P intersects the pitch circle C_pch at the point Q below (rearward rotation) the point O_cp. Here, if an angle formed by sequentially connecting the pitch point O_cp, the rotor center O, and the point Q is defined as θ, in this tooth profile, when the male rotor 2 is rotated by θ in a rotor rotation direction from the moment shown in FIG. 6, the point Q coincides with the pitch point O_cp.

According to gear meshing theory, when the point P comes into contact with the female rotor 3, the normal line L1 needs to pass through the pitch point O_cp, and therefore, the state in which the male rotor 2 is advanced by an angle θ from the state shown in FIG. 6 corresponds to a state in which “the point P becomes the point of contact”, namely, the point P coincides with a point on the seal line on this cross-section. Conversely speaking, at the point in time in FIG. 6, the rotor rotation angle is θ ahead of the angle at which the point P comes into contact.

Meanwhile, since a normal line drawn from the tooth tip 21t passes through the pitch point O_cp in the state shown in FIG. 6, the tooth tip 21t becomes a contact point at this moment. Since the contact point is located on the reference line L, the contact point coincides with a lower end point U (see FIG. 5) of the space V_res on the seal line S1 shown in FIG. 5. In contrast, since the point P comes into contact on a cross-section where the point U exists when advanced by the angle θ, at the timing of the seal line S1, the point P exits which corresponds to a contact point (a point on the seal line) on a cross-section that is moved toward the suction side by a distance corresponding to the angle θ from the cross-section where the point U.

Therefore, it can be seen that the seal line corresponding to the point P is located behind (on the suction side) the lower end point U of the space V_res in FIG. 5 and a space protruding toward the suction side, such as the space V_res shown in FIG. 5, is formed. Similarly, when the cross-sectional tooth profile in the vicinity of the tooth tip 21t and on the advancing surface side with respect to the tooth tip 21t coincides with or is located outside the pitch point-centered circle CP, the seal line does not protrude toward the suction side on the rear side (left side) of the seal line S1 and above the reference line L in FIG. 5, and therefore, a space that can become a residual volume is not generated.

Furthermore, when the female rotor 3 rotates and advances by the angle θ, the distance that the seal line moves in the axial direction is the product of a lead Ld of the male rotor 2 and θ, and therefore, if the point P is taken as the point where θ (positive in the rotor rotation direction) is at its minimum (at its negative maximum) when moved along the tooth profile, an axial width Lv of the space V_res shown in FIG. 5 can be expressed as Lv=Ld×θ using θ at that time. Here, the lead Ld indicates a distance advanced in the axial direction by one rotation of the male rotor. Therefore, the condition that the transition section I_t described above is larger than the width of the space V_res can be rephrased as the transition section I_t being larger than Ld×θ.

In view of the above description, returning to FIG. 5 to describe the manner of change in the seal lines S1, S2, and S3, in the seal line S1 existing in the section I₂, the cross-sectional tooth profile in the vicinity of the tooth tip of the male rotor 2 and on the advancing surface side is located inside the pitch point-centered circle CP, and as described above, the space V_res that can become a residual volume is generated. Meanwhile, in the section I₁ adjacent to the discharge-side end face 21c (see FIG. 1), the cross-sectional tooth profile in the vicinity of the tooth tip of the male rotor 2 and on the advancing surface side is not located inside the pitch point-centered circle CP, and therefore, the space V_res does not exist in the seal line S3. In the transition section I_t connecting these sections, the cross-sectional tooth profile is formed to be continuously interpolated, so that, as shown by the seal line S2, the size of the space V_res is changed to decrease as the space V_res approaches the discharge side.

As described above, in the sections I₂ and I_t, by allowing the existence of the space V_res that can become a residual volume, and thinning the tooth thickness in the vicinity of the tooth tip of the male rotor 2, frictional loss caused by the shear force of the fluid between the tooth tip and the casing is reduced. Furthermore, since the tooth profile is designed such that the space V_res disappears in the section I₁ including the discharge-side end face, vibration or power loss caused by abnormal compression of the fluid in the residual volume, which is a problem in conventional fluid devices, can be eliminated.

Second Embodiment

Next, a second embodiment of the present invention will be described with reference to FIG. 7. In examining the present invention, the inventors have found that, depending on how the transition section I_t is set, the space V_res can become a residual volume, which is a closed space, even at locations other than the discharge end face, and have devised means for avoiding such a situation. FIG. 7 is a view showing, for comparison, seal lines when the condition of the first embodiment of the present invention is not satisfied (when the transition section I_t is smaller than Ld×θ) in the same configuration as that in FIG. 5. The common point with the seal lines shown in FIG. 5 is that at the end face (discharge-side end face 21c) in contact with a discharge end of the male rotor 2, the tooth profile has the shape in the section I₁, and on a side in contact with the suction-side end face, the tooth profile has the shape in the section I₂, and the difference point is that, in FIG. 7, the width of the transition section I_t is shorter than the width Lv of the space V_res in the section I₁. In FIG. 7, the shape of the seal line S3 immediately after passing through the transition section I_t is shown by a dashed line, and a seal line S3′ when the seal line S1 reaches the same position of the seal line S3 while maintaining its shape is shown by a solid line.

Since the seal line S3 is a seal line in the section I₁, the seal line is actually formed in this shape, and a working chamber C3 is partitioned. Meanwhile, since the cross-sectional tooth profile of the seal line S3′ is different from that of the seal line S1 in the sections I_t and I₁, the seal line S3′ is not actually formed; however, in a portion of the seal line S3′ that overlaps the section I₁, the cross-sectional tooth profile has the same shape as the seal line S1, and therefore, the seal line S3′ is actually formed.

Accordingly, a working chamber partitioned by the seal line S3 and existing on the discharge side (the right side in FIG. 7) with respect to the seal line S3 and C3′ surrounded by the seal line S3′ existing in the section I₁ are separated from the working chamber C3 by the seal line S3. Actually, since the tooth profile changes continuously in the transition section I_t, a seal line connected to the seal line S3′ in the section I₁ is also formed in the transition section I_t, and C3′ becomes a closed space, the volume of which is reduced, thereby causing the same problem as the residual volume generated at the so-called discharge end face.

The cause of such a problem is that since the width Lv of the space V_res in the section I₁ is larger than the width of the transition section I_t, a part of the space V_res remains even after the seal line S3 is formed. Therefore, in the example shown in FIG. 7, the effect of the present invention cannot be maximized; however, depending on the shape or number of teeth of the male rotor 2, the influence of the above-described residual volume generated in the vicinity of the transition section I_t is considered to be small enough to be tolerable, and the male rotor 2 having seal lines as shown in the second embodiment can also be implemented. In the present invention, by adding, as one of the conditions, that the axial length of the transition section I_t is set to be larger than the width Lv, as shown in the first embodiment, the problem of the residual volume being generated at locations other than the discharge end face can be effectively solved.

According to the second embodiment, in the sections I₁ and I_t, by allowing the existence of the space V_res that can become a residual volume, and thinning the tooth thickness in the vicinity of the tooth tip of the male rotor 2, frictional loss caused by the shear force of the fluid between the tooth tip and the casing can be reduced, and in the section I₁ including the discharge-side end face, the space V_res disappears and no residual volume is generated. Furthermore, since the generation of a residual volume that can be generated due to a change in the tooth profile cross-section in the axial direction can be avoided, vibration or power loss caused by abnormal compression of the fluid in the residual volume can be eliminated, and a highly efficient and reliable screw compressor can be realized.

Third Embodiment

In the first and second embodiments described above, the male rotor 2 is divided into three sections in the axial direction, in the first section I₂ when viewed from the suction side, the tooth thickness in the vicinity of the tooth tip is thinned, and in the second I₁, the tooth thickness in the vicinity of the tooth tip is thickened; however, the number of the divided sections I may be further increased. For example, the tooth thickness in the vicinity of the tooth tip of the male rotor 2 in the vicinity of the suction end face may be thickened. FIG. 8 shows the state of the rotor or seal lines in this case; however, here, new sections I₃ and I_t2 are provided upstream (suction side) of I₁. Here, for example, the tooth profile in the section I₃ can be made the same as the tooth profile in the section I₁, and the transition section I_t2 can be configured as a transition section in which the tooth profile changes continuously similarly to a transition section I_t1. Namely, as can be seen from seal lines S4 and S3, in the sections I₃ and I₁, the space V_res that can become a residual volume is not generated. Meanwhile, in the section I₂, the space V_res that can become a residual volume is generated. In the section I_t2, the tooth thickness in the vicinity of the tooth tip is changed to continuously connect the section I₃ and the section 12. In this manner, it is preferable that the tooth profile of the male rotor 2 is formed such that the tooth thickness in the vicinity of the tooth tip is thinned in some regions in the rotation axis direction, particularly, in the vicinity of the suction end face and in the vicinity of an intermediate portion excluding the vicinity of the discharge end face. This effect is particularly noticeable in a liquid-supply screw compressor. Incidentally, in the liquid-supply screw compressor, since the amount of supplied liquid immediately after the suction-side end face 21b may be insufficient and it may be better not to excessively thin the vicinity of the tooth tip, the present invention can achieve higher efficiency while dealing with such a compressor.

The present invention has been described above based on the embodiments; however, the present invention is not limited to the above-described embodiments, and can be changed in various forms without departing from the concept of the present invention. For example, in the above-described embodiments, the male rotor 2 is divided into three or five sections in the axial direction; however, the number of sections of the male rotor 2 may be equal to or larger than this number. In addition, the present invention can be similarly applied not only to an oil-supply screw compressor to which oil is supplied, but also to an oil-free screw compressor.