GEAR PUMP

Abstract

A gear pump having a suction port, first and second discharge ports, an inner rotor having external teeth, an outer rotor having internal teeth such that the number of the internal teeth is larger than that of the external teeth of the inner rotor, and inter-tooth chambers defined by the external teeth and the internal teeth. The first and second discharge ports are in communication with the inter-tooth chamber whose volume is decreased along with rotation of the inner rotor etc. An inter-tooth chamber which has been brought out of communication with the second discharge port is brought into communication with the suction port while the volume is decreasing. The volume of the inter-tooth chamber which has been brought out of communication with the second discharge port is increased after at least a part of the inter-tooth chamber has been brought into communication with the suction port.

Description

TECHNICAL FIELD

The present disclosure relates to a gear pump that includes an inner rotor having a plurality of external teeth and an outer rotor having a plurality of internal teeth and disposed eccentrically with respect to the inner rotor.

BACKGROUND ART

There has hitherto been known a gear pump that includes an inner rotor having n external teeth, an outer rotor having (n+1) internal teeth meshed with the external teeth, and a casing formed with a suction port and a discharge port (see Patent Document 1, for example). In the gear pump, a first angle formed by a first line that connects between the center of rotation of the inner rotor and the tooth tip of the external teeth and a second line that connects between the center of rotation and a meshing portion of the external teeth is 1.4 times or more and 1.8 times or less a second angle formed by a third line that connects between the center of rotation and the tooth bottom of the external teeth and the second line. The width of the external teeth at the meshing portion along the rotational direction is equivalent to the distance between the rear end, in the rotational direction of the rotors, of the suction port and the front end, in the rotational direction, of the discharge port, that is, the width of a partition between the ports.

RELATED-ART DOCUMENTS

Patent Documents

[Patent Document 1] Japanese Patent No. 4889981

SUMMARY OF THE DISCLOSURE

Patent Document 1 teaches that it is possible to prevent occurrence of a so-called fluid containment, in which a cell with the smallest volume, among a plurality of cells (inter-tooth chambers), positioned at a meshing position, at which the rotors are meshed with each other and a rotational drive force is transferred from the external teeth to the internal teeth, is tightly closed, by determining the first angle as 1.4 times or more and 1.8 times or less the second angle. Even if the configuration described in Patent Document 1 is adopted, however, it is still difficult to completely suppress an inflow of a fluid into the cell with the smallest volume, from which the fluid has been discharged to the discharge port, through a gap between the case and the rotors (a gap in the axial direction of the gear pump). Thus, with the gear pump according to Patent Document 1, cavitation may be caused because of a rapid inflow of the fluid into the inter-tooth chamber, which is brought out of communication with the discharge port and which is brought into communication with the suction port, through the gap between the case and the rotors.

Thus, it is an aspect of the present disclosure to provide a gear pump that can suppress well occurrence of cavitation in an inter-tooth chamber that is brought out of communication with a discharge port and that is brought into communication with a suction port.

The present disclosure provides a gear pump including a suction port, a discharge port, an inner rotor having a plurality of external teeth, an outer rotor which has a plurality of internal teeth such that the number of the internal teeth is larger than that of the external teeth of the inner rotor and which is disposed eccentrically with respect to the inner rotor, and a plurality of inter-tooth chambers defined by the plurality of external teeth and the plurality of internal teeth, characterized in that the discharge port is in communication with the inter-tooth chamber whose volume is decreased along with rotation of the inner rotor and the outer rotor, the inter-tooth chamber which has been brought out of communication with the discharge port is brought into communication with the suction port while the volume of the inter-tooth chamber is decreasing, and the volume of the inter-tooth chamber which has been brought out of communication with the discharge port is increased after at least a part of the inter-tooth chamber has been brought into communication with the suction port.

In the gear pump, the inter-tooth chamber which has been brought out of communication with the discharge port is brought into communication with the suction port while the volume of the inter-tooth chamber is decreased along with rotation of the inner rotor and the outer rotor. Consequently, a fluid is discharged from the inter-tooth chamber which has been brought out of communication with the discharge port to the suction port with the volume of the inter-tooth chamber decreased along with rotation of the inner rotor etc. The volume of the inter-tooth chamber which has been brought out of communication with the discharge port is increased after the inter-tooth chamber has been brought into communication with the suction port. That is, the volume of the inter-tooth chamber which has been brought out of communication with the discharge port becomes smallest after the inter-tooth chamber has been brought into communication with the suction port. As a result, a rapid inflow of a fluid into the inter-tooth chamber which has been brought out of communication with the discharge port from a gap between the inner rotor and the outer rotor and a member that houses the inner rotor and the outer rotor (a gap in the axial direction) can be regulated well by a fluid that flows out of the inter-tooth chamber to the suction port. Thus, with the gear pump, it is possible to suppress well occurrence of cavitation in the inter-tooth chamber which is brought out of communication with the discharge port and which is brought into communication with the suction port.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a schematic configuration of a gear pump according to the present disclosure.

FIG. 2 is a diagram illustrating a schematic configuration of external teeth of an inner rotor included in the gear pump according to the present disclosure.

FIG. 3 is a diagram illustrating procedures for forming the external teeth of the inner rotor included in the gear pump according to the present disclosure.

FIG. 4 is a diagram illustrating procedures for forming internal teeth of an outer rotor included in the gear pump according to the present disclosure.

FIG. 5 is an enlarged view illustrating operation of the gear pump according to the present disclosure.

FIG. 6 is an enlarged view illustrating operation of the gear pump according to the present disclosure.

FIG. 7 is an enlarged view illustrating operation of the gear pump according to the present disclosure.

FIG. 8 is a chart illustrating the relationship between the rotational angle of the inner rotor about the center of rotation and the volume of an inter-tooth chamber that is brought out of communication with a discharge port.

FIG. 9 is an enlarged view illustrating operation of a gear pump according to another embodiment of the present disclosure.

PREFERRED EMBODIMENTS

Now, an embodiment of the invention according to the present disclosure will be described with reference to the drawings.

FIG. 1 is a diagram illustrating a schematic configuration of a gear pump 1 according to an embodiment of the present disclosure. The gear pump 1 illustrated in the drawing is constituted as an oil pump to be mounted on a vehicle (not illustrated), for example, and suctions working oil (ATF) stored in an oil pan and pumps the working oil to a hydraulic control device (none of such components is illustrated). The gear pump 1 includes a pump housing constituted from a pump body fixed to a transmission case of an automatic transmission and a pump cover fastened to the pump body (none of such components is illustrated), for example, and an inner rotor (drive gear) 2 and an outer rotor (driven gear) 3 that are rotatably disposed in a gear housing chamber (not illustrated) defined by the pump housing. The gear pump 1 may be constituted as an on-vehicle pump (e.g. an engine oil pump) other than an oil pump that pumps working oil for a transmission, and may be applied for use other than an on-vehicle pump.

The inner rotor 2 is fixed to a rotary shaft 4 coupled to a crankshaft of an engine mounted on the vehicle (none of such components is illustrated), and rotationally driven by power applied to the rotary shaft 4. A plurality of (e.g. eleven in the embodiment) external teeth 20 are formed on the outer periphery of the inner rotor 2. Meanwhile, a number (e.g. twelve in the embodiment) of internal teeth 30 are formed on the inner periphery of the outer rotor 3, the number being larger than the total number of the external teeth 20 of the inner rotor 2 by one. One or a plurality of the internal teeth 30 of the outer rotor 3 positioned on the lower side in FIG. 1 are meshed with corresponding external teeth 20 of the inner rotor 2. The outer rotor 3 is rotatably disposed in the gear housing chamber eccentrically with respect to the inner rotor 2. A plurality of inter-tooth chambers (pump chambers) 5 are each formed between the inner rotor 2 and the outer rotor 3, basically by two adjacent external teeth 20 and two adjacent internal teeth 30.

Consequently, when the inner rotor 2 is rotated in the direction of the thick arrow in FIG. 1 by power from the rotary shaft 4, the outer rotor 3 is rotated in the same direction as the inner rotor 2 about a center of rotation 3c, which is shifted from a center of rotation 2c of the inner rotor 2 by an amount of eccentricity e, with some of the plurality of internal teeth 30 meshed with some of the plurality of external teeth 20. When the inner rotor 2 and the outer rotor 3 are rotated, the volume of each of the inter-tooth chambers 5 is increased (the inter-tooth chambers 5 are expanded) along with rotation of the inner rotor 2 etc. in a region on the rear side in the rotational direction (see the thick arrow in FIG. 1) of the inner rotor 2 and the outer rotor 3, that is, a region mainly on the right half in FIG. 1. When the inner rotor 2 and the outer rotor 3 are rotated, meanwhile, the volume of each of the inter-tooth chambers 5 is decreased (the inter-tooth chambers 5 are contracted) along with rotation of the inner rotor 2 etc. in a region on the front side in the rotational direction of the inner rotor 2 etc., that is, a region mainly on the left half in FIG. 1.

In the pump housing (not illustrated) of the gear pump 1, a suction port 6, a first discharge port 7, and a second discharge port 8 that extend generally arcuately are formed. The suction port 6 communicates with (faces) such inter-tooth chambers 5, of the plurality of inter-tooth chambers 5 defined by the external teeth 20 and the internal teeth 30, that the volume of each of the inter-tooth chambers 5 is increased along with rotation of the inner rotor 2 and the outer rotor 3. The first and second discharge ports 7 and 8 are separated by a partition wall 9 to be independent of each other, and communicate with (face) such inter-tooth chambers 5, of the plurality of inter-tooth chambers 5, that the volume of each of the inter-tooth chambers 5 is decreased along with rotation of the inner rotor 2 and the outer rotor 3. In the embodiment, the first discharge port 7 which is positioned on the rear side in the rotational direction of the inner rotor 2 etc. serves as a low pressure port, and the second discharge port 8 which is positioned on the front side in the rotational direction serves as a high pressure port.

The first and second discharge ports 7 and 8 may be connected to different oil passages, or may be connected to a common oil passage. The suction port 6 and the first and second discharge ports 7 and 8 may be formed on both sides (in both of the pump body and the pump cover) in the axial direction of the inner rotor 2 and the outer rotor 3, or may be formed on one side (in one of the pump body and the pump cover) in the axial direction of the inner rotor 2 and the outer rotor 3. Furthermore, the suction port 6 may be formed on one side in the axial direction of the inner rotor 2 etc., and the first and second discharge ports 7 and 8 may be formed on the other side in the axial direction of the inner rotor 2 etc., for example. The first discharge port 7 may be formed on one side in the axial direction of the inner rotor 2 etc., and the second discharge port 8 may be formed on the other side in the axial direction of the inner rotor 2 etc.

FIG. 2 is a diagram illustrating a schematic configuration of the external teeth 20 of the inner rotor 2. FIG. 3 is a diagram illustrating procedures for forming the external teeth 20. As illustrated in the drawings, each of the external teeth 20 of the inner rotor 2 includes a tooth tip portion 21 in a convex curve shape, a tooth bottom portion 22 in a concave curve shape, a first intermediate portion 23 positioned between the tooth tip portion 21 and the tooth bottom portion 22 on the front side, in the rotational direction (see the thick arrow in FIG. 3) of the inner rotor 2, with respect to the tooth tip portion 21, and a second intermediate portion 24 positioned between the tooth tip portion 21 and the tooth bottom portion 22 on the rear side, in the rotational direction of the inner rotor 2, with respect to the tooth tip portion 21. As illustrated in the drawings, the external teeth 20 are formed to be horizontally asymmetric with respect to a tooth shape center line Lc that passes through a top portion 21t positioned on the radially outermost side of the tooth tip portion 21 and the center of rotation 2c of the inner rotor 2.

As illustrated in FIG. 3, the tooth tip portion 21 is formed in a convex curve shape by an epitrochoid curve (a portion other than a loop portion). The trochoid coefficient of the curve which is obtained by dividing a radius rde of a first drawn point by a radius re of an outer rolling circle Co is larger than a value of 1 (e.g. a value of about 1.2). The epitrochoid curve which forms the tooth tip portion 21 is obtained by keeping the radius rde of the first drawn point at a first value Rde (constant value) and rolling, without slipping, the outer rolling circle Co having the radius re which is smaller than the first value Rde while circumscribing a base circle BCt having a center O that is common to the center of rotation 2c of the inner rotor 2.

The tooth bottom portion 22 includes an intermediate portion formed by a hypotrochoid curve (a portion other than a loop portion) and two rising portions formed by a curve such as an arc. The trochoid coefficient of the hypotrochoid curve which is obtained by dividing a radius rdh of a second drawn point by a radius rh of an inner rolling circle Ci is larger than a value of 1. The hypotrochoid curve which forms the intermediate portion of the tooth bottom portion 22 has the base circle BCt in common with the epitrochoid curve which forms the tooth tip portion 21, and is obtained by keeping the radius rdh of the second drawn point at a second value Rdh (constant value) and rolling, without slipping, the inner rolling circle Ci having the radius rh which is smaller than the second value Rdh while inscribing the base circle BCt. In the embodiment, the radius rde of the first drawn point, that is, the first value Rde, for drawing the epitrochoid curve which forms the tooth tip portion 21 and the radius rdh of the second drawn point, that is, the second value Rdh, for drawing the hypotrochoid curve which forms the tooth bottom portion 22 are determined as the same value Rd. Similarly, the radius re of the outer rolling circle Co and the radius rh of the inner rolling circle Ci are also determined as the same value R. Thus, the relations Rde=Rdh=Rd, re=rh=R, and tooth height=Rde+re+Rdh+rh=2·e are met for the inner rotor 2.

The two rising portions of the tooth bottom portion 22 extend from the intermediate portion, which is formed by a hypotrochoid curve, toward the first and second intermediate portions 23 and 24 so as to be smoothly continuous with the intermediate portion. The rising portion on the rear side in the rotational direction of the inner rotor 2 is formed to be smoothly continuous with the first intermediate portion 23 at an end portion 23f of the first intermediate portion 23 on the front side in the rotational direction. The rising portion on the front side in the rotational direction of the inner rotor 2 is formed to be smoothly continuous with the second intermediate portion 24 at an end portion 24r of the second intermediate portion 24 on the rear side in the rotational direction. Consequently, the intermediate portion, which is formed by a hypotrochoid curve, of the tooth bottom portion 22 is offset toward the center O (the center of rotation 2c of the inner rotor 2) with respect to the base circle BCt. Furthermore, the tooth bottom portion 22 includes an intersection portion 22x with a line segment Le obtained by rotating the tooth shape center line Lc forward or rearward in the rotational direction by half (ϕ/2) an angle ϕ (360°/number of the external teeth 20) corresponding to one external tooth 20. In the inner rotor 2, as illustrated in FIGS. 2 and 3, a range between two intersection portions 22x that interpose the tooth shape center line Lc is determined as a range corresponding to one external tooth 20.

As illustrated in FIGS. 2 and 3, the first intermediate portion 23 is formed between the tooth tip portion 21 and the tooth bottom portion 22 on the front side of the tooth tip portion 21 in the rotational direction of the inner rotor 2. In the embodiment, the first intermediate portion 23 is formed by an involute curve determined such that a tangent to an end portion 21f of the tooth tip portion 21 on the front side in the rotational direction is the same as a tangent to the epitrochoid curve at the end portion 21f. Consequently, the tooth tip portion 21 and the first intermediate portion 23 can be smoothly continuous with each other at the end portion 21f. In the embodiment, the length of the involute curve which forms the first intermediate portion 23, that is, the distance from the end portion 21f of the tooth tip portion 21 to the end portion 23f of the first intermediate portion 23, is determined to be longer than the length of a curve that forms the second intermediate portion 24, that is, the distance from an end portion 21r of the tooth tip portion 21 to the end portion 24r of the second intermediate portion 24.

As illustrated in FIGS. 2 and 3, the second intermediate portion 24 is formed between the tooth tip portion 21 and the tooth bottom portion 22 on the rear side of the tooth tip portion 21 in the rotational direction of the inner rotor 2. The second intermediate portion 24 includes an outer intermediate portion 24o positioned on the tooth tip portion 21 side with respect to an intersection portion 24x with the base circle BCt, and an inner intermediate portion 24i positioned on the tooth bottom portion 22 side with respect to the intersection portion 24x. In the embodiment, as illustrated in FIG. 3, the outer intermediate portion 24o, that is, a range from the intersection portion 24x to the end portion (boundary) 21r of the tooth tip portion 21 on the rear side in the rotational direction of the inner rotor 2, is formed by a first curve obtained by rolling, without sliding, the outer rolling circle Co which circumscribes the base circle BCt while varying the radius (see the dotted line in the drawing) of the first drawn point. Meanwhile, as illustrated in FIG. 3, the inner intermediate portion 24i, that is, a range from the intersection portion 24x to the end portion 24r, is formed by a second curve obtained by rolling, without sliding, the inner rolling circle Ci, which inscribes the base circle BCt, while varying the radius (see the dash-double-dot line in the drawing) of the second drawn point. See Japanese Patent Application Publication No. 2014-181619 (JP 2014-181619 A) for procedures for varying the radius of the first or second drawn point of the outer rolling circle Co or the inner rolling circle Ci.

FIG. 4 is a diagram illustrating procedures for forming the internal teeth 30 of the outer rotor 3 included in the gear pump 1. As illustrated in the drawing, the tooth shape (contour) of the outer rotor 3, which is defined by the plurality of internal teeth 30, is determined on the basis of an envelope drawn for a plurality of tooth shape lines (the contour of the inner rotor 2; see the dash-double-dot line in FIG. 3) obtained by revolving the center of rotation 2c of an inner rotor 2Z, which is based on the inner rotor 2, around by a predetermined angle δ at a time on a circumference having a diameter of 2·e+t and centered on the center of rotation 3c of the outer rotor 3, and rotating the inner rotor 2Z by a rotational angle δ/N while the center of rotation 2c is revolved by the predetermined angle δ. It should be noted, however, that “t” is the clearance (tip clearance) between the top portion 21t of the tooth tip portion 21 of the external teeth 20 and the top portion of the tooth tip portion of the internal teeth 30 at the time when the center of rotation 2c of the inner rotor 2Z, the center of rotation 3c of the outer rotor 3, the top portion 21t, and the top portion of the internal teeth 30 are positioned on one line, and has a value of about 0.03 to 0.07 mm, for example.

The inner rotor 2Z for determining the tooth shape of the outer rotor 3 corresponds to the inner rotor 2 in which the tooth bottom portion 22 has been replaced with a tooth bottom portion 22z indicated by the dash-double-dot line in FIGS. 2 and 3. As illustrated in FIGS. 2 and 3, the tooth bottom portion 22z includes a portion that is from the end portion 24r of the second intermediate portion 24 to a boundary portion 22y indicated in FIGS. 2 and 3 and formed by a hypotrochoid curve (a portion other than a loop portion) that is the same as the hypotrochoid curve which forms the intermediate portion of the tooth bottom portion 22, and a portion that is from the boundary portion 22y to the end portion 23f of the first intermediate portion 23 and formed by a smooth curve (e.g. an arc). Consequently, it is possible to easily obtain the outer rotor 3 which can be adequately meshed with the inner rotor 2. It should be noted, however, that the tooth shape (contour) of the outer rotor 3 may be the envelope itself, or may be determined to be positioned on the outer side with respect to the envelope. The internal teeth of the outer rotor 3 may be formed using a gear cutting having a shape that is generally the same as that of the inner rotor 2Z.

In the gear pump 1, the inner rotor 2 (specifications of the external teeth 20), the outer rotor 3, the suction port 6, and the first and second discharge ports 7 and 8 are configured such that an inter-tooth chamber 5x (see FIG. 1) that has been brought out of communication with the second discharge port 8 communicates with the suction port 6 while the volume of the inter-tooth chamber 5x is decreasing and the volume of the inter-tooth chamber 5x is increased after at least a part of the inter-tooth chamber 5x and the suction port 6 has been brought into communication with each other. In the gear pump 1, additionally, the plurality of external teeth 20 of the inner rotor 2 are formed such that, while one of the external teeth 20 that is the closest to the top dead center (a position at which the top portion of the tooth tip portion 21 of the external teeth 20 and the top portion of the tooth tip portion of the internal teeth 30 face (contact) each other on a line) contacts a corresponding one of the internal teeth 30, the external tooth 20 which is positioned on the rear side, in the rotational direction, of the one external tooth 20 by one tooth contacts a corresponding one of the internal teeth 30. By determining the specifications of the tooth tip portion 21, the tooth bottom portion 22, the first and second intermediate portions 23 and 24, etc. so as to meet such conditions, it is possible to suppress well occurrence of cavitation in the inter-tooth chamber 5 (5x), and to reduce vibration or noise by stabilizing the behavior of the inner rotor 2 and the outer rotor 3 during operation of the gear pump 1.

Next, operation of the gear pump 1 will be described with reference to FIGS. 5 to 8. FIGS. 5 to 7 are each an enlarged view illustrating operation of the gear pump 1. FIG. 8 is a chart illustrating the relationship between a rotational angle θ of the inner rotor 2 about the center of rotation 2c and a volume V of the inter-tooth chamber 5x which is brought out of communication with the second discharge port 8. The rotational angle θ of the inner rotor 2 is a rotational angle, about the center of rotation 2c, of a line portion that connects between the bottommost portion (deepest portion) of the tooth bottom portion 22 of a certain external tooth 20 and the center of rotation 2c, and is measured counterclockwise in FIG. 1 with the state in which the bottommost portion of the tooth bottom portion 22 of the external tooth 20 is positioned directly below the center of rotation 2c of the inner rotor 2 in the drawing defined as 0°.

In the gear pump 1, the volume V of each of the inter-tooth chambers 5 in communication with the second discharge port 8 is decreased along with rotation of the inner rotor 2 and the outer rotor 3. When the rotational angle θ of the inner rotor 2 reaches a first angle θ1 (see FIG. 8), as illustrated in FIG. 5, a meshing portion E between the external tooth 20 on the rear side in the rotational direction and the internal tooth 30 which define the inter-tooth chamber 5x which was in communication with the second discharge port 8 overlaps a peripheral edge 8e of the second discharge port 8 as seen in the axial direction of the inner rotor 2, so that communication between the inter-tooth chamber 5x and the second discharge port 8 is blocked. Furthermore, when the inter-tooth chamber 5x has been brought out of communication with the second discharge port 8 with the meshing portion E overlapping the peripheral edge 8e of the second discharge port 8, as illustrated in FIG. 5, a tooth surface (the tooth bottom portion 22 or the second intermediate portion 24) of the external tooth 20 which is on the front side, in the rotational direction of the inner rotor 2, by one tooth with respect to the external tooth 20 including the meshing portion E slightly rides on a peripheral edge 6e of the suction port 6 as seen in the axial direction of the inner rotor 2. Consequently, the inter-tooth chamber 5x is brought into communication with the suction port 6 substantially at the same time as the inter-tooth chamber 5x is brought out of communication with the second discharge port 8.

After the inter-tooth chamber 5x has been brought out of communication with the second discharge port 8 and has been brought into communication with the suction port 6, as illustrated in FIG. 8, the volume V of the inter-tooth chamber 5x is further decreased along with rotation of the inner rotor 2 and the outer rotor 3. As illustrated in FIG. 6, the area of communication between the inter-tooth chamber 5x and the suction port 6 as seen in the axial direction of the inner rotor 2 is gradually increased along with rotation of the inner rotor 2 and the outer rotor 3. In the embodiment, further, when the rotational angle θ of the inner rotor 2 reaches a second angle θ2 (see FIG. 8), as illustrated in FIGS. 7 and 8, the entire inter-tooth chamber 5x is communicated with the suction port 6 (the entire inter-tooth chamber 5x overlaps the suction port 6 as seen in the axial direction), and the volume V of the inter-tooth chamber 5x reaches a minimum value Vmin.

When the volume of the inter-tooth chamber 5x has reached the minimum value Vmin, as illustrated in FIG. 7, the tooth bottom portion 22 between the two external teeth 20 that define the inter-tooth chamber 5x approximates to (substantially contacts) an inner peripheral edge 6ie of the suction port 6 without projecting toward the center of rotation 2c from the inner peripheral edge 6ie as seen in the axial direction of the inner rotor 2. After the volume V of the inter-tooth chamber 5x has reached the minimum value Vmin, as illustrated in FIG. 8, the volume V of the inter-tooth chamber 5x is increased along with rotation of the inner rotor 2 and the outer rotor 3, so that working oil is suctioned from the suction port 6 into the inter-tooth chamber 5x.

In the gear pump 1, as discussed above, the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 is brought into communication with the suction port 6 while the volume V of the inter-tooth chamber 5x is decreased along with rotation of the inner rotor 2 and the outer rotor 3. Consequently, working oil remaining in the inter-tooth chamber 5x is discharged to the suction port 6 as the volume V of the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 is decreased along with rotation of the inner rotor 2 etc. The volume V of the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 starts increasing after the inter-tooth chamber 5x is fully communicated with the suction port 6. That is, the volume V of the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 reaches the minimum value Vmin after the inter-tooth chamber 5x is fully communicated with the suction port 6.

As a result, a rapid inflow of working oil (leaked oil) into the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 and which is narrow from a gap between the inner rotor 2 and the outer rotor 3 and at least one of the pump body and the pump housing (a gap in the axial direction of the inner rotor 2) can be regulated well by working oil that flows out of the inter-tooth chamber 5x to the suction port 6. Thus, with the gear pump 1, it is possible to suppress well occurrence of cavitation in the inter-tooth chamber 5x which is brought out of communication with the second discharge port 8 and which is brought into communication with the suction port 6.

In the gear pump 1, the inter-tooth chamber 5x whose volume V is decreased along with rotation of the inner rotor 2 and the outer rotor 3 is brought out of communication with the second discharge port 8 when the meshing portion E between the external tooth 20 and the internal tooth 30 which define the inter-tooth chamber 5x overlaps the peripheral edge 8e of the second discharge port 8 as seen in the axial direction of the inner rotor 2. In the gear pump 1, when the meshing portion E overlaps the peripheral edge 8e of the second discharge port 8 as seen in the axial direction of the inner rotor 2, a tooth surface (the second intermediate portion 24 or the tooth bottom portion 22) of the external tooth 20 which is on the front side, in the rotational direction of the inner rotor 2, by one tooth with respect to the external tooth 20 including the meshing portion E overlaps the peripheral edge 6e of the suction port 6 as seen in the axial direction. Consequently, the inter-tooth chamber 5x whose volume V is decreased along with rotation of the inner rotor 2 etc., can be brought into communication with the suction port 6 immediately after the inter-tooth chamber 5x has been brought out of communication with the second discharge port 8 to allow working oil in the inter-tooth chamber 5x to flow out to the suction port 6. Thus, it is possible to regulate significantly well a rapid inflow of working oil (leaked oil) into the inter-tooth chamber 5x, which has been brought out of communication with the second discharge port 8 and which is narrow, from a gap between the inner rotor 2 and the outer rotor 3 and at least one of the pump body and the pump housing.

In the gear pump 1, further, the volume V of the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 starts increasing after the entire inter-tooth chamber 5x is communicated with the suction port 6. Consequently, the area of communication between the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 and the suction port 6 at the time when working oil starts flowing from the suction port 6 into the inter-tooth chamber 5x in accordance with an increase in the volume V can be prevented from being narrowed. As a result, it is possible to suppress an increase in flow rate of working oil that flows from the suction port 6 into the inter-tooth chamber 5x, and to suppress well occurrence of cavitation that accompanies suctioning of working oil into the inter-tooth chamber 5x.

In the gear pump 1, as discussed above, the intermediate portion, which is formed by a hypotrochoid curve, of each of the tooth bottom portions 22 of the inner rotor 2 is offset toward the center O (center of rotation 2c) with respect to the base circle BCt, and the tooth bottom portion 22 is deeper than a tooth bottom portion corresponding to the internal teeth 30 of the outer rotor 3. Additionally, as illustrated in FIG. 7, the tooth bottom portion 22 between the two external teeth 20 that define the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 approximates to the inner peripheral edge 6ie of the suction port 6 without projecting toward the center of rotation 2c from the inner peripheral edge 6ie as seen in the axial direction of the inner rotor 2 when the volume V of the inter-tooth chamber 5x has reached the minimum value Vmin. As a result, the area of communication between the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 and the suction port 6 at the time when working oil starts flowing from the suction port 6 into the inter-tooth chamber 5x in accordance with an increase in the volume V can be increased. Thus, it is possible to suppress an increase in flow rate of working oil that flows from the suction port 6 into the inter-tooth chamber 5x, and to suppress significantly well occurrence of cavitation that accompanies suctioning of working oil into the inter-tooth chamber 5x.

In the gear pump 1, further, the tooth tip portion 21 of each of the external teeth 20 of the inner rotor 2 is formed by a portion of an epitrochoid curve whose trochoid coefficient is larger than a value of 1 and which is other than a loop portion. Additionally, the tooth bottom portion 22 of the inner rotor 2 is formed by a portion of a hypotrochoid curve whose the base circle BCt is common to the epitrochoid curve and whose trochoid coefficient of is larger than a value of 1, and which is other than a loop portion. Consequently, by increasing the radii rde and rdh of the first and second drawn points, that is, the first and second values Rde and Rdh, while keeping the radii re and rh of the outer rolling circle Co and the inner rolling circle Ci (« radius of base circle BCt/number of teeth) small, it is possible to determine the shape of the tooth tip portion 21 and the tooth bottom portion 22 using a single base circle BCt, and to easily increase the tooth height of the external teeth 20 while keeping the outside diameter of the base circle BCt, that is, the outside diameter of the inner rotor 2, small.

By increasing the tooth height of the external teeth 20 in this way, the meshing portion E (the locus of the meshing portion E indicated by the dotted line in FIGS. 5 to 7) can be shifted toward the rear side, in the rotational direction of the inner rotor 2 etc., with respect to a line (see the dash-and-dot line which extends in the up-down direction in FIG. 1) that passes through the center of rotation 2c of the inner rotor 2 and the center of rotation 3c of the outer rotor 3. Consequently, it is possible to easily bring the inter-tooth chamber 5x which has been brought out of communication with the second discharge port 8 into communication with the suction port 6 while the volume V is decreasing by approximating an end portion of the suction port 6 on the rear side in the rotational direction to an end portion of the second discharge port 8 on the front side in the rotational direction.

By making the external teeth 20 asymmetric by making the length of a curve that forms the first intermediate portion 23 of the external teeth 20 longer than the length of a curve that forms the second intermediate portion 24, the end portion 21r of the tooth tip portion 21 (epitrochoid curve) on the rear side in the rotational direction can be approximated to the tooth bottom portion 22, and the end portion 21f of the tooth tip portion 21 on the front side in the rotational direction can be brought closer to the outer side in the radial direction of the inner rotor 2. By approximating the end portion 21r of the tooth tip portion 21 on the rear side in the rotational direction to the tooth bottom portion 22, it is possible to totally reduce the minimum value of the clearance between the external teeth 20 and the internal teeth 30 which define the inter-tooth chambers 5 in communication with the first and second discharge ports 7 and 8. By bringing the end portion 21f of the tooth tip portion 21 on the front side in the rotational direction closer to the outer side in the radial direction of the inner rotor 2, the minimum value of the clearance between the external teeth 20 and the internal teeth 30 which define the inter-tooth chambers 5 in communication with the suction port 6 can be totally increased. As a result, it is possible to reduce the minimum value (discharge-side clearance) of the clearance between the external teeth 20 and the internal teeth 30 which overlap the partition wall 9 separating the first and second discharge ports 7 and 8 from each other while improving the degree of freedom in determining the position of the partition wall 9, that is, the distribution ratio of the discharge flow rates from the first and second discharge ports 7 and 8. Additionally, it is possible to suppress well occurrence of cavitation in the inter-tooth chamber 5 whose the amount of variation in volume is the largest, by making the minimum value (suction-side clearance) of the clearance in the inter-tooth chamber 5 sufficiently large.

In the gear pump 1, further, the first intermediate portion 23 which is positioned on the front side, in the rotational direction of the inner rotor 2, of the tooth tip portion 21 is formed by an involute curve. Consequently, it is possible to smoothly mesh the external teeth 20 of the inner rotor 2 and the internal teeth 30 of the outer rotor 3 with each other, and to make the rotational speed ratio between the inner rotor 2 and the outer rotor 3 constant. It should be understood, however, that the first intermediate portion 23 may be formed by a curve other than an involute curve, such as an n-th order function (n is an integer having a value of 1 or more), an arc, a desired polynomial, a trigonometric function, an easement curve, or a combination thereof.

It should be understood, however, that the second intermediate portion 24 may also be formed by a curve other than an involute curve, such as an n-th order function (n is an integer having a value of 1 or more), an arc, a desired polynomial, a trigonometric function, an easement curve, or a combination thereof. The gear pump 1 may have a single discharge port. Furthermore, each of the external teeth 20 of the inner rotor 2 may be formed symmetrically with respect to the tooth shape center line Lc. The timing when the inter-tooth chamber 5x is brought into communication with the suction port 6 may be slightly later than the timing when the inter-tooth chamber 5x is brought out of communication with the second discharge port 8 so that the inter-tooth chamber 5x is not in communication with the suction port 6 while the inter-tooth chamber 5x is in communication with the second discharge port 8. That is, it is not necessary that the timings should perfectly coincide with each other. Furthermore, as illustrated in FIG. 9, the inner rotor 2, the second discharge port 8, and the input port 6 may be formed such that the inter-tooth chamber 5x is brought into communication with the suction port 6 before the meshing portion E between the external tooth 20 and the internal tooth 30 which define the inter-tooth chamber 5x overlaps the peripheral edge 8e of the second discharge port 8 as seen in the axial direction of the inner rotor 2. That is, the timing when the inter-tooth chamber 5x is brought into communication with the suction port 6 may be slightly earlier than the timing when the inter-tooth chamber 5x is brought out of communication with the second discharge port 8 to the extent that the discharge pressure from the second discharge port 8 is not significantly affected. Consequently, the inter-tooth chamber 5x whose the volume is decreased along with rotation of the inner rotor 2 etc., can be brought into communication with the suction port 6 before the inter-tooth chamber 5x is brought out of communication with the second discharge port 8 to allow an appropriate amount of working oil in the inter-tooth chamber 5x to flow out to the second discharge port 8 and the suction port 6. As a result, it is possible to hold the pressure of working oil in the inter-tooth chamber 5x so as not to be raised more than necessary, and to suppress occurrence of vibration due to a rise in pressure of working oil in the inter-tooth chamber 5x.

As has been described above, the present disclosure provides a gear pump (1) including a suction port (6), a discharge port (7, 8), an inner rotor (2) having a plurality of external teeth (20), an outer rotor (3) which has a plurality of internal teeth (30) such that the number of the internal teeth (30) is larger than that of the external teeth (20) of the inner rotor (2), and which is disposed eccentrically with respect to the inner rotor (2), and a plurality of inter-tooth chambers (5) defined by the plurality of external teeth (20) and the plurality of internal teeth (30), characterized in that the discharge port (7, 8) is in communication with the inter-tooth chamber (5) whose volume (V) is decreased along with rotation of the inner rotor (2) and the outer rotor (3), the inter-tooth chamber (5x) which has been brought out of communication with the discharge port (8) is brought into communication with the suction port (6) while the volume (V) of the inter-tooth chamber (5x) is decreasing, and the volume (V) of the inter-tooth chamber (5x) which has been brought out of communication with the discharge port (8) is increased after at least a part of the inter-tooth chamber (5x) has been brought into communication with the suction port (6).

In the gear pump, the inter-tooth chamber which has been brought out of communication with the discharge port is brought into communication with the suction port while the volume of the inter-tooth chamber is decreased along with rotation of the inner rotor and the outer rotor. Consequently, a fluid is discharged from the inter-tooth chamber which has been brought out of communication with the discharge port to the suction port as the volume of the inter-tooth chamber is decreased along with rotation of the inner rotor etc. The volume of the inter-tooth chamber which has been brought out of communication with the discharge port is increased after the inter-tooth chamber has been brought into communication with the suction port. That is, the volume of the inter-tooth chamber which has been brought out of communication with the discharge port becomes smallest after the inter-tooth chamber has been brought into communication with the suction port. As a result, a rapid inflow of a fluid into the inter-tooth chamber which has been brought out of communication with the discharge port from a gap between the inner rotor and the outer rotor and a member that houses the inner rotor and the outer rotor (a gap in the axial direction) can be regulated well by a fluid that flows out of the inter-tooth chamber to the suction port. Thus, with the gear pump, it is possible to suppress well occurrence of cavitation in the inter-tooth chamber which is brought out of communication with the discharge port and which is brought into communication with the suction port.

The volume (V) of the inter-tooth chamber (5x) which has been brought out of communication with the discharge port (6) may start increasing after the entire inter-tooth chamber (5x) is communicated with the suction port (6). Consequently, the area of communication between the inter-tooth chamber which has been brought out of communication with the discharge port and the suction port at the time when a fluid starts flowing from the suction port into the inter-tooth chamber in accordance with an increase in the volume can be prevented from being narrowed. As a result, it is possible to suppress an increase in flow rate of a fluid that flows from the suction port into the inter-tooth chamber, and to suppress well occurrence of cavitation that accompanies suctioning of the fluid into the inter-tooth chamber.

The inner rotor (2) may be formed such that a tooth bottom portion (22) that defines the inter-tooth chamber (5x) which has been brought out of communication with the discharge port (8) approximates to an inner peripheral edge (6ie) of the suction port (6) without projecting from the inner peripheral edge (6ie) toward a center of rotation (2c) of the inner rotor (2) as seen in an axial direction of the inner rotor (2) when the volume (V) of the inter-tooth chamber (5x) has become smallest. Consequently, the minimum volume of the inter-tooth chamber, that is, the area of communication between the inter-tooth chamber which has been brought out of communication with the discharge port and the suction port at the time when a fluid starts flowing from the suction port into the inter-tooth chamber in accordance with an increase in the volume, can be increased. As a result, it is possible to suppress an increase in flow rate of a fluid that flows from the suction port into the inter-tooth chamber, and to suppress significantly well occurrence of cavitation that accompanies suctioning of the fluid into the inter-tooth chamber. In this case, by deepening the tooth bottom portion of the external teeth of the inner rotor (offsetting the tooth bottom portion toward the center of rotation of the inner rotor), for example, the tooth bottom portion can be approximated to the inner peripheral edge of the suction port when the volume of the inter-tooth chamber reaches the minimum value, and the minimum volume (area of communication) of the inter-tooth chamber which has been brought out of communication with the discharge port can be increased adequately.

The inner rotor (2) may be formed such that the inter-tooth chamber (5x) whose volume (V) is decreased is brought into communication with the suction port (6) after a meshing portion (E) between the external tooth (20) and the internal tooth (30) defining the inter-tooth chamber (5x) overlaps a peripheral edge (8e) of the discharge port (8) as seen in an axial direction of the inner rotor (2). Consequently, the inter-tooth chamber whose volume is decreased along with rotation of the inner rotor etc. can be brought into communication with the suction port substantially at the same time as the inter-tooth chamber has been brought out of communication with the discharge port to allow a fluid in the inter-tooth chamber to flow out to the suction port. Thus, it is possible to regulate significantly well an inflow of the fluid into the inter-tooth chamber which has been brought out of communication with the discharge port from a gap between the inner rotor and the outer rotor and a member that houses the inner rotor and the outer rotor.

The inner rotor (2) may be formed such that the inter-tooth chamber (5x) whose volume (V) is decreased is brought into communication with the suction port (6) before a meshing portion (E) between the external tooth (20) and the internal tooth (30) defining the inter-tooth chamber (5x) overlaps a peripheral edge (8e) of the discharge port (8) as seen in an axial direction of the inner rotor (2). Consequently, the inter-tooth chamber whose volume is decreased along with rotation of the inner rotor etc. can be brought into communication with the suction port before the inter-tooth chamber is brought out of communication with the discharge port to allow an appropriate amount of fluid in the inter-tooth chamber to flow out to the discharge port and the suction port. As a result, it is possible to hold the pressure of a fluid in the inter-tooth chamber so as not to be raised more than necessary, and to suppress occurrence of vibration due to a rise in pressure of the fluid in the inter-tooth chamber.

Each of the external teeth (20) of the inner rotor (2) may include a tooth tip portion (21) formed by an epitrochoid curve obtained by rolling, without sliding, an outer rolling circle (Co) having a radius (re) that is smaller than a radius (rde) of a drawn point while circumscribing a base circle (BCt). That is, by increasing the radius of a drawn point of the epitrochoid curve while keeping the radius of the outer rotating circle (∝ radius of base circle/number of teeth) small, it is possible to easily increase the tooth height of the external teeth while keeping the outside diameter of the base circle, that is, the outside diameter of the inner rotor, small. By increasing the tooth height of the external teeth, the meshing portion (the locus of the meshing portion) between the external teeth and the internal teeth can be shifted rearward in the rotational direction of the inner rotor etc. with respect to a line that passes through the center of rotation of the inner rotor and the center of rotation of the outer rotor. Consequently, it is possible to easily bring the inter-tooth chamber which has been brought out of communication with the discharge port into communication with the suction port while the volume is decreasing by approximating an end portion of the suction port on the rear side in the rotational direction to an end portion of the discharge port on the front side in the rotational direction of the inner rotor etc.

Each of the external teeth (20) of the inner rotor (2) may include a first intermediate portion (23) formed by a desired curve and positioned between the tooth tip portion (21) and a tooth bottom portion (22) positioned on a front side, in a rotational direction of the inner rotor (2), with respect to the tooth tip portion (21), and a second intermediate portion (24) formed by a desired curve and positioned between the tooth tip portion (21) and a tooth bottom portion (22) positioned on a rear side, in the rotational direction of the inner rotor (2), with respect to the tooth tip portion (21); and a length of the curve which forms the first intermediate portion (23) may be longer than a length of the curve which forms the second intermediate portion (24).

By making the external teeth asymmetric by making the length of a curve that forms the first intermediate portion longer than the length of a curve that forms the second intermediate portion in this way, the end portion of the epitrochoid curve which forms the tooth tip portion on the rear side in the rotational direction can be approximated to the tooth bottom portion, and the end portion of the epitrochoid curve on the front side in the rotational direction can be brought closer to the outer side in the radial direction of the inner rotor. By approximating the end portion of the epitrochoid curve which forms the tooth tip portion on the rear side in the rotational direction to the tooth bottom portion, it is possible to totally reduce the minimum value of the clearance between the external teeth and the internal teeth which define the inter-tooth chambers in communication with the discharge port. By bringing the end portion of the epitrochoid curve which forms the tooth tip portion on the front side in the rotational direction closer to the outer side in the radial direction of the inner rotor, the minimum value of the clearance between the external teeth and the internal teeth which define the inter-tooth chambers in communication with the suction port can be totally increased.

The first intermediate portion (23) may be formed by at least an involute curve. Consequently, it is possible to smoothly mesh the external teeth and the internal teeth with each other, and to make the rotational speed ratio between the inner rotor and the outer rotor constant.

The discharge port may include a first discharge port (7) and a second discharge port (8) separated from the first discharge port (7) by a partition wall (9) and disposed on a front side, in a rotational direction of the inner rotor (2), with respect to the first discharge port (7).

The present disclosure is not limited to the embodiment described above in any way, and it is a matter of course that the embodiments may be modified in various ways without departing from the range of the extension of the present disclosure. Furthermore, the mode for carrying out the invention described above is merely a specific of an embodiment described in the “SUMMARY OF THE DISCLOSURE” section, and does not limit the elements of the embodiments described in the “SUMMARY OF THE DISCLOSURE” section.

INDUSTRIAL APPLICABILITY

The disclosure according to the present disclosure is applicable to the gear pump manufacturing industry.

Claims

1. A gear pump including a suction port, a discharge port, an inner rotor having a plurality of external teeth, an outer rotor which has a plurality of internal teeth such that the number of the internal teeth is larger than that of the external teeth of the inner rotor and which is disposed eccentrically with respect to the inner rotor, and a plurality of inter-tooth chambers defined by the plurality of external teeth and the plurality of internal teeth,

wherein the discharge port is in communication with the inter-tooth chamber, whose volume is decreased along with rotation of the inner rotor and the outer rotor, the inter-tooth chamber which has been brought out of communication with the discharge port is brought into communication with the suction port while the volume of the inter-tooth chamber is decreasing, and the volume of the inter-tooth chamber which has been brought out of communication with the discharge port is increased after at least a part of the inter-tooth chamber has been brought into communication with the suction port.

2. The gear pump according to claim 1, wherein

the volume of the inter-tooth chamber which has been brought out of communication with the discharge port starts increasing after the entire inter-tooth chamber is communicated with the suction port.

3. The gear pump according to claim 1, wherein the inner rotor is formed such that a tooth bottom portion that defines the inter-tooth chamber which has been brought out of communication with the discharge port approximates to an inner peripheral edge of the suction port without projecting from the inner peripheral edge toward a center of rotation of the inner rotor as seen in an axial direction of the inner rotor when the volume of the inter-tooth chamber has become smallest.

4. The gear pump according to claim 1, wherein the inner rotor is formed such that the inter-tooth chamber whose volume is decreased is brought into communication with the suction port after a meshing portion between the external tooth and the internal tooth defining the inter-tooth chamber overlaps a peripheral edge of the discharge port as seen in an axial direction of the inner rotor.

5. The gear pump according to claim 1, wherein the inner rotor is formed such that the inter-tooth chamber whose volume is decreased is brought into communication with the suction port before a meshing portion between the external tooth and the internal tooth defining the inter-tooth chamber overlaps a peripheral edge of the discharge port as seen in an axial direction of the inner rotor.

6. The gear pump according to claim 1, wherein each of the external teeth of the inner rotor includes a tooth tip portion formed by an epitrochoid curve obtained by rolling, without sliding, an outer rolling circle having a radius that is smaller than a radius of a drawn point while circumscribing a base circle.

7. The gear pump according to claim 6, wherein:

each of the external teeth of the inner rotor includes a first intermediate portion formed by a desired curve and positioned between the tooth tip portion and a tooth bottom portion positioned on a front side, in a rotational direction of the inner rotor, with respect to the tooth tip portion, and a second intermediate portion formed by a desired curve and positioned between the tooth tip portion and a tooth bottom portion positioned on a rear side, in the rotational direction of the inner rotor, with respect to the tooth tip portion; and

a length of the curve which forms the first intermediate portion is longer than a length of the curve which forms the second intermediate portion.

8. The gear pump according to claim 7, wherein the first intermediate portion is formed by at least an involute curve.

9. The gear pump according to claim 1, wherein

the discharge port includes a first discharge port and a second discharge port separated from the first discharge port by a partition wall and disposed on a front side, in a rotational direction of the inner rotor, with respect to the first discharge port.

10. The gear pump according to claim 2, wherein the inner rotor is formed such that a tooth bottom portion that defines the inter-tooth chamber which has been brought out of communication with the discharge port approximates to an inner peripheral edge of the suction port without projecting from the inner peripheral edge toward a center of rotation of the inner rotor as seen in an axial direction of the inner rotor when the volume of the inter-tooth chamber has become smallest.

11. The gear pump according to claim 2, wherein the inner rotor is formed such that the inter-tooth chamber whose volume is decreased is brought into communication with the suction port after a meshing portion between the external tooth and the internal tooth defining the inter-tooth chamber overlaps a peripheral edge of the discharge port as seen in an axial direction of the inner rotor.

12. The gear pump according to claim 2, wherein the inner rotor is formed such that the inter-tooth chamber whose volume is decreased is brought into communication with the suction port before a meshing portion between the external tooth and the internal tooth defining the inter-tooth chamber overlaps a peripheral edge of the discharge port as seen in an axial direction of the inner rotor.

13. The gear pump according to claim 2, wherein

each of the external teeth of the inner rotor includes a tooth tip portion formed by an epitrochoid curve obtained by rolling, without sliding, an outer rolling circle having a radius that is smaller than a radius of a drawn point while circumscribing a base circle.

14. The gear pump according to claim 13, wherein:

each of the external teeth of the inner rotor includes a first intermediate portion formed by a desired curve and positioned between the tooth tip portion and a tooth bottom portion positioned on a front side, in a rotational direction of the inner rotor, with respect to the tooth tip portion, and a second intermediate portion formed by a desired curve and positioned between the tooth tip portion and a tooth bottom portion positioned on a rear side, in the rotational direction of the inner rotor, with respect to the tooth tip portion; and

a length of the curve which forms the first intermediate portion is longer than a length of the curve which forms the second intermediate portion.

15. The gear pump according to claim 14, wherein the first intermediate portion is formed by at least an involute curve.

16. The gear pump according to claim 2, wherein the discharge port includes a first discharge port and a second discharge port separated from the first discharge port by a partition wall and disposed on a front side, in a rotational direction of the inner rotor, with respect to the first discharge port.

17. The gear pump according to claim 10, wherein the inner rotor is formed such that the inter-tooth chamber whose volume is decreased is brought into communication with the suction port after a meshing portion between the external tooth and the internal tooth defining the inter-tooth chamber overlaps a peripheral edge of the discharge port as seen in an axial direction of the inner rotor.

18. The gear pump according to claim 17, wherein each of the external teeth of the inner rotor includes a tooth tip portion formed by an epitrochoid curve obtained by rolling, without sliding, an outer rolling circle having a radius that is smaller than a radius of a drawn point while circumscribing a base circle.

19. The gear pump according to claim 18, wherein:

each of the external teeth of the inner rotor includes a first intermediate portion formed by a desired curve and positioned between the tooth tip portion and a tooth bottom portion positioned on a front side, in a rotational direction of the inner rotor, with respect to the tooth tip portion, and a second intermediate portion formed by a desired curve and positioned between the tooth tip portion and a tooth bottom portion positioned on a rear side, in the rotational direction of the inner rotor, with respect to the tooth tip portion; and

a length of the curve which forms the first intermediate portion is longer than a length of the curve which forms the second intermediate portion.

20. The gear pump according to claim 19, wherein the first intermediate portion is formed by at least an involute curve.