UNIAXIAL ECCENTRIC SCREW PUMP

Abstract

A uniaxial eccentric screw pump includes a stator having an insertion hole with an inner peripheral surface being internally threaded, and a rotor including shaft placed through the insertion hole in the stator. The insertion hole is, in a cross section, an opening including a middle area and two end areas. An interference amount between an outer peripheral surface of the rotor and the inner peripheral surface of the insertion hole in the stator is smaller in the middle area than in the two end areas.

Description

TECHNICAL FIELD

The present invention relates to a uniaxial eccentric screw pump.

BACKGROUND ART

A known uniaxial eccentric screw pump includes a stator having an insertion hole with its inner peripheral surface internally threaded and a rotor including an externally threaded shaft placed through the insertion hole in the stator (refer to, for example, Patent Document 1).

In the known uniaxial eccentric screw pump, the rotor and the stator have, between them, substantially the same interference value when the rotor is located in two end areas in an opening that is the cross section of the insertion hole in the stator and when the rotor is located in a middle area in the opening.

When the interference amount is sufficiently large, the rotor uses higher torque and a greater driving force to rotate.

However, the interference amount decreased to facilitate movement of the rotor can lower the sealing tightness, causing inappropriate delivery of a fluid.

The inventors have noticed that the interference amount can be relatively small in the middle area when the interference amount is sufficient in the two end areas, and have developed a uniaxial eccentric screw pump according to one or more aspects of the present invention.

PRIOR ART DOCUMENTS

Patent Documents

• • Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-344587

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

One or more aspects of the present invention are directed to a uniaxial eccentric screw pump with the sealing tightness and the driving force for rotating a rotor adjustable as appropriate.

Solutions to the Problems

In response to the above issue, a uniaxial eccentric screw pump according to an aspect of the present invention includes a stator having an insertion hole with an inner peripheral surface being internally threaded, and a rotor including a shaft being externally threaded and placed through the insertion hole in the stator. The insertion hole is, in a cross section, an opening including a middle area and two end areas. An interference amount between an outer peripheral surface of the rotor and the inner peripheral surface of the insertion hole in the stator is smaller in the middle area than in the two end areas.

In this structure, the rotor touches the inner surface of the insertion hole in the stator with a predetermined interference amount when the rotor is located in the two end areas of the opening. This achieves sufficient sealing tightness (pressure tightness) in the two end areas of the opening. When the rotor is located in the middle area of the opening, the rotor less closely touches the inner surface of the insertion hole in the stator with a smaller interference amount than in the two end areas. The rotor thus generates less friction in the middle area than in the two end areas of the opening and uses a reduced driving force to rotate.

The opening in the stator may have an elliptical profile with a ratio of a minor diameter to a major diameter being 0.83 to 0.96 inclusive.

In this structure, the opening in the stator has an optimum shape that achieves tight sealing and also facilitates rotation of the rotor. More specifically, when the ratio of the minor diameter to the major diameter is less than 0.83, the fluid can leak with an insufficient interference amount in middle portions of the respective two end areas (at the two ends of the opening). When the ratio is greater than 0.96, the fluid cannot be delivered in the smaller-volume cavities defined between the rotor and the stator.

The opening in the stator may have a profile expressed with a single formula defining a condition to cause a smaller interference amount in the middle area than an opening having a profile with a shape of a racetrack including semicircles and straight lines.

In this structure, the rotor is contacted by the stator less closely in the middle area and uses a reduced driving force to rotate.

The opening in the stator may have a profile expressed with a single formula defining a condition to cause a larger interference amount in the two end areas than an opening having a profile with a shape of a racetrack including semicircles and straight lines.

In this structure, the rotor is contacted by the stator more closely in the two end areas and can achieve tighter sealing for more reliable delivery of the fluid.

The opening in the stator may have a profile including, in the middle area, a portion with a first profile and including, in each of the two end areas, a portion with a second profile different from the first profile.

In this structure, the insertion hole has the opening shape to achieve intended performance in the two end areas and in the middle area.

The opening in the stator may have a profile including a part of a perfect circle.

The opening in the stator may have a profile including a part of an ellipse.

The opening in the stator may have, in the middle area, an outwardly curved profile.

The opening in the stator may have a profile including at least one straight portion.

The at least one straight portion may include parallel straight portions facing each other in the middle area.

This structure facilitates designing of the insertion hole and changing of the interference amount based on the rotational position and the outer shape of the rotor as the rotor moves in the middle area.

The stator may consist of a stator body formed from an elastic material.

This structure includes fewer components and facilitates manufacture at low costs.

Effects of the Invention

The uniaxial eccentric screw pump according to the above aspects of the present invention has the sealing tightness and the driving force for rotating the rotor adjustable as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view of a uniaxial eccentric screw pump according to an embodiment.

FIG. 2 is a longitudinal sectional view of the uniaxial eccentric screw pump in

FIG. 1.

FIG. 3 is a cross-sectional view of a stator in FIG. 1, showing an example opening that is a cross section of an insertion hole in the stator.

FIG. 4 is a cross-sectional view of a rotor located in a middle area of the opening in FIG. 3.

FIG. 5 is a cross-sectional view of the rotor located in one of two end areas of the opening in FIG. 3.

FIG. 6 is a cross-sectional view of the stator in FIG. 1, showing another example opening that is a cross section of an insertion hole in the stator.

FIG. 7 is a cross-sectional view of the stator in FIG. 1, showing another example opening that is a cross section of an insertion hole in the stator.

FIG. 8 is a cross-sectional view of the stator in FIG. 1, showing another example opening that is a cross section of an insertion hole in the stator.

FIG. 9 is a cross-sectional view of the stator in FIG. 1, showing another example opening that is a cross section of an insertion hole in the stator.

FIG. 10 is a cross-sectional view of the stator in FIG. 1, showing another example opening that is a cross section of an insertion hole in the stator.

FIG. 11 is a cross-sectional view of the stator in FIG. 1, showing another example opening that is a cross section of an insertion hole in the stator.

FIG. 12 is a cross-sectional view of the stator in FIG. 1, showing another example opening that is a cross section of an insertion hole in the stator.

FIG. 13 is a cross-sectional view of the stator in FIG. 1, showing another example opening that is a cross section of an insertion hole in the stator.

FIG. 14 is a partial longitudinal sectional view of a uniaxial eccentric screw pump according to another embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described with reference to the accompanying drawings. The terms for specific directions or positions (e.g., terms including up, down, side, and end) are used herein as appropriate to facilitate understanding of the present invention with reference to the drawings. However, such terms do not limit the technical scope of the present invention. The embodiments described below are mere examples and are not intended to limit the scope of the present invention and its applications or uses. The drawings are schematic and are not drawn to scale relative to the actual size of each component.

FIG. 1 is a front view of a uniaxial eccentric screw pump as an example of a rotary positive-displacement pump. FIG. 2 is a sectional view (longitudinal sectional view) taken along line A-A in FIG. 1. The uniaxial eccentric screw pump includes a casing 1, a drive (not shown) at one end of the casing 1, and a stator 2, a rotor 3, and an end stud 4 at the other end of the casing 1.

The casing 1 is tubular and is formed from a metal material. The casing 1 accommodates a coupling rod 5. The coupling rod 5 has one end connected to a coupling 6 to receive a driving force from the drive (not shown). The casing 1 has a first opening 7 in its outer peripheral surface at one end. The first opening 7 is connected to a connection tube 8. The connection tube 8 carries a fluid (e.g., a viscous material such as mayonnaise) from, for example, a tank (not shown) to feed the fluid into the casing 1. The stator 2 includes an outer cylinder 9 and a stator body 10. The outer cylinder 9 and the stator body 10 may be fixed to each other with an adhesive or by pressure welding.

The stator body 10 is tubular (e.g., cylindrical) and is formed from an elastic material. Examples of the elastic material include nitrile rubber, fluorine rubber, ethylene-propylene rubber, styrene-butadiene rubber, silicone rubber, and fluorosilicone rubber. Any of these elastic materials may be selected as appropriate for the material (fluid) to be transferred. The stator 2 has an insertion hole 14 in its center. The insertion hole 14 has an inner peripheral surface including a single or multiple internally threaded portions with an n-start thread (a two-start thread in this example).

The rotor 3 is a shaft formed from a metal material and includes a single or multiple externally threaded portions with an (n−1)-start thread (a single-start thread in this example). The rotor 3 is received in the insertion hole 14 in the stator 2 to define transfer spaces 15 continuous with one another in the longitudinal direction. The rotor 3 has one end connected to the coupling rod 5 in the casing. The rotor 3 rotates in the insertion hole 14 in the stator 2 under a driving force from the drive (not shown) and revolves along the inner peripheral surface of the insertion hole 14. In other words, the rotor 3 rotates eccentrically in the insertion hole 14 in the stator 2. As viewed in the cross section of the stator 2, the rotor 3 is displaced in a reciprocating manner between two opposite ends of an opening 16, which is the cross section of the insertion hole 14. The distance of this movement is four times the eccentricity of the rotor 3 that is rotating. Such eccentric rotation of the rotor 3 transfers the fluid in the transfer spaces 15 in the longitudinal direction.

The end stud 4 is tubular and is formed from a metal material. The end stud 4 has a second opening 17 at its distal end.

The casing 1 and the end stud 4 are connected to each other with stay bolts 18. The stay bolts 18 are tightened to join the casing 1, the stator 2, and the end stud 4 with the stator 2 between the casing 1 and the end stud 4. In the joined state, a flow path is defined to extend from the first opening 7 in the casing 1 through the insertion hole 14 in the stator 2 to the second opening 17 in the end stud 4.

In the present embodiment, the opening 16, or the cross section of the insertion hole 14 in the stator 2, has a characteristic shape.

In FIG. 3, the opening 16 as the cross section of the insertion hole 14 in the stator body 10 is elliptical. The opening 16 may have a ratio Sd/Ld of a minor diameter Sd to a major diameter Ld satisfying 0.83 to 0.96 inclusive, or preferably 0.88 to 0.95 inclusive. As shown in FIG. 4, the opening 16 includes a middle area 19 that is touched by the rotor 3 having the center aligned with the center of the opening 16, and two end areas 20 that are closer to its two ends than the middle area 19 along the major axis.

The pump can have an insufficient interference amount in the two end areas 20 at the ratio Sd/Ld less than 0.88, and can have no sealing contact at the ratio Sd/Ld less than 0.83 and thus cannot function. This results from an unintended gap x created between the rotor 3 and one of the two end areas 20 as shown in FIG. 6 after the rotor 3 moves from the middle area 19 toward the end area 20 and can no longer move under an increased elastic force from the stator 2. The ratio Sd/Ld is thus set greater than or equal to 0.83 to allow the pump to function. At the ratio Sd/Ld greater than or equal to 0.88, the pump has sufficient sealing tightness.

At the ratio Sd/Ld greater than 0.95, the rotor 3 has the circumferential length closer to the circumferential length of the opening 16, leaving smaller-volume cavities between the rotor 3 and the stator 2. The smaller-volume cavities limit the capacity of the fluid to be delivered. At the ratio Sd/Ld greater than 0.96, the rotor 3 has the circumferential length substantially equal to the circumferential length of the opening 16, leaving substantially no cavities between the rotor 3 and the stator 2. With no cavities, the fluid cannot be delivered.

The elliptical opening 16 may include a straight portion. For example, the opening 16 may include two parallel straight portions at the two sides along the major axis.

With the opening 16 being elliptical, the stator 2 and the rotor 3 have, between them, an interference amount 82 in the middle area 19 of the opening 16 smaller than an interference amount 81 in each of the two end areas 20 as shown in FIGS. 4 and 5. As the rotor 3 rotates eccentrically and moves from one of the two end areas 20 to the middle area 19 of the opening 16, the interference amount between the stator 2 and the rotor 3 decreases gradually. In other words, the rotor 3 generates less friction against the stator 2 and uses a reduced driving force to rotate.

Opening Shapes in Other Embodiments

The opening 16 as the cross section of the insertion hole 14 in the stator body 10 may have a shape other than an ellipse described above.

(1) The shape of the opening 16 may be expressed with a single formula defining a condition to cause a smaller interference amount in the middle area 19 than in the two end areas 20 or a larger interference amount in the two end areas 20 than in the middle area 19 as compared with an opening 16 with an oval shape (racetrack) including parallel straight lines and a pair of semicircles connecting the corresponding ends of the straight lines. For example, the opening 16 may have the shape of a superellipse expressed with the formula below.

${❘\frac{x}{a}❘}^n+{❘\frac{y}{b}❘}^n=1$

• • (a, b, and n: positive numbers)

In this case, when the value n is about 2 to 3, the rotor 3 is expected to move smoothly in the middle area 19 and also achieve sufficient sealing tightness in the two end areas 20. FIG. 7 shows an opening shape with n being 2.5. As shown in the figure, the opening 16 with this shape allows the rotor 3, which has a cross section with the shape of a perfect circle, to move with a smaller interference amount in the middle area 19 of the opening 16. The rotor 3 can thus move easily and use a less driving force to rotate. In the two end areas 20, the rotor 3 can move with a larger interference amount than in the middle area 19. This achieves tighter sealing for reliable delivery of the fluid.

(2) The shape may be other than a superellipse and may be, for example, a curve satisfying the formula below expressed in polar coordinates.

$r\left(\theta \right)=a\left(\theta \right)+f\left(\theta \right)$

• • a(θ): perfect circle or ellipse
  • f(θ): curve symmetric about θ=α+π rad (α≤θ≤α+2π rad)
  • curve symmetric about θ=α+3π/2 rad (α+π≤θ≤α+2π rad)

The formula f(θ) may be, for example, any of the formulas below.

$f\left(\theta \right)=a\times {❘\cos \theta ❘}^m$

• • a: positive number
  • m: real number (preferably, greater than or equal to 1.5)

$f\left(\theta \right)=b\times \left(\cos 2\theta +c\right)$

• • c: positive number

$f\left(\theta \right)=d\times x^{10}+e\times x^9$

• • d, e, . . . : positive numbers

(3) The opening 16 may include, in the middle area 19, a portion with a first profile, and include, in each of the two end areas 20, a portion with a second profile different from the first profile to have the interference amount smaller in the middle area 19 than in the two end areas 20. For example, the opening 16 may include multiple lines or curves connected together.

For example, the shape may include multiple straight portions (line segments) connected together. The straight portions may define a part of the opening 16 or the entire opening 16. Preferably, the straight portions may connect smoothly to each other with, for example, circular arcs.

The multiple lines or curves included in the opening 16 may be parts of a perfect circle (circular arcs) or parts of an ellipse (elliptical arcs), in place of or in addition to the multiple straight portions. The shape may combine multiple circular arcs with different curvature radii or combine parts of multiple ellipses with different major diameters and different minor diameters. The shape may combine circular arcs and elliptical arcs.

FIG. 8 shows an opening 16 including two pairs of circular arcs with different curvature radii. More specifically, the opening 16 may include circular arcs 3a at the two sides spaced from each other in the vertical direction and extending along the horizontal axis and circular arcs 3b at the two sides spaced from each other in the horizontal direction and extending along the vertical axis. The circular arcs 3a may have a first curvature radius R1, and the circular arcs 3b may have a second curvature radius R2 smaller than the first curvature radius R1.

FIG. 9 shows an opening 16 including two types of elliptical arcs with different curvature radii. More specifically, the opening 16 may include first curves 3c at the two sides spaced from each other in the vertical direction and extending along the horizontal axis and second curves 3d at the two sides spaced from each other in the horizontal direction and extending along the vertical axis. The first curves 3c may be parts (parts on the both sides along the horizontal axis) of a first ellipse. The second curves 3d may be parts (parts on the both sides along the vertical axis) of a second ellipse and have a curvature radius different from the curvature radius of the first curves 3c. In the example in FIG. 9, the first curves 3c are two parts of a single first ellipse 11, and the second curves 3d are two parts of a single second ellipse 12, with the centers of the first ellipse 11 and the second ellipse 12 aligned with each other. However, this is not limitative. The first curves 3c may simply be symmetric about the horizontal axis, and the second curves 3d may simply be symmetric about the vertical axis. For example, the center of the first ellipse 11 defining the first curves 3c may be offset from a center O in opposite directions (upward and downward in FIG. 9), or in other words, in the positive direction and the negative direction along the vertical axis, by the same distance. The same applies to the second curves 3d.

FIG. 10 shows an opening 16 including straight lines and parts of an ellipse (elliptical arcs). In the middle area 19, the straight lines may be parallel to each other and spaced from each other at a predetermined interval. In the two end areas 20, the elliptical arcs may be located inward from semicircles with the major axis of the ellipse aligned with the horizontal axis. This structure facilitates designing of the insertion hole 14 and changing of the interference amount based on the rotational position and the outer shape of the rotor 3 as the rotor 3 moves in the middle area 19. In the two end areas 20, the interference amount is larger, thus achieving tighter sealing for reliable delivery of the fluid. The elliptical arcs may be replaced with a combination of multiple circular arcs. The opening shape may combine circular arcs and elliptical arcs (e.g., circular arcs and circular arcs, circular arcs and elliptical arcs, or elliptical arcs and elliptical arcs).

The opening may be curved outwardly in the middle area 19. In this structure, the rotor 3 generates less friction against the stator 2 as the rotor 3 moves toward the center of the opening 16, and uses a further reduced driving force to rotate. FIG. 11 shows an opening 16 having the shape of a racetrack with its straight portions in the middle area 19 partially or entirely replaced with circular arcs 3e having a curvature radius R3. Each circular arc 3e has a center O1 farther from the circular arc 3e than the center O from the circular arc 3e on the minor axis.

The opening shape may include curves located inward from semicircles in the two end areas 20. In this structure, the rotor 3 in the two end areas 20 can more closely touch the inner surface of the insertion hole 14 in the stator 2 than in a structure with semicircles. FIG. 12 shows an opening 16 having the shape of a racetrack with its semicircles replaced with two circular arcs 3f located inward from the semicircles. Each circular arc 3f has a greater curvature radius than the semicircle and has a center O3 farther from the circular arc 3f than a center O2 of the semicircle from the circular arc 3f along the major axis and the minor axis.

(4) The opening 16 may be asymmetric about the longitudinal axis in either of the two end areas 20. FIG. 13 shows an opening 16 having the shape of a racetrack with its semicircles including inward portions 16a located inward from the semicircles. The inward portions 16a are straight lines at diagonal positions with respect to the center of the stator 2 and asymmetric about the horizontal axis. The straight lines are parallel to each other. The opening that is asymmetric about the longitudinal axis in either of the two end areas 20 increases the design flexibility as appropriate for the use.

OTHER EMBODIMENTS

The present invention is not limited to the structures described in the above embodiments and may be modified variously.

In the above embodiments, the casing 1 receives a fluid through the first opening 7 and discharges the fluid through the second opening 17. In some embodiments, the casing 1 may receive a fluid through the second opening 17 and discharge the fluid through the first opening 7 by rotating the rotor 3 in the reverse direction.

In the above embodiments, the stator 2 includes the outer cylinder 9 and the stator body 10. In some embodiments, the stator 2 may include the stator body 10 without the outer cylinder 9 as shown in FIG. 14. The casing 1 is stepped at one end and includes a radially-inward end portion 21 that is cylindrical and protruding from the end. The end stud 4 has a central hole 22 in one surface and a recess 23 surrounding the central hole 22. The stator body 10 includes a flange 24 at one end. The recess 23 on the end stud 4 receives the flange 24 on the stator body 10 and also receives the radially-inward end portion 21 of the casing 1. The end stud 4 can thus fix the stator 2 (stator body 10) with the flange 24 tightly held between the bottom surface of the recess 23 and the end face of the radially-inward end portion 21.

In this structure, the stator 2 (stator body 10) is deformable radially outward and cantilevered with the tightly held portion. This simple structure facilitates manufacture at low costs. Unlike in the above embodiments, this structure eliminates an adhesive to fix the outer cylinder 9 and the stator body 10 to each other, thus eliminating concerns about, for example, the resistance of the adhesive to the fluid.

EXPLANATION OF REFERENCES

• • 1 casing
  • 2 stator
  • 3 rotor
  • 4 end stud
  • 5 coupling rod
  • 6 coupling
  • 7 first opening
  • 8 connection tube
  • 9 outer cylinder
  • 10 stator body
  • 14 insertion hole
  • 15 transfer space
  • 16 opening
  • 17 second opening
  • 18 stay bolt
  • 19 middle area
  • 20 end area
  • 21 radially-inward end portion
  • 22 central hole
  • 23 recess
  • 24 flange

Claims

1. A uniaxial eccentric screw pump, comprising: a stator having an insertion hole with an inner peripheral surface being internally threaded; anda rotor including a shaft being externally threaded and placed through the insertion hole in the stator,wherein the insertion hole is, in a cross section, an opening including a middle area and two end areas, and an interference amount between an outer peripheral surface of the rotor and the inner peripheral surface of the insertion hole in the stator is smaller in the middle area than in the two end areas.

2. The uniaxial eccentric screw pump according to claim 1, wherein the opening in the stator has an elliptical profile with a ratio of a minor diameter to a major diameter being 0.83 to 0.96 inclusive.

3. The uniaxial eccentric screw pump according to claim 1, wherein the opening in the stator has a profile expressed with a single formula defining a condition to cause a smaller interference amount in the middle area than an opening having a profile with a shape of a racetrack including semicircles and straight lines.

4. The uniaxial eccentric screw pump according to claim 1, wherein the opening in the stator has a profile expressed with a single formula defining a condition to cause a larger interference amount in the two end areas than an opening having a profile with a shape of a racetrack including semicircles and straight lines.

5. The uniaxial eccentric screw pump according to claim 1, wherein the opening in the stator has a profile including, in the middle area, a portion with a first profile and including, in each of the two end areas, a portion with a second profile different from the first profile.

6. The uniaxial eccentric screw pump according to claim 5, wherein the opening in the stator has a profile including a part of a perfect circle.

7. The uniaxial eccentric screw pump according to claim 5, wherein the opening in the stator has a profile including a part of an ellipse.

8. The uniaxial eccentric screw pump according to claim 3, wherein the opening in the stator has, in the middle area, an outwardly curved profile.

9. The uniaxial eccentric screw pump according to claim 1, wherein the opening in the stator has a profile including at least one straight portion.

10. The uniaxial eccentric screw pump according to claim 9, wherein the at least one straight portion includes parallel straight portions facing each other in the middle area.

11. The uniaxial eccentric screw pump according to claim 1, wherein the stator consists of a stator body formed from an elastic material.

12. The uniaxial eccentric screw pump according to claim 3, wherein the opening in the stator has a profile including at least one straight portion.

13. The uniaxial eccentric screw pump according to claim 3, wherein the stator consists of a stator body formed from an elastic material.

14. The uniaxial eccentric screw pump according to claim 3, wherein the opening in the stator has a profile expressed with a single formula defining a condition to cause a larger interference amount in the two end areas than an opening having a profile with a shape of a racetrack including semicircles and straight lines.