GB2442477A - Rotary positive-displacement machine - Google Patents

Abstract

A machine comprises a casing 102 defining an enclosing wall spherical about a centre, a first rotor rotatable about a first axis 303 and a second rotor rotatable about a second axis 316. Each rotor has a circumferential face 302 conformed to its respective wall and a working face 305 with a plurality of vanes 306-308 intermeshing with those of the other rotor. The first and second axes intersect at the centre at a first angle whereby as the rotors rotate cavities 313-315 between the vanes alternately expand and contract admit and discharge fluid through inlet and outlets ports 104, 105. The rotor working face is defined with reference to a spline defined by a trace on the hemispherical envelope of one rotor of a point on the hemispherical envelope of the other rotor as the two rotors are rotated together through a first angle and a neutral interface surface is swept between the centre and the spline. Each vane comprises a first working surface that is a polar offset of the neutral interface surface. Vane tips have a lobular surface formed as a rotation of the end of the first working surface that is on the surface of the hemispherical envelope through a second angle 1606.

Description

Rotary Positive-Displacement Machine

Technical Field

The present invention relates to a rotary positive-displacement machine, in particular a pump.

Background of the Invention

Positive-displacement machines typically include two rotors within a housing, with either both driven or one driven and one free rotor. The rotors interlock and are in constant contact to create chambers that move fluid between an inlet port and an outlet port. In machines where both rotors are driven, any error of manufacture in either rotor causes the rotors to scrape together at certain points in the rotation. In machines where one rotor is free, pressure imbalances can cause the rotation to be uneven, again resulting in rotors scraping each other. This scraping causes wear on the rotors, and thus the chambers produced by the interlocking of the rotors are no longer sealed, with the result that the machine no longer functions efficiently. The only solution at that point is to replace the rotors, which is expensive and time-consuming.

Brief Summary of the Invention

According to an aspect of the present invention, there is provided a rotary positive-displacement machine comprising a casing defining a working space with an enclosing wall that is generally spherical about a centre, a first rotor rotatable about a first axis within a first part of the working space and a second rotor rotatable about a second axis within a second part of the working space, each said rotor having a circumferential face about its axis of rotation that is conformed to its respective part of the wall of the working space and a working face formed with a plurality of vanes that intermesh with those of the other rotor, the first axis and the second axis intersecting, at said centre, at a first angle, whereby as the rotors rotate cavities between the vanes alternately expand and contract, an inlet port to admit fluid to the working space where the cavities expand and an outlet port to discharge fluid from the working space where the cavities contract. The working face of each rotor is defined with reference to a spline defined by a trace on the hemispherical envelope of the rotor of a point on the hemispherical envelope of the other rotor as the Iwo rotors are rotated through a first angle, wherein a neutral interface surface is swept between the centre and the spline, and each vane is formed with a first working surface that is a polar offset of the neutral interface surface.

Brief Description of the Several Views of the Drawings Figure 1 illustrates a rotary positive-displacement pump; Figure 2 shows the pump illustrated in Figure 1 from the side; Figure 3 shows an internal view of the pump illustrated in Figure 1; Figure 4 is a side view of a rotor shown in Figure 2; Figure 5 is an isometric view of a rotor shown in Figure 2; Figure 6 illustrates the rotor system shown in Figure 3; Figure 7 shows the rotor system illustrated in Figure 6 with half of each rotor cut away; Figure 8 shows diagrammatically the first stage in the design of the rotors shown in Figure 2; Figure 9 illustrates the trace of a point shown in Figure 8; Figure 10 shows the equations defining the trace illustrated in Figure 9; Figure 11 shows a spline that is an offset of the trace shown in Figure 9; Figure 12 shows how a second spline is defined; Figure 13 shows how a cavity closure curve is defined; Figure 14 illustrates the surfaces defined by the curves shown in Figure 13; Figure 15 shows a point about which another point is rotated to create a lobe curve; Figure 16 shows how a lobe curve is defined; Figure 17 illustrates an alternative rotor; and Figure 18 shows the rotor illustrated in Figure 17 meshing with an identical rotor to create an alternative rotor system.

Description of the Best Mode for Carrying out the Invention Figure 1 A rotary positive-displacement machine, in this example a pump 101, is shown in Figure 1. The housing 102 of the pump includes a spherical casing 103, a cylindrical inlet port 104, a cylindrical outlet port 105, an open-ended cylindrical housing 106 through which a first rotor driveshaft 107 passes into casing 103, and a second closed-ended cylindrical housing 108 that houses a second rotor driveshaft.

In use, a pipe is attached to each of inlet port 104 and outlet port 105 and first rotor driveshaft 107 is caused to turn. Fluid is then moved from the inlet port 104 through the casing 103 to the outlet port 105.

Figure 2 Pump 101 is shown from the side in Figure 2. Casing 103, inlet port 104, first rotor driveshaft 107, first driveshaft housing 106 and second driveshaft housing 108 can be seen from this angle. A drive rotor 201 and a free rotor 202 can be seen within casing 103 through inlet port 104.

Figure 3 Figure 3 illustrates pump 101 with half the housing 102 removed. Thus casing 103, inlet port 104, outlet port 105, first driveshaft housing 106 and second driveshaft housing 108 are shown in cross-section along the line 203 in Figure 2. Casing 103 encloses a substantially spherical working space 301 in which are housed first rotor 201 and second rotor 202.

Drive rotor 201 has a circumferential face 302 about its axis of rotation 303 that is a partial sphere and thus conforms to the wall of the working space 301. The centre point of rotor 201 is defined as the centre point of the partial sphere, ie the point which is equidistant from every point on the circumferential face 302. Rotor 201 further has a nonworking face 304 that is substantially flat and to which first rotor driveshaft 107 is perpendicularly attached, and a working face 305 formed with a plurality of substantially identical vanes, which in this example is six. For example, drive rotor 201 includes vanes 306, 307 and 308. Between each pair of vanes is a cavity, such as cavities 309, 310 and 311.

Free rotor 202 is substantially identical to first rotor 201. It is mounted within casing 103 such that its driveshaft 312 is housed within housing 108 and such that its vanes mesh with the cavities of drive rotor 201 and vice versa.

Each meshing vane-cavity pair forms a chamber, such as chambers 313, 314 and 315. The axis of rotation 316 of free rotor 202 is at an angle 317 to the axis of rotation 303 of drive rotor 201. In this example angle 317 is 15 , with free rotor 202 inclined towards outlet port 105. Thus the chambers between the rotors are smallest at outlet port 105 and largest at inlet port 104.

When driveshaft 107 is caused to turn around axis of rotation 303, drive rotor 201 turns also. Driveshaft 312 is freely mounted within housing 108 and thus free rotor 202 turns around axis of rotation 316 at the same speed and in the same direction as drive rotor 201. Because the axes of rotation 303 and 316 are at an angle to each other, the chambers between the rotors expand and contract as the rotors turn. Fluid is taken into a large chamber at inlet port 104, and expelled from a small chamber at outlet port 105. This creates a pumping action.

This type of two-rotor system can be used in other applications, for example as an engine, turbine or flow meter.

Figure 4 A side view of drive rotor 201 is shown in Figure 4. Rotor 201 comprises six vanes and six cavities between the vanes. Each vane includes two lobes; for example vane 308 includes lobes 401 and 402.

Each vane and cavity is defined by a number of surfaces, each of which is defined by sweeping a line that extends from the centre point of the rotor 201 to the partially spherical circumferential face 302. For example, vane 308 consists of two working surfaces 403 and 404, one on each side of the vane, a lobular surface 405 adjacent to working surface 406 that defines lobe 401, a lobular surface 406 adjacent to working surface 404 that defines lobe 402, and an interlobular surface 407 that joins lobular surfaces 405 and 406.

Similarly each cavity is defined by a working surface of each adjacent vane and a further surface. For example, cavity 310 is defined by the working surface 403 of vane 308, by working surface 408 of vane 307, and by a cavity closure surface 409.

Figure 5 Drive rotor 201 is again shown in Figure 5. Here the six substantially identical vanes 306, 307, 308, 501, 502 and 503 are seen, along with the six substantially identical cavities defined by the vanes and cavity closure surfaces, namely cavities 309, 310, 311, 504, 505 and 506.

At the centre of the rotor is a partially spherical cavity 507 configured to house a spherical bearing.

Figure 6 Figure 6 shows the rotor system comprising drive rotor 201 and free rotor 202. The partially spherical shape of the circumferential face 302 of drive rotor 201 and the circumferential face 601 of free rotor 202 can be seen in this Figure. This partially spherical shape ensures that each rotor can be mounted at any angle within casing 103, provided that the centre point of each rotor (defined as the point that is equidistant from every point on the circumferential face) is located at the centre point of the spherical working space 301. As the rotors turn, in this example in the direction indicated by arrows 611 and 612, the two circumferential faces constantly define a partial sphere.

The working space 301 defined by casing 103 does not need to be entirely spherical. For example, the casing could follow the contours of the non-working faces of the rotors to provide a working space that completely encloses the meshed rotors, rather than leaving space free around the driveshafts as in the present example, as long as the working space that interacts with the circumferential faces of the rotors is partially spherical.

Further, the shape of the non-working face of either rotor can be any shape. In this example it is flat and perpendicular to the axis of rotation, but in other embodiments it could be a partial sphere, continuing the contour of the circumferential face, or any other shape that can fit inside an appropriately-shaped housing.

Each vane-cavity pair defines a chamber. For example, vane 602 of free rotor 202 meshes with cavity 310 of rotor 201 to form chamber 314.

Chamber 314 is bounded on the forward side (in the direction of rotation) by the interface between lobe 603 of vane 601 of free rotor 202 and working surface 403 of vane 308 of drive rotor 201. On the trailing side, chamber 314 is bounded by the interface between lobe 604 of vane 307 of drive rotor 201 and working surface 605 of vane 602 of free rotor 202. In order to keep chamber 314 closed, these interfaces must continually exist as the rotors rotate towards the point at which the chamber is fully expanded.

Once the chamber is expanded it starts to contract again, and the bounding interfaces become the interfaces between the lobes and working surfaces that are not touching as the chamber expands; that is, the forward interface is between lobe 401 of vane 308 and working surface 606 of vane 602, and the trailing interface is between lobe 607 of vane 602 and working surface 408 of vane 307.

In order to ensure that the chambers are constantly closed, the lobes and working surfaces that form the interfaces must be constantly in contact.

This is ensured by the shape of the working surfaces. However, the interfaces between the rotors are actually separated by a fluid film which lubricates the rotors. Because the separation is minimal no fluid passes between the chambers. Further, because rotor 202 is free it is held apart from drive rotor 201 by the film and thus does not scrape against rotor 201 at any point.

However, in an alternative embodiment both rotors could be driven.

Figure 7 Figure 7 shows the rotor system of Figure 6 with each rotor cut down its axis of rotation. Spherical bearing 701 fits into a partially spherical cavity in the centre of each rotor, such as cavity 507. Bearing 701 has the same centre point as each of the rotors. Thus when rotors 201 and 202 rotate about their respective axes of rotation they are held in place by bearing 701.

Figure 8 The shape of a working surface of a vane on, for example, the free rotor 202 is defined by the trace that a point on the edge of the drive rotor 201 makes on the free rotor 202 as the rotors turn about their respective axes of rotation 303 and 316. The design of the rotors is shown conceptually in Figure 8. Two hemispheres 801 and 802 with equal radius 803 are considered to be overlapping in space, with their centre points together at point 804. The axis of rotation 805 of the first hemisphere 801 and the axis of rotation 806 of the second hemisphere 802 are at an angle 807 to each other. In this example the angle 807 is 15 . Hemisphere 801 is the hemispherical envelopes of rotor 801, with the circumferential face 302 rotor 201 being a portion of hemisphere 801 and the centre of rotor 801 being the centre of hemisphere 801. Similarly, hemisphere 802 is the hemispherical envelope of free rotor 802.

A point 808 is marked on the surface of hemisphere 801. This point is at an angle 809 to an line 810 which is normal to the axis of rotation 805 of hemisphere 801. Angle 809 is equal to angle 807 and thus the point 808 is positioned on the edge of hemisphere 802.

As hemispheres 801 and 802 turn in the same direction about their respective axes of rotation 805 and 806, the point 808, fixed on hemisphere 801, will move with respect to hemisphere 802 and conceptually draw a trace on the surface of hemisphere 802.

Figure 9 The trace 901 drawn on the surface of hemisphere 802 is shown in Figure 9. The extent of the trace is dependent upon the angle 902 through which hemispheres 801 and 802 are rotated, which in this example is 190 .

This angle has been shown to be effective.

Figure 10 The equations defining the shape of trace 901 are shown in Figure 10.

The parameters are the radius 803 of hemisphere 801 (equal to the radius of hemisphere 802), angle 806 (the angle between the axes of rotation 805 and 806) and angle 902 (the angle of rotation of the hemispheres to create trace 901). The equations define the position of a point in three dimensions for any value of an angle 1001, but in fact the angle 1001 is limited to between zero and the angle of rotation 807.

The point's position in the first dimension is given by equation 1002 which is the product of the hemispheres' radius 803 and the sum of the following: the product of the squared cosine of angle 807, the cosine of half of angle 1001 and the cosine of angle 1001; the product of the cosine of angle 807, the sine of half of angle 1001 and the sine of angle 1001; the cosine of half of angle 1001; and the product of minus one, the cosine of half of angle 1001 and the squared cosine of angle 807.

The point's position in the second dimension is given by equation 1003 which is the product of minus one, the hemispheres' radius 803, the sine of angle 807 and the sum of the following: the product of the cosine of angle 807, the cosine of half of angle 1001 and the cosine of angle 1001; the sine of half of angIe 1001 and the sine of angle 1001; and the product of minus one, the cosine of half of angle 1001 and the cosine of angle 807.

The point's position in the third dimension is given by equation 1004 which is the product of the hemispheres' radius 803 and the sum of the following: the product of the cosine of angle 807, the cosine of half of angle 1001 and the sine of angle 1001; and the product of minus one, the sine of half of angle 1001 and the cosine of angle 1001.

Figure 11 Spline 901 is defined in three dimensions by equations 10011 1002 and 1003. The actual working surface of a vane, however, is defined by spline 1101 which is a polar offset on the surface of hemisphere 802 of spline 901 by an angIe 1102, which in this example is 2 .

Figure 12 In order to create the opposing working surface of a cavity, spline 1101 is mirrored and then rotated about the axis of rotation 806 of hemisphere 802, in the same direction that the hemispheres were rotated to create spline 902, by an angle 1201, to create spline 1202. Angle 1201 is calculated as shown in equation 1203, by subtracting the polar offset angle 1102 from the result of dividing 360 by the number of vanes required, which in this example is six.

Thus in this example angle 1201 is 60-2=58.

Figure 13 The cavity closure curve 1301 is tangential to splines 901 and 1202. In this example, cavity closure curve 1301 is defined as follows. Spline 1101 has a tangent line 1302 at its final point 1303, and similarly spline 1202 has a tangent line 1304 at its final point 1305. Cavity closure curve 1301 is defined as a Bezier curve that has point 1303 and tangent line 1302 at one end, and point 1305 and tangent line 1304 at the other end, with tangent handles of equal length. Experimentation shows that tangent handles having a magnitude equal to the reciprocal of the number of vanes are suitable, but other values are possible. The depth of the cavity closure line can vary substantially.

Other methods of calculating a cavity closure line that is tangential to splines 1101 and 1202 are envisaged.

Figure 14 The surfaces of the cavity thus defined are shown in Figure 14. Working surface 1401 is swept between the centre 803 of hemisphere 802 and spline 1101, working surface 1402 is swept between the centre 803 of hemisphere 802 and spline 1202, and cavity closure surface 1403 is swept between the centre 803 of hemisphere 802 and spline cavity closure curve 1301.

Since spline 1101 is a polar offset of trace 902, working surface 1401 is a polar offset by the same angle of a neutral interface surface (not shown) that is swept between the centre 803 of hemisphere 802 and trace 902.

Figure 15 In order to define the lobe of the vane that has a working surface defined by spline 1101, the initial point 1501 of spline 1101 is rotated anticlockwise about an axis through centre point 803 of hemisphere 802 and point 1502, which is a rotation about axis 806 by an angle 1503 from point 1501. AngIe 1503 is equal to offset angle 1102.

Figure 16 The lobe curve 1601 thus created is outside hemisphere 802 and therefore the shape is shown on the surface of a sphere 1602 in Figure 15.

The angle 1603 through which point 1501 is rotated is in this example 1300, which has been shown to work well.

A mirrored lobe curve 1604 is created for spline 1202. The entire curve 1605, consisting of two splines 1101 and 1202, cavity closure curve 1301 and two lobe curves 1601 and 1604, is patterned about the sphere the required number of times, which in this example is six. This can be considered as performing the calculations detailed with respect to Figures 8 to 15 for six points around the hemisphere 801, each offset from the previous one by 60 .

Thus each vane has a working surface that is defined with reference to a spline traced on the hemispherical envelope of the rotor by a point on the hemispherical envelope of the other rotor as the two rotors are rotated through a second angle.

Interlobular curves are created in a similar way to cavity closure curve 1301. Thus each interlobular curve is defined by creating a curve between endpoints of facing lobe curves that is tangential to the ends of the lobe curves. In order for the rotors to intermesh, the interlobular curve should not be shallower than the cavity closure curve 1301.

Similarly to the working surfaces and cavity closure surfaces, each lobular surface is swept between the centre of the sphere and a lobe curve, and each interlobular surface is swept between the centre of the sphere and an interlobular curve. All of the surfaces may then be offset with respect to the centre of the sphere by a small amount to allow a fluid film clearance between the rotors. This amount is half the required fluid film clearance.

Finally, the partially spherical cavity 507 is defined to complete the rotor surfaces. The definitions of all these surfaces can then be used in a suitable apparatus, such as a computer-controlled lathe, to produce drive rotor 201 and free rotor 202.

Figures l7and 18 Figure 17 shows an example of an alternative rotor 1701 that has three vanes instead of six. The design parameter that is altered to create this rotor is angle 1201, which is calculated by subtracting the polar offset angle from the result of dividing 360 by the number of vanes required. Thus in this example angle 1201 is 120-2=118. Further, once the entire curve 1605 is defined it is only patterned three times around the sphere. Rotor 1701 intermeshes with identical rotor 1801 as shown in Figure 18.

In this way rotors of any number of vanes that intermesh as required to produce a rotary positive-displacement machine can be created.

Claims (8)

1. Claims 1. A rotary positive-displacement machine comprising: a casing

   defining a working space with an enclosing wall that is generally spherical about a centre; a first rotor rotatable about an axis within a first part of the working space and a second rotor rotatable about a second axis within a second part of the working space, each said rotor having a circumferential face about its axis of rotation that is conformed to its respective part of the wall of the working space and a working face formed with a plurality of vanes that intermesh with those of the other rotor, and the first axis and the second axis intersecting, at said centre, at a first angle, whereby as the rotors rotate cavities between the vanes alternatively expand and contract; and an inlet port to admit fluid to the working space where the cavities expand and an outlet port to discharge fluid from the working space where the cavities contract; wherein said working face of each rotor is defined with reference to a spline defined by a trace on the hemispherical envelope of the rotor of a point on the hemispherical envelope of the other rotor as the two rotors are rotated through a first angle, wherein a neutral interface surface is swept between said centre and said spline; and each said vane is formed with a first working surface that is a polar offset of said neutral interface surface.
2. 2. A rotary positive-displacement machine as claimed in claim 1, wherein for each of said first working surfaces, the opposing working surface on the adjacent vane is a mirror image of said first working surface rotated about the axis of the rotor by a second angle.
3. 3. A rotary positive-displacement machine as claimed in claim 2, wherein the second angle is equal to the polar offset subtracted from the result of dividing 360 by the number of vanes on the rotor.
4. 4. A rotary positive-displacement machine as claimed in either of claims 3 or 4, wherein a cavity between a first vane and a second vane adjacent to said first vane is defined by the first working surface of said first vane, the opposing working surface on the second vane, and a cavity closure surface tangential to the working surfaces.
5. 5. A rotary positive-displacement machine as claimed in any preceding claim, wherein the polar offset is approximately 2 .
6. 6. A rotary positive-displacement machine as claimed in any of claims I to 5, wherein the first angle is approximately 190 .
7. 7. A rotary positive-displacement machine as claimed in any preceding claim, wherein said machine is a pump.
8. 8. A rotary positive-displacement machine as claimed in claim 7, wherein the first rotor is driven and the second rotor is free.