US3208391A

US3208391A - Screw pump

Info

Publication number: US3208391A
Application number: US314024A
Authority: US
Inventors: Lindberg Gustav Rudolf
Original assignee: Flygts Pumpar AB
Current assignee: Flygts Pumpar AB
Priority date: 1963-04-23
Filing date: 1963-10-04
Publication date: 1965-09-28
Anticipated expiration: 1982-09-28
Also published as: AT247155B; ES298683A1; BE646869A; DK112423B; GB972420A; NL6404512A; FI40440B; CH412586A

Description

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Sept. 28, 1965 G. R. LINDBERG SCREW PUMP 6 Sheets-Sheet 1 Filed Oct. 4, 1963 ,gw A TTOPNEPS G R E B D m L R a Sept. 2, 195

m U P. w E R c S 6 Sheets-Sheet 2 Filed Oct. 4, 1963 //VV N TOE 61/5 7141/ PUDOZF UNDBHPG A TTOP/VEVS ca. R. LLLLLL RG 3,08,39'1

SSSSSSS MP Sept. 28 1965 G. R. LINDBERG SCREW PUMP 6 Sheets-Sheet 4 Filed Oct. 4, 1965 m m m w m WOLU QWW %N iOk $3 A TTOENEVS G R E B D m L R G Sept. 1%

m U P w E R C s 6 Sheets-Sheet 5 Filed Oct. 4, 1963 ATTOEA/EVS United States Patent 4 6 Claims. (Cl. 103-417 The present invention relates to screw pumps of the type incorporating a single rotor acting in conjunction with a fixed stator.

A known pump of this type is the one known as the Moineau pump (cf, for example, US. Patent 1,892,217), comprising a rotor with an external single helical thread, i.e., a thread with a single starting point, the said rotor revolving while simultaneously moving eccentrically in a fixed stator member having an internal double helical thread, i.e., a thread with two starting points 180 apart, the internal thread acting in conjunction with the rotor screw and having a pitch twice that of the rotor thread. The thread of the stator member is executed in some elastic material, generally rubber, the shape of the two pump members resulting in the formation of closed voids or pockets, which are fed axially through the pump by continous movement without pulsations.

Known pumps of this type have the disadvantage of being expensive and difficult to manufacture owing to the intricate shape of the helical members. A section cut at right angles to the axis through the stator of a Moineau pump will reveal at each point a pump-cavity profile of an oval shape comprised of two half circles at the ends of a rectangular section. The internal envelope surface of the pump cavity is thus generated by an oval as described being moved progressively along an axis while at the same time being rotated. However, the shape of an oval structure like this is unwieldy in a mathematical sense and it is difficult to manufacture a pump cavity of the said form of thread, which, in addition, will have comparatively sharp and thin thread tips that will be subjected to considerable wear. A similar section cut through the rotor, on the other hand, will reveal a very simple figure, viz., a circle, but taking into consideration that the object is a screw which at each point would have to display the same said sectional profile, unusual for screw-shaped bodies as it would be, it is obvious that for making the rotor it would be necessary to adopt special procedures and machinery designed for this particular purpose (of, for example, French Patent No. 846,489).

The present invention aims at producing a pump of the said general design the geometrical form of which can in its entirety be traced to, or generated by, a straight line, an ellipse and a cylindrical helix. The basic features of the invention is to be found in the replacement of the stator profile of the conventional Moineau pump, i.e., the two half circles connected by two parallel straight lines, by a mathematically easily defined ellipse, .a replacement that has led to far-reaching consequences as concerns simplified manufacturing methods, more reliable operation, and increased life.

The said advantages are gained and the above mentioned disadvantages are eliminated by the screw pump according to the present invention having been given the characteristics specified in the claims.

The invention is illustrated by the appended drawings and will be described in detail on the basis thereof:

FIG. 1 is a vertical longitudinal section through a screw pump according to the invention, giving a general representation of the basic features of the said pump.

3,2fi8,39l Patented Sept. 28, 1965 FIG. 2 is a longitudinal section through part of the said pump stator, showing its internal surface.

FIG. 2A is a sectional view taken along the line IIAIIA in FIG. 2.

FIG. 2B is a sectional view IIB-IIB in FIG. 2.

FIG. 3 is a lateral view of part of the said pump rotor, showing its external surface.

FIG. 4 is a longitudinal section, similar to FIG. 2, through the pump stator but including also the corresponding area of the pump rotor.

FIG. 4A is a sectional view taken along the line IVA-IVA in FIG. 4.

FIG. 5 shows one of the closed voids forming between the pump stator and rotor, and progressively moving in an axial direction during the rotation of the rotor.

FIG. 6 shows how the inner surface of the stator can be generated by a single helical line executing a certain planetary movement.

FIG. 7 shows part of the rotor including a co-ordinate system for determining the rotors external envelope surface.

FIG. 8 is a section cut along line VIII-VIII in FIG. 7.

FIG. 9 is a simplified representation of one way of producing the rotor.

FIG. 10 is a simplified representation of another way of achieving the same result.

The screw pump shown in FIG. 1 comprises a rotor 10 arranged in such a way as to revolve in a stator 12. The latter is generally made of rubber 14 or other elastic material supported by a surrounding external sleeve 16. The rotor which moves in an eccentric path within the stator is connected to its drive shaft 18, which is journalled in fixed bearings, by means of a suitable type of universal joint, e.g., by making the rotor hollow so as to receive in its internal cavity a short pivoted axle 20 at one end connected by a universial joint 22 to the driving shaft 18 and at its other end to the rotor by means of another universal joint 24, the latter connection for obvious reasons being tight with respect to the surrounding chamber.

The stator 12 is enclosed in an adequately formed external pump casting 26 having an inlet 30 and an outlet 28. Theoretically, the flow through the pump may be either way, depending on the direction of the rotation of the rotor, but in practice the pressure is generally applied at the packing-box end of the pump to avoid a vacuum there. The drive shaft 18 is mounted in a conventional way in a bearing arranged in an extension 32 to the pump casting 26, said extension also supporting the packing box 34 and other sealing devices.

So far the screw pump described is conventional and it should be understood that its external design and construction do not form part of the present invention.

The pumping action of the new pump will now be explained on the basis of FIGS. 25. FIG. 2 shows part of the stator 12 and FIG. 3 (immediately below) the corresponding part of the rotor 10, the latter being turned so as to make its profile correspond with the stator profile. If s denotes the pitch of the stator the pitch of the rotor will, consequently, be s/2. The figures show 3 turns of the rotor thread and 1 /2 of the stators. For the purpose of description the rotor and stator enveloping surfaces as shown are divided into 6 portions separated by 7 stations 1-7 indicated by the figures. FIG. 4 shows the combination of rotor and stator. Strictly geometrically the following conditions are then satisfied.

(1) The tip of thread of the rotor, i.e., the helical taken along the line r-r at each point must be forming a contact with the inner surface of the stator independent of the position of the rotor.

(2) The tips of thread g-g and hh of the stator at each point must be reaching, and establishing contact with, the rotor. It should be noted here that the contact with the rotor surface is. not by the very crest g-g and h-h but that the actual line of contact runs beside the crest line, intersecting with this. For the purpose of description and to simplify matters the crest lines of the stator surface, i.e., the two helical lines g-g and l1h connecting the points on the stator surface proximate to the axis here will be regarded as lines of contact.

Obviously, since the pitch of the rotor is only one-half of that of the stator the contact line r-r must of necessity intersect with the two other lines of contact gg and hh. In fact closed pockets or voids form between the lines, the said pockets progressing continuously in an axial direction during the rotation of the rotor.

These conditions are illustrated in FIG. 5, showing the mode of spreading of a void of the character described over 3 turns of the rotor thread and 1 /2 turns of the stators. The void, commencing at station 1 where the tip of rotor thread rr and a tip of stator thread gg (dash-dot line) intersect, broadens between the said lines of contact until between

stations

3 and 5. Here a new intersection occurs between rotor and stator thread tips and in its further course the void becomes enclosed between the other stator thread tip h-h (dotted line) and the rotor thread tip rr, yet on the other side of the latter, to finally become narrower when approaching station '7. Three axially displaced voids as described are illustrated and it is obvious from what has been said that the axial length of the rotor-stator system must be in excess of 1.5s.

Of the said sealing lines, g-g and hh on one hand and r-r on the other, the latter is of particular interest. Continuous sealing along the tip of the rotor thread in accordance with condition (1) as stated above is achieved by the present invention in the following manner:

The inner enveloping, or helical, surface of the stator is generated by an ellipse (see FIGURES 2A and 213) with axes 2a and 2b that moves along an axis perpendicular to the plane of the ellipse, said axis passing through the centre thereof, the said ellipse simultaneously executing a continuous turning movement about the said axis, the axial movement being directly proportional to the turning about the axis. Hence the previously mentioned pitch s of the helical surface corresponds to the axial displacement of the ellipse after turning 360. The geometrical form of the stator is, thus, determined with absolute certainty by measures a, b and s as indicated in FIGS. 2 and 4.

If the rotor is placed in the stator in accordance with FIG. 4 and it is assumed that its thread tip is a single cylindrical helix with a constant pitch of s/2. and that the said tip of thread always contacts the stator then it can easily be seen from FIG. 4 that the diameter of the helical line must be a+b (cf. measures indicated at stations 5 and 6). Further, the rotor will be eccentric in relation to the stator and it is easy to realize that the eccentricity e must equal cf., for example, station 3 in FIG. 4; radius of screw minus semi-minor axis b=distance between stator and rotor axis during which the rotor completes one revolution round its own axis while the said axis moves in a circular path with radius e as indicated by the arrows in FIGURE 4A.

According to the present invention it now happens surprisingly that independent of the position of the rotor Within the stator the tip of thread of the former will always be in contact with, or touch, the inner surface of the stator. This applies exactly and not only approximately.

To prove this we first have the general expression for the envelope surface of the stator as generated by an ellipse with axes 2a and 2b in the manner already described. To facilitate mathematical treatment co-ordinate axes are drawn in a sensible way, for example with the x and y axis in the initial stage coinciding with the major and minor axis, respectively, of the ellipse and with the z axis at right angles to that plane, i.e., coinciding with the stator axis. Cf. FIG. 4 where the co-ordinate system is to be found at station 2. In the initial stage the equation of the ellipse will be (parametric form) x=a cost y=b sin t (1) 2 0 After the ellipse having turned through the angle v an arbitrary point (x, y, z) will have the co-ordinates x=a cos t cos vb sin t sin a y=a cos t sin v-l-b sin t cos 1) (H) This system of equations defines the surface generated by the ellipse, i.e., the helical surface of the stator memher.

It will now be shown that an identical surface is generated by a cylindrical helix with the diameter a-I-b and pitch s/Z (a helical line corresponding to the tip of the rotor thread) revolving about its own axis which at the same time travels in the opposite direction in a circular path with the diameter 2e=a-b. Here, rotation is bound, i.e., if the rotor turns through a certain angle about its own axis this will travel through the same angle along the eccentric (i.e. rotor axis) circle (a natural consequence of the rigidity of the universal joint between driving shaft and rotor).

The helical line is now fitted into the same co-ordinate system as the stator member, meaning that its axis will intersect the x-y plane (station 2, FIG. 4) at the point A with its co-ordinates In this initial position the equation of the helical line will be (parametric form) y=- sin ti (III) After a rotation through the angle 5 we get cos (it-H) sin (14-1-5) (IV).

Hence, systems VI and II show complete coincidence, meaning that the two surfaces are identical and, further, that during its rotation within the stator the rotor will always seal against that member along the crest r-r of the rotor thread.

FIG. 6 shows the manner in which the helical line r-r generates the above mentioned envelope surface. The line is drawn to indicate the position after every 40 turn, visualizing its envelope, i.e., the surface thus generated which, as has just been proved, is identical with that generated by an ellipse screwed in a like manner.

As concerns the two other sealing lines gg and hh between stator and rotor conditions are more complicated as previously indicated and no attempt at analyzing their exact course will be made. It is necessary, however, to determine the envelope of the rotor in the area between the crests of the rotor threads. From FIG. 4 it is easy to see that the minimum radius of this surface (cf., for example, station 3) must be as the tip of the stator thread has to touch the rotor at its thread bottom where the rotor axis is proximate to the thread tip of the stator as clearly illustrated in FIG. 4. It has been found that the envelope surface of the rotary screw may be designed in various ways without losing its full sealing capacity, provided that the basic conditions stated are fulfilled i.e., the crest of the thread shall have a diameter of a a-l-b and a pitch of s/ 2 while the thread bottom shall have a radius of FIGS. 7 and 8 show the basic features of one way of generating the envelope of the rotor, a method that can easily and economically be adapted to quantity production of rotors.

Starting from the previously defined crest of the rotor thread r-r, i.e., a cylindrical helix of pitch s/Z and diam eter a+b, two points n, n located on adjacent turns of the helical line are connected with each other the relative positions of the said points being chosen so as to make the line n-n touch a cylinder coaxial with the said helical line and having a diameter of 3b-a (cf. FIG. 8), the tangent point thus representing a point at the rotor thread bottom. Now if this line is allowed to glide with its ends along the helical line, and hence continuously touching the said cylinder, this will generate a helicoid surface that lends itself very well to the purpose of the present invention, i.e., as an enveloping surface for the pump rotor. The condition prescribed is also fulfilled by a line m-m (cf. FIG. 7) which generates a very similar surface that can also be used for the said purpose. To establish the equation of the helicoid surface we 6 choose as before a co-ordinate system x, y, z, in this case letting the z axis coincide With the axis of the helical line. The equation of the said line is then (parametric form;

cf. Equation III) 5 b a x cos a b 11 sin ti (VIII) 10 i u 41r where u is a parameter representing the angle through which the radius of the helical line turns about the z axis from an initial position.

Let us suppose the parameter value u=v to yield point T in FIG. 8, i.e., a point on the helical line straight behind and opposite the point of intersection between n-n and m-m in FIG. 7. To obtain points n, n and m, m

we give the parameter the value via where 0: denotes a second-quadrant (points m, m) or third-quadrant (points n, n) angle, the angular distance between. T and either of points m, m being in the range, 90180 and the angul-ar distance between T and either of points n, n being within the range, ISO-270. If points m, m and n, 11 respectively, are connected and if (x, y, z) is a point on one of the lines so situated that it divides the said line in the proportion A:(l where 0A 1 (this condition excludes a continuation of the lines beyond points m and n, respectively) then we obtain This is the equation of the helicoid surface with v and A as parameters (a still remaining to be determined). However, the equation can be simplified considerably by changing parameters.

Let us first introduce parameter p with a variation range of l pl by putting x= g (c0s 1) cos a-l-p sin a sin a) y= (sin 1) cos 04- cos 1) sin a) (X) Let us then substitute 1 for p, putting t=p-tan a. The variation range of 1 will then be tan attan 0c.

Summing up W get g cos zx(COS v-l-t sin 0) y= g cos a(Si 1 v-t cos When r is minimum, however, lines m-m and n:n are bisected into equal parts, thus making /z. From Equation IX We then get COS 2) 005 a +1) (XIV) y= sin 1) cos a va +y cos a) (Xv) Note that a is in the second or third quadrant, hence the minus sign. From Equations XIII and XV we now obtain COS (I: a+b W a+b Hence c follows from the condition 3b-a COS 0a b The mathematical description of the enveloping surface of the rotor screw will thus be x= cos 0: (cos v+t sin 0) y= g cos a (sin v-t cos a) (XVIII) t a 2 4w tan a C08 3ba a a+b where t and v are parameters of which t has the variation range, tan u t tan a.

It was pointed out in the introductory part of this description that one advantage of the screw pump according to the invention is the simple manner in which it can be made. The stator is manufactured by first producing a core whose surface is identical with the inner enveloping surface of the stator. A rubber casting is then made round the core which is screwed away when the rubber has cured. Hence the core may be considered a tool which can be used for manufacturing a plurality of stators. When producing the core use is made of the above analyzed fact that the inner envelope of the stator and, consequently, also the outer surface of the core can be generated by a single helical line turning while simultaneously executing an eccentric movement. FIG. 6 suggests how a core as described may be produced. A blank is mounted in a combined longitudinal and circular feed device and is fed in such a way as to permit a cutting tool continuously to follow the single helical line rr with reference to the blank, a technique that can be easily applied. Allowing the cutting tool to travel along its path rr a great number of times, care being taken that the blank is each time advanced slightly in accordance with certain rules, the tool will in the end have elaborated a core characterized by the desired profile as can be seen in FIG. 6 assuming that the dense helical lines rr are mutually identical although in a specific way displaced working marks of the tool.

It is a matter of still greater importance, however, that the pump rotor according to the invention can be manufactured rapidly and employing simple means in contrast to the rotors of previously known pumps of the Moineau type. FIG. 7 suggests one way of manufacturing the rotor. A blank is mounted in a combined longitudinal and circular feed device of substantially the same type as the one used for machining stator blanks and is then treated in a shaper or planer the reciprocating cutting-off tool travelling in a path coincident with either of lines m-m or n--n while the blank is being advanced gradually by the feed device.

A better and more rapid method is illustrated in FIG. 9 according to which a blank 34, which is mounted in a longitudinal and circular feed device, is being machined by a rotating shank end mill 36 while at the same time being rotated and advanced the longitudinal feed being s/2 for each turn. It should be noted here that the helicoid surface obtained in this way and the one previously defined are not mathematically identical although they are very similar. For all practical purposes a quite satisfactory sealing effect is obtained between the tips of the stator thread or the immediately adjacent area, on one hand, and the surface produced as described, on the other.

FIG. 10 shows diagrammatically another method of producing the rotor, which closely resembles the method shown in FIG. 9. In this case the shank end mill 36 is replaced by an ordinary cutter or by a cylindrical cutter 38 of larger diameter the said cutter being set in such a way as to form an angle with the axis of the blank that equals the pitch angle of the rotor thread. Naturally, the longitudinal feed is 5/2 for each turn even in this case.

Practical tests have shown the pump according to the invention to work extremely well and the stator-rotor system has shown unusually little wear even after extended periods of operation. Pressures of the order of magnitude of 7-8 kp./crn. are rapidly built up in the pump and show no tendency of dropping as time goes on.

The above comparatively detailed description of the vital parts of the pump according to the invention, viz., its stator and rotor, has been made with the purpose of exemplifying, but not of limiting, the invention, and it is possible within the scope of the said invention to carry out certain modifications and changes of the subject matter, such modifications and changes being extensively provided for as can be seen from the appended claims.

I claim:

1. A screw pump comprising a pump casing with one inlet and one outlet, a stator with an internal double helical thread fixed within the pump casing between said inlet and said outlet, and a rotor with an external single helical thread the pitch of which is one-half that of the stator thread, said rotor being mounted in bearings for rotation within the stator in co-operation with the helical stator thread in such a way as to form closed voids between said rotor and said stator, said voids gradually and continuously moving axially from the inlet to the outlet while conveying the pumped medium, said rotor being driven by a driving shaft through the intermediary of a universal joint means so as to permit the rotor to execute a compound movement composed of a rotation about its own axis and an eccentric movement about the stator axis, the latter movement having the same angular velocity but in the opposite direction as said rotation, the internal enveloping surface of said stator being generated by an ellipse travelling along an axis perpendicular to the plane of said ellipse and passing through the centre thereof while simultaneously turning continuously about said axis, the rate of travel being directly proportional to the rate of turning.

2. A screw pump in accordance with claim 1 in which the pitch of the stator thread is equal to s and the elliptic cross-section has a major axis equal to Zn and a minor axis equal to 2b, wherein the internal envelope of the stator is analytically described by the following system of equations which relates to a rectangular co-ordinate system (x, y. z) so arranged as to make the z axis coincide with the longitudinal axis of said stator and having the x and y axes perpendicular thereto:

x=a cos t cos v-b sin tsin v y=a cos i sin v-I-b sin 25 cos where t and v are parameters.

3. A screw pump in accordance with claim 1, in which the major diameter of the rotor equals the sum of the lengths of the major and minor axes of the elliptic stator cross-section, and the minor diameter of the rotor equals one-and-a-half times the length of said minor axis minus one-half the length of said major axis.

4. A screw pump in accordance with claim 3 wherein the root surface of the rotor is generated by a straight line connecting two points on adjacent turns of the rotor thread, said points being so situated as to make said straight line touch said root surface of said rotor, said root surface being generated by said line as it moves along the tip of the rotor thread.

5. A screw pump in accordance with claim 4 wherein the external envelope of the rotor is described analytically by the following system of equations which relates to a rectangular co-ordinate system (x, y, z) so arranged as to make the z axis coincide with the longitudinal axis of said rotor and having the x and y axes perpendicular thereto:

cos a(sin v-t cos 1)) where t and v are parameters of 6. A screw pump comprising:

a pump casing having an inlet and an outlet spaced from each other;

a stator fixed within the pump casing between said inlet and said outlet, said stator having an internal double helical thread;

a rotor mounted within said stator for rotation about its own axis and for eccentric movement about the stator axis, said rotor having a single helical thread whose tip continuously contacts the internal surface of said stator, the threads of said stator also continuously contacting said rotor, the pitch of said rotor thread being one-half that of said stator thread so that when said rotor rotates closed voids are formed between said stator and said rotor and said voids gradually and continuously move axially from the inlet to the outlet for thereby conveying the medium being pumped;

a driving shaft and universal joint means connecting said driving shaft to said rotor so as to permit the rotor to execute a compound movement composed of a rotation about its own axis and an eccentric movement about the stator axis, the eccentric movement having the same angular velocity as the rotation of the rotor but being in the opposite direction;

the internal surface of said stator at any point along its length defining an internal opening of elliptical cross-section, the internal surface of said stator being generated by an ellipse traveling along an axis perpendicular to the plane of the ellipse and passing through the center thereof while simultaneously turning continuously about said axis, the rate of axial travel of the ellipse being directly proportional to the rate of turning, so that the angular position of the major axis of said internal opening with respect to the axis of the stator continuously and smoothly changes at a uniform rate from one axial end of the stator to the other axial end thereof.

No references cited.

DONLEY J. STOCKING, Primary Examiner. JOSEPH H Bn NsoN, JR., Examiner,

Claims

1. A SCREW PUMP COMPRISING A PUMP CASING WITH ONE INLET AND ONE OUTLET, A STATOR WITH AN INTERNAL DOULBE HELICAL THREAD FIXED WITHIN THE PUMP CASING BETWEEN SAID INLET AND SAID OUTLET, AND A ROTOR WITH AN EXTERNAL SIGNAL HELICAL THREAD THE PITCH OF WHICH IS ONE-HALF THAT OF THE STATOR THREAD, SAID ROTOR BEING MOUNTED IN BEARINGS FOR ROTATION WITHIN THE STATOR IN CO-OPERATION WITH THE HELICAL STATOR THREAD IN SUCH A WAY AS TO FORM CLOSED VIODS BETWEEN SAID ROTOR AND SAID STATOR, SAID VOIDS GRADUALLY AND CONTINUOUSLY MOVING AXIALLY FROM THE INLET TO THE OUTLET WHILE CONVEYING THE PUMPED MEDIUM, SAID ROTOR BEING DRIVEN BY A DRIVING SHAFT THROUGH THE INTERMEDIARY OF A UNIVERSAL JOINT MEANS SO AS TO PERMIT THE ROTOR TO EXECUTE A COMPOUND MOVEMENT COMPOSED OF A ROTATION ABOUT ITS OWN AXIS AND AN ECCENTRIC MOVEMENT ABOUT THE STATOR AXIS, THE LATTER MOVEMENT HAVING THE SAME ANGULAR VELOCITY BUT IN THE OPPOSITE DIRECTION AS SAID ROTATION, THE INTERNAL ENVELOPING SURFACE OF SAID STATOR BEING GENERATED BY AN ELLIPSE TRAVELLING ALONG AN AXIS PERPENDICUALR TO THE PLANE OF SAID ELLIPSE AND PASSING THROUGH THE CENTRE THEREOF WHILE SIMULTANEOUSLY TURNING CONTINUOUSLY ABOUT SAID AXIS, THE RATE OF TRAVEL BEING DIRECTLY PROPORTIONAL TO THE RATE OF TURNING.