GB2401401A

GB2401401A - Three rotor screw pump with smaller central rotor

Info

Publication number: GB2401401A
Application number: GB0310592A
Authority: GB
Inventors: Robert William Beaven
Original assignee: Automotive Motion Technology Ltd
Current assignee: Automotive Motion Technology Ltd
Priority date: 2003-05-08
Filing date: 2003-05-08
Publication date: 2004-11-10
Also published as: GB0310592D0

Abstract

A screw pump including three rotors 12, 14a, 14b each being provided with a generally helical screw thread 20, 20', 26a, 26a' 26b, 26b' with the rotors arranged such that a central rotor 12 is located between the other two outer rotors 14a, 14b and the screw threads mesh with rotation of one rotor causing rotation of the others. The thread 20, 20' of the central rotor is a helical groove extending radially inwardly of the central rotor and the threads of the outer rotors are a helical ridge extending radially outwardly of the rotor and the outer diameter of the central rotor is smaller than the outer diameter of the outer rotors. Provides two fluid delivery chambers between the pump housing and the outer rotors to increase output. Thread pitch and depth may be respectively less than 1.6 to 0.5 and 0.2-0.1 times outer rotor diameter.

Description

Title: Pump

Description of Invention

The present invention relates to a pump, more particularly to a pump in which pumping is effected by means of at least two intermeshing screw threads, i.e. an intermeshing screw pump.

Pumps in which the pumped fluid is carried between the screw threads on one or more rotors such that the liquid is displaced in a direction generally parallel to the axis of rotation of the or each rotor, are known, and are generally referred to as screw pumps.

Where more than one rotor is provided, the pump is generally known as an intermeshing screw pump. In this case, one rotor is provided with one or more helical grooves and another rotor is provided with one or more corresponding helical ridges. One of the rotors (the power rotor) is driven by a motor, which when activated causes the power rotor to rotate along its longitudinal axis. The rotors are mounted in a housing such that their helical screw threads mesh and rotation of the power rotor causes the other rotor or À À. rotors (the idler rotor or rotors) to rotate about its/their longitudinal axis or axes. À e.

Ale. Fluid is drawn into the pump at an inlet or suction end of the pump À.e between the counter-rotating screw threads. As the rotors turn, the meshing of À. À

À the threads produces fluid chambers bounded by the threads and the pump Àe À housing. Fluid becomes trapped in the fluid chambers and continued rotation À of the screws causes the fluid chambers to move from the inlet end of the pump À. . : to the high pressure outlet end of the pump. Fluid is ejected from the pump at the outlet end as fluid is displaced from the fluid chambers.

It is known to provide intermeshing screw pumps with three rotors, a central rotor (usually the power rotor) being arranged between the other two outer rotors (usually the idler rotors). In such conventional intermeshing screw pumps, the central rotor is generally cylindrical with the or each thread forming a helical ridge which extends around and radially outwardly of the rotor along substantially the entire length of the rotor. The outer rotors are, in this case, also generally cylindrical with the or each thread forming a helical groove which extends around and radially inwardly of the rotor along substantially the entire length of the rotor.

The main fluid chambers are thus formed between the thread(s) of the central rotor and the pump housing, and thus the central rotor acts as the main fluid carrying element.

According to a first aspect of the invention, we provide a pump including three rotors each being provided with a generally helical screw thread, the rotors being arranged such that a central rotor is located between the other two outer rotors and the screw threads mesh such that rotation of one rotor causes rotation of the other rotors, wherein the thread of the central rotor is a generally helical groove which extends radially inwardly of the central rotor, and the thread of the outer rotors is a generally helical ridge which extends radially outwardly of the rotorand the outer diameter of the central rotor is smaller than the outer diameter of the outer rotors.

Thus, in a pump according to the invention, the main fluid chambers are Àe

formed between the thread or threads of the outer rotors and the pump housing, À...DTD: and as there are two such rotors, there are twice as many main fluid carrying À À chambers as in a conventional screw pump. Thus, by virtue of the invention, À e.

the volume output of the pump may be increased. À

A: Whilst the volume output of the pump may be increased by increasing À. .: the thread depth, as this also increases the volume of the main fluid carrying chambers, this has been found to have an adverse effect on the volumetric efficiency of the pump. By virtue of the invention, for a given pump speed, the volume output of the pump may be increased whilst retaining satisfactory volumekic efficiency.

Moreover, since the rotors are arranged side by side, the number of main fluid carrying chambers may be doubled, and hence the volume output of the pump increased, without increasing the length of the pump. Reduction of the central rotor outer diameter relative to the outer diameter of the outer rotors reduces the overall diameter of the pump, and thus a pump assembly according to the first aspect of the invention is particularly compact.

The pitch of the threads may be less than 1.6 times the outer diameter of the outer rotors.

In known ntermeshing screw pumps, the pitch of the threads, i.e. the axial distance between corresponding points on adjacent turns of the thread, is typically twice the outer diameter of the main fluid carrying rotor, and may be up to 2.4 times the outer diameter of this rotor. Thus, for a given pump length, more fluid chambers are formed in a pump according to the invention than in a conventional pump, i.e. for a given number of fluid chambers, a pump according to the invention is shorter than a conventional pump. Since the pressure of fluid output from an intermeshing screw pump depends on the number of fluid chambers formed by the screw threads of the rotors, reduction of the pitch also permits a further reduction in pump length for a given pressure compared to a conventional pump.

Preferably the pitch of the threads is less than the outer diameter of the À À À..

outer rotors, and may, for example, be 0.8 times the outer diameter of the outer À:.

rotors. À..

. :Preferably the pitch of the threads is at least 0.5 times the outer diameter À À :of the outer rotors.

Preferably the thread depth of the screw threads is less than 0.2 times the outer diameter of the outer rotors.

In conventional screw pumps, the thread depth of the screw threads is greater than 0.2 times the diameter of the main fluid carrying rotor. Whilst, decreasing the thread depth decreases the volume of each fluid chamber, and thus tends to decrease the volume output of the pump, use of a reduced thread depth has particular advantages.

One advantage of reducing the thread depth is that decreasing the thread depth also decreases the area of leakage paths which permit leakage of fluid from the fluid chambers, and thus reduces leakage from the fluid chambers and hence increases the volumetric efficiency of the pump for a given output flow rate. In addition, for a given rotor root diameter (the rotor outer diameter minus twice the thread depth), the overall diameter of a pump according to the invention may be reduced. Rotors with threads of lower depth are also easier and thus less expensive to machine. Thus, a more compact and more efficient pump may be produced at reduced manufacturing cost.

Any reduction in output volume may be compensated for by increasing the speed of rotation of the rotors.

Preferably the thread depth of the screw threads is less than 0.175 times the outer diameter of the outer rotors.

The thread depth of the screw threads may be less than 0.15 times the outer diameter of the outer rotors. À. .

Preferably the thread depth of the screw threads is at least 0.1 times the outer diameter of the outer rotors.

Preferably each rotor is provided with two generally helical screw À: threads. À..

À : Preferably the central rotor is connected to a driving means operation of À À À: which causes rotation of the central rotor about a longitudinal axis of the rotor.

An embodiment of the invention will now be described with reference to andlor as shown in the accompanying drawings in which: FIGURE 1 is a side sectional illustrative view of a pump according to the invention; FIGURE 2 is an enlarged illustrative view of the rotors of the pump of Figure 1, the rotors being arranged in an inoperative position, side by side; FIGURE 3 is an illustrative and cross-sectional view through the rotors and housing of the pump shown in Figure 1.

Referring now to the figures, there is shown a pump 10 including a central rotor 12 and two outer rotors 14a, 14b, all mounted on either side of the central rotor 12 for rotation about their longitudinal axes in a housing 16. In this example the central rotor 12 is connected to a driving means, by means of a drive shaft which is supported in a bearing assembly 28. In this case the driving means is an electric motor (not shown), which when activated, causes the central rotor 12 to rotate about its longitudinal axis A. The central rotor 12 will thus hereinafter be referred to as the power rotor, and the outer rotors 14a, 14b as idler rotors.

Each rotor 12, 14a, 14b is provided with a generally helical screw thread, and the rotors 12, 14a, 14b are arranged in the housing 16, with the power rotor 12 between the two idler rotors 14a, 14b, such that the screw threads mesh.

The longitudinal axes A, B and C of the rotors 12, 14_ are generally all parallel, À:. and thus rotation of the power screw about axis A causes the idler rotors 14a, Àe À . 14b to rotate about their longitudinal axes, B and C respectively.

In this example, the rotors 12, 14a, 14b are all provided with two À . À À..

generally helical threads or flights which each extend along substantially the À: entire length of the rotor 12, 14a, 14b, and which are interposed such that when Àe : the rotor 12, 14a, 14b is viewed in transverse cross-section, as shown in Figure À.

À 3, one thread is diametrically opposite the other.

The power rotor 12 has the shape of a generally cylindrical shaft 22 with the threads 20, 20', in the form of two generally helical grooves, extending radially inwardly into the shaft 22. The idler rotors 14a, 14b each have the shape of a generally cylindrical shaft 24a, 24b with the threads 26a, 26a', 26b, 26b', in the form of two generally helical ridges, extending radially outwardly of each shaft 24a, 24b.

The outer diameter OD of the power rotor 12 is smaller than the outer diameter OD of the idler rotors 14a, 14b. Typically, the outer diameter OD of the idler rotors 14a, 14b are 1.2 times the outer diameter OD of the power rotor 12. For example, for idler rotor 14a, 14b outer diameters of the order of 10mm, the power rotor 12 outer diameter OD is of the order of 7mm.

The power rotor 12 is mounted in a bearing provided in the pump housing 16.

An inlet port (not shown) is provided in the pump housing 16 adjacent a first end of the rotors 12, 14a, 14b and an outlet port 30 is provided in the pump housing 16 adjacent asecond,oppositeendofthe rotors 12, 14a, 14b.

The pump is operated as follows.

The motor is activated to cause rotation of the power rotor 12 about axis A, which in turn causes rotation of the idler rotors 14a, 14b in the housing 16 about axes B and C respectively. Fluid is drawn into the inlet 28 between the threads 20, 20', 26a, 26a', 26b, 26b' at the first ends of the rotors. As the rotors turn, the meshing of the threads produces main fluid chambers bounded by the thread roots R and the thread flanks F of the two idler rotors 14a, 14b and the . pump housing 16. Fluid becomes trapped in the fluid chambers and continued À..

rotation of the screws causes the fluid chambers to move from the first end of Àe À À the rotors 12, 14a, 14b to the second end of the rotors 12, 14a, 14b. Fluid is À..

À ejected from the pump 10 via the outlet port 30 as a consequence of fluid being displaced from the fluid chambers as the screw threads at the second end of the À.

À. .: rotors 12, 14a, 14b mesh.

Thus, fluid is drawn into and ejected from the pump 10 via two fluid chambers at any one time.

In contrast, in a conventional screw pump, the threads 20, 20' of the power rotor 12 are formed by two helical ridges, whereas the threads 26a, 26a', 26b, 26b' of the idler rotors 14a, 1 4b are formed by two helical grooves. In this case, the main fluid chamber is formed between the thread roots and thread flanks of the power rotor 12 and the pump housing 16, and thus only one main fluid chamber is available at any one time to draw fluid into and eject fluid from the pump 10.

The pressure of fluid output from the pump 10 increases with the increased number of main fluid chambers, and the provision of large diameter idler rotors 14a, 14b, further increases the volume of the fluid chambers which also increases the volume output of the pump 10. It is therefore possible to produce a pump which operates at the same pressure and volume output as a conventional pump, but which has shorter rotors. Thus the space occupied by the pump 10 is reduced.

Thus the pump 10 can be used where high output pressure is required and space is restricted, such as in automotive applications, for example in an electrically operated power pack in which the pump is activated to produce ressurised fluid and the pressurized fluid is used to move an actuator member.

Such an electrically powered power pack may be required for applications such as power steering.

It is advantageous to use a screw pump in such applications as screw pumps are relatively quiet compared with vane and gear pumps, for examples, and require only a relatively small motor in order to run at the high speeds, e.g. over 7,500 rpm, required to produce the fluid volume output needed for such I: applications.

À À ' .: The provision of a smaller pump 10 also has a further advantage that less material is required to manufacture the pump 10, and thus the cost of the unit is reduced.

The provision of a smaller diameter power rotor 12 has a further advantage that forces exerted on the bearing by the power rotor 12 as a result of fluid pressure within the pump 10 are reduced. Reduction of the forces on the bearing is desirable as it reduces energy losses as a result of frictional forces between the bearing and the power rotor 12, and reduces wear on the bearing, thus increasing the life of the bearing.

The pitch of each thread 20, 20', 26a, 26a', 26b, 26b', i.e. the distance between corresponding points on adjacent loops of the thread, marked as P on Figure 2, is less than 1.6 times the outer diameter of the outer rotors 14_, 14b, marked as OD in Figure 3, and is preferably less than the outer diameter OD of the outer rotors 14_, 14b, but at least 0.5 times the outer diameter OD of the outer rotors 14_, 14_.

For example, for a outer rotor outer diameter OD of 9mm, the pitch P of the threads 20, 20', 26a, 26a', 26b, 26b' is typically from 7 up to 9mm.

The depth of each thread 20, 20', 26a, 26a', 26b, 26b', marked on Figure 3 as TD, is less than 0.2 times the outer diameter of the outer rotors 14a, 14_.

In this example, the outer diameter OD of the outer rotors 14a, 14b are 9mm and the thread depth TD is between 1.4 and 1.7mm inclusive.

In known intermeshing screw pumps, the pitch P of the threads 20, 20', 26a, 26a', 26b, 26b' is typically twice the outer diameter OD of the power rotor ., 12, and may be up to 2.4 times the outer diameter OD of the power rotor 12, whereas the thread depth TD is 0.2 times the outer diameter OD of the power À rotor 12.

Thus, for a given pump length, more fluid chambers are formed in a Àe À : pump 10 according to the invention than in a conventional pump, or, put . ' another way, for a given number of fluid chambers, the pump 10 is shorter than a conventional pump. Since the pressure of fluid output from an intermeshing screw pump 10 depends on the number of fluid chambers formed by the screw threads 20, 20', 26a, 26a', 26b, 26b' of the rotors 12, 14a, 14b, for a given pressure, the pump 10 may be shorter than a conventional pump.

Moreover, since the thread depth TD is lower than for a conventional pump for a given rotor 12, 14a, 14_ root diameter RD, the overall pump diameter may be smaller than for a conventional pump.

The reduction in thread depth TD described above does have a consequence of reducing the volume of each fluid chamber in the pump 10, which in turn reduces the volume output of the pump, but this can be compensated for by increasing the speed of operation of the pump.

Use of the screw thread form described above also improves the efficiency of the pump 10. A screw pump using a conventional thread form which was scaled down to produce a pump of the same dimensions as a pump I lO according to the invention, had a relatively low efficiency (less than 20%), whereas relatively high efficiency (greater than 60%) has been achieved using the screw thread form described above.

During operation of the pump 10 leakage of fluid from the fluid chambers occurs along leakage paths between the flanks F of the meshing threads 20, 20', 26a, 26a', 26b, 26b', and between the exterior surfaces of the I rotors 20, 14a, 14b and the housing 16 or the thread roots R. Such leakage reduces the efficiency of the pump 10.

À . Reduction of the thread depth TD reduces the size of the leakage path between the flanks F of meshing threads 20, 20', 26a, 26a', 26b, 26b', and reduction of the pitch reduces the size of the leakage paths between the exterior surfaces of the rotors 20, 14a, 14b and the housing 16, and it is understood that À. i' : this contributes towards the improved efficiency of the pump 10.

Use of the above described screw thread form also decreases the costs of manufacturing the pump 10.

The rotors 12, 14a, 14b are typically made by machining the thread forms into a cylindrical metal rod, and the tolerances must be tight in order to ensure that the threads mesh properly without leaving large fluid leakage paths and without the meshing threads becoming jammed during rotation of the rotors 12, 14a, 14b. The longer the rotor, the more difficult it becomes accurately to control a machine tool to produce a tight tolerance thread over the entire rotor length. Thus, for a given number of thread turns, it is easier, and hence less expensive, to manufacture a tight tolerance thread on the rotors 12, 14a, 14b of the present invention than it would be to manufacture a longer rotor with a conventional thread form.

In addition, the complexity and hence cost of machining a tight tolerance thread form decreases with a reduced thread depth. This is at least partly because a reduction in root diameter RD increases the likelihood of the rotor 12, 14a, 14b bending during machining, and thus more care must be taken to I produce a thread form of the required tight tolerance. For a given rotor outer diameter OD, the root diameter RD of the rotors 12, 14a, 14b of the present invention is correspondingly larger than the root diameter RD of rotors of conventional design.

The above embodiment is described by way of example only, and various modifications may be made to the pump 10 within the scope of the I invention.

For example, the rotors 12, 14a, 14b may be provided with fewer or Àe A. : more than two threads or flights per rotor. It would be possible, for example to A. À provide three interposed threads on each rotor 12, 14a, 14b each having a pitch : . and thread depth as described above. À Àe

It is also possible to provide more than two idler rotors 14a, 14b.

Finally, it is not necessary for the central rotor 12 to be the power rotor, À À . and the outer rotors 14a, 14b the idler rotors. Any of the outer rotors 14a, 14b À À may be connected to driving means such as an electric motor, which is operable to rotate the outer rotor 14a, 14b about its longitudinal axis B. C. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. À:e' Àe À. À Àe À À À À À À À Àe

Claims

1. A pump including three rotors each being provided with a generally helical screw thread, the rotors being arranged such that a central rotor is located between the other two outer rotors and the screw threads mesh such that rotation of one rotor causes rotation of the other rotors, wherein the thread of the central rotor is a generally helical groove which extends radially inwardly of the central rotor, and the thread of the outer rotors is a generally helical ridge which extends radially outwardly of the rotor, and the outer diameter of the central rotor is smaller than the outer diameter of the outer rotors.

2. A pump according to claim 1 wherein the pitch of the threads is less than 1.6 times the outer diameter of the outer rotors.

3. A pump according to claim 2 wherein the pitch of the threads is less than the outer diameter of the outer rotors.

4. A pump according to claim 3 wherein the pitch of the threads is 0.8 times the outer diameter of the outer rotors. À:e À A. À

À

5. A pump according to any preceding claim wherein the pitch of the threads is at least 0.5 times the outer diameter of the outer rotors. À:.

.

6. A pump according to any preceding claim wherein the thread depth of À..

the screw threads is less than 0.2 times the outer diameter of the outer rotors. ÀÀ

7. A pump according to claim 6 wherein the thread depth of the screw threads is less than 0.175 times the outer diameter ofthe outer rotors.

8. A pump according to claim 7 wherein the thread depth of the screw threads is less than 0.15 times the outer diameter of the outer rotors.

9. A pump according to any preceding claim wherein the thread depth of the screw threads is at least 0.1 times the outer diameter of the outer rotors.

10. A pump according to any preceding claim wherein each rotor is provided with two interposed generally helical screw threads.

11. A pump according to any preceding claim wherein the central rotor is connected to a driving means operation of which causes rotation of the central rotor about a longitudinal axis of the rotor.

12. A pump substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

13. Any novel feature or novel combination of features described herein and/or in the accompanying drawings. Àe À. À e À. À À À À. À: À e. À À À À À À À À