N/N +1 lobed rotary fluid pumps

Abstract

 In an N, N+1 lobe type pump, dirt clearances are provided at the bases of the interlobe roots 22 of the annulus 14 and the shaping and dimensioning of the inner rotor and annulus are such that elsewhere there is a normal running clearance at the zones of drive contact between the two rotors. To prevent the creation of excessive clearances in the positions of full mesh especially at position D, the inner rotor 10 and annulus 14 are conjugate (as defined) such that at positions B and D normal running clearances are maintained.

Description

[0001] This invention relates to rotary fluid pumps of the N and N+1 lobed type disclosed in for example prior Patent No. 1032862.

[0002] In the practical design of such pumps, in addition to allowing normal running clearance between the inner and outer rotors, it is necessary to provide rotor clearances in each of those regions where a lobe of the inner rotor comes into full mesh with the interlobe roots of the outer rotor. This is because, in practice, the liquid being pumped will inevitably contain particles of dirt and other foreign matter which would cause excessive wear at the full mesh positions and possibly even seizure of the pump unless sufficient clearance is provided at the interlobe roots of the outer rotor to allow the dirt particles to pass around the pump during operation. Thus, whilst not actually illustrated in Patent No. 1032862, a practical version of the pump disclosed therein would of necessity be produced with dirt clearances at the outer rotor roots larger than normal running clearances, which are typically upto about 0.0055 inch (0.014cm) depending on the size of the pump.

[0003] In the past, it has been recognised that drive between the two rotors will not take place around the entire contour of the interlobe roots of the outer rotor: instead the area of driving contact between each lobe of the inner rotor and the internal periphery of the outer rotor transfers instantaneously across the interlobe roots of the latter from one side to the other. In view of this, it has long been thought that substantial clearance for dirt passage can be provided beyond those positions at which the area of driving contact transfers from one side to the other, without any deleterious affect on the volumetric efficiency of the pump.

[0004] The object of the present invention is to provide an improved practical form of pump of the type disclosed in Patent No. 1032862.

[0005] According to the present invention we provide a pump of this type wherein clearance is provided at the roots of the outer rotor in excess of normal running clearance characterised in that the inner and outer rotors are conjugate (as defined herein) at at least one of the positions of full mesh whereby in the or each such position a substantially normal running clearance or clearances is maintained at the opposite side of the inner rotor axis to the position of full mesh.

[0006] As used herein, the expression "normal running clearance" refers to the separation that is present between the driving surfaces of the rotor forms during those parts of each rotor revolution wherein the rotors are not substantially fully meshed. In practice, while such surfaces will be in drive transmitting relation they will be separated by a film of the oil which is being pumped. Typically, as mentioned above, normal running clearances range up to about 0.0055 inch (about 0.014cm) depending upon pump size. Calculations for a prior art 6/7 lobed pump, indicate that normal running clearances will prevail for about 55% of the time and for the remaining 45% of the time, the clearances will become excessive. Thus, the present invention allows normal running clearances to be attained for the remaining 45% of the time with improved volumetric efficiency.

[0007] The present invention is based on the realisation that, contrary to previous thinking, volumetric efficiency is affected by providing for dirt clearance at the interlobe roots of the outer rotor despite transfer of the area of driving contact from one side to the other. The deleterious affect is primarily attributable to two factors: firstly the relatively large dirt clearances at the interlobe roots of the outer rotor; and secondly the fact that in pumps of the type with which the present invention is concerned, the outer rotor does not rotate about a precisely fixed centre, the practice being to mount the outer rotor in a housing which permits a limited radial floating movement to compensate for tolerances in manufacture etc. Consequently as the outer rotor rotates, its axis of rotation tends to follow a fairly complex non-circular path about the substantially fixed rotational axis of the inner rotor primarily dictated by contact with the inner rotor rather than the outer rotor housing.

[0008] As a result, when an interlobe root of the outer rotor comes fully into registry with a lobe of the inner rotor, the outer rotor can undergo a radial displacement relative to the inner rotor which leads to an increased separation, exceeding normal running clearance, between the inner rotor and the outer rotor on the opposite side of the inner rotor axis with the consequently increased leakage gap which adversely affects volumetric efficiency.

[0009] The present invention avoids this phenomenon by reducing the clearance between the registering lobe and root at that point where the outer rotor would otherwise be able to shift radially and create a leakage gap or gaps at the opposite side of the inner rotor axis.

[0010] In the prior art for convenience of manufacture and for provision of dirt clearance, it is known to form the "base" of each interlobe root of the outer rotor as a part cylindrical surface but in the past the clearance gap between the inner rotor lobes and each root has always been in excess of normal running clearance even at the zone of closest approach between the two. In accordance with the present invention, the inner and outer rotors are conjugate such that the clearance at this zone is limited to a value which ensures that the clearances on the opposite side of the inner rotor axis are substantially normal running clearances.

[0011] In the case where the inner rotor has an even number of lobes such that at a position of full mesh the diametrically opposite inner rotor lobe registers with the midpoint of a contiguous outer rotor lobe, by "conjugate" we mean that that diameter of the circle circumscribing the tips of the inner rotor lobes is less than the diametral distance from said zone of closest approach to the tip of the diametrically opposite outer rotor lobe by an amount no greater than that necessary to assure a substantially normal running clearance at the position of "contact" between said inner and outer rotor lobes. In the case where the inner rotor has an odd number of lobes and hence where at the side opposite the full mesh position a pair of inner rotor lobes "contact" the two outer rotor lobes disposed either side of the plane passing through the inner rotor axis and the full mesh position, "conjugate" means that the diameter of the previously mentioned circumscribing circle is less than the diametral distance from the zone of closest approach to the circle inscribing the tips of the outer rotor lobes by an amount no greater than that necessary to secure substantially normal running clearances at the positions of "contact" between the pairs of inner and outer rotor lobes opposite the position of full mesh. In general, the running clearance or clearances in both cases will be in the range normally adopted in the design of pumps of the type with which the invention is concerned, i.e. upto 0.0055 inch.

[0012] In order to promote further understanding of the invention, reference is now made to the accompanying drawings in which:

Figure 1 is an axial section through a pump of the type with which the present invention is concerned;

Figure 2 is a diagrammatic end view showing the relationship between the inner and outer rotors at the position of full mesh between the inlet and outlet ports of the pump;

Figure 3 is an enlarged schematic view illustrating the position of full mesh between an inner rotor lobe and an outer rotor root;

Figure 4 is a similar view to that of Figure 2 with the inner periphery of the outer rotor housing shank and employing an inner rotor with an odd number of lobes, this Figure serving to illustrate the full mesh condition in a prior art pump; and

Figure 5 is a similar view to that of Figure 4 but for a pump incorporating the improvement of the present invention.

[0013] The type of pump shown in Figures 1 and 2 is well known in the art (for instance see Patent 14o. 1032862, the disclosure of which is incorporated herein by reference) and will not be described in detail except to identify the main parts, namely the inner rotor 10 fast with the drive shaft 12, annulus 14 mounted eccentrically with respect to the fixed axis 16 of shaft 12, annulus housing 15, inlet port 18 and outlet port 20. The direction of rotation is indicated by arrow X.

[0014] The base of each of the interlobe roots 22 of the annulus 14 are formed as part cylindrical surfaces which afford dirt clearance pockets for passage of dirt particles through the pump. In conventionally designed pumps of this type, the clearance at point 24 of each root 22 is greater than the normal running clearance allowed at other points around the peripheries of the rotor 10 and annulus 14. This has not been considered undesirable in the past as explained hereinbefore.

[0015] Referring to Figure 3, the approximate positions at which the point of driving contact between the inner and outer rotors "jumps" across the interlobe root 22 are indicated at 26,28. Although at these positions the clearances A are normal running clearances, it will be observed that, considered in a direction parallel to the line 29 passing through the point 24 and the inner rotor axis 16, the clearance B is larger than A approximately by a factor equivalent to the reciprocal of the sine of the angle of inclination C in this region. Thus, if angle C is of the order of 30°, B will be approximately twice the clearance A. Consequently, if as in conventional pumps of this type, a large dirt clearance is present at 24 and the annulus 14 is permitted a limited amount of floating movement, the annulus can undergo a displacement of the order of 2A thereby creating a larger than normal running clearance on the opposite side, i.e. adjacent the tip of the annulus lobe 32 (see Figure 1) and consequent loss of pressure/flow.

[0016] In accordance with the invention, the inner rotor 10 is conjugate with the annulus 14 such that, in the illustrated embodiment having an inner rotor with an even number of lobes, the diametral distance between the tips of the inner rotor lobes is less than the distance from the point 24 (the zone of closest approach) and the annulus lobe tip 32 by an amount such that the clearances 24 and 32 are normal running clearances, i.e. no greater than 0.0055 inch (0.014cm).

[0017] Figure 4 serves to illustrate further the loss of pump efficiency that occurs with prior art pumps. This Figure also shows, on a somewhat exagerrated scale, the radial clearance or play 40 provided between the outer rotor 14 and its housing 15. Such clearance is necessary in practice to allow for the manufacturing tolerances involved in the production of the rotors 10,14. The maximum. clearance between the outer rotor 14 and its housing 15 will in general exceed the normal running clearance between the rotors and will typically be of the order of 0.006 - 0.013 inch (0.015 - 0.033cm). The same reference numerals are used in Figure 4 as in Figures 1-3 and it will be seen that the excessive dirt clearance at point 24 results in large separations (exceeding normal running clearances) at points 32 at the opposite side of the axis 16.

[0018] Figure 5 corresponds to Figure 4 but illustrates the improvement that results when the clearance that can occur at point 24 is reduced. In this case, the clearances at points 32 can be constrained to normal running clearances thereby providing effective sealing between the rotors and avoiding loss of pump efficiency. As well as maintaining effective sealing between the rotors, the improvement provided by the present invention also leads to smoother running of the outer rotor since its freedom to float radially is constrained to a greater extent in each position of full mesh.

[0019] The improved sealing obtained with pumps in accordance with the invention is particularly noticeable with inner rotors having an even number of lobes and while the effect is not as significant, in terms of volumetric efficiency, for odd lobed inner rotors, the fact that the outer rotor is constrained to a greater extent by the inner rotor results in improved pump operation.

Claims

1. A pump of the N,N+1 lobed type with an inner rotor and an eccentrically disposed outer rotor wherein clearance is provided at the roots of the outer rotor in excess of normal running clearance, characterised in that the inner and outer rotors are conjugate (as defined herein) at at least one of the positions of full mesh whereby in the or each such position a substantially normal running clearance or clearances is maintained at the opposite side of the inner rotor axis to the position of full mesh.

2. A pump as claimed in Claim 1 in which the clearance or clearances at said opposite side of the inner rotor axis is no greater than 0.0055 inch (0.014cm).

3. A pump as claimed in Claim 1 or 2 in which each interlobe root of said outer rotor is of part cylindrical configuration.

4. A pump of the 14, N+1 lobed type with an inner rotor and an eccentrically disposed outer rotor wherein the rotors are radially movable relative to one another to a limited degree to compensate for manufacturing tolerances and wherein clearance is provided at the roots of the outer rotor in excess of normal running clearance, characterised in that, in the regions of full mesh between said rotors, the two rotors can move radially relative to one another but only to such an extent that the clearance in the full mesh position is reduced by an amount compatible with maintaining a substantially normal running clearance on the opposite side to the positions of full mesh.

5. A pump as claimed in Claim 4 in which the inner rotor has an even number of lobes.

6. A pump as claimed in Claim 4 in which the inner rotor has an odd number of lobes.

7. A pump as claimed in Claim 4, 5 or 6 in which the outer rotor is mounted for rotation in a housing with sufficient clearance to permit said limited degree of radial movement between the rotors, the latter-mentioned clearance bieng greater than normal running clearance between the rotors.

Drawing