US3814557A

US3814557A - Fluid displacement apparatus having helical displacement elements

Info

Publication number: US3814557A
Application number: US00158479A
Authority: US
Inventors: H Volz
Original assignee: Allweiler AG
Current assignee: Allweiler GmbH
Priority date: 1970-07-04
Filing date: 1971-06-30
Publication date: 1974-06-04
Anticipated expiration: 1991-06-04
Also published as: CH524068A; DE2033201B2; SE375830B; DE2033201A1; DE2033201C3

Abstract

A fluid displacement apparatus, such as a pump or motor in which three helically splined elements are arranged in parallel and in meshing engagement. Profile discs are mounted on the elements to provide a predetermined pitch and a possible variation in the delivery surface. The discs are secured for relative movement with respect to the outer housing or the element on which they are mounted.

Description

United States Patent 1191 Volz I FLUID DISPLACEMENT APPARATUS HAVING HELICAL DISPLACEMENT ELEMENTS [75] Inventor: Hermann V012, Konstanz, Germany [73] Assignee: Allweiler AG, Radolfzell, Germany [22] Filed: June 30, 1971 [2]] Appl. No.: 158,479

1 30] Foreign Application Priority Data July 4. 1970 Germany 203320] 521 [5. CI. 418/197, 418/201 {51] Int. Cl. F0lc 1/16, F03c 3/00, F040 1/10 [58] Field of Search 418/l921, 418/197, 201-203 [56] References Cited UNITED STATES PATENTS 612.304 111/1898 Blazer .1 418/201 1.940.410 12/1933 Fitch et '41.... 418/107 2,095,167 10/1937 Burghauser 418/197 2.231.357 2/1941 Burghauser et a1 418/201 1 1 June 4, 1974 2.325.617 8/1943 Lysholm et al. 418/201 2.462.924 3/1949 Un gar 418/201 2.652.192 9/1953 Chilton l 418/197 2.745.643 5/1956 Kleinlein 418/201 FOREIGN PATENTS OR APPLICATIONS 875.145 4/1953 Germany 418/203 596.122 12/1947 Great Britain 418/201 384.355 12/1932 I Great Britain... 418/21 839.289 5/1952 Germany 418/197 Prinuzr) E.\'uminer-Car1ton R. Croyle Assistant Examiner-John J. Vrablik 15 7 l ABSTRACT A fluid displacement apparatus, such as a pump or motor in which three helically splined elements are arranged in parallel and in meshing engagement. Profile discs are mounted on the elements to provide a prede- '1 .termined pitch and a possible variation in the delivery surface. The discs are secured for relative movement with respect to the outer housing or the element on which they are mounted.

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FLUID DISPLACEMENT APPARATUS HAVING HELICAL DISPLACEMENT ELEMENTS The invention relates generally to a fluid displacement apparatus such as a pump or motor having at least two helical displacement elements arranged with their axes parallel in intermeshing relationship and in which at least one element is driving or driven and the other acts as a seal. The elements are tightly enclosed by a housing, and the apparatus is effective to provide a constant, or nearly constant, displacement over its working range.

In displacement apparatus of this type, the specific throughput of fluid is the product of the cross section of the displacement element for the delivery of the fluid, formed by the gaps between the teeth of the helical thread and the pitch thereof. Conventional displacement apparatus of this category have a constant delivery cross section and constant pitch within their operating areas. In order to obtain the highest possible specific fluid throughput, the pitch and the delivery cross-section must therefore be large. For reasons related to wear, the delivery cross-section is dependent upon the specific dimensions of the helical thread. In this connection it is customary to use for the displacement elements a helical-thread profile in which the specific design of the width of the threads and the magnitude of the gaps between the teeth results in the displacement elements, which serve as seals, which are driven hydraulically, so that for practical purposes no torque is transferred by the flanks of the helical thread. If one were to depart from these relatively ideal dimensions, at least to any great extent, the flanks of the threads would be subjected to very heavy abrasion which would lead to rapid wear in the apparatus. Basically, therefore, if it is desired to increase the specific fluid throughput in a apparatusof this kind, it is only possible to increase the pitch. However, in the case of displacement apparatus used as pumps, the pitch cannot be increased at will, since pump suction depends upon the pitch. Thus, large pitches within a pump produce high axial velocities resulting in low suction lifts. Since in addition to the highest possible specific throughput, the greatest possible specific suction-lift is required, existing designs of displacement apparatus are a compromise which, from the point of view of maximal fluid throughput and maximal suction is unsatisfactory.

Also known are infinitely adjustable displacement apparatus. In such displacement apparatus the delivery cross section is constant, but the pitch is not. In apparatus of this kind, the displacement elements are made longer than the housing and maybe moved axially in relation to the housing. Depending upon the degree of movement, the effective portion of the spindle has a larger or smaller pitch, and this changes the fluid fluid throughput constant, and furthermore, to improve throughput. However, since the delivery cross section 1 remainsconstant, a squeezing action occurs if the pitch is larger on the input side than on the output side, whereas cavitation occurs if the pitch is smaller on the input side than on the output side. In order to overcome to some extent the resulting disadvantages, the helical displacement elements must operate with greater play between them, and this leads to relatively poor volumetric efficiency.

It is the primary object of the invention to provide a displacement apparatus which is effective to increase the above-mentioned adjustable displacement apparatus in such a manner that a constant or almost constant displacement occurs within the operating area at each control setting.

This object is accomplished by a displacement apparatus in which the effective pitchof the helical thread of the displacement element, and the delivery surface formed by the gaps between the teeth of the helical threads, are not constant over the operating range; but in which the displacement is inversely proportional, or almost inversely proportional to pitch and delivery surface; and in which the product of the delivery surface and the effective pitch of the helical thread gives the same, or almost the same, value for any desired cross section in the operating area.

The displacement apparatus in accordance with the present invention accomplishes these aims because the pitch of the helical thread of the displacement element and the delivery surface do not need to be constant within the operating area in order to achieve uniform displacement. It is possible to vary the pitch within the displacement apparatus and adapt it to the particular operating conditions. Another characteristic of the invention resides in that the delivery surface is altered by altering the width of the teeth of the helical thread in the displacement element. Assuming that the displacement elements are provided with helical threads which differ in their diameter ratios while maintaining their cylindrical shape, this approach ensures the changing of the delivery surface. If a change in delivery surface were to be effected, for instance by changing the diameter of the addendum circle in the operating area, the cylindrical shape of the spindles could no longer be maintained. The production of the displacement elements, and of the housing closely surrounding them would be technically extremely difficult.

In order to improve the suction lift of displacement apparatus of this kind which are used as pumps, it is provided, in accordance with the invention, to arrange the displacement .elements in such a manner that the pitch of the helical thread on the pressure side is greater than on the suction side. The result of this is that the working medium on the suction side enters the operating area at a lower axial velocity than that at which it leaves the pressure side. Acceleration of the working medium from input velocity to output velocity takes place within the operating area, when the delivery chambers are already closed by the reduction in delivery surface.

In the case of displacement apparatus working as pumps, a slight reduction in displacement is often desired within the operating area from the suction side towards the pressure side, for example in order to compress gas bubbles or air inclusions slowly, or to dissolve them in the working medium. It has been found that in displacement pumps in which no compression takes place internally, these gas or air bubbles on the pressure side collapse violently under the action of the delivery pressure or become dissolved in the working medium, producing a considerable amount of noise or establishing a destructive phenomena resembling cavitation, even though the pump is not actually operating in cavitation. In order to overcome this defect, in the displacement apparatus according to'the invention, espe- '3 cially when they are being used as pumps, provision is made for the change in pitch and delivery surface to take place in such a manner that the product thereof decreases within the operating area from the suction side towards the pressure side.

Except in the case of infinitely adjustable displacement apparatus, the displacement elements according to the invention may be made of solid material. However, the production of the helical thread is relatively difficult and requires special apparatus capable of producing flanks for threads not of constant pitch. In order to avoid this difficulty, the invention furthermore provides that the displacement elements consist, as known per se, of tooth-profile discs axially aligned in rows and twisted in relation to each other to correspond to the pitch. These tooth-profile discs may be placed onto a shaft having the pitch and representing the core of the displacement element, and they may be aligned to the correct pitch and clamped into position. The pitch applied to the shaft may be constant, but this will make it necessary for the clamping connection to change its angular position in accordance with the change in pitch from one tooth-profile disc to' another.

It is also possible to place the tooth-profile disc onto a shaft of non-constant pitch which doesnot correspond to the desired pitch, while the tooth-profile discs are provided with a clamping. connection, the angular position of which is staggered from disc to disc in such a manner that the effective pitch of the helical thread, resulting from the pitch on the shaft and the angular position of the disc-clamping connection, produces the pitch desired.

The tooth-profile discs may, however, also be provided, as is known per se, with a multi-groove profile, especially a notched-tooth profile, or a splined-shaft profile, and may be placed on an appropriately designed shaft staggered in relation to each other at angles corresponding to the pitch.

As regards that portion of the object of the invention which is intended to provide an infinitely adjustable pump with constant displacement, this aim is accomplished, especially if the basic characteristic of the invention is used as well as the additional characteristic that the displacement elements consist, as is known per se, of profile discs arranged axially in rows and staggered in relation to each other according to the pitch, the tooth-profile discs in each displacement element being placed onto a shaft provided with the pitch, and being arranged thereon and clamped in accordance with this pitch, in that the shafts provided with the pitch are made longer than the length of the rows of toothprofile discs, the discs being arranged in such a manner that they may be moved axially along the shafts.

Since the tooth-profile discs can be moved axially along the shafts, they may be brought into engagement with different pitch areas. If the profile discs are in engagement with a large pitch, the specific delivery volume is large; on the other hand, if they are in engagement with a small pitch, the specific delivery volume is small.

Actually, many embodiments of the infinitely adjustable displacement apparatus according to the invention are possible. 'In one embodiment, the tooth-profile discs are fixed axially in relation to the surrounding housing, the tooth-profile discs, together with the surrounding housing, being displaceable on the axially stationary shafts.

.the tooth-profile discs, bring into play another pitch area and thus another specific delivery volume.

Since the apparatus according to the invention must be accurately positioned axially in relation to each other, provision is also made for mutual axial positioning bf the shafts and displacement elements to be effected, as known per se, by means of shaft collars engaging each other. In order to make certain that this axial positioning can be accurately adjusted, at least the shaft collar on one of the cooperating displacement elements is infinitely adjustable.

In order to prevent the transfer of torque by the thread flanks of the tooth-profile discs, which might be brought about, for example, by fluid friction within the pump, it may be advisable to effect the axial positioning, not by engaging shaft collars, but by herring-bone gearwheels, in which case, for the reasons mentioned above, at least one of the two meshing gearwheels is infinitely adjustable axially by means of a positive connection. This characteristic, namely the mutual axial positioning of the shafts and displacement elements, has the same significance for the infinitely adjustable displacement apparatus as for displacement apparatus which are not infinitely adjustable.

It is furthermore provided to arrange the displacement elements of the above described apparatus with helical-thread profiles, the geometrical mean of which, from the widths at the start and finish of the thread, corresponds approximately to the width at which no torque is transferred to the displacement element serving as the seal, through the flanks of the thread. The result of this design is that two opposed hydraulic torques act upon the displacement elements and largely cancel each other out. Thus no substantial torques are transferred through the flanks of the helical thread, or through the herring-bone gearwheels, if any.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is a longitudinal section of a displacement pump according to the invention having solid displacement elements;

FIG. 2 is an enlarged cross section of the displacement elements;

FIG. 3 is a graphic illustration of the relationship between the delivery surface, the tooth angle resulting from the width of the helical thread, and the pitch of the axial extension of the displacement elements;

FIG. 4 is further example of a displacement apparatus according to the invention;

FIGS. 5 and 6 are cross sections through the displacement apparatus according to FIG. 4, taken along the lines CC and DD, respectively;

FIG. 7 shows a further modification of a displacement apparatus according to the invention;

FIGS. 8 and 9 are transverse cross sections of the displacement apparatus according to FIG. 7;

FIG. is a longitudinal cross section of an infinitely adjustable displacement apparatus according to the invention;

FIGS. 11 and 12 are top views of the displacement elementsin the displacement apparatus shown in FIG. 10, in the maximal and minimal displacement position;

FIG. 13 is a graphic illustration of the relationship between the delivery surface, the pitch, the tooth angle, and the specific delivery volume when the displacement elements are shifted within the displacement apparatus shown in FIG. 10;

FIGS. 14 and 15 are cross sections through the infinitely adjustable pump taken along lines EE and F-F, respectively, of FIG. 10; and

FIG. 16 is another longitudinal section of the device staggered through 90 in relation to the device shown in FIG. 10.

In the displacement apparatus illustrated in FIG. 1,

displacement elements

1,2,3 are enclosed with very little play by a housing 4 within the operating area. For greater clarity, the outside diameters of the displacement elements are shown longitudinally batched in the drawings. Central displacement element 1, which has a stub-shaft 6 fitted with a key 5 so that it may drive or be driven, carries at one end twoshaft collars 7,8 which locate in the axial

direction shaft collars

9,10 fitted to

displacement elements

3,2 on each side, serving as seals.

Displacement elements

1,2,3 are designed in such a manner that the pitch increases from the shaftcollar end towards stub-shaft end 6. At the same time, the width of the teeth of helical thread 11 of the central displacement element decreases, whereas the width of the teeth of helical threads 12 of lateral displacement elements 2 increases.

When the illustrated displacement apparatus is used as a pump and is driven in the direction of arrow 13, the working medium is drawn in through threaded connection 14 at the shaft-collar end, and is expelled from the other end of the apparatus through a threaded connection 15. Suction end 16 of the displacement apparatus is closed off by means of a cover 17, and at the pressure end 18 by a cover I9. The central displacement element is retained axially and radially in cover 19 by means of a ball-bearing 20. A piston 21 relieves displacement element 1 hydraulically ofthrust. In order to make it possible to install ball-bearing 20, cover 19 has another cover 22 carrying radial sealing rings 23 to seal off the central displacement element. Space 24 between balI-bearing and cover 22 communicates with the pump suction chamber through bore 25 in the central displacement element, and is thus relieved of pressure. Axial adjustment of

lateral displacement elements

2,3 is achieved by means of shims 26.

In FIG. 2, the full lines show the cross section of the displacement elements along the line A-A, whereas the dotted lines show the cross section of the displace- AF= A-y (1r/72O)(2 D,, 2 D D, D it is possible to calculate the relative change in the de- 6 livery surface, i.e. the amount by which the delivery surface changes in the area between A and B. In this connection: y the tooth and tooth-gap angle in relation to the dotted pitch circle, the diameter of which is the same for all three displacement elements in the example illustrated.

D, the outside diameterof the central displacement element.

D the inside diameter of the central displacement element.

D the outside diameter of the lateral displacement elements.

D the inside diameter of the lateral displacement elements.

Thus, in relation to initial delivery cross section F delivery cross section F F, AF. For constantdisplacement delivery, the pitch must also change, and this change will occur in accordance with the formula:

F, S, F S Within the axial extension of the displacement elements, the changes in delivery surface and pitch should be continuous, being in the form of a geometrical progression. If the distance between A and B is divided into a specific number of stages n, the

following equation may be set up:

For reasons of continuity, the following is obtained for the pitch:

The relationships to the axial extension are shown in the diagram in FIG. 3. The values relate to a displacement apparatus as shown in FIG. 1, having one central and two lateral displacement elements, based on the following dimensions:

D, 3 cm D 2 cm D 2.2 cm

D 1.2 cm

F, 3.4 cm also A F= 0.72 cm F 4.12 cm The change from to and thus from area F, to F should take place in n 20 jumps. The jump is calculated from the exponential equation:

wherein q 1.0096. Moreover, in the range between n 0 and n 20, the pitch decreases according to the formula:

and the initial value of 79.9 mm, to 66 mm. If the individual values at each jump are multiplied by multiplied F and s, the delivery surface and the pitch, then over the whole range between n O and n 20 there will be a constant line F's, i.e. the specific throughput of fluid will not change within the range between n 0 and n 20. The significance of the dotted lines in FIG. 3 will be dealt with hereinafter.

The displacement apparatus illustrated in FIG. 4 is of approximately the same design as that shown in FIG. 1,

ments

27,28,29, the functions of which, however, cor

respond to those of

elements

1,2,3 in the displacement apparatus shown in FIG. 1.

Displacement elements

27,28,29 consist of tooth-

profile discs

30,31,32 arranged axially in rows which,'as shown in the upper half of the figure, are fitted to

shafts

35,36 having

helical grooves

33,34. In this illustration the tooth-profile discs are shown symbolically only by the outside and core diameters. The pitches of

helical grooves

33,34 are not constant, but behave in accordance with the formula:

As may be seen from FIGS. and 6, tooth-

profile discs

30,31,32 are positively aligned with

helical grooves

33,34 by means of

cylindrical pins

37,38. Since the angular settings of the cylindrical pins in relation to the tooth center of the tooth-profile elements does not alter, the pitch of

helical threads

11,12, formed by the tooth-profile elements, and shown in thin, full lines, corresponds to the pitch of

helical grooves

33,34 on

shaft

35,36. According to the formula:

F". Fl .qn the widths of the teeth vary from a to a and from b to b, between the cross sections shown in FIGS. 5 and 6.

What cannot be readily illustrated is that the width of the teeth, and therefore the tooth angle and/or the pitch, cannot behave exactly in accordance with the formulae. There is no difficulty in performing as shown by the dotted lines in FIG. 3. In this case, when the suction end is at n and the machine is used as a pump, a decrease in the specific delivery volume occurs. This produces compression which results in particularly quiet operation.

The displacement apparatus shown in FIG. 7 corresponds substantially to that shown in FIG. 4, the difference being that the tooth-

profile discs

39,40,41, as may be seen very clearly in FIGS. 8 and 9, are fitted to

splined shafts

42,43, 44 in staggered relationship to each other which depends upon the pitch desired.

In the infinitely adjustable displacement apparatus shown in FIG. 10, three

shafts

46,47,48 are arranged in a tubular housing 45, the shafts being provided with

helical grooves

49,50,51. The housing 45 is closed off by covers 52 and 53. The central shaft 46 passes through cover 52 and is equipped with a stub-shaft 55 having a key-groove 54, so that it may drive or be driven. A ball- 1 bearing 56 retains the shaft axially and radially. Also mounted on shaft 46 is a herringbone gearwheel 57 which meshes with herring-

bone gearwheels

58,59 fitted to

shafts

47,48. In order to permit accurate alignment of

helical grooves

49,50,51, herring-

bone gearwheels

58,59 are infinitely adjustable on

shafts

47,48 by means of clamping

elements

60,61. The

shafts

47,48 are each provided with an extension 62,63 mounted in cover 52 for radial guidance, the front areas thereof communicating, through bore 64 in the cover, and through bore 65 in the central shaft, with pressure chamber 66 in the displacement apparatus. Internal bore 67 in housing 45 encloses, with very little play, another axially displaceable housing 68 which, in turn encloses tooth-

profile discs

69,70,71, fitted to

shafts

46,47,48. The toothprofile discs are located axially in housing 68 by means of

discs

74,75 provided with

perforations

72,73. As may be seen from FIGS. 14,15,16,

discs

74,75 are held in place by means of screws 76. Housing 68, may be moved axially by means of a threaded shaft 77 fitted with a handwheel 78 and rotatably mounted in cover 53. Since the tooth-profile discs are aligned by means of

cylindrical pins

79,80 according to the pitch of

helical grooves

49,50,51, the said tooth-profile discs adapt themselves automatically to the pitch of the said helical grooves when housing 68v is moved axially. The tooth-profile discs thus form displacement elements made in accordance with the embodiment shown in FIG. 4.

FIG. 14 shows a view of cover disc 75, indicating the shape of the perforations 73 through which the working medium flowing through the displacement machine can enter or leave the apparatus. Cover disc 74 is of substantially the same design as disc 75.

As already indicated, turning handwheel 78 causes housing 68 to be displaced in housing 45. If, in the embodiment illustrated, the housing is moved as far as

splined gears

57,58,59 then tooth-

profile discs

69,70,71 assume the shape shown in FIG. 11 of the helical threads with a larger pitch. In this connection, the thin, full lines show the paths of

helical threads

11,12 of the displacement elements marked 82,83,84. The tooth-profile discs therefore assume a portion corresponding to a helical thread having a smaller pitch if, as shown in FIG. 12, housing 68 assumes the position in the vicinity of cover 53 illustrated in FIG. 10.

The diagram in FIG. 13 shows the relationship between delivery surface, pitch and the axial length of

shafts

46,47,48. The diagram is based on the same tooth-profile dimensions as the diagram in FIG. 3, i.e. within 20 jumps, the delivery surface alters from 3.4 cm to 4.12 cm When the rows of tooth-profile discs covering the 20 steps are moved to the right to the shafts covering, as shown, 44 steps,- the effective pitch becomes smaller and the specific delivery volume decreases, but remains constant within the row of toothprofile discs. For instance, with the dimensions given for the infinitely adjustable displacement apparatus, the specific fluid throughput may be steplessly adjusted from about 27 cm to about 21.5 cm. The values given are not limit values for the range of adjustment of a pump of this kind. By altering the dimensions of the displacement elements, or by increasing the number of jumps per length of helical groove, or by decreasing the delivery surface in larger jumps, the specific delivery volume may be altered within still wider limits.

In the illustrated displacement elements the geometrical mean of tooth-angle amounts to as at approximately this angle no appreciable torque is transferred, by the flanks of the helical thread, to the lateral displacement elements, provided a helical profile of the type shown herein is used.

While there have been described what are at present considered to be the preferred embodiments of this in-- vention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A fluid displacement apparatus, such as a pump or motor, comprising: a housing; at least two helical displacement elements arranged within said housing in meshing engagement and with their axes parallel to each other, at least one element thereof being adapted to drive or be driven and another being effective to act as a seal, said elements having helical threads in which the effective pitch and the delivery surface formed by the space between the teeth in the helical threads, within the operating area of the elements, are variable; the change in the pitch and the change in the delivery surface being substantially inversely proportional, and the product of said delivery surface and of the effective pitch of said helical threads providing substantially the same value at any desired cross-section of the operating area.

2. A fluid displacement apparatus, such as a pump or motor, comprising: a housing; at least two helical displacement elements arranged within said housing in meshing engagement and with their axes parallel to each other, at least one element thereof being adapted to drive or be driven and another being effective to act as a seal, said elements having helical threads in which the effective pitch and the delivery surface formed by the space between the teeth in the helical threads, within the operating area of the elements are variable; the change in the pitch and the change in the delivery surface being substantially inversely proportional, and the product of said delivery surface and of the effective pitch of said helical threads providing essentially the same value at any desired cross-section of the operating area.

3. A fluid displacement apparatus according to claim 2, wherein said elements have threads with an inside and outside diameter of different dimension and the variation in the delivery surface is effected by changing the tooth widths of the helical threads of said elements.

4. A fluid displacement apparatus according to claim 2, wherein said housing has an inlet and outlet providing a pressure and suction region with the pitch of the helical threads of said elements being larger in the pressure region than in the suction region.

5. A fluid displacement apparatus according to claim 2, wherein the common axial location of said displacement elements is effected by means of shaft collars in engagement with each other, at least the shaft collar on one of two cooperating displacement elements being steplessly adjustable in an axial direction.

6. A fluid displacement apparatus according to claim 2, wherein the diaplacement elements comprise multishaped tooth-profile discs arranged axially in rows and staggered in relation to each other and to said pitch.

7. A fluid displacement apparatus according to claim 6, wherein said shaft has a non-constant pitch, with the angular position of said discs being staggered relative to each other so that the effective pitch resulting from the pitch of the shaft and the angular position of the tooth-profile discs coincides with the desired predetermined pitch of the helical thread.

8. A fluid displacement apparatus according to claim 6, wherein said elements comprise a central and two side elements with the geometrical mean value of the tooth-widths at the forward and rear ends of said central displacement element corresponding approximately to the tooth-width at which no torque is trans ferred to said side displacement elements through the flanks of the helical threads.

9. A fluid displacement apparatus according to claim 6, wherein each of said elements comprises a shaft and said tooth-profile discs are mounted on said shaft, the latter having a predetermined pitch, and said discs are aligned relative to said pitch.

10. A fluid displacement apparatus according to claim 9, wherein said shaft is grooved, and pins projecting into said grooves securing said discs to said shaft.

11. A fluid displacement apparatus according to claim 9, wherein said shaft has a constant pitch with the angular position of said discs varying in relation to the pitch.

12. A fluid displacement apparatus according to claim 9, wherein the common axial location of said shafts is effected by shaft collars in engagement with each other, at least the shaft collar on one of two cooperating shafts being steplessly adjustable in an axial direction.

13. A fluid displacement apparatus according to claim 9, wherein said shafts have a common axial location effected by means of herring-bone gearwheels with at least one of two engaging gears being steplessly adjustable in an axial direction, and clamping means between said two gears.

14. A fluid displacement apparatus according to claim 9, wherein said tooth-profile discs are formed with a multi-groove profile. I

15. A fluid displacement apparatus according to claim 14, wherein said multi-groove profile has a splined shaft configuration.

16. A fluid displacement apparatus according to claim 14, wherein said multi-groove profile has a serrated tooth configuration.

17. A fluid displacement apparatus according to claim 9, wherein said shafts are longer than the length of the rows of tooth-profile discs, and said tooth-profile discs are arranged to be axially displaceable along said shafts.

18. A fluid displacement apparatus according to claim 17, wherein said tooth-profile discs are axially fixed in relation to said housing, and said tooth-profile discs and said housing are axially displaceable along and relative to said shafts.

Claims

1. A fluid displacement apparatus, such as a pump or motor, comprising: a housing; at least two helical displacement elements arranged within said housing in meshing engagement and with their axes parallel to each other, at least one element thereof being adapted to drive or be driven and another being effective to act as a seal, said elements having helical threads in which the effective pitch and the delivery surface formed by the space between the teeth in the helical threads, within the operating area of the elements, are variable; the change in the pitch and the change in the delivery surface being substantially inversely proportional, and the product of said delivery surface and of the effective pitch of said helical threads providing substantially the same value at any desired cross-section of the operating area.

8. A fluid displacement apparatus according to claim 6, wherein said elements comprise a central and two side elements with the geometrical mean value of the tooth-widths at the forward and rear ends of said central displacement element corresponding approximately to the tooth-width at which no torque is transferred to said side displacement elements through the flanks of the helical threads.

14. A fluid displacement apparatus according to claim 9, wherein said tooth-profile discs are formed with a multi-groove profile.