CA2393411A1 - Internal-axis screw displacement machine - Google Patents

Abstract

In internal axis displacement machines in prior art, screw rotors with parallel axes operate with varying pitches and/or varying transverse profile s to achieve an internal compression at rotational speed ratios of (x+1):x without an internal spacer (= GE-Rotor) in a manner which is satisfactory in many respects. However, the geometry of said machines requires complex production processes, such as machining/erosion involving high production costs and leading to problems related to assembly, alignment and servicing. The invention provides the basis for significant economies in rotor manufacture by defining novel rotor geometry, thus solving the problems related to assembly and servicing. The inventive variation of the gap betwee n rotors, made possible by axially offsetting the rotors in relation to one another, overcomes the problems of alignment and allows the rotational speed , pressure differential, temperature and other process data to be adapted duri ng operation. The capacity to achieve intense compression rates from 1:1 (= isochore) up to approximately 1:10 opens up a wide range of uses for pumps o f between approximately 10 m3/h and 100 m3/h in the chemical/pharmaceutical, packaging and semiconductor manufacturing industries.

Description

INTERNAL-AXIS SCREW DISPLACEMENT MACHINE
The invention relates to a displacement machine, in particular for use as a vacuum pump, with screw rotors, with a thread number ratio of (x+1 ):x, in mutual, meshing engagement, movably disposed in an axis-parallel way in a housing, which screw rotors form variable chambers, axially staggered, shifting during operation, and which screw rotors are designed in an internal-axis construction as hollow outer rotor with inner spiral thread and inner rotor with outer spiral thread, with varying or constant pitches as well as varying screw rotor transverse proftles in each case along the rotor axes.
Z 0 In the following, external-axis and internal-axis screw-type displacement machines are compared to one another, with reference to some typical sets of problems. Internal-axis machines having the same output can be built more compactly than external-axis ones. With both types, the synchronization of the rotors can be achieved via toothed wheels or synchronous belts, for example, whereby dry operation, aimed-far nowadays for many applications, is easier to achieve with internal-axis machines. Rotor cooling is also easier to achieve with internal-axis machines, and pumps of this type are thermally less critical. Manufacture of the rotors is relatively costly for both types of screw-type displacement machines. Metallic materials are primarily used, which are not infrequently cast, metal-removing machining being practically 20 always necessary, however. With internal-axis machines sometimes at least one of the rotors has to be manufactured in multiple pieces so that the rotors can be mounted at all. With external-axial, screw-type displacement machines an inner compression of up to about 1:3 is attainable without any difficulty. Internal-axis machines, on the other hand, achieve an inner compression of up to about 1:5 without any difficulty. Equipping external-axis machines with slides is known, when a limitation of pressure is supposed to be achieved, which can be advisable with high inner compression in order to avoid an overheating of the machine.
In the case of internal-axis machines, exhaust lead channels and corresponding valves can be installed for this purpose. Adjustment of the gap between rotor outer diameter and housing is known with external-axis machines, for example in order to take into consideration the thermal expansion during operation. Such an 30 adjustment is extremely expensive. Adequate adjustment in the case of internal-axis machines is not known to the applicant. The measuring and adjusting of the AMENDED PAGE

play between the rotors tends to be more difficult to accomplish with internal-axis machines than with external-axis machines. Mounting of the rotors in the housing takes place relatively simply with external-axis machines, at least with rotors having a cylindrical outer generated surface. With state-of the-art internal-axis s machines, the inner rotor must be inserted into the outer rotor with a rotating movement. With rotors having variable pitch, however, this is usually not possible, so it is often necessary to construct one of the rotors in multiple pieces.
Finally, for the reasons just mentioned, access to the pump chamber for repair and maintenance purposes is usually easier with external-axis machines than with lo internal-axis ones.
Other sets of problems, such as dynamic sealing or rotor balancing are very similar for both of the said types of machines. All in all, however, for the reasons explained above, the internal-axis machines offer a better point of departure for attaining the goals mentioned further below.
is Disclosed in the French patent document 695539 is a displacement machine usable as a pump or a motor, in which a hollow outer rotor with inner spiral thread and a solid inner rotor with at least one outer spiral thread are in meshing engagement with each other. The rotors are disposed axis-parallel, and the outer rotor has a number of threads per unit greater by one compared to that 20 of the inner rotor.
The Swedish patent document 85331 shows, for example in Figure 14, a displacement machine with rotors disposed axis-parallel in mutual meshing engagement, which are designed as hollow outer rotor with inner spiral thread and inner rotor with outer spiral thread, in an internal-axis construction.
2s Achieved in none of these machines, however, is a variable pitch or an absence of axial undercutting of the spiral threads of the rotors.
Disclosed also in the German published patent application 2434782 is a displacement machine, in which a hollow outer rotor with inner spiral thread and an inner rotor with outer spiral thread are in meshing engagement with each other, so AMENDED PAGE

2a the profiles over the length of the rotor having an alternating pitch. So that these rotors can be mounted, the outer rotor is designed in two pieces.
Besides the above-described internal-axis screw-type displacement machines, internal-axis machines are also known in which the rotor profile has s spirals with axially progressive profile contour. Examples of such machines are described in the patents U.S. 5,603,614 and CH 263 376. Unlike machines of the screw type, the rotors in these machines turn in the ratio 1:1.
With the state of the art as the point of departure, the objects of the invention are as follows. The manufacture of the rotors should be greatly to simplified, mounting and dismounting of the rotors being improved at the same time, whereby access to the pump chamber for maintenance and cleaning AMENDED PAGE

purposes is also made easier. Furthermore, adjustment of the gap should be possible in a simple way, and an adjustment of pressure should be achievable at high rates of compression with minimal effort. These objects should be achieved through the invention without all too negative consequences for the construction s size and the inner compression of the displacement machine, or for the synchronization and cooling of the rotors.
These objects are attained through a displacement machine which has the characterizing features of claim 1.
More specifically, these objecfs are attained through a displacement io machine, in particular for use as a vacuum pump, with rotors in mutual, meshing engagement, movably disposed in an axis-parallel way in a housing, which rotors form variable chambers, axially staggered, shifting during operation, and which rotors are designed in an internal-axis construction as hollow outer rotor with inner spiral thread and inner rotor with outer spiral thread, with selectively varying is pitches as well as varying rotor transverse profiles in each case along the rotor axes, wherein that the axial projections of any two transverse profiles of one and the same rotor in the same plane have no common points, the rotors are thus free of any axial undercutting, and as such can be placed in the working positions in a simple way through axial movements, and in engagement can be adjusted axially 2o with respect to one another.
Besides the aforementioned advantage of simple mounting, the absence of axial undercutting of the rotors brings still further advantages.
The pitch is also freely definable for one-piece rotors, so that, in interaction with the cross-sectional course, a course for the volume can be determined in order to 2s achieve a desired compression. The rotors can be produced in one piece in very simple fashion by means of moulds, for example through casting. The moulds for both the outer rotor and the inner rotor can be separated in such a way that the lines of separation do not run over the rotor profiles, which brings about a minimization of the necessary finishing work. Finally, even during operation, with 3o a corresponding configuration of the machine, the play between the rotors can be adjusted very easily through axial adjustment of the relative rotor positions.
Special embodiments of the displacement machine according to the invention are described in the dependent claims.

' - CA 02393411 2002-06-04 The invention will be described more closely in the following, with reference to the embodiment examples shown in the drawings. Shown are:
Figure 1 a view, in perspective, partially in section, of the rotor pair;
Figure 2 a longitudinal section of the outer rotor;
s Figure 3 a view in perspective of the inner rotor;
Figure 4 the geometric relationships and mutual arrangement of inner rotor, outer rotor and support;
Figure 5 a cross-section through the outer rotor;
Figure 6 a cross-section through the inner rotor, in the same plane as the cross-io section through the outer rotor according to Figure 5; and Figure 7 Cross-sectional evolution and possible course of pitch of the outer rotor.
In the view in perspective of Figure 1, the outer rotor 1 is shown half cutaway so that the inner rotor 11 is visible. The spiral threads of the outer rotor 1 is are designated by 2, whereas those of the inner rotor 11 are designated by 12.
Located on the upper, suction-side end of the outer rotor 1 is a cylindrical projection 4 with which the outer rotor 1 is borne in a support.
Figure 2 shows the outer rotor 1 in an axial longitudinal section in which the inner spiral threads 2 are more visible than in Figure 1.
2o The view in perspective, according to Figure 3, of the inner rotor 11 substantially corresponds to that of Figure 1.
With reference to an example, Figure 4 shows diagrammatically the geometric relationships and mutual arrangement of outer rotor 1 and inner rotor 11. Assumed in the example is a double-threaded outer rotor 1 with oval, inner 2s cross-section, in which a single-threaded inner rotor 11 meshes. The reference numeral 3 designates the axis of the outer rotor 1. The reference numeral 13 designates the axis of the inner rotor 11, and the reference numeral 14 the centre of the profile section of the inner rotor 11. The inner, oval area of the outer rotor 1 is designated by 5, and the outer circular area of the inner rotor 11 bears the reference numeral 15.
The symbols used in the formulas which follow have the following s significance:
a - angle of wrap at the outer rotor Fa - cross-sectional area at the location a s - eccentricity R<a> radius R dependent upon a =

io as differential quotient -R; - reference radius R at the location a = 2~

1 <_ undercut factor k -T - constant (see [4]) W - axial position of a cross-section is L; reference increment (in a = 2n) -Lj - reference pitch (in a = 2n) 2~

dW - dynamic pitch = differential quotient of W

da a2 - upper limit of the integral - pi (3.9495...).
2o For this profile, the chamber cross-sectional area is Fa =4s(1-cosa~t<a> [1]
The requirement of absence of axial undercutting is expressed mathematically as dR z 2Eda . The function selected here, namely R < a >= R~ +2kE(a-2n), fulfils this condition for all k >_ 1. There results Zs therefrom R < a > =1 + T(a - 2n) [2]
R~
with R~ = R < 2~ > [3]

> CA 02393411 2002-06-04 T - 2ks [4]
R~
The curve corresponding to formula [2] and the conditions [3] and [4] is designated by 31 in the diagram according to Figure 7.
Valid for a linear course of pitch is L; _dw _ ~;
s W < a >= 2~ a' da 2~
Generally valid for a non-linear course of pitch is _dw __ ~j g < a > and therefrom da 2~
dw=2~g<a>da [5]
wherein g<a> is a pure function >0.
io The chamber volume is generally calculated «~
VW = f Fadw [6]
az-2n with insertion of [5] ~
L. «z Vw =-'' jF«g < a > da (general) [6a]
27L "z-2n and with insertion of [1 ]
2sL. "~
Is V~,h = ' J(1-cosa)R < a > g < a > da (profile-specific) [6b]
«z-2n This formula [6b] applies for the chamber volume with stationary rotors in the starting position for a2=2~,4~ and 6n.

For any desired rotor positions with az=2~ ... oca the formula is «, Vin - 2ELi ['(1- cos(a -a2 ))R < a > g < a > da [6c]
n «~J-zn With setting g < a >= Ri ~ h < a >, whereby h<a> is again a pure R<a>
function, one obtains Vdyn - 2sLiRi «~(1- ~s(a - az )~'t < a > da [6d]
«,-zn W= ~i Jg<axla= ~i j h<a' da [5a]
2~c 2~ R < a >
Ri From the formula [6d], the following adequate condition can be derived for an isochoric machine:
h < a >;$o= constant ~ h < a >;so= g < °~' R < °~'' = constant =
A

1o and if dW <a=2n>= ~' g<2~>, R<2~>=Ri, g<2~>=1 ~
da 2~
1~R.
h<a>;so=A= R' =1 ~
i LiRi 1 a W;~ _ (' ~a ~
2~ ~R<a>
LiRi 2kE
W < a >;SO= 4k~E I 1 + Rl - 4k~cs a k.ze 2n.w a. - Ri - 4k~t~ a R; ~ L; _ 1 2ks g L;R; R
W~so = 4k~s In R j - 4knE
_2kE.2n_W
R~so = ~R1- 4k~ts~~ a R' R. R
g < a > ~so = ' ~ h < a >,$o = ' R<a> R<a>
g < a >iso= 1 [7]
1 + 2k R (a - 2n~
l R L
s V;~o = 4sL;R~ = 4E3 Ej E
Figure 5 shows a cross-section through the outer rotor 1 with the inner recess (area 5), of oval cross-section, and the axis 3, the ends of the oval having the radius R. Figure 6 shows a cross-section through the inner rotor 11 in the same plane as the cross-section through the outer rotor 1 according to Figure 5.
io The radius R of the circular cross-sectional area (area 15) corresponds to the aforementioned radius R at the inner rotor 11.
Figure 7 illustrates, using a diagram, possible courses of pitch for the rotor spiral threads of the outer rotor as a function of the angle of wrap, the angle of wrap a being plotted on the abscissa. The curve 31 corresponds to a change is in cross-section with an increase in radius as defined under [2] to achieve the absence of undercutting. The curve 32 corresponds to the formula [7], and is proportional to the course of pitch of the outer rotor of an isochorically operating machine. The curve 33 is proportional to the course of pitch of the outer rotor of a machine in which compression rates of over 1:1 to 1:5 are achieved in 2o combination with cross-sectional changes corresponding to curve 31. Suction side at a=0.
The curve 34 is proportional to the course of pitch of the outer rotor of a machine in which compression rates of 1:2 to 1:10 are achieved in combination with cross-sectional changes corresponding to curve 31. Suction side at a=aa.

Claims (12)

Claims

1. Displacement machine, in particular for use as a vacuum pump, with screw rotors (1, 11), with a thread number ratio of (x+1):x, in mutual, meshing engagement, movably disposed in an axis-parallel way in a housing, which screw rotors form variable chambers, axially staggered, shifting during operation, and which screw rotors are designed in an internal-axis construction as hollow outer rotor (1) with inner spiral thread (2) and inner rotor (11) with outer spiral thread (12), with varying or constant pitches as well as varying screw rotor transverse profiles in each case along the rotor axes (3, 13), characterized in that the axial projections of any two transverse profiles of one and the same screw rotor in the same plane have no common points, the screw rotors (1, 11) are thus free of any axial undercutting and are without steps, and as such can be placed in the working positions in a simple way through axial movements, and in engagement can be adjusted axially with respect to one another.

2. Displacement machine according to claim 1, characterized in that the screw rotors (1, 11), at least in the region of mutual engagement, have no separation seams, and in that at the same time the pitch for any one of the two screw rotors (1, 11) is freely definable as a variable dependent upon the angle of wrap.

3. Displacement machine according to claim 2, characterized in that small transverse profiles and large pitches are achieved on the suction side, and on the pressure side large transverse profiles and small pitches, whereby the displacement effect is further supported by centrifugal force, in particular at high rotational speeds and to an intensified extent with media having a high proportion of liquid.

4. Displacement machine according to claim 3, characterized in that the cross-section and the pitches of the screw rotors (1, 11) are designed in such a way that the volumes of the chambers remain constant during the axial movement despite change of shape, whereby the displacement machine works with isochoric compression.

5. Displacement machine according to claim 3, characterized in that the cross-section and the pitches of the screw rotors (1, 11) are designed in such a way that the volumes of the chambers during the axial movement decrease in such a way that compression rates between 1:1 and 1:5 result.

6. Displacement machine according to claim 2, characterized in that large transverse profiles and large pitches are achieved on the suction side, and small profiles and small pitches on the pressure side, whereby compression rates of 1:2 to 1:10 are attained.

7. Displacement machine according to claim 1 or 2 or 3 or 6, characterized in that the gaps between the chambers, disposed axially staggered, can be changed by changing the relative axial position of both screw rotors (1, 11) with respect to each other at standstill or also during operation.

8. Displacement machine according to claim 7, characterized in that the change of the relative axial position of the two screw rotors (1, 11) with respect to each other causes small changes in the cross-section of the gap on the suction side and large changes in the cross-section of the gap on the pressure side, and that a continuously adjustable pressure difference control is achieved to such an extent.

9. Displacement machine according to claim 1 or 2 or 3 or 6, characterized in that the screw rotors (1, 11) are fabricated in two-piece moulds from a moldable mass, whereby the mould separation does not run over the rotor surfaces in mutual engagement, but is situated in each case at the rotor end.

10. Displacement machine according to claim 9, characterized in that the rotor material is ceramic and/or synthetic material.

11. Displacement machine according to one of the claims 1 to 10, characterized in that one of the two screw rotors is fixed centrally to the housing, and the other screw rotor in each case is borne eccentrically is a centrally borne supporting rotor, and rotates driven in a planetary way.

12. Displacement machine according to one of the claims 1 to 10, characterized in that both screw rotors (1, 11) are rotatably borne in the housing.