EP1151199A1

EP1151199A1 - Twin delivery screws provided for installation in positive-displacement machines, especially pumps

Info

Publication number: EP1151199A1
Application number: EP99973677A
Authority: EP
Inventors: Ulrich Becher
Original assignee: Ateliers Busch SA
Current assignee: Ateliers Busch SA
Priority date: 1999-02-08
Filing date: 1999-12-07
Publication date: 2001-11-07
Also published as: JP2002536593A; CN1334904A; CA2362075A1; WO2000047897A1; AU1371400A; EP1026399A1

Abstract

Prior art multithread twin delivery screws comprise point-symmetric profiles, small inclines, and very large angles of wrap and, due to the residual blow-hole losses between the working cells, have to be operated in a high-speed manner. Prior art single-thread embodiments seal-off work cells without the use of blow-hole connections and produce a good vacuum and a good level of efficiency when operated at average and low rotational speeds, but have a tendency to produce unwanted bending vibrations at high rotational speeds. Due to geometric reasons, it is not possible to provide an effective inner cooling for any of these embodiments. The invention eliminates this deficiency and combines the advantages of both aforementioned embodiments. The invention provides a uniform solution for a broad range of rotational speeds by using delivery screws (1, 2) which are not point-symmetric and which comprises multithread front profiles with a centrical center-of-gravity. Delivery screws of this type function without blow-hole losses, are also suited for high-speed operation by virtue of the centrical center-of-gravity, and, due to the lack of balancing cavities with average inclines, can be equipped with channels for inner cooling. These favorable adaptations possibilities with regard to process control permit the invention to be used in the fields of medicine, chemicals and semiconductor technology.

Description

Twin conveyor screws for installation in positive displacement machines, especially pumps

The invention relates to twin conveyor screws for installation in positive displacement machines, in particular pumps, for axially parallel, opposing external engagement with wrap angles> 720 ° and carried out with the same end profiles, each with a cycloid-shaped hollow flank, which form a well-sealed axial chamber sequence without blow hole connections in a housing.

Various types of known twin feed screws and their properties are explained below.

In a first type of twin screw of this type, each feed screw has a plurality of screw bodies arranged axially one behind the other on a common shaft.

The twin screws shown in documents EP 0496 170 A2 and DE 43 16 735 A1 have screw bodies with different winding directions. The volume flow is divided, which saves critical dynamic seals. Dynamic balancing is also guaranteed for catchy, non-point-symmetrical profiles.

Documents JP 63-36086 and DE 19522 559 A1 also show screw pairs of this first type, but here the screw bodies arranged one behind the other have different pitches. As a result, the area of the compression is axially expanded, which increases the heat exchange area of the housing, which leads to an improvement in the external cooling.

The screw pump according to DE 196 14 562 A1 combines the two features mentioned, namely different winding directions and different pitches of the screw bodies arranged one behind the other on a shaft. The disadvantages of this first type of twin screw consist in particular in an increased construction effort compared to those with a single screw body per spindle. In addition, with the exception of patent application DE 19522 559 A1, all of the publications mentioned above have catchy rotors which, despite being 100% balanced due to their great length, have a strong tendency to vibrate, which precludes the operation of corresponding pumps at high speeds.

In a second type of twin conveyor screw, the screws are multi-start and the profiles are point-symmetrical. These include, in particular, the SRM profiles named after the company "Svenska Rotor Maskiner Aktiebolag", in which the two rotors have an uneven number of teeth and in which one of the rotors is referred to as "male" and the other as "female" Profiles do not have any imbalance problems because they are point-symmetrical. This applies both to the profile called "symmetrical" with mirror-image tooth flanks and to that called "asymmetrical" with different tooth flanks.

The disadvantages of this second type of twin screw can be seen in the fact that they are not able to form an axial chamber sequence in a pump housing, because a so-called blow hole forms at the housing edge, which is located at the intersection of the two housing bores. The screws have a large pitch and have small wrap angles, which means that a good final vacuum and good efficiency can only be achieved at high speeds. In addition, front end covers are required for functional reasons in compressors with such twin screws.

Cooling options for such rotors are, on the one hand, the generally known and widespread “fluid injection”, which is not suitable for all applications, on the other hand, and on the other hand an internal cooling system in which a cooling medium is passed through the rotors. The patent documents DE 914 886, US 2,714,314 and US 2,441,771 show cooling systems of the latter type. The twin screws of a third type described below also belong to the point-symmetrical profiles.

The patent applications DE 195 22 559 A1 and DE 19522 560 A1 describe twin screws with a two-start rectangular profile. The pump concept realized with these feed screws is the most advanced and most flexible concept at the moment. The two-course rectangular profile allows high speeds without bending vibrations and operates without a front end cover. On the other hand, precisely because of this rectangular profile, the formation of sufficiently well partitioned, axial chamber sequences with little leakage is only achieved at low gradients and large wrap angles in combination with high speed (8000 min ^"1 ), ie operation in the low and medium speed range results The pump is cooled here on the stationary part conventionally by coolant. The rotors are cooled according to the principle of heat radiation in the area of the pressure-side coaxial bores. This rotor geometry offers no space for an effective internal cooling system with flowing cooling medium.

In the twin screws described in the patent specification DE 4224 969 C1 with the same pitch depth, the same number of gears and symmetrical flank profiles on both sides, attempts are made to reduce the loss gaps by correcting the flank profiles. However, there is no information about improvements that can be achieved through this measure.

The fourth and last type of twin screw to be described here has catchy profiles.

A known pump with rotors of this type is manufactured by Kashiyama and has rotors mounted on one side with a catchy rectangular profile. With regard to geometry, speed, leakage, final vacuum and efficiency, the same problems occur with this pump as described above in connection with the two-start rectangular profile of documents DE 19522 559 A1 and DE 195 22 560 A1. The additional problem of balancing is solved by recesses in the core area at the front. In the opinion of the manufacturer, the severely restricted area of application and the small size make cooling unnecessary.

In the documents GB 112 104, GB 670 395, GB 746628 and WO 97/21926, twin screws with catchy profiles with a hollow flank are disclosed. By forming one flank of the catchy profiles as an elongated cycloid (hollow flank), symmetrical lines of engagement are alternately formed, which run from the inner edges of the housing to the core circles along the outer contours of the screws. These lines of engagement divide the interior of the pump into axially moving work cells with twice the length of the slope in an overlapping arrangement. Versions with large wrap angles (> 720 °) bring the good isolation without a blow hole to full effect and good efficiency and very good final vacuum are also achieved for medium (3000 min ^"1 ) and low speeds. The output control is not necessarily carried out via a front cover The screws themselves can do this alone. The balancing method is important in the implementation of such catchy conveyor screws - by forming frontal and / or internal cavities, cutting away outer, insignificant screw parts, using materials of different densities and varying the wrap angle, static and dynamic Documents representing the prior art with regard to such methods are JP 62 291 486, WO 97/21925, JP 0 230 5393 and WO 98/11351.

Bending vibration-free operation cannot be achieved, especially at high speeds and slim rotors. Effective internal rotor cooling is also not possible here, since this would require essential areas of the balancing cavities.

Starting from this prior art, the present invention has for its object to provide a set of conveyor screws, which is suitable for problem-free operation in a wide speed range (approx. 1'000 - 800 min ^"1 ) and with which a good vacuum even at low speeds and a vibration-free operation can be achieved up to the upper range. In addition, the conveyor screws are intended to install an effective Enable internal rotor cooling, which is becoming increasingly important, especially for larger pumps (> 200 m ³ / h).

At this point, it should also be noted that certain solutions that are offered are only false solutions. These include the rotors with point-symmetrical profiles with several hollow flanks, which are described in another context in documents US Pat. No. 4,224,016 and AU 261 792. With such profiles, screw rotors can be formed, but these do not form an axial chamber sequence in a housing, but rather all partial volumes are connected to one another, so that no displacement machine can be realized with them.

To achieve the object, the twin conveyor screws according to the invention are characterized in that the end profile is multi-start but not point-symmetrical, that exactly one flank of a single tooth is designed as a cycloid-shaped hollow flank and that all other flanks are essentially convexly curved and that they are formed accordingly individual stimulus profile limitation curves there is agreement between the center of gravity and the pivot point.

According to one possible embodiment, helical channels are provided in the interior of the rotor, in the areas of the teeth, through which a cooling medium flows.

Exemplary embodiments of the invention are described in more detail below with reference to the accompanying drawings. In the drawings shows

FIG. 1 shows a set of twin conveyor screws in a housing, in an end cut, along the line I - 1 from FIG. 3,

Figure 2 shows a twin screw set with unequal arches, in an end cut and 3 shows a twin screw set with internal cooling channels, in a partially sectioned view in the direction of arrow III in FIG. 1.

FIG. 1 schematically shows a cross section through a housing, indicated by 3, in which there is a first exemplary embodiment of the twin conveyor screws 1 and 2 according to the invention. The view according to FIG. 1 is cut along the line I - I in FIG. 3, but the channel denoted by 11 in FIG. 3 is not shown. Each of the screws shown has a uniform, coherent screw body and is preferably intended for a dry-running pump with an externally arranged synchronization device. The profile of the screws is composed of the following limitation curves: A single hollow flank 6 merges at its radially outer end over an edge 7 into a circular arc K. At its radially inner end, the cycloid-shaped hollow flank 6 merges tangentially into a circular arc F. At the other end of the base arc, a first convex flank 8 is connected via a transition radius, at the radially outer end of which a further tip arc K is connected. At the other end of this further tip arc there is a second convex flank 9, which in turn merges into a further root arc F via a transition radius. At the other end of the further base arc, the contour merges again via a transition radius into a third convex flank 10, which abuts the already mentioned tip circle at its radially outer end. The shapes and the mutual position of the limiting curves mentioned are selected so that the center of gravity of the cross-sectional profile of the end face exactly coincides with the axis of rotation of the screw in question. As a result, measures for balancing are superfluous and cooling ducts can be arranged in the interior of the feed screws, as is shown in FIG. 3 with the reference number 11.

FIG. 2 shows, based on a second exemplary embodiment of the twin conveyor screws according to the invention, that the radii rK1, rK2 of the two circular arcs bK1 and bK2 of each conveyor screw are not the same as in FIG have to be. According to Figure 2 is rK1 is greater than rK2, so that there is a clear gap between the head arc K adjacent to the convex flanks 8 and 9 and the inner wall of the housing. However, this gap is of minimal importance for the partitioning of the work cells, because this takes place at the edge 7 and the adjacent circular arc. The different root circle radii rF1, rF2 inevitably result from the center distance and the tip circle radii rK1, rK2. With this embodiment, the surfaces of the housing 3 and the rotors 1, 2 forming the working space are enlarged, which is advantageous in combination with the internal rotor cooling.

Figure 3 shows the twin screw set according to Figure 1 in a partially sectioned view in the direction of arrow III in Figure 1. The two-speed rotors are mirror images and are in engagement with each other. A helical channel 11 is formed in its interior, through which a cooling medium flows during operation. Each rotor is made in one piece. Instead, of course, each rotor could also be composed of a rotor shaft 4 and a screw body 5, which can be made of the same or different materials. Because of the favorable end profile geometry, materials of low strength can also be used in this case. Non-castable special materials can also be used if the internal rotor cooling is not used.

Claims

claims

1.Twin screws (1, 2) for installation in positive displacement machines, especially pumps, for axially parallel, opposing external engagement with wrap angles> 720 ° and executed with the same end profiles, each with a cycloid-shaped hollow flank, which has a well-sealed axial in a housing (3) Form chamber sequence without blow hole connections, characterized in that the end profile is multi-start but not point-symmetrical, that exactly one flank of a single tooth is designed as a cycloid-shaped hollow flank (6) and that all of the at least three other flanks (8, 9, 10) in the are essentially convexly curved and that the profile center of gravity and the pivot point correspond by appropriate design of the individual face profile limitation curves.

2. Twin screws according to claim 1, characterized in that the stimulus profile has two teeth, formed from said cycloid-shaped hollow flank (6), three convex flanks (8, 9, 10), two radius-equal arches (K) and two radius-equal Arch of feet (F).

3. twin screw according to claim 2, characterized in that the first convex flank (8), which connects to the same base arc (F) as the hollow flank (6) and the third convex flank (10), to the same head arc (K) as the hollow flank (6), are congruent.

4. Twin conveyor screws according to claim 3, characterized in that the second convex flank (9) is a mirror image of the first (8) and third convex flank (10).

5. Twin screw head according to claim 1, characterized in that not all of the circular arcs (K) and consequently not all of the circular arches (F) are the same radius and that the largest, corresponding to the housing bore head circle the tooth with the hollow flank (6) assigned.

6. Twin conveyor screws according to one of claims 1 to 5, characterized in that they are designed in monobloc construction.

7. Twin conveyor screws according to one of claims 1 to 5, characterized in that they are assembled in several pieces.

8. Twin conveyor screws according to claim 7, characterized in that the rotor shaft (4) and screw body (5) are made of different materials.

9. twin conveyor screws according to one of claims 1 to 8, characterized in that in the screw interior, in the areas of the teeth, helical channels (11) are provided, which are intended to be flowed through by a cooling medium.

10. Twin conveyor screws according to one of claims 1 to 9, characterized in that the slope varies along the axis.

11. Twin screws according to claim 10, characterized in that the profile varies along the axis.