EP0472751B1 - Rotor for a lobe rotor type vacuum pump - Google Patents

Description

The invention relates to a rotor for a Roots vacuum pump with two identical rotors, each of which has an essentially eight-shaped profile contour consisting of four base profile sections and four head profile sections.

Roots vacuum pumps with rotors of this type have been widely used. The rotary movement of the rotors, which are synchronized by a gearbox, follows at the same speed and takes place without contact with one another and towards the housing wall. The rotors are not subject to mechanical wear and can be operated at high speeds. The gap width between the housing wall and the rotors and between the rotors themselves is a few tenths of a millimeter.

1:

Rotoren

2:

Drehachsen der Rotoren

3:

kurze Achse der Rotoren

4:

lange Achse der Rotoren

5:

Wälzkreis

6:

Fußprofil des Rotors (innerhalb des Wälzkreises 5 liegende Rotorkontur)

7:

Kopfprofil des Rotors (außerhalb des Wälzkreises 5 liegende Rotorkontur)

8, 9:

Fußprofilabschnitte

11, 12:

Kopfprofilabschnitte

13:

Umhüllende bzw. Kontur des Schöpfraumes

a:

Wälzkreisdurchmesser bzw. Abstand der Drehachsen 2

B:

kurzer Durchmesser des Rotors (Taillenbreite)

C:

Berührungspunkt der Wälzkreise 5

D:

langer Durchmesser des Rotors

E₁-E₄:

Eingriffslinien

F:

Rotorschöpffläche

Q:

Querschnittsfläche des Schöpfraumes

t_o, t:

Tangenten durch einen "Berührungspunkt"

x, y:

rotorfestes Koordinatensystem (Figur 4)

x_f,yf:

orts-(gehäuse-)festes Koordinatensystem (Figur 3)

φ:

Zentridrehwinkel

α(φ):

Steigung der Tangente an die Profilkontur im "Berührungspunkt" der beiden Rotoren im Koordinatensystem x/y

Die Figuren 2, 3, 4 zeigen, daß sich die Profilkontur eines Rotors 1 aus vier Fußprofilabschnitten 8, 9 und vier Kopfprofilabschnitten 11, 12 zusammensetzt. Die Fußprofilabschnitte 8, 9 erstrecken sich vom Fußgrund (Schnittpunkt der Kontur mit der kurzen Rotorachse 3) bis zum Schnittpunkt der Kontur mit dem Wälzkreis 5. Die vier Fußabschnitte sind paarweise (Paarungen 8, 8 und 9, 9) einander gleich bzw. spiegelsymmetrisch (Paarungen 8, 9). Die Kopfprofilabschnitte erstrecken sich jeweils vom Schnittpunkt der Kontur mit der langen Rotorachse 4 bis zum Schnittpunkt mit dem Wälzkreis 5. Auch die vier Kopfprofilabschnitte sind paarweise einander gleich (Paarungen 11, 11 und 12, 12) bzw. spiegelsymmetrisch (Paarungen 11, 12). Die in den Figuren 1a und 2 bis 4 dargestellten Profilkonturen entsprechen der Kontur von Rotoren nach dem Stand der Technik (Evolvente), welche in der Vakuumtechnik vielfache Anwendung gefunden haben.Figures 1 to 5 show sections through known rotors or rotor pairs of Roots pumps of the type concerned here. FIG. 1a shows a rotor profile contour developed from an involute. The rotor profile contour according to Figure 1b is known from CH-PS 389 817 (Figures 3, 6, "straight" rotors). The profile contour according to FIG. 1c has sections which correspond to the shape of a cycloid. Figures 2, 3 and 4 again show the profile contour according to Figure 1a. The letters and numbers entered in these figures have the following meanings:

1:

Rotors

2:

Rotation axes of the rotors

3:

short axis of the rotors

4:

long axis of the rotors

5:

Pitch circle

6:

Base profile of the rotor (rotor contour lying within the pitch circle 5)

7:

Head profile of the rotor (rotor contour lying outside the pitch circle 5)

8, 9:

Foot profile sections

11, 12:

Head profile sections

13:

Enveloping or contour of the scoop space

a:

Pitch circle diameter or distance of the rotary axes 2

B:

short diameter of the rotor (waist width)

C:

Contact point of the pitch circles 5

D:

long diameter of the rotor

E₁-E₄:

Lines of intervention

F:

Rotor head area

Q:

Cross-sectional area of the scoop space

t _o , t:

Tangents through a "point of contact"

x, y:

rotor-fixed coordinate system (Figure 4)

x _{f, yf} :

fixed (housing) coordinate system (Figure 3)

φ:

Center rotation angle

α (φ):

Slope of the tangent to the profile contour at the "contact point" of the two rotors in the coordinate system x / y

Figures 2, 3, 4 show that the profile contour of a rotor 1 is composed of four base profile sections 8, 9 and four head profile sections 11, 12. The foot profile sections 8, 9 extend from the foot base (intersection of the contour with the short rotor axis 3) to the intersection of the contour with the pitch circle 5. The four foot sections are in pairs (pairings 8, 8 and 9, 9) the same or mirror-symmetrical ( Pairings 8, 9). The head profile sections each extend from the intersection of the contour with the long rotor axis 4 to the intersection with the pitch circle 5. The four head profile sections are also identical in pairs (pairings 11, 11 and 12, 12) or mirror-symmetrical (pairings 11, 12). The profile contours shown in FIGS. 1a and 2 to 4 correspond to the contour of rotors according to the prior art (involutes), which have been used many times in vacuum technology.

α(φ) ist die Steigung der Tangente in einem Berührungspunkt an die Profilkontur. Bei einem Zentrierwinkel φ = 0 ist beispielsweise die Steigung der Tangente t_o im Berührungspunkt an die Profilkontur (Fig. 3) gleich 0.With each position of the two rotors in the scoop, there is a constriction which - although the rotors do not touch each other - shall be referred to below as the point of contact. At the point of contact, a point of a foot profile section of the first rotor with a point of a corresponding head profile section of the second rotor form the constriction. In a Cartesian xy coordinate system fixed to the rotor (FIG. 4), each value of the central angle corresponds to φ

$\text{f (φ) = Y (φ) = α ()}$

α (φ) is the slope of the tangent at a point of contact with the profile contour. With a centering angle φ = 0, for example, the slope of the tangent t _o at the point of contact with the profile contour (FIG. 3) is 0.

During the operation of the pump or during the rotation of the rotors, the point of contact moves on a closed line. The typical line of engagement for the rotor profile contour according to FIGS. 1a, 2 to 4 (involute) is shown in FIG. 3 and designated E₁. It has an eight-like shape with a center labeled C. The line of engagement E 1 is shown in a stationary or housing-fixed coordinate system x _f , y _f , the 0 point of which lies in the axis 2 of a rotor. The coordinate system x _f , y _f is shown in FIG. 3.

wobei n die Drehzahl der Kolben ist. Je größer F (und damit V) bei fest vorgegebener Querschnittsfläche Q ist, desto größer ist der Flächennutzungsgrad µ und damit Q_th. Bei vorgegebenem Saugvermögen führt deshalb ein großer Flächennutzungsgrad µ zu kleinen und kompakten Wälzkolbenvakuumpumpen, was Auswirkungen auf die Material- und Herstellkosten und damit auf den Preis der Pumpen hat.A characteristic value for the properties of a Roots vacuum pump of the type concerned here is the area utilization factor µ, defined as the ratio of four times the rotor head area F to the cross-sectional area Q of the scooping area 13. The volume V delivered with each half rotation of the rotors is equal to the product of the rotor scooping area F and the length l of the scooping area 13, so that the following applies to the theoretically conveyable amount of gas (suction capacity):

${\text{Q}}_{\text{th}}\text{= 4 · V · n = 4 · F · l · n}$

where n is the speed of the pistons. The larger F (and thus V) for a fixed cross-sectional area Q, the greater the area utilization factor µ and thus Q _th . For a given pumping speed, a large area efficiency µ leads to small and compact Roots vacuum pumps, which has an impact on the material and manufacturing costs and thus on the price of the pumps.

Another characteristic value for the properties of a Roots vacuum pump is the volumetric efficiency η. It is defined as the ratio of the effectively extracted gas quantity Q _eff to the theoretically extractable gas quantity Q _th . Because of the gaps which are necessarily present in a tumbler vacuum pump of the type concerned here (contact-free movement of the rotors), backflows of the gases are inevitable and therefore Q _{eff is} always less than Q _th . The larger η is, the better the compression behavior of a Roots vacuum pump. A relatively large η could be achieved, for example, by small gap widths at the "point of contact" on the one hand and between the rotors and the wall of the suction chamber on the other. Small gap widths result in a high temperature sensitivity of the pump. The reason for this is that the heat removal from the pistons rotating in a vacuum is limited. In the case of small gap widths, a slight increase in the temperature of the rotors leads to a gap consumption and thus to the rotor starting.

With regard to the optimization of the properties of a Roots pump, the largest possible values for µ and η are desirable.

At the same time, it must be taken into account that, in particular for the manufacture of the rotors, complex machining processes are required because of their special profile contours and because of the small gap widths. Because of these complex machining processes, the production of larger Roots machines with less favorable µ and η values was often cheaper in the past than the production of smaller machines with better µ and η values, but otherwise the same performance data.

With regard to the state of the art, reference is made to the book by G. Niemann and H. Winter "Maschinenelemente", vol. 2, 1985, and to the content of the above-mentioned CH-PS 389 817. On the one hand, these references show that a large number of rotor profile contours are known (CH-PS), which for the most part have not become established on the market. On the other hand, these references show that the profile contour of the rotors of Roots blowers or pumps introduced on the market are generated with the aid of roll curves (cycloids, involutes) (CH-PS 389 817, FIG. 2; "Machine elements", Vol. 2, P. 142). Roots pumps with such rotors have an area efficiency µ that does not significantly exceed 50 percent.

CH-PS 389 817 also discloses a rotor profile contour with straight, parallel longitudinal sides for the foot profile (straight rotors). According to the information in the Swiss patent mentioned, this contour enables a relatively low final pressure to be achieved. However, a significant reduction in the area utilization rate is accepted, since the rotor surface area F is larger in pumps with waisted pistons than in pumps with straight pistons (cf. surfaces 9 and 32 in FIGS. 4 b and 6 b of the CH-PS 389 817). Machines with rotor profile contours of the type proposed in CH-PS 389 817 therefore build relatively large, are correspondingly heavy and are therefore relatively expensive.

The present invention has for its object to improve the profile contour of the rotors for a Roots vacuum pump.

According to the invention, this object is achieved by the characterizing features of the claims. By using these features, the contour of a foot profile section of a first rotor and the contour of a corresponding head profile section of a second rotor are obtained. Since these sections are identical to or mirror-symmetrical to the other base or head profile sections of the rotors because of the identity of the rotors, the entire profile contour of the rotors can be developed for a Roots machine.

For a Roots machine of this type it applies that it has a high area utilization factor µ (62% and higher). As a result of this high degree of space utilization, a given small suction capacity results in a relatively small cross-sectional area Q and thus a compact and inexpensive construction.

At the same time, a Roots vacuum pump with rotors of the type according to the invention has a relatively high volumetric efficiency η. The reason for this is that the profile slope in the bottom of the waist (with small φ values) is kept low. This results in favorable osculation values both between the rotors themselves and between the rotors and the wall of the suction chamber. The osculation is to be understood as the ratio of the radii of curvature of the surfaces forming the gaps. With favorable osculation values, the radii of curvature differ only slightly from one another. This practically lengthens the gaps between the pistons themselves and between the pistons and the wall of the pump chamber and thus a lower backflow rate. This extension of the column does not affect the temperature behavior of the pump.

To reduce the backflow rates and thus to improve the volumetric efficiency further contributes to the fact that, despite high μ values, the "contact point" on the line of engagement carries out a constant movement, ie a movement without regression or jumps (sudden skipping of larger profile contour sections), so that there are no dead volumes or other contaminated areas during the entire pass-through, and thus volume inclusions or volume carryovers do not occur.

The rotor profile contour according to the invention has a uniform and continuous course, so that there are considerable manufacturing advantages. There are no abrupt changes in incline. A minimum (tool-related) radius of curvature is not undercut.

Further advantages and details of the invention will be explained with reference to FIGS. 5 to 7.

Figure 5 shows a pair of rotors with a profile contour according to the invention. The area utilization rate µ is 64%. The slope at the bottom of the waist is small over a relatively wide range. It can therefore be seen that the cuddling of the rotors to one another and to the envelope is better than with the rotors according to the prior art.

ist gestrichelt eingezeichnet.Figure 6 shows the course of the curve α (φ) depending on the central angle φ from 0 ° to 45 °. Except for the start and end range (each about 5 °), α (φ) is larger than φ. Straight $\text{α (φ) = φ}$

is shown in dashed lines.

Figure 7 represents lines of engagement E₁, E₂, E₃ and E₄, in the fixed coordinate system x _f , y _f . Half the waist width B and half the pitch circle diameter are shown. The lines of engagement E₁, E₂, E₃ belong to the rotor profiles according to the prior art, namely

The outer, sine-like line of engagement E₄ characterizes a particularly advantageous embodiment of the rotor profile contour according to the invention, i.e. it is expedient if the rotor profile contour according to the invention is chosen so that its associated line of engagement E₄ has the greatest possible amplitude and a sine-like shape.

After the profile contour sections 8, 11 have been shaped in accordance with the present invention, the shape of the other contour sections 9, 12 and thus the complete rotor profile is fixed, since the other contour sections are identical to or mirror-symmetrical to the sections 8, 11. To set application-specific clearances (gap widths), the profile contour can be provided with an equidistant depending on the angle of rotation.

The use of rotors with the profile contour according to the invention in Roots vacuum pumps leads to an optimized solution with significant improvements compared to the prior art. Despite the relatively large area utilization, a continuous contour can be achieved and minimal radii of curvature avoided, which not only results in a compact design (smaller, lighter), but also in simplified production. The improved osculation process and the relatively even profile contact point movement lead to a favorable volumetric efficiency. Overall, an improvement in the pump properties is achieved with a simultaneous reduction in manufacturing costs.

Claims (3)

1. Roots vacuum pump with two identical rotors which have a profile contour which is essentially shaped like a figure eight and consists of four root profile sections (8, 9) and four tip profile sections (11, 12), characterised in that

   a) the root profile section (8) of the rotors (1) has the following properties in a coordinate system (x, y, φ) which is fixed with respect to the rotors

   - the y value corresponding to half the minor rotor diameter B when
   
   $\text{φ = x = o}$

   

   
   
   lies between 0.5 a (a = rolling circle diameter) and a (1 - √2/2),

   - the following applies for the values between φ = 0° and φ = 45° for the tangent gradient angle α(φ):
   up to ≈ 5° : α(φ) < φ
   from ≈ 5° to ≈ 40° : α(φ) > φ
   from ≈ 40° upwards : α(φ) < φ
   and

   b) the contour of the tip profile (11) of the second rotor (1) which corresponds to the root profile contour (8) of the first rotor (1) corresponds to a generation of the contour of the root profile (8) of the first rotor.
2. Roots vacuum pump according to claim 1, characterised in that the following applies for the tangent gradient angle α(φ) as a function of φ:
   α(φ) > 40° for φ ≧ 15°.
3. Roots vacuum pump according to claim 1, 2 or 3, characterised in that an equidistance is taken into account to form rotor clearance values.

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