Abstract:
A pump ( 1 ) is provided with a housing ( 2 ), having an inlet ( 28 ) and an outlet ( 29 ), a drive ( 5 ), a fixed cylinder ( 2 ) centered on a mid-axis ( 9 ), a displacer ( 18 ), rotating eccentrically within the cylinder ( 2 ), a crank drive ( 13 ) for the displacer ( 18 ), a circumferential sickle-shaped pumping chamber ( 26 ) between the cylinder ( 2 ) and displacer ( 18 ) and a helical sealing element ( 27, 27′, 27″, 39 ) in the pumping chamber ( 26 ). The pump is a dry vacuum pump, whereby the displacer ( 18 ) circulates in the cylinder ( 2 ) without making contact.

Description:
BACKGROUND 
   The invention relates to a pump with a housing, having an inlet and an outlet, a fixed cylinder central to a mid-axis of the pump, a displacer, planetating eccentrically within the cylinder, a crank drive for the displacer, a circumferential sickle-shaped pumping chamber between the cylinder and displacer and a helical sealing element in the pumping chamber. Moreover, the present invention relates to a method for operating such a pump. 
   A pump having the characteristics mentioned is known from U.S. Pat. No. 5,174,737. It has the function of a compressor and is preferably intended for compressing the gas of a refrigerant circuit. 
   It is the task of the present invention to design a pump of the aforementioned kind such that it may be employed as a dry running vacuum pump. 
   Over the past years, the customers have required from the manufacturers of vacuum pumps, dry running vacuum pumps at an increasing rate. These are to be understood as pumps, the pumping chambers of which are free of lubricant. In the instance of pumps of this kind there no longer exists the risk of hydrocarbons diffusing into the chambers to be evacuated by the pumps and thereby impairing the processes (semiconductor production, evaporation processes, chemical processes etc.) being performed within the chambers. 
   Dry running rotary vane pumps are known. The parts (vanes, inside wall of the pumping chamber) which slide under friction exhibit a comparatively high relative velocity. For this reason, the service life of the vanes and thus the pumps themselves is limited. Scroll vacuum pumps are better suited for dry operation. These comprise a fixed and a revolving component which support helical pumping elements engaging into each other. Their manufacturing costs are high. Moreover, they need to be subjected to maintenance frequently so as to ensure reliable continuous operation. Also dry piston vacuum pumps are offered on the market. Their manufacturing costs are also high, their construction volume is large. Other disadvantages are noise production and the unavoidable vibrations. Finally, dry two-shaft vacuum pumps (screw, Roots, claws vacuum pumps) are known. These offer pumping capacities commencing at approximately 20 m 3 /h. Manufacture and deployment of vacuum pumps of this kind is usually, however, no longer economical at pumping capacities below 50 m 3 /h. 
   SUMMARY 
   The eccentric vacuum pump in accordance with the present invention does no longer exhibit the disadvantages detailed. Friction is substantially limited only to the movement of the helical sealing element in its groove. Significantly less is the friction between the sealing element and the inside wall of the cylinder or the outside surface of the displacer, depending on the location of the groove guiding the pumping element. Since the displacer orbits, the relative velocities between the friction partners are, however, not high so that the wear is negligible, in particular when employing suitable materials. 
   Further advantages and details of the present invention shall be explained with reference to the schematically presented examples of embodiments in the drawing  FIGS. 1 to 5 . 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. 
     drawing  FIG. 1  depicts a sectional view through a vacuum pump in accordance with the present invention of single flow design with the displacer being supported by bearings at both sides, 
     drawing  FIG. 2  depicts a sectional view through a vacuum pump in accordance with the present invention of single flow design with a cantilevered displacer, 
     drawing  FIG. 3  depicts a partial sectional view through a vacuum pump in accordance with the present invention of double flow design, 
     drawing  FIG. 4  depicts a partial sectional view through a vacuum pump in accordance with the present invention with two stages and cantilevered displacer 
     drawing  FIGS. 5   a ,  5   b , and  5   c  depict sectional views through the helical sealing element, and, 
     drawing  FIG. 6  depicts a variant ballast gas supply. 
   

   DETAILED DESCRIPTION 
   The vacuum pump  1  depicted in drawing  FIG. 1  has a cylindrical housing  2  with cap or bearing pieces  3  and  4 . Associated with the cap piece  3  is the drive motor  5 . The motor shaft  6  penetrates the cap piece  3  and is supported in the bearing  7 . The motor shaft  6  is a component of a planetary system  8 , the mid-axis of rotation of which is designated as axis  9  and which is supported by means of a shaft connection piece  11  via the bearing  12  in the cap piece  4 . 
   A further component of the planetating system  8  is a crank  13  which is located at the level of the cylindrical housing  2 . e designates the eccentricity. The end sections  14  and  15  of the crank  13  are equipped with bearings  16  and  17  which support a hollow (hollow space  20 ) planetating displacer  18 . The revolving movement of the substantially cylindrical displacer  18  is effected about the mid-axis  9 . The crank axis is designated as  19 . For the purpose of securing the axial position of the displacer  18 , one of the two bearings  16 ,  17 —in this instance bearing  16 —is designed by way of a spherical roller bearing. 
   The cylindrical housing  2  which simultaneously has the function of a cylinder stator of pump  1  is arranged centrally with respect to the mid-axis  9 . The diameter of the displacer  18  is selected such that it does not make contact with the inner wall of housing  2 . The smallest distance between housing  2  and displacer  18  shall be as small as possible, expediently significantly less than 1 mm, 0.2 mm for example. 
   In order to prevent the turning motion of a planetating displacer it is known to employ torque supports (Oldham coupling, leaf springs, wire springs or alike). In the embodiment in accordance with drawing  FIG. 1 , an additional revolving eccentric is provided and designated as  21 . It is supported by stubs in the displacer  18  and in the cap piece  4 . For rotary bearing support of the eccentric within the displacer  18  and within the cap piece  4 , dry plain bearings or grease lubricated rolling bearings (not depicted) may be employed, for example. For attaining an unambiguous kinematic condition for the displacer  18 , at least two eccentrics  21  need to be employed which are, for example, arranged offset by 120°. The depicted kinematics result in a planetary motion of the displacer  18  relative to crank  13  with axis of rotation  19 . 
   The middle, substantially cylindrical section  22  of the crank  13  with its axis  23  is also arranged eccentrically with respect to mid-axis  9 , specifically exhibiting eccentricity E. The directions of the eccentricities e and E are opposed to each other. The eccentricity E and the mass of the middle section  22  are selected such that unbalance forces causing the masses of the rotating crank sections  14  and  15  with bearings  16  and  17  as well as the mass of the planetating displacer  18  during operation of the pump  1 , are compensated. 
   Located between the housing  2  and the displacer  18  is the sickle-shaped pumping chamber  26 . A helical sealing element or band  27  forms the pumping chambers which move from the inlet  28  of the pump  1  to the outlet  29 . On the inlet side, pumping chambers are created continuously which close during the rotary movement of the displacer  18  and which only open again on the outlet side. In the embodiment depicted in drawing  FIG. 1  the inlet  28  is located at cap piece  4 . An outlet chamber  29  is located in cap piece  3 . An adjacent outlet port is not depicted. 
   The sealing element  27  is a helical, flexible rectangular band, the cross-section of which is long stretched out. It is guided in a groove  30  in the displacer  18 . In the relaxed state the sealing element  27  exhibits an outside diameter which is slightly larger than the inside diameter of the bore in cylinder  2 . Thus, in the fitted state it is subjected to an initial tension acting radially towards the outside, so that leak tight resting of the sealing element  27  against the inside wall of the housing  2  is ensured. The radial width b of the sealing element  27  is greater than twice the magnitude of the eccentricity e. Thus the closed state of the pumping chambers during their motion from inlet  28  to outlet  29  as well as reliable guidance of the sealing element  27  within the groove  30  is ensured, and reverse flows are prevented. Play of the sealing element  27  within the groove  30  should be as small as possible, for example 0.2 mm. 
   Although there exists between housing  2  and the sealing element  27  no significant friction, torque caused by friction between sealing element  27  and groove  30  is exerted on the sealing element  27  during operation of the pump  1 . A therefrom resulting axial shift of the sealing element  27  is expediently prevented by barriers. Such a barrier may, for example, be designed by way of a stop within the groove  30  of the displacer  18 . Another possibility exists in that an end section of the sealing element  27  is affixed at the housing  2  or at one cap piece  3 ,  4  in such a manner that the end section cannot turn about the axis  9 , but nonetheless exhibits in the axial direction a slight amount of play (see drawing  FIG. 2 ). 
   In the embodiments depicted in drawing  FIG. 1  the pitch of the groove  30  in displacer  18  decreases steadily and thereby also the pitch of the sealing element  27  decreases steadily from the inlet  28  to the outlet  29 . The same applies also to the volumes of the pumping chambers moving from the inlet  28  to the outlet  29  so that a compression of the sucked in gases is effected. In order to avoid, at the beginning of an evacuation phase, inadmissibly high overpressures within the pump, a relief valve  32  is provided. It is located between inlet  28  and outlet  29  and opens a bore  33  within housing  2 , should inadmissibly high pressures occur. The relief is effected through channels  34 ,  35  which run directly to the outlet  29 . 
   In the example of the embodiment according to drawing  FIG. 1  the chance that the hollow space  20  of the displacer  18  creates a short-circuit between inlet  28  and outlet  29  and that hydrocarbons from this hollow space  20  enter into the area of the inlet is to be prevented. These tasks are fulfilled firstly by the seals  41 ,  42  which seal off the passages of the end sections  14 ,  15  of the crank  13  through the face side openings within displacer  18 . Moreover, it is expedient to employ for lubricating the bearings  16 ,  17  a grease which is free of hydrocarbons. Finally it is expedient to maintain within the inner chamber  20  of the displacer a low pressure, 80 mbar for example. This may be effected by means of a bore  43  within the displacer wall. The bore opens out into the pumping chamber  26  specifically within the area in which the desired internal pressure within the hollow space of the displacer prevails. Through this provision the pressure difference present at the seal  42  is considerably reduced. 
   The embodiment depicted in drawing  FIG. 2  differs from the embodiment in accordance with drawing  FIG. 1  in that the planetating system  8 , as well as the thereby supported displacer  18 , are supported in a cantilevered manner on the shaft  6 . The shaft  6  in turn is supported by the bearing  7  in the pump housing  2  and a further, not depicted, bearing in the motor housing. This provision offers the advantage that the hollow inside space  20  of the displacer  18  can be sealed off tightly (cover  44 ) on the intake side. For the purpose of preventing the turning movement of the displacer  18 , an Oldham coupling  45  is provided. The sealing element  27  is affixed by means of an axial pin  46  at cap piece  4 . The pin  46  penetrates a bore  47  in the sealing element  27  which prevents the sealing element from rotating about the axis  9 , permitting, however, play in the axial direction. 
   Two variants for a gas ballast supply are depicted. In the first variant, the ballast gas enters through a line  51  from outside through a bore, not specifically depicted, in housing  2  into the pumping chamber  26 . In the line  51  there are present a blocking valve  52 , a non-return valve  53  and a differential pressure valve  54 . A gas ballast facility of this kind is known from U.S. Pat. No. 6,776,588. 
   In the second variant, the ballast gas is additionally (drawing  FIG. 2 ) or alternately (drawing  FIG. 6 ) supplied through the hollow space  20  of the displacer  18 . A system of channels  55  in the planetating system  8  forms the link to the outside. 
   Ballast gas (arrows  56 ) supplied through the system of channels passes through a bore  57  (depicted by dashed lines) in the displacer wall into the pumping chamber  26 . The advantage of this embodiment is such that the displacer is cooled from the inside by the ballast gas. 
   In the embodiment in accordance with drawing  FIG. 2  the gases pumped by the pump exit the pumping chamber  26  through a bore  59  in housing  2 . The bore opens out into the channel  34  which is linked to the outlet  29  of the pump. The planetary movement of the displacer  18  and the pitch of the helical groove  30  are so selected that during operation of the pump  1 , the individual pumping chambers in the pumping chamber  26  move from the inlet  28  to the bore  59  (arrow  61 ). In the instance of the embodiments depicted, the displacer  18  with its section  62  extends over the bore  59 . The same also applies to the groove  30 . However, the pitch of the groove  30  is so selected that a further, independent sealing element  27 ′ forms pumping chambers which oppose (arrow  63 ) the direction of the pumping action between inlet  28  and bore  59 . Ultimately the pump is of a double flow design. It exhibits two pumping stages which provide a pumping action from the respective face sides in the direction of bore  61 . If a link is provided between the hollow space  20  of the displacer and the suction side of the section  62  (arrows  64 ), then there exists the possibility of maintaining a low pressure within the hollow space  20 . Moreover, effective cooling of the pump can be implemented. Cooling gas flowing through the system of channels  55  in the planetating system  8  into the hollow space  20  passes on to the suction side of the section  62  and is removed from the pumping chamber  26  jointly with the pumped gas through the bore  59  and the outlet  29 . In this manner it is also prevented that gas can pass from the inlet  28  of the pump into the hollow space  20  and the therein located bearings  7 ,  16  and  17 . This is, for example, desirable when corrosive or caustic gases shall be pumped. 
   Drawing  FIG. 3  depicts a double flow design with a center inlet  28  and two face side outlets  29  and  29 ′ indicated only by arrows. Located to the side of inlet  28  are two pump sections of which only one is depicted. The section not visible is designed as a reversed image with respect to the visible section. The two pumping sections provide a pumping action each from the inlet  28  to the outlets  29 ,  29 ′ respectively. The rotating system  8  (axis  9 ) as well as the rotating displacer  18  extend over the entire length of the pump  1 . Driving is effected through the motor  5  and a vacuum coupling not depicted in detail. Two sealing elements  27 ,  27 ′ form the pumping chambers which move from inside to outside. In contrast to the embodiment in accordance with drawing  FIG. 1 , the grooves  30 ,  30 ′ guiding the sealing elements  27 ,  27 ′ are located in housing  2 . The respective inner narrow side of the sealing element  27 ,  27 ′ rests against the cylindrical outer wall of the displacer  18 . This is attained in that the helical sealing elements  27 ,  27 ′ have, in the relaxed state, a diameter which is smaller than the outside diameter of the displacer  18 . 
   The special advantage of the embodiment in accordance with drawing  FIG. 3  is that the two outlets  29 ,  29 ′ are arranged on the face sides. The two face sides of the displacer need no longer to be sealed off in a vacuum-tight manner. There even exists the possibility of modifying the pump such that a cooling agent—for example, cooling air supplied by a fan—flows through the hollow space  20 . A further advantage is that no significant axial forces are exerted on the bearings because axial gas forces and friction forces cancel each other. 
   In the embodiment in accordance with drawing  FIG. 4 , a two-stage pump  1  according to the present invention is presented. It has an outer housing  2  with two helical grooves  30  and  30 ″, in which a sealing element  27 ,  27 ″ is guided in each one. The arrangement corresponds to that of a double thread. The sealing elements  27 ,  27 ″ rest against the cylindrical outer surfaces of the rotating displacer  18 . These form pumping chambers which in the sickle-shaped pumping chamber  26  move from the free side face  31  of the housing  2  to the outlet  29  of the pump  1 . 
   Both the crank  13  (crank section  14 ) and also the rotating displacer  18  are cantilevered such that in the area of the side face  31  bearings are no longer required. The crank section  14  exhibits a step. The displacer  18  is supported in a cantilevered manner by the two bearings  16 ,  17  having different diameters. 
   In the example of the depicted two-stage version, a further pump stage is located upstream of the pump stage formed by the sealing elements  27 ,  27 ″ and the outside wall of the displacer  18 . To this end, the displacer  18  is designed according to the type of a double pot. 
   Located in one of the hollow spaces on the face side are the crank  13  as well as the bearings  16 ,  17 . Located in the second—opposite—hollow space  36  with the side face  31 , is a further pumping stage. In the housing  2 , a cylindrical component  35  is affixed centrally with respect to axis  9  by means of a flange  34 , the cylindrical component extending into the inner space  36  of the displacer  18 . The diameter of the cylindrical component is so selected that its outside wall and the inside wall of the displacer  18  form a further sickle-shaped pumping chamber  37 . The outside wall of the cylindrical component  35  (or the inside wall of the displacer  18 ) is equipped with a helical groove  38  in which a further sealing element  39  is guided. 
   The pump stage formed by component  35 , displacer  18  and the sealing element  39  serves as the first stage of a two-stage pump  1  in accordance with the present invention. It pumps from the bearing side in the direction of the side face  31 . In this area, the pumping chambers  37  and  26  are linked to each other. The inlet  28  is formed by a central bore  60  in component  35 . The pitches of the groove  38  in the component  35  and the grooves  30 ,  30 ′ in housing  2  are constant (easy to manufacture) but selected to differ in size. The pitch of the groove  38  is greater than the pitch of the grooves  30 ,  30 ′. During the passage through the two-stage pump  1  a compression of the pumped gases is effected. A special advantage of the embodiment detailed is that the high-pressure stage is located outside. The heat mostly generated in the high-pressure stage can be simply dissipated, be it through cooling channels in housing  2  or—as shown—through heat sinks  51  having a relatively large surface area. 
   The helical sealing element  27 ,  27 ′,  27 ″,  39  has the task of mutually sealing the pumping chambers moving from the intake side to the delivery side. Moreover, the frictional resistance between the sealing element and the involved components  2 ,  18 ,  35  is minimal. In the drawing  FIGS. 5   a  to  5   c  embodiments of the sealing elements  27  are depicted. In the embodiment in accordance with drawing  FIG. 5   a  the sealing element  27  rests flush against the inner side of the stator housing  2  with a substantially axially oriented sealing lip  71 . The recess  72  located under the sealing lip  71  is open towards the side at the higher pressure so that a flexible and reliable contact of the sealing lip  71  is ensured. The embodiments of the sealing element  27  in accordance with drawing  FIGS. 5   b  and  5   c  exhibit in the area of the groove  30  radially oriented sealing lips  73 ,  74  differing in length. These have the effect of a reduced friction resistance between the sealing element and the side walls of the groove. 
   The examples of embodiments detailed differ chiefly with respect to their bearings as well as with respect to the number, pitch and selection of the location of the guide grooves for the sealing element(s). As a precaution it is pointed out that the variants detailed here can be implemented in any of the examples of embodiments detailed. The present invention permits, at low manufacturing cost, the production of a compact, dry running, low noise and low vibration vacuum pump which is also economical at low pumping capacities (under 50 m 3 /h). It suffices when the rotational speed of the planetating components is between 1500 and 3600 rpm. Cooling of the pump is simple since all important components are in contact with the atmosphere. 
   Of importance to the service life of the pump is the selection of the materials for the components between which there is friction. For the helical sealing element  27 ,  27 ′,  39 , PTFE or a PTFE compound is well proven, as employed also in piston or scroll vacuum pumps. The displacer  18  and/or the housing  2  as well as the component  35  consist expediently of an aluminium material, preferably of a hard anodized aluminum alloy, AlMgSi1, for example. When employing these or similar materials it is possible, in spite of the absence of lubricants in the pumping chamber, to permit high sliding velocities between the sealing element(s) and the related grooves. The sliding velocity depends on the rotational speed of the crank and on the degree of eccentricity e. The higher these values are, the more compact a pump offering a certain pumping performance can be manufactured. Expediently, planetating speed and eccentricity are so selected that the sliding velocity ranges between 1 and 5 m/s, preferably 4 and 5 m/s. 
   The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.