Abstract:
A vane pump includes a casing having a cylindrical inner bore and a rotor disposed in the inner bore with an eccentric relation thereto, forming a circular pump chamber between the rotor and the inner bore. The circular pump chamber is divided by vanes held in the rotor into pump chambers changing their capacities according to rotation of the rotor. A gravity center of the rotor is shifted from the rotational center of the rotor by removing or adding some weight, so that an imbalanced centrifugal force is applied to the rotor. The driving shaft and the rotor are tightly coupled to each other by the imbalanced centrifugal force even if there is a small gap or dimensional errors therebetween.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims benefit of priority of Japanese Patent Application No. 2004-321909 filed on Nov. 5, 2004, the content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a vane pump for compressing fluid. 
   2. Description of Related Art 
   An example of a vane pump having a rotor rotating in an eccentric relation with respect to an inner bore of a casing is disclosed in JP-A-2003-222089. The rotor rotates in the inner bore of the casing at a high speed. If there is dimensional errors in the housing and the rotor, or there is an inclination in coupling the rotor with a driving shaft, it is possible that the rotor contacts the inner bore of the casing. If this happens, noises will be generated, and the rotor and the casing will be damaged by abrasion. In the worst case, the rotor will be locked. Further, the rotor may be deformed in a process of press-fitting the driving shaft into the rotor. 
   In order to avoid these troubles, all the components of the vane pump, i.e., the driving shaft, the rotor and the casing have to be machined with a high precision. Alternatively, it would be preferable to loosely couple the driving shaft to the rotor with a certain gap therebetween to absorb dimensional errors. However, if there is a gap between the driving shaft and the rotor, the rotor vibrates relative to the driving shaft when it is rotated, generating noises and making compression pressure unstable. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved vane pump which is easily manufactured and stably operates with low noises. 
   The vane pump is composed of a casing having a cylindrical inner bore, a rotor disposed in the inner bore to form a circular pump chamber between the inner bore and rotor, and a driving shaft for rotating the rotor. The rotor is disposed in the inner bore of the casing so that a rotational center of the rotor is positioned eccentric with respect to the center of the inner bore. Vanes are slidably held in grooves formed in the rotor, and radial ends of the vanes contact the inner bore by a centrifugal force generated according to rotation of the rotor. The circular pump chamber is divided by the vanes into a few pump chambers changing their capacities according to rotation of the rotor. Fluid is introduced into the pump chamber and compressed therein, and the compressed fluid is pumped out of the vane pump. 
   The driving shaft is coupled to the rotor with a small gap therebetween. A gravity center of the rotor is positioned to be eccentric with respect to the rotational center of rotor, so that an imbalanced centrifugal force is applied to the rotor when the rotor is rotated. The driving shaft and the rotor are closely coupled to each other by the imbalanced centrifugal force notwithstanding the small gap therebetween. 
   The gravity center of the rotor maybe shifted by removing some weight from the rotor. A depression or depressions may be formed in the rotor to remove some weight. A cutout groove or grooves may be made on the circumferential surface of the rotor. Alternatively, some weight may be added to the rotor to place the gravity center eccentrically with the rotational center. In this case, a material heavier than the rotor material may be disposed in holes formed on the rotor. 
   According to the present invention, it is not required to machine the driving shaft and the rotor with a high precision or to press-fit the driving shaft into the rotor. A small gap between the driving shaft and the rotor or some dimensional errors are overcome by the imbalanced centrifugal force, and the driving shaft is tightly coupled to the rotor. The vane pump can be operated under a stable pressure with low noise. 
   Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view showing a vane pump as a first embodiment of the present invention, viewed from direction I shown in  FIG. 2 , removing an upper plate; 
       FIG. 2  is a cross-sectional view showing the vane pump, along line II-II shown in  FIG. 1 ; 
       FIG. 3  is a schematic view showing a rotor of the vane pump; 
       FIG. 4  is a plan view showing a vane pump as a second embodiment of the present invention; and 
       FIG. 5  is a plan view showing a vane pump as a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A first embodiment of the present invention will be described with reference to  FIGS. 1-3 . A vane pump pressurizes fluid such as liquid or gas sucked thereinto. In this particular embodiment shown in  FIG. 1 , fluid is pressurized therein. A vane pump  10  includes: a casing composed of a ring  20  and a pair of plates  31 ,  32 ; a rotor  40 ; vanes  41 ; and a driving shaft  13 . The rotor  40  disposed in an inner bore  21  of the ring  20  is coupled to the driving shaft  13  and rotated by a motor  11 . The motor  11  may be an electric motor such as a direct current motor or an alternating current motor. The motor  11  is contained in a cover  12 . 
   The ring  20  is cylinder-shaped and has a cylindrical inner bore  21 . The inner bore  21  may be formed in an oval form. Both axial ends of the ring  20  are closed with a pair of plates  31  and  32 . The plate  31  is a lower plate positioned at the motor side and the plate  32  is an upper plate, as shown in  FIG. 2 . The rotor  40  is disposed in the inner bore  21  of the ring  20 . A rotational center of the rotor  40 , where a center hole  42  coupled to the driving shaft  13  is formed, is positioned in an eccentric relation with respect to a center of the inner bore  21 . A space between the rotor  40  and the inner bore  21  of the ring  20  closed with the plates  31 ,  32  constitutes a circular pump chamber  22 . A capacity of the pump chamber  22  is not uniform in its circular direction, but continuously changes as shown in  FIG. 1  because of the eccentric positioning of the rotor  40  relative to the inner bore  21 . 
   As shown in  FIG. 2 , an inlet port  23  communicating with the pump chamber  22  is formed in the lower plate  31 , and an outlet passage  24  communicating with the pump chamber  22  is formed between a groove  25  of the lower plate  31  and the ring  20 . According to rotation of the rotor  40 , fluid is sucked into the pump chamber  22  from the inlet port  23 , pressurized in the pump chamber  22  and pumped out through the outlet passage  24 . 
   The rotor  40  has a center hole  42  formed in the rotational center of the rotor  40 . The driving shaft  13  is inserted into the center hole  42 . As shown in  FIG. 2 , the center hole  42  has a circular cross-section from the lower end up to its middle portion and has a half circular cross-section from the middle portion to the upper end, thereby forming a step  43  at the middle portion. The driving shaft  13  has a cross-section corresponding to the cross-section of the center hole  42 . That is, a lower portion of the driving shaft  13  has a circular cross-section and its upper portion has a half circular cross-section, forming a step  14  at its middle portion. 
   The half circular cross-section of the driving shaft  13  is composed of an arc portion and a chord portion that are coupled to those of the half circular cross-section of the center hole  42 . When the driving shaft  13  is coupled to the rotor  40 , the step  14  of the driving shaft  13  abuts the step  43  of the center hole  42 . The outer diameter of the driving shaft  13  is made a little smaller than that of the center hole  42 , so that a small gap exists between the driving shaft  13  and the center hole  42 . 
   The rotor  40  has grooves  44 , formed in its outer periphery, extending in the axial direction. In this particular embodiment, four grooves  44  are formed at an equal interval. The number of the vanes  41  is not limited to four but it may be variously selected. A vane  41  is disposed in each groove  44  so that the vane  41  is able to reciprocally move in the groove  44  in the radial direction. A distance between the outer periphery of the rotor  40  and the inner bore of the ring  20  changes according to rotation of the rotor  40  because the rotor  40  is eccentric with respect to the inner bore  21 . An outer end of each vane  41  contacts the inner bore  21  by a centrifugal force generated according to rotation of the rotor  40 . As the distance between the outer periphery of the rotor  40  and the inner bore  21  changes according to rotation of the rotor  40 , the vane  41  slidably moves in the groove  44  in the radial direction. 
   The rotor  40  has depressions  45  extending in its axial direction up to a middle portion of the rotor  40 . In this particular embodiment, two depressions  45  are formed. By making the depressions  45 , some of the rotor mass is removed. As shown in  FIG. 3 , the rotor weight of the left half with respect a symmetric line “p” in  FIG. 3  becomes lighter than that of the right half. In other words, a gravity center of the rotor  40  moves from a rotational center “q” to a point “c” (a new gravity center). 
   When the rotor  40  rotates around the rotational center q, a centrifugal force is applied to the rotor. Since the gravity center c is positioned at the right side of the rotational center q, a centrifugal force f 2  toward the right side is larger than a centrifugal force f 1  toward the left side. Therefore, the rotor  40  is pushed against the driving shaft  13  in the rightward direction (in  FIG. 3 ). This means that the rotor  40  is pushed against the arc portion of the half circular cross section of the driving shaft  13 . In this manner, the rotor  40  is tightly coupled to the driving shaft  13  although the small gap is provided between the driving shaft  13  and the center hole  42  of the rotor  40 . 
   In this particular embodiment, the depressions  45  are formed at the arc portion side of the half circular cross-section. Therefore, the arc portion of the center hole  42  closely contacts the arc portion of the driving shaft  13 . The rotor  40  and the driving shaft  13  are more closely coupled to each other than they are coupled by making a contact between the chord portions. It is not required to machine the center hole  42  and the driving shaft  13  with a high precision because the rotor  40  and the driving shaft  13  are tightly coupled to each other with a help of the imbalance of the centrifugal force. 
   Operation of the vane pump  10  will be briefly described. Fluid is sucked into the pump chamber  22  through the inlet port  23  and compressed in the pump chamber  22 , and then the compressed fluid is pushed out through the outlet passage  24 . The pump chamber between a pair of the neighboring vanes is the largest at the position of the inlet port  23 . The pump chamber  22  becomes gradually smaller according to rotation of the rotor  40  and becomes the smallest at the outlet passage  24 . The radial outer ends of the vanes  41  always contact the inner bore  21  of the ring  20  because of the centrifugal force applied to the vanes  41 . Accordingly, the fluid is continuously compressed in the pump chamber  22  and delivered out through the outlet passage  24 . 
   Advantages of the present invention will be summarized below. The gravity center c is shifted from the rotational center q by removing some of the rotor mass. When the rotor  40  is rotated, the rotor  40  is pushed toward the driving shaft  13  by the imbalance of the centrifugal force imposed on the rotor  40 . The rotor  40  is tightly coupled to the shaft  13  even if there is a small gap between the center hole  42  of the rotor  40  and the driving shaft  13 . Therefore, it is not required to machine the driving shaft  13  and the center hole  42  with a high precision. It is not necessary to press-fit the driving shaft  13  into the center hole  42  of the rotor  40 . Since the rotor  40  is tightly coupled to the driving shaft  13 , a stable pumping pressure is obtained while suppressing noises in operation. The gravity center c can be arbitrarily positioned by selecting the depth, the number and the position of the depressions  45 . 
   A second embodiment of the present invention is shown in  FIG. 4 . In this embodiment (a vane pump  50 ), cutout grooves  46  are formed on the outer periphery of the rotor  40  in place of the depressions  45  in the first embodiment. Other structures are the same as those of the first embodiment. A certain mass of the rotor  40  are removed by making the cutout grooves  46 . The gravity center of the rotor  40  is shifted from the rotational center. The rotor  40  is tightly coupled to the driving shaft  13  by the imbalance of the centrifugal force in the same manner as in the first embodiment. 
   A third embodiment of the present invention is shown in  FIG. 5 . In this embodiment (a vane pump  60 ), some weight is added to the rotor  40  instead of removing some weight. Other structure is the same as those of the first embodiment. As shown in  FIG. 5 , depressions  62  are formed in the axial direction, and the depressions  62  are filled with weights  61  that are heavier than the rotor material. In this manner, the gravity center of the rotor  40  is shifted from its rotational center. In this particular embodiment shown in  FIG. 5 , the gravity center is positioned at the right side of the rotational center. The rotor  40  can be tightly coupled to the driving shaft  13  in the same manner as in the first embodiment. The weight  61  is tightly kept in the depression  62  not to contact the upper plate  32 . 
   The present invention is not limited to the embodiments described above, but it may be variously modified. For example, though the depressions  45  are formed at the side of the upper plate  32  in the first embodiment, they may be formed at the side of the lower plate  31 . Similarly, the weights  61  in the third embodiment may be positioned at the side of the lower plate  31 . The rotor  40  may be made of a resin mold. In this case, the weights  61  in the third embodiment may be made of a heavy metallic material and embedded in the molded resin. 
   While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.