Abstract:
The present invention relates to a centrifuge including a balancer which contains balls and liquid and enables a rotor to rotate steadily. More concretely, the centrifuge comprises a motor, a motor shaft protruded from the motor, a rotor, and a balancer which includes a space constructed with a balancer body and a cover unit combining with the balancer body and formed to contain balls and liquid.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Korean patent application No. 10-2008-0066371 filed on Jul. 9, 2008, all of which is incorporated herein by reference in its entirety for all purposes. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a centrifuge equipped with a balancer, particularly, to a centrifuge equipped with the balancer which contains balls and liquid to reduce force and moment generated due to the weight imbalance among samples loaded a rotor and makes the rotor rotate more steadily. The balancer reduces vibration of the centrifuge, prolongs lifetime of the centrifuge and the rotor, and enhances the efficiency of the centrifugal separation. 
     2. Description of the Related Art 
     To accelerate settling of materials melt in fluid or suspended materials contained in a suspension, centrifugal force is used instead of gravity. This process is referred as centrifugal separation. 
     A centrifuge used for the centrifugal separation is an apparatus using a phenomenon that high density particles move to the edge and low density particles concentrate on the center in a suspension due to the centrifugal force. An example of a structure of the centrifuge is shown in  FIGS. 1   a  and  1   b.    
     As shown in  FIGS. 1   a  and  1   b , a centrifuge is configured with a buffer member  30  such as the anti-vibration rubber and damper set up on first supporting plate  15  formed in the inner side of a case  10  of the centrifuge and a bracket or second supporting plate  35  installed on the buffer member  30 . 
     Also, a configuration of the centrifuge includes a motor  50  beneath the bracket or the second supporting plate  35  and a rotor  200  or  200   a  on a shaft  40  protruded from the motor  50 . 
     The centrifuge uses different types of rotors according to the usage and there are two general types of rotors such that a swing-out rotor  200   a , which rotates perpendicularly to the shaft of the motor, and a fixed-angle rotor  200  which has a space rotating with fixed angle. The fixed-angle rotor  200  may include a plurality of chambers  60 . 
     The motor  50  in the centrifuge rotates at high speed and gives a big strong centrifugal force to samples within bottles or test tubes loaded into the swing-out rotor or the fixed-angle rotor. Therefore, the centrifuge separates the materials contained within the samples by the difference of the centrifugal forces due to the difference of densities between the materials. 
     A big centrifugal force has to be given to the samples for the separation of the materials within the samples. High speed rotation of the rotor is generally required in order to generate a big centrifugal force to the samples and particularly vibration should not be generated by the high speed rotation of the rotor. 
     However, during the high speed rotation of the centrifuge, vibration is generated by a bending motion of the shaft of the motor, a whirling motion due to the weight imbalance of the rotor, and influences by the other external factors. And the whirling motion by the weight imbalance of the rotor is the main factor among these reasons of the vibration. 
     Accordingly, in the centrifuge without a balancer, an operator measures independently the weight of each sample in advance before the centrifugal separation operation in order to remove the weight imbalance of the rotor generated due to the difference in the number of samples loaded into the rotor or due to the difference in the weight of each sample. Therefore, there has been an inconvenience that an operator should perform the centrifugal separation that after removing the weight difference between the opposite-side samples. If the weight imbalance between the opposite-side samples exists, materials within the samples are not separated due to the vibration generated during the centrifugal separation process. Although materials might be separated, the materials might be mixed again by the vibration. 
     Furthermore, during the centrifugal separation process, some noise may be generated by the vibration. 
     In the centrifuge, there has been a problem that a force or a moment is generated due to the weight imbalance among samples and it causes a disorder of the centrifuge itself. 
     To resolve the problem of the noise and the vibration generated during the centrifugal separation process, the buffer member such as damper and rubber may be included. But the buffer member still has a problem that noise and vibration are not absorbed enough. 
     Therefore, to resolve the problems of noise and vibration generated due to the weight imbalance among samples, the centrifuge equipped with a balancer including balls has been proposed. 
     The ball balancer (hereinafter, it is referred as Conventional Technology  1 ) shown in  FIG. 2   a  contains a plurality of balls  420  in the case  400  forming the balancing space  410  shaped as a circular ring and has an axis hole  430  at the center to fix the shaft of the motor. 
     Thus, the ball balancer includes balls  420  to fill some portion of the balancing space  410  formed inside of the case  400  and has the advantage that if the rotational speed of the motor (not illustrated) is above the resonance speed then rotation is stable because the balls move to the opposite side of the weight imbalance amount and the rotor (not illustrated) is balanced. 
     But there is a disadvantage that if the rotational speed of the rotor is below the resonance speed then the rotor is more unstable because the balls  420  rather move to the side where the weight imbalance exists. 
     To resolve the problem of the ball balancer of Conventional Technology  1 , the ball balancer shown in  FIG. 2   b  (hereafter, it is referred as Conventional Technology  2 ) has been proposed. 
     The ball balancer has the balancing space  410   a  formed to be inclined from the center of a case  400   a  having a hollow to the edge and includes balls  420   a  which fill up the groove part of the edge of the balancing space  410   a  formed within the case  400   a.    
     The ball balancer has an axis hole  430   a.    
     The ball balancer may prevent the unstable rotation occurred at the time of low-speed rotation under the resonance speed because the balls  420   a  locate near the center of rotation at that time. 
     Furthermore, if the motor rotates at high speed above the resonance speed then balls are floated by the centrifugal force and move to the opposite side to compensate the weight imbalance amount. Thus, the ball balancer has an advantage of vibration and noise reduction because the rotor rotates at a stable state. 
     However, in case of the ball balancer shown in  FIG. 2   b , if the rotational speed of the motor increases over the resonance speed starting from the initial low speed then it takes some time for the balls to move to the opposite side to compensate weight imbalance amount. Therefore, the ball balancer has a disadvantage that it does not have sufficient effect of vibration attenuation because vibration is created at this moment. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The following description is proposed to resolve the problems described above. Provided is a centrifuge equipped with a balancer which can compensate for the imbalance amount with accurate when the rotational speed of a rotor is not only below the resonance speed but also near or above the resonance speed. 
     Technical Solution 
     In order to achieve the above object, a centrifuge comprises a motor, a motor shaft protruded from the motor, a rotor combined to the motor shaft, and a balancer, wherein the balancer has a balancing space which is formed by combining a cover unit with a balancer body including an annular shaped space inside and contains balls and liquid altogether. 
     Advantageous Effects 
     (1) Both balls and liquid are used for a balancer. Therefore, since the balls which rapidly move to the opposite side of the position at which the weight imbalance exists and fixed by the viscosity of the liquid during the centrifugal separation process, the balancing effect is excellent. 
     (2) A problem of the conventional ball balancer that balls are continuously moving by the inertial force after the balls moved to the opposite side of the position at which the weight imbalance exists is resolved, wherein the problem is the disadvantage of the conventional ball balancer. 
     (3) The present invention prevents the instability of the overall system according to the abnormal vibration of the liquid during high speed rotational motion. (4) Since each ball moving to the outer side of the balancing space by the centrifugal force contact with, the inner wall of the outer side of the balancer body at least at two or more points, each ball is fixed in some degree along the vertical direction. Therefore, the vibration attenuation effect along the vertical direction is excellent. 
     (5) The balancing space containing balls and liquid takes a shape of wave by the balancer body or the cover unit. Thus, because balls can reach the space taking the shape of wave safe and sound after the balls move to the opposite side of the position at which the weight imbalance amount among samples exists, the centrifuge according to the present invention can achieve more stable balancing effect. 
     (6) The balancing space can contain different balls in size. Because relatively bigger balls compensate for the major weight imbalance amount and relatively smaller balls play a role of compensating for the minor imbalance, the centrifuge according to the present invention can achieve faster and more accuracy balancing effect. 
     (7) Since the balancing space is formed by multilayer, moments existing in the up and down parts of rotor due to the weight imbalance among samples can be offset. In addition, each of multilayered balancing spaces independently can compensate for the imbalance amount of samples. 
     (8) Each of distances between the center of the balancer and each of balls in the balancer can be made differently by dividing the balancing space with bulkheads. The balancing space can contain different balls in size. Because the balls positioned at the outermost of the balancing space compensate for the major weight imbalance amount and the balls located at the inside of the balancing space play a role of compensating for the minute imbalance, the centrifuge according to the present invention can compensate accurately for imbalance. 
     (9) Consequently, the noise and the vibration generated due to the high speed rotation of the motor during the centrifugal separation process are reduced. Therefore, the damage of the bottle or the test tube where samples are contained can be prevented and lives of the rotor and the centrifuge can be extended. 
     (10) It is unnecessary for the operator to measure directly the weight of the samples and adapt the number of samples. Therefore, the time to be taken on the centrifugal separation process can be minimized. It is possible to improve the centrifugal separation work efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1   a  is a cross sectional view showing a conventional centrifuge with a fixed angle type rotor; 
         FIG. 1   b  is a cross sectional view showing a conventional centrifuge with a swing-out type rotor; 
         FIG. 2   a  is a perspective view of a ball balancer according to one embodiment of Conventional Technology  1 ; 
         FIG. 2   b  is a perspective view of a ball balancer according to one embodiment of Conventional Technology  2 ; 
         FIGS. 3   a  and  3   b  are cross sectional views of centrifuges equipped with a balancer according to one embodiment of the present invention; 
         FIGS. 3   c  and  3   d  are cross sectional views of centrifuges equipped with a balancer according to one embodiment of the present invention; 
         FIG. 4   a  is a cross sectional view of a balancer according to one embodiment of the present invention; 
         FIG. 4   b  is a perspective view of a balancer according to one embodiment of the present invention; 
         FIG. 5   a  is a cross sectional view of a balancer according to another embodiment of the present invention; 
         FIG. 5   b  is a perspective view of a balancer according to another embodiment of the present invention; 
         FIG. 6   a  is a plane view of a balancer according to another embodiment of the present invention; 
         FIG. 6   b  is a cross sectional view of a balancer according to another embodiment of the present invention; 
         FIGS. 7   a  and  7   b  are cross sectional views of multilayer type balancers according to embodiments of the present invention; 
         FIG. 8  is a cross sectional view of a multilayer type balancer according to another embodiment of the present invention; 
         FIGS. 9   a  and  9   b  are cross sectional views of bulkhead type balancers according to embodiments of the present invention; 
         FIG. 10  is an experimental graph showing the balancing effect according to the quantity of balls. 
         FIG. 11  is an experimental graph showing the balancing effect according to the combination of balls. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, preferred embodiments according to the present invention are described in detail with reference to the accompanying drawings as follows. 
       FIG. 1   a  is an embodiment of a conventional centrifuge with a fixed-angle type rotor and  FIG. 1   b  is an embodiment of another conventional centrifuge with a swing-out rotor type. 
       FIG. 2   a  is an embodiment of a ball balancer according to Conventional Technology  1  and  FIG. 2   b  is an embodiment of a ball balancer according to Conventional Technology  2 . 
       FIGS. 3   a - 3   d  are cross sectional views of centrifuges equipped with a balancer according to embodiments of the present invention. 
       FIG. 4   a  is a cross sectional view of a balancer according to an embodiment of the present invention and  FIG. 4   b  is a perspective view thereof. 
       FIG. 5   a  is a cross sectional view of a balancer according to another embodiment of the present invention and  FIG. 5   b  is a perspective view thereof. 
       FIG. 6   a  is a plane view of a balancer according to another embodiment of the present invention and  FIG. 6   b  is a cross sectional view thereof; 
       FIGS. 7   a - 7   b  and  8  are cross sectional views of multilayer type balancers according to embodiments of the present invention and  FIG. 9   a - b  are cross sectional views of bulkhead type balancers according to embodiments of the present invention; 
       FIG. 10  is an experimental graph showing the balancing effect according to the quantity of balls and  FIG. 11  is an experimental graph showing the balancing effect according to the combination of balls. 
     As shown in drawings, the centrifuge equipped with a balancer according to the present invention comprises a motor  50 , a motor shaft protruded from motor  40 , a rotor  200  or  200   a , and a balancer  100 . 
     Hereinafter, the present invention is illustrated on reference to  FIGS. 3   a - 3   d  in detail. 
     The centrifuge of the present invention sets up a rotor  200  or  200   a  on the motor shaft  40  protruded from the motor  50  installed at the supporting plate  15  inside the outer case  10 . 
     It is preferable that the motor  50  is supported by the buffer member  30  such as damper and rubber, and a second supporting plate  35 . 
     The buffer member  30  absorbs some portion of the vibration and the noise generated at the centrifuge by the high speed rotation of the motor. 
     The shaft  40  of the motor protruded from the motor  50  unites with the fixed-angle rotor  200  in which a plurality of chambers  60  are formed. 
     As shown in  FIGS. 3   a  and  3   b , the lower part (not illustrated) of the chamber  90  formed in the fixed-angle rotor  200  inclines about the center “of shaft  40  .of the motor to outer. 
     Moreover, as shown in  FIGS. 3   c  and  3   d , the centrifuge sets up the swing-out rotor  200   a  as another preferred embodiment and the swing-out rotor  200   a  rotates perpendicularly to the shaft  40  of the motor. 
     The swing-out rotor  200   a  comprises a ring (not illustrated) hung with a bucket (not illustrated) loading samples. 
     As shown in  FIG. 3   a , given the length L of the shaft  40  of the motor and the distance R between the shaft  40  of the motor and the center of mass (not illustrated) of each sample contained into the rotor  200 , it is preferable that the condition of L/R&lt;2.6 is satisfied during the centrifugal separation process. 
     That is, the condition described in the above means that if the length L of the shaft  40  of the motor is much longer than the distance R between the center of mass (not illustrated) of samples and the shaft  40  of the motor, during the centrifugal separation process, the rotational instability of the rotor  200  by the rotation of the motor  50  is aggravated and the balancing effect by the balancer  100  reduces. 
     According to experiment, it was found that if the condition of L/R&lt;2.6 is satisfied, then the rotational instability of the rotor  200  is reduced and the balancing effect is excellent. 
     In the centrifuge comprised as the above, the balancer  100  to be described later is installed at a part of the shaft  40  of the motor or the rotor  200 . 
     Hereinafter, referring to  FIGS. 4   a  and  4   b , the balancer  100  according to the present invention is illustrated in detail. 
     As shown in the figures, the balancer  100  is formed by combining a cover unit  120  with a balancer body  130  and has the balancing space  150  of annular form inside. 
     The balancer  100  has a joint  110  in which the penetration hole  105  is formed to join to the shaft of the motor or the rotor. 
     The balancing space  150  includes a plurality of balls  160  and liquid  170 . 
     The balancing space  150  formed in the balancer  100  takes a shape of annular and is formed by combining the cover unit  120  with the balancer body  130 . 
     The cover unit  120  and the balancer body  130  can be combined by several coupling methods including grooves and protrusions (not illustrated) or screws (not illustrated) formed at the corresponding locations. 
     These coupling methods between of two members are widely known. Therefore, the detailed description is omitted. 
     At the center of the balancer  100 , there is a joint  110  in which the penetration hole  105  is formed to combine with a part of the shaft of the motor or the rotor. 
     In the balancing space  150 , a plurality of balls  160  and liquid  170  are included to balance the weight imbalance amount among samples during the centrifugal separation process. 
     Not only water but also oil can be used as the liquid  170 . 
     During the centrifugal separation process, the liquid  170  plays the role that the liquid fixes the balls  160  not to move by using the viscosity which is One of characteristics of the liquid  170  after the balancing completion f 01 ′ the weight imbalance amount by moving of the balls  160  to the opposite side of the weight imbalance amount. 
     Accordingly, the balancing effect for the weight imbalance amount is excellent in comparison with balancers which contain only balls or only liquid in the balancing space  150 . 
     According to the condition of the centrifugal separation process, the amount and the viscosity of the liquid  170  stored in the balancing space  150  are controlled to the optimal level. 
     If the balancing space  150  stores excessive quantity of the liquid  170 , the liquid  170  rotates at high speed continuously by the centrifugal force working on during the centrifugal separation process. Thus, the excessive quantity of the liquid works on the centrifuge as an unstable factor. 
     Accordingly, the abnormal vibration rather can be generated in the rotor. Therefore, it is desirable to limit the amount of the liquid to the optimal level. 
     Furthermore, according to the working condition, the amount of the balls  160  stored in the balancing space  150  can be controlled to the optimal level. 
     As shown in  FIG. 10 , according to experiments, when the number of balls stored in the balancing space  150  is limited to about 20%-70% of the maximum number of the balls that can be stored in the balancing space  150 , the excellent balancing effect can be obtained. 
     As shown in  FIGS. 4   a  and  4   b , the balls  160  and the liquid  170  stored in  25  the balancer  100  is pushed to the lateral wall surface  135  of the balancer body  130  by the centrifugal force generated during the centrifugal separation process. 
     The shape of the lateral wall surface  135  of the balancer body  130  can be differentiated. It can form the inner wall  131   a  of the lateral wall surface  135  of the balancer body  130  to be inclined so that each of the balls  160  contacts the inner wall  131  of the lateral wall surface  135  of the balancer body  130  in at least two or more points by the centrifugal force. 
     Hereinafter, referring to  FIGS. 5   a  and  5   b , another embodiment of the balancer is illustrated in detail. 
     The lateral wall surface  135   a  of the balancer body  130  based on the horizontal direction central axis of the balancer  100  is inclined to outside. 
     The sloped wall is formed from the most outer part  141  of the cover unit  120  to the part  142  which has the maximum radius of the balancer  100 . 
     Similarly, the sloped wall is formed from the most outer  142  of the floor side of the balancer body  130  to the part  142  which has the maximum radius of the balancer  100 . 
     The lateral wall surface  135   a  and inner wall  131   a  of the balancer body are inclined by this sloped wall. 
     Each of the balls  160  pushed by the generated centrifugal force during the centrifugal separation process contacts with the inner wall  131   a  of the lateral wall surface  135   a  at least at two or more points. 
     Although the vibration along the vertical direction is generated at the rotor during the centrifugal separation process, each of the balls  160  balancing to the opposite direction of the weight imbalance amount contact with the inner wall  131   a  of the lateral wall surface  135   a  at least at two or more points. 
     Since the contacts working on each of the balls  160  at least at two or more points help to suppress the vertical motion of the balls  160 , more stable balancing is possible. 
     The balancing space  150  of the balancer  100  can store a combination of the balls  160  with two sizes. 
     That is, in case it stores only large size balls in the balancing space  150 , the balls may not be ideal and may be inclining to the fixed angle to balance the rotor. 
     Accordingly, the principal balancing effect and the principal balancing force are increased by using the relatively bigger sized balls. 
     The relatively smaller sized balls provide the rotor with better rotation stability because they are planned to give the minute balancing effect. 
       FIG. 11  is a comparative experimental result between a case using the balls of only one size and another case using combination of the balls with two sizes. 
     The vibration acceleration G is vibration generated during the centrifugal separation process. The lower the vibration acceleration is, the less the vibration and the noise of the rotor are. 
     As shown in  FIG. 11 , in case of using the combination of the balls with two sizes, the frequency of generating the vibration  25  acceleration at the rotor from 0.15 G to O. 20 G occupies about 50% and the vibration .acceleration above. 0.35 G is not generated. 
     However, in case of using the balls of only one size, the frequency of generating vibration acceleration at the rotor from 0.2 G to 0.40 G occupies about 70% and even 0.55 G of vibration acceleration is measured. 
     According to these experimental results, the balancing which is the effect of the decreased vibration is better in case of using the combination of the balls with two sizes than in case of the balls of only one size. 
     To store the combination of the different balls in size into the balancing space  150  can be applied to the multilayer type balancer and the bulkhead type balancer to be described later. 
     Hereinafter, referring to  FIGS. 6   a  and  6   b , another embodiment of balancer is illustrated in detail. 
     The balancer forms the lateral wall surface of the balancer body or the lateral wall surface of the cover unit as the shape of wave. Then, the space part  175  in which the balls  160  moving to the opposite side of the weight imbalance amount can stay can be formed. 
     The balancing space  150  formed in the balancer, that is, the lateral wall surface  135   b  of the balancer body takes a shape of wave shape when looked at the front side. 
     Looking the cross section of the balancing space  150 , the penetration hole is made at center and the whole section takes a shape of a horn. 
     The floor  300  of the balancer body is plane. 
     At the part  310  at which the plane part of the floor  300  of the balancer body is finished, a declined part  320  is started to outside from the central axis of the balancer. 
     There may be a round part  340  at the part  330  at which the declined part is finished since the declined part  320  may be formed into the straight line or curve. 
     The space part  175  for the balls  160  and the liquid is formed inside the balancer owing to the round part  340 . 
     If the centrifugal force works on the balls  160  located on the floor  300  of the balancer body, the balls  160  move to the outermost of the floor  300  of the balancer body comprising a part of the balancing space  150  by the centrifugal force and are positioned at the part  310  at which the declined part  320  of the balancer body starts. 
     As the rotational speed of the motor continuously increases, the centrifugal force acting on the balls  160  is increased and the balls  160  move along the declined part  320 . 
     The balls  160  moving along the declined part  320  are positioned at the opposite direction of the weight imbalance amount for balancing the rotor. 
     At this time, the balls  160  safely reach a space part  175  formed by the round part  340  initiated from the part  330  at which the declined part  320  is finished inside the balancer. 
     Since the balls  160  reaching the space part  175  formed safely, after balancing at the opposite side of the weight imbalance amount, the balls are not influenced although the vibration along the vertical direction is generated at the rotor, the balancing effect is excellent. 
     The space part  175  can be formed .not only by making the balancer body of the balancer but also by making the lateral wall surface (not illustrated) of the cover unit as a wave shape. 
     It decides to omit the detailed description about this. 
     As shown in  FIGS. 7   a  and  7   b , the balancing space formed in the balancer can be partitioned into multilayer. 
     It is possible to independently compensate for the weight imbalance amount about the rotor by each of balancing spaces  151  and  152 . Therefore, the rotation of the rotor can be stabilized in the fast time. 
     Moreover, not only the force but also the moment caused by the weight imbalance among samples included in the rotor can be offset. 
     Hereinafter, referring to  FIGS. 7   a  and  7   b , an embodiment of multilayer type balancer is illustrated in detail. 
     The balancing space can be partitioned by the bulkheads  180  which are parallel to the floor (not illustrated) of the balancer body  130  and have the shape of the circular plate on the whole. 
     The number of balancing spaces  151  and  152  can be adjusted by installing one or more the bulkheads  180  according to the working condition. 
     As shown in  FIG. 7   b , the size of the balls that are included in each of balancing spaces  151  and  152  is adjusted. While contributing to the maximum compensation mass by using the balls  162  of relatively long diameter in the lower layer  152  of the balancing space, the balls  161  of relatively short diameter are used in the upper layer  151  of the balancing space to compensate for the minute imbalance amount. The stable—balancing effect—can be obtained quickly. 
     As shown in  FIG. 8 , the distances between the balls  160  stored in each of balancing spaces  151   a  and  152   a  and the center of the balancer  100  or the shaft  40  of the motor can be differently formed. 
     Thus, if the distances between the balls  160  and the center of the balancer or the shaft of the motor is differently formed, the balls  160  stored in the balancing space  151   a  which relatively is positioned at outside from the center of the balancer or the shaft of the motor contribute to the maximum compensation mass. 
     The balls  160  stored in the balancing space  152   a  which relatively is positioned at inside from the center of the balancer or the shaft of the motor compensate for minute imbalance amount. Therefore, the stable balancing effect can be obtained in the fast time. 
     As shown in  FIGS. 7   a ,  7   b  and  8 , the balancing space in the balancer is formed with a bulkhead into the upper and lower layers. However, the balancing spaces of the upper and lower layers also can be formed by combining two or more balancers. 
     That is, the balancing spaces of the upper and the lower layers can be formed by combining two or more balancers with a part of the shaft of the motor or the rotor. 
     As shown in  FIGS. 9   a  and  9   b , the balancer is formed into a plurality of balancing spaces  153  and  154  divided by the bulkheads  190 . 
     In this way, a plurality of balancing spaces  153  and  154  formed with the bulkheads  190  can be composed with different distances between the balls  160  stored in a plurality of balancing spaces  153  and  154  and the center of the balancer or the shaft of the motor. 
     The balls  160  existing in the balancing space  153  formed at the outermost one among a plurality of balancing spaces  153  and  154  divided with the bulkhead  190  mainly contribute to the maximum compensation mass of the balancer. The balls existing in the balancing space  154  near the center of the balancer play a role of compensating for the minute imbalance. 
     Therefore, if one balancing space is formed into a plurality of balancing spaces  153  and  154  divided by the bulkhead  190 , it is possible to compensate for the imbalance amount more exactly and quickly. 
     Hereinafter, referring to  FIGS. 9   a  and  9   b , an embodiment of bulkhead type balancer is illustrated in detail. 
     The bulkhead  190  has the common center with the balancer, and is shaped as a ring on the whole, and is installed in the balancing space. 
     Under the necessity, one or two or more the bulkheads  190  may be installed in the balancing space. Therefore, the number of balancing spaces  151  and  152  can be adjusted. 
     Furthermore, as described above, the size of the balls stored in a plurality of balancing spaces  153  and  154  divided with the bulkhead  190  is adjusted. The balls  162   a  of long diameter stored in the balancing space  153  that are positioned at relatively outside from the center of the balancer or the shaft of the motor compensate for the main imbalance amount. 
     The balls  161   a  of relatively short diameter stored in. the balancing space  154  that are positioned at relatively inside from the center of the balancer or the shaft of the motor compensate for the minute imbalance amount. Therefore, it is possible to compensate for the imbalance amount more exactly and quickly. 
     Furthermore, as the balancer having one balancing space  150  is formed into a plurality of balancing spaces  153  and  154  with the bulkheads  190 , a plurality of balancing spaces  153  and  154  can be formed by combining the balancer. 
     That is, a balancer (not illustrated) of relatively short diameter can be combine with the inner space (not illustrated) of a balancer of relatively long diameter by the corresponding grooves, protrusions and screws. Then, the combined balancer can unite with the shaft of the motor or the rotor. 
     As described above, although the present invention is described with reference to the preferred embodiment of the present invention, the person skilled in the art can modify or change the present invention in many ways without departing from the spirit or scope of the present invention described within the following claims.