Abstract:
A balancing method and apparatus is used for dynamically balancing an out of balance condition in a rotating body caused by resistance forces acting tangentially to the body. A device having a rotatable component and automatic or dynamic balancing includes a housing, a shaft rotatably mounted in the housing, the shaft supporting the component near one end of the shaft, at least one counterweight fixedly mounted on the shaft and at least one automatically adjusting balancer mounted on the shaft. The automatically adjusting balancer includes one or more compensating masses contained to move about a path relative to the shaft to compensate for variable imbalanced forces acting on the component.

Description:
This application claims priority to U.S. Provisional Patent Application No. 60/379,765, filed May 14, 2002, the entire disclosure of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to a balancing method and apparatus and, more particularly, to a balancing method and apparatus used for dynamically balancing an out of balance condition in a rotating body caused by resistance forces acting tangentially to the body. 
   BACKGROUND OF THE INVENTION 
   Many different apparatus for balancing an out of balance condition in a rotating body are known. Such apparatus generally include a counterweight having a weight of a predetermined value which is located at a predetermined position from the axis of rotation to oppose an imbalance in the rotating body. The magnitude of the imbalance is generally known and, accordingly, the necessary weight and position of the counterweight can be calculated so that the weight is positioned where it will act to counter the known imbalance. When the mass imbalance remains substantially constant during rotation of the machine, a conventional method for removing such imbalance involves attaching or positioning counterweights equal to the amount of imbalance to the rotating body, and in a position directly opposite to the imbalance. 
   Under dynamic conditions, that is, when a body is rotating about an axis and an imbalance in the rotating body develops because of external conditions or otherwise, techniques are known wherein a vibration dampening assembly is provided with a plurality of annular grooves or races located about the periphery of the assembly and extending axially there along. A plurality of balls or rollers are located in each of the races. Such balls or rollers are free to move along the races and thereby counteract the imbalance forces. 
   Out of balance conditions can develop in rotating bodies resulting from rotating resistance forces. Such out of balance conditions result from an interaction of the rotating body with the surrounding fluid and/or stationery surfaces. In particular, such out of balance conditions may occur when the rotating body is not substantially symmetric. This non-symmetricity gives rise to detrimental unbalanced rotating resistance forces. Machines that experience these unbalanced rotating resistance forces include orbital sanders, marine and aircraft propellers, mixers, processors, fans, etc. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, a balancing apparatus is provided to remove imbalance in a rotating body wherein the imbalances tend to change during use, such as imbalances resulting during a sanding operation using an orbital sander. According to another aspect of the invention, the tangential imbalanced forces that are compensated for result in particular with devices having a rotatable component that encounters resistance in a tangential direction as a result of a medium the component is moved through during use. The rotating resistance forces that develop during the use of an orbital sander or other device cannot be predetermined in advance and tend to change greatly during the operation of the machine. Accordingly, such balancing inherently leads to residual imbalances and associated vibrations. These residual vibrations can be particularly high on high-speed machines. The imbalances which change during operation of the machine, can lead to severe vibrations and damage. 
   According to one embodiment of the invention, a device having a rotatable component and automatic or dynamic balancing includes a housing, a shaft rotatably mounted in the housing, the shaft supporting the component near one end of the shaft, at least one counterweight fixedly mounted on the shaft and at least one automatically adjusting balancer mounted on the shaft. The automatically adjusting balancer includes one or more compensating masses contained to move about a path relative to the shaft to compensate for variable imbalanced forces acting on the component. 
   According to a further aspect of the invention, the at least one counterweight fixedly mounted on the shaft is mounted in a different plane then the plane within which the at least one automatically adjusting balancer is mounted. 
   According to yet a further aspect of the invention, the at least one automatically adjusting balancer is mounted on the shaft closer to a plane within which the variable imbalanced forces are acting on the component than the at least one counterweight. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       FIG. 1  is a side sectional view of an orbital sander including an automatic balancing apparatus according to an embodiment of the invention. 
       FIG. 1A  is a further side sectional view of the automatic balancing apparatus shown in FIG.  1 . 
       FIG. 1B  is a perspective view of the automatic balancing apparatus shown in FIG.  1 . 
       FIG. 1C  is a further perspective view of the automatic balancing apparatus shown in FIG.  1 . 
       FIG. 2  is a side sectional view of an automatic balancing apparatus for use with an orbital sander according to a second embodiment of the invention. 
       FIG. 2A  is a perspective view of the automatic balancing apparatus shown in FIG.  2 . 
       FIG. 3  is a side sectional view of an automatic balancing apparatus for an orbital sander according to a third embodiment of the invention. 
       FIG. 4A  is a perspective view of the automatic balancing apparatus according to the first embodiment of the invention and illustrating various forces acting on the balancing apparatus. 
       FIG. 4B  is a top plan view, partially sectioned, of the automatic balancing apparatus shown in FIG.  4 A. 
       FIG. 4C  is a diagrammatic representation of the various unbalance and counterweight forces acting on the automatic balancing apparatus shown in FIG.  4 A. Specifically,  FIG. 4C  shows the typical forces for a normal sanding operation. 
       FIG. 4D  is a further perspective view of the automatic balancing apparatus according to the first embodiment of the invention and showing various forces acting on the balancing apparatus. 
       FIG. 4E  is a top plan view, partially sectioned, of the balancing apparatus shown in  FIG. 4D , and illustrating various forces acting on the components of the balancing apparatus. 
       FIG. 4F  is a diagrammatic representation of the various forces acting on the balancing apparatus shown in FIG.  4 D. Specifically,  FIG. 4F  shows various forces for a light sanding operation. 
       FIG. 4G  is a further perspective view of an automatic balancing apparatus according to the first embodiment of the invention and illustrating various forces acting on the balancing apparatus. 
       FIG. 4H  is a top plan view, partially sectioned, of the balancing apparatus shown in  FIG. 4G , and illustrating various forces acting on the components of the balancing apparatus. 
       FIG. 4I  is a diagrammatic representation of the various unbalance and counterweight forces acting on the balancing apparatus shown in  FIG. 4G  during a heavy sanding operation. 
       FIG. 4J  illustrates an exemplary balancing apparatus that does not have automatic balancing features, showing various forces acting on the balancing apparatus. 
       FIG. 4K  is a top plan view, partially sectioned, of the balancing apparatus shown in FIG.  4 J. 
       FIG. 4L  is a diagrammatic representation of the various forces acting on the balancing apparatus shown  FIG. 4J  during a normal sanding operation. 
       FIG. 4M  is a further perspective view of a balancing apparatus such as that shown in  FIG. 4J , and not having any automatic balancing features. 
       FIG. 4N  is a top plan view, partially sectioned, of the balancing apparatus shown in FIG.  4 M. 
       FIG. 4O  is a diagrammatic representation of the various forces acting on the balancing apparatus shown in  FIG. 4M  during a light sanding operation. 
       FIG. 4P  is a further perspective view of a balancing apparatus that does not have any automatic balancing features. 
       FIG. 4Q  is a top plan view, partially sectioned, of the balancing apparatus shown in FIG.  4 P. 
       FIG. 4R  is a diagrammatic representation of the various forces acting on the balancing apparatus shown in  FIG. 4P  during a heavy sanding operation. 
       FIG. 5A  illustrates a propeller having a balancer according to an embodiment of the invention. 
       FIG. 5B  is a front view of the propeller shown in FIG.  5 A. 
       FIG. 6A  is an illustration of a mixer having a balancer according to an embodiment of the invention. 
       FIG. 6B  is a top plan view, in partial section, of the mixer shown in FIG.  6 A. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring initially to  FIG. 1 , an automatic balancing apparatus according to one embodiment of the invention is shown mounted within a housing for an orbital sander  20 . The automatic balancing apparatus includes a main central shaft  30  that is rotatably mounted within the housing  24  of the orbital sander  20 . An eccentrically mounted sanding pad  44  is mounted at the lower end of the shaft  30 , with the axis  45  about which the sanding pad  44  rotates being offset from the central axis  22  of the shaft  30 . The shaft  30  is rotatably mounted within an upper bearing  32  and a lower bearing  39 , and an additional friction pad bearing  42  rotatably mounts the friction pad  44  eccentrically to the central axis  22  of the shaft  30 . 
   In the embodiment shown in  FIGS. 1 ,  1 A,  1 B, and  1 C, the central shaft  30  of the automatic balancing apparatus supports an upper automatic balancer  34  and a lower automatic balancer  40  along with a first counterweight  37  and a second counterweight  38 . In the embodiment shown in  FIGS. 1 ,  1 A,  1 B and  1 C, the upper automatic balancer  34  is a smaller “trim” balancer, and the lower automatic balancer  40  is a larger, main balancer. A motor  36 , which can be electric, pneumatic or hydraulic, is mounted to the shaft  30  to rotatably drive the shaft  30  within the upper and lower bearings  32 ,  39 . 
   As best seen in  FIGS. 1 and 1A , the upper automatic balancer  34  includes a balancer housing  34   a , which defines an annular cavity within which one or more compensating masses  34   b  and optional balancer fluid  34   c  are contained. Although the compensating masses  34   b  shown in the embodiment of  FIG. 1  are illustrated as spherically shaped masses, these compensating masses could be provided in any of a variety of different shapes such as spherical, disc-like or cylindrical. The compensating masses  34   b  are freely movable within the cavity defined by balancer housing  34   a , and an appropriate lubricant or balancer fluid  34   c  can be provided within the cavity to reduce the friction between the compensating masses and the balancer housing  34   a , as well as to reduce noise made by the compensating masses when the balancing apparatus is in operation. The balancing fluid  34   c  also creates an appropriate amount of viscous dampening on the compensating masses. 
   In the illustrated embodiments, a main balancer  40  containing larger sized compensating masses, is also mounted to the shaft  30  in a position closer to the plane of the eccentrically mounted sanding pad  44 . The main balancer  40  includes a balancer housing  40   a , which defines a cavity within which the compensating masses  40   b  are free to move. An optional balancer fluid  40   c  can also be contained within the cavity defined by balancer housing  40   a.    
     FIGS. 4A-4I  show the automatic balancing apparatus of  FIGS. 1-1C  in different applications wherein different forces are acting on the automatic balancer. As shown in  FIGS. 4A-4I , as the automatic balancer is rotated about shaft  30 , a first counter balance force F 1  is created by the counterweight  38 , and a second counter balance force F 2  is created by the counterweight  37 . The first and second counterweights  38 ,  37  are selected to balance the orbital sander for an average or mean operation, wherein the first counter balance force F 1  and second counter balance force F 2  are sufficient to compensate for all imbalances created during an average or mean operation. An eccentric imbalance force F i  is created as a result of the rotation of the sanding pad  44  about the eccentric axis  45 , and the rotation of eccentric axis  45  about the central axis  22  of the main shaft  30 . A rotating friction force F f  is created as a result of the contact between the sanding pad  44  and a work piece that is being sanded. This friction force F f  acts tangentially to the main shaft  30  since it is directed from the eccentric axis  45 , as shown in  FIGS. 4B ,  4 E,  4 H,  4 K,  4 N and  4 Q. In the situation shown in  FIGS. 4A-4C , the counterweights  37 ,  38  are compensating for all of the dynamic forces acting on the automatic balancing apparatus, with the automatic balancers  34 ,  40  adding nothing to the balanced situation, as illustrated by the even distribution of the compensating masses  34   b  in the smaller balancer  34 , and the even distribution of the compensating masses  40   b  in the main balancer  40 . The counterweights  37  and  38  are typically set at the factory to provide suitable compensation for the unbalanced forces resulting from a typical use. Any deviation from such a typical use or any change in the sanding conditions would result in the change of the friction force F f  and would thus result in the unbalanced condition which in turn is compensated for by the automatic balancers. 
   Specifically,  FIGS. 4D-4F  illustrate a situation wherein the automatic balancing apparatus of an orbital sander is being used in a light sanding operation. During this situation, the rotating friction force F f  is relatively small compared to the friction force F f  under normal sanding operation, as shown in  FIG. 4F , so the counter balance forces F 1  and F 2  created by counterweights  38 ,  37  over-compensate for the imbalance forces. This gives rise to the overall unbalanced force  47   a  (FIG.  4 F). Furthermore, since all the unbalanced forces lie in different planes along the shaft  22 , an unbalanced moment or couple results. In this situation the compensating masses  40   b  of the main balancer  40  and the compensating masses  34   b  of the trim balancer  34  position themselves substantially so as to counteract the overall unbalanced force  47  and the associated unbalanced moment or couple. A main balancer force F bd  is generated by the large compensating masses  40   b  within the main balancer  40 , and a smaller or trim balancer force F bt  is generated by the compensating masses  34   b  within the smaller or trim balancer  34  resulting in the effective counterbalance force  47   b . As a result of the movement and positioning of the compensating masses within the balancers  34  and  40 , the overcompensation of counterweights  37 ,  38  is counteracted to balance the entire balancing system as shown in FIG.  4 F. Because the trim balancer  34  is mounted in a different plane than the main balancer  40 , the forces generated by the compensating masses within the balancers counteract the imbalance forces and the imbalance moments created by the friction force F f , the imbalance force F i  and the counterweight forces F 1  and F 2 . In accordance with embodiments of the present invention, it has been discovered that the trim balancer force F bt  and the main balancer force F bd  acting in different planes along the main shaft  30 , are effective in counterbalancing the tangential imbalanced forces generated by the resistance between the rotating component, e.g., the orbital sanding pad, and the medium upon which the rotating component is acting, e.g., a workpiece being sanded. 
   As shown in  FIGS. 4D and 4E , the main balancer force F bd  acts in the opposite direction from the trim balancer force F bt , as well as acting in a plane closer to the plane within which the imbalanced forces F f  and F i  are acting than the plane of the trim balancer force F bt . The arrangement of counterweights and automatic balancers in axially spaced positions along the shaft  30  therefore results in the dynamic balancing of radial imbalanced forces, tangential imbalanced forces, and the associated imbalance moments or couples. 
     FIGS. 4G-4I  illustrate the situation wherein the automatic balancing system is used during a heavy sanding operation. In this situation the rotating friction force F f  is relatively large as compared to the friction force F f  for normal sanding operation, as shown in FIG.  4 I. Accordingly, the counter balance forces F 1 , F 2 , created by counterweights  38 ,  37  under-compensate for the dynamic forces created by operation of the orbital sander. This results in the overall unbalanced force  48   a  and the associated unbalanced moment or couple. In this case, the large compensating masses  40   b  within main balancer  40  and the small compensating masses  34   b  within trim balancer  34  shift their positions within the balancers so as to counteract the overcompensation by counterweights F 1 , F 2 , with the effective counterbalance force  48   b , and bring the entire automatic balancing system into equilibrium, as shown in FIG.  4 I. As illustrated in  FIGS. 4G and 4H , the large tangential imbalanced friction force F f  is counteracted by the main balancer force F bd , which acts in the opposite direction and in a plane close to the plane of the imbalanced forces F f  and F i . The additional trim balancer force F bt  acts in a plane farther away from the plane of the imbalanced forces than the main balancer, and in the opposite direction from the main balancer force F bd . As a result, both the radial and tangential imbalanced forces and moments are counteracted, and the system is brought into equilibrium. 
     FIGS. 4J-4L  show the system in the same situation as the automatic balancing system according to an embodiment of the invention shown in  FIGS. 4D-4F , but with the automatic balancers  34 ,  40  being removed. In this situation the dynamic forces created by a rotating friction force F f  and imbalance force F i  are perfectly compensated by the counter balance forces F 1 , F 2  created by counterweights  37 ,  38 . 
     FIGS. 4M-4O  show the system in the same situation as the automatic balancing system according to an embodiment of the invention shown in  FIGS. 4D-4F , but with the balancer not including automatic balancers  34 ,  40 . In this situation the orbital sander is being used in a light sanding application such that the rotating friction force F f  is relatively small compared to the normal friction force F f . Accordingly, without the presence of automatic balancers, the counterweights F 1 , F 2  overcompensate for the dynamic forces F f , F i  thereby giving rise to an unbalanced force F U  such as shown in FIG.  4 O. 
     FIGS. 4P-4R  illustrate the system in the same situation as the automatic balancing system according to an embodiment of the invention shown in  FIGS. 4G-4I , but with the balancer not including automatic balancers  34 ,  40 . In this situation the rotating friction force F f  is relatively large compared to the normal friction force F f , and therefore the counter forces F 1 , F 2  created by counterweights  37 ,  38  under compensate for the dynamic forces, giving rise to an unbalanced force F U  as shown in FIG.  4 R. 
   An alternative embodiment of an automatic balancing system according to the invention is illustrated in  FIGS. 2 and 2A . In this embodiment the smaller trim balancer  34  is eliminated, with a large main balancer  40  being located in a plane close to the plane of the orbital sander  44 , and counterweights  37 ,  38  being mounted to shaft  30  in axially spaced positions farther away from the sanding pad  44 . It is noted here that a preferred embodiment includes two automatic balancers such as the embodiment of  FIG. 4A ,  4 D, or  4 G. However, for cases where the unbalanced moments or couples are small as compared to the mass of the system, it is possible to achieve acceptable balancing with a single automatic balancer as shown in  FIGS. 2 and 2A . 
   A further embodiment is shown in  FIG. 3 , wherein a smaller trim balancer  34  and a larger main balancer  40  are provided, but only one counterweight  38  is mounted on the shaft  30 . As with the other embodiments discussed above, the main balancer  40  is located in a plane close to the plane of the sanding pad  44 , with the trim balancer  34  being located on shaft  30  as far away from the plane of the sanding pad  44  as possible. The one counterweight  38  is mounted on shaft  30  in between the balancers  34 ,  40 . 
   Similar principles and mechanisms can be used for providing dynamic balancing of a propeller, such as shown in the embodiment of  FIGS. 5A and 5B . Propeller blades  144  extend radially from a propeller hub  145 , and are driven to rotate about shaft  130 . As the propeller is rotated, a drag force F D  is generated by the friction between the blades  144  and the surrounding fluid. The shape of the propeller blades  144  results in the creation of a radial force F L , and an axial thrust force F T . The rotating unbalanced forces created by drag force F D  and the radial unbalanced force F L  can be compensated for by an automatic balancer  140 , which includes one or more compensating masses that are free to move about an annular race defined by the housing of the balancer  140 . Although only one automatic balancer is illustrated, it will be recognized that alternative embodiments can include additional automatic balancers and/or fixed counterweights, positioned in different planes and providing a desired combination of forces to counteract any imbalanced forces generated during operation of the device. 
   In the embodiment shown in  FIGS. 6A and 6B , a mixer uses similar balancing mechanisms and principles, with an automatic balancer  240  mounted to the main shaft  230 . As the shaft  230  rotates within a support bearing  232 , imbalance forces F U  are created by the friction between the mixing arm  244  and the substance being mixed. These imbalanced forces can be compensated for by movement of the compensating masses  240   b  within the housing of the automatic balancer  240 . As with the embodiment illustrated in  FIGS. 5A and 5B , although only one automatic balancer is illustrated, it will be recognized that alternative embodiments can include additional automatic balancers and/or fixed counterweights, positioned in different planes and providing a desired combination of forces to counteract any imbalanced forces and moments generated during operation of the device. 
   While specific embodiments of the invention have been described, such embodiments should be considered as illustrative of the invention only and not as limiting its scope as defined in accordance with the following claims.