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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a damper device for use on the door of a container to apply damping forces to the angular movement of the door in terminal ranges of opening and closing movements of the door, or for use on an electrically powered tool such as an electrically powered saw or plane to reduce shocks in terminal ranges of reciprocating movements thereof. 
     2. Description of the Related Art 
     Heretofore, some containers are equipped with a damper device mounted on the door for preventing the door from banging against the door frame when the door is closed. Since the damper device operates only when the door is closed, however, the damper device is not active when the door is quickly opened. When the door is quickly opened, therefore, the door tends to hit a stop, producing undesirable noise or causing damage to itself or the stop. For producing a damping action in a terminal range of the opening movement of the door, the door needs to incorporate another separate damper device separately from the existing damper device which operates only when the door is closed. 
     There has not been known any example in which the above damper device is applied to a reciprocally movable electrically powered tool. Heretofore, it has been customary for the user of a reciprocally movable electrically powered tool to empirically control forces produced by the tool in terminal regions of its reciprocating actions to avoid unwanted impacts or damage to the tool. However, controlling forces produced by the tool in terminal regions of its reciprocating actions needs a skilled experience on the part of the user, and is physically fatiguing to the user. 
     Installing two damper devices on one door is highly costly. In addition, it is a complex task to install two damper devices on one door and also to perform maintenance on the two damper devices mounted on the door. There has been a demand for a single damper device which is capable of producing a damping force in both terminal ranges of opening and closing movements of a door on which the damper device is installed. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a damper device which is capable of generating a damping force in both terminal ranges of opening and closing movements or reciprocating movements of an apparatus on which the damper device is installed. 
     To achieve the above object, there is provided a damper device comprising a cylindrical casing, a rotor partly housed in the casing, a viscous fluid filled in the cylindrical casing around the rotor, and torque generating means for generating a torque during a rotating stroke of the rotor, the torque generating means including fluid torque adjusting means for producing a relatively large torque in a terminal range of each of the rotating strokes in normal and reverse directions of the rotor and a relatively small torque in other range of each of the rotating strokes. Therefore, if the damper device is installed on a door, then the damper device can produce a damping force in terminal periods of opening and closing movements of the door, thereby preventing the door from banging against a stopper or a door frame when the door is fully closed or opened. Furthermore, since the damper device generates a relatively small torque in the rotating strokes in the normal and reverse directions, it is possible not to apply a damping force in a period other than the terminal periods of opening and closing movements of the door. As a result, the door can be opened and closed with a small force. 
     The torque generating means may have a first ridge extending axially on an outer surface of the rotor and having a radially outer surface held in sliding contact with an inner surface of the casing, and the fluid torque adjusting means may comprise a first land extending axially on the inner surface of the casing, a pair of first axial grooves defined axially in a radially inner end of the first land and spaced circumferentially from each other, a pair of first needle valves loosely fitted in the first axial grooves, respectively, for movement in the width direction of the first axial grooves, and a first circumferential groove defined circumferentially on the outer surface of the rotor, the first circumferential groove being positioned out of facing relation to at least one of the first needle valves in the terminal range of each of the rotating strokes. When the at least one of the needle valves is held against the outer surface of the rotor in the terminal range, the one of the first needle valves and the first ridge jointly divide an interior space of the casing into two chambers, for effectively preventing the viscous fluid from moving between the chambers to produce the relatively large torque. The fluid torque adjusting means of the above structure can easily be incorporated in the damper device, and can reliably generate a relatively large torque in the terminal range of each of the rotating strokes. 
     Alternatively, the torque generating means may have a second land extending axially on an inner surface of the casing and having a radially inner surface held in sliding contact with an outer surface of the rotor, and the fluid torque adjusting means may comprise a third land extending axially on the outer surface of the rotor, a pair of second axial grooves defined axially on a radially outer end of the third land on the rotor and spaced circumferentially from each other, a pair of second needle valves loosely fitted in the second axial grooves, respectively, for movement in the width direction of the second axial grooves, and a second circumferential groove defined circumferentially in the inner surface of the casing, the second cylindrical groove being positioned out of facing relation to at least one of the second needle valves in the terminal range of each of the rotating strokes. When the at least one of the second needle valves is held against the inner surface of the casing in the terminal range, the one of the second needle valves and the second land of the casing jointly divide an interior space of the casing into two chambers for effectively preventing the viscous fluid from moving between the chambers to produce the relatively large torque. The fluid torque adjusting means of the above structure can easily be incorporated in the damper device, and can reliably generate a relatively large torque in the terminal range of each of the rotating strokes. 
     Further alternatively, the torque generating means may have a fourth land extending axially on an inner surface of the casing and having a radially inner surface held in sliding contact with a outer surface of the rotor, and the fluid torque adjusting means may comprise a pair of second ridges extending axially on the outer surface of the rotor, a pair of valve bodies loosely mounted on the second ridges, respectively, and a third circumferential groove defined circumferentially in the inner surface of the casing, the third cylindrical groove being positioned out of facing relation to at least one of the needle valves in the terminal range of each of the rotating strokes. When the at least one of the valve bodies is held against the inner surface of the casing in the terminal range, the one of the valve bodies and the second ridges jointly divide an interior space of the casing into two chambers for effectively preventing the viscous fluid from moving between the chambers to produce the relatively large torque. The fluid torque adjusting means of the above structure can easily be incorporated in the damper device, and can reliably generate a relatively large torque in the terminal range of each of the rotating strokes. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a damper device according to a first embodiment of the present invention; 
     FIG. 2 is an enlarged exploded perspective view of a rotor of the damper device shown in FIG. 1; 
     FIG. 3 is an enlarged fragmentary perspective view of a cylindrical casing of the damper device shown in FIG. 1; 
     FIGS. 4A through 4I are cross-sectional view taking along line IV—IV of FIG. 1, showing the manner in which a fluid torque adjuster operates; 
     FIG. 5 is a cross-sectional view of a damper device according to a second embodiment of the present invention; 
     FIG. 6 is an enlarged fragmentary perspective view of a cylindrical casing of the damper device shown in FIG. 5; 
     FIG. 7 is an enlarged perspective view of a rotor of the damper device shown in FIG. 5; 
     FIGS. 8A through 8I are cross-sectional view taking along line VIII—VIII of FIG. 5, showing the manner in which a fluid torque adjuster operates; 
     FIG. 9 is a cross-sectional view of a damper device according to a third embodiment of the present invention; 
     FIG. 10 is an enlarged fragmentary perspective view of a cylindrical casing of the damper device shown in FIG. 9; 
     FIG. 11 is an enlarged exploded perspective view of a rotor of the damper device shown in FIG. 9; 
     FIGS. 12A through 12I are cross-sectional view taking along line XII—XII of FIG. 9, showing the manner in which a fluid torque adjuster operates; 
     FIG. 13 is a graph showing how a torque is generated when the rotor rotates in a normal direction; 
     FIG. 14 is a graph showing how a torque is generated when the rotor rotates in a reverse direction; 
     FIGS. 15A and 15B are cross-sectional views of a damper device according to a modification of the first embodiment; 
     FIGS. 16A and 16B are cross-sectional views of a damper device according to a modification of the second embodiment; 
     FIGS. 17A and 17B are cross-sectional views of a damper device according to a modification of the third embodiment; 
     FIGS. 18A and 18B are cross-sectional views of a damper device according to another modification of the first embodiment; 
     FIGS. 19A and 19B are cross-sectional views of a damper device according to another modification of the second embodiment; and 
     FIGS. 20A and 20B are cross-sectional views of a damper device according to another modification of the third embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Like or corresponding parts are denoted by like or corresponding reference characters throughout views. 
     FIG.  1  through FIGS. 4A-4I show a damper device according to a first embodiment of the present invention. 
     As shown in FIG. 1, the damper device, generally denoted by the reference numeral  1 , comprises a cylindrical casing  2 , a rotor  3  having a portion rotatably mounted in the  2 , and a viscous fluid  5  such as silicone oil filled in the casing  2  around the rotor  3 . The casing  2  has a closed end  7  and an opposite open end fitted with an annular cap  6  having an opening  61  through which an axial protrusion  32  of the rotor  3  projects out of the casing  2 . A sealing member  10  such as an O-ring is mounted between the rotor  3  and the cap  6  for preventing the viscous fluid  3  from leaking out of the casing  2  along the outer surface of the rotor  3 . The casing  2  includes flanges  21  projecting radially outwardly from the open end thereof. 
     As shown in FIG. 2, the rotor  3  has an axial ridge  31  projecting radially outwardly from an outer surface thereof and fitted between circumferentially spaced radial walls  41 ,  41 ′ of a spacer  4  which is of a substantially U-shaped cross section. The spacer  4  is placed on the radially outer end of the ridge  31  with substantially no play in circumferential and radial directions between the spacer  4  and the ridge  31 . Substantially no play between the spacer  4  and the ridge  31  means that the spacer  4  is snugly mounted on the ridge  31  so that the spacer  4  will not wobble on the ridge  31 , but the spacer  4  and the ridge  31  are not required to be dimensionally accurate as with precision parts. 
     When the rotor  3  rotates, the spacer  4  rotates in unison with the ridge  31  with a very small clearance present between the outer circumferential surface of the spacer  4  and the inner surface of the casing  2 . 
     The spacer  4  may be integrally formed with the ridge  31  of the rotor  3 . 
     As shown in FIG. 2, the rotor  3  has a first groove  33  defined in the outer surface thereof at an axially intermediate position, the first groove  33  having a constant width and depth and extending from a starting end S to a terminal end E. The starting end S is spaced circumferentially a given distance from the ridge  31 . The groove  33  extends circumferentially from the starting end S away from the ridge  31  to the terminal end E at a position that is diametrically opposite to the starting end S and which is spaced a given distance from the ridge  31 . The groove  33  angularly extends about 190° around the central axis O (FIG. 1) of the rotor  3 . The width and depth of the groove  33  may be varied circumferentially and/or axially to obtain a desired torque from the damper device  1 . 
     As shown in FIG. 3, the cylindrical casing  2  has an axial land  8  projecting radially inwardly from the inner surface thereof and angularly extends about 130° around the central axis O (FIG. 1) of the casing  2  which is aligned with the central axis O of the rotor  3 . When the rotor  3  rotates, the spacer  4  also rotates until it abuts against one of circumferentially opposite ends  81 ,  81 ′ of the land  8 , whereupon the rotor  3  is stopped against rotation. 
     It is more preferable to stop the rotation of the rotor  3  with stoppers positioned outside of the damper device  1  before the spacer  4  abuts against one of circumferentially opposite ends  81 ,  81 ′ because use of such stoppers is effective in increasing the durability of the spacer  4  and the land  8 . 
     The land  8  has a pair of axial grooves  82 ,  82 ′, and needle valves  9 ,  9 ′ having smaller diameters than the widths of the axial grooves  82 ,  82 ′ are loosely fitted in the axial grooves  82 ,  82 ′, respectively. The groove  82  has a depth progressively greater circumferentially toward the end  81 , and the groove  82 ′ has a depth progressively greater circumferentially toward the end  81 ′. The needle valves  9 ,  9 ′ are movable between shallowest groove portions  82   a,    82   a ′ of the grooves  82 ,  82 ′ and deepest groove portions  82   b,    82   b ′ of the grooves  82 ,  82 ′. 
     As shown in FIGS. 4B through 4E, when the rotor  3  rotates counterclockwise with respect to the casing  2 , the needle valve  9  is positioned in the deepest groove portion  82   b,  and the needle valve  9 ′ is positioned in the shallowest groove portion  82   a ′. As shown in FIGS. 4F through 4I, when the rotor  3  rotates clockwise with respect to the casing  2 , the needle valve  9  is positioned in the shallowest groove portion  82   a,  and the needle valve  9 ′ is positioned in the deepest groove portion  82   b′.    
     While the needle valve  9  or  9 ′ is being positioned in the shallowest groove portion  82   a  or  82   a ′, when the needle valve  9  or  9 ′ face the groove  33  in the rotor  3 , a clearance which is as deep as the groove  33  is present between the needle valve  9  or  9 ′ and the rotor  3 , and when the needle valve  9  or  9 ′ does not face the groove  33 , the needle valve  9  or  9 ′ contact the outer surface of the rotor  3 . 
     While the needle valve  9  or  9 ′ is being positioned in the deepest groove portion  82   b  or  82   b ′, when the needle valve  9  or  9 ′ face the groove  33  in the rotor  3 , a clearance which is deeper than the groove  33  is present between the needle valve  9  or  9 ′ and the rotor  3 . 
     The damper device  1  is installed on a container or the like as follows: The flanges  21  of the casing  2  are fastened to a container body such as a frame for the door of the container. The door is pivotally supported on the container body by a hollow shaft or the like. The protrusion  32  of the rotor  3  is fitted in the hollow shaft and lockingly secured to the hollow shaft, so that the rotor  3  is coupled to the door by the hollow shaft. 
     Conversely, the flanges  21  may be fastened to the door of the container, and protrusion  32  of the rotor  3  may be fitted in a hollow shaft mounted on a container body such as a frame for the door of the container, and lockingly secured to the hollow shaft, so that the rotor  3  is coupled to the door by the hollow shaft. 
     According to the first embodiment as described above, a fluid torque adjuster comprises the needle valves  9 ,  9 ′ disposed axially on the inner surface of the casing  2 , and the first groove  33  defined circumferentially in the outer surface of the rotor  3 . 
     Operation of the fluid torque adjuster according to the first embodiment will be described below with reference to FIGS. 4A through 4I. First, an action of the fluid torque adjuster when the door of the container is closed from a fully open, still position will be described below. 
     When the door of the container with the dapper device  1  mounted thereon is fully open, the parts of the dapper device  1  are in the position shown in FIG.  4 A. The wall  41  of the spacer  4  mounted on the ridge  31  is held against the end  81  of the land  8 . Since the viscous fluid does not flow at this stage, the needle valves  9 ,  9 ′ are in a free state in the respective grooves  82 ,  82 ′. 
     When the rotor  3  slightly rotates counterclockwise from the position shown in FIG. 4A, the needle valve  9  moves counterclockwise into the deepest groove portion  82   b  and the needle valve  9 ′ moves counterclockwise into the shallowest groove portion  82   a ′ as shown in FIG.  4 B. 
     As the rotor  3  rotates from the position shown in FIG. 4B to the position shown in FIG. 4C, the needle valve  9  is positioned in the deepest groove portion  82   b  though it does not face the groove  33 . Therefore, a passage for the viscous fluid  5  is provided between opposite sides of the needle valve  9 . Inasmuch as the needle valve  9 ′ faces the groove  33 , a passage for the viscous fluid  5  is also provided between opposite sides of the needle valve  9 ′. Since passages for the viscous fluid  5  are thus provided between opposite sides of the needle valves  9 ,  9 ′, the fluid torque adjuster generates a relatively small torque. 
     When the rotor  3  rotates from the position shown in FIG. 4C to the position shown in FIG. 4D, the needle valve  9 ′ positioned in the shallowest groove  82   a ′ passes the terminal end E of the groove  33  and contact the surface of the rotor  3 . Therefore, as the passages for the viscous fluid  5  between opposite sides of the needle valve  9 ′ is interrupted, the fluid torque adjuster generates a relatively large torque. 
     When the rotor  3  further rotates counterclockwise from the position shown in FIG. 4D, the other wall  41 ′ of the spacer  4  abuts against the other end  81 ′ of the land  8  as shown in FIG. 4E, whereupon the rotor  3  is stopped against rotation. While the rotor  3  is rotating from the position shown in FIG. 4D to the position shown in FIG. 4E, the needle valve  9 ′ does not face the groove  33  and is positioned in the shallowest groove portion  82   a ′, and hence is in contact with the outer surface of the rotor  3 . Since no passage for the viscous fluid  5  is provided between opposite sides of the needle valve  9 ′, the fluid torque adjuster generates a relatively large torque. 
     Consequently, when the door of the container is closed from the fully open position, the fluid torque adjuster generates a relatively large torque in a terminal range of the rotating stroke of the rotor  3 , and generates a relatively low torque from a starting range of the rotating stroke of the rotor  3  prior to the terminal range of the rotating stroke thereof. 
     Now, an action of the fluid torque adjuster when the door of the container is opened from a fully closed position will be described below. 
     When the door of the container with the damper device  1  mounted thereon is fully closed, the parts of the damper device  1  are in the position shown in FIG.  4 E. 
     When the rotor  3  slightly rotates clockwise from the position shown in FIG. 4E, the needle valve  9  moves clockwise from the deepest groove portion  82   b  into the shallowest groove portion  82   a  and the needle valve  9 ′ moves clockwise from the shallowest groove portion  82   a ′ into the deepest groove portion  82   b ′ as shown in FIG.  4 F. 
     As the rotor  3  rotates from the position shown in FIG. 4F to the position shown in FIG. 4G, the needle valve  9 ′ is still positioned in the deepest groove portion  82   b ′ while not facing the groove  33 , and the needle valve  9  faces the groove  33 . Therefore, the passages for the viscous fluid  5  are thus kept between opposite sides of the needle valves  9 ,  9 ′, and the fluid torque adjuster generates a relatively small torque. 
     When the rotor  3  rotates from the position shown in FIG. 4G to the position shown in FIG. 4H, the needle valve  9  positioned in the shallowest groove  82   a  passes the starting end S of the groove  33  and contact the circumferential surface of the rotor  3 , while the needle valve  9 ′ faces the groove  33 . Therefore, as the passages for the viscous fluid  5  between opposite sides of the needle valves  9  is thus interrupted, the fluid torque adjuster generates a relatively large torque. 
     As the rotor  3  further rotates from the position shown in FIG. 4H, when the wall  41  of the spacer  4  abuts against the end  81  of the land  8  as shown in FIG. 4I, whereupon the rotor  3  is stopped against rotation. 
     As the rotor  3  rotates from the position shown in FIG. 4H up to the position shown in FIG. 4I, the needle valve  9 ′ is positioned in the deepest groove portion  82   b ′ while facing the groove  33 . Therefore, the clearance is still kept between the needle valve  9 ′ and the rotor  3 . However, the needle valve  9  is no longer facing the groove  33  and contacts the outer surface of the rotor  3 . Since no passage for the viscous fluid  5  is produced between opposite sides of the needle valve  9 , the fluid torque adjuster generates a relatively large torque. 
     Consequently, when the door of the container is opened from the fully closed position, the fluid torque adjuster generates a relatively large torque in a terminal range of the rotating stroke of the rotor  3 , and generates a relatively low torque from a starting range prior to the terminal range of the rotating stroke thereof, as when the door of the container is closed from the fully open position. 
     A damper device according to a second embodiment of the present invention will be described below with reference to FIGS.  5  through  8 A- 8 I. 
     As shown in FIG. 5, the damper device, generally denoted by the reference numeral  101 , comprises a cylindrical casing  102 , a rotor  103  having a portion rotatably mounted in the casing  102 , and a viscous fluid  105  filled in the casing  102  around the rotor  103 . An axial protrusion  132  of the rotor  103  projects out of an opening  161  in an annular cap  106  in the open end of the casing  102 . A sealing member  110  prevents the viscous fluid  103  from leaking out of the casing  102 . 
     As shown in FIG. 6, the casing  102  has a first land  122  projecting radially inwardly from an inner surface thereof. When the rotor  103  rotates, the outer surface of the rotor  103  slides against a radially inner surface of the land  122 . When a second land  135  (described later on) of the rotor  103  abuts against one of circumferentially opposite ends  122   a,    122   a ′ of the land  122 , the rotor  103  is stopped against rotation. 
     As shown in FIG. 6, the casing  102  has a second groove  134  defined in the inner surface thereof, the second groove  134  having a constant width and depth and extending from a starting end S to a terminal end E. The second groove  134  extends diametrically opposite to the land  122 . Specifically, the distance between the starting end S and the end  122   a  is equal to the distance between the terminal end E and the other end  122   a ′. Alternatively, the distance between the starting end S and the end  122   a  may be different from the distance between the terminal end E and the other end  122   a ′. The groove  134  angularly extends about 140° around the central axis O (FIG. 5) of the casing  102 . The width and depth of the groove  134  may be varied circumferentially and/or axially to obtain a desired torque from the damper device  101 . 
     As shown in FIG. 7, the rotor  103  has a second land  135  extending axially on the outer surface thereof. The land  135  has a pair of axial grooves  136 ,  136 ′ defined in an outer surface thereof, and also has a pair of circumferentially opposite ends  137 ,  137 ′. The land  135  angularly extends about 80° around the central axis O (FIG. 5) of the rotor  103 . Needle valves  109 ,  109 ′ having smaller diameters than the widths of the axial grooves  136 ,  136 ′ are loosely fitted in the axial grooves  136 ,  136 ′, respectively. The groove  136  has a depth progressively greater circumferentially toward the end  137 , and the groove  136 ′ has a depth progressively greater circumferentially toward the end  137 ′. The needle valves  109 ,  109 ′ are movable between shallowest groove portions  136   a,    136   a ′ and deepest groove portions  136   b,    136   b ′ of the grooves  136 ,  136 ′. 
     As shown in FIGS. 8B through 8E, when the rotor  103  rotates counterclockwise with respect to the casing  102 , the needle valve  109  is positioned in the deepest groove portion  136   b,  and the needle valve  109 ′ is positioned in the shallowest groove portion  136   a ′. As shown in FIGS. 8F through 8I, when the rotor  103  rotates clockwise with respect to the casing  102 , the needle valve  109  is positioned in the shallowest groove portion  136   a,  and the needle valve  109 ′ is positioned in the deepest groove portion  136   b′.    
     While the needle valve  109  or  109 ′ is being positioned in the shallowest groove portion  136   a  or  136   a ′, when the needle valve  109  or  109 ′ face the groove  134  in the casing  102 , a clearance which is as deep as the groove  134  is created between the needle valve  109  or  109 ′ and the casing  102 , and when the needle valve  109  or  109 ′ does not face the groove  134 , the needle valves  109  or  109 ′ contact the inner surface of the casing  102 . 
     While the needle valve  109  or  109 ′ is being positioned in the deepest groove portion  136   b  or  136   b ′, when the needle valve  109  or  109 ′ faces the groove  134  in the casing  102 , a clearance which is deeper than the groove  134  is created between the needle valve  109  or  109 ′ and the casing  102 . 
     According to the second embodiment as described above, a fluid torque adjuster comprises the grooves  136 ,  136 ′ defined axially in the land  135  on the outer surface of the rotor  103 , and the second groove  134  defined circumferentially in the inner surface of the casing  102 . 
     The damper device  101  according to the second embodiment is connected to the door of a container or the like as with the damper device according to the first embodiment. 
     Operation of the fluid torque adjuster according to the second embodiment will be described below with reference to FIGS. 8A through 8I. First, an action of the fluid torque adjuster when the door of the container is closed from a fully open position will be described below. 
     When the door of the container with the damper device  101  mounted thereon is fully open, the parts of the damper device  101  are in the position shown in FIG.  8 A. The end  137  of the land  135  is held against the end  122   a  of the land  122 . Since the viscous fluid does not flow at this stage, the needle valves  109 ,  109 ′ are in a free state in the respective grooves  136 ,  136 ′. 
     When the rotor  103  slightly rotates counterclockwise from the position shown in FIG. 8A to the position shown in FIG. 8B, the needle valve  109  moves clockwise into the deepest groove portion  136   b  and the needle valve  109 ′ moves clockwise into the shallowest groove portion  136   a′.    
     As the rotor  103  rotates from the position shown in FIG. 8B to the position shown in FIG. 8C, the needle valve  109  is positioned in the deepest groove portion  136   b  though it does not face the groove  134 . Therefore, a passage for the viscous fluid  105  is provided between opposite sides of the needle valve  109 . Inasmuch as the needle valve  109 ′ faces the groove  134  a passage for the viscous fluid  105  is also provided between opposite sides of the needle valve  109 ′, and as a result, the fluid torque adjuster generates a relatively small torque. 
     When the rotor  103  rotates from the position shown in FIG. 8C to the position shown in FIG. 8D, the needle valve  109 ′ positioned in the shallowest groove  136   a ′ comes to the terminal end E of the groove  134  and contacts the inner surface of the casing  102  while the needle valve  109  faces the groove  134 . Therefore, since the passages for the viscous fluid  105  between opposite sides of the needle valve  109 ′ is thus interrupted, the fluid torque adjuster generates a relatively large torque. 
     When the rotor  103  further rotates counterclockwise from the position shown in FIG. 8D, the other end  137 ′ of the land  135  abuts against the other end  122   a ′ of the land  122  as shown in FIG. 8E, whereupon the rotor  103  is stop against rotation. While the rotor  103  is rotating from the position shown in FIG. 8D to the position shown in FIG. 8E, the needle valve  109 ′ does no longer face the groove  134  and is positioned in the shallowest groove portion  136   a ′, and hence is in contact with the inner surface of the casing  102 . Since no passage for the viscous fluid  105  is provided between opposite sides of the needle valve  109 ′, the fluid torque adjuster generates a relatively large torque. 
     Consequently, when the door of the container is closed from the fully open position, the fluid torque adjuster generates a relatively large torque in a terminal range of the rotating stroke of the rotor  103 , and generates a relatively low torque from a starting range of the rotating stroke of the rotor  103  prior to the terminal range of the rotating stroke thereof. 
     Now, an action of the fluid torque adjuster when the door of the container is opened from a fully closed position will be described below. 
     When the door of the container with the damper device  101  mounted thereon is fully closed, the parts of the dapper device  101  are in the position shown in FIG.  8 E. 
     When the rotor  103  slightly rotates clockwise from the position shown in FIG. 8E, the needle valve  109  moves counterclockwise from the deepest groove portion  136   b  into the shallowest groove portion  136   a  and the needle valve  109 ′ moves counterclockwise from the shallowest groove portion  136   a ′ into the deepest groove portion  136   b ′ as shown in FIG.  8 F. 
     As the rotor  103  rotates from the position shown in FIG. 8F to the position shown in FIG. 8G, the needle valve  109 ′ is still positioned in the deepest groove portion  136   b ′ while not facing the groove  134 , and the needle valve  109  faces the groove  134 . Therefore, the passages for the viscous fluid  105  are thus kept between opposite sides of the needle valves  109 ,  109 ′, the fluid torque adjuster generates a relatively small torque. 
     When the rotor  103  rotates from the position shown in FIG. 8G to the position shown in FIG. 8H, the needle valve  109  positioned in the shallowest groove  136   a  comes to the starting end S of the groove  134  and contacts the inner surface of the casing  102  while the needle valve  109 ′ faces the groove  134 . Therefore, as the passages for the viscous fluid  105  between opposite sides of the needle valve  109  is thus interrupted, the fluid torque adjuster generates a relatively large torque. 
     As the rotor  103  further rotates from the position shown in FIG. 8H, when the end  137  of the land  135  abuts against the end  122   a  of the land  122  as shown in FIG. 8I, whereupon the rotor  103  is stopped against rotation. In the position shown in FIG. 8I, the needle valves  109 ,  109 ′ are in the initial free state as shown in FIG.  8 A. 
     As the rotor  103  rotates from the position shown in FIG. 8H up to the position shown in FIG. 8I, the needle valve  109 ′ is positioned in the deepest groove portion  136   b ′ while facing the groove  134 . Therefore, the clearance is still kept between the needle valve  109 ′ and the casing  102 . However, the needle valve  109  positioned in the shallowest groove portion  136   a  is no longer facing the groove  134  and contacts the inner surface of the casing  102 . Since no passage for the viscous fluid  105  is provided between opposite sides of the needle valve  109 , the fluid torque adjuster continues to generate a relatively large torque. 
     Consequently, according to the second embodiment, as with the first embodiment, when the door of the container is opened from the fully closed position, the fluid torque adjuster generates a relatively large torque in a terminal range of the rotating stroke of the rotor  103 , and generates a relatively low torque from a starting range prior to the terminal range of the rotating stroke thereof, as when the door of the container is closed from the fully open position. 
     A damper device according to a third embodiment of the present invention will be described below with reference to FIG.  9  through  12 A- 12 I. 
     As shown in FIG. 9, the dapper device, generally denoted by the reference numeral  201 , comprises a cylindrical casing  202 , a rotor  203  having a portion rotatably mounted in the casing  202 , and a viscous fluid  205  filled in the casing  202  around the rotor  203 . An axial protrusion  232  of the rotor  203  projects out of an opening  261  in an annular cap  206  in the open end of the casing  202 . A sealing member  210  prevents the viscous fluid  203  from leaking out of the casing  202 . 
     As shown in FIG. 10, the casing  202  has a land  222  projecting radially inwardly from an inner surface thereof. When the rotor  203  rotates, the outer surface of the rotor  203  slides against a radially inner surface of the land  222 . When the rotor  203  rotates clockwise in FIGS. 12F-12H, an end  241   a  of a valve body  204   a  abuts against an end  222   a  of the land  222 , whereupon the rotor  203  is stopped against rotation. When the rotor  203  rotates counterclockwise in FIGS. 12B-12D, an end  241   b  of a valve body  204   b  abuts against an opposite end  222   a ′ of the land  222 , whereupon the rotor  203  is stopped against rotation. 
     As shown in FIG. 10, the casing  202  has a third groove  234  defined in the inner surface thereof, the third groove  234  having a constant width and depth and extending from a starting end S to a terminal end E. The third groove  234  extends diametrically opposite to the land  222 . Specifically, the distance between the starting end S and the end  222   a  is equal to the distance between the terminal end E and the other end  222   a ′. Alternatively, the distance between the starting end S and the end  222   a  may be different from the distance between the terminal end E and the other end  222   a ′. The groove  234  angularly extends about 190° around the central axis O of the casing  202 . The width and depth of the groove  234  may be varied circumferentially and/or axially to obtain a desired torque from the damper device  201 . 
     As shown in FIG. 11, the rotor  203  has a pair of circumferentially spaced axial ridges  235   a,    235   b  projecting radially outwardly from an outer surface thereof and having respective recesses p, q defined axially centrally in radially outer ends thereof. The ridges  235   a,    235   b  extend respectively along planes angularly spaced about 50° from each other about the central axis O of the rotor  203 . 
     Valve bodies  204   a,    204   b,  each of a substantially U-shaped cross section, are loosely fitted over the respective ridges  235   a,    235   b  for rotation with the ridges  235   a,    235   b  upon rotation of the rotor  203 . The valve body  204   a  has a pair of circumferentially spaced radial walls  241   a,    242   a,  and the valve body  204   b  has a pair of circumferentially spaced radial walls  241   b,    242   b.  The circumferential distance between the walls  241   a,    242   a  and the circumferential distance between the walls  241   b,    242   b  are greater than the circumferential widths of the ridges  235   a,    235   b.  Therefore, the ridges  235   a,    235   b  can move between the walls  241   a,    242   a  and between the walls  241   b,    242   b.    
     The walls  242   a,    242   b  which are positioned adjacent to each other have respective recesses r, s defined axially centrally therein. The walls  241   a,    241   b  have no such recesses. 
     As shown in FIGS. 12B through 12E, when the rotor  203  rotates counterclockwise with respect to the casing  202 , the ridge  235   a  rotates with its forward face held against the wall  242   a  of the valve body  204   a,  and the ridge  235   b  rotates with its forward face held against the wall  241   b  of the valve body  204   b.  As shown in FIGS. 12F through 12I, when the rotor  203  rotates clockwise with respect to the casing  202 , the ridge  235   a  rotates with its forward face held against the wall  241   a  of the valve body  204   a,  and the ridge  235   b  rotates with its forward face held against the wall  242   b  of the valve body  204   b.    
     According to the third embodiment, a fluid torque adjuster comprises the valve bodies  204   a,    204   b  loosely fitted over the respective ridges  235   a,    235   b,  and the third groove  234  defined circumferentially in the inner surface of the casing  202 . The valve bodies  204   a,    204   b  include the radial walls  242   a,    242   b,  respectively, each having a recess and the radial walls  241   a,    241   b,  respectively, each having no recess. 
     The damper device  201  according to the third embodiment is connected to the door of a container or the like as with the damper devices according to the first and second embodiments. 
     Operation of the fluid torque adjuster according to the third embodiment will be described below with reference to FIGS. 12A through 12I. First, an action of the fluid torque adjuster when the door of the container is closed from a fully open position will be described below. 
     When the door of the container with the damper device  201  mounted thereon is fully open, the parts of the damper device  201  are in the position shown in FIG.  12 A. The wall  241   a  of the valve body  204   a  has an inner surface held against the lower surface, as shown in FIG. 12A, of the ridge  235   a,  and an outer surface held against the end  222   a  of the land  222 . The radially outer surface of the valve body  204   a  is held against the inner surface of the casing  202 . The wall  242   b  of the valve body  204   b  has an inner surface held against the lower surface, as shown in FIG. 12A, of the ridge  235   b.  The radially outer surface of the valve body  204   b  faces the groove  234 . A clearance which is as deep as the groove  234  is present between the radially outer surface of the valve body  204   b  and the inner surface of the casing  202 . 
     When the rotor  203  slightly rotates counterclockwise from the position shown in FIG. 12A to the position shown in FIG. 12B, the valve bodies  204   a,    204   b  do not rotate until the inner surface of the wall  242   a  of the valve body  204   a  is brought into contact with the upper surface, as shown in FIG. 12B, of the ridge  235   a,  and the inner surface of the wall  241   b  of the valve body  204   b  is brought into contact with the upper surface, as shown in FIG. 12B, of the ridge  235   b.    
     In the position shown in FIG. 12B, as with the position shown in FIG. 12A, the radially outer surface of the valve body  204   a  is held against the inner surface of the casing  202 , and the radially outer surface of the valve body  204   b  in its entirety faces the groove  234  while the wall  242   b  having the recess r being held against the forward face of the ridge  235   a.  Therefore, a passage for the viscous fluid is provided between the radially outer surface of the valve body  204   b  and the inner surface of the casing  202 , and through the recesses r, p of the wall  242   a  and the ridge  235   a,  respectively. 
     As the rotor  203  rotates from the position shown in FIG. 12B to the position shown in FIG. 12C, a passage for the viscous fluid  205  extends from the groove  234  which faces the valve body  204   b,  through the recess r in the wall  242   a  and through the recess p in the ridge  235   a,  to a space between the inner surface of the wall  241   a  and one side surface of the ridge  235   a.  As a result, the fluid torque adjuster produces a relatively small torque. 
     When the rotor  203  rotates from the position shown in FIG. 12C to the position shown in FIG. 12D, the wall  241   b  of the valve body  204   b  comes to the terminal end E of the groove  234  and a front end of the radially outer surface of the valve body  204   b  contacts the inner surface of the casing  202  which has no groove. As a result, a fluid communication through the groove  234  is interrupted at the terminal end E of the groove  234 . Since no recess is defined in the wall  241   b  of the valve body  204   b,  no fluid communication is established between the wall  241   a  of the valve body  204   a  and the wall  241   b  of the valve body  204   b.  Therefore, the fluid torque adjuster now produces a relatively large torque. 
     When the rotor  203  further rotates from the position shown in FIG. 12D, the outer surface of the wall  241   b  of the valve body  204   b  abuts against the other end  222   a ′ of the land  222 , as shown in FIG. 12E, whereupon the rotor  203  is stopped against rotation. During the rotation of the rotor  203  from the position shown in FIG. 12D to the position shown in FIG. 12E, as the radially outer surface of the valve body  204   b  is held against the inner surface of the casing  202 , the fluid torque adjuster continues to produce a relatively large torque. 
     Consequently, according to the third embodiment, as with the first embodiment, when the door of the container is closed from the fully open position, the fluid torque adjuster generates a relatively large torque in a terminal range of the rotating stroke of the rotor  203 , and generates a relatively low torque from a starting range of the rotating stroke of the rotor  203  prior to the terminal range of the rotating stroke thereof. 
     Now, an action of the fluid torque adjuster when the door of the container is opened from a fully closed position will be described below. 
     When the door of the container with the damper device  201  mounted thereon is fully closed, the parts of the damper device  201  are in the position shown in FIG.  12 E. The wall  241   b  of the valve body  204   b  has an inner surface held against the lower surface, as shown in FIG. 12E, of the ridge  235   b,  and an outer surface held against the end  222   a ′ of the land  222 . The radially outer surface of the valve body  204   b  is held against the inner surface of the casing  202 . The wall  242   a  of the valve body  204   a  has an inner surface held against the lower surface, as shown in FIG. 12E, of the ridge  235   a.  The radially outer surface of the valve body  204   a  faces the groove  234 . 
     When the rotor  203  slightly rotates clockwise from the position shown in FIG. 12E to the position shown in FIG. 12F, the valve bodies  204   a,    204   b  do not rotate until the inner surface of the wall  242   b  of the valve body  204   b  is brought into contact with the upper surface, as shown in FIG. 12F, of the ridge  235   b,  and the inner surface of the wall  241   a  of the valve body  204   a  is brought into contact with the upper surface, as shown in FIG. 12F, of the ridge  235   a.    
     In the position shown in FIG. 12F, as with the position shown in FIG. 12E, the radially outer surface of the valve body  204   b  is held against the inner surface of the casing  202 , and the radially outer surface of the valve body  204   a  faces the groove  234 . Therefore, a clearance which is as deep as the groove  234  is provided between the radially outer surface of the valve body  204   a  and the inner surface of the casing  202 . 
     As the rotor  203  rotates from the position shown in FIG. 12F to the position shown in FIG. 12G, a passage for the viscous fluid  205  extends from the groove  234  which faces the valve body  204   a,  through the recess s in the wall  242   b  and through the recess q in the ridge  235   b,  to a space between the inner surface of the wall  241   b  and one side surface of the ridge  235   b.  As a result, the fluid torque adjuster produces a relatively small torque. 
     When the rotor  203  rotates from the position shown in FIG. 12G to the position shown in FIG. 12H, the wall  241   a  of the valve body  204   a  reaches the starting end S of the groove  234 , and a front end of the radially outer surface of the valve body  204   a  contacts the inner surface of the casing  202  which has no groove. As a result, a fluid communication through the groove  234  is interrupted at the starting end S of the groove  234 . Since no recess is defined in the wall  241   a  of the valve body  204   a,  no fluid communication is established between the wall  241   a  of the valve body  204   a  and the wall  241   b  of the valve body  204   b.  Therefore, the fluid torque adjuster now produces a relatively large torque. 
     When the rotor  203  further rotates from the position shown in FIG. 12H, the outer surface of the wall  241   a  of the valve body  204   a  abuts against the end  222   a  of the land  222 , as shown in FIG. 12I, whereupon the rotor  203  is stopped against rotation. During the rotation of the rotor  203  from the position shown in FIG. 12H to the position shown in FIG. 12I, as the radially outer surface of the valve body  204   a  is held against the inner surface of the casing  202 , the fluid torque adjuster continues to produce a relatively large torque. 
     Consequently, according to the third embodiment, as with the first and second embodiments, when the door of the container is closed from the fully open position, the fluid torque adjuster generates a relatively large torque in a terminal range of the rotating stroke of the rotor  203 , and generates a relatively low torque from a starting range of the rotating stroke of the rotor  203  prior to the terminal range of the rotating stroke thereof. 
     FIGS. 13 and 14 show how a torque is generated in the rotating strokes of the rotor  3  ( 103 ,  203 ), in normal and reverse directions. FIG. 13 shows a generated torque when the rotor  3  ( 103 ,  203 ) rotates in a counterclockwise stroke in FIGS. 4A-4E,  8 A- 8 E, and  12 A- 12 E, and FIG. 14 shows a generated torque when the rotor  3  ( 103 ,  203 ) rotates in a clockwise stroke in FIGS. 4E-4I,  8 E- 8 I, and  12 E- 12 I. It can be seen from FIGS. 13 and 14 that a higher torque is generated in the terminal range of each of the counterclockwise and clockwise strokes of the rotor  3  ( 103 ,  203 ). 
     Various modifications of the first through third embodiments will be described below. 
     FIGS. 15A and 15B show a modification of the first embodiment. As shown in FIGS. 15A and 15B, two spacers  304  are mounted respectively on diametrically opposite ridges  331  on the rotor  303 , and two diametrically opposite pairs of needle valves  309 ,  309 ′ are disposed axially on the inner surface of the cylindrical casing  302 . Two first grooves  333  are defined in diametrically opposite relation in the outer surface of the rotor  303 . Therefore, the various parts are provided in two sets. 
     FIGS. 16A and 16B show a modification of the second embodiment. As shown in FIGS. 16A and 16B, two diametrically opposite axial lands  422  project radially inwardly from the inner surface of the cylindrical casing  402 , and two diametrically opposite pairs of needle valves  409 ,  409 ′ are loosely fitted in respective grooves defined in lands  435  of the rotor  403 . Two grooves  434  are defined in diametrically opposite relation in the inner surface of the casing  402 . Therefore, the various parts are provided in two sets. 
     FIGS. 17A and 17B show a modification of the third embodiment. As shown in FIGS. 17A and 17B, two diametrically opposite axial lands  522  project radially inwardly from the inner surface of the cylindrical casing  502 , and two diametrically opposite pairs of valve bodies  404   a,    404   b  are circumferentially loosely fitted over respective ridges  435   a,    435   b  of the rotor  403 . Two grooves  434  are defined in diametrically opposite relation in the inner surface of the casing  402 . Therefore, the various parts are provided in two sets. 
     FIGS. 15A,  16 A, and  17 A show the positions of the parts when the rotors  303 ,  403 ,  503  rotate counterclockwise, and FIGS. 15B,  16 B, and  17 B show the positions of the parts when the rotors  303 ,  403 ,  503  rotate clockwise. 
     Fluid torque adjusters according to these modifications operate in the same manner as with the fluid torque adjusters where the parts are provided in one set. The parts of the modified fluid torque adjusters may be dimensioned and positioned differently from those illustrated. While the parts are provided in two sets according to the modifications shown in FIGS. 15A,  15 B,  16 A,  16 B, and  17 A,  17 B, the parts may be provided in three or more sets. 
     Other modifications of the first through third embodiments will be described below. 
     FIGS. 18A and 18B show another modification of the first embodiment. As shown in FIGS. 18A and 18B, a pair of diametrically opposite axial lands  608 ,  608 ′ is disposed on the inner surface of the cylindrical casing  602 , and radially movable needle valves  609 ,  609 ′ are loosely fitted in respective grooves  682 ,  682 ′ defined axially in the lands  608 ,  608 ′. A pair of diametrically opposite ridges  631 ,  631 ′ is disposed on the outer surface of the rotor  603 , and spacers  604 ,  604 ′ are mounted on the respective ridges  631 ,  631 ′. Two grooves  633 ,  633 ′ are defined in diametrically opposite relation in the outer surface of the rotor  603 . In the modification shown in FIGS. 18A and 18B, the various parts are provided in two sets. However, the grooves  682 ,  682 ′ and the needle valves  609 ,  609 ′ loosely fitted therein are provided one in each set. 
     FIGS. 19A and 19B show another modification of the second embodiment. As shown in FIGS. 19A and 19B, a pair of diametrically opposite first axial lands  722 ,  722 ′ is disposed on the inner surface of the cylindrical casing  702 , and a pair of diametrically opposite second axial lands  735 ,  735 ′ is disposed on the outer surface of the rotor  703 . Radially movable needle valves  709 ,  709 ′ are loosely fitted in respective grooves  736 ,  736 ′ defined axially in the second lands  735 ,  735 ′. Two grooves  734 ,  734 ′ are defined in diametrically opposite relation in the inner surface of the casing  702 . In the modification shown in FIGS. 19A and 19B, the various parts are provided in two sets. However, the grooves  736 ,  736 ′ and the needle valves  709 ,  709 ′ loosely fitted therein are provided one in each set. 
     FIGS. 20A and 20B show another modification of the third embodiment. As shown in FIGS. 20A and 20B, a pair of diametrically opposite first axial lands  822 ,  822 ′ is disposed on the inner surface of the cylindrical casing  802 , and a pair of diametrically opposite axial ridges  835   a,    835   b  is disposed on the outer surface of the rotor  803 . Valve bodies  804 ,  804 ′ are loosely mounted on the respective ridges  835   a,    835   b,  and two grooves  834 ,  834 ′ are defined in diametrically opposite relation in the inner surface of the casing  802 . In the modification shown in FIGS. 20A and 20B, the various parts are provided in two sets. However, the ridges  835   a,    835   b  and the valve bodies  804   a,    804   b  mounted thereon are provided one in each set. 
     FIGS. 18A,  19 A, and  20 A show the positions of the parts when the rotors  603 ,  703 ,  803  rotate counterclockwise, and FIGS. 18B,  18 B, and  20 B show the positions of the parts when the rotors  603 ,  703 ,  803  rotate clockwise. 
     Fluid torque adjusters according to these modifications operate in the same manner as with the fluid torque adjusters where the parts are provided in one set. The parts of the modified fluid torque adjusters may be dimensioned and positioned differently from those illustrated. While the parts are provided in two sets according to the modifications shown in FIGS. 18A,  18 B,  19 A,  19 B, and  20 A,  20 B, the parts may be provided in three or more sets. 
     The damper device according to the present invention has been described as being applied to the door of a container for damping the door in terminal ranges of opening and closing movements of the door. However, the damper device may be used in other applications. For example, the damper device may be used in combination with reciprocally movable devices for applying damping forces to back-and-forth movements thereof in terminal ranges. 
     One of the other applications is as an electrically powered saw. Specifically, when the electrically powered saw is pushed to cut a piece of wood, the damper device applies damping forces to the saw in a terminal range of its stroke before the saw hits a rear stop, and when the electrically powered saw is pulled, the damper device also applies damping forces to the saw in a terminal range of its stroke before the saw hits a front stop. 
     In each of the above embodiments and modifications, the casing is fixed in position and the rotor is rotatable. However, the rotor may be fixed in position and the casing may be rotatable. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Summary:
A damper device includes a casing, a rotor partly housed in the casing, a viscous fluid filled in the casing around the rotor, and a torque generator for generating a torque during a rotating stroke of the rotor. The torque generator includes a fluid torque adjuster for producing a relatively large torque in at least a terminal range of the rotating strokes in normal and reverse directions of the rotor.