Patent Publication Number: US-2022212320-A1

Title: Impact tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-001036, filed on Jan. 6, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an impact tool such as an impact driver. 
     2. Description of the Background 
     For example, an impact driver described in Japanese Unexamined Patent Application Publication No. 2019-936 includes a motor in its rear and an output unit in its front including an anvil drivable by the motor for rotational striking. The output unit further includes a spindle rotatable as the motor rotates and a hammer connected to the spindle with a cam with balls in between. The hammer is urged to a forward position by a coil spring externally mounted on the spindle to have its tabs on the front surface engaged with arms of the anvil in the rotation direction. 
     When the motor is driven to rotate the spindle, the anvil rotates with the hammer, allowing a screw to be screwed with a bit attached to the anvil. As the screw is tightened and increases the torque of the anvil, the hammer retracts against the urging force from the coil spring while rolling the balls along cam grooves in the spindle. After the tabs are disengaged from the arms, the hammer rotates forward along the cam grooves under the urging force from the coil spring. This then causes the tabs to be re-engaged with the arms, causing the anvil to generate a rotational impact force (impact). This process is repeated for further tightening of the screw. 
     BRIEF SUMMARY 
     For tightening a screw in a high load state with this impact tool, the hammer may retract under the reaction force from the impact to a rearmost position until the balls reach the rear ends of the cam grooves. The hammer retracting to the rearmost position is urged further to rotate with the rotational energy, thus causing an overloaded state in which a shock load applied to the spindle through the balls reaches internal components in the preceding stage including planetary gears. This may lower the durability of the impact tool. 
     One or more aspects of the present disclosure are directed to an impact tool that effectively reduces durability deterioration caused by a shock load. 
     A first aspect of the present disclosure provides an impact tool, including: 
     a motor; 
     a carrier including a reduction assembly and rotatable by the motor; 
     a shaft configured to receive rotation of the carrier, the shaft being rotatable relative to the carrier in an overloaded state; 
     a hammer held by the shaft; and 
     an anvil configured to be struck by the hammer in a rotation direction. 
     The impact tool according to the above aspect of the present disclosure effectively reduces durability deterioration caused by a shock load. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of an impact driver. 
         FIG. 2  is a longitudinal central sectional view of the impact driver. 
         FIG. 3  is an enlarged view of a hammer case in  FIG. 2 . 
         FIG. 4  is an exploded perspective view of the hammer case. 
         FIG. 5  is an enlarged cross-sectional view taken along line A-A in  FIG. 3 . 
         FIG. 6  is an enlarged cross-sectional view taken along line B-B in  FIG. 3 . 
         FIG. 7  is an enlarged cross-sectional view taken along line C-C in  FIG. 3 . 
         FIG. 8A  is a perspective view of a striking assembly with a hammer at a forward position. 
         FIG. 8B  is a longitudinal cross-sectional view of the striking assembly with the hammer at the forward position. 
         FIG. 9A  is a perspective view of the striking assembly with the hammer at a rearmost position. 
         FIG. 9B  is a longitudinal cross-sectional view of the striking assembly with the hammer at the rearmost position. 
         FIG. 10A  is a perspective view of the striking assembly with a cam assembly in operation. 
         FIG. 10B  is a longitudinal cross-sectional view of the striking assembly with the cam assembly in operation. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will now be described with reference to the drawings. 
       FIG. 1  is a side view of a rechargeable impact driver as an example of an impact tool.  FIG. 2  is a longitudinal central sectional view of the impact driver. 
     An impact driver  1  includes a body  2  and a grip  3 . The body  2  includes a central axis extending in the front-rear direction. The grip  3  protrudes downward from the body  2 . The impact driver  1  includes a housing including a body housing  4 , a rear cover  5 , and a hammer case  6 . The body housing  4  includes a motor housing  7 , a grip housing  8 , and a battery mount  9 . The motor housing  7  is cylindrical and defines a rear portion of the body  2 . The grip housing  8  defines the grip  3 . The battery mount  9  receives a battery pack  10 , which serves as a power supply. 
     The body housing  4  and the rear cover  5  are formed from resin. The body housing  4  includes left- and right-half housings  4   a  and  4   b . The left- and right-half housings  4   a  and  4   b  are joined together with multiple screws  11  placed from the right. The rear cover  5  is a cap. The rear cover  5  is joined to the motor housing  7  from the rear with two screws, or right and left screws. 
     The hammer case  6  is formed from metal. The hammer case  6  is joined to a front portion of the motor housing  7 . The hammer case  6  defines a front portion of the body  2 . Lamps (not shown) for illuminating ahead are located on the right and left of the hammer case  6  between the hammer case  6  and the motor housing  7 . 
     The body  2  accommodates, from the rear, a brushless motor  12 , a reduction assembly  13 , a spindle  14 , and a striking assembly  15 . The brushless motor  12  is accommodated in the motor housing  7  and the rear cover  5 . The reduction assembly  13 , the spindle  14 , and the striking assembly  15  are accommodated in the hammer case  6 . The striking assembly  15  includes an anvil  16 . The anvil  16  has a front end protruding frontward from the hammer case  6 . 
     The grip  3  accommodates a switch  17  in its upper portion. A trigger  18  protrudes in front of the switch  17 . 
     A forward-reverse switch lever  19  for the brushless motor  12  is located between the hammer case  6  and the switch  17 . A mode switch  20  is located in front of the forward-reverse switch lever  19 . The mode switch  20  faces frontward and has a button exposed on the front surface. The button is repeatedly pressed to switch impact forces or registered striking modes. 
     The battery mount  9  accommodates a terminal base  21  and a controller  22 . The terminal base  21  is electrically connected to multiple battery cells encased in the battery pack  10 . The controller  22  is located above the terminal base  21 . The controller  22  includes a control circuit board  23  receiving, for example, a microcomputer and switching elements. A display panel  24  is located on the upper surface of the battery mount  9 . The display panel  24  is electrically connected to the control circuit board  23 . The display panel  24  displays the rotational speed of the brushless motor  12  and the remaining battery level of the battery pack  10 . The display panel  24  also allows other operations including switching the on-off state of the lamps. 
     The brushless motor  12  is an inner-rotor motor including a stator  25  and a rotor  26 . The stator  25  includes a stator core  27 , insulators  28 , and coils  29 . The insulators  28  are on the front and the rear of the stator core  27 . The coils  29  are wound around the stator core  27  with the insulators  28  in between. 
     The front insulator  28  receives a sensor circuit board  30 . The sensor circuit board  30  includes three rotation detectors (not shown). The three rotation detectors detect the position of a sensor permanent magnet  34  in the rotor  26  and output rotation detection signals. 
     The rotor  26  includes a rotational shaft  31 , a cylindrical rotor core  32 , a permanent magnet  33 , and the sensor permanent magnet  34 . The rotational shaft  31  is aligned with the axis of the rotor  26  and extends in the front-rear direction. The permanent magnet  33  is cylindrical and surrounds the rotor core  32 . The sensor permanent magnet  34  is in front of the rotor core  32 . 
     The rear cover  5  holds a bearing  35  in the center portion of its rear inner surface. The bearing  35  axially supports the rear end of the rotational shaft  31 . The rotational shaft  31  receives a fan  36  for cooling the motor in front of the bearing  35 . The rear cover  5  has multiple outlets  37  in its circumferential surface outward from the fan  36 . The motor housing  7  has multiple inlets  38  in its right and left side surfaces in front of the outlets  37 . 
     A bearing box  40  is held in front of the brushless motor  12  in the motor housing  7 . The bearing box  40  is a disk having a stepped shape with a center portion protruding rearward. The motor housing  7  includes an engagement rib  41  on its inner surface. The engagement rib  41  is engaged with the bearing box  40 . 
     The bearing box  40  receives the rotational shaft  31  through its center. The bearing box  40  holds a bearing  42  in its rear portion. The bearing  42  supports the rotational shaft  31 . The rotational shaft  31  receives a pinion  43  at its front end. 
     As shown in  FIG. 3 , the bearing box  40  includes an inner wall  44  on its outer circumference. The inner wall  44  is annular and extends frontward. The inner wall  44  has a thread on its outer circumferential surface. The hammer case  6  has an internal thread on its inner circumference at the rear. The inner wall  44  is screwed to the hammer case  6 . The hammer case  6  includes a projection  45  on its lower surface. The projection  45  is held between the left- and right-half housings  4   a  and  4   b . The hammer case  6  is thus locked in a nonrotatable manner in the motor housing  7 . The hammer case  6  is also positioned in the front-rear direction with the engagement rib  41 . 
     An internal gear  46  is held inside the inner wall  44 . The internal gear  46  forms the reduction assembly  13 . As shown in  FIG. 4 , the internal gear  46  includes, on its outer circumferential surface, multiple protrusions  47  protruding frontward. The protrusions  47  are held between the inner wall  44  and the hammer case  6 . The hammer case  6  includes multiple recesses  48  on its inner circumferential surface. The recesses  48  are fitted with the respective protrusions  47 . As shown in  FIG. 5 , the internal gear  46  is restricted from rotating by the protrusions  47  and the recesses  48  engaged with each other. An O-ring  49  is located inside the inner wall  44 . The O-ring  49  receives the rear end of the internal gear  46 . 
     The hammer case  6  is cylindrical and tapered frontward. A bearing  50  is at the front end of the hammer case  6 . The bearing  50  supports the anvil  16 . The anvil  16  includes a pair of arms  51  behind the bearing  50 . A receiving ring  52  is on the inner wall of the hammer case  6  in front of the arms  51 . The receiving ring  52  receives the arms  51 . 
     The spindle  14  is dividable into a shaft  55  at the front and a carrier  56  at the rear. The carrier  56  is hollow and disk-shaped. The carrier  56  includes, at its center, a cylindrical portion  57  that opens rearward. The cylindrical portion  57  is held in the bearing box  40  with the bearing  58 . The pinion  43  on the rotational shaft  31  protrudes into the cylindrical portion  57 . The carrier  56  includes three planetary gears  59 . The planetary gears  59  mesh with internal teeth on the internal gear  46 . The planetary gears  59  are rotatably supported by pins  60 . The planetary gears  59  mesh with the pinion  43 , forming the reduction assembly  13 . 
     The carrier  56  has, at the center of its front surface, a cam projection  61  protruding frontward. The cam projection  61  protrudes into a rear portion of the shaft  55 . As shown in  FIG. 6 , the cam projection  61  has three rear cam recesses  62  on its circumferential surface. The rear cam recesses  62  are cutouts on the front end of the cam projection  61  toward the rear. The rear cam recesses  62  each have an inner surface extending in the circumferential direction of the cam projection  61  and a bottom. The three rear cam recesses  62  are arranged at equal intervals in the circumferential direction of the cam projection  61 . The three rear cam recesses  62  receive three cam balls  63 . The cam balls  63  are restricted from moving outward in the radial direction of the cam projection  61  by expanded portions  77  of a cam  75  (described later), and are thus rollable circumferentially in the rear cam recesses  62 . 
     The carrier  56  has a joint  64  around the cam projection  61  on its front surface. The joint  64  is annular and protrudes frontward concentrically with the cam projection  61 . The joint  64  has an outer recess  65  along its entire inner circumferential surface. 
     The shaft  55  is a cylinder having an outer diameter smaller than the inner diameter of the joint  64 . The shaft  55  has its rear end between the cam projection  61  and the joint  64 . The shaft  55  has an inner recess  66  along its entire outer circumferential surface at the rear end. The inner recess  66  faces the outer recess  65  on the joint  64 . As shown in  FIG. 5 , multiple connecting balls  67  are fitted in the outer recess  65  and in the inner recess  66 . The shaft  55  is thus prevented from slipping off the carrier  56 , and is also coaxially connected to the carrier  56  in a rotatable manner. 
     The shaft  55  has a cam reception hole  68  that opens rearward. The cam reception hole  68  has a stepped-diameter including a front small diameter hole  69  and a rear large diameter hole  70 . The shaft  55  includes a flange  71  having a larger diameter than the joint  64  in front of the inner recess  66 . 
     The cam reception hole  68  receives the cam  75 . The cam  75  includes a front shaft  76  and the expanded portions  77 . The front shaft  76  is placed into the small diameter hole  69 . The cam  75  includes three expanded portions  77  arranged circumferentially. The expanded portions  77  are placed into the large diameter hole  70 . 
     As shown in  FIG. 7 , the front shaft  76  has three inner grooves  78  on its outer circumferential surface. The inner grooves  78  extend in the front-rear direction. The three inner grooves  78  are arranged at equal intervals in the circumferential direction of the front shaft  76 . The small diameter hole  69  facing the inner grooves  78  has three outer grooves  79  on its inner circumferential surface. The outer grooves  79  extend frontward from the rear end of the small diameter hole  69 . Three coupling balls  80  are fitted in the inner grooves  78  and in the outer grooves  79 . The coupling balls  80  cause the cam  75  to be integrally coupled to the shaft  55  in the rotation direction. The cam  75  is movable relative to the shaft  55  in the front-rear direction within the range in which the coupling balls  80  roll back and forth in the inner grooves  78  and in the outer grooves  79 . 
     The expanded portions  77  have three front cam recesses  81  on the rear ends. The front cam recesses  81  each have an arc shape recessing frontward. The front cam recesses  81  are fitted with the cam balls  63  placed in the rear cam recesses  62  from the front. 
     Multiple disc springs  82  are externally mounted on the front shaft  76 . The disc springs  82  are arranged between the step at the front end of the large diameter hole  70  and the front surfaces of the expanded portions  77 , urging the cam  75  rearward. The front cam recesses  81  are engaged with the cam balls  63  under the urging force from the disc springs  82 . The rotation of the cam projection  61  is thus transmitted to the cam  75 . 
     A hammer  85  is externally mounted on the shaft  55 . The hammer  85  includes a pair of tabs  86  on its front surface. The hammer  85  has a pair of outer cam grooves  87  on its inner circumferential surface. The outer cam grooves  87  extend rearward from the front end of the hammer  85 . The pair of outer cam grooves  87  are point-symmetric to each other about the axis of the hammer  85 . The shaft  55  has a pair of inner cam grooves  88  on its outer circumferential surface. The pair of inner cam grooves  88  are point-symmetric to each other about the axis of the shaft  55 . The pair of inner cam grooves  88  are each inverted V-shaped with the tip being the front. Two balls  89  are fitted in the outer cam grooves  87  and in the inner cam grooves  88 . With the balls  89  in between, the hammer  85  and the shaft  55  are coupled together in the rotation direction. 
     The hammer  85  has an annular groove  90  on its rear surface. The groove  90  receives multiple spring balls  91  on its bottom. A washer  92  is behind the spring balls  91 . 
     A coil spring  93  is externally mounted on the shaft  55 . The coil spring  93  is tapered to have a diameter gradually decreasing toward the rear. The rear end of the coil spring  93  is in contact with the flange  71  on the shaft  55 . The front end of the coil spring  93  is in contact with the washer  92  in the groove  90 . The hammer  85  includes a central cylindrical portion  94  that defines the inner circumferential surface of the groove  90 . Similarly to the coil spring  93 , the central cylindrical portion  94  is tapered to have a diameter gradually decreasing toward the rear. The central cylindrical portion  94  protrudes more rearward than the outer diameter portion of the hammer  85  that defines the outer circumferential surface of the groove  90 . 
     The hammer  85  is thus urged to a forward position shown in  FIGS. 8A and 8B  by the coil spring  93 . At the forward position, the balls  89  are at the rear ends of the outer cam grooves  87  and the tips of the inner cam grooves  88 . 
     The shaft  55  has a fitting recess  95  in the center of its front end. The anvil  16  includes a fitting protrusion  96  at the center of its rear surface. The fitting protrusion  96  is fitted in the fitting recess  95 . The shaft  55  has an axial communication hole  97 . The communication hole  97  allows the fitting recess  95  and the cam reception hole  68  to communicate with each other. A receiving ball  98  is fitted to the front end of the communication hole  97 . The receiving ball  98  receives the rear end of the fitting protrusion  96 . 
     The shaft  55  has a front grease supply hole  99  and a rear grease supply hole  100 . The front grease supply hole  99  communicates with the communication hole  97  between the inner cam grooves  88  and is open in the outer circumferential surface of the shaft  55 . The rear grease supply hole  100  communicates with the small diameter hole  69  in the cam reception hole  68  and one of the outer grooves  79 , and is open in the outer circumferential surface of the shaft  55 . The front grease supply hole  99  and the rear grease supply hole  100  are orthogonal to each other when viewed from the front. 
     In the impact driver  1  according to the present embodiment, the trigger  18  is pressed to turn on the switch  17  after a bit (not shown) is attached to the anvil  16 . The brushless motor  12  is then powered to rotate the rotational shaft  31 . More specifically, the microcomputer in the control circuit board  23  receives, from the rotation detectors in the sensor circuit board  30 , rotation detection signals (rotation detection signals indicating the position of the sensor permanent magnet  34  in the rotor  26 ), and determines the rotational state of the rotor  26 . The microcomputer then controls the on-off state of each switching element in accordance with the determined rotational state, and applies a current through the coils  29  in the stator  25  sequentially to rotate the rotor  26 . 
     When the rotational shaft  31  rotates, the planetary gears  59 , which mesh with the pinion  43 , revolve in the internal gear  46 . This causes the carrier  56  to rotate at a lower speed. The rotation of the cam projection  61  integral with the carrier  56  is transmitted to the cam  75  through the cam balls  63  in between rolling to the circumferential ends of the rear cam recesses  62 , as indicated with the two-dot chain line in  FIG. 6 . The rotation of the cam  75  is transmitted to the shaft  55  through the coupling balls  80  in between. The hammer  85  then rotates together with the shaft  55  with the balls  89  in between, thus rotating the anvil  16  with the arms  51  engaged with the tabs  86 . This allows tightening a screw with the bit. 
     When the screw is tightened and increases the torque of the anvil  16 , the hammer  85  retracts against the urging force from the coil spring  93  while rolling the balls  89  along the corresponding inner cam grooves  88  on the shaft  55 . After the tabs  86  are disengaged from the arms  51 , the hammer  85  rotates forward along the inner cam grooves  88  under the urging force from the coil spring  93 . This then causes the tabs  86  to be re-engaged with the arms  51 , thus causing the anvil  16  to generate a rotational striking force (impact). This process is repeated for further tightening of the screw. 
     When the screw is tightened in a high load state, the balls  89  may roll to the rear ends of the inner cam grooves  88  along with the retracting hammer  85  as shown in  FIGS. 9A and 9B . This state is referred to as the hammer  85  at a rearmost position. In this state, the rear end of the central cylindrical portion  94  in the hammer  85  is not in contact with the flange  71  on the shaft  55 . 
     When the rotational energy does not decrease with the hammer  85  at the rearmost position, the hammer  85  and the shaft  55  are urged to rotate further. Thus, the rotational energy of the shaft  55  exceeds the engagement force between the cam  75  and the cam projection  61  caused by the disc springs  82 . As shown in  FIGS. 10A and 10B , the cam  75  integral with the shaft  55  in the rotation direction then rolls the cam balls  63  relatively to the circumferential ends of the front cam recesses  81 , compresses and deforms the disc springs  82 , and moves forward against the urging force from the disc springs  82  while rotating. The cam projection  61  and the shaft  55  may have a phase shift between them as the cam  75  moves forward and compresses and deforms the disc springs  82 . This can decrease the rotational energy. Thus, when the hammer  85  retracts to the rearmost position, a shock load is not transmitted to the carrier  56 . 
     When the hammer  85  at the rearmost position starts moving forward under the urging force from the coil spring  93 , the cam  75  retracts under the urging force from the disc springs  82  to roll the cam balls  63  relatively to the circumferential centers of the front cam recesses  81 . This eliminates the phase shift between the cam projection  61  and the shaft  55 . 
     The impact driver  1  according to the present embodiment includes the brushless motor  12  (motor), the carrier  56  including the planetary gears  59  (reduction assembly) and rotatable by the brushless motor  12 , and the shaft  55  to receive the rotation of the carrier  56  and rotatable relative to the carrier  56  in an overloaded state. The impact driver  1  further includes the hammer  85  held by the shaft  55  and the anvil  16  to be struck by the hammer  85  in the rotation direction. 
     This structure allows the carrier  56  and the shaft  55  to rotate relative to each other in an overloaded state, thus absorbing the rotational energy. This effectively reduces durability deterioration caused by a shock load. This also decreases the urging force from the coil spring  93 , which urges the hammer  85 . Thus, the first impact occurs earlier during further screwing. This reduces the likelihood of camming out (the tip of the bit separates and slips out of the screw head). 
     The shaft  55  extends frontward. The hammer  85  is held by the shaft  55  with the balls  89  in between. The balls  89  roll in the inner cam grooves  88  (cam grooves) on the outer circumferential surface of the shaft  55 . This causes the hammer  85  to be movable back and forth between the forward position at which the hammer  85  is engaged with the anvil  16  in the rotation direction and a rearward position at which the hammer  85  is disengaged from the anvil  16  in the rotation direction. The hammer  85  is urged to the forward position by the coil spring  93  externally mounted on the shaft  55 . The shaft  55  rotates relative to the carrier  56  in response to an overload occurring at the rearward position for the hammer  85  at which the balls  89  reach the rearmost ends of the inner cam grooves  88 . 
     The structure of the spindle  14  dividable into the shaft  55  and the carrier  56  allows the relative rotation in an overloaded state. 
     A cam assembly (the cam projection  61 , the cam  75 , and the disc springs  82 ) is located between the carrier  56  and the shaft  55 . The cam assembly transmits the rotation of the carrier  56  to the shaft  55  and rotates the carrier  56  and the shaft  55  relative to each other in the overloaded state of the shaft  55 . 
     Thus, the carrier  56  and the shaft  55  are easily rotated relative to each other with the cam assembly. 
     The cam assembly includes the cam projection  61  protruding frontward from the center of the carrier  56 , the cam  75  coupled to the shaft  55  in a manner rotatable together with the shaft  55  and movable back and forth relative to the shaft  55 , and the disc springs  82  (urging members) to urge the cam  75  to a rearward position. The cam  75  is engageable with the cam projection  61  at the rearward position to transmit the rotation of the carrier  56  to the shaft  55 , and rotates the carrier  56  and the shaft  55  relative to each other at the forward position. 
     This structure transforms a shock load from the hammer  85  at the rearmost position into deformation of the disc springs  82 , thus effectively reducing the rotational energy. 
     The cam projection  61  and the cam  75  are engaged with each other with the cam balls  63  in between. The cam projection  61  and the cam  75  transmit the rotation of the carrier  56  to the shaft  55 . Thus, the rotation of the carrier  56  is smoothly transmitted to the cam  75 . 
     The cam projection  61  includes the rear cam recesses  62  holding the cam balls  63  on its outer circumferential surface. The cam  75  includes, on its rear end, the front cam recesses  81  engaged with the cam balls  63 . This facilitates transmission of the rotation from the cam projection  61  to the cam  75  as well as deformation of the disc springs  82  as the cam  75  moves forward. 
     The structure includes the three cam balls  63 , the three rear cam recesses  62 , and the three front cam recesses  81 . This allows transmission of the rotation from the cam projection  61  to the cam  75  as well as deformation of the disc springs  82  in a well-balanced manner as the cam  75  moves forward. 
     The cam  75  is coupled to the shaft  55  with the coupling balls  80  in a manner rotatable together with the shaft  55  and movable back and forth relative to the shaft  55 . This reliably allows switching between transmission of the rotation from the cam  75  to the shaft  55  and relative rotation. 
     The shaft  55  is cylindrical and has the rear end with an opening. The cam  75  and the disc springs  82  are accommodated in the shaft  55 . Thus, the cam assembly can be located in a small space using the shaft  55 . 
     The shaft  55  internally has the cam reception hole  68  including a rear portion with a larger diameter than a front portion. The cam  75  is a shaft having a stepped-diameter including the front shaft  76  (smaller-diameter portion) placed in the front portion of the cam reception hole  68  and the expanded portions  77  (larger-diameter portions) placed in the rear portion of the cam reception hole  68 . 
     The urging members include the multiple disc springs  82  externally mounted on the front shaft  76 . Thus, the urging members can be included in a small space in the shaft  55 . 
     The shaft  55  receives the cam projection  61  in its rear end and is coupled to the carrier  56  at its rear end in a rotatable manner. Thus, the shaft  55  and the carrier  56  can be integrated into the dividable spindle  14  in a space-saving manner. 
     The carrier  56  includes, on its front surface, the joint  64  that is annular and concentric with the cam projection  61 . The shaft  55  is connected to the inner surface of the joint  64  at its rear end in a rotatable manner. Thus, the shaft  55  can be easily connected using the joint  64 . 
     The joint  64  and the rear end of the shaft  55  are connected to each other with the multiple connecting balls  67  arranged in the circumferential direction of the joint  64  and the shaft  55 . Thus, the shaft  55  and the carrier  56 , which are rotatable relative to each other, can be reliably connected. 
     The shaft  55  includes the flange  71  receiving the rear end of the coil spring  93 . This allows the coil spring  93  and the shaft  55  to rotate together. 
     Modifications will now be described. 
     In the embodiment, the carrier includes the cam projection and the cam includes the expanded portion covering the cam projection. In some embodiments, the cam may include the cam projection in its rear portion and the carrier may include the expanded portion covering the cam projection on its front surface. The structure may include more or fewer front cam recesses, rear cam recesses, and balls than in the illustrated example. 
     The number of disc springs to urge the cam may be changed as appropriate. The urging members may be, for example, coil springs other than disc springs. 
     The structure may include more or fewer inner grooves, outer grooves, and balls to couple the shaft and the cam than in the illustrated example. The shaft and the cam may be key-coupled or splined, without using the balls. 
     The reduction assembly may include more or fewer planetary gears than in the illustrated example. 
     The motor is not limited to a brushless motor. The power source is not limited to a battery pack but may be utility power. 
     The present disclosure is also applicable to impact tools other than an impact drive, such as an angle impact driver. 
     REFERENCE SIGNS LIST 
     
         
           1  impact driver 
           2  body 
           3  grip 
           4  body housing 
           6  hammer case 
           12  brushless motor 
           13  reduction assembly 
           14  spindle 
           15  striking assembly 
           16  anvil 
           22  controller 
           31  rotational shaft 
           43  pinion 
           55  shaft 
           56  carrier 
           59  planetary gear 
           61  cam projection 
           62  rear cam recess 
           63  cam ball 
           64  joint 
           67  connecting ball 
           68  cam reception hole 
           71  flange 
           75  cam 
           76  front shaft 
           77  expanded portion 
           81  front cam recess 
           82  disc spring 
           85  hammer 
           89  ball 
           93  coil spring