Patent Publication Number: US-11648653-B2

Title: Driving tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese patent application serial number 2020-063846, filed on Mar. 31, 2020, the contents of which are incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present disclosure generally relates to a driving tool for driving a material, such as a nail or a staple, into a workpiece such as, for example, a wooden material. 
     Driving tools, such as a gas-spring type driving tool and a mechanical-spring type driving tool, are well known. For instance, in Japanese Patent No. 6260944, a gas-spring type driving tool may include a driver for driving a driving material, a rack for moving the driver upward, and a wheel. The rack is provided in the driver and the driver is formed integral with a piston. When the wheel rotates, the rack, the driver and the piston move upward, thereby increasing the pressure of a gas in an accumulation chamber communicating with a cylinder. By utilizing the pressure of gas in the accumulation chamber, the piston and the driver moves downward, thereby causing the driving material to be driven by the driver. 
     The wheel includes a plurality of engaging portions, a roller adjacent to the engaging portions, and a region where there is no engaging portion in a circumferential direction of the wheel. During the time when the driver moves upward, the plurality of engaging portions engage the rack one after another and finally the roller engages the rack. When the wheel rotates further, the roller disengages from the rack. Because of the presence of a region where there is no engagement portion, the wheel allows the rack and the driver to move downward. By use of the roller, the wheel disengages from the rack with a small frictional resistance. Then, the driver moves downward smoothly due to the pressure of the gas in the accumulation chamber. 
     However, a larger force is applied to the last engaging portion engaged with the rack. i.e., the roller corresponding to the last engaging portion, in comparison with the other engaging portions. In more detail, when the driver and the rack are moved upward by the wheel, the pressure of gas in the accumulation chamber increases. Owing to this, a larger force is applied to the last engagement portion, i.e., the roller corresponding to the last engaging portion, in comparison with the other engagement portions. A mechanical-spring type driving tool works in a similar manner to a gas-spring type driving tool. In the mechanical-spring type driving tool, when the driver and the rack are moved upward by the wheel, an elastic energy stored by a spring, which provides the driver with a driving force, increases. Immediately before the wheel disengages from the rack, only the last engaging portion or the roller engages with the rack. Therefore, a large force is applied to the last engaging portion. Because of this, wear of the last engaging portion may be increased in comparison with the other engaging portions. Thus, there is a need to provide a mechanism for the driving tool in which the last engagement portion is less subject to wear. 
     SUMMARY 
     According to one feature of the present disclosure, a driving tool comprises a driver provided in a housing so as to be movable in an up-down direction, thereby driving a driving member. The driving tool also comprises a driving mechanism in which driving energy is stored by movement of the driver in an upward direction, a first rack and a second rack provided in the driver to move the driver in the upward direction, a first wheel including a plurality of first engaging portions engageable with the first rack, and a second wheel including one second engaging portion engageable with the second rack. Furthermore, the second engaging portion of the second wheel is disposed such that: (i) in a stage of an initial upward movement of the driver, the second engaging portion of the second wheel engages the second rack at a same time as or before the first engaging portion of the first wheel engages the first rack; and (ii) in a stage of a final upward movement of the driver, the second engaging portion of the second wheel disengages from the second rack at a same time as or after the first engaging portion of the first wheel disengages from the first rack. 
     Because of this configuration, by rotation of the first wheel and the second wheel, the driver returns upward. In the stage of the initial upward movement of the driver, when the first engaging portion of the first wheel engages the first rack, the second engaging portion of the second wheel engages the second rack. In more detail, in the stage of the initial upward movement of the driver, the second engaging portion engages the second rack at the same time or before the first engaging portion of the first wheel engages the first rack. When the driver reaches the top dead center, the second engaging portion of the second wheel engages the second rack (which may be in the stage of the final upward movement of the driver). In other words, the second engaging portion of the second wheel engages the second rack both in the stage of the initial upward movement and in the stage of the final upward movement. After the driver reaches the top dead center, the second engaging portion disengages from the second rack by further rotation of the second wheel. Furthermore, in the stage of the final upward movement of the driver, the second engaging portion of the second rack disengages from the second rack at the same time as or after the first engaging portion of the first wheel disengages from the first rack. Then, the driver drives the driving member by the driving mechanism. The second engaging portion of the second wheel has high durability and wear resistance. Because of this configuration, a cost of production can be reduced, in comparison with a case where a total mechanism, including the first engaging portions of the first wheel, is provided with high strength and wear resistance. Furthermore, as a degree of wear progresses, it may be sufficient to replace only the second wheel or only the second engaging portion of the second wheel. The original first wheel can continue to be used. As a result, a cost of maintenance can be reduced. Also, the second engaging portion of the second wheel may work as a common engaging portion for both starting and releasing the engagement with the second rack. In this respect, further cost reductions can be obtained. 
     According to another feature of the present disclosure, the first wheel is disposed on one lateral side of the driver and the second wheel is disposed on the other lateral side of the driver. Thus, both sides of the driver are engaged by the first wheel and the second wheel. As a result, the driver can be prevented from being displaced to one side when the driver moves upward. 
     According to another feature of the present disclosure, both the first wheel and the second wheel are disposed on one lateral side of the driver. Because of this, a driving nose from which the driving member is driven by the driver can be made more compact. 
     According to another feature of the present disclosure, the second engaging portion of the second wheel has a roller structure configured to rotate relative to the second rack. Because of this, high frictional resistance by the second engaging portion serving as the common engaging portion can be obtained. 
     According to another feature of the present disclosure, a ratio of rotation number of the second wheel to the first wheel is configured to be an integer. Because of this, a mechanism for moving the driver can be simple and reliable. 
     According to another feature of the present disclosure, a rotation speed of the second wheel is higher than that of the first wheel. Because of this, the second wheel can be compact while the second wheel cooperates with the first wheel. 
     According to another feature of the present disclosure, the second engaging portion of the second wheel has a higher strength than the first engaging portions of the first wheel. Because of this, the first engaging portions of the first wheel are allowed to have a weaker strength in durability and wear resistance than that of the second engaging portion of the second wheel. As a result, cost reduction can be obtained. 
     According to another feature of the present disclosure, an engaging tooth of the second rack engageable with the second engaging portion of the second wheel in the stage of the final upward movement of the driver has a higher strength than engaging teeth of the first rack engageable with the first engaging portions of the first wheel. Thus, a reduction in the total cost can be obtained by only increasing strength for the required portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a longitudinal sectional view of a driving tool according to a first embodiment of the present disclosures, showing a state where a driving operation is performed. 
         FIG.  2    is a cross-sectional view taken along line II-II of  FIG.  1   , showing a transversal cross-sectional view of a reduction gear. 
         FIG.  3    is a cross-sectional view taken along line III-III of  FIG.  1   , showing a transversal cross-sectional view of a drive-returning mechanism. 
         FIG.  4    is a cross-sectional view taken from line IV-IV of  FIG.  2   , showing a longitudinal cross-sectional view of a driving section. 
         FIG.  5    is a perspective view of the driver-returning mechanism according to the first embodiment. 
         FIG.  6    is a perspective view of a driver according to the first embodiment. 
         FIG.  7    (A) to  FIG.  7    (E) respectively shows an operating state of the driver-returning mechanism according to the first embodiment. 
         FIG.  8    is a longitudinal sectional view of a driving tool according to a second embodiment of the present disclosures. 
         FIG.  9    is a cross-sectional view taken along line IX-IX of  FIG.  8   , showing a transversal cross-sectional view of a reduction gear. 
         FIG.  10    is a cross-sectional view taken along line X-X of  FIG.  8   , showing a transversal cross-sectional view of a drive-returning mechanism. 
         FIG.  11    (F) to  FIG.  11    (K) respectively shows an operating state of the driver-returning mechanism according to the second embodiment. 
         FIG.  12    is a longitudinal sectional view of a driving tool according to a third embodiment of the present disclosures. 
         FIG.  13    is a cross-sectional view taken along line XIII-XIII of  FIG.  12   , showing a transversal cross-sectional view of a reduction gear. 
         FIG.  14    is a cross-sectional view taken along line XIV-XIV of  FIG.  12   , showing a transversal cross-sectional view of the reduction gear. 
         FIG.  15    is a cross-sectional view taken along line XV-XV of  FIG.  12   , showing a transversal cross-sectional view of the driver-returning mechanism on a side of a first wheel. 
         FIG.  16    is a cross-sectional view taken along line XVI-XVI of  FIG.  12   , showing a transversal cross-sectional view of the driver-returning mechanism on a side of a second wheel. 
         FIG.  17    is a perspective view of a driver-returning mechanism according to the third embodiment of the present disclosures. 
         FIG.  18    is a perspective view of a driver according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below, when considered with the appended drawings, is intended to be a description of exemplary embodiments of the present invention and is not intended to be restrictive and/or to represent the only embodiments in which the present invention can be practiced. The term “exemplary” used throughout this description means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other exemplary embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the exemplary embodiments of the invention. It will be apparent to those skilled in the art that the exemplary embodiments of the invention may be practiced without these specific details. In some instances, these specific details refer to well-known structures, components, and/or devices that are shown in block diagram form in order to avoid obscuring significant aspects of the exemplary embodiments presented herein. 
     A driving tool  1  according to a first embodiment to a third embodiment of the present disclosures will be described with reference to  FIGS.  1  to  18   . In the first embodiment to the third embodiment, a gas-spring type driving tool, which utilizes a pressure of a gas filled in an accumulation chamber as a driving force for driving a driving member n, will be exemplified as the driving tool  1 .  FIGS.  1  to  4    shows the driving tool  1  according to the first embodiment of the present disclosures. In the following explanation, a driving direction of the driving member is a downward direction, and a direction opposite to the driving direction is an upward direction. When a driver  11 , which will be discussed later in detail, moves downward, the driving member n may be driven. After that, the driver  11  may move upward to be returned to its original position. In the following explanation, a direction perpendicular to a paper surface in  FIG.  1    is referred to as a width direction of the driving tool  1 . 
     The driving tool  1  of the first embodiment may include a tool main body  10 . The tool main body  10  may house a cylinder  3  within a tubular main body housing  2 . The cylinder  3  may support a driving piston  4  so that the driving piston  4  may reciprocate in an up-down direction. A driver  11  for driving a driving member n may be provided to extend downward from a center of a lower surface of the driving piston  4 . The driving piston  4  and the driver  11  may be integrally provided, so that they may reciprocate within the cylinder  3  in the up-down direction. The driver  11  may extend long in the downward direction. A tip end of the driver  11  may enter a driving passage  5   a  of a driving nose  5  provided at a lower end of the tool main body  10 . A lower end of the driving nose  5  may be an injection port  5   b , from which the driving member n is driven out.  FIGS.  1  to  3    show a state where the driving piston  4  moves to its movement lower end and the driving member n is driven out from the injection port  5   b . Further,  FIGS.  1  to  3    show that the driver  11  moves downward in the driving passage  5   a  and the tip end of the driver  11  may slightly protrude from the injection port  5   b.    
     A magazine  6 , which may be loaded with a plurality of driving members n, may be linked to the driving nozzle  5 . A handle  7  for a user to hold may be provided on a lateral side of the main body housing  2 . A switch lever  8  for the user to pull with a fingertip may be provided in a base portion of the handle  7 . A battery pack  9  serving as a power source may be attached to an end of the handle  7 . The battery pack  9  may be rechargeable and may be detached from the driving tool  1  in order to use it as a power source of another electric power device. The main body housing  2 , the handle  7 , the switch lever  8 , and the battery pack  9  are shown in only  FIG.  1    and are omitted in other figures. 
     The driving nose  5  may be provided with a driver-returning mechanism  20 . The driver-returning mechanism  20  may be configured to return the driving piston  4  as well as the driver  11  upward to its original position. When the driving piston  4  returns upward by the driver-returning mechanism  20 , the gas pressure in the accumulation chamber  3   a  disposed above the upper surface of the driving piston  4  may increase. The driving piston  4  may move downward owing to the increased gas pressure in the accumulation chamber  3   a . As a result, the driver  11  may drive the driving member n. The driver-returning mechanism  20 , may be an example of a driving mechanism of the driving tool  1 . The driving mechanism of the driving tool  1  may have a configuration for storing a driving energy (e.g., a thrust power in the accumulation chamber  3   a ) obtained due to an upward movement of the driver  11 . A damper  3   b  may be disposed at a lower end of the cylinder  3  in order to absorb an impact the driving piston  4  receives when the driving piston  4  moves toward its lowermost end. 
     The driver-returning mechanism  20  may include an electric motor  21 , a reduction gear train  22 , and a wheel mechanism  30 . The electric motor  21  may be powered by the battery pack  9  serving as the electric power source. The electric motor  21  may be actuated by a pulling operation of the switch lever  8 . A rotation output of the electric motor  21  may be reduced by the reduction gear train  22 , which may comprise a planetary gear train arranged in two rows, and may be output to the wheel mechanism  30 . The electric motor  21  may be housed in a tubular motor case  21   a . Also, the reduction gear train  22  may be housed in a tubular gear case  22   a . The tubular motor case  21   a  may be coaxially connected to an end portion of the gear case  22   a.    
     The wheel mechanism  30  may include a mechanism case  31 . As shown in  FIG.  2   , a contour of the mechanism case  31  may form a shape of a numeral eight (8) in cross-section, in such a manner that a large circular tube combines with a small circular tube. The mechanism case  31  may be integrally provided with the driving nose  5 . The mechanism case  31  may be linked to the gear case  22   a .  FIG.  5    shows a perspective view of the wheel mechanism  30  of this embodiment in detail. The mechanism case  31  is omitted in  FIG.  5   . The wheel mechanism  30  may include a first wheel  32 , a second wheel  33 , and an interlocking gear train  23 , in addition to the mechanism case  31 . The first wheel  32 , the second wheel  33 , and the interlocking gear train  23  may be housed in the mechanism case  31 . A driving shaft  34  of the wheel mechanism  30  may be connected to the reduction gear train  22 . 
     As shown in  FIGS.  1  and  4   , the driving shaft  34  may be rotatably supported by the mechanism case  31  via two bearings  34   a . The second wheel  33 , which may be integral with a second interlocking gear  25 , may be attached to the driving shaft  34 . A driven shaft  35  may be provided parallel to the driving shaft  34  supported by the mechanism case  31 . As shown in  FIG.  4   , the driven shaft  35  may be rotatably supported by the mechanism case  31  via two bearings  35   a . The first wheel  32 , which may be integral with a first interlocking gear  24 , may be attached to the driven gear  35 . 
     As shown in  FIGS.  3  and  4   , the driving shaft  34  may be disposed on one side in a width direction of the driver  11 , and the driven shaft  35  may be disposed on the other side in the width direction of the driver  11 . That is, the driving shaft  34  may be disposed apart from the driven shaft  35  by a predetermined length. The driving shaft  35  may face the driven shaft  35 , with the driver  11  interposed therebetween. Because of this configuration, the first wheel  32  and the first interlocking gear  24  may be disposed on the one side in the width direction of the driver  11  (e.g., a right side in  FIGS.  3  and  4   ). Also, the second wheel  33  and the second interlocking gear  25  may be disposed on the other side in the width direction of the driver  11  (e.g., a left side in  FIGS.  3  and  4   ). 
     A spur gear may be used for both the second interlocking gear  25  on the driving shaft  34  and for the first interlocking gear  24  on the driven shaft  35 . The second interlocking gear  25  may engage the first interlocking gear  24 . A gear ratio of the first interlocking gear  24  to the second interlocking gear  25  may be configured to be two to one. Because of this configuration, the driven shaft  35  may rotate in accordance with the driving shaft  34  with a ratio of rotation speed being reduced by one-half. In other words, when the driving shaft  34  rotates twice, the driven shaft  35  may rotate once. 
     Because a ratio of the rotation number of the driving shaft  34  to the driven shaft  35  is two to one, a ratio of the rotation number of the first wheel  32  to the second wheel  33  may be one to two. In other words, when the second wheel  33  rotates twice, the first wheel  32  rotates once. 
     As shown in  FIG.  3   , the first wheel  32  may include a total of eight first engaging portions  32   a . As shown in  FIG.  5   , each of the first engaging portions  32   a  may form a round column shape. Also, each end of the first engaging portions  32   a  may be supported by unnumbered parts of the first wheel  32 , for instance as shown in  FIG.  5   . The eight first engaging portions  32   a  may be disposed at equal intervals along a same circumferential arc. Furthermore, the eight first engaging portions  32   a  may be disposed in an area around approximately half the circumference of the driven shaft  35 . The area where the eight first engaging portions  32   a  are disposed in the circumferential direction may correspond to a first circumferential engaging area. In contrast, a remaining almost-semicircular area where the eight first engaging portions  32   a  are not disposed may correspond to a first circumferential non-engaging area. When the driver  11  moves downward, engaging teeth  12   a  of a first rack  12  may face the first circumferential non-engaging area. Because of this, when the driver  11  moves downward, the engaging teeth  12   a  of the first rack  12  may not interfere with the first engaging portions  32   a  of the first wheel  32 . Thus, the driver  11  may move downward in a smooth manner and the thrust power in the accumulation chamber  3   a  may not be dissipated. 
     The second wheel  33  may include one second engaging portion  33   a . Similar to the first engaging portions  32   a , the second engaging portion  33   a  may form a round column shape and each end of the second engaging portion  33   a  may be supported by unnumbered parts of the second wheel  33 , for instance as shown in  FIG.  5   . The second engaging portion  33   a  may work as an engaging portion when an upward movement of the driver  11  starts. The second engaging portion  33   a  may also work as an engaging portion when the upward movement of the driver  11  is released. For this reason, the second engaging portion  33   a  may correspond to a common engaging portion. The second engaging portion  33   a  of the second wheel  33  may have a roller structure in which a roller is rotatably supported via a shaft. A predetermined circumferential area where the second engagement portion  33   a  is disposed may correspond to a second circumferential engaging area. When the driver  11  moves upward, the second rack  13  may face the second circumferential engaging area. In contrast, a remaining circumferential area where the second engaging portion  33   a  is not disposed may correspond to a second circumferential non-engaging area. When the driver  11  moves downward, the second rack  13  may face the second circumferential non-engaging area for at least a part of the downward movement. Because of this, when the driver  11  moves downward, engaging teeth  13   a ,  13   b  of the second rack  12  may not interfere with the second engaging portions  33   a  of the second wheel  32 . Thus, the driver  11  may move downward in a smooth manner. 
     As shown in  FIGS.  3 ,  5 , and  6   , the driver  11  may include a first rack  12  engageable with the first wheel  32  and the second rack  13  engageable with the second wheel  33 . The first rack  12  may be provided along the lateral surface of one side of the driver  11  in the width direction of the driver  11  (e.g., on the side where the first wheel  32  is disposed). The first rack  12  may include a plurality of engaging teeth  12   a  (e.g., eight engaging teeth  12   a  in  FIGS.  3    and  6 ). The second rack  13  may be provided along the lateral surface of the other side of the driver  11  in the width direction of the driver  11  (e.g., on the side where the second wheel  33  is disposed). As shown in  FIG.  3   , the second rack  13  may include two engaging teeth  13   a ,  13   b.    
     The engaging teeth  13   a ,  13   b  of the second rack  13  may have a higher strength and wear resistance than the engaging teeth  12   a  of the first rack  12 . For example, a heat treatment or surface treatment may be locally applied to the engaging teeth  13   a ,  13   b  of the second rack  13  in order to increase their strength and wear resistance. 
     As shown in  FIG.  6   , the lower engaging tooth  13   a  of the second rack  13  may be disposed below a lowermost engaging tooth  12   a  of the first rack  12  in the up-down direction. Also, the upper engaging tooth  13   b  of the second rack  13  may be disposed above an uppermost engaging tooth  12   a  of the first rack  12  in the up-down direction. By rotation of the first wheel  32  and the second wheel  33 , an engaging position of the engaging teeth  12   a ,  13   a ,  13   b  of the first rack  12  and the second rack  13  may vary to return the driver  11  upward. 
       FIGS.  7 (A) to  7 (E)  show a series of states where the driver  11  and the driving piston  4  return upward by an operation of the driver-returning mechanism  20 .  FIG.  7    (A) shows a standby state (initial state). In this state, the driving piston  4  may be disposed slightly below a top dead center. Also in this state, a hindmost of the first engaging portions  32   a  in the first circumferential engaging area of the first wheel  32  may engage the lowermost engaging tooth  12   a  of the first rack  12  from below, for example as shown in  FIG.  7    (A). Also, the second engaging portion  33   a  of the second wheel  33  may engage the lower engaging tooth  13   a  of the second rack  13  from below. In this state, the first wheel  32  and the second wheel  33  may engage the first rack  12  and the second rack  13 , respectively, at the same time. 
     In this standby state, when the switch lever  8  is pulled, the driver-returning mechanism  20  may be actuated. When the switch lever  8  is pulled to activate the electric motor  21 , the second wheel  33  may rotate, for instance in the clockwise direction as indicated by an arrow in the figures. In accordance with the rotation of the second wheel  33 , the first wheel  32  may rotate, for instance in the counterclockwise direction, via the interlocking gear train  23 . The first wheel  32  may rotate in a direction opposite to the second wheel  33 , for instance at half the rotation speed of the second wheel  33 . 
     When the first wheel  32  rotates, for instance in the counterclockwise direction, the hindmost first engaging portion  32   a  may disengage from the lowermost engaging tooth  12   a  of the first rack  12 . At the same time, the second wheel  33  may also rotate, for instance in the clockwise direction, engaging the second engaging portion  33   a  with the lower engaging tooth  13   a  of the second rack  13  from below. As a result, the driver  11  and the driving piston  4  may move upward. Because of this movement, the driving piston  4  may return to its top dead center, for instance as shown in  FIG.  7    (B).  FIG.  7    (B) shows a state before a driving operation is performed (which may be a final state of an upward movement of the driver  11 ). In this state, one driving member n, for instance from the magazine  6 , may be supplied, or may have already been supplied, into the driving passage  5   a.    
     When the driving piston  4  reaches the top dead center, the gas pressure in the accumulation chamber  3   a  may have increased. Owing to this, in the state before the driving operation is performed, a large load (a thrust power in the accumulation chamber  3   a  and a rotation power of the second wheel  33 ) may be applied to the second engaging portion  33   a  of the second wheel  33 . For instance, the thrust power in the accumulation chamber  3   a  may be applied to the second engaging portion  33   a  via the lower engaging tooth  13   a  of the second rack  13 . 
     When the driving piston  4  reaches or is approximately at the top dead center and the second wheel  33  further rotates, for instance in the clockwise direction, from the state before the driving operation is performed, for example from the state shown in  FIG.  7    (B), the second engaging portion  33   a  may disengage from the lower engaging tooth  13   a  of the second rack  13 . That is, a returning operation of the driver  11  being performed by the driver-returning mechanism  20  (which may be related to the engagement state of the second engaging portion  33   a ) may be released. At this time, a large friction force may be applied to the second engaging portion  33   a . After the second engaging portion  33   a  disengages from the lower engaging tooth  13   a  of the second rack  13 , the driving piston  4  may move downward, owing to the gas pressure in the accumulation chamber  3   a , which serves as the thrust power, for instance as shown in  FIG.  7    (C). In accordance with the downward movement of the driving piston  4 , the driver  11  may move downward in the driving passage  5   a . During the downward movement of the driver  11 , the one driving member n supplied into the driving passage  5   a  may be driven by the tip end of the driver  11 . As a result, the driving member n may be driven from the injection port  5   b  and into a workpiece W. 
     While the driving piston  4  moves downward, the electric motor  21  may continue to rotate. During this period, the first wheel  32  may rotate in a direction opposite to the second wheel  33  of the driver-returning mechanism  20 . While the driver  11  moves downward, the circumferential non-engaging area of the first wheel  32  (e.g., the almost-semicircular area where there is no first engaging portion  32   a ) may face one surface of the driver  11 , and the circumferential non-engaging area of the second wheel  33  (e.g., the circumferential area where there is no second engaging portion  33   a ) may face the other surface of the driver  11 . Because of this configuration, the first engaging portions  32   a  of the first wheel  32  and the second engaging portion  33   a  of the second wheel  33  may be prevented from interfering with the driver  11 . As a result, the driver  11  may be allowed to move downward in a smooth manner. 
     When the driving piston  4  reaches its a lower position, the driving operation may be completed. During this period, the first wheel  32  and the second wheel  33  may continue to rotate. After the driving operation has been completed, the second engaging portion  33   a  of the second wheel  33  may engage the upper engaging tooth  13   b  of the second rack  13  from below, for instance as shown in  FIG.  7    (C). As previously mentioned, after the second engaging portion  33   a  of the second wheel  33  disengages from the lower engaging tooth  13   a  of the second rack  13 , the downward movement of the driving piston  4  may be started. After the driving piston  4  reaches a lower position, the second engaging portion  33   a  may engage the upper engaging tooth  13   b  of the second rack  13  from below. Because of this configuration, the second engaging portion  33   a  of the second wheel  33  may work as a release-engaging portion that allows the driving piston  4  to move downward as well as a start-engaging portion that assists with moving the driver  11  upward. That is, the second engaging portion  33   a  may work as a common engaging portion for assisting with both starting and releasing the engagement with the second rack  13 . 
     After the driving piston  4  reached a lower position, for instance shown in  FIG.  7    (C), the electric motor  21  may continue to rotate. Accordingly, the driver-returning mechanism  20  may continue to be operated.  FIG.  7    (D) shows a return-start stage (e.g., a stage of the upward movement of the driver  11 ) where the second wheel  33  rotates, for instance in the clockwise direction, while the second engaging portion  33   a  of the second wheel  33  engages the upper engaging tooth  13   b  of the second rack  13  from below. As a result, the driver  11  may move upward. Also, the first wheel  32  may rotate, for instance in the counterclockwise direction, and a first engaging portion  32   a , for instance the foremost first engaging portion  32   a  in the first circumferential engaging area, may engage the uppermost engaging tooth  12   a  of the first rack  12  from below. 
     As shown in  FIG.  7    (E), by rotation of the first wheel  32 , for instance in the counterclockwise direction, and the second wheel  33 , for instance in the clockwise direction, an engagement state of the driver  11  may be transferred from the engagement of the second engaging portion  33   a  with the second rack  13  to only the engagement of the first engaging portion  32   a  with the first rack  12 . 
     After the transfer of the engagement state shown in  FIG.  7    (E), the driver  11  may continue to move upward, owing to continuous engagement of one or more of the first engaging portions  32   a  of the first wheel  32  with the engaging tooth/teeth  12   a  of the first rack  12 . Then, the driving piston  4  may return to its standby position, for instance the position shown in  FIG.  7    (A). During this period, in accordance with the rotation of the second wheel  33 , for instance in the clockwise direction, the first wheel  32  may also rotate, for instance in the counterclockwise direction. Owing to the rotation of the first wheel  32  and the second wheel  33 , the second engaging portion  33   a  of the second wheel  33  may again engage the lower engaging tooth  13   a  of the second rack  13  from below. 
     When the driving piston  4  returns to its standby position shown in  FIG.  7    (A), the electric motor  21  may automatically stop to complete one driving cycle. When the switch lever  8  is pulled again, or when another driving cycle is to be performed, the driver-returning mechanism  20  may start again or continue to operate. Accordingly, a series of operations may be performed according to the following procedure: (A) standby state→(B) start driving operation→(C) driving operation completed→(D) start returning→(E) transfer of engagement. 
     According to the driving tool  1  of the first embodiment discussed above, the driver  11  may return upward by rotation of the first wheel  32  and the second wheel  33 . While the driving piston  4  is returning to or is near its top dead center, the second engaging portion  33   a  of the second wheel  33  may engage the lower engaging tooth  13   a  of the second rack  13 . After the driving piston  4  reaches or is near its top dead center, further rotation of the second wheel  33  may cause the release of the engagement of the second engaging portion  33   a  with the lower engaging tooth  13   a  of the second rack  13 . Then, the driver  11  may be moved downward by the driving mechanism (in this embodiment by the thrust power in the accumulation chamber  3   a ) to drive the driving member n. Thus, high durability and wear resistance may be required for the second engaging portion  33   a  of the second wheel  33 , which works as the engaging portion for releasing the engaging state from the second rack  13 . 
     Furthermore, the second engaging portion  33   a  of the second wheel  33  may work as the starting engaging portion for moving the driver  1 I upward when the driving piston  4  returns to its standby position (e.g., during the positions shown  FIG.  7    (C) to  FIG.  7    (D)). In this state, a relatively large load and friction force may be applied to the second engaging portion  33   a . In this way, by applying the large load and friction force to the second engaging portion  33   a  of the second wheel  33  when the engagement starts and releases, a load applied to the first engaging portions  32   a  of the first wheel  32  may be reduced. Thus, by providing the second engaging portion  33   a  of the second wheel  33  with the necessary strength and wear resistance, a cost of production may be reduced in comparison with a case where a total mechanism, for instance including the first wheel  32 , is provided with high strength and wear resistance. When a degree of wear progresses, it may be sufficient to merely replace the second engaging portion  33   a  of the second wheel  33 . The original first wheel  32  may continue to be used. As a result, a cost of maintenance may be reduced. 
     Furthermore, according to the driving tool  1  of the first embodiment discussed above, the first wheel  32  may be disposed on one lateral side of the driver  11  in the width direction of the driver  11  and the second wheel  33  may be disposed on the other lateral side of the driver  11 . Because of this configuration, the driver  11  may engage the first wheel  32  and the second wheel  33  from both sides in the width direction of the driver  11 . As a result, when the driver  11  moves in the up-down direction, the driver  11  may be restricted from excessively displacing (tilting) to one side in the width direction of the driver  11 . 
     Furthermore, the second engaging portion  33   a  of the second wheel  33  may work as the common engaging portion for both the start-engaging portion and release-engaging portion. Because of this configuration, simplification of structure may be obtained. The second engaging portion  33   a  of the second wheel  33  may have a roller structure in which the roller is rotatably supported via the shaft. Because of this configuration, wear resistance of the second engaging portion  33   a  may be further improved. 
     Furthermore, according to the driving tool  1  of the first embodiment discussed above, the ratio of the rotation speed of the first wheel  32  to the second wheel  33  may be configured to one to two (or at another integer ratio). Because of this configuration, the driver returning mechanism  20  may be simple and reliable. Especially, rotation speed of the second wheel  33  may be configured to be twice as large as that of the first wheel  32 . Thus, the second wheel  33  may be compact, while still being able to cooperate with the first wheel  32 . 
     Furthermore, the engaging teeth  13   a ,  13   b  of the second rack  13 , both of which engage the second engaging portion  33   a  of the second wheel  33 , may have a higher strength as compared to the engaging teeth  12   a  engageable with the first engaging portion  32   a  of the first wheel  32 . By increasing the strength of only the necessary portion(s), a required cost of production, as well as of maintenance, may be reduced. 
     It is noted that the present teachings are not limited to the above-discussed embodiment, and it is understood that variations and modifications may be effected without departing from the spirit and scope of the present teachings. For example, the first embodiment shows that the ratio of rotation number of the first wheel  32  to the second wheel  33  is configured to be one to two, via the interlocking gear train  23 . However, a ratio of rotation number of the first wheel  32  to the second wheel  33  may be modified in various ways, as long as the ratio is an integer ratio. For example,  FIGS.  8  to  11    show a driving tool  1  according to a second embodiment, in which a ratio of rotation number of the first wheel  41  to the second wheel  42  is configured to one to three. The driving tool  1  according to the second embodiment 2 primarily differs in a wheel structure  40  of the driver-returning mechanism  20 . Descriptions of the members and configurations that do not need to be substantially modified and are essentially in common with the first embodiment are omitted by use of the same reference numerals. 
     A driver-returning mechanism  20  according to the second embodiment includes an electric motor  21  powered by a battery pack  9  serving as the electric power source. The driver-returning mechanism  20  also includes a reduction gear train  22  that reduces the rotation output of the electric motor  21 . These components are essentially the same as those of the first embodiment. An output of the reduction gear train  22  may be input to a driving shaft  44  of the wheel mechanism  40 . The wheel mechanism  40  may include a mechanism case  48  that has approximately the same shape as the mechanism case  31  of the first embodiment. A first wheel  41 , a second wheel  42 , and an interlocking gear train  43  may be housed in the mechanism case  48 . The first wheel  41  may be disposed on one side of the driver  11  in the width direction of the driver  11 , and the second wheel  42  may be disposed on the other side of the driver in the width direction of the driver. This arrangement is basically the same as in the first embodiment. 
     The interlocking gear train  43  may include a first interlocking gear  46  and a second interlocking gear  47 , each of which is a spur gear. The second interlocking gear  47  may be linked to a driving shaft  44 , and the first interlocking gear  46  may be connected to a driven shaft  45 . A number of teeth of the first interlocking gear  46  on the driven side may be configured to be three times that of the second interlocking gear  47  on the driving side. Thus, a rotation speed of the driven shaft  45  may be one third of that of the driving shaft  44 . 
     Similar to the first embodiment, a second wheel  42  may be attached to the driving shaft  44  and a first wheel  41  may be attached to the driven shaft  45 . Thus, a rotation speed of the first wheel  41  may be configured to one third of that of the second wheel  42 . When the first wheel  41  rotates once, the second wheel  42  may rotate three times. Similar to the first embodiment, each of the driving shaft  44  and the driven shaft  45  of the second embodiment may be rotatably supported by the mechanism case  48 , via bearings. 
     As shown in  FIG.  10   , the first wheel  41  may include a plurality, for example a total of seven, first engaging portions  41   a . Each of the first engaging portions  41   a  may form a round column shape. Also, each end of the first engaging portions  41   a  may be supported by unnumbered parts of the first wheel  41 , in a similar fashion as in the first embodiment. The seven first engaging portions  41   a  may be disposed at equal intervals along a same circumferential direction of the first wheel  41 . Furthermore, the seven first engaging portions  41   a  may be disposed in an area slightly larger than an area of approximately half the circumference around the driven shaft  41   a . Thus, the first engaging portions  41   a  may be disposed over a slightly broader area of the first wheel  41 , in comparison to the first portions  32   a  of the first wheel  32  of the first embodiment. Because of this configuration, the seven first engaging portions  41   a  of the second embodiment may be disposed at larger intervals in the circumferential direction than the eight first engaging portions  32   a  according to the first embodiment. The area where the seven first engaging portions  41   a  of the second embodiment are disposed in the circumferential direction of the first wheel  41  may correspond to the first circumferential engaging area. The remaining almost-semicircular area of the first wheel  41  where the seven first engaging portions  41   a  are not disposed may correspond to the first circumferential non-engaging area. In this way, the first wheel  41  according to the second embodiment may include a total of seven first engaging portions  41   a , while the first wheel  32  according to the first embodiment may include a total of eight first engaging portions  32   a.    
     As discussed above, the total number and the disposed interval (within the area of the first circumferential engaging area) of the first engaging portions  41   a  may differ from those of the first embodiment. Because of this difference, a ratio of the rotation number of the first wheel  41  to the second wheel  42  may be one to three, whereas that of the first embodiment may be one to two. 
     The second wheel  42  of the second embodiment may include one second engaging portion  42   a . Similar to the first engaging portions  41   a , the second engaging portion  42   a  may form a round column shape and each end of the second engaging portion  42   a  may be supported by unnumbered parts of the second wheel  42 . The second engaging portion  42   a  may work as an engaging portion when an upward movement of the driver  11  is performed. The second engaging portion  42   a  may also work as an engaging portion when the upward movement of the driver  11  is released. For this reason, the second engaging portion  33   a  may correspond to a common engaging portion. The second engaging portion  42   a  of the second wheel  42  may have a roller structure in which a roller is rotatably supported by a shaft. The second wheel  42  according to the second embodiment may have substantially the same general configuration as the second wheel  33  according to the first embodiment. 
     As shown in  FIG.  10   , the driver  11  of the second embodiment may include a first rack  12  engageable with the first wheel  41  and a second rack  13  engageable with the second wheel  42 . The first rack  12  may be provided along a lateral surface on one side of the driver  11  in the width direction of the driver  11  (e.g., on a side where the first wheel  41  is disposed). The first rack  12  of the second embodiment may include a total of seven engaging teeth  12   a . Thus, the first rack  12  of the second embodiment may differ from that of the first embodiment in that the first rack  12  of the second embodiment includes a total of seven, rather than eight, engaging teeth  12   a . The second rack  13  may be provided along a lateral surface of the driver  11  on another side of the driver  11  in the width direction of the driver  11  (e.g., on a side where the second wheel  42  is disposed). As shown in  FIG.  10   , the second rack  13  may include two engaging teeth  13   a ,  13   b.    
     Similar to the first embodiment, the engaging teeth  13   a ,  13   b  of the second rack  13  of the second embodiment may have a higher strength and wear resistance than the engaging teeth  12   a  of the first rack  12 . For example, a heat treatment or surface treatment may be locally applied to the engaging teeth  13   a ,  13   b  in order to increase their strength and wear resistance. 
     As shown in  FIG.  10   , the lower engaging tooth  13   a  of the second rack  13  may be disposed offset, for example below, a lowermost engaging tooth  12   a  of the first rack  12  in the up-down direction. Also, the upper engaging tooth  13   b  of the second rack  13  may be disposed above an uppermost engaging tooth  12   a  of the first rack  12  in the up-down direction. 
     By rotation of the first wheel  41  and the second wheel  42 , an engaging position of the engaging teeth  12   a ,  13   a  of the first rack  12  and the second rack  13  may vary to return the driver  11  upward, which is similar to that of the first embodiment.  FIGS.  11    (F) to  7 (K) show a series of states where the driver  11  and the driving piston  4  return upward by an operation of the driver-returning mechanism  20  according to the second embodiment.  FIG.  11    (F) shows a standby state (for instance an initial state) where the driving piston  4  is disposed slightly below a top dead center. In this state, a first engaging portion  41   a , for instance a hindmost first engaging portion  41   a  in the first circumferential engaging area of the first wheel  41 , may engage the lowermost engaging tooth  12   a  of the first rack  12  from below, for example as shown in  FIG.  11    (F). Also in this state, the second engaging portion  42   a  of the second wheel  42  may engage the lower engaging tooth  13   a  of the second rack  13  from below. In this state, the first wheel  41  and the second wheel  42  may engage the first rack  12  and the second rack  13 , respectively, at the same time. 
     In this standby state, when the switch lever  8  is pulled, the driver-returning mechanism  20  may be actuated. When the switch lever  8  is pulled to activate the electric motor  21 , the second wheel  42  may rotate, for instance in the clockwise direction as indicated by an arrow in the figures. In accordance with the rotation of the second wheel  42 , the first wheel  41  may rotate, for instance in the counterclockwise direction, via the interlocking gear train  43 . The first wheel  41  may rotate in a direction opposite to the second wheel  42  at one third the rotation speed of the second wheel  42 . 
     When the first wheel  41  rotates, for example in the counterclockwise direction, the hindmost first engaging portion  41   a  may disengage from the lowermost engaging tooth  12   a  of the first rack  12 . At the same time, the second wheel  42  may also rotate, for instance in the clockwise direction, while continuing to engage the second engaging portion  42   a  with the engaging tooth  13   a  of the second rack  13  from below. As a result, the driver  11  and the driving piston  4  may move upward. Because of this movement, the driving piston  4  may return to or near its top dead center, for instance as shown in  FIG.  11    (G).  FIG.  11    (G) shows a state before a driving operation is performed. In this state, one driving member n may be or have already been supplied into the driving passage  5   a  from the magazine  6 . 
     When the driving piston  4  reaches the top dead center, the gas pressure in the accumulation chamber  3   a  may increase. Owing to this, in the state before the driving operation is performed, a large load (for example due to the thrust power in the accumulation chamber  3   a  and the rotation power of the second wheel  42 ) may be applied to the second engaging portion  42   a  of the second wheel  42 . For example, the load from the thrust power in the accumulation chamber  3   a  may be transferred to the second engaging portion  42   a  via the lower engaging tooth  13   a  of the second rack  13 . 
     When the driving piston  4  reaches or is near the top dead center and the second wheel  42  further rotates clockwise from the state before the driving operation is performed, for example that shown in  FIG.  11    (G), the second engaging portion  42   a  may disengage from the lower engaging tooth  13   a  of the second rack  13 . That is, a returning operation of the driver  11  by the driver-returning mechanism  20  (which may correspond to an engagement state of the second engaging portion  42   a ) may be released. At this time, a large friction force may be applied to the second engaging portion  42   a . When the second engaging portion  42   a  disengages from the lower engaging tooth  13   a  of the second rack  13 , the driving piston  4  may move downward owing to the gas pressure in the accumulation chamber  3   a  serving as the thrust power, for instance to enter a state similar to that shown in  FIG.  11    (H). In accordance with the downward movement of the driving piston  4 , the driver  1 I may move downward in the driving passage  5   a . During the downward movement of the driver  11 , the one driving member n supplied into the driving passage  5   a  may be driven by the tip end of the driver  11 . As a result, the driving member n may be driven from the injection port  5   b  and into the workpiece W. 
     While the driving piston  4  moves downward, the electric motor  21  may continue to rotate. During this period, the first wheel  41  may rotate in a direction opposite to the second wheel  42  of the driver-returning mechanism  20 . While the driver  11  moves downward, the circumferential non-engaging area of the first wheel  41  (which may correspond to the semicircular area where there is no first engaging portions  41   a ) may face one surface of the driver  11 . The circumferential non-engaging area of the second wheel  42  (which may correspond to the circumferential area where there is no second engaging portion  42   a ) may face the other surface of the driver  11 . Because of this configuration, the first engaging portions  41   a  of the first wheel  41  and the second engaging portion  42   a  of the second wheel  42  may be prevented from interfering with the driver  11 . As a result, the driver  11  may be allowed to move downward in a smooth manner. 
     When the driving piston  4  reaches an end position, the driving operation may be completed. During this period, the first wheel  41  and the second wheel  42  may continue to rotate. After the driving operation has been completed, the second engaging portion  42   a  of the second wheel  42  may engage the upper engaging tooth  13   b  of the second rack  13  from below, for instance as shown in  FIG.  11    (H). As described above, when the second engaging portion  42   a  of the second wheel  42  disengage from the lower engaging tooth  13   a  of the second rack  13 , the downward movement of the driving piston  4  may start. Then, when the driving piston  4  reaches a lower end, the second engaging portion  42   a  may engage the upper engaging tooth  13   b  of the second rack  13  from below. Because of this configuration, the second engaging portion  42   a  of the second wheel  42  may work as a release-engaging portion that allows the driving piston  4  to move downward as well as a start-engaging portion that assists with moving the driver  11  upward. That is, the second engaging portion  42   a  may work as the common engaging portion for assisting with both starting and releasing the engagement with the second rack  13 . 
     After the driving piston  4  has reached its lower end, for instance the position shown in  FIG.  11    (H), the electric motor  21  may continue to rotate. Accordingly, the driver-returning mechanism  20  may continue to be operated.  FIG.  11    (I) shows a return-start stage, where the second wheel  42  rotates, for instance in the clockwise direction, while the second engaging portion  42   a  of the second wheel  42  engages the upper engaging tooth  13   b  of the second rack  13  from below. As a result, the driver  11  may move upward. Also, the first wheel  41  may rotate, for instance in the counterclockwise direction, and a first engaging portion  41   a , for instance the foremost first engaging portion  41   a  in the first circumferential engaging area, may engage the uppermost engaging tooth  12   a  of the first rack  12  from below. 
     As shown in  FIG.  11    (J), by rotation of the first wheel  41 , for instance in the counterclockwise direction, and by the rotation of the second wheel  42 , for instance in clockwise direction, an engagement state may be transferred from the engagement of the second engaging portion  42   a  with the second rack  13  to only the engagement of the first engaging portion  41   a  with the first rack  12 . 
     After the transfer of the engagement state, for instance to that shown in  FIG.  11    (J), the driver  11  may move upward owing to the continuous engagement of the first engaging portions  41   a  of the first wheel  41  with the engaging teeth  12   a  of the first rack  12 , as shown in  FIG.  11    (K). Then, the driving piston  4  may return to its standby position, for instance the position shown in  FIG.  11    (F). During this period, in accordance with the rotation of the second wheel  42 , for instance in the clockwise direction, the first wheel  41  may also rotate, for instance in the counterclockwise direction. Owing to the rotation of the first wheel  41  and the second wheel  42 , the second engaging portion  42   a  of the second wheel  42  may again engage the lower engaging tooth  13   a  of the second rack  13  from below. 
     After the driving piston  4  has returned to its standby position, for instance the position shown in  FIG.  11    (F), the electric motor  21  may automatically stop, thereby completing one driving cycle. When the switch lever  8  is pulled again or the driving cycle is otherwise started again, the driver-returning mechanism  20  may start or continue, and a series of operation may be performed according to the following procedure: (F) standby state→(G) start driving operation→(H) driving operation completed→(I) start returning→(J) transfer of engagement. 
     According to the driving tool  1  of the second embodiment discussed above, by applying a large load and friction force mainly to the second engaging portion  42   a  of the second wheel  42  when the engagement starts and releases, a load applied to the first engaging portions  32   a  of the first wheel  32  may be reduced. Thus, by providing the second engaging portion  42   a  of the second wheel  42  with the necessary strength and wear resistance, a cost of production may be reduced in comparison with a case where a total mechanism, including the first wheel  41 , is provided with high strength and wear resistance. When a degree of wear progresses, it may be sufficient to merely replace the second engaging portion  41   a  of the second wheel  42 . The original first wheel  41  may continue to be used. As a result, a cost of maintenance may be reduced. 
     Furthermore, according to the driving tool  1  of the second embodiment discussed above, the ratio of the rotation speed of the first wheel  41  to the second wheel  42  may be configured to one to three (or any other integer ratio). Because of this, the second wheel  42  and the second interlocking gear  47  may be made more compact (e.g., have a reduced diameter), in comparison with the second wheel  33  and the second interlocking gear  25  according to the first embodiment. As a result, the wheel mechanism  40  may be made more compact, in comparison with the wheel mechanism  30  according to the first embodiment. Thus, the driving tool  1  of the second embodiment may be made more compact, especially in the width direction. 
     Furthermore, according to the driving tool  1  of the second embodiment discussed above, the first wheel  41  may be disposed on one lateral side of the driver  11  in the width direction of the driver  11  and the second wheel  42  may be disposed on the other lateral side of the driver  11 . Because of this configuration, the driver  11  may be engaged with the first wheel  41  and the second wheel  42  from both sides in the width direction. As a result, when the driver  11  moves upward and downward, the driver  11  may be restricted from excessively displacing (tilting) to one side in the width direction of the driver  11 . 
     Furthermore, the second engaging portion  42   a  of the second wheel  42  may work as a common engaging portion for both the start-engaging portion and release-engaging portion. Because of this configuration, simplification of structure may be obtained. The second engaging portion  42   a  of the second wheel  42  may have a roller structure, in which the roller is rotatably supported via a shaft. Because of this configuration, wear resistance of the second engaging portion  42   a  may be further improved. 
     The driving tool  1  according to the first embodiment and the second embodiment may be further modified. In the first embodiment and the second embodiment, the first wheel  32 ,  41  may be disposed on one lateral side of the driver  11  in the width direction of the driver  11  and the second wheel  33 ,  42  may be disposed on the other lateral side of the driver  11  in the width direction of the driver  11 . That is, a structure in which the first and second wheels are disposed on opposite sides relative to the driver  11  may be adopted in the first and second embodiments. However, in other embodiments, both the first wheel and the second may be disposed on one lateral side of the driver  11  in the width direction. 
       FIGS.  12  to  18    show the driving tool  1  according to a third embodiment. In the third embodiment, the wheels are disposed on one lateral side of the driver  11 . A wheel mechanism  50  of the driving tool  1  according to the third embodiment may differ from the wheel mechanism  30  of the first embodiment and the wheel mechanism  40  of the second embodiment. Descriptions of the members and configurations that do not need to be substantially modified and are basically in common with the first and second embodiments are omitted by use of the same reference numerals. 
     The wheel mechanism  50  according to the third embodiment may include a mechanism case  51 . The mechanism case  51  may be linked to the driving nose  5 . A driving shaft  52 , a first driven shaft  53 , and a second driven shaft  54  may be rotatably supported by the mechanism case  51  via bearings  52   a ,  53   a ,  54   a , respectively. The rotational output of an electric motor  21  may be transferred to the driving shaft  52  via a reduction gear train  22 . 
     A first wheel  55  and a first interlocking gear  56  may be connected to the driving shaft  52 . As shown in  FIG.  13   , the first interlocking gear  56  may engage a second interlocking gear  57 . The second interlocking gear  57  may be connected to the first driven shaft  53 . As shown in  FIGS.  13  and  14   , the second interlocking gear  57  and a third interlocking gear  58  may be connected to the first driven shaft  53 . The third interlocking gear  58  may engage a fourth interlocking gear  59 . The fourth interlocking gear  59  may be connected to the second driven shaft  54 . The fourth interlocking gear  59  and the second wheel  60  may be connected to the second driven shaft  54 . 
     As discussed above, the wheel mechanism  50  of the third embodiment may be disposed on one lateral side of the driver  61  in the width direction of the driver  61 . A rotation of the driving shaft  52  may be transferred to the second driven shaft  54  with a rotation number of the second driven shaft  54  being increased via the first interlocking gear  56  to the fourth interlocking gear  59 . In the third embodiment, the number of teeth of the first interlocking gear  56  to the fourth interlocking gear  59  may be configured such that a rotation speed of the second driven shaft  54  becomes three times as fast as that of the driving shaft  52 . In other words, a ratio of the rotation number of the first wheel  55  to the second wheel  60  may be configured to be one to three in the third embodiment. 
     Furthermore, since the four interlocking gears, i.e., the first interlocking gear  56  to the fourth interlocking gear  59 , may be interposed, the first wheel  55  may rotate in the same direction as the second wheel  56 . In this respect, the third embodiment may differ from the first and second embodiments. 
     As shown in  FIG.  15   , the first wheel  55  may include a plurality, for example a total of seven, first engaging portions  55   a . The seven first engaging portions  55   a  may be disposed at equal intervals along a circumferential direction of the first wheel  55 . An almost-semicircular area where the first engaging portions  55   a  are disposed may correspond to a first circumferential engaging area. A remaining circumferential area where the first engaging portions  55   a  are not disposed may correspond to a first circumferential non-engaging area. As shown in  FIG.  16   , the second wheel  60  may include one second engaging portion  60   a . An area where the second engaging portion  60   a  is disposed may correspond to a second circumferential engaging area. A remaining area where the second engaging portion  60   a  is not disposed may correspond to a second circumferential non-engaging area. In this respect, the second wheel  60  of the third embodiment may be similar to the second wheel  33  in the first embodiment. 
     As shown in  FIG.  18   , the driver  61  of the third embodiment may be connected to a lower surface of the driving piston  4 , extending from a center thereof. The driver  61  may include a first rack  62  and a second rack  63 . In the third embodiment, the first rack  62  may be disposed on the same lateral side as the second rack  63  in the width direction of the driver  61 . As shown in  FIG.  18   , in a thickness direction of the driver  61 , the first rack  62  may be disposed on one lateral side of the driver  61  and the second rack  63  may be disposed on the other side of the driver  11 . 
     As shown in  FIG.  18   , the first rack  62  may be disposed along an end edge on the left side in the thickness direction. The first rack  62  may include a plurality, for example seven, engaging teeth  62   a . The seven engaging teeth  62   a  may be disposed at equal intervals approximately on a side of the lower half of the driver  61 . 
     As shown in  FIG.  18   , the second rack  63  may be disposed along an end edge on the right side in the thickness direction. The second rack  63  may include a plurality, for example two, engaging teeth. One of the engaging teeth may be an upper engaging tooth  63   b  and another may be a lower engaging tooth  63   a . As shown in  FIG.  18   , the lower engaging tooth  63   a  may be disposed on a right side of an area where the first rack  62  is disposed. On the other hand, the upper engaging tooth  63   b  may be disposed above the area where the first rack  62  is disposed. 
     According to the wheel mechanism  50  of the third embodiment discussed above, when the electric motor  21  is activated to rotate the driving shaft  52 , the first wheel  55  may rotate in the same direction as the second wheel  60 . A ratio of the rotation speed of the first wheel  55  to the second wheel  60  may be one to three. As shown in  FIG.  16   , after the driving piston  4  reaches the moving lower end to complete a driving operation, the second engaging portion  60   a  of the second wheel  60  may engage the upper engaging tooth  63   b  from below. From the state shown in  FIG.  16   , the second wheel  60  may rotate clockwise and the driver  61  may move upward. Thus, the second engaging portion  60   a  may work as a starting engaging portion by which the driver  61  moves upward. This stage may generally correspond to  FIG.  7 (C)  of the first embodiment and to  FIG.  11 (H)  of the second embodiment. 
     When the driver  61  moves upward from the moving lower end, one of the first engaging portions  55   a  of the first wheel  55  may engage a corresponding engaging tooth  62   a  of the first rack  62 . This stage may generally correspond to  FIGS.  7 (D) and  7 (E)  of the first embodiment and to  FIGS.  11 (I) and  11 (J)  of the second embodiment, which show a transfer of the engagement. Due to the rotation of the first wheel  55 , for example in the counterclockwise direction in  FIG.  15   , the driver  61  may move upward further. When the first wheel  55  rotates by about one-third, the second wheel  60  may rotate approximately once. During these rotations, the driving piston  4  moves toward a standby position. When the driving piston  4  returns to its standby position, the second engaging portion  60   a  of the second wheel  60  may engage the lower engaging tooth  63   a  of the second rack  63 . This stage may be a standby state, which may correspond to  FIG.  7 (A)  of the first embodiment and to  FIG.  11 (F)  of the second embodiment. 
     In the standby state, the electric motor  21  may stop and the driving piston  4  may be held in the standby position. When the switch lever  8  is pulled or when another driving cycle is otherwise started, the electric motor  21  may start to activate the driver-returning mechanism  20 . Then, the driver  61  may move upward and the driving piston  4  may move toward a top dead center. The electric motor  21  may continue rotating, and when the driver  61  further moves upward, the second engaging portion  60   a  of the second wheel  60  may disengage from the lower engaging tooth  63   a  of the second rack  63 . Owing to this, the driving piston  4  may move downward by the thrust power in the accumulation chamber  3   a  to perform a driving operation. 
     According to the wheel mechanism  50  of the third embodiment discussed above, similar to the first embodiment and the second embodiment, by primarily applying a large load and/or friction force to the second engaging portion  60   a  of the second wheel  60  when the engagement starts and releases, a load applied to each of the first engaging portions  55   a  of the first wheel  55  may be reduced. Thus, by providing the second engaging portion  60   a  of the second wheel  60  with the necessary strength and wear resistance, a cost of production may be reduced in comparison with a case where a total mechanism, including the first wheel  55 , is provided with high strength and wear resistance. When a degree of wear progresses, it may be sufficient to merely replace the second engaging portion  60   a  of the second wheel  60 . The original first wheel  55  may continue to be used. As a result, a cost of maintenance may be reduced. 
     Furthermore, according to the third embodiment discussed above, the wheel mechanism  50  may be disposed on one side of the driver  61  in the width direction of the driver  61 . Thus, the wheel mechanism  50  may be made more compact in the width direction of the driving nose  5 . Furthermore, the ratio of the rotation number of the first wheel  55  to the second wheel  60  may be configured to one to three (or any other integer ratio). Because of this, the second wheel  60  may be made more compact, in comparison with a case where the ratio is set to one to two. In this respect, the wheel mechanism  50  may be made more compact. 
     The driving tool  1  according to the first embodiment to the second embodiment may be further modified. For example, the second engaging portion  33   a ,  42   a ,  60   a  may be a columnar body with high wear resistance instead of a roller rotatably supported via a shaft. 
     Furthermore, a gas-spring type driving tool  1  was exemplified in the first to third embodiments, in which the gas pressure stored in an accumulation chamber  3   a  is used for the thrust power. However, the present disclosure may be applied to mechanical-spring type driving tools, in which a compression spring is used for the trust power.