Abstract:
According to an aspect of the present invention, there is provided a nailing machine including: a housing; a motor that is accommodated in the housing; a plunger that plunges a nailing member; and a spring that is expanded or compressed by the motor to store an elastic energy therein and that accelerates the plunger by releasing the elastic energy, wherein a spring constant of the spring at one end of the spring in an expanding/compressing direction is set smaller than that of the spring at a center in the expanding/compressing direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims a priority from prior Japanese Patent Application No. 2007-109066 filed on Apr. 18, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An aspect of the present invention relates to a nailing machine. 
     2. Description of the Related Art 
     Spring-driven nailing machines pushing up a plunger, which is urged by a nailing spring, against the urging force of the nailing spring to a top dead center, accelerating the plunger by opening the nailing spring, and driving a nail into a target member with the plunger have already been produced. In the nailing machines, the labor for pushing up the plunger is reduced by pushing the plunger by the use of the motor built in the housing. Specifically, as described in JP-9-295283-A, plural rotating gears coupled to the motor with a reduction gear therebetween are disposed opposite each other and a driving pin fixed to a position departing from the rotation centers of the rotating gears is made to engage with an engaging protrusion formed in the plunger. The plunger is pushed up by a predetermined stroke by the use of the driving pin. 
     A nailing spring used in the conventional nailing machines is urged at the position of a top dead center by a motor, etc. to accumulate elastic energy and is opened at the time of hitting to move toward a bottom dead center. When an end of the nailing spring reaches the position of the bottom dead center, the end of the nailing spring starts being compressed, and the other end of the nailing spring moves to the bottom dead center, whereby the elastic energy is accumulated in the nailing spring at the position of the bottom dead center again. When the nailing spring is compressed to some extent, the nailing spring moves toward the top dead center with the accumulated elastic energy and the other end of the nailing spring reaches the position of the top dead center, whereby the elastic energy is accumulated in the nailing spring again. 
     By repeating this operation at the positions of the top dead center and the bottom dead center, the nailing spring is attenuated to return to the before-nailing state. Accordingly, the ends of the nailing spring always reach the position of the top dead center and the position of the bottom dead center and thus both ends of the nailing spring essentially suffer from the greatest impact and thus are expanded and compressed. Therefore, the nailing spring might be deteriorated from the ends. As a countermeasure, it can be considered that the strength of the nailing spring is enhanced by enlarging the nailing spring. However, in this case, the nailing machine increases in size, the spring constant is enhanced, and the hitting power becomes too strong, thereby damaging the workability. An object of the invention is to provide a nailing machine having a small size and a long lifetime. 
     SUMMARY OF THE INVENTION 
     In order to accomplish the above-mentioned object, the invention provides a nailing machine including: a housing; a motor disposed in the housing; a plunger hitting a nailing member; and a spring that is expanded or compressed by the motor to accumulate elastic energy and that accelerates the plunger by opening the elastic energy, wherein a spring constant of the spring is smaller at ends in the expanding and compressing direction than at the center in the expanding and compressing direction. 
     It is preferable that at least one end of the ends of the spring has a spring constant smaller than that of the center thereof. 
     According to the above-mentioned configuration, since the spring constant of the ends of the spring is small, the ends of the spring can be easily expanded and compressed. Accordingly, even when an impact is applied to the ends of the spring, it is possible to easily absorb the impact. As a result, it is possible to improve the durability of the ends of the spring against the expansion and compression. Since the center portion of the spring has a great spring constant, it is possible to accumulate the elastic energy required for the hitting of the plunger. Accordingly, the hitting can be suitably performed without using a large spring, thereby accomplish a decrease in size of the nailing machine. 
     It is preferable that the spring is an unequal-pitch coil spring. 
     It is preferable that the unequal-pitch coil spring is formed of a winding wire and the radial length in the section of the winding wire in the radial direction of the unequal-pitch coil spring is greater than the axial length of the unequal-pitch coil spring in the section. It is also preferable that the spring includes end turn regions and an effective turn region and the effective turn region has an unequal pitch. 
     According to the above-mentioned configuration, in the spring accumulating constant elastic energy, it is possible to reduce the length of the most compressed spring. Accordingly, since it is possible to reduce the length of the spring, it is possible to reduce the size of the nailing machine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a cross-sectional view illustrating a nailer according to an embodiment; 
         FIG. 2  is a side view illustrating a coil spring of the nailer according to the embodiment; 
         FIG. 3  is a cross-sectional view illustrating the nailer according to the embodiment where the coil spring moves to a bottom dead center; and 
         FIG. 4  is a cross-sectional view illustrating the nailer according to the embodiment where the coil spring moves to a top dead center. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a nailing machine according to an embodiment of the invention will be described with reference to  FIGS. 1 to 4 . A nailer  1  which is a nailing machine shown in  FIG. 1  is of an electromotive type and is used to insert a nail, which is a fixing member not shown, into a target member W such as timber and plaster. The nailer  1  roughly includes a housing  2 , a motor  3 , a clutch mechanism  4 , a transmission portion  5 , a coil spring portion  6 , a nose portion  7 , and a magazine  8 . In the following description, a direction in which a plunger  63  described later moves is defined as a vertical direction and a direction in which the plunger  63  is urged by a coil spring  62  of the coil spring portion  6  described later to hit a nail not shown is defined as a downward direction. 
     The housing  2  is made of resin such as nylon and polycarbonate and has the motor  3  built therein. A handle  21  is disposed in an upper portion of the housing  2  and a trigger  21 A controlling the motor  3  is disposed in the handle  21 . A detachable battery  22  supplying power to the motor  3  is disposed at an end of the handle  21 . 
     The motor  3  is disposed below the handle  21  in the housing  2  and a rotation shaft thereof is perpendicular to the vertical direction. The rotation shaft of the motor  3  is provided with a first pulley  31  and a belt  32  is suspended on the first pulley  31 . The belt  32  extends upward from the first pulley  31  and is suspended on a second pulley  33  at the top end of the belt  32 . The second pulley  33  is rotatably supported by the housing  2  and a first gear  34  coaxially rotating is disposed in parallel thereto. A second gear  35  engaging with the first gear  34  is disposed above the first gear  34 . The second gear  35  is rotatably supported by the housing  2  and a third gear  36  coaxially rotating is disposed in parallel thereto. A fourth gear  37  engaging with the third gear  36  is disposed above the third gear  36 . The number of rotations of the motor  3  is reduced by the first gear  31  to the fourth gear  37  and the rotary power is transmitted to the clutch mechanism  4 . 
     One end of the clutch mechanism  4  is connected to the fourth gear  37  so as to be coaxially rotatable and the other end is connected to a drum  51  described later so as to be coaxially rotatable. One end and the other end of the clutch mechanism  4  are connected to each other to coaxially rotate in a bundle, until one end thereof is made to rotate by a predetermined angle (an angle required for the plunger  63  to reach the top dead center, about 270°) by the fourth gear  37 . In a state where one end rotates by the predetermined angle, the clutch mechanism  4  intercepts the connection between one end and the other end thereof, whereby the other end (the drum  51  side) can freely coaxially rotate relative to one end (the fourth gear  37  side). 
     The transmission portion  5  roughly includes a drum  51  and the wire  52 . The drum  51  has substantially a disk shape and is disposed in the axis of a blade  63 B described later. One end of the wire  52  is fixed into a groove formed in the outer periphery of the drum  51  and the other end is connected to an urging portion  63 A described later. The wire is formed by bundling fiber-like steel wires and the surface thereof is coated with resin. 
     The coil spring portion  6  roughly includes a spring guide  61 , a coil spring  62 , and a plunger  63 . The spring guide  61  is separated from the housing  2 , is made of aluminum or resin such as nylon and polycarbonate, and has a cylinder shape. The spring guide is disposed in the housing  2  so that the axial direction thereof is parallel to the vertical direction. 
     The coil spring  62  has a configuration in which a steel wire is wound and the outer diameter thereof is slightly smaller than the inner diameter of the spring guide  61 , whereby the coil spring is inserted into the spring guide  61 . As shown in  FIG. 2 , end turn regions E are defined in both ends of the coil spring  62  and an effective turn region A is defined between one end turn region E and the other end turn region E. The effective turn region A is formed of an unequal-pitch spring in which both ends are dense to have a small spring constant and the center is sparse to have a great spring constant. Accordingly, both ends of the coil spring  62  can easily absorb an impact and the center thereof can absorb a great impact, whereby the coil spring  62  can accumulate the same elastic energy as an equal-pitch spring as a whole. In the coil spring  62 , an end, which has a small spring constant, which is close to the bottom dead center, and which includes one end turn region E, is defined as a first end  62 A. An end, which has a small spring constant, which is close to the top dead center, and which includes the other end turn region E, is defined as a second end  62 B. An approximately central portion, which includes the effective turn region A and which is a portion of the coil spring  62  having a great spring constant, is defined as a center portion  62 C. 
     In a conventional nailer, an additional-end-turn coil spring having an unequal pitch in the first turn at the ends is generally used to enhance the close adhesion between the end of the coil spring driving the plunger and the plunger. However, since the second or later turn has an equal pitch, it is not greatly different in size and mass from a coil spring of which the entire turns have an equal pitch. 
     On the contrary, in the coil spring  62  according to this embodiment, the unequal-pitch region (effective turn region A) is disposed in about the second or later turn from the ends. A portion closer to the center has a greater pitch in the unequal-pitch region (effective turn region A). By employing this shape, it is possible to more markedly reduce the weight and size than the conventional coil spring. 
     As shown in  FIG. 1 , the plunger  63  includes the urging portion  63 A and the blade  63 B. The urging portion  63 A is located at a lower end of the coil spring  62 , is formed in a disk shape having substantially the same outer diameter as the outer diameter of the coil spring  62 , and is connected to the other end of the wire  52  inserted into the coil spring  62  at the center thereof. Accordingly, the urging portion  63 A is pulled by the wire  52  to move upward in the spring guide  61  against the urging force of the coil spring  62 , thereby compressing the coil spring  62 . A position where the urging portion  63 A is urged and usually located by the coil spring  62  is defined as the bottom dead center. A position where the urging portion  63 A is pulled up to the uppermost is defined as the top dead center. 
     The blade  63 B is formed in a longitudinal plate shape and extends downward from the position which is the substantial center of the urging portion  63 A and which is the rear surface of the connection position of the wire  52 . A bumper  64  made of resin such as soft rubber and urethane is disposed below the urging portion  63 A in the housing  2 . 
     The nose portion  7  is disposed below the coil spring portion  6 . A passage which is not shown but through which the blade  63 B passes is formed in the nose portion  7 . 
     The magazine  8  is attached to the nose portion  7  at a position below the motor  3  and has plural nails not shown therein. The nails not shown are supplied into the passage not shown and disposed in the nose portion  7  from the magazine  8 . 
     At the time of driving a nail with the nailer  1  a worker grips the handle  21  to keep the nailer  1  substantially perpendicular to the top surface of the target member W and actuates the motor  3  by pulling the trigger  21 A. The drum  51  rotates by the clutch mechanism  4  interlocking with the actuation of the motor  3  to wind the wire  52 , the plunger  63  moves up to the top dead center, and the coil spring  62  is compressed to accumulate the elastic energy. 
     When the plunger  63  reaches the top dead center, the clutch mechanism  4  is intercepted and thus the drum  51  is rotatable. Accordingly, the urging force of the urging portion  63 A to the coil spring  62  disappears and the elastic energy accumulated in the coil spring  62  is released, thereby rapidly pushing down the plunger  63  to the bottom dead center. Then, the nail supplied into the nose portion  7  from the magazine  8  is hit with the blade  63 B, thereby driving the nail into the target member W. 
     The blade  63 B goes down to the bottom dead center to hit the nail and stops its motion by colliding with the bumper  64 . However, as shown in  FIG. 3 , the coil spring  62  moves to the bottom dead center at the time of hitting, the first end  62 A reaches the position of the bottom dead center, the first end  62 A starts being compressed, and then the second end  62 B further goes to the bottom dead center, whereby the coil spring  62  is compressed as a whole and the elastic energy is accumulated at the position of the bottom dead center. After it is compressed to some extent, as shown in  FIG. 4 , the coil spring  62  goes up to the top dead center with the accumulated elastic energy and the second end  62 B reaches the position of the top dead center, whereby the elastic energy is accumulated in the coil spring  62  again. 
     Since the coil spring  62  is oscillated and attenuated by repeating the above-mentioned action at the position of the top dead center and the position of the bottom dead center, the ends first reach the position of the top dead center and the position of the bottom dead center. Accordingly, both ends of the coil spring  62  are essentially expanded and compressed at the greatest speed. Therefore, the ends of the coil spring  62  suffer from an impact the most strongly. However, since both ends have a small spring constant, the ends easily absorbs the impact to suppress the deterioration of the coil spring  62 . After one end of the coil spring  62  is compressed, the center portion  62 C starts being compressed. However, since the center portion  62 C has a great spring constant, it can sufficiently accumulate the elastic energy. Accordingly, since the coil spring  62  can accumulate the same elastic energy as the conventional spring not having an unequal-pitch as a whole, it is possible to prevent the deterioration of the coil spring  62  while maintaining the elastic force sufficient for nailing and to elongate the life time of the nailer  1 . 
     The nailer according to the invention is not limited to the above-mentioned embodiment, but may be modified and improved in various forms without departing from the scope of the claims. Although the spring having different spring constants at both ends and at the center like the unequal-pitch spring has been suggested in the embodiment, for example, the spring constant may be made to vary by performing heat treatment on both ends and the center of an equal-pitch spring or varying a wire diameter thereof. 
     According to an aspect of the present invention, it is possible to reduce the size of the nailing machine and to elongate the lifetime thereof.