Abstract:
A combustion type power tool having a motor suspension arrangement capable of buffering a shock applied to a motor. An integral buffer member made from an elastic material is fixed to a head cap. The buffer member includes a motor storage section in which a motor casing is stored, and a plurality of buffer segments extending radially outwardly from the motor storage section and integrally therewith. Each radially outer end portion of each buffer segment is fixed to the head cap for floatingly supporting the motor storage section.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a combustion-type power tool, and more particularly, to such power tool having an improved motor mount structure.  
         [0002]     In a conventional combustion-type driving tool such as a nail gun, a gaseous fuel injected into a combustion chamber is ignited to cause gas expansion in the combustion chamber, which in turn causes a linear momentum of a piston. By the movement of the piston, a nail is driven into a workpiece. In order to improve combustion, a fan is disposed in the combustion chamber for agitating a combustible gas. Such conventional combustion-type driving tool is disclosed in U.S. Pat. Nos. 4,483,280, Re 32,452 and 5,197,646.  
         [0003]     A rotation drive unit such as a motor is disposed in a frame of the driving tool for rotating the fan. By the rotation of the fan, a turbulent flow is generated in the combustion chamber to promote combustion within the combustion chamber. Thus volumetric expansion occurs in the combustion chamber which in turn occurs impact. The impact is propagated to an entirety of the tool. Thus, the motor is also subjected to impact force.  
         [0004]     In case of the combustion type fastener driving tool, the volumetric expansion generates movement of the piston for driving the fastener at the time of combustion. Surplus energy of the piston is absorbed at a bumper disposed within and one end of a cylinder in which the piston is slidingly moved. Acceleration is imparted to the entirety of the driving tool when the piston impacts against the bumper, and this acceleration is also transmitted to the motor.  
         [0005]     Generally, the motor is not a shock proof precision instrument. Therefore, performance of the motor may be lowered due to structural damage caused by the repeated application of impact or shock, and finally the motor may be destroyed. To avoid this drawback, U.S. Pat. No. 6,520,397 discloses a cushioning member interposed between an outer frame and the motor so as to protect the motor against the shock. Thus, moderated shot transmission to the motor results.  
         [0006]     More specifically, as shown in  FIGS. 6 and 7 , a combustion type fastener driving tool  101  includes a housing  102  having an upper end provided with a head cap  111  covered by a head cover  103 . A motor  118  is supported to the housing  102  through the head cap  111 . The motor  118  includes a motor case  118   a  serving as an outer casing, and a motor shaft  118   b , and a fan  119  is fixed to a tip end of the motor shaft  118   b . Further, an ignition plug  112  protruding into a combustion chamber  126  is supported to the head cap  111  at a position adjacent to the motor  118 .  
         [0007]     A pair of annular grooves are formed at an outer peripheral surface of the motor case  118   a . The annular grooves are spaced away from each other in an axial direction of the motor case  118   a . As shown in  FIG. 6 , retaining rings  114  are fitted into the pair of annular grooves, and an inner ring  113   a  which is a constituent of a suspension member  113  is interposed between the retaining rings  114 , 114 .  
         [0008]     As shown in  FIG. 7 , the suspension member  113  includes the inner ring  113   a , an outer fixing metal  113   c  and a rubber member  113   b  fixed between the inner ring  113   a  and the fixing metal  113   c  by baking. The fixing metal  113   c  is fixed to the head cap  111 . Thus, the motor  118  is connected to the head cap  111  through the suspension member  113 .  
         [0009]     If the driving tool  101  is subjected to impact force, the impact is transmitted to the fixing metal  113   c , but is moderately transmitted to the inner ring  113   a  and to the motor  118  because of the damper effect of the rubber member  113   b.    
       SUMMARY OF THE INVENTION  
       [0010]     However, the present inventors recognized the following disadvantages in the conventional suspension arrangement. That is, in order to fix the motor  118  to the suspension member  113 , the annular grooves must be formed at the motor case  118   a . In other words, an ordinary available motor cannot be employed, but a motor having a special specification must be required as the motor  118 , which increases a production cost. Further, since the suspension member  113  is made from two different materials, i.e., two metal rings  113   a ,  113   c  and the rubber member  113   b  and these members must be integrally connected by baking, reliability as to the connection may be lowered. These members may be separated from each other if the baking is insufficient. Furthermore, the baking process increases production cost.  
         [0011]     Further, the rubber member  113   b  cannot be continuously distributed between the two rings  113   a  and  113   c , since the ignition plug  112  is positioned adjacent to the motor  118 . In other words, the rubber member  113   b  is cut off by the ignition plug  112 . As a result, buffer function cannot be evenly provided over an entire region of the suspension member  113  against the shock imparted to the motor. Consequently, tensile stress is locally concentrated at the rubber member  113   b  nearby the ignition plug  112 , to damage to the rubber member  113   b.    
         [0012]     It is therefore an object of the present invention to provide a combustion type power tool having a motor support structure having sufficient durability and capable of being produced easily at low cost.  
         [0013]     This and other object of the present invention will be attained by a combustion-type power tool including a housing, a head portion, a cylinder, a nose, a push lever, a piston, a combustion-chamber frame, a motor, and a buffer member. The head portion is disposed at one end of the housing and is formed with a fuel passage. The cylinder is disposed in and fixed to the housing. The nose is positioned at the other end of the housing and extending from the cylinder. The push lever is movable along the nose in the longitudinal direction of the housing when the push lever is in pressure contact with a workpiece. The piston is reciprocally movable in the longitudinal direction and is slidable relative to the cylinder. The piston divides the cylinder into an upper space above the piston and a lower space below the piston. The combustion-chamber frame is disposed in the housing and is movable in the longitudinal direction in interlocking relation to the push lever. The combustion chamber frame is abuttable on the head portion to provide a combustion chamber in cooperation with the head portion and the piston. The motor includes a motor case disposed at the head portion at a position opposite to the combustion chamber, and an output shaft extending from the motor case and protruding into the combustion chamber. The buffer member is made solely from an elastic material and includes a motor storage section that supports the motor case, and a fixing section integrally extending form the motor storage section and fixed to the head portion.  
         [0014]     In another aspect of the invention, there is provided a support structure for a motor that rotates a fan rotatable in a combustion chamber in a combustion-type power tool for driving a fastener into a workpiece. The power tool includes a tool body and generates an acceleration of the motor in an axial direction of the fan upon combustion in the combustion chamber. The acceleration causes the motor to move in the axial direction relative to the tool body. The support structure includes the buffer member made solely from an elastic material. The buffer member includes a motor storage section that stores the motor, and a fixing section integrally extending from the motor storage section and fixed to the tool body. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     In the drawings;  
         [0016]      FIG. 1  is a vertical cross-sectional view showing a combustion type nail gun embodying a combustion type power tool according to an embodiment of the present invention;  
         [0017]      FIG. 2  is a cross-sectional plan view of the nail gun particularly showing a motor support arrangement according to the embodiment of the present invention;  
         [0018]      FIG. 3  is a cross-sectional view taken along the line III-III of  FIG. 2  particularly showing an arrangement at a head cap of the nail gun according to the embodiment;  
         [0019]      FIG. 4  is a plan view showing a damper employed in the nail gun according to the embodiment;  
         [0020]      FIG. 5  is a plan view showing a support plate employed in the nail gun according to the embodiment;  
         [0021]      FIG. 6  is a cross-sectional view showing a conventional combustion type nail gun; and  
         [0022]      FIG. 7  is a cross-sectional plan view of the conventional nail gun particularly showing a motor support arrangement.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     A combustion-type power tool according to an embodiment of the present invention will be described with reference to  FIGS. 1 through 5 . The embodiment pertains to a combustion type nail gun. The combustion type nail gun  1  has a housing  2  constituting an outer frame. A head cover  3  formed with an intake port  3   a  is mounted on the top of the housing  2 . A handle  4  is attached to the housing  2  and extends from a side of the housing  2 . The handle  4  has a trigger switch  5  and accommodates therein a battery  4 A. A canister housing  29  is provided in the handle  4  at a position immediately beside the housing  2 . A gas canister (not shown) containing therein a combustible gas is detachably disposed in the canister housing  29 . A magazine  6  is provided at a lower side of the handle  4 . The magazine  6  contains nails (not shown). The housing  2  has a lower portion formed with an exhaust port  2   a  for discharging a combustion gas to the atmosphere.  
         [0024]     A nose  7  extends from a lower end of the housing  2 . The nose  7  is formed integrally with a cylinder  20  (described later) and has a tip end in confrontation with a workpiece  28 . The nose  7  is adapted for guiding sliding movement of a drive blade  23 A (described later) and for setting the nail to a predetermined position. A push lever  9  is movably provided and has a lower portion slidable with respect to the lower end portion of the nose  7 . The push lever  9  is coupled to an arm member (not shown) that is engaged with a combustion-chamber frame  10  which will be described later through a pin (not shown). A compression coil spring  22  is interposed between the arm member and the cylinder  20  for normally urging the push lever  9  in a protruding direction from the housing  2 . When the housing  2  is pressed toward a workpiece  28  while the push lever  9  is in abutment with the workpiece against a biasing force of the compression coil spring  22 , an upper portion of the push lever  9  is retractable into the housing  2 .  
         [0025]     A head cap  11  is secured to the top of the housing  2  for closing the open top end of the housing  2 . The head cap  11  supports a motor  18  at a position opposite to a combustion chamber  26  described later as shown in  FIG. 3 . Further, an ignition plug  12  is also supported to the head cap  11  at a position adjacent to the motor  18 . The ignition plug  12  has an ignition spot exposed to the combustion chamber  26 . The ignition plug  12  is ignitable upon manipulation to the trigger switch  5 . An injection rod (not shown) is provided at the head cap  11 .  
         [0026]     The motor  18  has a motor case  18   a  and an output shaft  18   b , and is supported by a buffer member  13  made from an elastic material such as a rubber. The buffer member  13  includes a cylindrical motor storage section  14  and a fixing section  15 . The motor storage section  14  is adapted to surround the motor case  18   a  in intimate contact therewith, and has a bottom wall formed with a through-hole through which the output shaft  18   b  extends toward the combustion chamber  26 . As shown in  FIG. 4 , the motor storage section  14  has a motor insertion section in opposition to the bottom wall. The motor insertion section is formed with an open end  14   a  whose inner diameter is smaller than an outer diameter of the motor case  18   a . Further, four slots  14   b  are formed in a radial direction of the motor insertion section in a cruciform fashion. Each radially inner end of the slot  14   b  is open to the open end  14   a . Since the buffer member  13  is made from the elastic material, and since four slots  14   b  are formed at the motor insertion section, an area of the open end  14   a  can be easily enlarged for facilitating insertion and removal of the motor  18  into and from the motor storage section  14 .  
         [0027]     An annular groove is formed at an outer peripheral surface of the motor insertion section. A choke ring  17  such as a C-ring is fitted into the annular groove for maintaining the open end  14   a  at its closed fashion so as to avoid accidental removal of the motor  18  from the motor storage section  14 .  
         [0028]     As shown in  FIG. 4 , the fixing section  15  is provided integrally with an outer peripheral portion of the motor storage section  14 . The fixing section  15  includes three buffer segments  15 A protruding radially outwardly from the motor storage section  14  with an equal interval in a circumferential direction thereof. As shown in  FIG. 3 , each buffer segment  15 A has a radially outer portion  15 B serving as a fixed portion and formed with a fixing hole  15   a , and has a radially inner portion  15 C serving as a flex portion. The radially inner portion  15 C has a thickness smaller than that of the radially outer portion  15 B. Further, flange portions  15 D protrude radially outwardly from the outer peripheral surface of the motor storage section  14 . Each flange portion  15 D is positioned between the neighboring buffer segments  15 A. Radially protruding length of the flange portion  15 D is far smaller than that of the buffer segment  15 A. The motor storage section  14  has a flexibility less than that of the fixing section  15 , particularly the flex portion  15 C.  
         [0029]     A fixing plate  16  is attached to the head cap  11  by a screw  31  to fix the fixing section  15  to the fixing plate  16 . The fixing plate  16  is in the form of equilateral triangle plate. Each apex portion of the fixing plate  16  is formed with a hole  16   a  in alignment with the fixing hole  15   a . Thus, the screw  31  extends through the holes  16   a  and  15   a  and is threadingly engaged with the head cap  11 . The fixing plate  16  is formed with a central hole  16   b  having an inner diameter greater than the outer diameter of the motor storage section  14  when the motor  18  is stored in the motor storage section  14 , so that an annular space providing a radial distance “a” ( FIG. 2 ) is provided between the central hole  16   b  and the motor storage section  14 . Further, an opening  16   c  in communication with the central bore  16   b  is formed at one side of the fixing plate  16  for allowing the ignition plug  12  to extend through the opening  16   c . Therefore, as shown in  FIG. 2 , when the plate  16  is fixed to the head cap  11 , mechanical interference between the plate  16  and the motor storage section  14  and between the plate  16  and the ignition plug  12  does not occur.  
         [0030]     For assembly, as shown in  FIG. 3 , each radially outer portion  15 B of the buffer segment  15 A of the fixing section  15  is interposed between the head cap  11  and the fixing plate  16 . Then, the screws  31  are inserted through the holes  16   c  and  15   a  and are threadingly engaged with the head cap  11 . Therefore, the fixing plate  16  is fixed to the head cap  11  interposing the buffer segment  15 A therebetween. In this case, since the distance “a” is provided between the inner peripheral surface of the central hole  16   b  and the outer surface of the motor storage section  14 , accidental nip of the flex portion  15 C between the fixing plate  16  and the head cap  11  can be prevented, but the flex portion  15 C is freely deformable. Further, since the flexibility of the motor storage section  14  is lower than that of the fixing section  15 , particularly the flex portion  15 C, and the inner peripheral surface of the motor storage section  14  is in intimate contact with the motor  18 , excessive deformation of the motor storage section  14  can be prevented. Moreover, since the ignition plug  12  extends through the opening  16 C, the ignition plug  12  is positioned between the neighboring buffer segments  15 A and  15 A. In other words, the ignition plug  12  can be positioned at radially outer side of the flange portion  15 D whose protrusion amount is far smaller than that of the buffer segment  15 A. Thus, mechanical interference between the ignition plug  12  and the fixing section  15  can also be prevented.  
         [0031]     A head switch (not shown) is provided in the housing  2  for detecting an uppermost stroke end position of the combustion-chamber frame  10  when the nail gun  1  is pressed against the workpiece  28 . Thus, the head switch can be turned ON when the push lever  9  is elevated to a predetermined position for starting rotation of the motor  18 .  
         [0032]     The head cap  11  has a handle side in which is formed a fuel ejection passage  25  which allows a combustible gas to pass therethrough. One end of the ejection passage  25  serves as an ejection port that opens at the lower surface of the head cap  11 . Another end of the ejection passage  25  serves as a gas canister connecting portion  25 A in communication with the injection rod.  
         [0033]     The combustion-chamber frame  10  is provided in the housing  2  and is movable in the lengthwise direction of the housing  2 . The uppermost end of the combustion-chamber frame  10  is abuttable on the lower peripheral side of the head cap  11 . Since the arm member connects the combustion-chamber frame  10  to the push lever  9 , the combustion-chamber frame  10  is movable in interlocking relation to the push lever  9 . The cylinder  20  is fixed to the housing  2 . The inner circumference of the combustion-chamber frame  10  is in sliding contact with an outer peripheral surface of the cylinder  20 . Thus, the sliding movement of the combustion-chamber frame  10  is guided by the cylinder  20 . The cylinder  20  has an axially intermediate portion formed with an exhaust hole  21 . An exhaust-gas check valve (not shown) is provided to selectively close the exhaust hole  21 . Further, a bumper  24  is provided on the bottom of the cylinder  20 .  
         [0034]     As shown in  FIG. 1 , a piston  23  is slidably and reciprocally provided in the cylinder  20 . The piston  23  divides an inner space of the cylinder  20  into an upper space above the piston  23  and a lower space below the piston  23 . The driver blade  23 A extends downwards from a side of the piston  23 , the side being at the cylinder space below the piston  23 , to the nose  7 . The driver blade  23 A is positioned coaxially with the nail setting position in the nose  7 , so that the driver blade  23 A can strike against the nail during movement of the piston  23  toward its bottom dead center. The bumper  24  is made from a resilient material. When the piston  23  moves to its bottom dead center, the piston  23  abuts on the bumper  24  and stops. In this case, the bumper  24  absorbs a surplus energy of the piston  23 .  
         [0035]     When the upper end of the combustion-chamber frame  10  abuts on the head cap  11 , the head cap  11 , the combustion-chamber frame  10 , the upper cylinder space above the piston  23  define in combustion the combustion chamber  26 . When the combustion-chamber frame  10  is separated from the head cap  11 , a first flow passage in communication with an atmosphere is provided between the head cap  11  and the upper end of the combustion-chamber frame  10 , and a second flow passage in communication with the first flow passage is provided between the lower end portion of the combustion-chamber frame  10  and the upper end portion of the cylinder  20 . These flow passages allow a combustion gas and a fresh air to pass along the outer peripheral surface of the cylinder  20  for discharging these gas through the exhaust port  2   a  of the housing  2 . Further, the above-described intake port  3   a  is formed for supplying a fresh air into the combustion chamber  26 , and the exhaust hole  21  is adapted for discharging combustion gas generated in the combustion chamber  26 .  
         [0036]     The fan  19  is disposed in the combustion chamber  26 . Rotation of the fan  19  performs the following three functions. First, the fan  19  stirs and mixes the air with the combustible gas as long as the combustion-chamber frame  10  remains in abutment with the head cap  11 . Second, after the mixed gas has been ignited, the fan  19  causes turbulent combustion of the air-fuel mixture, thus promoting the combustion of the air-fuel mixture in the combustion chamber  26 . Third, the fan  19  performs scavenging such that the exhaust gas in the combustion chamber  26  can be scavenged therefrom and also performs cooling to the combustion-chamber frame  10  and the cylinder  20  when the combustion-chamber frame  10  moves away from the head cap  11  and when the first and second flow passages are provided.  
         [0037]     A plurality of ribs (not shown) are provided on the inner peripheral portion of the combustion-chamber frame  10  which portion defines the combustion chamber  26 . The ribs extend in the lengthwise direction of the combustion-chamber frame  10  and project radially inwardly toward the axis of the housing  2 . The ribs cooperate with the rotating fan  19  to promote stirring and mixing of air with the combustible gas in the combustion chamber  26 .  
         [0038]     Operation of the combustion type nail gun  1  will next be described. In the non-operational state of the combustion type nail gun  1 , the push lever  9  is biased downward in  FIG. 1  by the biasing force of the compression coil spring  22 , so that the push lever  9  protrudes from the lower end of the nose  7 . Thus, the uppermost end of the combustion-chamber frame  10  is spaced away from the head cap  1  because the arm member connects the combustion-chamber frame  10  to the push lever  9 . Further, a part of the combustion-chamber frame  10  which part defines the combustion chamber  26  is also spaced from the top portion of the cylinder  20 . Hence, the first and second flow passages are provided. In this condition, the piston  23  stays at its top dead center in the cylinder  20 .  
         [0039]     With this state, if the push lever  9  is pushed onto the workpiece  28  while holding the handle  4  by a user, the push lever  9  is moved upward against the biasing force of the compression coil spring  22 . At the same time, the combustion-chamber frame  10  which is coupled to the push lever  9 , is also moved upward in  FIG. 1 , closing the above-described flow passages. Thus, the sealed combustion chamber  26  is provided. In accordance with the movement of the push lever  9 , the gas canister is tilted toward the head cap  11  by an action of a cam (not shown). Thus, the injection rod is pressed against the connecting portion  25 A of the head cap  11 . Therefore, the liquidized gas in the gas canister is ejected once into the combustion chamber  26  through the ejection port of the ejection passage  25 .  
         [0040]     Further, in accordance with the movement of the push lever  9 , the combustion-chamber frame  10  reaches its uppermost stroke end whereupon the head switch is turned ON to energize the motor  18  for starting rotation of the fan  19 . Rotation of the fan  19  stirs and mixes the combustible gas with air in the combustion chamber  26 .  
         [0041]     In this state, when the trigger switch  5  provided at the handle  4  is turned ON, spark is generated at the ignition plug  12  to ignite the combustible gas. The combusted and expanded gas pushes the piston  23  to its bottom dead center. Therefore, a nail in the nose  7  is driven into the workpiece  28  by the driver blade  23 A until the piston  23  abuts on the bumper  24 .  
         [0042]     After the nail driving, the piston  23  strikes against the bumper  24 , and the combustion gas is discharged out of the cylinder  20  through the exhaust hole  21  of the cylinder  20  and through the check valve (not shown) provided at the exhaust hole  21 . When the inner space of the cylinder  20  and the combustion chamber  26  becomes the atmospheric pressure, the check valve is closed.  
         [0043]     By the combustion and expansion of the air-fuel mixture, the fan  19  is subjected to back pressure impact. Thus, acceleration is to be imparted on the motor  18  connecting to the fan  19 . Further, the piston  23  consumes surplus kinetic energy as a result of impingement onto the bumper  24  in addition to the fastener driving energy. In this instance, acceleration due to the surplus energy is imparted on the entire nail gun  1 , and therefore, the acceleration is to be also transmitted to the motor  18 . Thus, a combined acceleration is to be imparted on the motor  18 . However, since the motor  18  is supported to the head cap  11  only through the elastic fixing section  15 , the energy can be absorbed at the fixing section  15 , particularly at the thin flexed portion  15 C. Consequently, excessive impact is not applied to the motor  18  in spite of the acceleration.  
         [0044]     Further, since the radially outer portion  15 B (fixed portion) is thicker than the radially inner portion  15 C (flex portion), expansion and contraction of the fixed portion  15 B can be restrained regardless of the expansion and contraction of the flex portion  15 C. Thus, a positional displacement of the fixed portion  15 B can be prevented. Accordingly, positional displacement of the motor  18  relative to a space defined by the head cover  3  and the housing  2  can be restrained at timings immediately before and after the shock absorption by the fixing portion  15 .  
         [0045]     Further, since the plurality of buffer segments  15 A are arranged symmetrically with respect to the motor storage section  14 , impact to be applied to the motor  18  can be buffered in a well-balanced manner. In other words, stress applied to each buffer segment  15 A is equal to each other, to avoid local fatigue, thereby prolonging service life of the fixing member  15 .  
         [0046]     Further, since the motor storage section  14  stores the motor  18  in intimate contact therewith and has a sufficient thickness, the motor  18  is not violently moved within the motor storage section  14 . Further, since expansion and contraction of the motor storage section  14  does not occur during impact, but only the buffer segments  15 A are deformed because of the difference in flexibility therebetween. Further, accidental enlargement of the opening  14   a  due to the application of the shock can be prevented because of the provision of the choke ring  17 . Thus, accidental removal or projection-out of the motor  18  through the opening  14   a  can be avoided.  
         [0047]     Combustion gas still remaining in the cylinder  20  and the combustion chamber  26  has a high temperature at a phase immediately after the combustion. However, the high temperature can be absorbed into the walls of the cylinder  20  and the combustion-chamber frame  10  to rapidly cool the combustion gas.  
         [0048]     Thus, the pressure in the sealed space in the cylinder  20  above the piston  23  further drops to less than the atmospheric pressure (creating a so-called “thermal vacuum”). Accordingly, the piston  23  is moved back to the initial top dead center position.  
         [0049]     Then, the trigger switch  5  is turned OFF, and the user lifts the combustion type nail gun  1  from the workpiece  28  for separating the push lever  9  from the workpiece  28 . As a result, the push lever  9  and the combustion-chamber frame  10  move downward due to the biasing force of the compression coil spring  22  to restore a state shown in  FIG. 1 . In this case, the fan  19  keeps rotating for a predetermined period of time in spite of OFF state of the trigger switch  5  because of an operation of a control portion (not shown). In the state shown in  FIG. 1 , the flow passages are provided again at the upper and lower sides of the combustion chamber  26 , so that fresh air flows into the combustion chamber  26  through the intake port  3   a  formed at the head cover  3  and through the flow passages, expelling the residual combustion gas through the exhaust port  2   a . Thus, the combustion chamber  26  is scavenged. Then, the rotation of the fan  19  is stopped to restore an initial stationary state. Thereafter, subsequent nail driving operation can be performed by repeating the above described operation process.  
         [0050]     In the combustion type nail gun  1 , since the buffer member  13  is an integral product and can be produced by a molding, the buffer member  13  can be produced easily. Further because of the integral product, a problem of separation and reduction in strength and durability is avoidable thereby enhancing reliability. Further, an ordinary available motor can be used without any modification.  
         [0051]     While the invention has been described in detail and with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modification may be made therein without departing from the scope of the invention. For example, the present invention is not limited to the nail gun but is available for any kind of power tools in which a combustion chamber and a piston are provided, and as long as expansion of gas as a result of combustion of air-fuel mixture in the combustion chamber causes reciprocal motion of the piston. Further, the numbers of the buffer segments  15 A is not limited to three, but at least two buffer segments are required.