S&F Ref: 717763D1 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name and Address Hitachi Koki Co., Ltd., of No. 15-1, Konan 2-chome of Applicant: Minato-ku, Tokyo, 108-6020, Japan Actual Inventor(s): Haruhisa Fujisawa Kunio Yamamoto Yoshitaka Akiba Tomomasa Nishikawa Shinki Ohtsu Address for Service: Spruson & Ferguson St Martins Tower Level 35 31 Market Street Sydney NSW 2000 (CCN 3710000177) Invention Title: Combustion-type power tool The following statement is a full description of this invention, including the best method of performing it known to me/us: 5845c(5231706_1) 1 COMBUSTION-TYPE POWER TOOL BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a combustion-type power tool, such as 5 combustion-type nail driver for striking fastening members such as nails or studs into a workpiece wherein acceleration applied to a motor when combustion explosion occurs or a piston impinges upon a bumper is suppressed. Description of the Related Art A combustion-type nail driver ignites air-fuel mixed gas confined in a 1o combustion chamber and translates voluminal expansion of the gas into power. A fan is disposed within the combustion chamber to stir air and fuel to enhance the combustion property of the mixed gas. The fan is rotated by a motor. The fan generates turbulence of the mixed gas in the combustion chamber and promotes combustion of the gas. Occurrence of explosive 15 combustion in the combustion chamber brings the voluminal expansion of the gas and generates impact. The impact thus generated is transmitted to the body of the nail driver, and so to the motor for rotating the fan. A piston that translates the voluminal expansion of the gas into power strikes a nail into a workpiece. A kinetic energy in excess of the energy required for striking the 20 nail into the workpiece is absorbed into a bumper disposed in the cylinder along which the piston slidingly moves when the piston impinges upon the bumper. At this time, acceleration generated when the piston impinges upon the bumper is applied to the body of the nail driver, and the acceleration thus generates is transmitted to the motor. Because the motor is a precise device and is week against vibration. The motor 25 may be damaged by the impact repeatedly applied to the motor, resulting in degradation of the property of the motor. In order not to transmit the impact to the motor, a buffer material is used for a motor holding member. The motor holding member separates the motor from the body of the nail driver, thus transmission of the impact to the motor can be prevented, as disclosed in U.S. Patent No. 6,520,397. 30 More specifically, as shown in Figs. I and 2, a motor 118 is mounted on a cylinder head 111. The cylinder head 111 is disposed at one end portion of a housing 102 of the nail driver 101 and covered by a head cover 103. A fan 119 is attached to the tip end of the output shaft 118b of the motor 118. A spark plug 112 fixedly secured to the 2 cylinder head 111 is positioned in the vicinity of the motor 118 and has one end projected into the combustion chamber. Two circumferentially extending grooves are formed over the entire outer periphery of the motor 118 to be spaced apart in the axial direction of the motor 118. As 5 shown in Fig. 1, a retaining ring 114 is fitted into each of the two grooves. An inner ring 113a of a buffer member 113 is interposed and held between the two retaining rings 114. As shown in Fig. 2, the buffer member 113 includes the inner ring 113a, a fixing metal member 113c, and a rubber member 11 3b molded and coupled to both the inner ring 11 3b and the fixing metal member 11 3c to be integral therewith. The fixing metal to member I1 3c is fixedly secured to the cylinder head 111. As such, the motor 118 is supported on the cylinder head 111 via the buffer member 113. With the above-described structure, impact generated in the nail driver 101 is transmitted to the fixing metal member 113c of the buffer member 113. However, due to the presence of the rubber member 11 3b, the impact transmitted to the inner ring 11 3a is and to the motor 118 supported by holding the inner ring 113 with the retaining rings 114 is suppressed. However, as described above, with the conventional nail driver, the grooves need to be formed in the outer periphery of the motor 118 in order to fix the motor 118 to the buffer member 113. Therefore, general-purpose motors cannot be employed but motors 20 manufactured based on a special specification, which are expensive in cost, are required. The buffer member 113 is an integral member in which the two metal rings 11 3a and 11 3c are connected together with the rubber member 11 3b interposed therebetween. Due to the different materials forming the integrated buffer member 113, reliability of rubber mold coupling is low and there is a possibility that the different material segments are 25 separated if the rubber molding condition is not good. From the structural requirement, the spark plug 112 is positioned in the vicinity of the motor. Accordingly, the rubber member 113 cannot extend to the position of the spark plug 112. Continuity of the rubber member 113 is thus interrupted by the spark plug 112 and the rubber member 113 is separated at the position of the spark plug 112. 30 The buffer member 113 is incapable of equally suppressing the impact to be imparted upon the motor 118. Tensile stress is thus focused on a position near the spark plug securing position, so that the rubber member 11 3b is liable to be damaged. In the nail driver of the type described above, continuously performed nail driving operations accumulate heat generated at the time of explosive combustion. The 35 combustion chamber 26 and the cylinder (not shown) are the primary sources of heat 3 generation. The heat thus generated is transmitted to and raises the temperature of the nail driver including the motor 118. Driving the motor 118 also generates heat in the motor coil, so further raises the temperature of the motor 118. A problem that temperature rise of the motor 118 may cause burning of the motor 118 has conventionally s been solved by employing highly durable motors. However, such motors are expensive in cost. OBJECT OF THE INVENTION It is the object of the present invention to substantially overcome or ameliorate one or more of the disadvantages of the prior art, or at least provide a useful alternative. 10 A further object of the invention, at least in its preferred form(s), is to provide a combustion-type power tool with an easily manufacturable and less-damageable motor supporting structure. Another object of the present invention, at least in its preferred form(s), is to provide a combustion-type power tool that is inexpensive in cost and has a motor is supporting structure of an improved cooling efficiency. SUMMARY OF THE INVENTION Accordingly, in a first aspect, the present invention provides a combustion-type power tool comprising: a housing; 20 a cylinder head disposed at one end of the housing; a cylinder disposed in the housing; a piston slidably disposed in the cylinder; a driver blade connected to the piston; a combustion-chamber provided in the housing; 25 a fan disposed in the combustion chamber; a motor having a cylindrical portion and an output shaft extending from the cylindrical portion and coupled to the fan; an elastic member disposed in an area superposed with the cylindrical portion in an axial direction of the output shaft, and 30 a cylindrical member disposed in the cylinder head for receiving the motor. Preferably, the cylindrical member has a plurality of protrusions which attenuates the impact to be imparted upon the motor by friction caused when the motor moves relative to the cylinder head.
4 Preferably, the combustion-type power tool described above further comprises a heat shielding member disposed between the motor and the bottom portion of the cylinder head. BRIEF DESCRIPTION OF THE DRAWINGS s In the drawings: Fig. 1 is a cross-sectional side view showing a conventional cylinder head and its associated components; Fig. 2 is a cross-sectional top view showing the conventional cylinder head and its associated components; 10 Fig. 3 is a cross-sectional side view showing a combustion-type nail driver according to a first embodiment of the present invention; Fig. 4 is a side view showing the combustion-type nail driver according to the first embodiment of the present invention; Fig. 5 is a cross-sectional top view showing the combustion-type nail driver 15 according to the first embodiment of the present invention; Fig. 6 is a cross-sectional view cut along a line III-III in Fig. 5 showing a cylinder head and its associated components according to the first embodiment of the present invention; Fig. 7 is a cross-sectional view cut along a line IV-IV in Fig. 5 showing a 20 cylinder head and its associated components according to the first embodiment of the present invention; Fig. 8 is a cross-sectional view showing a cylinder head and its associated components according to a second embodiment of the present invention; Fig. 9 is a cross-sectional view showing a cylinder head and its associated 25 components according to a third embodiment of the present invention; Fig. 10 is a cross-sectional view showing a cylinder head and its associated components according to a fourth embodiment of the present invention; Fig. 11 is a cross-sectional side view showing a combustion-type nail driver according to a fifth embodiment of the present invention; 30 Fig. 12 is a partial enlarged diagram of the combustion-type nail driver shown in Fig. 11; Fig. 13 is a cross-sectional view showing a cylinder head and its associated components according to the fifth embodiment of the present invention; and 5 Fig. 14 is a top view showing a head protector as viewed from a direction D shown in Fig. 12. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A combustion-type power tool according to an embodiment of the invention will s be described with reference to Figs. 3 through 7. The embodiment pertains to a combustion-type nail driver. In the following description, the terms "upper", "lower", "above", "below", "upward", "downward" and the like will be used assuming that the combustion-type nail driver is disposed in an orientation in which a nail is fired vertically downward. 10 The combustion-type nail driver I shown in Fig. 3 has a housing 2 constituting an outer frame. A head cover 3 formed with an intake port 3a is mounted on the top of the housing 2. A handle 4 extends from one side of the housing 2. The handle 4 has a trigger switch 5 and detachably accommodates therein a battery pack 4a. The combustion-type nail driver 1 also has a canister housing 29 at one side of the housing 2 15 from which the handle 4 extends. A gas canister 30 containing therein a combustible liquidized gas is detachably installable in the canister housing 29. A magazine 6 accommodating therein a bundle of nails (not shown) is disposed below the handle 4. A nose 7 extends from near the lower end of the housing 2. The nose 7 is integral with a cylinder 20 described later and has a tip end abuttable on a workpiece 28. 20 The nose 7 is adapted for guiding sliding movement of a driver blade 23a described later and for guiding the nail driven into the workpiece 28. A push lever 9 is reciprocally slidingly movably supported to the nose 7, and projects from the tip end 7a of the nose 7. The push lever 9 has an upper end abuttingly associated with an arm section 8 fixed to a combustion-chamber frame 10 described later. A compression coil spring 22 serving as a 25 biasing member is interposed between the arm section 8 and the cylinder 20. Thus, the push lever 9 abuttingly associated with the arm section 8 is urged downwardly by the biasing force of the compression coil spring 22. A cylinder head 11 is fixedly secured to the top of the housing 2 and substantially covers the open top end of the housing 2. As shown in Fig. 5, a motor 18 is 30 disposed at one side of the cylinder head 11 opposite the combustion chamber 26 as will be described later. An ignition plug 12 is disposed in the vicinity of the motor 18 and the ignition position is directed toward the combustion chamber 26. As shown in Fig. 6, a motor holder 13 serving as a motor accommodating member is in the form of a hollow cylinder made from resin such as plastic. The motor 6 holder 13 has a hole through which lead lines of the motor 18 are drawn upward. A motor holder receiving portion 11 a is formed in the cylinder head 11 for receiving the motor holder 13. The motor holder 13 is formed with a groove 13a at the lower inner periphery thereof adapted to receive an end portion of a coil spring 15. The motor holder 5 receiving portion I Ia is also formed with a groove 1 lb at the lower inner periphery thereof to receive another end portion of the coil spring 15. The motor 18 is inserted into the motor holder 13 from its lower end. Then, one end portion of the coil spring 15 is inserted into the groove 13a of the motor holder 13 and the other end portion into the groove 1 b of the motor holder receiving portion I Ia. As shown in Fig. 7, the other end 1o portion of the coil spring 15 inserted into the groove 11 b of the motor holder receiving portion I Ia is fixedly held by a screw 32. The coil spring 15 interposed between the motor 18 and the cylinder head II serves to suppress the initial acceleration which may be imparted upon the motor 18. The motor holder 13 has a dimension such that the inner diameter thereof is is slightly larger than the outer diameter of the motor holder receiving portion 1 a, thereby allowing the motor holder 13 to slidingly move along the inner periphery of the motor holder receiving portion 11 a. Impact generated at the time of firing the nails is imparted upon the motor holder 13 to move the latter back and forth. However, due to friction between the outer periphery of the motor holder 13 and the inner periphery of the motor 20 holder receiving portion 11 a, little impact is received at the motor 18. A disk 14 serving as a heat shielding member is disposed in the motor holder 13 and in a position between the body of the motor 18 and the combustion chamber 10. More specifically, the disk 14 is formed with a center hole into which the output shaft 18b of the motor 18 is fitted. The disk 14 is disposed at a position remote from the end 25 portion of the output shaft 18b to which the fan 19 is attached. The disk 14 prevents heat wind generated in the combustion chamber 26 from entering into the motor 18 through a gap between the cylinder head 11 and the motor output shaft 18b. Referring back to Fig. 3, a switch 33 is provided in the housing 2 for detecting an uppermost stroke end position of the combustion-chamber frame 10 described later when 30 the nail driver I is pressed against the workpiece 28. Thus, the switch 33 can be turned ON when the push lever 9 is elevated to a predetermined position for starting rotation of the motor 18. The cylinder head 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 35 25 opens at the lower surface of the cylinder head 11. Another end of the ejection 7 passage 25 serves as a gas canister connecting portion 25a in communication with a gas canister 30. 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 5 chamber frame 10 is abuttable on the lower surface of the cylinder head 11. A combustion chamber includes a chamber 1 Oa and a chamber head I Ob connected integrally using a bolt (not shown). Since the arm section 8 is connected to the combustion-chamber frame 10, the combustion-chamber frame 10 is moved in accordance with the movement of the push lever 9. The cylinder 20 is fixed to the 10 housing 2. An outer peripheral surface of the cylinder 20 is in sliding contact with the inner circumference of the combustion-chamber frame 10 for guiding the movement of the combustion-chamber frame 10. 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. IS As shown in Fig. 3, a piston 23 is slidably and reciprocally movably 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 23a extends downwards from the lower surface of the piston 23 to the nose 7, so that the tip end of the driver blade 23a can strike against the nail (not shown). A bumper 24 made 20 from an elastic material such as rubber is disposed at a lower side of the cylinder 20. The piston 23 strikes against the bumper 24 when the piston 23 is moved downward toward a bottom dead center. When the upper end of the combustion-chamber frame 10 abuts the cylinder head 11, the cylinder head 11, the combustion-chamber frame 10, and the upper cylinder 25 space above the piston 23 define a combustion chamber 26. When the combustion chamber frame 10 is separated from the cylinder head 11, a first flow passage in communication with the atmosphere is provided between the cylinder head 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 inner peripheral 30 surface of the combustion-chamber frame 10 and the outer peripheral surface of the cylinder 20. The housing 2 has a lower portion formed with an exhaust port 2a. The first and second flow passages allow a combustion gas and a fresh air to pass along the outer peripheral surface of the cylinder 20 for discharging these gases through the exhaust port 2a of the housing 2. Further, the above-described intake port is formed for supplying a 8 fresh air into the combustion chamber 26, and the exhaust hole 21 discharges combustion gas generated in the combustion chamber 26. A fan 19 is attached to the lower end of the motor output shaft 18b with two nuts. The fan 19 is disposed in the combustion chamber 26. Rotation of the fan 19 5 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 cylinder head 11. Second, after the mixed gas has been ignited, the fan 19 causes turbulence of the air-fuel mixture, thus promoting the turbulent combustion of the air-fuel mixture in the combustion chamber 26. Third, the fan 19 performs scavenging such that io the exhaust gas in the combustion chamber 26 can be scavenged therefrom and also performs cooling of the cylinder 20 when the combustion-chamber frame 10 moves away from the cylinder head 11 and when the first and second flow passages are provided. Operation of the combustion-type nail driver 1 will next be described. In the non-operational state of the combustion-type nail driver 1, the push lever 9 is biased 15 downward 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 cylinder head 11 because the combustion chamber frame 10 is in association with the push lever 9 through the arm section 8. Further, a part of the combustion-chamber frame 10 which part defines the combustion 20 chamber 26 is also spaced apart 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 the top dead center in the cylinder 20. 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 25 compression coil spring 22. At the same time, the combustion-chamber frame 10 which is connected to the push lever 9 through the arm section 8 is also moved upward, closing the first flow passage and hermetically sealing the combustion chamber 26. In accordance with the movement of the push lever 9, the gas canister 30 is tilted toward the cylinder head 11. Thus, the injection rod 30a of the gas canister 30 is pressed 30 against a gas canister connecting portion 25a of the cylinder head 11. Therefore, the liquidized combustible gas in the gas canister 30 is ejected once from the ejection port of the fuel ejection passage 25 into the combustion chamber 26. Further, in accordance with the movement of the push lever 9, the combustion chamber frame 10 reaches the uppermost stroke end whereupon the switch 33 is turned 35 ON to supply electric power to the motor 18 and start rotation of the fan 19. Rotation of 9 the fan 19 in the combustion chamber 26 in which a hermetically sealed space is provided, stirs and mixes the ejected combustible gas with air in the combustion chamber 26. In this state, when the trigger switch 5 provided at the handle 4 is turned ON, 5 spark is generated at the ignition plug 12 to ignite the combustible gas. As a result of combustion, volumetric expansion of the combustion gas occurs within the combustion chamber 26 to move the piston 23 downwardly. Accordingly, the driver blade 23a drives the nail held in the nose 7 into the workpiece 28 until the piston 23 strikes against the bumper 24. 10 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. A check valve (not shown) is 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. 15 Impact is imparted upon the fan 19 when the air-fuel mixed gas is ignited. The motor 18 connected to the fan 19 is applied with resultant acceleration. Striking the piston 23 against the bumper 24 consumes kinetic energy of the piston 23 in excess of energy necessary for driving the nail. Acceleration resulting from the excessive energy is applied to the nail driver I including the motor 18. The motor 18 is mounted on the 20 cylinder head I1 and is held thereon with only the elastically deformable spring 15. Therefore, although large acceleration is applied to the motor 18, expansion and compression behavior of the spring 15 absorb the energy to be applied to the motor 18. Thus, impact imparted upon the motor 18 is greatly reduced. Surface contact of the outer peripheral surface of the motor holder 13 with the inner wall of the motor holder 25 receiving portion 11 a suppresses transmission of the impact to the motor 18. Combustion gas still remaining in the cylinder 20 and the combustion chamber 26 has a high temperature at a phase immediately after the combustion. The heat is absorbed through the inner surfaces of the cylinder 20 and the combustion-chamber frame 10, and the temperature of these components is also increased. However, the absorbed 30 heat is released to the atmosphere through the outer surfaces of the cylinder 20 and the combustion-chamber frame 10. Combustion heat of the combustion gas is absorbed into such components as the cylinder 20, so that the combustion gas is abruptly cooled down and a volume of the combustion gas is decreased. Thus, the pressure in the sealed space in the cylinder 20 35 above the piston 23 further drops to less than the atmospheric pressure, creating a so- 10 called "thermal vacuum". Accordingly, the piston 23 is moved back to the initial top dead center position. Thereafter, the trigger switch 5 is turned OFF, and the user lifts the nail driver I until the push lever 9 is separated from the workpiece 28. As a result, the push lever 9 s and the combustion-chamber frame 10 move downward due to the biasing force of the compression coil spring 22. 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. 3, the first and second flow passages are provided at the upper side of the combustion-chamber frame 10, so that fresh 1o air flows into the combustion chamber 26 through the intake port 3a formed in the head cover 3 and the residual gas is expelled through the exhaust port 2a by the rotation of the fan 19. 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. 15 The coil spring 15 according to the above-described embodiment is covered with an elastic material such as rubber. Due to impact absorbing capability of the rubber, the impact imparted upon the motor can be attenuated or reduced without relying on the friction between the motor holder 13 and the motor holder receiving portion l Ia. The coil spring 15 may have an increasing diameter toward the lower portion of 20 the same. The use of such a coil spring 15 enables the vertical distance between the motor holder 13 and the bottom portion of the motor holder receiving portion 11 a to be shortened, thereby compacting the size of the nail driver 1. A sleeve formed from resin such as plastic may be interposed between the inner peripheral surface of the motor holder receiving portion 11 a and the outer peripheral 25 surface of the motor holder 13 in order to attain sliding movement between the sleeve and the motor holder 13. With the provision of the sleeve that contacts the motor holder 13, the motor holder 13 can slidingly move with less friction. The material of the sleeve is selected from low frictional materials. A motor holder employed in the combustion-type nail driver according to the 30 second embodiment will be described while referring to Fig. 8. Fig. 8 is a vertical cross sectional view showing the motor holder 13 and its associated components. A motor 18 is accommodated in the motor holder 13 formed from a material of a low friction factor such as plastic. The motor holder 13 is a hollow cylindrical member adapted to receive the motor 18. The motor 18 is supported by the motor holder 13 and the upper side of the 35 motor 18 is fixed by hooks 13b provided above the motor holder 13. Three protrusions 11 1 3c protrude radially outwardly from the intermediate portion of the motor holder 13. The protrusions 13c are formed at the time of mold shaping the motor holder 13. The protrusions 13c perform up and down movements while contacting the inner wall of the motor holder receiving portion 11 a formed in the cylinder head 11 when impact is 5 imparted upon the motor 18. Friction between the protrusions 13c and the motor holder receiving portion 11 a momentarily substantially absorbs the impact to be imparted upon the motor 18. A groove 13d is formed in the bottom of the motor holder 13, to which the upper end portion of the coil spring 15 is attached. The lower end portion of the coil spring 15 1o is attached to the groove 1 lb formed in the bottom of the motor holder receiving portion 1l a. The coil spring 15 interposed between the motor 18 and the cylinder head 11 serves to suppress the initial acceleration applied to the motor 18. A motor holder employed in the combustion-type nail driver according to the third embodiment will be described while referring to Fig. 9. Fig. 9 is a vertical cross is sectional view showing the motor holder and its associated components. In the third embodiment, one end of the coil spring 15 is force-fitted into the groove II b formed in the motor holder receiving portion formed in the cylinder head 11, thereby supporting the motor holder 13 on the cylinder head 11. According to the above-described holding method, machining the cylinder head II can be achieved easily because the shape of the 20 groove II b formed in the cylinder head 11 does not require a complicated spring capturing capability. A motor holder employed in the combustion-type nail driver according to the fourth embodiment will be described while referring to Fig. 10. Fig. 10 is a vertical cross-sectional view showing the motor holder and its associated components. The fourth 25 embodiment shows how to hold the coil spring 15 in the motor holder receiving portion 11 a. An annular end holding part I1d is formed in the upper end of the motor holder receiving portion 11 a for holding a sleeve 31. The sleeve 31 has an outer diameter approximately equal to the inner diameter of the motor holder receiving portion II a. The sleeve 31 inserted into the motor holder receiving portion 11 a holds the coil spring 15 in 30 the motor holder receiving portion 1. a. According to the above-described holding method, assembling of the motor 18 and its associated components can be performed easily because insertion of the sleeve 31 into the motor holder receiving portion 1 a enables holding of the coil spring 15. Further, by changing the material of the sleeve 31, friction between the sleeve 31 and the motor holder 13 can be adjusted.
12 A combustion-type nail driver according to the fifth embodiment will be described while referring to Figs. I1 through 14. The nail driver shown in Figs. I1 through 14 is basically the same in structure as that shown in Figs. 3 through 7 but differs therefrom in the material and shape of the motor holder 13. 5 The motor holder 13 according to the fifth embodiment is formed from a metal such as aluminum. The metallic motor holder 13 is imposed on a role of dissipating combustion heat generated at the time firing the nails and also heat generated from the motor 18. As shown in Fig. 13, the motor 18 is inserted from the upper end of the motor holder 13 and held therein with a pin 49 so as not to be detached therefrom. One end of to the coil spring 15 is inserted into and held by the groove 13a formed in the bottom of the motor holder 13. Another end of the coil spring 15 is inserted into the groove 1 l b engraved in the motor holder receiving portion 11 a and is fixed thereto by a screw 32. Fig. 11 shows an initial state of the nail driver 1, in which the nail driver I has not yet been operated. Air flow or combustion gas flow is created by the fan 19 at the is time of scavenging. Air is introduced into the intake port 3a from the upper side of the head cover 3, flows along the first flow passage 41, and then enters into the combustion chamber 26 via the fan 19. Combustion gas remaining in the combustion chamber 26 is expelled out of the combustion chamber 26, passes along the second flow passage 42, flows in the outer periphery of the cylinder 20, and discharged out of the housing 2 from 20 the exhaust port 2a formed in the lower portion of the housing 2. The motor 18 for rotating the fan 19 is accommodated in the motor holder 13 made from a metal such as aluminum. The motor holder 13 and the motor holder receiving portion 11 a formed in the cylinder head 11 are connected together with an elastically deformable coil spring 15 interposed therebetween. The coil spring 15 serves 25 to prevent impact generated at the time of firing the nails from being directly transmitted to the motor 18. The metallic motor holder 13 effectively dissipates heat generated at the time of operation of the nail driver and heat generated from the motor 18, thereby suppressing the temperature rise of the motor 18. Further, cooling fins 13e are formed in the outer 30 circumference of the motor holder 13 for enhancing the heat dissipation. Fig. 12 is a cross-sectional view showing the upper portion of the nail driver I as view from a direction in which the nail driver 1 is rotated 90 degrees about the motor output shaft from the state shown in Fig. 11. A head protector 44 is provided inside the head cover 3. Fig. 14 shows the head protector 44 as viewed from direction D shown in 35 Fig. 12. The head protector 44 is substantially cone-shaped and formed from plastic. The 13 head protector 44 includes a communication part 44c and a guide part 44b. A plurality of scavenging holes 44a is formed in the communication part 44c for allowing air to pass therethrough at the time of scavenging. The guide part 44b is such a portion where no scavenging holes 44a are formed. 5 When the fan 19 rotates to create an air flow, air is sucked from the intake port 3a formed in the head cover 3. The air thus sucked flows into the opening I Ia of the cylinder head 11 via the scavenging holes 44a formed in the head protector 44. If the scavenging holes 44a are positioned immediately above the opening 11 a of the cylinder head 11, air flows vertically downward. However, since the guide part 44b is positioned to immediately above the opening I Ia of the cylinder 11, air sucked from the intake port 3a flows into the opening 11 of the cylinder head 11 while passing by the motor holder 13. Accordingly, with the provision of the head protector 44, cooling the motor holder 13 can effectively be performed. With the structure as described above, and as shown in Fig. 12, the air flow at the is time of scavenging is that the air sucked from the intake port 3a passes by the cooling fins 13e formed in the upper portion of the motor holder 13, guided by the guide part 44b where no scavenging holes 44a are formed, and then introduced into the combustion chamber 26. As such, the fresh air passes by the upper portion of the motor holder 13, whereby the temperature rise of the motor 18 can further be suppressed. 20 While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein.