Combustion-type power tool

A combustion-type power tool includes a cylinder and a combustion chamber disposed on top of the cylinder that accommodates a gaseous mixture of existing air in the combustion chamber and fuel injected therein. A spark plug generates a spark to combust the gaseous mixture in the combustion chamber. A trigger switch produces the spark in the spark plug when operated. A piston is movably supported in the cylinder and driven by combustion in the combustion chamber. A driving blade is integrally formed with the piston for driving a fastener. A push lever is provided at one end of the housing and coupled with the combustion chamber frame. A temperature sensor is provided, for example, on the combustion chamber frame to sense the temperature thereof. A stopper is operated to adjust the position of the push lever to be shifted from an initial position when the sensed temperature is higher than a critical value. The push lever is positioned farther from another end of the housing when adjusted than when the push lever is in the initial position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a combustion-type power tool, such as combustion-powered fastener-driving tool for driving fasteners, such as nails, into a workpiece.

2. Description of the Related Art

Combustion-type power tools have been known in the art as disclosed in Japanese Patent Publication (B2) Nos. HEI-1-34753, HEI-4-48589, HEI-3-25307, HEI-4-11337, and SHO-64-9149.

Unlike a compressed-air type power tool that uses compressed air as a driving source, the combustion-type power tool requires no compressor and is, therefore, much easier to transport to a construction site or the like. Further, the combustion-type power tool can be conveniently provided with an internal power source, such as a battery, so that the tool can be used in any environment without requiring a commercial power supply.

Generally, the combustion-type power tool is made up of metal components, such as a cylinder, rubber components, such as shielding member, and plastic components, such as a housing. When fastener driving operations are successively performed with a short interval, heat generated by combustion is partly dissipated when a fan performs scavenging such that the exhaust gas in the combustion chamber can be scavenged therefrom, and is partly absorbed by the metal components, particularly by the cylinder and a combustion chamber frame. The temperature of these metal components gradually increases and finally they become overheated. When the cylinder and the combustion chamber frame become high temperature, the gas remaining in the combustion chamber is not sufficiently cooled down so that thermal vacuum in the combustion chamber cannot be attained. Hence, delay occurs in returning the piston to the original position after the fastener driving operation is performed. Due to this delay, a cycle speed for completing one-shot fastener driving operation is lowered and so the work efficiency is degraded.

If the fastener driving operation is continued, the housing and the handle disposed near the combustion chamber frame and the cylinder are also heated up. The operator grasping the handle may not be able to continue the fastener driving operation. If so, the work efficiency is further degraded.

The sealing member, which is made from rubber and used for sealing the combustion chamber, may thermally be damaged and sealing capability with the sealing member may not be maintained. If it is the case, the combustion chamber is communicated with external atmosphere and the flammable gas introduced into the combustion chamber will not be ignited. Then, the fastener driving operation can no longer be continued even if the trigger switch is turned on. The housing, which is made from plastic, may also be damaged or distorted by the heat. If the sealing member and/or the housing is damaged, the tool needs to be disintegrated and a damaged component has to be replaced with a new one.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to prevent thermal damage of a sealing member and/or a housing of a combustion-type power tool, thereby prolonging the service life of the tool.

It is another object of the present invention to provide a combustion-type power tool wherein work efficiency is improved without degrading operability of the tool.

In order to attain the above and other objects, a combustion-type power tool according to the present invention includes a housing, a head, a handle, a cylinder, a piston, combustion chamber frame, a push lever, a spark plug, a trigger switch, a driving blade, a temperature sensor, and a stopper. The housing has first and second end portions. The head is provided at the first end portion of the housing. The handle extends from the housing. The cylinder is fixedly disposed within the housing. The piston is movably supported in the cylinder. The combustion chamber frame is disposed within the housing to be movable along the cylinder. A combustion chamber is formed by the head, the cylinder, the piston, and the combustion chamber frame when the combustion chamber frame is in abutment with the head. The combustion chamber accommodates a gaseous mixture of existing air in the combustion chamber and fuel injected therein. The push lever is mounted on the second end portion of the housing and coupled with the combustion chamber frame. The push lever is movable into the housing, causing the combustion chamber frame to move together with the push lever, when the push lever is pushed against a workpiece. The spark plug is disposed in the combustion chamber and generates a spark to combust the gaseous mixture in the combustion chamber. The piston is driven by combustion in the combustion chamber. The trigger switch is provided to the handle and produces the spark in the spark plug when operated. The driving blade is integrally formed with the piston for driving a fastener. The temperature sensor is provided for sensing a temperature of at least one selected portion within the housing or on the handle. The stopper is operable to adjust a position of the push lever to be shifted from a first position to a second position when the temperature sensed by the temperature sensor is higher than a critical value.

The push lever is positioned farther from the head when the push lever is in the second position than when the push lever is in the first position.

The stopper is selectively movable between the first and second positions.

It is preferable to dispose the temperature sensor on the combustion chamber frame.

It is also preferable to operate the stopper by an electromagnetic solenoid.

The temperature sensor may be one of a thermistor, a thermocouple, and a bimetal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A combustion-powered fastener-driving tool according to a preferred embodiment of the invention will be described with reference to the accompanying drawings.

The following is a general description of a representative combustion-powered fastener-driving tool to which the present invention is applied. The combustion-powered fastener-driving tool drives fasteners, such as nails, rivets, staples, or the like. In the following description, nails will be taken as an example of the fasteners. Hereinafter, the terms “upward”, “downward”, “upper”, “lower”, “above”, “below”, “beneath” and the like will be used throughout the description assuming that the combustion-powered fastener-driving tool is disposed in an orientation in which it is used as shown inFIGS. 1 and 2.

FIGS. 1 and 2are vertical cross-sectional views showing a combustion-powered fastener-driving tool1, and particularly a nail-driving tool. The components and operations of the nail-driving tool are described below with reference toFIGS. 1 and 2.

As shown inFIG. 1, the fastener-driving tool1includes a housing with an elongated shape. The housing includes a main housing2A and a sub housing2B, in which the sub housing2B is coupled to the main housing2A along the longitudinal direction of the main housing2A. The main housing2A and the sub housing2B are integrally coupled together to form a framework of the fastener-driving tool1.

A head cover4is attached to the upper end of the main housing2A. An intake port (not shown) is formed in the head cover4and an exhaust port (not shown) is formed in the lower portion of the main housing2A. A gas cylinder5, which stores flammable gas (liquid gas), is detachably accommodated in the sub housing2B. A handle7provided with a trigger switch6is secured to the sub housing2B. A magazine8and a tail cover9are mounted below the main housing2A and the sub housing2B. The magazine8is filled with a plurality of the nails. The tail cover9guides the nails supplied from the magazine8and sequentially sets the nails in a predetermined position beneath a piston25.

The main housing2A houses therein a push lever10, a connection member12, a cylinder20, the piston25, a driving blade28extending from the piston25, a fan14, a motor3, a spark plug15, a combustion chamber frame11, and a head cap13. The push lever10projects downward from the lowermost portion of the main housing2A to be vertically movably supported thereon. The push lever10is coupled with the connection member12fixed to the combustion chamber frame11. A spring37urges the combustion chamber frame11downward in the drawing, that is, in a direction for driving the nail. Hence, the combustion chamber frame11is capable of moving axially with respect to the main housing2A.

When the push lever10is pressed against a workpiece W, such as a wood material, the push lever10opposes the urging force of the spring37. The upper portion of the push lever10retracts into the main housing2A and the combustion chamber frame11moves above the cylinder20, forming a combustion chamber26. Specifically, the combustion chamber26is a space enclosed by the combustion chamber frame11, the head cap13, and the piston25, in which a mixture of a flammable gas and air is burned. In order to form a hermetically sealed combustion chamber26, a first seal member19, such as an O-ring, is provided on the lower end of the head cap13. Also, a second seal member24, such as an O-ring, is provided on the upper portion of the cylinder20to seal the space between the lower inner periphery of the combustion chamber frame11and the upper outer periphery of the cylinder. This space is created when the combustion chamber frame11is brought into abutment with the head cap13.

The head cap13is fixed to the top end of the main housing2A. The combustion chamber26accommodates the tan14, which can be rotated by the motor3fixedly mounted on the head cap13, and the spark plug15for generating a spark when the trigger switch6is operated.

The main housing2A further accommodates a head switch16(seeFIG. 3) and a temperature sensor29, The head switch16is provided for detecting an uppermost stroke end position of the combustion chamber frame11when the fastener driving tool1is pressed against the workpiece W. The head switch16is turned on when the push lever10is elevated to a predetermined position so that rotation of the motor3and the fan14is started. The temperature sensor29, such as a thermistor, thermocouple, or bimetal, is affixed to the outer wall of the combustion chamber frame11for sensing the temperature of the combustion chamber frame11. The temperature sensor29may be disposed on the outer wall of the handle7or any other places where the temperature tends to rise as the nail driving operations are repeatedly performed.

The head cap13has a handle side in which is formed a fuel injection passage17which allows the flammable gas to pass therethrough. One end of the fuel injection passage17serves as an injection port18that opens at the lower surface of the head cap13. Another end of the fuel injection passage17is engaged with the gas cylinder-connecting portion in communication with the gas cylinder5. The injection port18injects flammable gas into the combustion chamber26from the gas cylinder5, which stores this flammable gas (liquid gas).

Provided below the cylinder20are an exhaust hole21, a check valve22for opening and closing the exhaust hole21, and the bumper23against which the piston25collides. The exhaust hole21is in communication with the exhaust port formed in the lower portion of the main housing2A. An exhaust-hole cover38provided to cover the exhaust hole21serves to change the flow direction of the exhaust gas. Specifically, with the exhaust-hole cover38, the exhaust gas out from the exhaust hole21is guided to flow in the axial direction of the cylinder20. When the piston25abruptly moves to its bottom dead center to drive the nail and collides with the bumper23, the bumper2deforms to absorb excess energy in the piston25.

The piston25is slidably and reciprocally provided in the cylinder20. The piston25divides an inner space of the cylinder20into an upper space above the piston25and a lower space below the piston25. The driver blade28extends downwards from a side of the piston25, the side being at the cylinder space below the piston25, to the tail cover9. The driver blade28is positioned coaxially with the nail setting position in the tail cover9, so that the driver blade28can strike against the nail during movement of the piston25toward its bottom dead center. When the piston25moves to its bottom dead center, the piston25collides against the bumper24and is stopped thereby. In this case, the bumper24absorbs a surplus energy of the piston25.

When the upper end of the combustion chamber frame11abuts the head cap13, the head cap13, the combustion chamber frame11, and the upper cylinder space above the piston25define a combustion chamber26. When the combustion chamber frame11is separated from the head cap13, a first flow passage S1in communication with an atmosphere is provided between the head cap13and the upper end portion of the combustion chamber frame11, and a second flow passage S2in communication with the first flow passage S1is provided between the lower end portion of the combustion chamber frame11and the upper end portion of the cylinder20. These flow passages S1, S2allow a combustion gas and a fresh air to pass along the outer peripheral surface of the cylinder20for discharging the mixture of the combustion gas and air through the exhaust port of the housing2. Further, the above-described intake port is formed for supplying a fresh air into the combustion chamber26, and the exhaust hole21is formed for discharging exhaust gas generated in the combustion chamber26. Fins27are also provided around the inner periphery of the combustion chamber26as ribs that protrude radially inward.

The fan14and the spark plug15are disposed within the combustion-chamber26, and the injection port18is open to the combustion chamber26. Rotation of the fan14performs the following three functions. First, the fan14stirs and mixes the air with the flammable gas as long as the combustion chamber frame11remains in abutment with the cylinder head11. Second, after the mixed gas has been ignited, the fan14causes turbulent combustion of the air-fuel mixture, thus promoting the combustion of the air-fuel mixture in the combustion chamber26. Third, the fan14performs scavenging such that the exhaust gas in the combustion chamber26can be scavenged therefrom and also performs cooling to the combustion chamber frame11and the cylinder20when the combustion chamber frame11moves away from the cylinder head11and when the first and second flow passages S1, S2are provided.

An ellipse-shaped stopper61is pivotally movably supported on a shaft62upstanding on the lower end side face of the connection member12to which coupled are the push lever10and the combustion chamber frame11. The connection member12, which is urged downwardly by means of the compression coil spring37, is normally in abutment with the stopper61, thereby placing the connection member12at its lowermost position. That is, the stopper61serves to position the push lever10and the combustion chamber frame11.

In a state when the tool1is not pressed against the workpiece W, the stopper61is held in a position so that the first flow passage S1has a channel height Lc. The stopper61is connected to both a temperature sensor29and a temperature switch control circuit45(seeFIG. 3) and is pivotally moved by, for example, an electromagnetic solenoid (not shown).

When the temperature of the combustion chamber frame11is below a critical value, the channel height of the first flow passage S1is held Lc whereas when the temperature of the combustion chamber frame11is higher than the critical value, the channel height of the first flow passage S1is held Lh (seeFIG. 2) greater than Lc. Above the critical temperature value, the rubber seal members19and24may be thermally deformed or damaged and thermal vacuum with a sufficient level may not be created.

FIG. 3is a block circuit diagram for controlling the fan14, the stopper61, and the spark plug15. The trigger switch6and the head switch16are connected to the input terminals of a first OR circuit41. The first OR circuit41has an output terminal connected to a first input terminal of a second OR circuit42. The second OR circuit42has an output terminal connected to a fan driver circuit43for rotating the fan14with the motor3. Accordingly, when at least one of the trigger switch6and the head switch16is turned on, the fan driver circuit43is energized to start rotating the motor3and the fan14connected thereto.

A fan timer44is connected between the output terminal of the first OR circuit41and a second input terminal of the second OR circuit42. The fan timer44is activated when both the trigger switch6and the head switch16are turned off. The fan timer44stops rotations of the fan14after elapse of a predetermined period of time from activation of the fan timer44.

The temperature sensor29is also connected to the output terminal of the first OR circuit41. Accordingly, the temperature sensor29performs the temperature sensing operation when at least one of the trigger switch6and the head switch16is turned on. The temperature switch control circuit45is connected to the temperature sensor29to receive a temperature signal from the temperature sensor29. The output of the temperature switch control circuit45is connected to one input terminal of an AND circuit46. Another input terminal of the AND circuit46is connected to the output terminal of the second OR circuit42,

When the temperature sensed by the temperature sensor29is lower than the critical value, the stopper driver circuit60is not operated so that the stopper61is held in a predetermined UP position. Therefore, the combustion chamber frame11, which is coupled to the push lever10through the connection member12, is held in a normal position where the channel height of the first flow passage S1is held Lc. On the other hand, when the temperature sensed by the temperature sensor29exceeds the critical value, the temperature switch control circuit45outputs a signal to the AND gate so as to enable the same. As a result, the stopper driver circuit60drives the stopper61to move to a predetermined DOWN position. The movement of the stopper61to the predetermined DOWN position causes the position of the combustion chamber frame11to a position lower than the normal position. Accordingly, the channel height of the first flow passage S1is held Lh as shown inFIG. 2.

A second AND circuit48has two input terminals connected to the trigger switch6and the head switch16. The output of the second AND circuit48is connected to a spark plug driver circuit50and the driver circuit50is in turn connected to the spark plug16. Accordingly, when both the trigger switch6and the head switch16are turned on, an ON signal is output to the spark plug driver circuit50, thereby igniting the spark plug16.

Next, operation of the above-described fastener driving tool1will be described.

In a static state where the tool1is not driven, the stopper61is held in the predetermined UP position as shown inFIG. 1. In this state, the push lever10is urged by the spring37to protrude lower than the bottom end of the tail cover9. The connection member12is in abutment with the stopper61, and hence the combustion chamber frame11coupled to the connection member12is in the predetermined upper position. At this time, a gap is formed between the top end of the combustion chamber frame11and the bottom of the head cap13, and another gap is formed between the combustion chamber frame11and the top end of the cylinder20. Thus, the first and second flow passages S1and S2are formed. The channel height of the first flow passage S1is set to be Lc as shown inFIG. 1. At this time, the piston25is held in its upper dead center in the cylinder20.

If a user grips the handle7and pushes the end of the push lever10against the workpiece W when the fastener-driving tool1is in this state, the push lever10moves upward against the opposing force of the spring37, causing the combustion chamber frame11, which is coupled to the push lever10, to rise to the position shown inFIG. 2. Raising the combustion chamber frame11to this position closes the first and second flow passages Si and S2and forms the combustion chamber26, which is hermetically sealed by the seal members19and24and thus closed off from the external air.

The gas cylinder5(fuel cell) is moved toward the head cap13by means of a cam (not shown) in accordance with the operation of the push lever10. Then, an injection rod39of the gas cylinder5is pressed against the gas cylinder connection portion formed in the head cap13, causing flammable gas to be injected through the injection port18into the combustion chamber26.

Further, when the combustion chamber frame11is further raised to its upper dead center in association with the upward movement of the push lever10, the head switch16is turned on. The driver circuit43of the motor3is then energized and the motor3drives the fan14to rotate. At the same time, the temperature sensor29is turned on and performs a temperature sensing operation. The flammable gas injected into the combustion chamber26is agitated and mixed with air in the combustion chamber26by the fan14rotating within the hermetically sealed combustion chamber26in cooperation with the fins27protruding inside the combustion chamber26. Here, the flammable gas stored in the gas cylinder5is a pressurized, liquid gas that becomes gasified when injected into the combustion chamber26. A measuring valve (not shown) is provided on the top end of the gas cylinder5for adjusting the amount of gas injected from the gas cylinder5through the injection port18.

After pressing the push lever10against the workpiece W, if the user pulls the trigger switch6provided on the handle7regardless of the temperature sensed by the temperature sensor29, then the spark plug15produces a spark for igniting and burning the gaseous mixture. The combusted gas expands to move the piston25downward and strike the nail in the tail cover9.

After striking the nail, the piston25contacts the bumper23, and the combusted gas is discharged from the cylinder20via the exhaust hole21. As described above, the check valve22is disposed in the exhaust hole21. This check valve is closed after the combusted gas has been discharged from the cylinder20and at the point that the interior of the cylinder20and the combustion chamber26have reached atmospheric pressure. While the gas remaining in the cylinder20and the combustion chamber frame11has just been combusted and is high in temperature, the heat from the combusted gas is absorbed by the inner walls of the cylinder20and combustion chamber frame11and by the fins27and the like, thereby rapidly cooling the gas. As a result, the pressure in the combustion chamber26drops to atmospheric pressure or below (thermal vacuum) and the piston25is drawn back to its initial upper dead center.

In this embodiment, in order to positively produce and maintain the thermal vacuum in the combustion chamber26, the combustion chamber frame11does not move downward immediately after the combustion. That is, the combustion chamber frame11is held in the upper dead center even if the lower end of the push lever10is separated from the workpiece W caused by the reaction force resulting from the nail striking operation, so that the first and second flow passages S1and S2are not created immediately after the combustion. To this end, the combustion chamber26is prohibited from being opened to atmosphere as far as the trigger switch6is held in on state.

When the user subsequently lifts the tool1so as to be separated from the workpiece W and then releases the trigger switch6(turns the trigger switch6off), the push lever10separates from the workpiece W, allowing the push lever10and the combustion chamber frame11to move downward by the urging force of the spring37and return to the position shown inFIG. 1. The downward movement of the combustion chamber frame11creates the first and second flow passages S1and S2. At this time, the fan14continues rotating for a prescribed time.

In the state shown inFIG. 1, the first and second flow passages S1and S2exist above and below the combustion chamber frame11so that the combustion chamber26is not hermetically sealed. In this state, the rotating fan14draws fresh air through an inlet formed in the top surface of the main housing2A and exhausts residual gas out through the discharge port formed in the bottom of the main housing2A, as indicated by arrows65through68, thereby scavenging the air in the combustion chamber26. Then, the fastener-driving tool1is returned to its initial state shown inFIG. 1.

In accordance with the turn-off operation of the trigger switch6and the downward movement of the combustion chamber frame11, the head switch16is turned off. Then, the fan timer44is activated, and after a prescribed time from the activation of the fan timer44, the fan14stops rotating. Stated differently, rotation of the fan14is not stopped even if the trigger switch6is turned off but continued for the prescribed time after the head switch16is turned off. Therefore, the air in the combustion chamber26can be scavenged through the first and second flow passages S1and S2by drawing fresh air from through the intake port and discharging the exhausts residual gas out through the discharge port.

After repetitive nail driving operations, the stopper driver circuit60does not operate as far as no signal is available from the temperature switch control circuit45. Thus, the stopper61is held in the upper position. Scavenging and cooling operations are performed with the same amount of fresh air each time the nail driving operation is carried out.

Continuous nail driving operations result in temperature rise in the combustion chamber frame11and the cylinder20. When the temperature of the combustion chamber frame11becomes higher than the critical value, the temperature switch control circuit45outputs a signal to the stopper driver circuit60through the first AND circuit46. As far as the fan14is rotating, the stopper61is controlled to be in a DOWN position. At this time, the channel height of the first flow passage S1is set to Lh and the subsequent nail driving operation is performed while maintaining this channel height of the first flow passage S1. Since Lh is greater than Lc, the cross-sectional area of the first flow passage S1increases in the state inFIG. 2as compared with the state inFIG. 1. Accordingly, with the state shown inFIG. 2, an amount of fresh air introduced into the combustion chamber26is increased, thereby enhancing the cooling effect on the combustion chamber frame11and the cylinder20. It should be noted that the amount of air increases in proportion to a cross-sectional area of the channel through which the air flows. Accordingly, the nail driving operations need not be interrupted for the purpose of cooling the tool1but can be continued.

When the sensed temperature falls below the critical value, no signal is output from the temperature switch control circuit45, and the stopper61returns to the initial UP position and the channel height of the first flow passage S1returns to Lc.

As described, according to the embodiment of the invention, the channel height of the first flow passage S1is changed from Lc to Lh (Lc<<Lh) or vice versa depending on the temperature of the combustion chamber frame11and/or the cylinder20. While it may be possible to fixedly set the channel height of the first flow passage S1to Lh without using the stopper61, this strategy is not realistic because the stroke of the push lever10increases by (Lh—Lc). The increased stroke of the push lever10opposes to the easy-to-use policy and urges the user to push the tool1a longer distance against the workpiece W. The present invention has adopted a strategy not to impose superfluous work upon the user by setting the stroke of the push lever10to a proper minimum length which can be changed depending on the temperature of the combustion chamber frame11or the cylinder20.

While the invention has been described in detail with reference to a specific embodiment thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein.

For example, while the embodiment describes the fastener driving tool in which the trigger switch6is turned on and off each time the nail driving operation is performed, the present invention is applicable to a fastener driving tool of a continuous type in which the fasteners are driven continuously by holding the trigger switch6in on state and repeatedly carrying out the push-and-release operations with respect to the workpiece. The block circuit diagram shown inFIG. 3is also available for the continuous type driving tool. When at least one of the head switch16and the trigger switch6is turned on, the fan14starts rotating and the temperature of the combustion chamber frame11or the cylinder20can be sensed. In accordance with the sensed temperature, the channel height of the first flow passage S1is determined by means of the stopper61. Due to the operation of the fan timer44, this channel height is maintained even if both the head switch16and the trigger switch6are turned off.

Further, the embodiment describes a temperature control based on a temperature detected at a particular point, the temperature control may be implemented based on a plurality of detections of temperature at various points, such as points on not only the combustion chamber frame but also the housing or other portions.

While the embodiment describes the electromagnetic solenoid as a means for driving the stopper61, the stopper may be formed from shape memory alloy to function as the temperature sensor as well.