Combustion-engined setting tool

A combustion-engined setting tool (10) for driving fastening elements in a workpiece includes a combustion chamber (11), a drive piston (15) displaceable in a piston guide (17) and driven by expanding gases produced in the combustion chamber (11), and a device for returning the drive piston (15) in its initial position as a result of pressure difference between chambers formed on opposite axial sides of the piston head (115), and including a pump (20).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a combustion-engined setting tool for driving fastening elements in a workpiece and including a combustion chamber for fuel, a piston guide, a drive piston displaceable in the piston guide, having a piston head and driven by expanding gases produced in the combustion chamber, and a device for returning the drive piston in its initial position as a result of pressure difference between chambers formed on opposite axial sides of the piston head.

2. Description of the Prior Art

Setting tools of the types described above can operate on gaseous or evaporated liquid fuels which are combusted in a combustion chamber and thereby drive a drive piston for driving fastening elements. After completion of a setting process, the drive piston should be returned to its initial position adjacent to the combustion chamber, so that the setting tool is ready for a next setting process.

German Publication DE 195 09 763 A1 discloses a combustion-engined setting tool having a drive piston displaceable in a piston guide and driven by propellant gases and which drives a fastening element in a constructional component with the piston shaft. The return of the drive piston in its initial position is effected by an elastomeric compression or tension spring.

The drawback of the setting tool disclosed in the German publication consists in that the drive piston at a certain state of wear does not return completely in its initial position, which leads to reduction of the available setting energy. On the other hand, particles of the elastomeric spring can become loose, interfering with the setting tool functions.

European Publication EP 0 056 989 A1 discloses a further combustion-engined tool with a piston displaceable in a piston guide and in which the drive piston is returned to its initial position by a pressure difference (or a differential pressure) between the environmental pressure that acts on the side of the drive piston remote from the combustion chamber, and the pressure created in the combustion chamber.

The drawback of the tool disclosed in the European Publication consists in that a faulty position of the piston can occur when, e.g., the friction of the drive piston increases as a result of contamination, and the piston does not return completely in its initial position, or when as a result of a too small difference between the environmental temperature and the power tool temperature, the pressure difference is not sufficient for a complete return of the drive piston in its initial position.

Accordingly, an object of the present invention is a setting tool in which a low-wear and reliable return of the drive piston in is initial position is possible.

SUMMARY OF THE INVENTION

This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a setting tool in which the returning device includes a pump. The pump produces a differential pressure for returning of the drive piston or at least reinforces the differential pressure. The provision of the pump insures always a complete return of the drive piston in its initial position. On the other hand, a high setting frequency can be achieved as a result of a more rapid return of the drive piston in its initial position. The pump can be a single source for producing the differential pressure, or can reinforce a thermally produced differential pressure.

Advantageously, the pump is formed as an electrical suction pump for producing a suction air flow and is connected with the combustion chamber. The pump produces underpressure in the combustion chamber, so that environmental pressure, which acts on a side of the drive piston remote from the combustion chamber, can return the drive piston to its initial position.

It is further advantageous when there is provided a control unit for controlling the pump based on data produced by sensors. This insures an efficient operation of the pump and energy saving.

Advantageously, the sensors include a piston sensor that detects the position of the drive piston, so that the control unit can turn the pump off after the drive piston has been returned in its initial position.

It is further advantageous when the sensors includes a temperature sensor, so that the control unit actuates the pump only when the temperature difference between the tool temperature or the combustion chamber temperature and the environmental temperature is below a certain value. This insures a complete return of the drive piston in its initial position. This also permits to achieve a high energy efficiency of the piston returning device.

It is also advantageous when an electrically controlled check valve is provided between the pump and the combustion chamber and which is controlled by the controlled unit. The check valve is closed by the control unit when the drive piston is returned in its initial position. This insures that pressure waves, which are produced in the combustion chamber during a setting process, do not reach the pump and damage it. Further, at the end of a setting process which is detected, e.g., by the piston sensor or is determined by time-control-means, the control unit can open the check valve, connecting the pump and the combustion chamber. Thereby, gases are aspirated from the combustion chamber, producing vacuum therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A setting tool10according to the present invention, which is shown in the drawing, can be operated with a fuel gas or an evaporated liquid fuel and includes a housing13and a setting mechanism located in the housing. The setting mechanism drives a fastening element such as nail, bolt, etc. in a workpiece W when the setting tool10is pressed with its bolt guide16against the workpiece and is actuated. For actuation of the setting tool10, there is provided, in addition to a safety switch in form of a contact switch, a trigger switch42. The trigger switch42is located on a handle of the setting tool10.

The setting mechanism includes, among others, a combustion chamber11, a piston guide17in which a drive piston15is displaceably supported, and the bolt guide16in which a fastening element can be displaced. The fastening element can be driven in a workpiece with a forward-movable, setting direction-side, end of the drive piston15. The bolt guide16adjoins, in the setting direction, the piston guide17. The drive piston15has, at its end adjacent to the combustion chamber11, a piston head115formed as a piston plate sealingly engaging the inner wall of the piston guide17upon displacement of the drive piston15, and separating, in the axial direction defined by the drive piston15, a first chamber and a second chamber from each other.

In the embodiment shown in the drawing, an ignition device14such as, e.g., a spark plug, is located in the combustion chamber14for igniting an oxidant-fuel mixture fed into the combustion chamber11for effecting a setting process. Feeding of the fuel into the combustion chamber14takes place from a fuel reservoir12or a fuel source through a fuel conduit19. In the embodiment shown in the drawing, a metering device18, e.g., a mechanical or electronic metering valve, is located in the fuel conduit19. A mechanical or electronic control device, not shown in the drawing, can regulate, via the metering device18, feeding of fuel to the combustion chamber11. During a setting process, the trigger switch42actuates the ignition device14that ignites the air-fuel mixture in the combustion chamber11. The expanding gases drive the drive piston15in the setting direction40, i.e., in the direction of the bolt guide16. The air in the first chamber between the piston head115and an end of the piston guide17which adjoins the bolt guide16, which is displaced by the drive piston15, can be exhausted through the exhaust41. The expanding combustion gases can likewise be evacuated from the combustion chamber11through the exhaust42as soon as the piston head115passes past the exhaust opening, whereby the exhaust opening becomes connected with the second chamber between the piston head115and the combustion chamber11. After the combustion gases have been evacuated and after the exhaust has been closed, as a result of cooling of the combustion chamber11, a differential pressure is produced between the first chamber on the side of the piston head115remote from the combustion chamber11, and the second chamber on the side of the piston head115adjacent to the combustion chamber11.

The setting tool10further includes a pump20of the device for returning the drive piston15. The pump20is formed, e.g., as a suction pump and includes a piston22driven by an electrical drive21and displaceable in a cylinder23. A connection rod24mechanically connects the piston22with the electrical drive23. The electrical energy for an electrical drive21is supplied from an electrical energy source32available in the setting tool10. The electrical energy source32can be, e.g., in form of an accumulator or a batter. A conduit27communicates the pump20with the combustion chamber11. In the conduit27, there is arranged a check valve28that is electrically controlled by a control unit30which also controls the operation of the pump20. The check valve28closes the conduit27in its locking position shown in the drawings, and provides for a pneumatic communication between the combustion chamber11and the pump20in the open position of the check valve28.

The cylinder23of the pump20has an inlet36in which a conduit27opens, and an outlet35open toward the environment and through which gases aspirated from the combustion chamber11are released into the environment. At the inlet36, there is arranged a valve26that enables entry of gases in the cylinder23. The valve26blocks the flow of air in an opposite direction from the cylinder23into the combustion chamber11. At the outlet35, there is provided a valve25that enables the flow of gases or air from the cylinder23and block the flow of environmental air into the cylinder23.

The control unit30is connected with a plurality of sensors at least one of which is a piston sensor31for determining the piston position. Preferably, the plurality of sensors includes also at least one temperature sensor33for determining a temperature difference between the power tool or combustion chamber temperature and the environmental temperature. Based on measurement data communicated by the piston sensor31and the temperature sensor33, the control unit30controls both the pump20and the check valve28. The control unit30actuates the pump20when the environmental temperature communicated by the temperature sensor33is so high that the temperature difference between the environmental temperature and the combustion chamber temperature falls short of a predetermined threshold, and the piston sensor31determines that the drive piston15is not in its initial position. Then, the control unit30displaces the check valve28to its open position, enabling communication between the pump20and the combustion chamber11. The pump20produces a suction flow from the combustion chamber11, increasing the pressure difference between the two chambers on opposite axial sides of the piston head115. Thereby, the drive piston15is referenced to its initial position. The drive piston is quasi “aspirated” to the combustion chamber11. As soon as the piston sensor31communicates a signal that the drive piston15is again in its initial position, the control unit30turns off the pump20and displaces the check valve28in its locking position (see the drawing figure).