Portable combustion powered, or so-called IMPULSE.RTM. brand tools for use in driving fasteners into workpieces are described in commonly assigned patents to Nikolich U.S. Pat. No. Re. 32,452, and U.S. Pat. Nos. 4,552,162, 4,483,473, 4,483,474, 4,403,722, and 5,263,439, all of which are hereby incorporated herein by reference. Similar combustion powered nail and staple driving tools are also available commercially from ITW-Paslode of Lincolnshire, Ill. under the IMPULSE.RTM. brand.
Such tools incorporate a tool housing enclosing a small internal combustion engine. The engine is powered by a canister of pressurized fuel gas, also called a fuel cell. A battery-powered electronic power distribution unit produces the spark for ignition, and a fan located in the combustion chamber provides for both an efficient combustion within the chamber, and facilitates scavenging, including the exhaust of combustion by-products. The engine includes a reciprocating piston with an elongate, rigid driver blade disposed within a piston chamber of a cylinder body.
A valve sleeve is axially reciprocable about the cylinder and, through means of a linkage, moves to close the combustion chamber when a work contact element at the end of the linkage is pressed against a workpiece. This pressing action also triggers a fuel metering valve to introduce a predetermined volume of fuel gas into the closed combustion chamber.
Upon the pulling of a trigger switch, which causes the ignition of a charge of gas in the combustion chamber of the engine, the piston and driver blade are shot downward so as to impact a positioned fastener and drive it into the workpiece. As the piston is driven downward, a displacement volume enclosed in the piston chamber below the piston is forced to exit through one or more exit ports provided at the lower end of the cylinder. After impact, the piston then returns to its original, or "ready" position through differential gas pressures within the cylinder. Fasteners are fed magazine-style into the nosepiece, where they are held in a properly positioned orientation for receiving the impact of the driver blade.
Combustion powered tools may be contrasted from conventional powder activated technology (PAT) tools, which employ a gunpowder powered cartridge to propel a driving member to drive a fastener into a workpiece. PAT tools generate an explosion in a combustion chamber which creates high pressures for propelling the driving member at a high velocity toward the fastener. The relatively small volume of the combustion chamber and the explosive combustion combine to create a rapid acceleration of the driving member for the velocity required for achieving proper fastener driving. In contrast, combustion powered tools typically provide a much slower acceleration of the driving member. This is due to the relatively large size of the combustion chamber, and to the requirement of the preferred fuel to obtain atmospheric oxygen for combustion (the powder in PAT tools incorporates its own oxygen). Thus, in combustion powered tools, the combustion event is a relatively gradual process. Commercially available combustion powered tools have relatively short cylinder bodies, so that the driving member is incapable of achieving velocities which are comparable to those of PAT tools.
A high velocity combustion powered tool of the type described above and featuring an extended piston chamber or cylinder is the subject of a co-pending patent application serial number 08/536,854, filed Sep. 29, 1995. The extended cylinder increases the stroke of the piston, thereby allowing for increased piston velocity and transfer of power from the driver blade to the fastener. In one embodiment, the extended length also allows an operator to stand generally upright while driving fasteners which are at foot level.
A number of factors influence piston velocity, including piston diameter and stroke, but these factors are fixed by design in a particular tool. One way to vary the power of a combustion powered tool is by means of controlling the speed of the fan in the combustion chamber, as described in copending U.S. application Ser. No. 08/337,289, filed Nov. 10, 1994. A circuit is used to vary fan speed, and increased fan speed produces additional power. However, in most conventional combustion powered tools, the piston velocity is fixed by design.
In conventional combustion powered tools, the fixed piston velocity prevents an operator from controlling the driving depth of the fastener being driven into a particular type of workpiece or substrate. In addition, depending on the composition of the workpiece or substrate, the lack of velocity control may prevent an operator from obtaining a desired consistent driving depth. An identical driver blade velocity, when translated into a force applied to a fastener being driven into wood, for instance, will result in a different depth when applied to a fastener being driven into a steel beam. Such velocity will result in still another depth when applied to a fastener being driven into sheet metal being fastened to a roof truss. Thus, depending on the design of the tool, there may be insufficient power to properly drive a fastener into all desired workpieces.
An additional problem, limited primarily to the high velocity, extended cylinder tools, concerns stability of the tool during operation. The increased stroke of extended length combustion tools, used to increase both velocity and power transfer, also increases the delay between combustion and the driving of a fastener into the substrate. This increased delay can reduce the amount of control and applied power of the tool, because the tool recoils in reaction to the combustion, causing the tool nosepiece to move with respect to the workpiece before the fastener is actually driven. Drawbacks of such operation are misaligned or incompletely driven fasteners.