Fastener driving tool with improved pneumatic piston retaining means

Claim 1

In a fastener driving tool of the type using an impulse of pressurized fluid for returning a piston from a displaced position to a normal position in which the piston is to be retained, said piston having a single effective diameter during its movement between said displaced and normal positions, said single effective diameter being located at an upper portion of said piston, a cylinder in which the piston is slidably mounted, said cylinder having a first end portion occupied by the piston in its normal position and a second end portion occupied by the piston in its displaced position, said cylinder having at its type using an impulse of a pressurized fluid for returning a piston from a displaced position to a normal position in which the piston is to be releasably retained, said piston having a single effective diameter during its movement between said displaced and normal positions, said single effective diameter being located at an upper portion of said piston, a cylinder in which the piston is slidably mounted, said cylinder having a first end portion occupied by the piston in its normal position and a second end portion occupied by the piston in its displaced position, said cylinder having at its first end portion and spaced from an intermediate portion of the cylinder through which the piston moves in moving to the displaced position an inner wall surface with greater and lesser diameter portions, the remaining inner surface of the cylinder in said intermediate portion being of a substantially uniform diameter generally corresponding to said single diameter, said cylinder having port means at said second end portion, a pair of spaced sealing means on the piston slidably engaging the inner wall surface of the cylinder and defining a fluidreceiving space therebetween, at least one of the sealing means being expansible and compressible, an air return space communicating with the interior of the cylinder at the second end portion through the port means, first stop means for arresting movement of the piston in said displaced position with the piston adjacent the port means, return means for providing pressurized fluid to said air return space which passes through said port means to provide said impulse of pressurized fluid to return the piston to its first end portion and spaced from an intermediate portion of the cylinder through which the piston moves in moving to the displaced position an inner wall surface with greater and lesser diameter portions, the remaining inner surface of the cylinder in said intermediate portion being of a substantially uniform diameter generally corresponding to said single diameter, a pair of spaced sealing means on the piston slidably engaging the inner wall surface of the cylinder and defining a fluid receiving space therebetween, at least one of the sealing means being expansible and compressible, piston return means for supplying an impulse of pressurized fluid for returning the piston from the displaced position to the normal position, stop means on the tool for arresting return movement of the piston in the normal position with the pair of spaced sealing means engaging the different diameter portions of the inner wall surface of the cylinder, and air-supplying means for supplying pressurized fluid to said space between the pair of sealing means which is effective by virtue of the different diameter portions of the inner wall surface of the cylinder and the corresponding different effective diameters of the sealing means to provide a fluid bias urging the piston toward the stop means which releasably retains the piston at the first end of the cylinder in its normal position.