AIR-DRIVEN HAMMERS
The ordinary rock drill is really a pneumatic hammer. The tool, which is chisel-shaped, is used as a hammer to pound a hole in the rock. The tool is driven up and down or in and out by compressed air bearing alternately on opposite sides of a piston. The elasticity of the compressed air acts as a cushion to relieve the machine from shock. The drill is mounted upon a tripod in such a way that it may be operated in any direction.
The pneumatic riveter is another form of compressed-air hammer. The machine is held in the hands, and the tool, which is rapidly reciprocated by air pressure, pounds the red-hot shank of a rivet. While this is being done an assistant holds a hammer against the head of the rivet. However, there is a type of riveter which has a U-shaped frame. One arm of the frame carries the reciprocating tool while the other reaches around and bears against the head of the rivet.
All sorts of tools have been built in which a small air-driven motor furnishes the motive power. There are hand drills in which the tool is revolved by a set of pistons, shears for shearing sheep which operate somewhat on the principle of hair clippers, and pneumatic chisels used for chipping stone. Pneumatic motors are widely used in operating cranes and air hoists. They consist of a simple cylinder and piston, and are used for short, direct lifts of all sorts.