pneumatic tool (no͝omăt`ĭk), instrument activated by air pressure. Pneumatic tools are designed around three basic devices: the air cylinder, the vane motor, and the sprayer. The air cylinder contains a piston that is pushed the length of the cylinder by compressed air and returned by air or by a spring. In a common type of pneumatic hammer, called a hammer drill, the piston is not connected to anything but runs freely in the cylinder. At one end of the power stroke the piston hits the top of the drill; an additional mechanism in the hammer drill turns the bit slightly after each blow. Light hand-held pneumatic hammers are used for chipping paint from metal, carving rock, and riveting. Much larger hammers are used in mining and quarrying; some of them are mounted on mechanically propelled vehicles. Hammers designed to clamp onto the side of a vat or other container are used to pack sand or concrete, the vibration causing the contents to settle. The vane motor is better adapted to rotary motion, and it can run at high speeds. In this motor, sliding vanes radiate from the end of a shaft extending into a cylinder. The center of the shaft is not at the center of the cylinder; consequently the pockets formed by the vanes and the cylinder wall are unequal in size. Air admitted through an opening in the cylinder wall at a point where the pockets are small tends to push the vanes around to the point where the pockets are large. There the air escapes through a second opening in the cylinder wall. The shaft is connected without gearing to wire brushes, drills, screwdrivers, and grinders, where high speeds are required; speeds of 10,000 to 20,000 rpm are common. With gearing, lower speeds and greater torque, or twisting force, are achieved for screw-thread tappers and for other heavy-duty applications. With suitable gearing the vane motor can drive a type of hoist to wind a cable or chain around a drum. The pneumatic sprayer applies not only paint but many other materials, such as cement and plaster in construction work. Insecticides, molten metal, and plastic fibers are also sprayed. Paint sprayers, also called air brushes, do faster work, spread a more even coat, and penetrate cracks better than do brushes wielded by hand. In a pneumatic sprayer, paint is drawn from its container by the reduced pressure created by a stream of air passing through a pipe connected with the container. The stream then entrains the paint and sprays it.