Another small class of pneumatic drills employ a three-cyl inder radial air motor, the three cylinders being in one plane with one crank provided for all three cylinders. The power from the crank is transmitted through a spindle and gears to the point where the drill chuck is attached. The main spindle also serves as a valve controlling the air to the three cylinders, greatly simplifying the construction of the tool. These tools are used for light drilling in metal, in the automobile assembly plants for running up nuts, and for running in screws. They are equipped with either feed screw, spade handle or breast plate. The close quarter type of pneumatic drill uses a two-cylinder, double acting motor to give a flat and compact construction. The power is transmitted from the crank-shaft to the spindle by a train of gears. A feed screw is located at the end of the spindle and can be turned by means of the ratchet handle. This type of drill is used for drilling, reaming and tapping in cramped spaces where the usual drill cannot be employed.
The pneumatic grinder is similar in general construction to the air drill except that a higher speed motor, operating at 3,00o to 6,000 r.p.m. is used. The motor is directly connected to a spindle upon which is mounted the grinding wheel. These tools are used in all kinds of service where a portable grinding tool, buffer or polisher is needed and are widely used in foundries and machine shops, monumental stone shops and automobile body plants.
Pneumatic diggers, of the same oper ating principle as a rivetter, are made in two styles. A short handle type is used in tunnel work or in close quarters and an extension-handle type for trench or shaft. Pneumatic diggers are a recent development for work where it is desired to replace the hand pick. The tools are used for loosening up clay, hard ground, etc. The pneumatic digger consists of a hammer having a piston which strikes against a blade or scoop. The blows of the piston drive the blade into the ground, prying it loose. One man with such a tool is the equal of six men using ordinary hand picks.
The removal of solid rock in excavation for foundations, trenches, highways, in mining and many other proj ects calls for the use of a vast number of compressed-air rock drills. These tools are made in a wide variety of sizes for all classes of work, depending on the size and depth of the hole to be drilled, the hardness of the rock and other factors. In the "Jackhammer" type of rock drill, compressed air enters to a throt tle valve in the handle and then passes to a flapper type of plate valve, which controls the movement of the piston. On the forward or downward movement of the piston it strikes against the shank of the drill steel, and imparts a cutting action to the points of the steel. On the back stroke of the piston it slides over a fluted rifle bar which turns the piston and causes it to turn the drill steel. Rotation of the drill steel is desired in order that the cutting edges on the end of the drill steel may be put in a new position with regard to the rock being drilled. A ratchet mechanism en sures rotation in one direction. To blow out the cuttings and dust from the hole in the rock, air is passed down through the rifle-bar and piston and through the drill steel to the cutting face.
These tools make it possible to drill holes in rock for inserting the explosives which blast out rock. One rock drill bores ioo to 15o ft. of hole in rock in an eight-hour day, while the best a man can do using a hand hammer and steel is not more than 8 to 10 ft. a day. Rock drilling requires the use of pointed drill
steels which must be resharpened as the cutting edges become worn and dulled by the rock. This is done by compressed-air-operated drill steel sharpening machines in which after the drill steel is heated and inserted quickly shapes and sharpens the steel by means of dies many times faster than is possible by hand methods.
The paving breaker is a tool somewhat similar in construction to the rock drill and is used in demolition work such as breaking up pavement, tearing out concrete walls, foundations, etc. This tool uses a pointed chisel or steel, but it is not rotated. The absence of the rotation mechanism simplifies the general construction of the paving breaker. Paving breakers are extensively used by public utilities for making openings in the pavement prior to installing or repairing sewer pipes and connections. They are usually operated from a portable air compressor, mounted on a motor truck, which can be quickly taken to the place where the work is to be done. One man operating one tool is able to break out more pavement in a given time than I 2 to 15 men using hand sledges and steels.
These are made in a range of sizes to handle loads from 500 lb. up to 20,000 pounds. This style of hoist is equipped with a high-powered air motor of the f our cylinder type which is geared to a hoisting drum. A feature of these hoists is the automatic brake which acts to hold the load at any point after the air pressure is shut off. When the motor is operating, air passes to the brake through a connecting tube releasing it. When the air is shut off at the throttle, the brake is automatically applied. These air hoists are widely used in industrial plants for handling material too heavy to be easily lifted by one man. In machine shops they lift and place heavy parts on lathes, milling machines and planers. (See MECHANICAL HANDLING.) Air hoists are particularly suited for foundry work because the operation is not affected by dust, dirt or fumes, and they are not damaged by overloading. Another type is the portable hoist, which can be mounted wherever convenient ; i.e., to a post or timber. It is used for pulling cars, skidding timbers, pulling a scraper on back-fill work, and dozens of other applications.
Special pneumatic tools have been developed for a number of applications in railroad track construction and maintenance. There are pneumatic tie tampers for tamping ballast under ties, pneumatic spike pullers, spike drivers, rail drills, pneumatic wrenches, rail bonding drills and others. Tamping ballast with pneumatic tampers is much easier than swinging a pick or tamping with a hand bar. The air tampers strike a uniform blow all day long and produce a uniform track. In removing old rail, the old spikes must be pulled. A compressed-air-operated spike puller draws 10 to 15 spikes per minute. One such machine does as much work as eight to ten men using hand claw bars. The pneumatic wrench removes the nuts or runs up nuts on the joint bolts much faster than is possible with a hand wrench. The air-operated spike driver puts down the spikes in a fraction of the time required by a hand hammer. A pneumatic rail drill bores a in. hole
through the web of a rail in from 25 to 3o seconds. Small holes for signal wires are drilled with pneumatic rail bonding drills at the rate of 6o per hour. (R. A. Lu.)