PILE-DRIVING MACHINES. Pile-driving machines may be classified according to the character of the driving power, which may be (1) a falling weight, (2) the force of an explosive, or (3) the erosive action of a jet of water. Piles are sometimes set in holes bored with a well-auger, and the earth rammed around them. This is quite common in the construction of small highway bridges in the prairie States, a 10- or a 12-inch auger being generally used. The various pile-driving machines will now be briefly described and compared.
The usual method of driving piles is by a succession of blows given with a heavy block of wood or iron— called a ram, monkey, or hammer which is carried by a rope or cable passing over a pulley fixed at the top of an upright frame and allowed to fall freely on the head of the pile. The machine for doing this is called a drop-hammer pile-driver, or a monkey pile-driver— usually the former. The machine is generally placed upon a car or a scow.
The frame consists of two uprights, called leaders, from 10 to 60 feet long, placed about 2 feet apart, which guide the falling weight in its descent. The leaders are either wooden beams or iron channel beams, usually the former. The hammer is generally a mass of iron weighing from 500 to 4,000 pounds (usually about 2,000) with grooves in its sides to fit the guides and a staple in the top by which it is raised. The rope employed in raising the hammer is usually wound up by a steam engine placed on the end of the scow or car, opposite the leaders.
A car pile-driver is made especially for railroad work, the leaders resting upon an auxiliary frame, by which piles may be driven 14 to 16 feet in advance of the end of the track; and the frame is pivoted so that piles may be driven on either side of the track. This method of pivoting the frame carrying the leaders is also sometimes applied to a machine used in driving piles for foundations.
In railroad construction, it is not possible to use the pile-driving oar with its steam engine in advance of the track; hence, in this kind of work, the leaders are often set on blocking and the hammer is raised by horses hitched directly to the end of the rope. Portable
engines also are sometimes used for this purpose. Occasionally the weight is raised by men with a windlass, or by pulling directly on the rope.
A machine used for driving' sheet piles differs from that described above in one particular, viz.: it has but one leader, in front of which the hammer moves up and down. With this construction, the machine can be brought close up to the wall of a coffer-dam (* 718 and 4 804), and the pile already driven does not interfere with the driving of the next one.
There are two methods of detaching the weight, i.e., of letting the hammer fall: (1) by a nipper, and (2) by a friction-clutch.
1. The nipper consists of a block which slides freely between the leaders and which carries a pair of hooks, or tongs, projecting from its lower side. The tongs are so arranged that when lowered onto the top of the hammer they automatically catch in the staple in the top of the hammer, and hold it while it is being lifted, until they are disengaged by the upper ends of the arms striking a inclined surfaces in another block, the trip, which may be placed between the leaders at any elevation, according to the height of fall desired.
With this form of machine, the method of operation is as follows: The pile being in place, with the hammer resting on the head of it and the tongs being hooked into the staple in the top of the hammer, the rope is wound up until the upper ends of the tongs strike the trip, which disengages the tongs and lets the hammer fall. As the hoisting rope is unwound the nipper block follows the hammer, and, on reaching it, the tongs automatically catch in the staple, and the preceding operations may be repeated. This method is objectionable owing to the length of time required (a) for the nipper to descend after the hammer has been dropped, and (b) to move the trip when the height of fall is changed. Some manufacturers of pile-driving machinery remove the last objection by making an adjustable trip which is raised and lowered by a light line pns.sing over the top of the leaders. This is a valuable improvement.