When the rope is wound rp by steam, the maximum speed is from 6 to 14 blows per minute, depending upon the distance the hammer falls. The speed can not be increased by the skill of the operator, although it could be by making the nipper block heavier.
2. The method by using a or friction-drum, as it is often called, consists in attaching the rope permanently to the staple in the top of the hammer, and dropping the hammer by setting free the winding drum by the use of a friction-clutch. The advan tages of this method are (a) that the hammer can be dropped from any height, thus securing a light or heavy blow at pleasure; and (b) that no time is lost in waiting for the nipper to descend or in adjusting the trip.
When the rope is wound up by steam, the speed is from 20 to 30 blows per minute, but is largely dependent upon the skill of the man who controls the friction-clutch. The hammer is caught on the rebound, is elevated very rapidly, and hence the absolute max imum speed is attained. The rope, by which the hammer is elevated, retards the falling weight; and hence, for an equal effect, this form requires a heavier hammer than when the nipper is used. Although the friction-drum pile-driver is much more efficient, it is not as generally used as the nipper driver. The former is a little more expensive in first cost.
As regards frequency of use, the next machine is probably the steam-hammer pile-driver. It consists essentially of a steam cylinder (stroke about 3 feet), the piston-rod of which carries a striking weight of 3,000 to 5,000 pounds. The steam-cylinder is fastened to and between the tops of two I-beams about 8 to 10 feet long, the beams being united at the bottom by a piece of iron in the shape of a frustum of a cone, which has a hole through it. The under side of this con necting piece is cut out so as to fit the top of the pile. The striking weight, which works up and down between the two I beams as guides, has a cylindri cal projection on the bottom which passes through the hole in the piece connecting the feet of the guides and strikes the pile. The steam to operate the hammer is conveyed from the boiler through a flexible tube. Fig. 87 shows one of the latest forms of steam-hammer.
The whole mechanism can be raised and lowered by a rope passing over a pulley in the top of the leaders. After a pile has been placed in position for driving, the machine is lowered upon the top of it and entirely let go, the pile being its only support. When steam is
admitted below the piston, it rises, carrying the striking weight with it, until it strikes a trip (on the back side of the hammer), which cuts off the steam, and the hammer of the newer form descends under its own weight and the pressure of the steam. At the end of the down stroke the valves are again automatically reversed, and the stroke repeated. By altering the adjustment of this trip-piece, the length of stroke (and thus the force of the blows) can be increased or dimin ished. The admission and escape of steam to and from the cylinder can also be controlled directly by the attendant, and the number of blows per minute is increased or diminished by regulating the supply of steam. The machine can give 60 to 80 blows per minute.
However, the effectiveness of a blow does not depend alone upon its energy. A considerable part of the energy is invariably lost by the compression of the materials of the striking surfaces, and the greater the velocity the greater this loss. An extreme illustration of this would be trying to drive piles by shooting rifle-bullets at them. A 1-ton hammer falling 45 ft. has 10 times the energy of a 1i-ton hammer falling 3 ft., but in striking, a far larger part of the former than of the latter is Lost by the compression of the pile head. In constructing the foundation of the Brooklyn dry-dock, it was prac tically demonstrated that "there was little, if any, gain in having the fall more than 45 feet." The loss due to the compression depends upon the material of the pile, and whether the head of it is bruised or not. The drop-hammer, using the pile-hood and the friction-drum, can drive a pile against a considerably harder resistance than the steam-hammer.