A Treatise On The Principles And Practice Of Dock Engineering

6o DOCK ENGINEERING. but the action is the reverse of ordinary usage. The piston (fig. 24) is kept stationary and in continuons contact with the pile head, while the blow is administered by the lower end of the heavy cast-iron cylinder, moving up and down under steam pressure. The movement of the cylinder is guided by rollers behind the main leaders, and the arrangement involves a sliding steam feed pipe (which is a special feature of the system), with a flexible rubber connection to the supply pipe from boiler. At the head of the cylinder is a two-way cock, regulated by a double-armed lever, which, when pulled down on one side, exhausts the cylinder, and on the other admits fresh steam. There is also a double-action machine, in which steam pressure is applied alternately to each side of the piston, thus increasing the force of the blow. The stroke is about 3 feet, and blows can follow one another with great rapidity. From observations of a Whitaker machine in single action with 80 to 90 Ibs. steam pressure, the author finds that 35 blows per minute can be delivered at full stroke, or 60 blows per minute with a stroke of 12 inches. In double action 45 blows per minute were delivered with a stroke of 2 feet. The weight of the cylinder was 1 ton. A similar machine, known as the Cram Pile Driver, manufactured in America, bas a hammer fastened to the lower end of the cylinder, and is supplied with steam through a hollow piston-rod. The original Nasmyth hammer is also used, in which the hammer is attached to the piston, the cylinder remaining stationary and being confined between the upper ends of two vertical and parallel I or channel beams, the lower ends of which enclose a hollow, conical bonnet casting, fitting over the head of the pile. This cast- ing is open at the top, and through it the blow is administered. When steam is admitted to the cylinder, the hammer is lifted about 30 to 40 inches and then allowed to fall, generally by the automatic opening of an escape valve. Piling machines of the steam-hammer type consume from 1 to 2 tons of coal per day, working with a boiler pressure of 50 to 75 Ibs. per square inch, and can deliver blows at the rate of about 60 per minute. They need three men in attendance. The disadvantage attending them is the liability of the pilehead to crushing or brooming, which, combined with the escape of moisture from the cylinder, reduces it, if in the least degree soft or sappy, to a saponaceous condition. The effect of this is to materially diminish the force of the blow, as is evidenced by the following particulars of the driving of a green Norway pile by a Nasmyth steam hammer * :— The 3rd foot of penetration required..........................5blows. » 4th „ „ „ 15 ,, ,, 5th „ ,, „ 20 „ ,, lOth ,, ,, .........................73 ,, 12th ,, ,, ,, 153 „ •> 14th „ „ „ 684 * Whittemore on “ The Effieieney of Pile Driving,” Min. Proc. Inst. C.E., vol. Ixxvi., p. 399.