A drop-hammer, as used in driving piles, usually consists of a solid casting, which is raised by means of a rope and allowed to drop upon the head of the pile. The hammer slides in guides upon the leads and should be so shaped as to give it a low center of gravity and a sufficient length to cause it to slide in the guides without rocking; it may be given a free fall by the use of nippers which engage a pin upon the hammer and are automatically disengaged at a certain height upon the leads. The more common method, however, is to raise and drop the hammer by the use of a hoisting drum with a friction clutch, the rope being permanently attached to the hammer—a more rapid system, which permits the operator to regulate readily the height of fall. The weight of hammer employed in ordinary work varies from about 2000 to 3500 pounds. For light work in small operations, light hammers may be used, while for heavy service and unusual conditions, heavier ones may be necessary. A heavy hammer with low fall is more effective in driving than a light hammer with high fall, as it may be operated more rapidly and causes less vibration in the ma chine.
A sit pile-hammer is one which is raised and dropped by a strain piston working in a cylinder attached to a frame which rests upon the head of the pile. The frame slides in the guides upon the leads, and the striking part or hammer is guided by the frame. In sonic of the steam hammers the pistons are attached to the striking weight; in others, the cylinders are the moving parts.
Steam pile-hammers are of two types—single acting, in which the weight is raised by the steam pressure and allowed to drop by gravity; double acting, in which the steam pressure is used to accelerate the downward motion of the hammer and increase the force of the blow. Single-acting hammers are made heavier and of longer stroke than double-acting ones for the same service, and are slower in action. For heavy service, single-acting hammers usually have strokes of 36 to 42 inches and strike 50 to 70 blows per minute, while the double acting kind have strokes from 12 to 24 inches and strike 120 to 200 blows per minute. Lighter machines may work much faster.
The blows of the steam hammer are so rapidly given that the motion of the pile is practically continuous and under ninny conditions the effectiveness of the driving is thereby greatly increased. There are few data giving definite information concerning the relative costs of driving by drop-hammer or steam-hammer, but the steam-hammer has seemed to be gradually replacing the drop-hammer in important operations. It has as advantages that of causing less damage to the
head of the pile; the driving may be accomplished at a more rapid rate, and more piles may usually be driven in the same time; the wear and tear upon the machine is much less than in the use of the drop hammer, although the first cost of the steam-hammer is consider ably greater.
Water jet pile drivers are fitted with appliances for discharging a jet of water at the foot of the pile. The water comes up around the pile, bringing with it much of the material cut from beneath the pile and lessening the friction resisting its descent. The water-jet equip ment is usually a straight piece of pipe, which may be held alongside the pile, with a nozzle at its lower end, the upper end being con nected by a flexible hose to a pump which supplies water under pres sure. The driver is equipped with leads and hammer, the latter being used to assist in sinking the pile by light blows and to settle it firmly into place after the jet is stopped.
The water jet is especially applicable to driving piles into sand, which usually offers considerable resistance to driving by the hammer alone. It may he used in any material which will he washed up by the jet and puddled about the pile, and frequently effects large savings in costs of driving. The pressure and volume of water required depend upon the kind of material to be penetrated. The pressure must be sufficient to cut the material and the volume enough to bring it up alongside the pilo. Pressures of 75 to 150 and volumes from about 50 to 200 gallons per minute are common.
194. Timber Piles.—A timber pile is usually the lower portion of the trunk of a tree, from which the branches and bark have been removed. It is nearly circular in section and tapers from butt to tip. Many hinds of timber arc employed for the purpose. The coni fers—yellow pine, Douglas fir, spruce, and cedar—are commonly obtainable in straight pieces of considerable length. White and post oak piles are not so straight, but are tough and hard and are suitable when requirements are severe. Cedar is valuable on account of its durability. For ordinary work in foundations, piles are usually required to be not less than 6 inches in diameter at the top, and commonly vary from 10 to 18 inches at the butt.
The specifications of the American Railway Engineering Associa tion name the following requirements for timber piles: