In these the chain or rope attached to the monkey, after passing over the pulley at the head of the frame, is connected with several short ropes, each of which is hauled on by a man until the monkey has been raised 3 or 4 feet, when upon a given signal the whole are let go at the same moment so as suddenly to release the monkey, which falls upon the pile.
Immediately after the blow is delivered the men pull the rope so as to tighten it and take advantage of the rebound of the monkey from the head of the pile.
Fig. 403 shows a very simple form of ringing engine adapted for use by seven or eight men.
The frame consists merely of an upright pole or leader supported by two side braces, and steadied by guys secured to an iron strap at the head.
The monkey, M, here shown is of cast-iron, weighing from 250 to 300 lb., and is guided in its descent by wrought-iron straps fixed to its sides, which embrace the "leader," and are secured at the back by a transverse bar passing through slits formed in the ends of the straps.
The rope shown round the head of the pile is intended to keep it close up to the engine so that it may not get out of place while it is being driven.
This engine is generally used for small piles; it delivers its blows with great rapidity, the monkey being raised only as far as the men can reach, some 3 or 4 feet each time, and the rope never being detached from it.
In some forms of this engine a monkey weighing from 600 to 800 lb. is used.
In these a stronger and more elaborate framing is required. Two parallel leaders, L L, are generally made use of, connected by a cross head, and further supported by framing.
In such engines the monkey may be provided with ears and projections cast on its sides, which travel in grooves formed on the inner sides of the leaders and thus guide the monkey during its fall.
Professor Rankine recommends that the men to work such an engine should be in the proportion of 1 to every 40 lbs. weight in the monkey, and states that they work most effectively when after every three or four minutes of exertion they have an interval of rest, and that under these circumstances they can give about 4000 or 5000 blows per day.
Crab, Engines are similar to the last described in their general arrangements, but the framing is much higher and the monkey is lifted to a height of 10 or 12 feet by means of a windlass or crab worked by men, horses, or steam-power.
In the commonest form the monkey is raised upon a hook, h (Fig. 404), attached to a counter-weighted lever, I, to the long arm of which is attached a rope, by pulling which the hook is pulled out and the monkey is permitted to fall The monkey can be released at any height by pulling the trigger rope C.
It is generally desirable that the height of fall should be the same for each stroke; this may be ensured by attaching the trigger-rope to the head of the pile.
Sometimes the rope is tied below, to the framing of the pile-driver, so as to cause the release of the monkey always at the same point, but in this case the height through which the monkey falls of course increases as the pile is driven further down.
The monkey should always descend in a line parallel to the direction of the pile. When that is vertical the guides are in the uprights of the framing, but if the pile is to be driven in an inclined position the guides must be similarly inclined, or if the framing will not permit this, temporary guiding pieces must be fixed at the required inclination.
Steam pile-drivers are those in which a small steam engine takes the place of the manual power applied to the crab. There are several forms of steam pile-drivers, but it is unnecessary to describe them in these Notes.
A " Punch," or "Dolly," is a short post or block interposed between the head of the pile and the monkey, either when the former would otherwise be out of reach, or when it is advisable, as in the case of cast-iron piles, to deaden the blow.
"According to some of the best authorities the test of a pile's having been sufficiently driven is that it shall not be driven more than i inch by 30 blows of a ram weighing 800 lb. and falling 5 feet at each blow. •
"It appears from practical examples that the limits of the safe load on piles are as follows : -
"In piles driven till they reach the firm ground, 1000 lb. per square inch of area of head.
" In piles standing in soft ground by friction, 200 lb. per square inch of area of head." - Rankine.