This section is from the book "American Library Edition Of Workshop Receipts", by Ernest Spon. Also available from Amazon: American Library Edition Of Workshop Receipts.
The left-hand part of the half-hitch is passed under the cross rope by A, while the free end of the rope is passed as described by B. b shows this phase of the operation. In thia way two loops are formed, one for each side. A's loop lies under the cross line, while B's loop comes outside of everything. All these operations are eieculed in a few seconds, no exact order being followed. The tightening process comes next. B begins to pull the rope backward and upward, grasping it at f, putting hia knee, or even foot, against the hook for a par-chase, while A takes in the slack as fast as given him from B's successive pulls, grasping and pulling the rope at g. When no more can be gained, and the poor brute is compressed as much as possible, A passes the loop on his side tightly around and partly underneath his half of the pack. Then B, grasping the rope at a, pulls diagonally backward and outward. This begins to "spread the diamond," He nest puts his loop in position, when A, taking hold of the free end of the rope, pulls it diagonally forward and outward, over the withers of the horse. This completes the spreading of the diamond, and it will be at once seen that this separation of the two lends of the rope tightens it with enormous power.
After A has given the final pull, he ties the free end of the rope wherever convenient, thus completing all. The final result is shown in c. The last two pulls consolidating the pack nearly double up the poor animal. The cinch, often cruelly narrow, is drawn up into his belly until the profile forms a double curve, his body being violently squeezed upward. After packing, the poor beast will sometimes go oft', as it were, on tiptoes, trying to relieve himself by motion. To untie the hitch, the end of the line is untied and cast loose, and withdrawn from under the cross lead of the rope. Then the whole being slackened, the bight is withdrawn from the hook, and the rope comes off without a knot. If a knot is formed, it is a sign that a mistake has been made in the tying.
The»tightness of the "lacing" to which the animals are subjected has an element of mercy in it, because, if the saddle shifts about, a sore back inevitably results.
For roping large, irregular bundles, the diamond hitch is well adapted, and its power in such cases is surprising. A simple loop tied on the end of the rope is made to serve instead of the cinch loop. (Scient. Amer.)
Blocks are used for changing the direction of ropes, and gaining power at the expense of time. They are made sometimes of wood frames, with a rope " strop " and thimble eye. The sheaves are usually brass, running on a wrought-iron centre pin, which passes through the frame and sheaves. Fig. 342 A chows a treble block. Generally blocks are made with light steel or sheet-iron plates for the sides, strengthened by wrought-iron links, where the strain is most direct. The suspending hooks are connected by a strong cross bar to the side links, and are capable of being easily moved, either round their own axis, or through an angle of 90° on each side. Blocks are called single, double, or treble, according to the number of sheaves which revolve on the centre pin. Snatch, or leading blocks, are single, with an opening on one side to admit a rope without passing its end through.
A simple " tackle " consists of one or more blocks rove with a single rope or " fall." When a tackle is in use, one end of the fall is made fast, and the other is hauled upon. The fixed end is called the "standing" end, the other the " running end." Each part of the rope contained between the blocks, or between either extremity and a block, is called a return of the fall. To overhaul a tackle is to separate the blocks. This should always be done from the standing, and not from the movable block: to round in is to bring the blocks closer together by hauling on the fall. When a rope is passed through the sheaves of a block, it is bent to a curve suiting the radius of each sheave passed over. The sheaves should be exactly the same diameter in each pair of blocks working together. Owing- to the stiffness of the rope, and Friction of sheaves on the pin, the theoretical power is considerably reduced. The weight any system of hlocks will lift is found by multiplying the power by the number of ropes attached to the movable block, including the standing end if fixed to it.
For example, suppose we have 3 sheaves in use in each block, then the additional bower acquired would be (theoretically) 6. The average additional power required for each sheave to compensate for friction is found to be about roughly 10 per cent. With 3 sheaves in use this would be 10 x 3, or 30 per cent, therefore, to raise a weight of one ton, a power of - 2240/3 + l.31b. = 970 lb. would be required. In hauling on a fall, men exert a pull of about 80 lb., or half their weight under favourable circumstances. In this case the least number of men required would be -
970/80= 12 1/10 or say 13 men
Therefore, in calculating the advantage gained by using blocks,1/10 of the weight to be lifted must be added for every sheave in use, as shown in B. Suppose it is required to lift a weight of 12 tons with a pair of treble-sheaved blocks threaded with a 5-in. rope, the theoretical gain of power is 6 to 1, and the power required will be 1/6 of R the total resistance to be overcome, which is compounded of W the weight to be raised, plus the resistance arising from stiffness of rope, and friction. Therefore we have W + 1/10 W for each sheave in use. Now 6 sheaves are in use, therefore. R = W + 6/10W.
P the power = R/6 = (W +6/10W)/6.
If 12 tons have to be lifted we havep = 12 +6/10 of 12 = 3 1/5.
A good rule for calculating the " safe working " strength of a new rope, of white hemp, is to square the circumference in inches, and divide by 8. For instance, the safe working strength of a 5-in. rope would be (5)2/8 = 3 1/8 tons. As another example we will calculate what weight can be raised by a " tackle" consisting of 2 treble blocks, rove with a fall of 6-in. rope, without exceeding the working strength of the rope. Here we have by formula