BLOCKS, TACKLE, ETC.
Blocks are of two kinds, made and mortised. A made block consists of four parts: shell, sheave, strap, and bush. A mortised block is made of a single block of wood, mortised out to receive a sheave.
Blocks are single, double, or treble, according to the number of sheaves.
A Tackle is a purchase formed by reeving a rope through two or more blocks for the purpose of hoisting.
A Whip is a purchase made by a rope rove through one single block.
A Gun-tackle Purchase is a rope rove through two single blocks, and made fast to the strap of the upper block. The parts of all tackles between the fasts and sheave are called the standing parts. The parts between the sheaves are the running parts, and the part which is taken hold of in hoisting is called the fall.
A Whip upon Whip is where the block of one whip is made fast to the fall of another.
A Luff-tackle Purchase is a single and a double block, the end of the rope being fast to the upper part of the single block, and the fall coming from the double block. A luff-tackle upon the fall of another luff-tackle is called luff upon luff.
A Watch-tackle or Tail-tackle is a luff-tackle purchase, with a hook in the end of the single block, and a tail to the upper end of the double block.
A Single Burton is composed of two single blocks, with a hook in the bight of the running part. Reeve the end of rope through the upper block, and make it fast to the strap of the fly-block. Then make fast your hook to the bight of the rope, and reeve the other end through the fly-block for a fall. The hook is made fast by passing the bight of the rope through the eye of the hook and over the whole. This is a very quick-working tackle and a strong purchase. Used for hoisting entirely.
When a very heavy weight is to be raised, the standing part should be attached to the slings by a fisherman's bend instead of to the block.
The size of blocks is expressed by the length of the shell in inches.
Tackles are also designated by the number of sheaves employed, as twofold (two single blocks), threefold (double and single block), etc.
The bight of a hook is the middle of the bend of the hook part.
Rope should always be stopped up, either with the end or with rope-yarn stops, to prevent its getting into a snarl. When using ropes for hauling, they should never be dragged upon the ground.
Before reeving a rope in a block the turn should be carefully taken out, to prevent twisting when the weight is lifted. This is done by stretching the rope out to its full length, and turning it in the opposite direction to that in which it is laid up until all the stiffness disappears.
Blocks should be overhauled very often to see that the sheaves are working properly on the pins, and that they work smoothly. If they do not, turn the pins end for end and rub a little black lead (graphite) on them to lubricate them; also on the sides of the sheaves where they rub against the shell.
When hoisting with tackles, they should never be allowed to twist. If they show a tendency to do so, insert a bar in the block or sling, and use it as a lever to hold it straight.
It frequently happens that the men cannot apply their full strength in the direction in which it would be most effective. In such cases hook a single block to some object about two feet above the ground and reeve the end of the fall through it, so that the men can add their strength to their weight, and more men can apply themselves.
Never trust the suspension of a weight to holding it by the unaided strength of men. If it is possible to get a turn around any fixed object, even in raising or hauling a weight, it is best to take a turn, as all that is gained is then saved.
Always select such blocks that the fall will run freely through them, and not ride upon the edges of the sheaves. If it does, it will be certain to cut. The rope should not quite fill the groove on the sheave. In this way excessive friction is avoided.
The power gained by using tackle is as follows:
Two single blocks, or gun-tackle: nearly doubled.
Luff-tackle, double and single block: doubled. If the double block is movable, trebled.
Two double blocks: power × 3⅓.
Double and treble blocks: power × 4.
Two treble blocks: power × 4½.
Whip upon whip (single Burton): trebled.
When one tackle is applied to another, the power obtained is found by multiplying their respective values together.
No advantage is gained by using a greater number of sheaves than two treble blocks in one fall.
The power is equal to the weight divided by the number of ropes (standing parts) attached to the lower block, or by twice the number of rising pulleys (sheaves).