To reeve blocks in simple tackle, you must first lay the blocks a few feet apart. The blocks should be placed down with the sheaves at right angles to each other and the becket bends pointing toward each other. To start reeving, lead the standing part of the falls through one sheave of the block that has the greatest number of sheaves. Begin at the block fitted with the becket. Next pass the standing part around the sheaves from one block to the other, making sure no lines are crossed until all sheaves have a line passing over them. Now secure the standing part of the falls at the becket of the block having the fewest number of sheaves, using a becket hitch for temporary securing or an eye splice for permanent securing.
When blocks have two or more sheaves, the standing part of the fall should be led through the sheave closest to the center of the block. This places the strain on the center of the block and prevents the block from toppling and the lines from being chafed and cut through by rubbing against the edges of the block.
Falls are normally reeved through 8-inch or 10-inch wood or metal blocks, in such away as to have the lower block at right angles to the upper. Two 3-sheave blocks are the traditional arrangement, and the method of reeving is shown in figure 6-6. The hauling part has to go through the middle sheave of the upper block or the block will tilt to the side and the falls will jam under load.
Figure 6-6.-Reeving two 3-sheave blocks.
If a 3- and 2-sheave block rig is used, the method of reeving is almost the same (fig. 6-6), but the becket for the deadman must be on the lower instead of the upper block.
You reeve the blocks before you splice in the becket thimble, or you will have to reeve the entire fall through from the opposite end. For the sake of appearance, if the becket block has a grommet, it is better to take it out and substitute a heart-shaped thimble. Splice it with a tapered eye splice, and worm, parcel, and serve the splice if you want a sharp-looking job.
TYPES OF TACKLE
SINGLE-WHIP tackle consists of one single sheave block (tail block), attached to a support with a
Figure 6-7.-A single-whip tackle.
line passing over the sheave (fig. 6-7). It has a mechanical advantage of 1, and if a load of 50 pounds were to be lifted, it would require 50 pounds of force to lift it, plus allowance for friction.
A RUNNER is a single sheave movable block that is free to move along the line for which it is rove. It has a mechanical advantage of 2.
A GUN TACKLE is made up of two single sheave blocks (fig. 6-8). The name of the tackle originated when it was being used in the old days of muzzle-loading guns. After the guns were fired and reloaded, this tackle was used to haul the guns back to the battery.
A gun tackle has a mechanical advantage of 2. Therefore, to lift a gun weighing 200 pounds requires a force of 100 pounds without considering friction.
By inverting any tackle, you should gain a mechanical advantage of 1. This occurs because the number of parts at the movable block has increased.
By inverting a gun tackle, as an example, you should gain a mechanical advantage of 3 (fig. 6-9). When a tackle is inverted, the direction of pull is always difficult. This can be overcome easily by using a snatch block, It changes the direction of pull but does not increase the mechanical advantage.
A SINGLE-LUFF TACKLE consists of a double and a single block (fig. 6-10). This type of tackle has a mechanical advantage of 3.
Figure 6-8.-A gun tackle.
Figure 6-9.-An inverted gun tackle.
A TWOFOLD PURCHASE tackle consists of two double blocks (fig. 6-11). It has a mechanical advantage of 4.
A DOUBLE-LUFF tackle consists of a triple block and a double block (fig. 6-12). It has a mechanical advantage of 5.
Figure 6-10.-A single-luff tackle.
Figure 6-11.-A twofold purchase.
Figure 6-12.-A double-luff tackle.
A THREEFOLD PURCHASE consists of two triple blocks and has a mechanical advantage of 6 (fig. 6-13).
A COMPOUND TACKLE is a rigging system using more than one line with two or more blocks. Compound systems are made up of two or more simple systems. The fall line from one simple system is secured to the hook on the traveling block of another simple system, which may have one or more blocks.
To determine the mechanical advantage of a compound tackle system, you must determine the mechanical advantage of each simple system in the compound system. Next, multiply the individual advantages to get the overall mechanical advantage. As an example, two inverted luff tackles, each has a mechanical advantage of 4. Therefore, the mechanical advantage of this particular compound system is 4 x 4 = 16.
Figure 6-13.-A threefold purchase.