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.
SINGLE-WHIP tackle consists of one single sheave block (tail block), attached to a support with a 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 6-7. - A single-whip tackle.
Figure 6-8. - A gun tackle.Continue Reading