The mechanical advantage of a tackle is the term applied to the relationship between the load being lifted and the power required to lift it. If the load and the power required to lift it are the same, the mechanical advantage is 1. However, if a load of 50 pounds requires only 10 pounds to lift it, then you have a mechanical advantage of 5 to 1, or 5 units of weight are lifted for each unit of power applied.
The easiest way to determine the mechanical advantage of a tackle is by counting the number of parts of the falls at the running block. If there are two parts, the mechanical advantage is two times the power applied (disregarding friction). A gun tackle, for instance, has a mechanical advantage of 2. Therefore, lifting a 200-pound load with a gun tackle requires 100 pounds of power, disregarding friction.
To determine the amount of power required to lift a given load by means of a tackle, determine the weight of the load to be lifted and divide that by the mechanical advantage. For example, if it is necessary to lift a 600-pound load by means of a single luff tackle, first determine the mechanical advantage gained by the tackle. By counting the parts of the falls at the movable block, you determine a mechanical advantage of 3. By dividing the weight to be lifted, 600 pounds, by the mechanical advantage in this tackle, 3, we find that 200 pounds of power is required to lift a weight of 600 pounds using a single luff tackle.
Remember though, a certain amount of the force applied to a tackle is lost through friction. Friction develops in a tackle by the lines rubbing against each other, or against the shell of a block. Therefore, an adequate allowance for the loss from friction must be added. Roughly, 10 percent of the load must be allowed for each sheave in the tackle.
Figure 4-24.-Single-whip and runner tackle.
TYPES OF TACKLE
Tackles are designated in two ways: first, according to the number of sheaves in the blocks that are used to make the tackle, such as single whip or twofold purchase; and second, by the purpose for which the tackle is used, such as yard tackles or stay
Figure 4-25.-Gun tackle.
tackles. In this section, we'll discuss some of the different types of tackle in common use: namely, single whip, runner, gun tackle, single luff, twofold purchase, double luff, and threefold purchase. Before proceeding, we should point out that the purpose of the letters and arrows in figures 4-24 through 4-30 is to indicate the sequence and direction in which the standing part of the fall is led in reeving. You may want to refer to these illustrations when we discuss reeving of blocks in the next sections.
A single-whip tackle consists of one singlesheave block (tail block) fixed to a support with a rope passing over the sheave (figure 4-24.) It has a mechanical advantage of 1. If a 100-pound load is lifted, a pull of 100 pounds, plus an allowance for friction, is required.
A runner (figure 4-24) is a single-sheave movable block that is free to move along the line on which it is reeved. It has a mechanical advantage of 2.
A gun tackle is made up of two single-sheave blocks (figure 4-25). This tackle got its name in the old days because it was used to haul muzzle-loading guns back into the battery after the guns had been fired and reloaded. A gun tackle has a mechanical advantage of 2. To lift a 200-pound load with a gun tackle requires 100 pounds of power, disregarding friction.
Figure 4-26.-Inverted gun tackle.
Figure 4-27.-Single-luff tackle.
By inverting any tackle, you always gain a mechanical advantage of 1 because the number of parts at the movable block is increased. By inverting
a gun tackle, for example, you gain a mechanical advantage of 3 (figure 4-26). When a tackle is inverted, the direction of pull is difficult. This can easily be overcome by adding a snatch block, which
Figure 4-28.-Double-luff tackle.
Figure 4-29.-Twofold purchase.
changes the direction of the pull, but does not increase the mechanical advantage.
A single-luff tackle consists of a double and single block as indicated in figure 4-27, and the double-luff tackle has one triple and one double
Figure 4-30.-Threefold purchase.
block, as shown in figure 4-28, The mechanical advantage of the single is 3, whereas the mechanical advantage of the double is 5.
A twofold purchase consists of two double blocks, as shown in figure 4-29, whereas a threefold purchase consists of two triple blocks, as shown in figure 4-30. The mechanical advantage of the twofold purchase is 4; the advantage of the threefold is 6.