Text 4. The Basic Hoisting Mechanism

Figure 1 shows a lifting device with a load attached to the lower block and the block in turnsupported by two ropes, or parts of line, suspended from the upper block. Each rope must therefore carry half the weight of the load; this gives the system a mechanical advantage of 2. Had the load been supported by five ropes, the mechanical advantage would have been 5. Mechanical advantage is governed by the number of ropes actually supporting the load. As parts of line are added, the force needed to raise or lower the load decreases, and load movement speed decreases as well.

The blocks contain pulleys, or sheaves, so that the rope is in one continuous piece from the end attached to the upper block to the winding drum. This makes the force in all parts of the rope uniform in a static system. The value of the rope load is found by dividing the weight of the lifted load by the mechanical advantage; in Figure 1 the lifted load would include the lower block, sometimes called the hook block. When the distance between the upper and lower blocks is great, it is necessary to include the weight of the parts of line as well.

The load in the rope is also equivalent to the force that must be generated at the winding drum in order to hold the load.

The effects of friction come into play as soon as the system is set into motion. Friction losses occur at the sheave shaft bearings and in the wire rope itself, where rope losses result when the individual wires rub together during passage over the sheave. These losses induce small differences in load between each rope segment (i.e., each section of rope from sheave to sheave). The loss coefficient can vary from a high of about 4.5% of rope load for a sheave mounted on bronze bushings to a low of as little as 0.9% for a sheave on precision ball or roller bearings. An arbitrary value of 2% is a reasonable approximation for sheaves on common ball or roller bearings when the rope makes a turn of 180°.

The tension in the rope at the winding drum is different when the load is raised and when it is lowered. Friction losses are responsible for this difference. When load-weighing devices that operate by reading the tension in the line to the drum are used, the variation is readily observed.

When an unloaded hook must be lowered, lowering will be resisted by friction, by the weight of the rope between the upper block and the deflector sheave, and by the inertia of the winding-drum mass. Mechanical advantage works in reverse in this case, as a mechanical disadvantage so to speak, so that the weight at the hook must exceed the rope weight multiplied by the mechanical advantage plus an allowance to overcome friction and inertia. If the weight at the hook is less than the result of this calculation, the hook will not lower; for that matter, if the weight is significantly less, the hook will rise on its own and will not stop until it strikes the upper block. To prevent this action, it is necessary to have a lower block with adequate weight or to add an overhauling weight (overhaul ball) so that the rope will overhaul through the system. Since the overhaul weight becomes part of the dead weight of the mechanism and remains in place throughout operations, it must be taken into account in operating plans. It is part of the lifted load.

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