This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol6", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
When it is necessary to lift large pieces of material, such as steel girders, etc., which are too heavy to be handled with the gin wheel, pulley blocks (B, Fig. 213) are brought into requisition. These consist of a top block having a hook for suspension purposes, wrought-iron or steel plate sides and frame, a ring for securing the end of the rope fall, and one, two, or three, or more pulleys or sheaves, grooved to receive the rope, and running freely on a common pin or shaft. Each pulley, when there are more than one, is separated from those next it by an additional steel plate. There is a bottom block which is substantially the same as the top one, with the exception of the ring, which is not required; and a fall or rope, which is fixed securely to the ring in the top block brought down and passed under the first sheave of the lower one, then up and over the first sheave of the top, and so on until the sheaves are full. The remainder of the rope will then hang free from the last sheave of the upper block.
This operation (known as reaving the blocks) having been carried out, the blocks are ready for use. As it is not usual to have the ordinary scaffolding of a building of sufficient strength to carry great weights, it is advisable to erect a special derrick or a set of shear legs, - full details of which will be given subsequently under separate headings, - to which the top block can be attached. As is well known, the gain in power by the use of these blocks is directly proportional to the number of sheaves employed, i.e. if one man can raise a certain weight with the gin wheel and fall he can raise twice the weight by exerting the same pull on the fall of two single-sheave blocks, although double the time will be consumed in raising the weight to the same height. In some cases the load to be raised may be so great that two or more men are needed to pull on the fall. In these circumstances it is best to bring the fall to the ground, and pass it through a single-sheave block or snatch-block (C, Fig. 213), which should be fixed securely to the foot of the derrick, or some other fixed object, as nearly as possible vertically under the top block. Although securely fixed, the snatch block should be free to turn in any direction, so that the men pulling on the fall, which passes through it, can stand in a convenient position one behind the other. In this way each man is enabled to exert his full strength on the rope, which would be impossible if all were to attempt to pull at once vertically. The snatch block is generally provided with a movable side plate, in order that the fall may be inserted at any point without having to be threaded through its whole length. The blocks most generally in use contain two or three sheaves each, as it has been found that the friction of the rope increases so rapidly with the number of pulleys that more power is lost in friction, when a larger number of sheaves is used, than is gained by the increased power due to them.