A block is a portable pulley, made of wood, metal, synthetic-resin bonded fibre (SRBF) or combinations of materials.
Blocks can be fitted with a hook as an alternative to an eye, andthe hook or eye can be fixed or swivel types. They may have more than one sheave; a single block has one sheave, a double block has two and a triple has three.
Wooden B locks are classified by their size, which is the length from crown to tail measured in millimetres round the shell; and it will take a rope one-tenth of its size. (Rope measured by its diameter in millimetres).
Metal Blocks are classified by the size of rope for which they are designed and this is normally marked on a plate affixed to the cheek.
SRBF Blocks are also classified by the size of rope for which they are designed and this is stamped on the binding of the block.
Study the part of blocks as shown in Figures
Types of Block:
Internal-bound Block (IB)
This type has a shell partly of wood and partly of metal, and is a version of a wooden block. The metal portion consists of a fork shaped steel fitting called `the binding’ which incorporates both the eye or hook and becket when fitted. The sheave is made of phosphor bronze which is less corrosive and does not create sparks as the pin is made of steel. This type of block can be used for rope or wire. A tally plate is found only on this type of block; it has two practical purposes, to hold in the pin and to supply information about the block. This information will contain the Pattern number (a computerised number to identify the block), the size of the rope for its use and the safe working loads for use. In addition to allow for lost or damaged plates, information about the block will also be stamped on the hook or eye. This will contain the pattern number, the size of the rope to be used, the tested weight (which will be higher than the safe working load) and the test date (month and year). All tests are normally carried out in HM Dockyard Test Centres.Very little maintenance is required for these blocks. Even if the wood is split or chipped it can be repaired by using a wood filler or synthetic resin paste. The swivel hook or eye, whichever is fitted, should be kept free of dirt and given a light oiling to ensure it swivels freely. As most of the metal parts are galvanised only the pin requires a light coating of grease to ensure free running of the sheave. The sheave should be checked for wear periodically, any rough edges should be filed down to prevent damage to rope.