In order to lift heavier weights, a system of blocks and tackles was gradually introduced. Originally they were made out of wood, but as the loads got heavier and they had to withstand harder work they were made out of steel. It is very rare these days to find a block made out of wood.

A block consists of two cheeks (if wooden) or two shells if steel, between which one or more sheaves are fitted to spin around a pin. The size and number of sheaves depends on the size of the rope/wire to be used and the intended weight to be lifted. It should be noted that the diameter of the sheave should be 5 to 7 times the diameter of the wire.

Parts of a block.

The main parts of a block are the following:

• The shell – The body of a block.
• The sheave(s). – The wheel(s) over which the rope runs.
• The pin on which the sheave turns.
• The bush or bearing between the sheave and the pin.
• The eye, hook, strop or other fitting by which the block is secured in the required position.
• The crown. – The top of the block where the eye or hook is fitted.
• The arse or tail. – The bottom of the block.
• The cheeks. – The sides of the shell.
• The score. – The groove made in the cheeks of some blocks to take the strop.
• The swallow. – The opening between the sheave and the shell through which the
  rope passes.
• The becket. – The eye sometimes fitted at the tail.

The following definitions are pertinent:

• A purchase – A block and tackle.
• To reeve a purchase – To lead a wire through a series of sheaves.
• To fleet a tackle – To stretch a tackle out to its fullest extent.

Classification of blocks.

Wooden blocks. They are classified by their size, which is the length measured from the crown to the tail along the score. An ordinary wooden block takes a rope 1/3 of its size.

Metal blocks. They are classified by the size of the rope/wire for which they are designed. This is marked on a plate which is affixed to one cheek. The blocks may have more than one sheave.

Sheaves:
Sheaves of wooden blocks are made of either Lignum Vitae or phosphor bronze, While those of metal blocks are either of mild steel or phosphor bronze. The pins of all of them are of mild steel. The bearings betseen the sheaves and pins are of various types; pin, roller or self lubricating. A mild steel sheave fitted with a plane bearing has a small brass bush to form the bearing, as otherwise steel bearing on steelis liable to seize. Roller bearings are fitted to life boat falls and other special types.

How blocks are described:

1. By size. (Wooden blocks only).
2. Number of sheaves.
3. Type of block (i.e. IB, Metal, common, etc) with details of the sheaves sometimes included).
4. Size and type of rope. (Metal blocks only)
5. Means of attachment (standing eye, etc).

Strength of blocks: The SWL (safe working load) is in general as follows:

An IB, metal or common block is stronger than the rope for which it is designed.

Clump, and snatch blocks are however much weaker, ie a snatch block is 1/3 as strong as an IB block used for the same size wire, while a clump block is only ¼ as strong.