There are several different types of blocks, each with a different use. Wooden and metal blocks are of the
same design except for the head or heel block which is only metal.
The single-sheave block has only one sheave and may or may not have a hook or becket.
The multiple-sheave block contains two or more sheaves. It may or may not have a hook or becket.
A fixed-hook block is a single or multiple sheave block with a stationary hook attached to the top of the strap.
A swivel-hook block is a single or multiple sheave block with a swivel hook that allows the lock to move in the direction of the load.
The snatch block has a hinged cheek on one side and differs from all other blocks. The advantage of a snatch block over the other types is that it can be opened and a bight of line placed over the sheave without passing the end of the line through the swallow. The snatch block also has a swivel hook. The function of the snatch block is to change the direction of the load or pull.
The head or heel block has a cast metal shell, roller bearings, and a grease fitting in the sheave pin. The cargo runner can pass over these blocks at the head and heel of the cargo boom. These high-speed blocks must be lubricated every time they are used.
Blocks are named for the purpose for which they are used, the places they occupy, or from a particular shape or type of construction. According to the number of sheaves, blocks are designated as single, double, or triple. A traveling block is attached to the load being lifted and moves as lifting occurs. A standing block is fixed to a stationary object.
Every tackle system contains a fixed block attached to some solid support and may have a traveling block attached to the load. The single rope leaving the tackle system is called the fall line. Personnel apply the pulling force to the fall line which may be led through a leading block.
SIZE OF BLOCKS - Users can determine the size of blocks by measuring the length of the cheek in inches. Blocks are designed for use with a specific line size. Bending line over a sheave that is too small causes distortion and strain, with the line wearing on the shell. You can use line smaller than that designated for a sheave with no damage, but should don't use line of a larger size.
To determine the size wooden block to use with line of a known size, you can use these formulas:
3 x circumference of line = shell size, 2 x circumference of line = sheave size
