Blocks and Tackles - Equipment On Board

Posted by : Jimmie Luthra Sunday, December 9, 2012

The purpose of a block is either to change the direction of pull on a rope or else to act, in conjuction with other blocks as a tackle for reducing the pull required to lift or shift a heavy object. The essential part of a block is sheave.

The sheave or sheaves revolve on a pin, And the pin itself is supported by the shell. The Shell consists of two or more cheeks connected at top and bottom by distance so as to keep the cheeks sufficiently apart for the sheaves to be slipped in and a metal binding which provides an eye car hook at the top and a becket at the bottom.The top of the shell is called crown and its bottom is called tail.

Types of Blocks

There are several different types of blocks, below are some different types of blocks.

Single Sheave: The single-sheave block has only one sheave and may or may not have a hook or becket.

Multiple Sheave: The multiple-sheave block contains two or more sheaves. It also may or may not have a hook or becket.

Fixed Hook: A fixed-hook block is a single- or multiple-sheave block with a stationary hook attached to the top of the strap.

Swivel Hook: 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.

Snatch Block: The snatch block has a hinged cheek on one side and differs from all the 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 primary function of the snatch block is to change the direction of the load or pull.

Head/Heel Block: 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. A good winch operator can pass the cargo runner over the sheaves of these blocks at a rate of 500 feet per minute.

Clump Block

Built Block

External Bound Block

Internal Bound Block

Non Toppling Block

Gin Block

Coaling Block

Stropped Block

Tail Block

Hook Block

Binding

The binding has holes in each of its arm through which the pin is passed. Very small wooden blocks may not have the binding.

In that case the outer cheeks are scored to take a strap for suspending the block. The space between the sheaves and the crown through which the ropes passes is called swallow. This block will take the rope one tenth the size of rope is measured by diameter in millimeters.

Tackles

A Tackle is a set of two single or multi-sheaved blocks with single continuous rope rove through both of them. Tackle are used to reduce the pull required to lift or shift a load.

For example: If ten men are required to drag a case of machinery across the deck, only two may be sufficient if a tackle is used.

One of the blocks is fixed to stationary point. It is called the standing block. The other block is called the moving block and it is hooked to the load which is to be lifted or shifted. The standing part of a rope is fastened to a becket in one of the blocks the other end of the rope is called the hauling part, running between the two blocks is called running parts.

Types of Tackles

Gun tackle. Place two single-sheave blocks about 3 feet apart with the hooks or straps facing outboard and both blocks in the same position, either on their face or cheek. Next, they should run the line through the first and second block, then splice it to the becket of the first block. Gun tackle has a 2 to 1 mechanical advantage.

Luff tackle. Position one single- and one double-sheave block in the same manner as with the gun tackle. Run the line through one of the sheaves of the double-sheave block first and then to the sheave of the single-sheave block. Next, run the line through the other sheave of the double-sheave block and splice the line to the becket of the single-sheave block. This tackle offers a 3 to 1 mechanical advantage.

Twofold purchase. Position two double-sheave blocks in the same manner as with the luff tackle. Reeve the line through the top or bottom block, stay in sequence, and never cross from one side to the other. After reeving the tackle, splice the standing line to the becket. Twofold tackle has a 4 to 1 mechanical advantage.

Double luff tackle. Obtain a double- and a triple-sheave block. Place the blocks 3 feet apart with the hooks or straps facing outboard and position the blocks so that one is face down and the other cheek down. When reeving a tackle that has one block with more sheaves than the other, always start with the block with the most sheaves. In this instance, start reeving through the center sheave, keeping the line parallel. Never cross from one side to the other. Double luff tackle has a 5 to 1 mechanical advantage.

Threefold purchase. Place two triple-sheave blocks 3 feet apart, with the hooks or straps facing outboard, positioning the blocks so one is face down and the other is cheek down. Start reeving in the center sheave of one block and finish in the center sheave on the other. Then splice the standing part to the becket. This tackle offers a 6 to 1 mechanical advantage.

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