Blocks, Swivels & Sheaves

Blocks, Swivels & Sheaves

Nobles offer a complete range of snatch and sheave blocks, hook blocks, sheave frames and swivels.

No matter what your lifting equipment needs, Nobles are the swivels, blocks and sheaves suppliers of choice in Australia.

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What are sheave and snatch blocks?

What are sheave and snatch blocks?

 

Sheave and snatch blocks are designed to deflect a loaded wire rope when a change in direction is required for a particular application or to increase the line pull or lifting capacity of a hoist or winch by multiplying the number of parts of line.

A sheave or snatch block usually consists of side plates, sheave and a head fitting or lower lifting fitting. These blocks are available in a wide variety of configurations and numerous sheave combinations.

Sheave Blocks

A sheave block refers to a block in which the side plates are fixed around the sheave or sheaves and the rope cannot be placed into the block at any point along its length. Sheave blocks, except snatch blocks, can only be reeved by feeding the end of the rope through the block. This type of block is most commonly used with wire rope in crane fall blocks. Sheave blocks and crane fall blocks often have numerous sheaves depending on the crane configuration.

Snatch Blocks

A snatch block usually only has one or two sheaves and is used more as a lifting accessory with wire rope slings or as a means of increasing line pull in a winching application as opposed to an integral part of a crane as is often the case with sheave blocks. The main feature common to snatch blocks is that the side plates open to allow the block to be placed on a wire rope at any point in a winching application or to allow wire rope slings to be placed into the block as in most circumstances it is impractical to feed a wire rope sling through a sheave block.

 Sheave & snatch blocks - Diagram 1

Loads on Snatch Blocks

Loads on Snatch Blocks

 

Working Load Limit (WLL) is the maximum load, which may be applied to the head fitting in normal conditions of use as defined in Australian Standard 1418. This shall be not greater than the value determined by means of the appropriate formula.


 Working Load Limit Calculation


Type of Sheave Block WLL Calculation
With becket (2N+1)PR / KS
Without becket 2N x PR / KS

 

Where:
N = number of sheaves
PR = minimum breaking load of rope in kilonewtons
KS = safety factor of rope

Loads on Snatch Blocks

The stress on a snatch block varies with the degree of angle between the lead and load lines. When the two lines are parallel, 1000kg on the lead line results in a load of 2000kg on the hook. As the angle between the lines increases, the stress on the hook is reduced as illustrated below.

Sheave & snatch blocks - Diagram 2

To determine the stress on a hook, multiply the pull on the lead line by a suitable factor from the following table adding 10% for friction.

Angle Factor
1.998
10º 1.99
15º 1.98
20º 1.97
25º 1.95
30º 1.93
35º 1.9
40º 1.87
45º 1.84
50º 1.81
55º 1.77
60º 1.73
65º 1.69
70º 1.64
75º 1.58
80º 1.53
85º 1.47
90º 1.41
95º 1.35
100º 1.29
105º 1.22
110º 1.15
115º 1.07
120º 1.00


(Eg.) 'S' when L =1.6t at 60° = 1.6t x 1.73+10% friction.

'S' = 3.04t (ie. stress on hook).