Tackle Block Warning, Use & Maintenance Information.


Sheave Size & Wire Rope Strength


Strength Efficiency
Bending wire rope reduces its strength. To account for the effect of bend radius on wire rope strength when selecting a sheave, use the table below:

Ratio A Strength Efficiency
Compared to Catalog
Strength in %
40 95
30 93
20 91
15 89
10 86
8 83
6 79
4 75
2 65
1 50
Ratio A = Sheave Diameter
----------------------
Rope Diameter

Example
To determine the strength efficiency of 1/2" diameter wire rope using a 10" diameter sheave:

Ratio A = 10" (Sheave Diameter)
---------------------------------
1/2" wire rope Diameter
= 20

Refer to ratio A of 20 in the table then check the column under the heading "Strength Efficiency Compared to Catalog Strength in ...%"...91% strength efficiency as compared to the catalog strength of wire rope.

Fatigue Life
Repeated bending and straightening of wire rope causes a cyclic change of stress called "fatiguing". Bend radius affects wire rope fatigue life. A comparison of the relative effect of sheave diameter on wire rope fatigue life can be determined as shown below:

Ratio B Relative Fatigue
Bending Life
30 10.0
25 6.6
20 3.8
18 2.9
16 2.1
14 1.5
12 1.1
Ratio B = Sheave Diameter
----------------------
Rope Diameter
Relative Fatigue
Bending Life
= Relative Fatigue Bending Life
(Sheave #1)
--------------------------------------------
Relative Fatigue Bending Life
(Sheave #2)

Example
To determine the extension of fatigue life for a 3/4" wire rope using a 22.5" diameter sheave versus a 12" diameter sheave:

 

Ratio B = 22.5" (sheave diameter)
------------------------------------
3/4" (wire rope diameter)
= 30
Ratio B = 12" (sheave diameter)
-----------------------------------
3/4" (wire rope diameter)
= 16

The relative fatigue bending life for a ratio B of 16 is 2.1 (see above Table) and ratio B of 30 is 10.

Relative Fatigue
Bending Life
= 10
----
2.1
= 4.7

Therefore, we expect extension of fatigue life using a 22.5" diameter sheave to be 4.7 times greater than that of a 12" diameter sheave.