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.