**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
EfficiencyCompared 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
FatigueBending 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
FatigueBending 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.