How to evaluate block stability due to rigging geometry.

Industry testing has been conducted to help you assess the block stability of your rigging configuration and rope selection. The bands on this graph approximate the block stability for three types of wire ropes tested in multi-part systems:

•  6-strand, right regular lay, IWRC.
• Category 3 (8-strand, rotation-resistant).
• Category 2 (19-strand, rotation-resistant).
• XLT4 & Category 1 (35x7 rotation resistant)

 

 Four independent variables are used in pairs to locate a reference point on the graph that indicates the stability of the lift being made. The ratios used include:

 

L/S = Length of fall (ft.) ÷ Spacing of the rope (ft.).

L = Length of fall measured from the centerline of the point sheave to the centerline of the traveling block sheave as shown in the diagram.

S = Average diagonal spacing of the rope at the boom point and the traveling block sheaves as shown in the diagram.

D/d = (D) Average pitch diameter of point and block sheaves (in.) ÷ (d) nominal rope diameter (in.).¸ (d) nominal rope diameter (in).

 

For 2-part reeving, S = average pitch diameter of point and block sheave

For 3-part reeving, S = 2/3 of 2-part

For 4-part reeving, S = diagonal distance of rope parts

For 5-part reeving, S = 4/5 of 4-part

For 6-part reeving, S = diagonal distance of rope parts

For 7-part reeving, S = 6/7 of six-part system

 

 When the reference point on the graph lies above the appropriate band, block rotation will probably occur. If the reference point lies below the band, then the lift will probably be stable without block rotation. If the point lies within the band, block rotation is uncertain.

Some graphs were developed in field research jointly conducted by Wire Rope Technical Board and the Power Crane and Shovel Association.