14.3 Rigging Blocks, Resultants and Reeving

Line pull is not always block load

A rigging block supports rope over a sheave. A snatch block can open to admit the rope without passing its end through the block. Blocks may redirect a line or form part of a mechanical-advantage system. The block body, sheave, attachment and anchor each need suitable rating and geometry.

Two rope forces act on a redirect block. Their vector sum determines the resultant at the block attachment, with block weight and other relevant effects also accounted for. In a simple static model with equal tension P in both legs, and included angle θ between the two forces pulling away from the block:

$$F=2P\cos(\theta/2)$$

The angle definition matters. When both legs leave in nearly the same direction, θ is near zero and the resultant is nearly twice line pull. When the legs form a right angle, the resultant is approximately 1.4142 times line pull. Do not use the rope’s change-of-direction angle without translating it into the angle between the force vectors.

Work a redirect example

A block redirects a line with assumed equal 8,000-lb tension through a 90-degree included force angle. The ideal rope-force resultant is approximately 11,314 lb. The anchor shackle, sling and supporting structure must be checked for that demand and applicable direction, along with relevant additional effects. Selecting them for only 8,000 lb would omit the second force.

The ideal zero at 180 degrees does not mean the installed block carries no weight or needs no attachment capacity. The rope forces cancel only in the defined ideal direction. Block self-weight, unequal tension, friction and actual geometry still matter. Real installations must follow rated product and equipment instructions.

Distinguish line and block ratings

Some product data describe allowable line pull, while other data describe total block load. Read the manufacturer’s definition. A label of “10 tons” cannot be interpreted without knowing what the rating covers. The anchor connection also has its own rating and permitted direction.

Check sheave diameter, groove size, rope compatibility, guards, side-plate closure, pins and retention. A sheave that accepts the rope physically may still provide an unsuitable bend radius or groove. Do not side load the block or let the rope leave the groove under an unapproved fleet angle.

Mechanical advantage is idealized

A multiple-part reeving system can reduce input line pull by distributing the supported load over several effective rope parts. In an ideal model, required line tension is the supported force divided by the number of supporting parts. Actual friction and equipment limits change the required input and permitted use.

Count parts supporting the moving block, not every visible rope segment anywhere in the system. A redirect added to the fixed lead may change direction without adding mechanical advantage. Include the moving block and carried equipment weight in the supported inventory where relevant.

A training 12,000-lb load with four ideal supporting parts has 3,000-lb tension per part. This does not establish real winch demand without friction and system data. It also does not rate the fixed anchors; their resultants depend on the attached rope directions and tensions.

Fleet angle and rope travel

Fleet angle describes lateral departure of rope between a sheave and drum relative to the intended drum alignment. Excessive angle can cause rubbing, poor spooling or groove departure. Limits depend on rope, drum and equipment design. Do not impose a universal 1.5-degree smooth-drum or 2-degree grooved-drum limit on every system without manufacturer support.

Follow the equipment’s permitted reeving and rope-path arrangement. Check that the rope cannot foul adjacent structures, rub against an edge or strike the termination. Keeping the payload clear does not establish a clear rope path.

Anchor-review scenario

A crew anchors a redirect block with a shackle whose WLL equals line pull. Both rope legs run nearly parallel toward the load and winch. The actual resultant approaches twice line pull, so the selection may be insufficient. Recalculate the vector demand and verify the supporting structure before tensioning.

During operation, stop for rope derailment, side-plate movement, binding or abnormal sounds. Never reach into a moving block or guide a tensioned rope by hand. After use, inspect sheaves, bearings, guards and retaining parts as instructed, and keep the complete assembly identifiable for future selection.

Compare supporting parts with a fixed redirect

Suppose a moving block has two effective vertical supporting rope parts and carries a total 6,000-lb ideal load. Each part carries 3,000 lb. Adding a fixed redirect to route the hauling end toward the operator does not create a third supporting part at the moving block; its ideal load share remains two parts.

The fixed redirect's anchor still requires a separate resultant calculation. If its equal-tension rope legs form a 90-degree included force angle, the ideal resultant is 3,000 × 1.4142, approximately 4,243 lb, before applicable additional effects. Counting three parts and selecting that anchor for only 2,000 lb would make both errors at once.

Sources: CCO block capacity and reference booklet, Crosby block application information.