What’s a hook block?

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A block is an assembly to which the hook of a tackle or crane is attached; it typically consists of a steel casing that houses a series of sheaves or sheaves that carry the ropes or chains that assist in lifting a load. While this sounds unnecessarily complex, it would be impossible for a crane or hoist to operate efficiently and safely if the crane hook was simply attached directly to a wire rope and then raised or lowered by the crane’s boom and winch system. A block allows considerable flexibility and safety in lifting operations compared to a direct link.

One of the most important functions of any block is to facilitate a swivel or rotary hook arrangement. When lifting loads, it is often necessary to rotate the load to position it in a new position or to avoid collisions with obstacles. A crane hook attached directly to the hoist ropes would cause the ropes to twist if the load was rotated from its original orientation. This would have a number of unwanted effects such as overstressing the boom ropes and sheaves, creating an unbalanced load and causing the load to swing back uncontrollably when released. A block allows loads to be rotated freely without changing the orientation of the hoist lines.

This rotation of loads can be facilitated by hand in the case of small and manageable loads or it can be achieved by means of a power hook which can be rotated remotely by the crane operator or ground personnel. This allows for very controlled and precise turning and positioning of very large and awkward loads. Powered blocks also mean that no ground personnel have to be involved in swinging loads, which greatly increases the safety of such operations.

The second major benefit of using one block is the use of multiple hoist sheaves. The rope used to lift the loads is rated capacity based on the crane’s overall rating. If a single rope were to be used, it would have to be extremely thick, which would create a number of problems relating to the design, construction and operation of the crane. It would also create safety issues as there would be no redundancy or safety margin in the event of a rope break. The use of multiple ropes means that each length, or drop, of rope is much thinner than a single rope, the load is better distributed across the top of the boom or jib, and there is a redundancy system in case of breaking of the rope.

The number of sheaves used in a davit block varies depending on the intended use and horsepower of the davit, and can range from two to eight or more sheaves. Hook blocks may also include peripheral devices such as load cells and scoreboards that display load mass, adjustable slip clutch assemblies for impact protection, and extended hook shanks for special applications. All fulfill the basic function of enabling easy and safe rotation of loads and the use of multiple thinner hoist cables for efficient and safe operation.

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