A hook block is an assembly to which the hook of a hoist or crane is attached; it typically consists of a steel enclosure housing a number of sheaves or pulleys that carry the ropes or chains that facilitate the lifting of a load. Although 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 rope and then raised or lowered by the cranes boom and winch system. A hook block allows for a considerable amount of flexibility and safety in lifting operations as opposed to a direct connection.
One of the most important functions of any hook block is facilitating of a free turning or rotating hook arrangement. When loads are lifted, it is often necessary to turn the load to position it in a new location or to avoid striking obstructions. A crane hook attached directly to the hoist ropes would cause the ropes to twist if the load was turned from its original orientation. This would have a number of undesirable effects such as over-stressing the ropes and boom pulleys, creating an unbalanced load, and causing the load to swing back in an uncontrolled fashion when released. A hook block allows loads to be freely rotated without changing the orientation of the hoist ropes.
This rotation of loads can be facilitated by hand in the case of small, manageable loads or may be achieved by a power hook that can be remotely rotated by the crane operator or ground personal. This allows for very controlled and precise rotation and placement of very large and unwieldy loads. Powered hook blocks also mean that no ground personal need be involved with the rotation of loads which greatly increases the safety of such operations.
The second major benefit of using a hook block is the use of multiple hoist sheaves. The rope used to hoist the loads is load rated according to the overall rating of the crane. If a single rope were to be employed, 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 margins should the rope break. The use of multiple ropes means that each length, or fall, of rope is far thinner than a single rope, the load is better distributed at the top of the boom or jib, and there is a system of redundancy should a rope breakage occur.
The number of sheaves used in a crane hook block differs according to the intended use and rating of the crane and may range from two to eight or more sheaves. Hook blocks may also include peripheral devices such as load cells and scoreboards which display the load mass, adjustable slip clutch assemblies for impact protection, and extended hook shanks for specialist applications. All fulfill the basic function of allowing for safe and easy rotation of loads and the use of thinner multiple hoist rope falls for efficient and safe operation.