A traveling block is a multi-sheave pulley used to raise or lower the drill string and casings into a well bore. The blocks typically consist of four to six individual sheaves over which the steel cables used to suspend the traveling black are passed. The cables are then attached to the fixed crown block at the top of the derrick, leaving the lower block free to move up and down the cable fall. A shock absorber and crane hook are attached to the bottom of the traveling block and are used to suspend the drill string. These block assemblies are most frequently encountered in the oil drilling applications and are often capable of handling loads in excess of 1,000,000 pounds (454,000 kg).
Lowering, lifting, and controlling the drill string in deep well bores generally requires an extraordinarily robust hoist arrangement. These hoists usually consist of a crown block mounted in a fixed position at the top of the well derrick and a traveling block at the bottom of the fall of rope. The traveling block is a multi-sheave device, generally featuring between four and six individual sheaves. These sheaves are flat disks with a deep groove machined around their circumference. When grouped together, as they are in the traveling block, they are collectively referred to as a pulley.
The function of the sheaves is to allow the drill line ropes to pass through the block with minimal friction. In the case of a traveling block, four or more sheaves are used to spread the often considerable loads carried by the block. They also serve to increase the overall mechanical advantage of the hoist system, as well as a redundancy measure in the case of the rope breakages. The sheaves are located on a central shaft as an assembly enclosed in a steel casing fitted with a bracket on its lower surface. The bracket is used to suspend a shock absorbing crane hook that, in turn, is used to support the designated loads.
The drill string and casings often extend for significant distances below the surface and exert extreme loads on the drill lines. This requires that traveling blocks be rated to safely suspend loads that often exceed 1,000,000 pounds (454,000 kg). These extreme loads, in conjunction with the stress placed on it by the drilling process, necessitates the use of highly robust and reliable components in the construction of the block.