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A crown block is a pulley situated at the top of an oil rig or derrick. It sits on the crown platform, which is a steel platform located along the upper portion of the rig. The crown block works in conjunction with a similar component, the traveling block, which is positioned just below the crown platform. Together, these two systems are known as the block and tackle. While the block and tackle system appears relatively simple to outsiders, it actually represents a critical component of the oil drilling process.
A crown block is a pulley that has a wire-rope drilling line running between it. While the crown block is fixed, the traveling block moves up and down between the crown block and the rig floor.
The use of a crown block and traveling block greatly enhances the power of the oil derrick. The position of the pulleys allows the cables to withstand tremendous levels of force, and helps workers drill deeper and extract more oil. Without a crown block, the oil derrick would require much thicker and stronger cables. It would also require a more powerful and substantial pumping system to operate successfully. The use of the block and tackle system provides a high degree of leverage to lift and lower the hoisting drum in order to maximize productivity and efficiency.
Depending on the size of the derrick and the depth it must drill to, an oil rig may use either a single or double crown block. While a single block utilizes only one set of pulleys, the double deck model includes two sets. These pulleys are situated at a right angle to one another to generate extra force and power.
One drawback to using the crown block design is its high maintenance requirements. These devices must remain lubricated to allow the sheaves to pass freely through the pulleys. Insufficient lubrication may result in excess friction, which could cause the cables to fail or break.
In simplest incarnation, there is one cable which is reeved (threaded, slipped) through the various sheaves. The use of multiple sheaves on the crown block and the travelling block gives mechanical advantage, allowing less tension (force) in the drill line to lift heavier loads (at the expense of length of line required).