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A crown block is a device 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.
Each crown block consists of a series of pulleys and steel cables, or sheaves. These cables and pulleys sit on a steel frame, which may be built into the structure of the derrick. The sheaves serve as drilling lines, and pass through the traveling block below to connect to the rig's hoisting drum. As the cables pass over the pulleys in one direction, they cause the oil drum to descend into the ground. When the cables pass over the pulleys in the other direction, they cause the oil drum to rise back up, bringing oil up for collection.
While the crown block itself is fixed and used to stabilize the sheaves, the traveling block moves up and down with the cables. This traveling block contains internal pulleys, which are surrounded by steel to protect both the pulleys and the cables from corrosion.
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.