A derrick truck is a vehicle that transports a boom arm with a drilling bit attached to its end. This bit is used to drill holes into the earth for a variety of purposes. Utility workers often use derrick trucks to dig holes for telephone poles or other utility features such as lamp posts, guard rails, underground lines or pipes, and so on. Most of the trucks themselves feature a heavy-duty diesel engine, and the trucks tend to be quite large to handle a wide variety of drilling applications.
While the primary function of the derrick truck is to dig holes, it can be used in other ways as well. Attachments can be affixed to the end of the boom arm that extends from the truck bed to make the lift a more versatile tool; a man cage or bucket, for example, can be affixed to the end of the boom arm to allow a person to be lifted to a height. This is useful for utility workers who must access the top of telephone poles or lights, or even utility lines such as phone lines. Some very highly specialized attachments can be used on the derrick truck, making it a versatile choice for a wide variety of applications.
When not in use, the long boom arm, which is hydraulically controlled, will drop down flush with the bed of the derrick truck where it can be secured in place. When the operator needs to use the boom arm, it can be controlled with hand levers on the ground level of the truck or in the cab of the truck. The boom arm swivels at its base, and the arm itself can extend outward, much like some types of cranes. Hydraulic cylinders will be mounted at strategic points along the length of the boom to control its position.
If the derrick truck is being used for drilling holes, the end of the boom will be outfitted with a large bit, usually made of steel. This bit will feature a hardened tip useful for penetrating the ground, and a spiral-cutting groove that extends upward on the bit. This spiral groove will be used for removing material from the ground and hollowing out the hole as it is drilled. The boom arm can control the depth of the bit at any given point throughout the drilling process, and it can be used to move the bit upward to dump material, then move it downward to continue cutting.