Drilling rigs are machines that creates holes (boreholes) and/or shafts in the ground. They are equipped with deck engines or truck engines, hoists or lifts, compressors, pumps, and pipe-handling equipment. Some drilling rigs have a power take-off (PTO), a device that transfers power from an engine to pump. Others are designed for drilling large-diameter, deep wells; mineral or petroleum exploration, surface-hole drilling, or directional drilling. Drilling rigs also differ in terms of compressor type. Choices include on-board, variable volume, and development. Often, the compressor has a clutch. Specifications for drilling rigs include table opening, pull-up, top head travel, and lift capacity. Drilling rigs are equipped with pumps that dispense drilling fluid or drilling mud to remove cuttings from holes. Drilling fluids also prevent other fluids from entering the hole and exerting enough pressure to cause a blowout. Drilling rigs use several basic types of drilling fluids. Examples include air, air/water and air/polymer mixtures. Compressed air is pumped down a bore-hole’s annular space, or the drill string itself. Air/water mixtures are used to provide increased viscosity, flush the hole, and control dust. Air/polymer mixtures are also used to improve drilling conditions for drilling rigs. Typically, the polymer is a foaming agent. Drilling fluids for drilling rigs include water and water-based mud (WBM). Typically, water is used to perform very specific functions. WBM includes clays and chemicals that provide viscosity control and shale stability. In its rock form, this clay is called shale and features one or more types of dissolved clay. Typically, drilling rigs in oilfields use an additive called bentonite. Bentonite is thin and free-flowing during pumping, but acquires a gel-like structure when motion stops. Drilling rigs that use hydraulic fluids are also available. Some drilling rigs are equipped with automated pipe-handling systems for positioning drill pipes and drill collars. Typically, these trailer-mounted systems are designed to lift pipes from a horizontal position and bring them into alignment with a tilting top-head. Features include hands-free operation, adjustable pipe racks, and hydraulic rack sets for loading pipes from either side. Typically, the hydraulic pipe racks are positioned between drilling rigs and pipe transport trailers or flat bed trucks. Specifications for automated pipe-handling systems include tubular length, tubular diameter with jaws removed, slide travel, load height, transport width with hydraulic pipe racks, transport width without hydraulic pipe racks, transport length, and weight with racks stowed.