The equipment for rotary drilling a borehole (Figure D-28). Usually consists of hoisting, rotating, circulation, blow-out prevention, and power systems. Hoisting usually involves a derrick (generally assembled at the site) or a mast (mostly preassembled) topped with a crown block (an assembly of pulleys or sheaves) and a traveling block (a pulley assembly that can move up and down that supports the swivel that allows the kelly to rotate freely). Drawworks for pulling the wire ropes are usually located on the rig floor. A rotary table and kelly bushing to rotate the kelly and drill stem is usually located in the center of the rig floor. The kelly is a heavy steel pipe (often square or hexagonal in shape) that passes through the rotary table and is connected to the drill stem; it transmits torque from the kelly bushing to rotate the drill stem (drill pipe). The rig floor is usually elevated to leave room for blow-out preventors (valves that can be closed in event high-pressure fluids threaten to eject material from the borehole). A drill bit (q.v.; see Figure D-26) to cut or bore the hole is located at the lower end of the drill stem. A heavy drill collar to put weight on the bit is located immediately above the bit and additional down-hole tools are often located immediately above the drill collar (Figure D-27). Circulation is accomplished by pumping fluid (mud, q.v.) down through the drill stem and bit and up in the annulus surrounding the drill stem. The mud that emerges from the annulus is sampled, its viscosity and density are measured, and gas and matter suspended in the mud are removed to clean up the mud and condition it for being pumped back through the drill stem. Other types of drills are also used, including the use of downhole motors often powered by the circulating mud.