KELLY MASTER BUSHING
A Kelly, a Kelly-drive bushing, a master bushing and a rotary table rotate the drill string and bit on some rigs. The Kelly is a heavy tubular device. It usually has either four or six sides. That is, it either has a square or a hexagonal cross-section. Square Kellies are less expensive than hexagonal ones but the hex Kellies are stronger so rigs drilling deep holes often use them. Whether four- or six-sided, crew members attach or make up the Kelly to the top joint of pipe in the drill string. Moreover, The Kelly, four sided or square in this example, moves through a square opening in the Kelly-drive bushing. The Kelly-drive bushing mates with the master bushing which the rotary table turns. This rotates the entire drill string and attached bit. The Kelly moves down as the hole deepens.