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short life. If the driller does not control the drill stem at all times
an easy fishing job becomes extremely difficult and a difficult one
becomes almost impossible. Because of this, several methods have
been tried in an attempt to ease the kelly through the drive
bushing. Hard metal liners have been used - set flush against the
kelly - in the bushing. Rollers, numbering from one to five on each
side of the kelly, are being employed, with many arguments
concerning their effectiveness.

Figure 7.3 – Master and drive bushing

Perhaps the greatest advancement since Kelly's name
became a part of rotary drilling is the advent of the hexagonal
kelly. Although the square Kelly is still in wider usage, the hex has
several distinctive advantages. Their manufacturers claim great
torque and tensional strength, greater adaptability all drilling

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