DRILLING BITS:
ROLLER CONE BITS:
As the name implies, roller cone bits are made up of (usually) three equal-sized cones and three
identical legs which are attached together with a pin connection. Each cone is mounted on
bearings which run on a pin that forms an integral part of the bit leg. The three legs are welded
together and form the cylindrical section which is threaded to make a pin connection. The pin
connection provides a means of attachment to the drill string Each leg is provided with an
opening for fluid circulation. The size of this opening can be reduced by adding nozzles of
different sizes. Nozzles are used to provide constriction in order to obtain high jetting velocities
necessary for efficient bit and hole cleaning. Mud pumped through the drillstring passes through
the bit pin bore and through the three nozzles, with each nozzle accommodating one third of the
total flow, if all the nozzles were of the same size.
There are two types of roller cone bits:
Milled tooth bits: Here the cutting structure is milled from the steel making up the cone
Insert bits: The cutting structure is a series of inserts pressed into the cones.
DESIGN FACTORS
The drill bit design is dictated by the type of rock to be drilled and size of hole. The three legs and
journals are identical, but the shape and distribution of cutters on the three cones differ. The design
should ensure that the three legs must be equally loaded during drilling. The following factors are
considered when designing an manufacturing a three-cone bit:
1-Journal angle.
2-Cone profile.
3-Offset between cones.
4-Teeth.
5-Bearings.
1-Journal angle: The bit journal is the bearing load-carrying surface. The journal angle is defined
as the angle formed by a line perpendicular to the axis of the journal and the axis of the bit, The
magnitude of the journal angle directly affects the size of the cone; the size of the cone decreases
as the journal angle increases. The journal angle also determines how much WOB the drill bit can
sustain; the larger the angle the greater the WOB. The smaller the journal angle the greater is the
gouging and scraping actions produced by the three cones. The optimum journal angles for soft
and hard roller cone bits are 33 degrees and 36 degrees, respectively.
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