STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

774 Drilling and Well Completions
Cone Cutter Design
To understand how cone geometry can effect the way rock bit teeth cut rock,
consider the moderate soft formation cone shown schematically in Figure 4-159
[45]. Such cones are designed to depart substantially from true rolling action
on the bottom of the borehole. They have two or more basic cone angles, none
of which has its apex at the center of bit rotation. The conical heel surface tends
to rotate about its theoretical apex and the inner row surface about the center
of its own apex. Since the cones are forced to rotate about the bit centerline,
they slip as they rotate and produce a tearing, gouging action. This action is
obtained by moving the cone centerline away from the center of bit rotation,
as shown in Figure 4-139. Bits for hard formation have cones that are more
nearly true rolling and use little or no cone offset. As a result, they break rock
primarily by crushing.
The bearing journal angle specified in Figure 4-139 (relative to horizontal) is
reduced for softer bits and increased for harder bits. This alters the cone profile
which in turn affects tooth action on the bottomhole and gauge cutter action
on the wall of the hole. No roller cone bit has truly conical-shaped cones, but
softer bits have more highly profiled, i.e., less-conical cones than harder bits.
This increases the scraping action of both bottomhole cutters and gauge
surfaces. The scraping action is beneficial for drilling soft formations but it will
result in accelerated tooth and gauge wear if the formation is relatively abrasive.
Scraping action is minimized on hard formation bits where strength and abrasion
resistance are emphasized in the design.
Bearing Design
The major bearing design used in present rock bits are shown in Fig. 4-140
[44]. Three styles of bearing designs are generally available: non-sealed roller
bearings, sealed roller bearings, and sealed friction bearings. Another name for
friction bearings is journal bearings. A fourth style features air-cooled non-sealed
roller bearings intended for air drilling applications.
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SOFT FORMATION CONE DESIGN
OFFSET
Figure 4-139. Roller bit cone design features. (Courtesy Canadian
Association of Oilwell Drilling Contractors.)