STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 775
Figure 4-140. Roller cone bit bearings design [44]. (Courtesy SPE.)
Sealed Friction Bearing (Journal Bearing). The journal bearing, developed
to match the life of carbide cutting structures, does not contain rollers; but
contains only a solid journal pin mated to the inside surface of the cone. This
journal becomes the primary load carrying element for the cone loads.
Advances in product design, metallurgy and manufacturing processes have
produced a journal-bearing featuring precisely controlled journal, pilot pin and
thrust-bearing surfaces. The bearing is designed and manufactured to ensure
that all bearing elements are uniformly loaded. Substantially higher weights and
rotary speeds can be run without decreasing bearing life. Sealed journal bearings
provide the best wear resistance at normal rotary speeds through a combination
of better load distribution and precision-machined surfaces.
Sealed Ball and Roller Bearing (Self-Lubricating). The sealed ball and roller
bearing was introduced in carbide tooth bits, but is now primarily in steel tooth
bits and generally lasts as long as the cutting structure. Some carbide tooth bits
of 12 $-in. and larger sizes also are available with this type bearing. Sealed roller
bearings are lubricated by clean grease rather than drilling mud and thus tend
to last longer than standard roller bearings.
Nonsealed Ball and Roller Bearings. The nonsealed ball and roller bearings
were introduced to replace the primitive friction journal bearing at a time when
only steel tooth bits were available. They operated well in mud, and in many
cases were adequate to last as long as or longer than the cutting structures they
served. Today, the nonlubricating bearings are used in steel tooth bits to drill
the top section of the hole where trip time is low and rotary speed are often high.
The major portion of the radial load on the cone cutter is absorbed by the
roller race, with the nose bearing absorbing a lesser amount. The thrust surface