The cones of a roller cone bit are mounted on journals as shown in Figure 6. There are three types of bearings used in these bits:
1. Roller bearings, which form the outer assembly and help to support the radial loading (or WOB)
2. Ball bearings, which resist longitudinal or thrust loads and also help to secure the cones on the journals
3. A friction bearing, in the nose assembly which helps to support the radial loading. The friction bearing consists of a special bushing pressed into the nose of the cone. This combines with the pilot pin on the journal to produce a low coefficient of friction to resist seizure and wear.
All bearing materials must be made of toughened steel which has a high resistance to chipping and breaking under the severe loading they must support. As with all rock bit components, heat treatment is used to strengthen the steel.
The most important factor in the design of the bearing assembly is the space availability. Ideally the bearings should be large enough to support the applied loading, but this must be balanced against the strength of the journal and cone shell which will be a function of the journal diameter and cone shell thickness. The final design is a compromise which ensures that, ideally, the bearings will not wear out before the cutting structure (i.e. all bit components should wear out evenly). However, the cyclic loading imposed on the bearings will, in all cases, eventually initiate a failure. When this occurs the balance and alignment of the assembly is destroyed and the cones lock onto the journals.
There have been a number of developments in bearing technology used in rock bits :
The bearing assemblies of the first roller cone bits were open to the drilling fluid. Sealed bearing bits were introduced in the late 1950s, to extend the bearing life of insert bits. The sealing mechanism prevents abrasive solids in the mud from entering and causing excess frictional resistance in the bearings. The bearings are lubricated by grease which is fed in from a reservoir as required. Some manufacturers claim a 25% increase in bearing life by using this arrangement (Figure 7).
Journal bearing bits do not have roller bearings. The cones are mounted directly onto the journal (Figure 8). This offers the advantage of a larger contact area over which the load is transmitted from the cone to the journal. The contact area is specially treated and inlaid with alloys to increase wear resistance. Only a small amount of lubrication is required as part of the sealing system. Ball bearings are still used to retain the cones on the journal.
Institute of Petroleum Engineering, Heriot-Watt University