Roller Cone Bit Design
The design of roller cone bits can be described in terms of the four principle elements of their design. The following aspects of the design will be dealt with in detail:
• Bearing assemblies
• Cones
• Cutting elements
• Fluid circulation
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:
• Roller bearings, which form the outer assembly and help to support the radial loading (or WOB)
• Ball bearings, which resist longitudinal or thrust loads and also help to secure the cones on the journals
• 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.
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Cone Shell Thickness |
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Friction Pin Radial Bearing Surface Outside Ball Bearing Flange _______ Friction Pin Thrust Bearing Surface Shirttail________________________ |
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BALL RETAINING PLUG |
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Figure 6 Details of bearing structure |
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Weld Groove |
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Ball Race Contour |
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Ball Loading Hole |
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.
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Figure7 Sealed bearing bit |
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Flexible Diaphragm |
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. Lubricant Passage |
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Shirttail Hardfacing Hard Metal Inlay Silver Infiltrated Bushing Gauge Insert |
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Figure 8 Journal bearing bit |
All three cones have the same shape except that the No. 1 cone has a spear point. One of the basic factors to be decided, in the design of the cones, is the journal or pin angle (Figure 9). The journal angle is formed between the axis of the journal and the horizontal. Since all three cones fit together, the journal angle specifies the outside contour of the bit. The use of an oversize angle increases the diameter of the cone and is most suitable for soft formation bits. Although this increases cone size, the gauge tip must be brought inwards to ensure the bit drills a gauge hole.
One important factor which affects journal angle is the degree of meshing or interfit (i. e. the distance that the crests of the teeth of one cone extend into the grooves of the other). The amount of interfit affects several aspects of bit design.
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Figure 9 Journal or pin angle |
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Heel
Figure 10 Cone slippage |
It allows increased space for tooth depth, more space for bearings and greater cone thickness
It allows mechanical cleaning of the grooves, thus helping to prevent bit balling
It provides space for one cone to extend across the centre of the hole to prevent coring effects
It helps the cutting action of the cones by increasing cone slippage.
In some formations, it is advantageous to design the cones and their configuration so that they do not rotate evenly but that they slip during rotation. This Cone slippage, as it is called, allows a rock bit to drill using a scraping action, as well as the normal grinding or crushing action.
Cone slippage can be designed into the bit in two ways. Since cones have two profiles: the inner and the outer cone profile, a cone removed from the bit and placed on a horizontal surface can take up two positions (Figure 10). It may either roll about the heel cone or the nose cone. When the cone is mounted on a journal it is forced to rotate around the centre of the bit. This “unnatural” turning motion forces the inner cone to scrape and the outer cone to gouge. Gouging and scraping help to break up the rock in a soft formation but are not so effective in harder formations, where teeth wear is excessive.
Cone slippage can also be attained by offsetting the axes of the cones. This is often used in soft formation bits (Figure 11). To achieve an offset the journals must be angled slightly away from the centre. Hard formation bits have little or no offset to minimise slippage and rely on grinding and crushing action alone.