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Roller cone bit components
The bearing, seal, and lubrication systems of a roller cone bit are important aspects of bit life and efficiency.
- 1 Bearing systems
- 2 Seal systems
- 3 Lubrication systems and lubricants
- 4 References
- 5 Noteworthy papers in OnePetro
- 6 External links
- 7 See also
- 8 Page champions
- 9 Category
Roller cone bearing systems are designed to be in satisfactory operating condition when the cutting structure of the bit is worn out. To achieve this standard of bearing performance, modern goals for seal and bearing system life are 1 million or more revolutions of a bit without failure, as opposed to ≈300,000 or fewer in the recent past. To achieve this goal, research into bearing, seal, and lubricant designs and into materials that improve seal and bearing life is ongoing.
Roller-cone bits primarily use two types of bearings: roller bearings and journal bearings, sometimes called friction bearings. Each type is normally composed of a number of separate components, including:
- Primary bearings
- Secondary bearings
- Seal system
- Features that resist thrust loading
- Cone retention balls
- A lubrication system (Fig. 1)
Primary bearings are as large as possible within the limits of available space. Secondary bearings are smaller, reduced-diameter bearings located adjacent to the interior apex area in a cone. Secondary bearings provide supplemental load-bearing capability. Primary and secondary bearings can individually be either roller bearings or journal bearings. It is not uncommon for a bearing system to be made up of combinations of the two.
Seals prevent cuttings and drilling fluids from entering the bearing system and prevent lubricant from escaping the bearing system. Thrust washers are located on the end of leg journals and between the primary and secondary bearing surfaces to resist axial loading.
Most roller-cone bits incorporate what appears to be a ball-type bearing. This is the cone retention feature. The balls prevent cones from separating from their journals. Finally, the lubrication system contains the lubricant that, throughout the life of the bearing system, provides lubrication to bearings and seals. These features are described below.
Roller bearings are a common bit-bearing system because they can reliably support large loads and generally perform well in the drilling environment (Fig. 2). They are typically used on larger-diameter bits (> 14 in.), which have more physical space to accommodate the rollers. To enhance bearing life, leading manufacturers continually research bearing materials, sizing, and shape.
Journal bearings consist of at least one rotating surface separated from the journal by a film of lubricant. The surfaces are specially designed so that the film of lubricant separates them; were they to touch, mating bearing components would gall or possibly fuse. As long as satisfactory lubrication is provided and loading remains within design limits, journal bearings are extremely efficient. Fig. 3 compares roller-bearing and journal-bearing assemblies.
Design of journal bearings
Journal bearings must provide a balanced bearing geometry and adequate journal strength and must maximize the thickness of the high-pressure lubricating film developed during hydrodynamic lubrication. The thickness of lubricating films (Fig. 4) is affected by:
- Surface areas
- Journal and cone diameters
- Clearances between journal and cone
Manufacturing tolerances must be precise so that surfaces run true. Roundness of journal and cone surfaces is important, and if any part of a bearing is out of round, the effectiveness of the lubrication regime will be adversely affected.
The metallurgy of a bearing must be balanced to minimize heat generation during boundary lubrication. Cone-bearing surfaces are steel. Soft, silver-plated sleeves are installed on the journal. Silver polishes easily, and minor surface irregularities from machining are quickly smoothed. This smoothness ensures low-friction operation and uniform lubricant flow over the bearing surface (Fig. 5).
Open bearing systems
Nonsealed roller bearings, referred to as open bearing systems, are typically used in large-diameter (> 20 in.) bits. These bits are often used to drill from surface to relatively shallow depths with a simple drilling fluid system (e.g., seawater). This drilling application does not necessitate the use of seals in the bits. They rely on the drilling fluid for cooling, cleaning, and lubrication of the bearings.
In general, seal systems are classified as either static or dynamic. Roller-cone bits use both types of seals. Dynamic seals involve sealing across surfaces that are moving in relationship to one another, as would be the case for a bearing seal. Seal parts or surfaces that do not move in relationship to one another during bit operation, such as the seal between a hydraulic nozzle and a bit to prevent leakage around the joint, are static seals.
Roller-cone bearing seals operate in an exceptionally harsh environment. Drilling mud and most cuttings are extremely abrasive. Drilling fluids often contain chemicals, and operating temperatures can be sufficiently high to break down the elastomers from which seals are made. Pressure pulses often occur in downhole drilling fluids that apply lateral loading on seals that must be resisted.
On a purely practical level, bearing seals have two functions: to prevent foreign materials, such as mud, cuttings, chemicals, and water, from entering the bearings and to prevent bearing lubricant from escaping the bit.
Visualize the difference in the nature of these two duties. On the interior side, the seal is excluding clean, functional lubricant from escaping the bit, while on the exterior side, the seal is excluding dirt and chemicals from penetrating the bit. The separation of these two extremely different functions takes place at a small point between the two sides of a seal. If either of these functions breaks down, the bearings and the bit could be destined for failure.
In a rotating bearing, the two working sides of a seal are called the static energizer and the dynamic wear face. These two parts are directly opposite each other, with the energizing portion bearing on the gland and the dynamic wear face bearing against the rotating unit. For the energizing portion of the seal to function properly, it must have a surface against which to react. This is provided by a channel-shaped groove called a seal gland.
The wearing portion of the seal must have the capability to withstand the heat and abrasion generated as the rotating surface passes over it. The energizer, when functioning correctly, is not a high-wear area. Ideally, it simply bears against the gland and provides the pushing energy that maintains firm contact between the wear surface and rotating cone.
Donut-shaped O-rings are used in many roller-cone bit applications. O-rings are manufactured from elastomers (synthetic rubbers) that withstand the temperatures, pressures, and chemicals encountered in drilling environments. They are a traditional, but still consistently reliable, seal system.
An O-ring is installed in a seal gland to form a seal system. The gland holds the O-ring in place and is sized so that the O-ring is compressed between the gland and the bearing hub at which sealing is required. It presses the interior wall of the O-ring against the hub and the exterior diameter of the O-ring against the gland. These latter forces prevent the seal from turning in the gland and experiencing wear on the outer surfaces by rotational contact with the gland (Fig. 6).
Lubrication of seals
Seals must be lubricated to prevent high wear rates and excessive temperatures that could lead to seal material failures. Lubricant for the bearings also lubricates the seals.
Lubrication systems and lubricants
Lubricants play a vital role in bearing performance. They provide lubrication for both bearings and seals, and they provide a medium for heat transfer away from the bearings. To achieve these functions, lubricants are specially engineered and continually improved. Lubrication systems are engineered to provide reserve storage, positive delivery to the bearing system, capacity for thermal expansion, and pressure equalization with fluids on the bit exterior.
Lubrication systems include a resupply reservoir large enough to ensure availability of lubricant for all lubrication functions throughout the life of the bit. A small positive pressure differential in the system ensures flow from reservoir to bearings. The system is vented to equalize internal and external reservoir pressures. Without equalization, a pressure differential between bit exterior and interior could be sufficient to cause seal damage, leading to bearing failure.
High drilling temperatures and high pressures in the lubrication system, together with the potential of exposure to water and chemicals, require high performance from lubricants. Most bit lubricants are specially formulated. Leading bit manufacturers employ scientists to develop and test lubricants. Better drill-bit lubricants are stable to temperatures > 300°F, and many function normally at temperatures down to ≈0°F. They are hydrophobic (repel water) and retain their stability if water penetrates the bit. Quality lubricants are also resistant to chemicals commonly found in drilling fluids, are environmentally safe, and do not contain the lead additives that have traditionally helped resist high pressures.
- Lubricant supply. Roller-cone bits typically contain one lubricant reservoir in each leg (Fig. 7). Thus, for a three-cone bit, there are three reservoirs. Each must have the capacity for sufficient reserves of lubricant for operation of the bearing assembly it serves throughout the bit’s life.
- Pressure equalization and relief. A column of drilling fluids and cuttings contained in a well exerts very high pressures on a bit operating at the well bottom. These high pressures are applied to the seal system and must be resisted by lubricant in the seal and bearing system. At installation, lubricant is at atmospheric pressure and cannot provide significant resistance to well-bottom pressures. Accordingly, internal lubrication system pressures equalize themselves with external bit pressures to prevent seal failure caused by differential pressure. Equalization is accomplished by a small relief valve installed in the lubricant reservoir system.