Friday, 10 November 2017

UNDERSTAND ALL ABOUT ROLLER CONES DRILL BIT BEARINGS SYSTEM

Drill Bit Bearings System

Drill Bits Bearing Systems

Various types of Roller Cone Bit lubrication systems are used for bearing protection. Early systems included using the drilling fluid as the lubricant, while more recent systems use greases and oils for lubrication. Drilling fluids containing abrasive solids soon proved to be a limiting factor relative to long bit runs and, as a result, are seldom used as the lubricant in present-day bits. 

Non-sealed roller ball bearings bit

Non-sealed roller ball bearings do not have any mechanism to prevent drilling fluids entry into the bearing nor to prevent the exit of bearing lubricants. These bearings are generally packed with a highly viscous grease, as shown in Fig.1 . The grease lubricates the bearings and, since it has a high viscosity, is not easily displaced by the drilling fluid. If mud does enter the bearing area, however, galling and pitting soon begin, causing bearing failure.

Rolling Cone Bit Bearing System
Fig.1: Non-sealed bearing lubrication

Sealed roller ball bearings bit

Sealed ball bearing bits have a seal between the cone back and the interior of the bit leg. The seal prevents mud entry into the bearing and thus prolongs bearing life. It has been estimated that roller sealed-bearing bits can be operated 30% longer than non-sealed bits due to the exclusion of mud from the bearing area.

The lubricant for the ball bearings on a sealed bit is a light oil, while a non-sealed bit uses viscous grease. The light oil provides a more eto leak, however, the sealed-bearing bit will quickly fail due to the ease of displacement ,of the light oil with the drilling fluid.

How Sealed Roller Ball Bearing Bit Protect its seal

The sealed-bearing Roller Cones Drill Bits uses a reservoir and pressure compensation system, as shown in Fig.2, to protect its seal's integrity. The compensation system balances internal oil and seal pressure with external mud hydrostatic pressure toffective lubricating action than the heavy grease, hence the extended bearing life. If the rubber seal begins minimize the differential pressure across the bearing seal. The primary components of the pressure compensation system are as follows:

  • compensator plug
  • locking mechanisms such as a snap ring
  • compensation recess or reservoir
  • connection holes to the bearing

In addition, a small reserve of oil is maintained in case a small amount is lost past the seals.

Rolling triCone Bit Sealed Bearing system components
Fig.2 Sealed Bearing system components

Why Still The Ball Bearing System In Roller Cones Bit Keep failure?

There is, however, one serious drawback to the roller-ball roller bearing system. The primary cause of roller bearing failure is journal spalling, which causes destruction of the rollers and the locking of the cone.

What Is The Journal Bearing System And How It Solves The Failure Of Roller Balling Bearing System?

To remedy this, instead of the standard roller bearing assembly, the “journal bearing” system utilizes solid metal bushings for direct cone to journal contact. This offers a distinct mechanical advantage over roller arrangements in that it presents a larger contact area at the load bearing point. This distribution of the load eliminated the chief cause of roller bearing assembly failure - spalling in the load portion of the bearing face.

Journal bearing systems in the tungsten carbide insert drill bits features a metal bearing surface combined with a hard faced journal and a lubricant. Specialized seals and reliable pressure equalization systems keeps the drilling fluid and formation contaminants out of bearings, and positively seals the graphite-based lubricant inside the bearing. Precision fit of the journal and cone distributes contact loading evenly throughout a near perfect arc. Bearing surfaces are finished to a carefully controlled surface texture to ensure optimum lubrication.

What Is The Roller Cone Drill Bit Journal Bearing System Components?

The manufacturing of the journal bearing system consists of having the journals either milled, grooved or pressed (depending on the drill bit company) to accommodate the bushing. Then the bushings are inlaid on the journal. Once the cone is fitted with teeth and gauge protection, the journal is then machine-pressed into the cone. To complete the seal between the cone and the journal, special rings (seals) have been developed.
Roller Cone Bit Journal bearing System
Fig.3: Roller Cone Drill Bit Journal bearing System


Journal Bearing System Seals

The first and still most popular seal is the radial seal (used mainly on the sealed roller bearing bits). The radial seal is a circular steel spring encased in rubber, which seals against the face of the shank and the face of the cone. The newer “O” ring seal is considered the most effective seal. The major problem confronting the “O” ring is tolerance, which must be precise in order to maintain an effective seal.

Journal Bearing Lubricating System

An understanding of lubricants and lubricating systems is necessary for successful drilling operations. The lubricating systems are essentially the same, and are composed of an external equalizer located under the drill bit or on back of the shanks, a grease reservoir with some sort of expandable diaphragm to distribute the grease, and some sort of distribution system to the bearings. In addition, there is a pressure relief valve to release any trapped pressure, which might otherwise rupture the seals.

To complete the journal-cone assembly, a positive seal is required to keep drilling fluid out, while allowing the graphite lubricant in, which keeps the bearings from overheating. The positive seal requires a relief valve to allow escape of excess pressure, which can overload the seal and cause seal failure.

Factors Affect Any Sealed Bearing Roller Cone Bits

The Effect Of Pressure Surges On Any Sealed Bearing Roller Cone Bits

Pressure surges can be detrimental to these sealed systems. As pressure and temperature increase, the viscosity of the lubricant in the roller cone drill bit increases. As a result, the journal bearing system cannot instantaneously compensate for abrupt changes in pressure due to surges (going into the hole, making connections, etc.) and small quantities of mud invade the system. With the close tolerance necessary for effective sealing, mud solids can be damaging.

The Effect Of Good Cleaning On Any Sealed Bearing Roller Cone Bits

Adequate cleaning is even more important with sealed bearing bits. If drilled cuttings are allowed to build up around the shirttail, seal damage and premature bearing failure may result. Gauge protection is also important to seal and bearing life, because seal damage can occur from shirttail wear caused by inadequate gauge protection. Any time a sealed bearing drill bit is rerun, the seals and shirttail should be carefully checked for excessive wear or grooving.

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